Principles of Machining Processes

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 1 4/19/2011 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 of manuftifacturing processes used to create prodtduct 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 precis ion of common casting, molding, and 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 p rocesses, which are g enerally q uicker and waste less material, but machining is often justified when precision is needed. 2 4/19/2011 Introduction Introduction 3 4/19/2011 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 chip is torn away from the part. The cutting 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. 4 4/19/2011 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 5 4/19/2011 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 also de term ines the width of each chip removed from the part as it is machined. 6 4/19/2011 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 exerts enough force on the workp iece to smoothly cut 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 and the workp iece. Mos t of the hea t is carrie d off in the chips, 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. 7 4/19/2011 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. 8 4/19/2011 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. 9 4/19/2011 Cutting Tool Materials High-Speed Steel Cutting Tools One hundred years ago hardened plain carbon tool steel was used for tools to cut metals.

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