4/19/2011

Principles of Machining Processes

Alessandro Anzalone, Ph.D. Hillsborough Community College, Brandon Campus

Agenda

1. Introduction 2. Motion and Parameters: Speed, Feed, and Depth of Cut 3. Machining: Shearing Chips from the Workpiece 4. Cutting Tool Materials 5. Cutting Tool Geometry 6. Cutting Fluids 7. References

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Introduction

Machining is essentially the process of removing unwanted material from wrought (rolled) stock, forgings, or castings to produce a desired shape, surface finish, and dimension. It is one of the four major types o f manu ftifacturing processes use d to crea te pro dtduct components. Machining is done by shaving away the material in small pieces, called chips, using very hard cutting tools and powerful, rigid machine tools. The cutting tool may be held stationary and moved across a rotating workpiece as on a lathe, or a rigidly held workpiece may move into a rotating cutting tool as on a milling machine.

Introduction

Few manufacturing technologies can achieve the precision of the various machining processes. The common 0.001 in. and 0.0001 in. precision that is possible for modern machining processes is far superior to the prec is ion o f common cas ting, mo lding, an d form ing operations. Large, heavy sections such as those used for dies are machined to precise dimensions, a virtual impossibility for many forming processes that handle only thin sheet materials.

Machining processes remove material in the form of chips that are disposed or recycled. Machining is more costly than casting, molding, and forming processes , which are g enerall y quicker and waste less material, but machining is often justified when precision is needed.

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Introduction

Introduction

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Introduction

Introduction

The performance of the cutting tool used to remove workpiece material determines the efficiency and cost of a machining operation. The geometry of the cutting edge controls the shearing action as a c hip is torn away from the par t. The cu tting too l material determines how fast the operation may progress, and since time is money in manufacturing activities this is an important factor in the cost of the operation.

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Motion and Parameters: Speed, Feed, and Depth of Cut

Machining operations require two basic simultaneous motions; one motion creates cutting speed, and the other is the feed motion. Cutting speed is the rate at which the workpiece moves past the tool or the rate at which the rotating surface of the cutting edge of the tool moves past the workpiece. Regardless of whether the tool rotates or the workpiece rotates, the relative motion between the two creates the cutting speed. English units for cutting speed are feet per minute (fpm), which is often called surface feet per minute (sfpm or sfm). Metric units are meters per minute (m/min). Higher cutting speed shortens the time required to complete the machining cut but can greatly shorten the useful life of the cutting tool. Cutting speeds that are too low tend to tear instead of cut, produce rough finishes, and distort the grain structure at the surface of the workpiece, all of which can cause early failure of a machined part. Speeds should be as high as can be maintained without causing the tool to wear out too quickly. Recommended cutting speeds for machining operations can be found in commonly available tables. These suggested speeds vary based on the workpiece material, cutting tool material, and type of machining operation.

Motion and Parameters: Speed, Feed, and Depth of Cut

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Motion and Parameters: Speed, Feed, and Depth of Cut

The feed motion is the advancement of the cutting tool along the workpiece (for lathes) or the advancement of the workpiece past the tool (for milling machines). The rate at which the tool or workpiece moves is the feed rate. The basic measure of the feed rate is the distance advanced per revolution of the machine spindle. Units for this feed per revolution are inches per revolution (ipr) or millimeters per revolution (mm/rev). Faster feeds shorten the time required to complete the machining cut but create rougher workpiece surfaces. Feed rates for roughing (rapid material removal) should be as heavy as the tool material and the machine tool can withstand without failure, and finishing feeds should be fine enough to produce the desired smooth surface finish on the workpiece.

The rate at which the workpiece advances (measured in inches or millimeters) per minute is an alternative way to specify the feed rate. The units are inches per minute (ipm) or millimeters per minute (mm/min). The feed per minute is calculated based on the spindle speed and the feed per revolution.

Motion and Parameters: Speed, Feed, and Depth of Cut

The tool must be engaged in the workpiece to remove material. The amount of engagement, called the depth of cut, is equal to the thickness of the layer of material removed from the workpiece. The depth of cut al so de term ines the w idth o f eac h c hip remove d from the part as it is machined.

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Machining: Shearing Chips from the Workpiece

Pressure, Heat, and Friction in Machining

Machining is similar to whittling wood with a sharp knife. The cutting too l exer ts enoug h force on the wor kp iece to smoo thly cu t away material in the form of chips. This action results in the continuous shear failure of the work material at the cutting edge of the tool, Imagine how much pressure is required to shear chips from a steel workpiece and make it appear to be as easy as cutting butter. This tremendously high pressure causes the tool and workpiece to flex or deflect away from each other. This movement affects precision, surface finish,,, and tool life, so deflection must be opp ppyosed by rigidity. The machining system is only as strong as its weakest link, so the workpiece, workholding tooling, machine tool, tool- holder, and cutting tool must all be extremely stiff to oppose deflection.

Machining: Shearing Chips from the Workpiece

The high forces in machining create a considerable amount of heat near the cutting edge. Most of this heat is generated within the shearing process, and some heat is created by friction between the too l an d the wor kp iece. Mos t o f the hea t is carr ie d o ff in the c hips, but the remainder stays in the tool and workpiece, creating a large amount of thermal stress and softening the tool. Cutting fluid (coolant) is often used to bathe the tool and workpiece to remove much of the heat and minimize damage to the tool and part.

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Machining: Shearing Chips from the Workpiece

Workpiece Materials and Their Effects

The workpiece material plays an important part in chip formation. Greater cutting forces are required to shear harder and higher-strength materials, causing more deflection between the cutting tool and workpiece. More heat is also generated, which creates higher operating temperatures in the tool and part. The increased friction and tool temperature encountered when machining hard or abrasive materials can accelerate tool wear, shortening the tool life, the amount of time that the tool will survive while cutting. Softer, lower-strength materials shear more easily and may be machined faster while causing less tool wear, but very soft materials such as soft pure aluminum or hot-rolled (HR) low-carbon steel are somewhat “gummy” when cut on machines and tend to cause built- up edge (BUE) on the cutting tool. Built-up edge is a common machining problem in which workpiece material becomes welded to the cutting edge of the tool, changing the geometry of the cutting edge. BUE causes chips to be torn away rather than cleanly cut, resulting in rough part surfaces, and it may damage the tool.

Machining: Shearing Chips from the Workpiece

The machinability of a workpiece material is the ease with which it can be machined. Machinability ratings compare various materials in terms of cutting ease or difficulty resulting from the forces required and heat generated in the shearing of a material. The relative machining difficulty of various materials may be compared by measuring the power required and cutting tool life for each workpiece material. Commonly machined materials have been machinability rated using a scale based on low- carbon soft steel (AISI B-1112), which was arbitrarily given a rating of 100 percent on the machinability scale. Metals that are more easily machined have a higher rating than 100 percent, and materials that are more difficult to machine are rated lower.

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Machining: Shearing Chips from the Workpiece

Cutting Tool Materials

Commonly machined materials include plastics, aluminum, many varieties of steels including heat-treated tool steels that are as hard as a knife blade, ceramics, and many others. Cutting tools must be capable of retaining their hardness at high temperatures (hot hardness). Better hot hardness permits tools to operate at higher cutting speeds, thereby improving productivity.

A variety of cutting tool materials are needed. Some must be very hard for long tool life and to machine hard workpiece materials. Others must be very shock resistant (tough) to withstand interrupted cuts (intermittent cutting action). An example of an interrupted cut would be turning a bar that has a hexagonal cross section. The cutting tool will, on the first cut, contact only the corners of the rotating bar, creating six impacts each time the bar rotates, The mechanical shock of this intermittent cutting action tends to fracture brittle cutting tools.

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Cutting Tool Materials

High-Speed Steel Cutting Tools

One hundred years ago hardened plain carbon tool steel was used for tools to cut metals. Cutting speeds were very slow because carbon steel would permanently lose its hardness if it got very hot. Today, plain carbon tool steel is used only for low-speed, low-temperature applications such as hand tools, including files, knives, and chisels. A type of tool steel developed around 1900 was called high-speed steel (HSS) because it could get hot, cool down, and retain its hardness. High-speed steel is less expensive than most cutting tool materials, and steel tools can be machined to complex shapes in their soft state, heat treated for hardness, and then ground to a sharp cutting edge. Because of its versatility and low cost, HSS is tod ay the most commonl y used cutti ng too l ma ter ia l in machining applications. High-speed steel drills, milling cutters, and lathe tools are widely used. After the cutting edge dulls, HSS tools are sharpened using a grinder, greatly increasing the useful life of the tool.

Cutting Tool Materials

Carbide Cutting Tools

Most cutting tools used for production machining are made of tungsten carbide. Compared with high-speed steel tools, carbide cutting tools have much better hot hardness, so they can machine at higher temperatures without softening and destroying the cutting edge. Cutting speeds are three to four times faster for carbides than for HSS tools. In the making of a carbide tool, tungsten carbide particles are mixed with a cobalt powder, compressed into a briquette of the required tool shape, and then sintered in a furnace, causing the cobalt to bind the tungsten carbide particles into a very hard, strong, solid material. Carbide is made in grades of varying hardness and toughness, and titanium carbide and tantalum carbide are sometimes a dde d to the m ix ture to prov ide grea ter har dness for wear resistance. Most carbide tools used today in manufacturing operations are throw-away inserts that have several indexable cutting edges. Carbide inserts are clamped into different toolholders for external and internal machining. Milling cutters, turning tools, boringbars, and many other types of toolholders equipped with carbide inserts have long life, and the inserts can be quickly rotated (indexed) to expose a fresh cutting edge or replaced when they wear out.

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Cutting Tool Materials

Ceramic Cutting Tools

Ceramic tools are often used to machine hard workpiece materials and have better hot hardness than carbide. These operate at much higher speeds than carbide inserts but require a very rigid setup (machine, cutting tool, workpiece, work-holding system) and will not endure the impact of heavy interrupted cuts. Ceramic cutting tools are manufactured in insert shapes similar to carbide inserts.

Coated Carbide and Cermet Cutting Tools

Tungsten carbide inserts coated with titanium nitride (TiN), ceramic (al um inum oxide ), and/or titan ium car bide com bine the qua lities of carbide and the wear resistance of the coating material to permit increased cutting speeds and added productivity. Cermet, a mixture of carbide and ceramic that is sintered into inserts, competes closely with the productivity of coated carbide tools.

Cutting Tool Materials

Diamond and CBN Cutting Tools

Diamond cutting tools can produce exceedingly smooth surface finishes and hold very close tolerances. Diamond tools are manufactured from polycrystalline powder (PCP), and it retains a sharp, stable cutting edge, but it is prohibitively expensive for many applications. Cubic boron nitride (CBN), a manmade material, is second in hardness to diamond. CBN is used to “hard turn” steel workpieces that are too hard to be machined by less exotic cutting tool materials, but it is also very expensive.

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Cutting Tool Geometry

Tool geometry (shape) varies considerably depending on the machining application. The shape and angles of the cutting tool must be accurate in order to have sufficient strength at the cutting edge, to ensure good chip formation and flow, and to provide sufficient relief (clearance) so the tool will cut into the work material and not rub. There are many shapes of cutting tools, including left-hand, threading, and form tools, but all of them must have correct angles for relief and rake. Chip breakers are sometimes needed on tools to control chip formation.

Cutting Tool Geometry

At what angle would you hold a knife blade when peeling an apple? If you hold the knife blade perpendicular to the surface of the apple (a neutral or zero rake angle) the knife will scrape the skin off the apple, requiring a large amount of force and cutting very poorly. If you hold the knife blade at an angle greater than 90º (a negative rake angle), which would seem to be backward, the blade will cut even less effectively, requiring a very large force to push the skin off, possibly crushing the apple. The most effective knife blade angle, of course, is the acute angle that one would normally use when peeling an apple (a positive rake angle). A positive rake angle will shear material smoothly and efficiently while requiring less force. This rake angle relationship also holds for machining operations, although the angles become more complex. Positive rake angles are always preferred, bu t the poor tens ile s treng th of many cu tting too l ma ter ia ls ma kes the use of negative or neutral rake angles necessary in many cases.

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Cutting Tool Geometry

Chip Control

Breaking chips is very important. Chips that do not break readily and form strings longer than the ideal 9-shaped chip create handling and safety problems. Chips are as sharp as knives or saws and can cause deep cuts. Chips should not be handled with bare hands, and long chips are especially difficult to handle and dispose of, creating a workplace hazard. Many automated manufacturing machines carry chips away for disposal on a conveyor. Short, broken chips are a necessity for this automatic machinery because stringy, wiry chips would entangle machine parts and eventually jam the conveyor. Chips tend to fly from the work at high speed, sometimes as far as 20 to 30 feet away. They can easily injure the eye if sa fe ty g lasses are no t worn, an d ho t c hips can pene tra te c lo thing or soft-soled shoes.

Cutting Tool Geometry

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Cutting Fluids

Cutting fluids are usually liquid, but air is sometimes used to avoid contamination for certain workpiece materials. in general, cutting fluids serve to dissipate heat, to lubricate between the tool and workpiece for lower friction, and to carry away chips from the cutting area. There are essentially two types of cutting fluids, cutting oils and coolants. Some cutting operations, such as using a tap to cut threads, create a considerable amount of friction and need a cutting oil for good lubricating qualities. Cutting oils can be either animal fats or petroleum-based. Higher- speed cutting operations require relatively less lubrication and more of the heat removal qualities that are found in a coolant. Coolants are usually water-based soluble oils that have excellent heat transfer qualities. Petroleum oils or waxes are treated so that they emulsify when mixed with wa ter to pro duce a milky white so lu tion. Other chem ica ls suc h as wetting agents, rust inhibitors, antibacterial agents, and polarizing agents are added to improve the coolant. Many synthetic coolants have been developed for use as emulsions or cutting oils for specific and general uses.

Cutting Fluids

Some machine tools used in tool rooms or for low- volume production are operated without coolant, but most production machines are fitted with coolant pumps and tanks. In some production machining operations, however, coolant is not used. For example, in an intermittent cutting setup with a carbide tool where there is alternate heating and cooling of the carbide, coolants would add to the thermal shock, which can crack the carbide insert, so no cutting fluid is used. Also, due to the risk of thermal shock and the superior hot hardness of ceramic, cutting fluids are not normally required with ceramic tools. This practice of “dry machining” is becoming more commonplace owing to environmental concerns involving the disposal of spent cutting fluid.

http://www.youtube.com/watch?v=VHTXaU7GZC0

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References 1. R Gregg Bruce, William K. Dalton, John E Neely, and Richard R Kibbe, , Modern Materials and Manufacturing Processes, Prentice Hall, 3rd edition, 2003, ISBN: 9780130946980 2. http://cdn.sheknows.com/articles/peeling-apple.jpg

Principles of Machining Processes

Alessandro Anzalone, Ph.D. Hillsborough Community College, Brandon Campus

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