PT8452 MOULD MANUFACTURING TECHNOLOGY
UNIT I - FUNDAMENTALS OF MOLD MAKING
Mold Making: selection of materials for mold making, Mechanism of metal cutting, types of tools, influence of tool angles, Cutting fluids. Basics of machining operations: Turning, Cylindrical Grinding, Surface Grinding & Vertical Milling.
UNIT II - ELECTRICAL DISCHARGE MECHINING
Electrical discharge machining – Principle, Types of EDM - Die Sinking & Wire Cut EDM, Machining Process, Requirements of dielectric fluid, Applications of EDM in mold making.
UNIT III - ELECTRO FORMING PROCESS
Electroforming for mold manufacturing - discussion of the process, materials for electroforming, design & materials for models, machining for electroformed mold cavities, Advantages, Disadvantages.
UNIT IV - HOBBING AND CHEMICAL TEXTURING
Hobbing for mold cavity making - Discussion of the hobbing process, elements of hobbing, materials used for cavity, lubrication, and depth of hobbing, advantages and disadvantages.
Surface Texturing of molds – Chemical Texturing, Process description, Advantages- Limitations of chemical texturing.
UNIT V - METOROLOGY AND INSPECTION
Metrology and inspection: Vernier caliper, Micrometer, Vernier height gauges, Surface plate, Slip gauges, Sine Bar, Rockwell Hardness, Optical profile projectors and Optical flat.
UNIT-I MOLD MANUFACTURING ENGINEERING
• Mold Making: Selection Of Materials For Mold Making,
• Mechanism Of Metal Cutting,
• Types Of Tools,
• Influence Of Tool Angles,
• Cutting Fluids.
• Basics Of Machining Operations:
• Turning (Lathe),
• Cylindrical Grinding, Surface Grinding
• Vertical Milling.
MOLD-DEFINITION:
Mold comprises core and cavity which forms an Impression to give shape to molten or hot liquid material when it cools and hardens. Core
A protrusion, or set of matching protrusions, in a mold which forms the inner surfaces of the molded articles Cavity A depression, or a set of matching depressions, in a mold which forms the outer surfaces of the molded articles.
Typically injection pressures range from 5000 to 20,000 psi. Because of high pressures involved, molds must be clamped shut during injection and cooling by clamping forces measured in tons.
MOLD Steel Properties Mold Component STEEL P-20 Pre-hardened, machines well, high carbon, Mold bases, ejector plates, and some general-purpose steel. cavities (if nickel or chrome plated to prevent rust). Disadvantage: May rust if improperly stored. H-13 Good general purpose tool steel. Can be Cavity plates and core plates. polished or heat-treated. Better corrosion resistance. S-7 Good high hardness, improved toughness, Cavity plates, core plates and general-purpose tool steel. Machines well, laminates, as well as thin wall shock resistant, polishes well. sections.
Disadvantage: Higher cost. A-2 Good high toughness tool steel. Heat-treats Ejector pins, ejector sleeves, and and polishes well. ejector blades. D-2 Very hard, high wear characteristics, high Gate blocks, gib plates to prevent vanadium content, somewhat brittle. galling, gate blocks to prevent wear.
Disadvantage: Difficult to machine. 420 SS Tough corrosion resistant material. Heat- Cavity blocks, ejector pins, sleeves, treats and polishes well. etc.
Disadvantage: High cost.
o Hot rolled steel materials vary from low carbon steel (A-36 or 1020) up to medium carbon steel (1045 or 1050). These steels are easy to machine and have reasonable tensile strength. They are easy to find and are very cost-friendly. They are usually chosen when the customer has very low production runs associated with the project. • Chrome-moly materials range from 4130, P20, 4140, etc., have a hardness range from 28- 34 HRC and have good mechanical properties. They are ideal for cavity and core plates as well as other plates required in the mold base. They can be machined fairly well; however, in some cases with heavy machining or grinding, a need to stress relieve the material may be required. • Stainless steel (a modified 420 F material that is used for holder block applications) is pre-hardened to 30-35 HRC. It offers good corrosion resistance and has very good machinability. The material is very stable and does not require stress relieving. In applications where humidity is a problem or corrosive material is being used this material works well. • Today there are two additional mold base materials to consider. The first material falls in the stainless steel category (see Chart 1). It is an improved 420 F material currently competing with 1.2085 in Europe. It has excellent machining qualities and does not need to be stress relieved. By lowering the carbon and chrome content, this material has improved machinability, and has 10- 15% better thermal conductivity than the 420 F material currently being used • This material also benefits from tightened rolling tolerances, which means a mill can deliver higher dimensional quality steel to the end user, saving them time and money. For example, currently the leading stainless material requires around 6 mm oversize to finish to the desired dimension. The improved rolling capabilities can reduce the amount of oversize needed by about 20 to 33 percent, lowering costs for customers. The mill can roll material up to 5.5” thick and up to 96” wide. • The second material is a P20-type material that has replaced 4140 and P20 by two large mold base manufacturers in Europe. The reason for the switch was better machinability, and because this material is very stable—even during heavy machining and grinding— there is no need for stress relieving. Gun drilling is also much easier because the material does not have hard spots. This material is supplied 30-35 HRC and possesses a unique chemistry. It also is heat treated differently than other types of quench and tempered steels. • A fast quench method of heat treatment is applied. The material comes off the rolling mill and immediately receives the fastest quench possible. There is no tempering process with this material. This process gives this material its unique properties
Mold Types
Compression Mold Types Injection Mold Types Open Flash Conventional Cold Runner Positive type Molds (two plate) two plate Semi positive type molds Three plate Runner Molds Transfer Mold Types Hot Runner Molds Plunger Type Insulated Runner Molds Pot Type Split Cavity Mold Rotational Mold
INJECTION MOLD & ITS PARTS
Two-Plate Mold:
A two plate cold runner mold is the easiest and least expensive type of mold to produce.
• Two plate molds have a single parting plane, and the mold splits into two halves at the plane • Because the runner system must be in line with the parting plane, the part can only be gated on its perimeter
Injection Mold elements
Mold comprises core and cavity which forms an Impression to give shape to molten or hot liquid material when it cools and hardens. Core
A protrusion, or set of matching protrusions, in a mold which forms the inner surfaces of the molded articles Cavity A depression, or a set of matching depressions, in a mold which forms the outer surfaces of the molded articles.
Register Ring : It is used to align injection molding machine with injection mold, usually made from medium carbon steel material case hardened. Sprue Bush : It will have 1.5° to 3° taper from injection molding machine nozzle the material enters into the mold through sprue bush, usually made from medium carbon steel material case hardened. Top Plate: It is used to clamp the top half of the mold with the moving half of the molding machine, usually made from mild steel material. Cavity Plate: The plate used to fit the cavity insert. The cavity insert will have gap of the details to fill the plastic material and form plastic component. Cavity plate usually of mild steel material cavity insert usually of hot die steel (through hardened) P20 direct use without hardening after cavity machining. Core Plate: The plate used to fit the core insert. The core insert will have projection and will create hallow portion in plastic component Core plate usually of mild steel material and core insert usually of hot die steel (through hardened) P20 material direct use without hardening after core machining. Sprue Puller Pin: Sprue puller pin pulls the sprue from the sprue bush, usually made of medium carbon steel material case hardened. Core Back Plate: It prevents the core insert coming out and act as stiffener to core plate usually of mild steel.
Guide Pillar and Guide Bush: The main purpose of a guide pillar and guide bush is to align fixed and moving halves of mold in each cycle. They are usually made of medium carbon steel and will have higher hardness. Ejector Plate: It is used to accommodate ejector pins usually of mild steel material. Ejector Back Plate: It prevents the ejector pins coming out usually of mild steel material. In addition to the cavity, there are other features of the mold that serve indispensable functions during the molding cycle. A mold must have a distribution channel through which the polymer melt flows from the nozzle of the injection barrel into the mold cavity.
CUTTING TOOL A cutting tool is any tool that is used to remove metal from the work piece by means of shear deformation. It is one of most important components in machining process. It must be made of a material harder than the material which is to be cut, and the tool must be able to withstand the heat generated in the metal cutting process. • Two basic types – Single point cutting tool (eg. Lathe, shaper, planer tools) – Multiple point tool (milling cutters, end mill, drill bits)
Tool Signature Convenient way to specify tool angles by use of a standardized abbreviated system is known as tool signature or tool nomenclature. It indicates the angles that a tool utilizes during the cut. It specifies the active angles of the tool normal to the cutting edge. The seven elements that comprise the signature of a single point cutting tool can be stated in the following order: Tool signature 5-7-6-8-15-16-0.8 1. Back rake angle (5°) 2. Side rake angle (7°) 3. End relief angle (6°) 4. Side relief angle (8°) 5. End cutting edge angle (15°) 6. Side cutting edge angle (16°) 7. Nose radius (0.8 mm)
CUTTING TOOL CHARACTERISTICS
Cutting Tool Geometry
Cutting Tool Size : It is determined by the width of shank, height of shank and overall length. Shank Shank is main body of a tool. It is held in a holder.
Flank : Flank is the surface or surfaces below and adjacent to cutting edge.
Heel: Heel is intersection of the flank and base of the tool.
Base: Base is the bottom part of the shank. It takes the tangential force of cutting.
Face: Face is surface of tool on which chip impinges when separated from workpiece.
Cutting Edge: Cutting edge is the edge of that face which separates chip from the workpiece. The total cutting edge consists of side cutting edge, the nose and end cutting edge.
Tool Point: That part of tool, which is shaped to produce the cutting edge and the face.
The Nose: It is the intersection of side cutting edge and end cutting edge.
Neck: Neck is the small cross section behind the point.
Side Cutting Edge Angle: The angle between side cutting edge and side of the tool shank is called side cutting edge angle. It is also called as lead angle or principle cutting angle.
End Cutting Edge Angle : The angle between the end cutting edge and a line perpendicular to the shank of tool is called end cutting edge angle. Side Relief Angle: The angle between the portion of the side flank immediately below the side cutting edge and line perpendicular to the base of tool measured at right angles to the side flank is known as side relief angle. It is the angle that prevents interference, as the tool enters the work material.
End Relief Angle: End relief angle is the angle between the portion of the end flank immediately below the end cutting edge and the line perpendicular to the base of tool, measured at right angles to end flank. It is the angle that allows the tool to cut without rubbing on the workpiece.
Back Rake Angle: The angle between face of the tool and a line parallel with the base of the tool, measured in a perpendicular plane through the side cutting edge is called back rake angle. It is the angle which measures the slope of the face of the tool from the nose toward the rear. If the slope is downward toward the nose, it is negative back rake angle. And if the slope is downward from the nose, it is positive back rake angle. If there is not any slope, back rake angle is zero.
Side Rake Angle : The angle between the face of the tool and a line parallel with the base of the tool, measured in a plane perpendicular to the base and side cutting edge is called side rake angle. It is the angle that measures the slope of the tool face from cutting edge. If the slope is towards the cutting edge, it is negative side rake angle. If the slope is away from the cutting edge, it is positive side rake angle. All the tool angles are taken with reference to the cutting edge and are, therefore, normal to the cutting edge.
Influence of Various Angles on Cutting Tool and their Significance
Side Cutting Edge Angle:
Relief Angles
MECHANICS OF METAL CUTTING
The machine tools involve various kinds of machines tools commonly named as lathe, shaper, planer, slotter, drilling, milling and grinding machines etc. The machining jobs are mainly of two types namely cylindrical and flats or prismatic. Cylindrical jobs are generally machined using lathe, milling, drilling and cylindrical grinding whereas prismatic jobs are machined using shaper, planner, milling, drilling and surface grinding. In metal cutting operation, the position of cutting edge of the cutting tool is important based on which the cutting operation is classified as orthogonal cutting and oblique cutting.
Orthogonal cutting is also known as two dimensional metal cutting in which the cutting edge is normal to the work piece. In orthogonal cutting no force exists in direction perpendicular to relative motion between tool and work piece.
Oblique cutting is the common type of three dimensional cutting used in various metal cutting operations in which the cutting action is inclined with the job by a certain angle called the inclination angle
• Metal cutting operation is illustrated in Figure. The work piece is securely clamped in a machine tool vice or clamps or chuck or collet. A wedge shape tool is set to a certain depth of cut and is forced to move in direction as shown in figure. All traditional machining processes require a cutting tool having a basic wedge shape at the cutting edge. The tool will cut or shear off the metal, provided
(i) the tool is harder than the metal, (ii) the tool is properly shaped so that its edge can be effective in cutting the metal, (iii) the tool is strong enough to resist cutting pressures but keen enough to sever the metal, (iv) provided there is movement of tool relative to the material or vice versa, so as to make cutting action possible. • Most metal cutting is done by high speed steel tools or carbide tools. In metal cutting, the tool does not slide through metal as a jack knife does through wood, not does the tool split the metal as an axe does a log.
• Actually, the metal is forced off the workpiece by being compressed, shearing off, and sliding along the face of the cutting tool. The way a cutting tool cuts the metal can be explained as follows.
• All metals in the solid state have a characteristic crystalline structure, frequently referred to as grain structure. The grain or crystals vary in size from very fine to very coarse, depending upon the type of metal and its heat-treatment.
• The cutting tool advances again in the work piece. Heavy forces are exerted on the crystals in front of the tool face. These crystals, in turn exert similar pressures on crystals ahead of them, in the direction of the cut or force applied by the cutter.
• As the tool continues to advance, the material at sheared point is sheared by the cutting edge of the tool or it may be torn loose by the action of the bending chip which is being formed.
• As the tool advances, maximum stress is exerted along sheared line, which is called the shear plane. This plane is approximately perpendicular to the cutting face of the tool. • There exists a shear zone on both sides of the shear plane, when the force of the tool exceeds the strength of the material at the shear plane, rupture or slippage of the crystalline grain structure occurs, thus forming the metal chip.
• The chip gets separated from the workpiece material and moves up along the tool face. In addition, when the metal is sheared, the crystals are elongated, the direction of elongation being different from that of shear. The circles which represent the crystals in the uncut metal get elongated into ellipses after leaving the shearing plane.
TYPES OF CHIPS
In a metal cutting operation is carried out in machine shop. Chips are separated from the workpiece to impart the required size and shape to the workpiece. The type of chips edge formed is basically a function of the work material and cutting conditions. The chips that are formed during metal cutting operations can be classified into four types: 1. Discontinuous or segmental chips 2. Continuous chips 3. Continuous chips with built-up edge. 4. Non homogenous chips
Types of Cutting Fluid
Functions of cutting fluid:
Cool the work piece and tool Reduce friction Protect work against rusting Provide anti-weld properties Wash away the chips
Types of Applications CUTTING FLUID Straight oils Not mixed used straight Petroleum based mineral oils are used in light duty machining where cooling Mineral oils is not a factor. Extends tool life and improves surface finish, good rust inhibitor Animal fat based oil used in medium duty machining where cooling is not a Fatty oils factor. Extends tool life and improves surface finish recommended for tapping and threading
Soluble oils Concentrate mixes with water
Suited for high speed machining where cooling is a factor improves surface Mineral Based finish and tool life, good rust inhibitor
Suited for high speed machining where cooling is a factor improves surface Fat Based finish and tool life
Synthetic oils Water based no minerals or animal fat
Rapid heat removal, good rust inhibitor, recommended for grinding, poor True Solution lubricant leaves sticky residue on work and machine after water evaporates
Rapid heat removal, good rust inhibitor, recommended for grinding, better Wet Solution lubricating leaves no sticky residue
CUTTING TOOL MATERIALS
Carbon steels, High Speed Steels, Cast Cobalt Alloys, Carbides, Coatings ,Cermets , Ceramics - Alumina , Ceramics -Silicon Nitride , Cubic Boron Nitride (cBN) , Diamond
1. Carbon Steels
• Carbon steels have been used since the 1880s for cutting tools. However carbon steels start to soften at a temperature of about 180 oC. This limitation means that such tools are rarely used for metal cutting operations. Plain carbon steel tools, containing about 0.9% carbon and about 1% manganese, hardened to about 62 Rc, are widely used for woodworking and they can be used in a router to machine aluminium sheet up to about 3mm thick.
2. High Speed Steel (HSS )
• HSS tools are so named because they were developed to cut at higher speeds. HSS are the most highly alloyed tool steels. • The tungsten (T series) were developed first and typically contain 12 - 18% tungsten, plus about 4% chromium and 1 - 5% vanadium. Most grades contain about 0.5% molybdenum and most grades contain 4 - 12% cobalt. • Molybdenum (smaller proportions) could be substituted for most of the tungsten resulting in a more economical formulation which had better abrasion resistance than the T series and undergoes less distortion during heat treatment. Consequently about 95% of all HSS tools are made from M series grades. These contain 5 - 10% molybdenum, 1.5 - 10% tungsten, 1 - 4% vanadium, 4% Chromium and many grades contain 5 - 10% cobalt. • HSS tools are tough and suitable for interrupted cutting and are used to manufacture tools of complex shape such as drills, reamers, taps, dies and gear cutters. Tools may also be coated to improve wear resistance. HSS accounts for the largest tonnage of tool materials currently used. Typical cutting speeds: 10 - 60 m/min.
3. Cast Cobalt Alloys
These alloys have compositions of about 40 - 55% cobalt, 30% chromium and 10 - 20% tungsten and are not heat treatable. Maximum hardness values of 55 - 64 Rc. They have good wear resistance but are not as tough as HSS but can be used at somewhat higher speeds than HSS. Now only in limited use.
4. Carbides
• Also known as cemented carbides or sintered carbides were introduced in the 1930s and have high hardness over a wide range of temperatures, high thermal conductivity, high Young's modulus making them effective tool and die materials for a range of applications. The two groups used for machining are tungsten carbide and titanium carbide, both types may be coated or uncoated. • Tungsten carbide particles (1 to 5 micro-m) are are bonded together in a cobalt matrix using powder metallurgy. The powder is pressed and sintered to the required insert shape. titanium and niobium carbides may also be included to impart special properties. • A wide range of grades are available for different applications. Sintered carbide tips are the dominant type of material used in metal cutting. The proportion of cobalt (the usual matrix material) present has a significant effect on the properties of carbide tools. 3 - 6% matrix of cobalt gives greater hardness while 6 - 15% matrix of cobalt gives a greater toughness while decreasing the hardness, wear resistance and strength. • Tungsten carbide tools are commonly used for machining steels, cast irons and abrasive non- ferrous materials. • Titanium carbide has a higher wear resistance than tungsten but is not as tough. With a nickel-molybdenum alloy as the matrix, TiC is suitable for machining at higher speeds than those which can be used for tungsten carbide. Typical cutting speeds are: 30 - 150 m/min or 100 - 250 when coated.
5. Cermets
• Developed in the 1960s, these typically contain 70% aluminium oxide and 30% titanium carbide. Some formulation contain molybdenum carbide, niobium carbide and tantalum carbide. Their performance is between those of carbides and ceramics and coatings seem to offer few benefits. Typical cutting speeds: 150 - 350 m/min.
6. Ceramics -Alumina
• Two classes are used for cutting tools: fine grained high purity aluminium oxide (Al 2O3) and silicon nitride (Si 3N4) are pressed into insert tip shapes and sintered at high temperatures. Additions of titanium carbide and zirconium oxide (ZrO 2) may be made to improve properties. • But while ZrO 2 improves the fracture toughness, it reduces the hardness and thermal conductivity. • Silicon carbide (SiC) whiskers may be added to give better toughness and improved thermal shock resistance. • The tips have high abrasion resistance and hot hardness and their superior chemical stability compared to HSS and carbides means they are less likely to adhere to the metals during cutting and consequently have a lower tendency to form a built up edge. • Their main weakness is low toughness and negative rake angles are often used to avoid chipping due to their low tensile strengths. Stiff machine tools and work set ups should be used when machining with ceramic tips as otherwise vibration is likely to lead to premature failure of the tip. Typical cutting speeds: 150 - 650 m/min.
6. Ceramics -Silicon Nitride
• These may also contain aluminium oxide, yttrium oxide and titanium carbide. SiN has an affinity for iron and is not suitable for machining steels. A specific type is 'Sialon', containing the elements: silicon, aluminium, oxygen and nitrogen. This has higher thermal shock resistance than silicon nitride and is recommended for machining cast irons and nickel based superalloys at intermediate cutting speeds.
7. Cubic Boron Nitride (cBN)
This is the second hardest material available after diamond. cBN tools may be used either in the form of small solid tips or or as a 0.5 to 1 mm thick layer of of polycrystalline boron nitride sintered onto a carbide substrate under pressure. In the latter case the carbide provides shock resistance and the cBN layer provides very high wear resistance and cutting edge strength. Cubic boron nitride is the standard choice for machining alloy and tool steels with a hardness of 50 Rc or higher. Typical cutting speeds: 30 - 310 m/min.
8. Diamond
• The hardest known substance is diamond. Although single crystal diamond has been used as a tool, they are brittle and need to be mounted at the correct crystal orientation to obtain optimal tool life. Single crystal diamond tools have been mainly replaced by polycrystalline diamond (PCD). This consists of very small synthetic crystals fused by a high temperature high pressure process to a thickness of between 0.5 and 1mm and bonded to a carbide substrate. The result is similar to cBN tools. The random orientation of the diamond crystals prevents the propagation of cracks, improving toughness. • Because of its reactivity, PCD is not suitable for machining plain carbon steels or nickel, titanium and cobalt based alloys. • PCD is most suited to light uninterrupted finishing cuts at almost any speed and is mainly used for very high speed machining of aluminium - silicon alloys, composites and other non - metallic materials. Typical cutting speeds: 200 - 2000 m/min.
9. COATED TOOLS
• Coatings are frequently applied to carbide tool tips to improve tool life or to enable higher cutting speeds. Coated tips typically have lives 10 times greater than uncoated tips. • Common coating materials include titanium nitride, titanium carbide and aluminium oxide, usually 2 - 15 micro-m thick. Often several different layers may be applied, one on top of another, depending upon the intended application of the tip. • The techniques used for applying coatings include chemical vapour deposition (CVD) plasma assisted CVD and physical vapour deposition (PVD). Diamond coatings are also in use and being further developed.
• Coatings have the following characteristics: 1. High hardness 2. Chemical stability and inertness 3. Low thermal conductivity 4. Compatibility and good bonding 5. Little or no porosity
Titanium-nitride Coatings • Have low friction coefficients, high hardness, resistance to high temperature and good adhesion to the substrate • Improve the life of high-speed steel tools and improve the lives of carbide tools, drill bits, and cutters • Perform well at higher cutting speeds and feeds
Titanium-carbide Coatings • Coatings have high flank-wear resistance in machining abrasive materials
Ceramic Coatings • Coatings have low thermal conductivity, resistance to high temperature, flank and crater wear
Multiphase Coatings • Desirable properties of the coatings can be combined and optimized with the use of multiphase coatings • Coatings also available in alternating multiphase layers
• Titanium carbonitride and titanium-aluminum nitride are effective in cutting stainless steels • Chromium carbide is effective in machining softer metals that tend to adhere to the cutting tool • More recent developments are nanolayer coatings and composite coatings
Coated Tools: Ion Implantation • Ions are introduced into the surface of the cutting tool, improving its surface properties • Process does not change the dimensions of tools • Nitrogen-ion implanted carbide tools have been used successfully on alloy steels and stainless steels
LATHE
Bed: - The bed of the turning machine is simply a large base that sits on the ground or a table and supports the other components of the machine.
Headstock assembly: - The headstock assembly is the front section of the machine that is attached to the bed. This assembly contains the motor and drive system which powers the spindle. The spindle supports and rotates the workpiece, which is secured in a workpiece holder or fixture, such as a chuck or collet.
Tailstock assembly - The tailstock assembly is the rear section of the machine that is attached to the bed. The purpose of this assembly is to support the other end of the workpiece and allow it to rotate, as it's driven by the spindle. For some turning operations, the workpiece is not supported by the tailstock so that material can be removed from the end.
Carriage - The carriage is a platform that slides alongside the workpiece, allowing the cutting tool to cut away material as it moves. The carriage rests on tracks that lay on the bed, called "ways", and is advanced by a lead screw powered by a motor or hand wheel.
Cross slide - The cross slide is attached to the top of the carriage and allows the tool to move towards or away from the workpiece, changing the depth of cut. As with the carriage, the cross slide is powered by a motor or hand wheel.
Compound rest: - The compound is attached on top of the cross slide and supports the cutting tool. The cutting tool is secured in a tool post which is fixed to the compound. The compound can rotate to alter the angle of the cutting tool relative to the workpiece.
Turret: Some machines include a turret, which can hold multiple cutting tools and rotates the required tool into position to cut the workpiece. The turret also moves along the workpiece, feeding the cutting tool into the material. While most cutting tools are stationary in the turret, live tooling can also be used. Live tooling refers to powered tools, such as mills, drills, reamers, and taps, which rotate and cut the workpiece.
Cutting parameters
In turning, the speed and motion of the cutting tool is specified through several parameters. These parameters are selected for each operation based upon the workpiece material, tool material, tool size, and more.
• Cutting feed - The distance that the cutting tool or workpiece advances during one revolution of the spindle, measured in inches per revolution (IPR). In some operations the tool feeds into the workpiece and in others the workpiece feeds into the tool. For a multi-point tool, the cutting feed is also equal to the feed per tooth, measured in inches per tooth (IPT), multiplied by the number of teeth on the cutting tool.
• Cutting speed - The speed of the workpiece surface relative to the edge of the cutting tool during a cut, measured in surface feet per minute (SFM).
• Feed rate - The speed of the cutting tool's movement relative to the workpiece as the tool makes a cut. The feed rate is measured in inches per minute (IPM) and is the product of the cutting feed (IPR) and the spindle speed (RPM).
SPECIFICATION OF LATHE
The size of a lathe is generally specified by the following means: (a) Swing or maximum diameter that can be rotated over the bed ways (b) Maximum length of the job that can be held between head stock and tail stock centres (c) Bed length, which may include head stock length also (d) Maximum diameter of the bar that can pass through spindle or collect chuck of capstan lathe
TURNING OPERATIONS
Lathe cutting Operations
During the process cycle, a variety of operations may be performed to the workpiece to yield the desired part shape. These operations may be classified as external or internal. External operations modify the outer diameter of the workpiece, while internal operations modify the inner diameter.
External cutting operations
Turning - A single-point turning tool moves axially, along the side of the workpiece, removing material to form different features, including steps, tapers, chamfers, and contours. These features are typically machined at a small radial depth of cut and multiple passes are made until the end diameter is reached. Facing - A single-point turning tool moves radially, along the end of the workpiece, removing a thin layer of material to provide a smooth flat surface. The depth of the face, typically very small, may be machined in a single pass or may be reached by machining at a smaller axial depth of cut and making multiple passes. Grooving - A single-point turning tool moves radially, into the side of the workpiece, cutting a groove equal in width to the cutting tool. Multiple cuts can be made to form grooves larger than the tool width and special form tools can be used to create grooves of varying geometries. Cut-off (parting) - Similar to grooving, a single-point cut-off tool moves radially, into the side of the workpiece, and continues until the center or inner diameter of the workpiece is reached, thus parting or cutting off a section of the workpiece. Thread cutting - A single-point threading tool, typically with a 60 degree pointed nose, moves axially, along the side of the workpiece, cutting threads into the outer surface. The threads can be cut to a specified length and pitch and may require multiple passes to be formed.
Internal cutting operations
Drilling - A drill enters the workpiece axially through the end and cuts a hole with a diameter equal to that of the tool. Boring - A boring tool enters the workpiece axially and cuts along an internal surface to form different features, such as steps, tapers, chamfers, and contours. The boring tool is a single- point cutting tool, which can be set to cut the desired diameter by using an adjustable boring head. Boring is commonly performed after drilling a hole in order to enlarge the diameter or obtain more precise dimensions. Reaming - A reamer enters the workpiece axially through the end and enlarges an existing hole to the diameter of the tool. Reaming removes a minimal amount of material and is often performed after drilling to obtain both a more accurate diameter and a smoother internal finish. Tapping - A tap enters the workpiece axially through the end and cuts internal threads into an existing hole. The existing hole is typically drilled by the required tap drill size that will accommodate the desired tap.
GRINDING MACHINES
Abrasive machining is a material removal process that involves the use of abrasive cutting tools.There are three principle types of abrasive cutting tools according to the degree to which abrasive grains are constrained,
Bonded abrasive tools : abrasive grains are closely packed into different shapes, the most common is the abrasive wheel . Grains are held together by bonding material. Abrasive machining process that use bonded abrasives include grinding , honing , superfinishing;
Coated abrasive tools : abrasive grains are glued onto a flexible cloth, paper or resin backing. Coated abrasives are available in sheets, rolls, endless belts. Processes include abrasive belt grinding , abrasive wire cutting ;
Free abrasives : abrasive grains are not bonded or glued. Instead, they are introduced either in oil-based fluids ( lapping , ultrasonic machining ), or in water ( abrasive water jet cutting ) or air (abrasive jet machining ), or contained in a semisoft binder ( buffing ).
Types of Grinding
• Surface Grinding • Cylindrical Grinding • Tool & Cutter Grinding • Profile Grinding
I. SURFACE GRINDING
Surface grinding is the most common of the grinding operations. It is a finishing process that uses a rotating abrasive wheel to smooth the flat surface of metallic or nonmetallic materials to give them a more refined look by removing the oxide layer and impurities on work piece surfaces. This will also attain a desired surface for a functional purpose.
The surface grinder is composed of an abrasive wheel, a workholding device known as a chuck, and a reciprocating or rotary table. The chuck holds the material in place while it is being worked on. It can do this one of two ways: ferromagnetic pieces are held in place by a magnetic chuck, while non-ferromagnetic and nonmetallic pieces are held in place by vacuum or mechanical means. A machine vise (made from ferromagnetic steel or cast iron) placed on the magnetic chuck can be used to hold non-ferromagnetic workpieces if only a magnetic chuck is available.
Types of surface grinders :
A.Vertical Spindle – a.Reciprocating Table, b. Rotary Table
The face of a wheel (cup, cylinder, disc, or segmental wheel) is used on the flat surface. Wheel-face grinding is often used for fast material removal, but some machines can accomplish high-precision work. The workpiece is held on a reciprocating table, which can be varied according to the task, or a rotary-table machine, with continuous or indexed rotation. Indexing allows loading or unloading one station while grinding operations are being performed on another
B. Horizontal Spindle- c.Reciprocating Table, d. Rotary Table
The periphery (flat edge) of the wheel is in contact with the workpiece, producing the flat surface. Peripheral grinding is used in high-precision work on simple flat surfaces; tapers or angled surfaces; slots; flat surfaces next to shoulders; recessed surfaces; and profiles
Grinding process • A surface grinder is a machine tool used to provide precision ground surfaces, either to a critical size or for the surface finish. • The typical precision of a surface grinder depends on the type and usage, however ±0.002 mm (±0.0001 in) should be achievable on most surface grinders.
• The machine consists of a table that traverses both longitudinally and across the face of the wheel. The longitudinal feed is usually powered by hydraulics, as may the cross feed, however any mixture of hand, electrical or hydraulic may be used depending on the ultimate usage of the machine (i.e., production, workshop, cost).
• The grinding wheel rotates in the spindle head and is also adjustable for height, by any of the methods described previously.
• Modern surface grinders are semi-automated, depth of cut and spark-out may be preset as to the number of passes and, once set up, the machining process requires very little operator intervention.
• Depending on the workpiece material, the work is generally held by the use of a magnetic chuck. This may be either an electromagnetic chuck, or a manually operated, permanent magnet type chuck; both types are shown in the first image.
• The machine has provision for the application of coolant as well as the extraction of metal dust (metal and grinding particles).
II. CYLINDRICAL GRINDING
The cylindrical grinder is a type of grinding machine used to shape the outside of an object. The cylindrical grinder can work on a variety of shapes, however the object must have a central axis of rotation. This includes but is not limited to such shapes as a cylinder, an ellipse, a cam, or a crankshaft. Cylindrical grinding is defined as having four essential actions:
1. The work (object) must be constantly rotating 2. The grinding wheel must be constantly rotating 3. The grinding wheel is fed towards and away from the work 4. Either the work or the grinding wheel is traversed with respect to the other.
Types: External grinding/Internal grinding/Centerless grinding
External cylindrical grinding
External grinding is grinding occurring on external surface a of an object between the centers. The centers are end units with a point that allow the object to be rotated. The grinding wheel is also being rotated in the same direction when it comes in contact with the object. This effectively means the two surfaces will be moving opposite directions when contact is made which allows for a smoother operation and less chance of a jam up.
Internal cylindrical grinding
ID grinding is grinding occurring on the inside of an object. The grinding wheel is always smaller than the width of the object. The object is held in place by a collet , which also rotates the object in place. Just as with OD grinding, the grinding wheel and the object rotated in opposite directions giving reversed direction contact of the two surfaces where the grinding occurs.
CENTERLESS GRINDING
Centerless grinding is a process for continuously grinding cylindrical surfaces in which the workpiece is supported not by centers or chucks but by a rest blade . The workpiece is ground between two wheels. The larger grinding wheel does grinding, while the smaller regulating wheel , which is tilted at an angle i, regulates the velocity Vf of the axial movement of the workpiece. Centerless grinding can also be external or internal , traverse feed or plunge grinding . The most common type of centerless grinding is the external traverse feed grinding , illustrated in the figure: III. TOOL AND CUTTER GRINDERS
A tool and cutter grinder is used to sharpen milling cutters and tool bits along with a host of other cutting tools. It is an extremely versatile machine used to perform a variety of grinding operations: surface, cylindrical, or complex shapes.
The image shows a manually operated setup, however highly automated Computer Numerical Control (CNC ) machines are becoming increasingly common due to the complexities involved in the process.
The operation of this machine (in particular, the manually operated variety) requires a high level of skill. The two main skills needed are understanding of the relationship between the grinding wheel and the metal being cut and knowledge of tool geometry.
The illustrated set-up is only one of many combinations available. The huge variety in shapes and types of machining cutters requires flexibility in usage. A variety of dedicated fixtures are included that allow cylindrical grinding operations or complex angles to be ground. The vise shown can swivel in three planes. The table moves longitudinally and laterally, the head can swivel as well as being adjustable in the horizontal plane, as visible in the first image. This flexibility in the head allows the critical clearance angles required by the various cutters to be achieved.
Today's tool and cutter grinder is typically a CNC machine tool, usually 5 axes, which produces endmills, drills, step tools, etc. which are widely used in the metal cutting and woodworking industries.
MODERN CNC TOOL AND CUTTER GRINDERS enhance productivity by typically offering features such as automatic tool loading as well as the ability to support multiple grinding wheels.
High levels of automation, as well as automatic in-machine tool measurement and compensation, allow extended periods of unmanned production. With careful process configuration and appropriate tool support, tolerances less than 5 micrometres (0.0002") can be consistently achieved even on the most complex parts.
Apart from manufacturing, in-machine tool measurement using touch-probe or laser technology allows cutting tools to be reconditioned. During normal use, cutting edges either wear and/or chip.
The geometric features of cutting tools can be automatically measured within the CNC tool grinder and the tool ground to return cutting surfaces to optimal condition. Significant software advancements have allowed CNC tool and cutter grinders to be utilized in a wide range of industries.
Advanced CNC grinders feature sophisticated software that allows geometrically complex parts to be designed either parametrically or by using third party CAD/CAM software. 3D simulation of the entire grinding process and the finished part is possible as well as detection of any potential mechanical collisions and calculation of production time. Such features allow parts to be designed and verified, as well as the production process optimized, entirely within the software environment.
Tool and cutter grinders can be adapted to manufacturing precision machine components. The machine, when used for these purposes more likely would be called a CNC Grinding System.
CNC Grinding Systems are widely used to produce parts for aerospace, medical, automotive, and other industries. Extremely hard and exotic materials are generally no problem for today's grinding systems and the multi-axis machines are capable of generating quite complex geometries.
MILLING MACHINES
Up Milling and Down Milling
Types of milling
1. Vertical milling 2. Horizontal Milling 3. Pantograph Milling 4.Copy Milling
VERTICAL MILLING MACHINE • The milling is a process of metal-removing by feeding the work past a rotating multipoint cutter. The diagram is shown below: • This machine can hold one or more, number of cutters at a time and rotates at high speed to remove the metal at a faster rate. • The metal removal rate is faster as compared to a lathe machine. • This machine is used to make mold cavities, mold plates, ejector plates, spacer and also used to drill the workpiece and produce slots.
Vertical milling machine. 1: milling cutter 2: spindle 3: top slide or overarm 4: column 5: table 6: Y-axis slide 7: knee 8: base In the vertical mill the spindle axis is vertically oriented. Milling cutters are held in the spindle and rotate on its axis. The spindle can generally be extended (or the table can be raised/lowered, giving the same effect), allowing plunge cuts and drilling. Machine Parts and its brief details:
Base:
• It is the foundation part of a milling machine and all other parts are joined on it. • It carries the entire load so it should have a high compressive strength and it is made up of cast iron. Column:
• It is mounted vertically on the base. • It supports the knee, table etc. Work as housing for the all the other driving member. • it is a hollow member which contains driving gears and sometimes motor for the spindle and the table. Knee:
• It is a casting that supports the saddle and table. • All gearing mechanism is enclosed within the knee. • It is fastened to the column by dovetail ways. • The knee is supported and adjusted by a vertical positioning screw (elevating screw). • The elevating screw is used to adjust the knee up and down by raising or lowering the lever either with the help of hand or power feed. Saddle:
• It is placed between the table and the knee, and work as an intermediate part between them, • It slides over the guideways provided situated on the knee which is perpendicular to the column face. • The main function is to provide motion in a horizontal direction to the workpiece. • It is also made by cast iron. Table:
• It is a rectangular casting which is present on the top of the saddle. • The table is situated over the knee. • It is the part of a machine which holds the workpiece while machining. • It is made by cast iron and has T slot cut over it.
Vertical Milling Operations:
There are two subcategories of Vertical milling: the bed mill and the turret mill.
• A turret mill has a stationary spindle and the table is moved both perpendicular and parallel to the spindle axis to accomplish cutting. The most common example of this type is the Bridgeport, described below. Turret mills often have a quill which allows the milling cutter to be raised and lowered in a manner similar to a drill press. This type of machine provides two methods of cutting in the vertical (Z) direction: by raising or lowering the quill, and by moving the knee. • In the bed mill , however, the table moves only perpendicular to the spindle's axis, while the spindle itself moves parallel to its own axis.
HORIZONTAL MILLING MACHINE
Horizontal milling machine. 1: base 2: column 3: knee 4 & 5: table (x-axis slide is integral) 6: overarm 7: arbor (attached to spindle)
• A horizontal mill has the same sort but the cutters are mounted on a horizontal spindle across the table. Many horizontal mills also feature a built-in rotary table that allows milling at various angles; this feature is called a universal table . • While endmills and the other types of tools available to a vertical mill may be used in a horizontal mill, their real advantage lies in arbor-mounted cutters, called side and face mills, which have a cross section rather like a circular saw, but are generally wider and smaller in diameter. Because the cutters have good support from the arbor and have a larger cross- sectional area than an end mill, quite heavy cuts can be taken enabling rapid material removal rates. These are used to mill grooves and slots. • Plain mills are used to shape flat surfaces. Several cutters may be ganged together on the arbor to mill a complex shape of slots and planes. Special cutters can also cut grooves, bevels, radii, or indeed any section desired. • These specialty cutters tend to be expensive. Simplex mills have one spindle, and duplex mills have two. It is also easier to cut gears on a horizontal mill. S • ome horizontal milling machines are equipped with a power-take-off provision on the table. This allows the table feed to be synchronized to a rotary fixture, enabling the milling of spiral features such as hypoid gears.
Horizontal Milling Operations:
• CNC milling machines (also called machining centers) are computer controlled vertical mills with the ability to move the spindle vertically along the Z-axis. T
• his extra degree of freedom permits their use in die sinking, engraving applications, and 2.5D surfaces such as relief sculptures.
• When combined with the use of conical tools or a ball nose cutter, it also significantly improves milling precision without impacting speed, providing a cost-efficient alternative to most flat-surface hand-engraving work.
DRILLING
This is the operation of making a circular hole by removing a volume of metal from the job by a rotating cutting tool called drill
Reaming This is the operation of sizing and finishing a hole already made by a drill. Reaming is performed by means of a cutting tool called reamer as shown in Fig. 22.7. Reaming operation serves to make the hole smooth, straight and accurate in diameter. Reaming operation is performed by means of a multitooth tool called reamer
Boring Fig. 22.8 shows the boring operation where enlarging a hole by means of adjustable cutting tools with only one cutting edge is accomplished. A boring tool is employed for this purpose
Counter-Boring Counter boring operation is shown in Fig. 22.9. It is the operation of enlarging the end of a hole cylindrically, as for the recess for a counter-sunk rivet. The tool used is known as counter-bore.
Counter-Sinking Counter-sinking operation is shown in Fig. 22.10. This is the operation of making a coneshaped enlargement of the end of a hole, as for the recess for a flat head screw. This is done for providing a seat for counter sunk heads of the screws so that the latter may flush with the main surface of the work.
Tapping
It is the operation of cutting internal threads by using a tool called a tap. A tap is similar to a bolt with accurate threads cut on it. To perform the tapping operation, a tap is screwed into the hole by hand or by machine. The tap removes metal and cuts internal threads, which will fit into external threads of the same size.
UNIT – II Electrical discharge machining
• Principle,
• Types of EDM - Die Sinking & Wire Cut EDM
• Machining Process, • Requirements of dielectric fluid, • Applications of EDM in mold making.
ELECTRICAL DISCHARGE MACHINING (EDM)
• When the voltage drops to about 12 volts, the spark discharge extinguishes and the dielectric fluid once again becomes deionized. The condensers start to recharge and the process steps itself
TYPES OF EDM:
1. CONVENTIONAL EDM /DIE SINKING EDM
2. WIRE CUT EDM
a) Die Sinking EDM
b) Wire Cut EDM
EDM-DIELECTRIC FLUID
Applications of EDM:
I.Die Sinking EDM Applications :
a) Straight Sinking (b) Helical Profile Drilling (c) 3-D Die Sinking of Injection Mold Cavity (d) Blind Sinking of Injection Mold Cavity (e) Threading of Blow Mold Cavity (Bottle cap) (f) Curved Hole Drilling
II.Wire-Cut EDM Applications :
Mostly used for making
o Extrusion Dies o Blanking Dies o Punches o Press Tools o Sintered Compacting dies
Applications of WireCut-ED M
EDM
Characteristics of EDM Process Characteristics of EDM
(a) The process can be used to machine any work material if it is electrically conductive (b) Material removal depends on mainly thermal properties of the work material rather than its strength, hardness etc http://www.slideshare.net/mohitgangwr/good-ppt-edm Metal Removal Rate (MRR): o MRR= Weight of material removed/Machining time (Unit:g/min) o MRR= 2.4/(Melting Point) 1.25 o Higher I,V, C, Pulse on-time, flushing flow rate causes higher MRR o EDM will cut Aluminium faster than Steel o Machining rate during roughing of steel with graphite electrode & 50A generator is about 400mm 3/min, and with 400A Generator is about 4800mm 3/min
(c) In EDM there is a physical tool and geometry of the tool is the positive impression of the hole or geometric feature machined (d) The tool has to be electrically conductive as well. The tool wear once again depends on the thermal properties of the tool material (e) Tool Wear Ratio (TWR):
Tool wear is the function of Metal removal rate, Material of Workpiece, Current setting, Machining area, Spark gap and Polarity of tool
o TWR= Volume of Workpiece material remocal/Volume of electrode consumed o Can be simplified to TWR = Depth of Cut/Decrease in usable length of electrode o Higher I,V, C, Pulse on-time, flushing flow rate causes higher TWR o Wear Ratio for Carbon electrode is upto 100:1, for Copper – 2:1, For Brass – 1:1
(f) Though there is a possibility of taper cut and overcut in EDM, they can be controlled and compensated Accuracy: • Sharp corner cannot be achieved • Tolerance value : ±0.05 mm for roughing, ±0.003mm for finishing cuts • Taper Value : 0.05 to 0.005mm per 100 mm depth (g) Though the local temperature rise is rather high, still due to very small pulse on time, there is not enough time for the heat to diffuse and thus almost no increase in bulk temperature takes place. Thus the heat affected zone is limited to 2 – 4µm of the spark crater
Surface Finish:
• Higher frequency spark and Lower I,V,C gives best surface finish • Economically achievable average surface roughness is 0.4µm (h) However rapid heating and cooling and local high temperature leads to surface hardening which may be desirable in some applications Important parameters of EDM
(a) Spark On-time (pulse time or T on ): The duration of time (µs) the current is allowed to flow per cycle. Material removal is directly proportional to the amount of energy applied during this on-time. This energy is really controlled by the peak current and the length of the on-time.
(b) Spark Off-time (pause time or T off ): The duration of time (µs) between the sparks (that is to say, on-time). This time allows the molten material to solidify and to be wash out of the arc gap. This parameter is to affect the speed and the stability of the cut. Thus, if the off-time is too short, it will cause sparks to be unstable.
(c) Arc gap (or gap): The Arc gap is distance between the electrode and workpiece during the process of EDM. It may be called as spark gap. Spark gap can be maintained by servo system
d) Discharge current (current I p): Current is measured in amp Allowed to per cycle. Discharge current is directly proportional to the Material removal rate.
(e) Duty cycle ( τ): It is a percentage of the on-time relative to the total cycle time. This parameter is calculated by dividing the on-time by the total cycle time (on-time pulse off-time).
(f) Voltage (V): It is a potential that can be measure by volt it is also effect to the material removal rate and allowed to per cycle. Voltage is given by in this experiment is 50 V.
Wire EDM machining (Electrical Discharge Machining) is an electro thermal production process in which a thin single-strand metal wire in conjunction with de-ionized water (used to conduct electricity) allows the wire to cut through metal by the use of heat from electrical sparks.
Due to the inherent properties of the process, wire EDM can easily machine complex parts and precision components out of hard conductive materials.
FUNCTIONS AND TECHNOLOGICAL PLANNING OF EDM
• In mould making the EDM technology has great importance, and the reasons for this are numerous. Certain geometrical features can not be manufactured by rotating tools, not even in larger size, because the tool can not touch the surfaces. • The 3D outer edges are a typical example for this type of feature. • The moulded plastic parts have more and more complicated and small geometric features, so the mould cores and cavities need more accurate and detailed manufacturing. • These demands cannot be satisfied by cutting technologies. • During EDMing special surface structure is generated, which considering the aesthetics is more favourable in case of visible surfaces than milled or burnished surfaces. • The EDM technology is not limited by hardness of the machined parts, so the parts can be machined in hard state after hardening. • During mould manufacturing there are two basic manufacturing methods that are used in order to produce mould inserts: milling and EDM. • In case of productivity the milling technology is preferred to the EDM technology (the latter is considered as secondary technology), yet based on previous mentioned reasons it has an important role in manufacturing process. • The first step of the planning process is to determine the surfaces that will be machined by EDM technology. • In this phase allowance has to be made for two circumstances: the surfaces that cannot be manufactured by milling and the surfaces that have EDM-ed surface structure. • In order to answer the first question, the process planning of milling technology (CAM) has to be performed, so that the milled and the final (original) surfaces can be compared. • The second step is the CAD modeling of the electrode, which is described in the next chapter. • After the modeling we have to check whether the electrode can be manufactured by milling and the milling cutter can machine all surfaces. • If it is not possible, we have to revisit the modelling phase and divide the electrode into more separate pieces. Following this, the offset and the number of pieces have to be determined.
The value of the offset is influenced by: the size of the surface, the value of the removable material, the character of the surface (steep or sloping), the surface roughness, the character of the manufacturing (roughing or finishing).
• The number of workpieces is determined by the volume of cut material, the character of the manufacturing and the number of needed inserts (number of cavities). A • fter the completion of the CAD model of EDM electrode the NC programs and the electrode manufacturing documentation have to be planned. • • The manufacturing documentation has four parts : the shop drawing of the electrode, which shows the overall size of it, the setup drawing, which shows the position of the electrode compared to the part, the NC work sheet, which contains the name of the programs, the tools, cutting parameters and the position of the coordinate system, and the NC programs
CNC MACHINES Computer numerical control (CNC ) is the automation of machine tools by means of computers executing pre-programmed sequences of machine control commands. This is in contrast to machines that are manually controlled by hand wheels or levers, or mechanically automated by cams alone.
In modern CNC systems, the design of a mechanical part and its manufacturing program is highly automated. The part's mechanical dimensions are defined using computer-aided design (CAD) software, and then translated into manufacturing directives by computer-aided manufacturing (CAM) software. The resulting directives are transformed (by "post processor" software) into the specific commands necessary for a particular machine to produce the component, and then loaded into the CNC machine.
Most machines need control systems to operate. There are many kinds of control systems, for example, manual control, automatic control, computer control or remote control . For the convenience of mass production, machines need to repeat precise, speedy and automatic actions continuously. These machines may use mechanical, pneumatic and electrical systems to control.
The production method that requires a computer to control the machines is called a computer aided manufacturing, simply called CAM. CAM is closely related to the computer- aided design (CAD) because the output information about the products from the CAD can assist the composing of production program. Tests and productions can start immediately. This simplifies the procedures from the designing to manufacturing of the product.
The advantages (merits) of computer numerical controlled (CNC) machine
The Disadvantages (Demerits) of CNC machine (i) The cost of the machine is so high that some small factories may not be able to afford. (ii) Operators need to be trained to compose computer control program. (iii) The control system is complicated and sophisticated, therefore the maintenance cost is high. The operation of CNC machine (a) Basic operation theory
Fig. Simplified operation procedures of CNC machine
Fig. shows the simplified operation procedures of a CNC machine. Firstly, the engineering drawing according to the design of the workpiece is prepared. CAD software can be used in this procedure. Then based on the information in the engineering drawing, the computer numerical controlled machinery program (CNC machinery program) will be composed. The CNC machinery program includes all the geometrical and technical information. The geometrical information decides the target position of the tools movement, cutting direction and movement priority, etc. Technical information includes the choice of tools, the rotational speed of the main axis, the rotating direction, cutting speed, etc. The CNC machinery program will input the geometrical and technical information into the digital controllers, while the input devices include paper tape, magnetic tape, external keyboard, etc. After the digital controller has processed all the information, the moving path of the tools and the suitable procedures will be available. Lastly, the digital controller will control the whole machine and its tools to process the materials into a required workpiece. The composition of machinery program of the CNC machine usually follows the internationally recognized ISO R358 standardized format. But the USA or some other nearby countries may use a similar US format EIA RS244.
The industrial applications of CNC machine The design of CNC machine CNC machine needs new designs to suit the needs of automation. Firstly, CNC machine needs a precise path measuring system, so that the computer can detect the position of tools and workpieces automatically. There are many designs of path measuring system, for example, using the photoelectric detecting device to input the electronic signals into the computer directly.
(a) Path measuring system (b) Photoelectric detecting device
Coordinate system
CNC machine needs a coordinate system to control the tools. Tools are needed to cut three dimensional work pieces. That means that the tools will move in a three-dimensional space. Therefore the coordinate system should have three perpendicular axes. The axes used are called x-axis, y-axis and z-axis respectively. Their respective position can be shown by making the thumb, index finger and middle finger of the right hand perpendicular to each other called the right-hand rule.
The rotational axis can be set by the main axes, A, B and C representing the rotational axes that rotate about X, Y and Z axes respectively. CNC electric-discharge machining
Electric discharge machining is a process that uses metal threat and workpiece as electrodes, and uses the electric spark generated between them to cut hard workpieces (Fig. 14a). CNC electro-discharge machine can use computer program to cut complicated shapes of workpieces (Fig. 14b).
CNC Programming There are many methods to compose a program by using ISO codes. The most common one is the ISO standardized word and address format. It divides the whole program into a number of blocks, and alphabets are used to represent different words within the blocks. The lengths of the block and word depend on the needs of the design.
Command Function Address Sequence Each block will be given a number for identification. N number Coordinate word Control the relative motions of both tool and the workpiece, X,Y,Z,A,B,C,U,V,W e.g. linear coordination and axial movement. Parameters for Insert the parameters of an arc lie between two points. I, J, K circular interpolation Feed function Describe the cutting speed. F Spindle function Describe the rate of spindle speed for the main axis. S Tool function Define the tools being used. T Preparatory Indicate the types of movement, e.g. rapid movement, G function the position of linear or curved insertion, etc. Miscellaneous Several functions are included, e.g. rotating direction of M function the main axis, the supplying switch of condenser, etc.
Some common G codes Some common M codes
G code Preparatory function M code Miscellaneous function G00 Point to point position at rapid M00 Program stop feed G01 Linear interpolation ...... G02 Circular interpolation, clockwise M03 Spindle rotation, clockwise G03 Circular interpolation, anti- M04 Spindle rotation, anti- clockwise clockwise ...... M05 Spindle stops G18 The specified ZX plane M06 Chang of tool
Sequence Block Explanation (N)
N040 G18 X100 Z50 ； 40 100 on the specified X-axis and 50 on the specified Z-axis
N080 G97 S1000 ； 80 The spindle speed of the specified main axis is1000 rpm
N090 M03 ； 90 Axial rotation (Clockwise)
N160 G00 X0 Z20 ； 160 Rapid movement to 0 on the X-axis and 20 on the Z-axis
N170 M08 ； 170 The supply of coolant starts
Application of CNC machines
UNIT III
Electroforming for mold manufacturing - discussion of the process, materials for electroforming, design & materials for models, machining for electroformed blanks, mold cavities, economy & service life.
ELECTROFORMING:
Electroforming is a metal forming process that forms parts through electrodeposition or electroplating on a model, known in the industry as a mandrel.
THE PRINCIPLE OF ELECTROFORMING
• The basic principles of both the electroforming and electroplating are, to all intents and purposes, identical. • Two electrodes, an anode and a cathode, are immersed in a suitable electrolyte containing a dissolved salt of a metal (for example nickel sulphate) in an appropriate container. • A direct current is applied between these electrodes causing the electrode connected to the positive pole of the current source, the anode, to dissolve. • This produces positively charged metallic ions, for example Ni2, in solution which migrate towards the negatively charged electrode, the cathode. • At the cathode surface the positively charged metallic ion is reduced to the elemental metallic state by electrons supplied from the direct current source. This causes the cathode to become coated with whatever metal is being deposited by the process.
Essential plant components are as follows:
A tank to contain the process solution The process solution Two electrodes -an anode and a cathode A controlled source of direct current between the electrodes Heaters and thermostats to control solution temperature Solution pumping to provide supply agitation to the solution and also to filter the solution continuously
a) model/mandrel b) nickel layer c) copper backup d) electroformed cavity e) Product
Process
As an example of electroforming, the manufacturing process of a micro-pipette injected impression, where electrode deposits containing nickels are electroformed into a male mold (i.e. spindle) to form a cavity. A male mold which can be made of metals or nonmetals resembles the products whereas the female mold is the contrary of a male mould. The clamping axis of a male mold may be provided with a plastic protective layer and a metal lead connecting to the negative pole of electroforming solutions, demanding surface treatment on the male mold to prevent nickels from clinging to the surface of the male mould. A metal mold is allowed. However, when plastics (insulation materials) and other nonmetals were employed, an electric conductor should be prepared, which is usually available by employing chemical silver plating technology with the thickness of the silver plated layer 0.5 µm (usually neglectable). After that, be ready to suspend the spindle at the slot of the electroforming solutions. Subjecting to factors like solution components, deposit parameters, current density, solution temperature, pH value etc, it takes 10-15 days to form a nickel layer with 2- 4mm thickness. If necessary, a reinforcing copper layer can be added at a copper solution pool. Subjecting to its surface sizes and other factors, a hard copper layer with deposit thickness as 5-20mm or thicker can be added until meeting the desired outline size. A finished impression by electroforming remains in the spindle when processing external shapes. The electroformed mold may be separated from the electroformed mold after finishing processing and the cavity can be assembled to a mold when a type is properly selected. Quality and precision in the surface of a male mold determine the quality of the surface of a finished mold and further determine the quality of moulded products therefrom. Manufacturing of modern plastics and related manufacturing processes rely on conditions such as temperature, pressure, injection mode and parting surface. Nickel sulfate solution and nickel sulfamic acid solution are two most frequently used electroforming solutions, which when added with some special additive may in deposition come out with a low stress nickel layer varied in hardness. A nickel layer evenly deposited in thickness at an undulant shape surface can be achieved through improving the diffusion performance of the electroforming solution. Since working temperature for manufacturing hardnickel cavities in slots with sulfamate solution may be as high as 300 °C, the decomposition of organic additive may cause trouble. Therefore, it is important to be fully aware of which plastic moulding technology to be adopted for a certain cavity so bring full play of nickel and processing method. Nickel is widely used in electroforming processing since it is an important alloy element in steels with high tension and corrosion resistance. Pure nickel is rarely used in manufacturing mold parts but frequently adopted in anticorrosive coating and decorative coating. Due to its special chemical, physical and mechanical performances, nickel has long been used in various products such as cover of electric shaver, parabola shape mirror surface, compressed container, filter screen, elaborate parts for aviation and making rockets. Thick nickel painting is helpful in repairing worn spare parts in lathe industry.
MODELS / MANDRELS for ELECTROFORMING
NICKEL in plastic processing has the following most important characteristics:
(1) Wear Resistance and Hardness Wear resistance of nickel is nearly the same as that of chrome alloy. Hardness of nickel for electroforming cavity that used for injection of mold is usually around 44~48HRC.
(2) Corrosion Resistance Nickel with anticorrosion performance performs well in anti-oxidation. However, what is more important is that it resists corrosive solutions like hydrochloric acid in manufacturing PVC.
(3) Good Release of Parts from mold: Plastic products requires high demoulding capacity while well-qualified electroforming nickel surface has the optimum polishing performance since it is soft and porous free and plastic products can be ejected easily. High passivity of nickel allows much less dosage of releasing agent to facilitate cleaning production and to come out with better surface as well.
(4)Precise Reproduction With electroforming, the most delicate counters of a model’s surface can be transferred precisely to the molded part such as real leather texture, find diamond cuts, mirror finishes ..etc
UNIT IV HOBBING AND CHEMICAL TEXTURING
Hobbing for mold cavity making
• Discussion of the hobbing process, • Elements of hobbing o materials used for cavity and Punch o Lubrication, and o Depth of hobbing,
• Advantages and Disadvantages.
Surface Texturing of molds
• Chemical Texturing, • Process description, • Advantages- Limitations ofchemical texturing.
UNIT-IV (Part-2):
SURFACE TEXTURING / ETCHING OF MOLD (SURFACE STRUCTURE)
• Texturing—sometimes referred to as graining or engraving—is the process of adding a pattern (the texture or grain) to the molding surface of a mold.
• This allows the mold to impress that pattern on each molded part. Compared to finishing each molded product, texturing or graining of a mold is a very economical way to impress complex designs or patterns on molded plastic parts.
• The accurate reproduction of the specified texture in the mold surface requires careful consideration of many factors.
• Creating a pattern on a mold cavity that can be reproduced on part.
SURFACE TEXTURING/ STRUCTURING/ETCHING can be done by
1. CHEMICAL ETCHING – a) DIP ETCHING b) SPRAY ETCHING 2. PHOTOCHEMICAL ETCHING 3. LASER TEXTURING
Pattern selection:
• Spray Patterns referred to as spray patterns are the least expensive. The resist is applied to the surface to be textured—with a sprayer.
• Transfer, or multi-layer patterns require more work and the patterns are transferred to the metal from sheets.
• Multi-layer patterns are achieved by iterating a transfer etching process. Multi-layer patterns might require handwork between each transfer
• Transfer patterns may require touching up areas by hand where the sheets blend together.
Types of Pattern
• Natural • Leather grains / hides • Images or logos • Woodgrains, slates • Organic • Geometric and linens • Matte finishes / stipples • Mixed reflective and low-gloss aspects • Technical • Layered texture effects creating new looks • Mult-gloss patterns • Graphics • Fusion
Patterns
A. CHEMICAL ETCHING/TEXTURING
Etchants: The texturing of most thermoplastic mold materials (including carbon steel, stainless steel, aluminums, coppers, bronzes) is often done with following chemical etchants:
• Ferric chloride (FeCl 3)
• Nitric acid (HNO 3),
• Sulphuric Acid (H 2SO 4), • Hydrochloric Acid (HCL) or • Sodium Hydroxide (NaOH)
CHEMICAL ETCHING PROCESS STEPS Chemical Etching is normally performed in a series of five steps: 1. Cleaning 2. Masking 3. Scribing 4. Etching 5. Demasking
• Defining the area of the mold to be chemically etched is done by masking off or protecting the areas and then etching to texture . • Mask resists the corrosive effects of the etchant and handling of the block during processing can be used. • Most often used masks are vinyl tapes and waxes, asphalt or other mastics that can be easily applied by hand with paint brushes. The tapes can be used to cover the broad areas and holes. The paintable mastics are used for the details.
1. Cleaning
• Cleaning is the preparatory process of ensuring that the surface to be etched is free of contaminants which could negatively impact the quality of the finished part. • The surface must be kept free from oils, grease, primer coatings, markings and other residue from the marking out process, scale (oxidation), and any other foreign contaminants. • For most metals, this step can be performed by applying a solvent to the surface to be etched, washing away foreign contaminants. The material may also be immersed in alkaline cleaners or specialized de-oxidizing solutions .
2. Masking
• Masking is the process of applying the maskant material to the surface to ensure that only desired areas are etched. • Liquid maskants may be applied via dip-masking, in which the part is dipped into an open tank of maskant and then the maskant dried. • Maskant may also be applied by flow coating
Maskant types (neoprene elastomers or isobutylene-isoprene copolymers)
• The maskant to be used is determined primarily by the chemical used to etch the material, and the material itself. • The maskant must adhere to the surface of the material, and it must also be chemically inert enough with regards to the etchant to protect the workpiece. • Most industries use maskants based upon neoprene elastomers or isobutylene-isoprene copolymers . Maskants to be used in etching processes must also possess the necessary light- reactive properties.
3.Scribing
• Scribing is the removal of maskant on the areas to be etched. Modern industrial applications may involve an operator scribing with the aid of a template or use computer numerical control to automate the process. • For parts involving multiple stages of etching, complex templates using colour codes and similar devices may be used. 4.Etching
• Etching is the actual immersion of the part into the chemical bath, and the action of the chemical on the part to be milled.
There are two application methods:
a. Immersion type
b. Spray type. a. With the immersion type , the part is immersed in the corrosive liquid, and the liquid is constantly stirred. Air injection is the widely used stirring method. b. The spray method of etching employs specially designed etching machines. Spray type machine has nozzle(s) that spray the etchant, and Paddle type has a rotating wheel with paddles that spray the etchant. Both types are made in vertical and horizontal configurations, as well as single sided etching and dual side etching types.
• The time spent immersed in the chemical bath determines the depth of the resulting etch; this time is calculated via the formula:
E = 8/t where E is the rate of etching (usually abbreviated to etch rate ), s is the depth of the cut required, and t is the total immersion time.
5.Demasking
• Demasking is the combined process of clearing the part of etchant and maskant. • Etchant is generally removed with a wash of clear, cold water (although other substances may be used in specialized processes). • A de-oxidizing bath may also be required in the common case that the etching process left a film of oxide on the surface of the material.
Etching aluminum: • Aluminum can be textured like steel, but does not corrode the same. • Aluminum actually reacts more quickly in these etchants, thereby leaving less room for error. • In addition, the chemical reaction taking place when etching aluminum is exothermic, meaning heat is produced during the process. • The heat given off during the process will heat up the etchant bath, thereby further speeding up the corrosion activity. The etchant solution temperature needs to be monitored and controlled where necessary. • Foundry-cast aluminums—like those used in some inexpensive blow mold tools as opposed to the wrought alloys—react at about the same rate, but react differently. • Porosity is an issue because those pores are exposed and often are deeper and more pronounced than the texture pattern applied. Those cases require blocking—or surface coating—the molding surface first. In general, the same texture applied to aluminum will not be as crisp as with steel.
Etching stainless steel: • Stainless steel is naturally more resistant to corrosion due to the content of Cr and Ni. • Stainless steel, by definition, is steel with about 11.5% Cr or greater. At that Cr content, the exposed surface of the alloy has enough tenacious Cr oxide to prevent the iron from oxidizing and the alloy therefore “stains less”. • Austenitic stainless steels (like the AISI 300 series) present difficulties due to the high Cr content in addition to the high Ni content—two alloying elements that improve corrosion resistance. • The martensitic AISI 420 is easier to texture. • Molds made from 420SS are often hardened, which affects corrosion resistance and therefore the ability to take a texture as well. • Corrosion resistance increases with hardness, and therefore, a 420SS block at Rc 52/54 will take longer than a block at RC 40.
Effects of hard-milled surfaces: • Hard milling leaves a surface with greater work hardening. Due to the variable cutter path over the entire surface and the nature of the programming and cutting, it is not a uniform hardness. • Work-hardened surfaces have different molecular structures from the parent steel and that affects corrosion resistance. This non-uniform hardness yields non-uniform corrosion resistance that leads to a non-uniform texture. The hard-milled, work-hardened surfaces need to be polished out to remove non-uniform texture.
B. PHOTOCHEMICAL TEXTURING
Representation Of The Working Process For Producing Photoetched Textures.
Surface texturing has been viewed as one of the effective surface engineering technologies to significantly improve tribological performance of mechanical parts. Photolithographic resists may be negative or positive. In positive resists , photochemical reactions caused by UV exposure weaken the polymer by scission of the main and side polymer chains. Thus, the exposed areas will be more soluble in developing solutions. Negative resists are strengthened by UV exposure, which promotes random crosslinking of the main or side of the resist. The exposed areas of the resist will be then less soluble in developing solutions.
Process:
• The process starts by printing the shape of the part onto optically clear and dimensionally stable photographic film. • The "photo-tool" consists of two sheets of this film showing negative images of the parts (meaning that the area that will become the parts is clear and all of the areas to be etched are black). • The two sheets are optically and mechanically registered to form the top and bottom halves of the tool. • The metal sheets are cut to size, cleaned and then laminated on both sides with a UV- sensitive photoresist. • The coated metal is placed between the two sheets of the phototool and a vacuum is drawn to ensure intimate contact between the phototool and the metal plate. • The plate is then exposed in UV light that allows the areas of resist that are in the clear sections of the film to be hardened. After exposure, the plate is "developed", washing away the unexposed resist and leaving the areas to be etched unprotected. • The etching line is a multi-chambered machine that has driven-wheel conveyors to move the plates and arrays of spray nozzles above and below the plates. • The etchant is typically an aqueous solution of acid, frequently ferric chloride, that is heated and directed under pressure to both sides of the plate. • The etchant reacts with the unprotected metal essentially corroding it away fairly quickly. After neutralizing and rinsing, the remaining resist is removed and the sheet of parts is cleaned and dried. • Surface texturing has been viewed as one of the effective surface engineering technologies to significantly improve tribological performance of mechanical parts. • Surface texturing can be applied to many products, such as mechanical seals, thrust bearings and piston rings. • It provides flexible control of the shape and size of a texture cell, and is able to process many kinds of materials. • Disadvantages include - heat affected zones, microcracks and redundant bulges are the shortages of the LST process.
TEXTURING COMPATIBLE MATERIALS • Aluminun (6061/3003) • Copper • Stainless Steel • Hastolloy C • Haynes 242 • Inconel 625, 718 • Titanium/Tantulum
APPLICATIONS
Composite to metal bonding, also known as COMELD Promotion of adhesive bonding Bonding and direct moulding of polymers to metal parts Manufacture of aerodynamically & hydrodynamically enhanced surfaces As a preparation prior to surface coating Manufacture of filters and other applications requiring shaped slots and holes Manufacture of surfaces with enhanced thermal properties Manufacture of tailored surfaces with specific wave interaction, absorption, emission and/or propagation properties Manufacture of locally alloyed functional surfaces with specific mechanical, electrical, magnetic & thermal chemical properties
Advantages • Simple process • Custom Engineered Products • Cycle Life Improvements • Total Cost of Ownership Improvement • Short Lead Times
DRAWBACKS OF CHEMICAL TEXTURING
• Chemicals are Harmful to operators • Limited Accuracy • It is difficult to texture complex texture patterns • In addition to adherence to strict government regulations, chemical etching also requires substantial manual labor. Each part undergoing the process must be carefully prepared to ensure that the chemicals only access the features to be textured. • Exposing other areas of the component to these chemicals can result in the entire piece being scrapped. • Additionally, the process requires readying the chemicals, administering the bath, cleaning the part, and disposing of the chemicals—all steps with high labor components.
LASER TEXTURING
• The process entails using a laser to sublimate material, turning it directly from solid to gas. It is already used in a variety of applications, ranging from surgical procedures to production of superconductors.
• Laser texturing centers can be equipped with pulsed, fiber-optic lasers of varying strengths and a variety of lenses with different focal lengths to maximize productivity. The process promises several key advantages to moldmakers requiring surface texturing.
• Laser texturing offers more precision and accuracy than the chemical etching process.
• No matter how much care is taken during etching, it is impossible to eliminate variation that results in a minimum of slight differences between workpieces.
• Laser texturing centers produce textures directly from digital files, allowing the creation of any number of identical components. Some manufacturers may think that variation is acceptable between components, but it becomes a much more apparent issue when thinking in terms of complex molds that require multiple inserts.
• In those instances, differences between the various inserts can result in a visibly inconsistent surface across the part. Laser texturing eliminates the need for this sacrifice in quality.
• This process also has no environmental impact. Even for manufacturers who are unconcerned about the environmental impact of their operations, it can provide substantial benefit. Because of the nature of the chemicals used for etching, U.S. companies conducting that process must comply with a host of regulations that substantially increase its price.
• As an alternative, parts can be shipped to companies doing business in countries with little or no environmental regulation, but this option brings with it substantial lead times and low flexibility in responding to urgent needs. In short, laser texturing will often be cheaper than domestic chemical etching and faster than international chemical etching.
• Laser texturing consumes very little labor. An operator sets the part in the machine, loads the program and walks away until the texturing is complete. While the actual process time of laser texturing will substantially exceed that of chemical etching, the ability to run fully unmanned minimizes labor utilization. The only true operating cost of the process is the cost of the electricity consumed. UNIT- V MEASUREMENTS
METOROLOGY AND INSPECTION
• Metrology and inspection: • Vernier Caliper, • Micrometer, • Vernier height gauges, • Surface plate, • Slip gauges, Sine Bar, • Rockwell Hardness, • Optical profile projectors and • Optical flat.
Optical Profile Projector-MECHANICAL OPTICAL COMPARATOR
Also known as an optical comparator or even called a shadowgraph
Useful for small parts machine shop or production line for the quality control inspection team.
The projector magnifies the profile of the specimen, and displays this on the built-in projection screen.
This utilizes a plunger tilted mirror, objective lens,prism and observing eye piece to provide a high degree of magnification The mirror is mounted on a knife-edge. It can be tilted about the fulcrum by any linear vertical movement of the contact plunger A beam of light passes through a graticule suitably engraved with a linear scale The movement of mirror causes this scale to move up (or) down past a translucent screen inside the observing hood of the instrument The eye placed near the eye piece views the image of a small scale engraved on scale after reflection from the plunger actuated mirror The” plan” view for the mirror is shown in fig In the focal plane of the eye piece , a fine reference line ( index 0) is provided The system of lenses is so arranged that the image of the scale is projected in the same focal plane Thus with movement of scale the image can be measured with reference to the fixed line The division of the scale image opposite the index line indicates the amount of movement of contact plunger The image of the scale and the index line could also be viewed through a projection system The overall magnification of the comparator is given by (2f/d)x eye-piece magnification Where `f` is the focal length of the lens and `d`is the distance between the knife-edge and the plunger The measurement is made by taking reading using master gauge of known accuracy and comparing with component reading It uses a controlled, pressurised jet of air to measure small dimensional variation in the size of component
Advantages
Optical magnification provides high degree of measuring precision due to reduction of moving members and better wear resistance qualities Optical magnification is also free from friction,bending,wear etc. An illuminated scale is provided that enables readings to be taken without regard to the room lighting conditions These are also used to magnify very small parts such as needles, saw teeth, screw threads etc
Applications of Profile projectors
For inspecting components with irregular shapes and sizes. Components which are generally used in different industries need to be analysed first. Not every component can be measured with a micrometre, sometimes due to the delicate nature of the product. Thus, this instrument cut down the challenge of measuring uneven shapes and surfaces. For inspecting shape and sizes of the PET bottles and their defects. There are some defects which cannot be analysed by naked eyes. Moreover, uneven shapes can be measured to bring uniformity into the whole batch. For inspecting surface deformities in the automotive industry. Surfaces of automobiles carry minute defects, like scratches and dents. These defects can be easily magnified using profile projector. For inspecting minute flaws in glassware. High-end glassware is crafted with precision.
OPTICAL FLAT
An optical flat is an optical-grade piece of glass lapped and polished to be extremely flat on one or both sides, usually within a few millionths of an inch (about 25 nanometres). They are used with a monochromatic light to determine the flatness of other optical surfaces by interference. When an optical flat's polished surface is placed in contact with a surface to be tested, dark and light bands will be formed when viewed with monochromatic light. These bands are known as interference fringes and their shape gives a visual representation of the flatness of the surface being tested. The surface flatness is indicated by the amount of curve and spacing between the interference fringes. Straight, parallel, and evenly spaced interference fringes indicate that the work surface flatness is equal to or higher than that of the reference surface.
Optical flats are cylindrical in form with the working surfaces flat and are of two types: a)TYPE A b) TYPE B
Type A —It has only one surface flat. The working surface of this type of flat is indicately by an arrow head on the cylindrical surface pointing towards the working surface.
Type B —It has both the surfaces flat and parallel to each other. Type A are used for testing the flatness of precision measuring surfaces of flats, slip gauges, measuring tables, etc.
Type B are used for testing measuring surfaces of micrometers, measuring anvils and similar length measuring devices for testing flatness and parallelism.
These are generally made of either fused quartz (whose co-efficient of linear expansion is not more than 0.6 x 10_6/degree at standard temperature of 20°G) or borosilicate glass (whose co- efficient of linear expansion is not more than 3.6 x 10-6/ degree at standard temperature of 20°C). This glass is clear and colourless and free from inclusions and defects like bubbles, internal strains and extraneous matter.
Applications
Measurement of the surface flatness of polished surfaces can be determined visually by comparing the variations between a work surface and the surface of an optical flat.
Optical flats are versatile optical components used in many applications, such as: inspection of gauge blocks for wear and accuracy, as well as the testing of various components including windows, prisms , filters, mirrors, etc.
They can also be used as extremely flat optical windows for demanding interferometry requirements.
OPTICAL FLAT - Flatness testing
An optical flat is usually placed upon a flat surface to be tested. If the surface is clean and reflective enough, rainbow colored bands of interference fringes will form when the test piece is illuminated with white light. If a monochromatic light is used to illuminate the work piece, such as helium, low-pressure sodium, or a laser, then a series of dark and light interference fringes will form. These interference fringes determine the flatness of the work piece, relative to the optical flat, to within a fraction of the wavelength of the light. If both surfaces are perfectly the same flatness and parallel to each other, no interference fringes will form. ANGLE PLATE
• An angle plate is a work holding device used as a fixture in metalworking. • An angle plate are used for supporting or setting up work vertically, and are provided with holes and slots through which securing bolts can be located. • It is made of cast iron and ground to a high degree of accuracy
SURFACE PLATE
• It is a solid, flat plate used as the main horizontal reference plane for precision inspection, marking out (layout), and tooling setup. • The surface plate is often used as the baseline for all measurements to the workpiece, therefore one primary surface is finished extremely flat with accuracy up to 0.00001 in or 250 nm for a grade AA or AAA plate. • Surface plates are a very common tool in the manufacturing industry and are often permanently attached to robotic type inspection devices such as a coordinate-measuring machine. • Plates are typically square or rectangular. One current British Standard includes specifications for plates from 160 mm x 100 mm to 2500 mm x 1600 m m.
Vernier caliper
1.Main Scale - The main scale is similar to that on a ruler, graduated in mm and cm on one side ; inches on the other side.
2.Vernier Scale – The vernier scale is a sliding scale .It slides parallel to the main scale and enables readings to be made to a fraction of a division on the main scale.
3.Screw -The vernier scale can be fixed at any position on the main scale with the help of a screw.
4.Jaws –It has two jaws. The lower jaws are called outside jaws and they are used to measure the length of a rod ,diameter of a sphere or the external diameter of a cylinder. The upper jaws are called the inside jaws which are used to measure the internal diameter of a hollow cylinder or pipe .
5.Strip - The thin strip is used to measure the depth of the objects like beakers .
The vernier, dial, and digital calipers give a direct reading of the distance measured with high accuracy and precision. They are functionally identical, with different ways of reading the result. These calipers comprise a calibrated scale with a fixed jaw, and another jaw, with a pointer, that slides along the scale. The distance between the jaws is then read in different ways for the three types.
The simplest method is to read the position of the pointer directly on the scale. When the pointer is between two markings, the user can mentally interpolate to improve the precision of the reading. This would be a simple calibrated caliper; but the addition of a vernier scale allows more accurate interpolation, and is the universal practice; this is the vernier caliper .
Least count = 0.02 mm
Measurement using Vernier:
1. Before measuring, close jaws and check that main scale zero mark lines up exactly with vernier scale zero mark. 2. Adjust jaws. 3. You may need to gently “rock” the jaws to get the right feel – not too tight, not too loose. 4. Lock the slide. 5. Read number of whole millimetres on main scale, before the vernier zero 6. Look for a vernier scale graduation which lines up exactly with any graduation on the main scale 7. Read fraction number on vernier scale where the marks line up 8. Add fraction to whole millimetres to get final measurement.
Uses of a vernier calipers :
Vernier callipers are used to measure
(i)The length of a rod or any object
(ii)The diameter of a sphere
(iii)The internal and external diameter of a hollow cylinder
(iv)The depth of a small beaker
VERNIER HEIGHT GAUGE :
The venier height gauge is used to measure the height of parts to an accuracy of 0.02 mm. This is also used for precision layout work. Scriber is used to mark out the job. A height gauge is a measuring device used either for determining the height of objects, or for marking of items to be worked on.
These measuring tools are used in metalworking or metrology to either set or measure vertical distances; the pointer is sharpened to allow it to act as a scriber and assist in marking out work pieces.
Construction:
A vernier height gauge consists of
(i) a finely grow and lapped base. The base is massive and robust in construction to ensure rigidity and stability.
(ii) A vertical graduated beam or column supported on a massive base
(iii) Attached to the beam is a sliding vernier head carrying the vernier scale and a clamping screw.
(iv) An auxiliary head which is also attached to the beam above sliding vernier head. It has fine adjusting and clamping screw.
(v) A measuring jaw or a scriber attached to the front of sliding vernier.
Uses of height gauge:
• The vernier height gauge is designed for accurate measurement and marking of vertical height above a surface plate datum. • It can also be used to measure differences in heights by taking the vernier scale reading each height and determining the difference by substraction. • It can be used for number of applications in tool room and inspection department.
VERNIER DEPTH GAUGE
It is used for measuring the depths of holes, slots , recessed areas . It has got one shoulder which acts as reference surface and is held firmly and perpendicular to the centre line of the hole.
SURFACE GAUGE /SCRIBER
A surface gauge is very useful when finding the centre of a piece of round section material. It is normally used to ‘scribe’ parallel lines. Its base is heavy and this means it is stable when in used. Surface gauges sometimes have magnetic bases and this means they can be locked onto metal surfaces making it easier to use. The diagram above shows the round section steel held in a vee block. The surface gauge is then moved across the surface of the steel, scribing a line. The steel is then rotated through 90 degrees and another line is scribed. This is repeated until a square is produced in the centre (see diagrams below). Diagonal lines are then drawn from each corner of the square to locate the exact centre of the circle.
SLIP GAUGES
• Slip gauges are rectangular block of high grade steel with exceptionally close tolerance.
• These blocks are suitably hardened up to 800 HRc through out to ensure maximum resistance to wear. • These are then stabilized by heating and cooling successively in stages so that hardening stresses are removed. • After being hardened they are carefully finished by high grade lapping to a high degree of finish flatness and accuracy.
• For successful use of slip gauges their working faces are made truly flat parallel. The cross section of the gauges are (i)9mmx30mm for sizes up to 10mm (ii)9mmx35mm for larger sizes. Any 2 slip gauges when perfectly clean may be wrung together. The dimensions are permanently marked on one of the measuring faces of gauges block.
5 grades of slip gauges are available as follows:
GradeII : Grade II gauge blocks are workshop grade and used for rough check. They are used for setting up machine tools, positioning milling cutters etc where the tolerance values are relatively wide.
Grade-I: These are used for more precise work such as setting up sine bars checking gap gauges and setting dial test indicators to zero.
Grade 0 (zero) : This is more commonly known as inspection grade and its use is confined to tool room or machine shop inspection.
Grade 00(zero zero): This grade gauges are placed in the standard room and used for highest precision work such as checking grade I &Grade II slip gauges.
Calibration grade: This is a special grade with the actual size of the slip calibration on a special chart supplied with a set. The chart must be referred while making up dimension.
Uses of Slip gauge blocks
(1) Direct precise measurement where accuracy is required. (2) For checking the accuracy of vernier calipers, micrometer etc (3) Setting up a comparator to a specific dimension. (4) It is used for angle measurement with sine bar. (5) The distance of plugs spigot etc on fixture are measured (6) To check gap between parallel locations such as in gap gauges or between 2 mating parts
MICROMETER
Most engineering precision works have to be measured to a much greater accuracy than this value in order to achieve the interchangeability of component parts. In order to achieve this greater precision measuring equipment of a greater accuracy and sensitivity must be used. Micrometer is one of the most common and most popular forms of measuring instrument for precise measurement with 0.01 mm accuracy. However micrometer with 0.001 mm accuracy are also available. Micrometer may be classified as (a) Outside micrometer (b) Inside micrometer (c) Screw thread micrometer (d) Depth gauge micrometer The micrometer is a precision measuring instrument, used by engineers. Each revolution of the rachet moves the spindle face 0.5mm towards the anvil face. The object to be measured is placed between the anvil face and the spindle face. The rachet is turned clockwise until the object is ‘trapped’ between these two surfaces and the rachet makes a ‘clicking’ noise. This means that the rachet cannot be tightened any more and the measurement can be read.
(a) Outside micrometer
The main parts of an outside caliper are: 1. U shaped steel frame 2. anvil & spindle 3. lock nut 4. sleeve or barrel 5. thimble 6. ratchet
1.U shaped steel frame : The outside micrometer has U shaped or C shaped frame. It holds all the micrometer parts together. The gap of the frame permits the maximum diameter or length of the job to be measured. The frame is generally made of steel, cast iron, maleable cast iron or light alloy. It is desirable that the frame of the micrometer be provided with conveniently placed finger grips of heat insulting materials. 2.Anvil & spindle The micrometer has a fixed anvil protruding 3mm from the left hand side frame. The diameter of the anvil is the same as the diameter of spindle. Another movable anvil is provided on the front of the spindle. The anvils are accuracy ground and lapped with its measuring faces flat and parallel to the spindle. These are also available with WC faces. The spindle is the movable measuring face with the anvil on the front side. The spindle engages with the nut. It should run freely and smoothly through out the length of its travel. There should be no backlash between the spindle screw and nut. There should be full engagement of nut& screw when the micrometer is at its full reading.
3.Lock nut A lock nut is provided on the micrometer spindle to lock it when the micrometer is at its correct reading. The design of the locknut is such that it effectively locks the spindle without altering the distance between the measuring faces. It thus retains the spindle in perfect alignment.
4.Sleeve or Barrel: The sleeve is accurately divided and clearly marked in 0.5mm division along its length which serves as a main scale. It is chrome plated and adjustable for zero setting.
5.Thimble : The thimble can be moved over the barrel, it has 50 equal divisions around its circumference. 6.Ratchet: The ratchet is provided at the end of the thimble. It is used to assure accurate measurement and to prevent too much pressure being applied to the micrometer. READING: 1. Reading the scale on the sleeve. The example clearly shows 12 mm divisions. 2. Still reading the scale on the sleeve, a further ½ mm (0.5) measurement can be seen on the bottom half of the scale. The measurement now reads 12.5mm. 3. Finally, the thimble scale shows 16 full divisions (these are hundredths of a mm). The final measurement is 12.5mm + 0.16mm = 12.66
Inside Micrometer
This is similar in structure to an outside micrometer and is used for measuring internal dimensions .
Thread Micrometer
the anvil may be shaped in the form of a segment of screw thread to measure screw dimensions
Depth Micrometer
A depth micrometer (figure 8) is used for measuring the depth of a hole, slot and keyway etc. A complete set of depth micrometer is equipped with spindles of different lengths, which can be interchanged to suit different measuring ranges.
Figure 8. Depth Micrometer
H ARDNESS TEST What is Hardness? Hardness is the property of a material that enables it to resist plastic deformation, usually by penetration. However, the term hardness may also refer to resistance to bending, scratching, abrasion or cutting.
Hardness Test Methods: Rockwell Hardness Test Rockwell Superficial Hardness Test Brinell Hardness Test Vickers Hardness Test Microhardness Test, Moh's Hardness Test Scleroscope and other hardness test methods
Rockwell Hardness Test
• The Rockwell scale is a hardness scale based on indentation hardness of a material. The Rockwell test determines the hardness by measuring the depth of penetration of an indenter under a large load compared to the penetration made by a preload. There are different scales, denoted by a single letter, that use different loads or indenters.
• The result is a dimensionless number noted as HRA, HRB, HRC, etc., where the last letter is the respective Rockwell scale. When testing metals, indentation hardness correlates linearly with tensile strength.
• This important relation permits economically important nondestructive testing of bulk metal deliveries with lightweight, even portable equipment, such as hand-held Rockwell hardness testers
• The Rockwell hardness test method consists of indenting the test material with a diamond cone or hardened steel ball indenter.
• The indenter is forced into the test material under a preliminary minor load F0 usually 10 kgf.
• When equilibrium has been reached, an indicating device, which follows the movements of the indenter and so responds to changes in depth of penetration of the indenter is set to a datum position.
• While the preliminary minor load is still applied an additional major load is applied with resulting increase in penetration (Fig. 1B). When equilibrium has again been reach, the additional major load is removed but the preliminary minor load is still maintained. • Removal of the additional major load allows a partial recovery, so reducing the depth of penetration (Fig. 1C). The permanent increase in depth of penetration, resulting from the application and removal of the additional major load is used to calculate the Rockwell hardness number.
Rockwell hardness tester classifications based on Rockwell scales Rockwell hardness tester: HRA, HRB, HRC • Superficial Rockwell hardness tester: 15N, 30N, 45N, 15T, 30T, 45T, 15W, 30W, 45W, 15X, 30X, 45X, 15Y, 30Y, 45Y • Plastic Rockwell hardness tester: HRE, HRL, HRM • Twin Rockwell hardness tester (also named as Rockwell & superficial Rockwell hardness tester): HRA, HRB, HRC,15N, 15T, 15W, 15X, 15Y, 30N, 30T, 30W, 30X, 30Y, 45N, 45T, 45W, 45X, 45Y
Scales and values There are several alternative scales, the most commonly used being the "B" and "C" scales. Both express hardness as an arbitrary dimensionless number .
Scale Abbreviation Load Indenter A HRA 60 kgf 120° diamond spheroconical
1 B HRB 100 kgf ⁄16 -inch-diameter (1.588 mm) steel sphere C HRC 150 kgf 120° diamond spheroconical D HRD 100 kgf 120° diamond spheroconical
1 E HRE 100 kgf ⁄8-inch-diameter (3.175 mm) steel sphere
1 F HRF 60 kgf ⁄16 -inch-diameter (1.588 mm) steel sphere
1 G HRG 150 kgf ⁄16 -inch-diameter (1.588 mm) steel sphere
1 H HRH 60 kgf ⁄8-inch-diameter (3.175 mm) steel sphere
1 K HRK 150 kgf ⁄8-inch-diameter (3.175 mm) steel sphere
Interpreting Rockwell Hardness Values
Rockwell hardness values consist of three parts. A typical value might be 50 HRB, of which “HR” indicates a Rockwell hardness of “50” using the “B” scale.
Testing of Sheet-Type Materials
A similar Rockwell scale exists for testing of thin sheet-type materials or surfaces subject to thin surface treatments, or parts that would not otherwise be able to produce meaningful readings using the standard Rockwell scale. Known as the Rockwell superficial test, it utilises lower loads including a preload of just 3kg and full load usually of 15 or 45kg depending on the hardness of the material. Readings for the superficial test also consist of three parts e.g. 15T-22. “15” indicates a load of 15kg was used and a reading of “22” was produced using the steel ball indenter, indicated by the “T”. The “T” would be substituted by a “N” if the diamond cone indenter was used.
Summary
The Rockwell technique can be performed quickly making it ideal for quality control. It also leaves only a tiny impression on the specimen and is capable of being used on a wide variety of materials and geometries.
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THE BRINELL HARDNESS TEST
• The Brinell scale characterizes the indentation hardness of materials through the scale of penetration of an indenter, loaded on a material test-piece. It is one of several definitions of hardness in materials science.
• The Brinell hardness test method consists of indenting the test material with a 10 mm diameter hardened steel or carbide ball subjected to a load of 3000 kg. For softer materials the load can be reduced to 1500 kg or 500 kg to avoid excessive indentation.
• The full load is normally applied for 10 to 15 seconds in the case of iron and steel and for at least 30 seconds in the case of other metals. The diameter of the indentation left in the test material is measured with a low powered microscope. The Brinell harness number is calculated by dividing the load applied by the surface area of the indentation.
where: BHN = Brinell Hardness Number (kgf/mm 2) P = applied load in kilogram-force (kgf) D = diameter of indenter (mm) d = diameter of indentation (mm)
• The diameter of the impression is the average of two readings at right angles and the use of a Brinell hardness number table can simplify the determination of the Brinell hardness.
• Compared to the other hardness test methods, the Brinell ball makes the deepest and widest indentation, so the test averages the hardness over a wider amount of material, which will more accurately account for multiple grain structures and any irregularities in the uniformity of the material. • This method is the best for achieving the bulk or macro-hardness of a material, particularly those materials with heterogeneous structures.
Brinell hardness numbers Material Hardness 5.0 HB (pure lead; alloyed lead typically can range
Lead from 5.0 HB to values in excess of 22.0 HB) Pure Aluminium 15 HB
Copper 35 HB
Mild steel 120 HB 18–8 (304) stainless 200 HB steel annealed
Glass 1550 HB Hardened tool steel 600–900 HB (HBW 10/3000)
VICKERS HARDNESS TEST
The Vickers hardness test method consists of indenting the test material with a diamond indenter, in the form of a right pyramid with a square base and an angle of 136 degrees between opposite faces subjected to a load of 1 to 100 kgf.
The full load is normally applied for 10 to 15 seconds. The two diagonals of the indentation left in the surface of the material after removal of the load are measured using a microscope and their average calculated. The area of the sloping surface of the indentation is calculated.
The Vickers hardness is the quotient obtained by dividing the kgf load by the square mm area of indentation.
When the mean diagonal of the indentation has been determined the Vickers hardness may be calculated from the formula, but is more convenient to use conversion tables. The Vickers hardness should be reported like 800 HV/10, which means a Vickers hardness of 800, was obtained using a 10 kgf force. Several different loading settings give practically identical hardness numbers on uniform material, which is much better than the arbitrary changing of scale with the other hardness testing methods.
The advantages of the Vickers hardness test are that extremely accurate readings can be taken, and just one type of indenter is used for all types of metals and surface treatments.
Although thoroughly adaptable and very precise for testing the softest and hardest of materials, under varying loads, the Vickers machine is a floor standing unit that is more expensive than the Brinell or Rockwell machines.
The Scleroscope Hardness Test
• The Scleroscope test consists of dropping a diamond tipped hammer, which falls inside a glass tube under the force of its own weight from a fixed height, onto the test specimen.
• The height of the rebound travel of the hammer is measured on a graduated scale. The scale of the rebound is arbitrarily chosen and consists on Shore units, divided into 100 parts, which represent the average rebound from pure hardened high-carbon steel.
• The scale is continued higher than 100 to include metals having greater hardness. The Shore Scleroscope measures hardness in terms of the elasticity of the material and the hardness number depends on the height to which the hammer rebounds, the harder the material, the higher the rebound.
Moh's Hardness Scale The Moh's hardness scale for minerals has been used since 1822. It simply consists of 10 minerals arranged in order from 1 to 10. Diamond is rated as the hardest and is indexed as 10; talc as the softest with index number 1. Each mineral in the scale will scratch all those below it as follows: Diamond 10 Corundum 9 Topaz 8 Quartz 7 Orthoclase (Feldspar) 6 Aptite 5 Fluorite 4 Calcite 3 Gypsum 2 Talc 1
The hardness is determined by finding which of the standard minerals the test material will scratch or not scratch; the hardness will lie between two points on the scale - the first point being the mineral which is scratched and the next point being the mineral which is not scratched. Some examples of the hardness of common metals in the Moh's scale are copper between 2 and 3 and tool steel between 7 and 8.
The Durometer
• The Durometer is a popular instrument for measuring the indentation hardness of rubber and rubber-like materials. • The most popular testers are the Model A used for measuring softer materials and the Model D for harder materials. • The operation of the tester is quite simple. The material is subjected to a definite pressure applied by a calibrated spring to an indenter that is either a cone or sphere and an indicating device measures the depth of indentation.
Microhardness Test • The term microhardness test usually refers to static indentations made with loads not exceeding 1 kgf.
• The indenter is either the Vickers diamond pyramid or the Knoop elongated diamond pyramid. The procedure for testing is very similar to that of the standard Vickers hardness test, except that it is done on a microscopic scale with higher precision instruments.
• The surface being tested generally requires a metallographic finish; the smaller the load used, the higher the surface finish required. Precision microscopes are used to measure the indentations; these usually have a magnification of around X500 and measure to an accuracy of +0.5 micrometres.
• Also with the sameobserver differences of +0.2 micrometres can usually be resolved. It should, however, be added that considerable care and experience are necessary to obtain this accuracy.
Knoop Hardness Indenter Indentation The Knoop hardness number KHN is the ratio of the load applied to the indenter, P (kgf) to the unrecovered projected area A (mm2) KHN = F/A = P/CL2 Where: F = applied load in kgf A = the unrecovered projected area of the indentation in mm2 L = measured length of long diagonal of indentation in mm C = 0.07028 = Constant of indenter relating projected area of the indentation to the square of the length of the long diagonal.
Vickers Pyramid Diamond Indenter Indentation
The Vickers Diamond Pyramid harness number is the applied load (kgf) divided by the surface area of the indentation (mm2)
Where: F= Load in kgf d = Arithmetic mean of the two diagonals, d1 and d2 in mm HV = Vickers hardness The Vickers Diamond Pyramid indenter is ground in the form of a squared pyramid with an angle of 136obetween faces. The depth of indentation is about 1/7 of the diagonal length. When calculating the Vickers Diamond Pyramid hardness number, both diagonals of the indentation are measured and the mean of these values is used in the above formula with the load used to determine the value of HV. Tables of these values are usually a more convenient way to look-up HV values from the measurements.
1. What are coated tools?
• Coatings are frequently applied to carbide tool tips to improve tool life or to enable higher cutting speeds. • Coated tips typically have lives 10 times greater than uncoated tips. • Common coating materials include titanium nitride, titanium carbide and aluminium oxide, usually 2 - 15 micro-m thick. Often several different layers may be applied, one on top of another, depending upon the intended application of the tip. • The techniques used for applying coatings include chemical vapour deposition (CVD) plasma assisted CVD and physical vapour deposition (PVD). • Coatings have the following characteristics: High hardness Chemical stability and inertness Low thermal conductivity Compatibility and good bonding Little or no porosity
2. Name few materials for mold making
MOLD Mold Component STEEL P-20 Mold bases, ejector plates, and some cavities (if nickel or chrome plated to prevent rust). H-13 Cavity plates and core plates. S-7 Cavity plates, core plates and laminates, as well as thin wall sections. A-2 Ejector pins, ejector sleeves, and ejector blades. D-2 Gate blocks, gib plates to prevent galling, gate blocks to prevent wear.
420 SS Cavity blocks, ejector pins, sleeves, etc.
3. Functions of electrolyte (Dielectric fluid) in EDM
4. Advantages of CNC MACHINES
5.What is electroforming ?
Electroforming is a metal forming process that forms parts/cold cavities through electrodeposition on a model or mandrel.
Principle of Electroforming
• The basic principles of both the electroforming and electroplating are, to all intents and purposes, identical. • Two electrodes, an anode and a cathode, are immersed in a suitable electrolyte containing a dissolved salt of a metal (for example nickel sulphate) in an appropriate container. • A direct current is applied between these electrodes causing the electrode connected to the positive pole of the current source, the anode, to dissolve. • This produces positively charged metallic ions, for example Ni2, in solution which migrate towards the negatively charged electrode, the cathode. • At the cathode surface the positively charged metallic ion is reduced to the elemental metallic state by electrons supplied from the direct current source. This causes the cathode to become coated with whatever metal is being deposited by the process.
6. Advantages of Hobbing
• Good surface finish • Higher rate of production • Precise reproduction • Better Dimensional Accuracy • Low stress in mold part • Used for making mold cavities 7. What is polish ability ?
• Polishability of mold steel also is a function of its chemical composition and its hardness level. Some common alloying elements include chromium, molybdenum and vanadium. These elements may be defined as carbide-formers, since they react with the carbon to form a combination of chromium, molybdenum and/or vanadium carbides. • The carbides that are present within the matrix provide the steel with its wear resistance. • The greater the amount of carbon and alloy elements, the greater will be the percentage of wear-resistant carbide particles that will form within the material.
8. Define Texture?
• Texturing—sometimes referred to as graining or engraving—is the process of adding a pattern (the texture or grain) to the molding surface of a mold. • This allows the mold to impress that pattern on each molded part. Compared to finishing each molded product, texturing or graining of a mold is a very economical way to impress complex designs or patterns on molded plastic parts. • The accurate reproduction of the specified texture in the mold surface requires careful consideration of many factors. • Creating a pattern on a mold cavity that can be reproduced on part. Surface Texturing/ Structuring/Etching Can Be Done By 1. Chemical Etching – A) Dip Etching B) Spray Etching 2. Photochemical Etching 3. Laser Texturing
9. What are Slip Gauges?
• Slip gauges are rectangular block of high grade steel with exceptionally close tolerance.
• These blocks are suitably hardened up to 800 HRc through out to ensure maximum resistance to wear. • These are then stabilized by heating and cooling successively in stages so that hardening stresses are removed. • After being hardened they are carefully finished by high grade lapping to a high degree of finish flatness and accuracy.
5 grades of slip gauges are available as follows:
GradeII , Grade-I, Grade 0 (zero) ,Grade 00(zero zero) & Calibration grade:
10. Limitataions of optical profile projector
Need of skilled operator Fixed device, cannot be moved to workplace Dust, moisture, oil etc., should not be allowed on apparatus. High instrument cost Precise maintenance required
11. Advantages of optical profile projector
Optical magnification provides high degree of measuring precision due to reduction of moving members and better wear resistance qualities Optical magnification is also free from friction,bending,wear etc. An illuminated scale is provided that enables readings to be taken without regard to the room lighting conditions These are also used to magnify very small parts such as needles, saw teeth, screw threads etc
12. Trueing and Dressing in grinding
Dressing When the sharpness of grinding wheel becomes dull because of glazing and loading, dulled grains and chips are removed (crushed or fallen) with a proper dressing tool to make sharp cutting edges and simultaneously, make recesses for chips by properly extruding to grain cutting edges. Thus, these operations are for the dressing.
Trueing When the grinding wheel is mounted to the grinding wheel spindle, the run-out on wheel operating surface is removed, the wheel during contour grinding is trued or worn grinding wheel is corrected. Thus, these operations are for the trueing 13.Applications of profile grinding
The principle of profile grinding is to divide the contour of the complex parts into several segments and arcs, and then piecewise grinding according to the process order, so that the connection is smooth, which meets the requirement of the product design. The profile grinding is a essential methods of fine machining in precision mold manufacturing, as it can improve the precision of mold manufacturing and shorten the processing time
14. Applications of wire cut EDM
ELECTRICALLY CONDUCTIVE- Any Material That Is Electrically Conductive Can Be Cut, Regardless Of Its Hardness. FRAGILE And Thin Sections Can Easily Be Machined Without Deformation. INTRICATE Contours Or Cavities In Hardened Steel Can Be Cut Without The Need For Heat Treatment To Soften And Re Harden. Better Dies And Molds Can Be Produced At A Lower Cost o Mostly used for making Extrusion Dies , Blanking Dies , Sintered Compacting dies
15. What is Surface Finish
Surface finish, also known as surface texture or surface topography, is the nature of a surface as defined by the three characteristics of lay, surface roughness, and waviness. It comprises the small, local deviations of a surface from the perfectly flat ideal (a true plane). Each manufacturing process (such as the many kinds of machining) produces a surface texture. The process is usually optimized to ensure that the resulting texture is usable. If necessary, an additional process will be added to modify the initial texture. The latter process may be grinding (abrasive cutting), polishing, lapping, abrasive blasting, honing, electrical discharge machining (EDM), milling, lithography, industrial etching/chemical milling, laser texturing, or other process 16. Applications of ANGLE PLATE
• An angle plate is a work holding device used as a fixture in metalworking. • An angle plate are used for supporting or setting up work vertically, and are provided with holes and slots through which securing bolts can be located. • It is made of cast iron and ground to a high degree of accuracy
17. Applications of toolmaker’s microscope
To measure the diameter of holes and distances between centres Position of keyway with reference to centre of bore Angles and pitch of threads Gear tooth spacing
18. Purpose of Milling ?
• Milling is the machining process of using rotary cutters to remove material from a workpiece by feeding in a direction at an angle with the axis of the tool.
• The milling cutter performs a rotary movement (primary motion) and the workpiece a linear movement (secondary motion). • The milling technique is used to produce, mainly on prismatic components, flat, curved, parallel, stepped, square and inclined faces as well as slots, grooves, threads and tooth systems. • It is one of the most commonly used processes in industry and machine shops today for machining parts to precise sizes and shapes.
19. What is split mold
• A split mould is a great way to cast complex shape of small to medium size quickly and effectively. It is used for producing parts with internal and external undercuts. Mold is actuated by Dog leg cam actuation and Finger cam actuation.
20. What is Die Sinking
• Die sinking is a process used to machine or create a specific size or shape cavity or opening in steel blocks.
• The openings in the steel blocks can then be used to mold plastic into different shapes. Such openings may also be used when doing forging, either hot or cold, or for coining or die-casting.
• Most often, die sinking is used to place names, numbers, and other sources of information onto metal.
• It can also be used to place such elements on a piece of wood, leather, or many other materials.
• The process for die sinking is fairly simple and something that anyone can do with a few simple tools.
21. Principle of EDM ?
Types of EDM:
1. Conventional EDM /Die Sinking EDM 2. Wire Cut EDM
22. What is electroforming
Electroforming is a metal forming process that forms parts through electrodeposition or electroplating on a model, known in the industry as a mandrel .
THE PRINCIPLE OF ELECTROFORMING
• The basic principles of both the electroforming and electroplating are, to all intents and purposes, identical. • Two electrodes, an anode and a cathode, are immersed in a suitable electrolyte containing a dissolved salt of a metal (for example nickel sulphate) in an appropriate container. • A direct current is applied between these electrodes causing the electrode connected to the positive pole of the current source, the anode, to dissolve. • This produces positively charged metallic ions, for example Ni2, in solution which migrate towards the negatively charged electrode, the cathode. • At the cathode surface the positively charged metallic ion is reduced to the elemental metallic state by electrons supplied from the direct current source. This causes the cathode to become coated with whatever metal is being deposited by the process. 23.Uses of Electrosonic polishing
• In ultrasonic machining, a tool of desired shape vibrates at an ultrasonic frequency (19 ~ 25 kHz) with an amplitude of around 15 – 50 µm over the workpiece. Generally the tool is pressed downward with a feed force. • Materials that are commonly machined using ultrasonic methods include ceramics, carbides, glass, precious stones and hardened steels. These materials are used in optical and electrical applications where more precise machining methods are required to ensure dimensional accuracy and quality performance of hard and brittle materials. • Ultrasonic machining is precise enough to be used in the creation of micro-system components such as micro-structured glass wafers • Polishing of mold cavities • Ultrasonic vibration machining is used for structural components because of the required precision and surface quality provided by the method
24. Define SURFACE ROUGHNESS
Surface Roughness often shortened to roughness , is a component of surface texture . It is quantified by the deviations in the direction of the normal vector of a real surface from its ideal form.
The geometrical characteristics of a surface include, 1. Macro-deviations, 2. Surface waviness, and 3. Micro-irregularities.
The surface roughness is evaluated by the height, Rt and mean roughness index R a of the micro- irregularities.
Surface roughness number (R a) is expressed in microns.
Ra = (h 1+h 2+-----+h n)/n
25. OPTICAL FLAT
An optical flat is an optical-grade piece of glass lapped and polished to be extremely flat on one or both sides, usually within a few millionths of an inch (about 25 nanometres). They are used with a monochromatic light to determine the flatness of other optical surfaces by interference . When an optical flat's polished surface is placed in contact with a surface to be tested, dark and light bands will be formed when viewed with monochromatic light
Types : i) TYPE A
ii) TYPE B
Type A—It has only one surface flat. The working surface of this type of flat is indicately by an arrow head on the cylindrical surface pointing towards the working surface.
Type B—It has both the surfaces flat and parallel to each other.
26. What is Comparator?
The general principle of comparator is to indicate the differences in size between the standard and the work piece being measured by means of some pointer on a scale with sufficient magnification. It thus not measure the actual dimension but indicates how much it differs from the standard dimension
27. Define mold?
Mold comprises core and cavity which forms an Impression to give shape to molten or hot liquid material when it cools and hardens. Core
A protrusion in a mold which forms the inner surfaces of the molded articles Cavity A depression in a mold which forms the outer surfaces of the molded articles.
Types: Injection mold, Compression mold, Transfer mold, Rotational mold, Blow mold 28. DRAWBACKS OF CHEMICAL TEXTURING
• Chemicals are Harmful to operators • Limited Accuracy • It is difficult to texture complex texturepatterns • In addition to adherence to strict government regulations, chemical etching also requires substantial manual labor. Each part undergoing the process must be carefully prepared to ensure that the chemicals only access the features to be textured. • Exposing other areas of the component to these chemicals can result in the entire piece being scrapped. • Additionally, the process requires readying the chemicals, administering the bath, cleaning the part, and disposing of the chemicals—all steps with high labor components.
29. Purpose of Drilling
This is the operation of making a circular hole by removing a volume of metal from the job by a rotating cutting tool called drill
30. What is Counter Sinking ?
Counter-Sinking Counter-sinking operation is shown in Fig. 22.10. This is the operation of making a coneshaped enlargement of the end of a hole, as for the recess for a flat head screw. This is done for providing a seat for counter sunk heads of the screws so that the latter may flush with the main surface of the work.