Basic Machine Tool Components.
Advances in machine-tool design and fabrication philosophy are quickly eliminating the differences between machine types. Fifty years ago, most machine tools performed a single function such as drilling or turning, and operated strictly stand-alone. The addition of automatic turrets, tool-changers, and computerized numerical control (CNC) systems allowed lathes to become turning centers and milling machines to become machining centers. These multiprocess centers can perform a range of standard machining functions: turning, milling, boring, drilling, and grinding
The machine tool frame supports all the active and passive components of the tool — spindles, table, and controls. Factors governing the choice of frame materials are: resistance to deformation (hardness), resistance to impact and fracture (toughness), limited expansion under heat (coefficient of thermal expansion), high absorption of vibrations (damping), resistance to shop-floor environment (corrosion resistance), and low cost.
Guide ways carry the workpiece table or spindles. Each type of way consists of a slide moving along a track in the frame. The slide carries the workpiece table or a spindle. The oldest and simplest way is the box way. As a result of its large contact area, it has high stiffness, good damping characteristics, and high resistance to cutting forces and shock loads. Box slides can experience stick-slip motion as a result of the difference between dynamic and static friction coefficients in the ways. This condition introduces positioning and feed motion errors. A linear way also consists of a rail and a slide, but it uses a rolling-element bearing, eliminating stick-slip. Linear ways are lighter in weight and operate with less friction, so they can be positioned faster with less energy. However, they are less robust because of the limited surface contact area.
Slides are moved by hydraulics, rack-and-pinion systems, or screws. Hydraulic pistons are the least costly, most powerful, most difficult to maintain, and the least accurate option. Heat buildup often significantly reduces accuracy in these systems. Motor-driven rack-and-pinion actuators are easy to maintain and are used for large motion ranges, but they are not very accurate and require a lot of power to operate. Motor-driven screws are the most common actuation method. The screws can either be lead screws or ballscrews, with the former being less expensive and the latter more accurate. The recirculating ballscrew has very tight backlash; thus, it is ideal for CNC machine tools since their tool trajectories are essentially continuous. A disadvantage of the ballscrew systems is the effective stiffness due to limited contact area between the balls and the thread.
Electric motors are the prime movers for most machine tool functions. They are made in a variety of types to serve three general machine tool needs: spindle power, slide drives, and auxiliary power. Most of them use three-phase AC power supplied at 220 or 440 V. The design challenge with machine tools and motors has been achieving high torque throughout a range of speed settings. In recent years, the operational speed of the spindle has risen significantly. For example, conventional speeds 5 years ago were approximately 1600 rpm. Today, electric motors can turn at 12,000 rpm and higher. Higher speeds cause vibration, which makes use of a mechanical transmission difficult. By virtue of improvement in motor design and control technology, it is now possible to quickly adjust motor speed and torque. Mechanical systems involving more than a three-speed transmission are becoming unnecessary for most high-speed and low-torque machines. Spindle motors are rated by horsepower, which generally ranges from 5 to 150 hp (3.7 to 112 kW) with the average approximately 50 hp (37 kW). Positioning motors are usually designated by torque, which generally ranges from 0.5 to 85 lb-ft (0.2 to 115 Nm).
The spindle delivers torque to the cutting tool, so its precision is essential to machine tool operation. The key factors influencing precision are bearing type and placement, lubrication, and cooling.
Cutting Tool Materials.
The selection of cutting tool materials is one of the key factors in determining the effectiveness of the machining process. During cutting, the tool usually experiences high temperatures, high stresses, rubbing friction, sudden impact, and vibrations. Therefore, the two important issues in the selection of cutting tool materials are hardness and toughness. Hardness is defined as the endurance to plastic deformation and wear; hardness at elevated temperatures is especially important. Toughness is a measure of resistance to impact and vibrations, which occur frequently in interrupted cutting operations such as milling and boring. Hardness and toughness do not generally increase together, and thus the selection of cutting tool often involves a trade-off between these two characteristics.
Cutting tool materials are continuously being improved. Carbon steels of 0.9 to 1.3% carbon and tool steels with alloying elements such as molybdenum and chromium lose hardness at temperatures above 400°F (200°C) and have largely been replaced by high-speed steels (HSS). HSS typically contains 18% tungsten or 8% molybdenum and smaller amounts of cobalt and chromium. HSSs retain hardness up to 1100°F (600°C) and can operate at approximately double the cutting speed with equal life. Both tool steels and HSS are tough and resistive to fracture; therefore, they are ideal for processes involving interrupted engagements and machine tools with low stiffness that are subject to vibration and chatter.
Powder metallurgy (P/M) high-speed tool steels are a recent improvement over the conventionally cast HSSs. Powder metallurgy processing produces a very fine microstructure that has a uniform distribution of hard particles. These steels are tougher and have better cutting performance than HSS. Milling cutters are becoming a significant application for these cutting tool materials.
Cast cobalt alloys, popularly known as Stellite tools, were introduced in 1915. These alloys have 38 to 53% cobalt, 30 to 33% chromium, and 10 to 20% tungsten. Though comparable in room temperature hardness to HSS tools, cast cobalt alloy tools retain their hardness to a much higher temperature, and they can be used at 25% higher cutting speeds than HSS tools.
Cemented carbides offered a four- or fivefold increase in cutting speeds over conventional HSS. They are much harder, but more brittle and less tough. The first widely used cemented carbide was tungsten carbide (WC) cemented in a ductile cobalt binder. Most carbide tools in use now are a variation of the basic WC-Co material. For instance, WC may be present as single crystals or a solid solution mixture of WC-TiC or WC-TiC-TaC. These solid solution mixtures have a greater chemical stability in the cutting of steel. In general, cemented carbides are good for continuous roughing on rigid machines, but should avoid shallow cuts, interrupted cuts, and less rigid machines because of likely chipping.
A thin layer of TiC, TiN, or Al2O3 can be applied to HSS or carbide substrate to improve resistance to abrasion, temperature, friction, and chemical attacks. The coated tools were introduced in the early 1970s and have gained wide acceptance since. Coated tools have two or three times the wear resistance of the best uncoated tools and offer a 50 to 100% increase in speed for equivalent tool life.
Ceramic tools used for machining are based on alumina (Al2O3) or silicon nitride (Si3N4). They can be used for high-speed finishing operations and for machining of difficult-to-machine advanced materials, such as superalloys (Komanduri and Samanta, 1989). The alumina-based materials contain particles of titanium carbide, zirconia, or silicon carbide whiskers to improve hardness and/or toughness. These materials are a major improvement over the older ceramic tools. Silicon nitride-based materials have excellent high-temperature mechanical properties and resistance to oxidation. These materials also have high thermal shock resistance, and thus can be used with cutting fluids to produce better surface finishes than the alumina tools.
These tools can be operated at two to three times the cutting speeds of tungsten carbide, usually require no coolant, and have about the same tool life at higher speeds as tungsten carbide does at lower speeds. However, ceramics lack toughness; therefore, interrupted cuts and intermittent application of coolants can lead to premature tool failure due to poor mechanical and thermal shock resistance.
Cermets are titanium carbide (TiC) or titanium carbonitride particles embedded in a nickel or nickel/molybdenum binder. These materials, produced by the powder metallurgy process, can be considered as a type of cemented carbide. They are somewhat more wear resistant, and thus can be used for higher cutting speeds. They also can be used for machining of ferrous materials without requiring a protective coating.
Cubic boron nitride (CBN) is the hardest material at present available except for diamond. Its cost is somewhat higher than either carbide or ceramic tools but it can cut about five times as fast as carbide and can hold hardness up to 200° C. It is chemically very stable and can be used to machine ferrous materials.
Industrial diamonds are now available in the form of polycrystalline compacts for the machining of metals and plastics with greatly reduced cutting force, high hardness, good thermal conductivity, small cutting-edge radius, and low friction. Recently, diamond-coated tools are becoming available that promise longer-life cutting edges. Shortcomings with diamond tools are brittleness, cost, and the tendency to interact chemically with workpiece materials that form carbides, such as carbon steel, titanium, and nickel.
Optimum speed and feed for machining depend on workpiece material, tool material, characteristics of the cut, cutting tool configuration, rigidity of setup, tolerance, and cutting fluid. Consequently, it is not possible to recommend universally applicable speeds and feeds.
Drilling and Reaming
Description and Applications. Drilling is the most widely used process for making circular holes of moderate accuracy. It is often a preliminary step to other processes such as tapping, boring, or reaming. Reaming is used to improve the accuracy of a hole while increasing its diameter. Holes to be reamed are drilled undersize.
Key System Components. Drills are classified by the material from which they are made, method of manufacture, length, shape, number and type of helix or flute, shank, point characteristics, and size series.
The most widely used drill is the general-purpose twist drill, which has many variations. The flutes on a twist drill are helical and are not designed for cutting but for removing chips from the hole.
Machining forces during reaming operations are less than those of drilling, and hence reamers require less toughness than drills and often are more fragile. The reaming operation requires maximum rigidity in the machine, reamer, and workpiece.
Most reamers have two or more flutes, either parallel to the tool axis or in a helix, which provide teeth for cutting and grooves for chip removal. The selection of the number of flutes is critical: a reamer with too many flutes may become clogged with chips, while a reamer with too few flutes is likely to chatter.
Machining Parameters. The optimal speed and feed for drilling depend on workpiece material, tool material, depth of hole, design of drill, rigidity of setup, tolerance, and cutting fluid. For reaming operations, hardness of the workpiece has the greatest effect on machinability. Other significant factors include hole diameter, hole configuration (e.g., hole having keyways or other irregularities), hole length, amount of stack removed, type of fixturing, accuracy and finish requirements, size of production run, and cost. Most reamers are more easily damaged than drills; therefore, the practice is to ream a hole at no more than two thirds of the speed at which it was drilled.
Capabilities and Process Limitations. Most drilled holes are 1/8 to 1 in. (3.2 to 40 mm) in diameter. However, drills are available for holes as small as 0.001 in. (0.03 mm) (microdrilling), and special drills are available as large as 6 in. (150 mm) in diameter. The range of length-to-diameter (L/D) of holes that can be successfully drilled depends on the method of driving the drill and the straightness requirements. In the simplest form of drilling in which a rotating twist drill is fed into a fixed workpiece, best results are obtained when L/D is <3. But by using special tools, equipment, and techniques, straight holes can be drilled with L/D = 8 or somewhat greater. Nonconventional machining processes can also generate high-aspect-ratio holes in a wide variety of materials.
Reaming and boring are related operations. Hole diameter and length, amount of material to be removed, and required tolerance all influence which process would be most efficient for a given application. Most holes reamed are within the size range of 1/8 to 1 inch (3.2 to 40 mm), although larger and smaller holes have been successfully reamed. For most applications with standard reamers, the length of a hole that can be reamed to required accuracy ranges from slightly longer to much shorter than the cutting edges of the reamer, but there are many exceptions to this general rule-of-thumb. Tolerances of 0.001 to 0.003 in. (0.03 to 0.08 mm) with respect to the diameter are readily achievable in production reaming operations. Surface finish for annealed steels can be held within the range of 100 to 125 µin. (2.50 to 3.20 µm), but a surface as smooth as 40 µin. (1 µm) can be obtained under appropriate processing conditions.
Turning and Boring
Description and Applications. Turning produces external cylindrical surfaces by removing material from a rotating workpiece, usually with a single-point cutting tool in a lathe. Boring is this same process applied for enlarging or finishing internal surfaces of revolution.
Key System Components. The basic equipment for turning is an engine lathe that consists of a bed, a headstock, a carriage slide, a cross slide, a tool holder mounted on the cross slide, and a source of power for rotating the workpiece. Engine lathes are often modified to perform additional types of machining operations through the use of attachments. Most turning machines can also perform boring operations, but boring machines may not be able to perform turning operations. Sizes of lathes range from fractional horsepower to greater than 200 hp.
Machines used for boring are noted for their rigidity, adaptability, and ability to maintain a high degree of accuracy. For extremely large workpieces, weighing thousands of pounds, the boring cutting tool is rotated and the workpiece is fixed.
Machine Tool and Machining Parameters. In turning and boring operations, a single-point tool is traversed longitudinally along the axisymmetric workpiece axis parallel to the spindle. A tangential force is generated when the cutting tool engages the rotating work. This force is generally independent of the cutting speed and directly proportional to the depth of cut for a particular material, tool shape (particularly side rake angle), and feed rate. That force, when multiplied by the surface speed of the workpiece, estimates the net horsepower required to remove material. Moving the tool longitudinally requires much less power.
To minimize the number of cuts required, the depth of cut should be as great as possible, which is limited by the strength of the part and the fixturing, and the power output of the machine tool. The feed rate is a function of the finish desired and the strength and rigidity of the part and machine tool.
Capabilities and Limitations. Components that range in size from those used in watches up to large steel propeller shafts more than 80 ft (24 m) long are regularly turned. Aluminum parts over 10 ft (3 m) in diameter have been turned. In practice, the weight of the workpiece per unit of volume determines the size of the workpiece that is practical to turn. Large, heavy parts can be turned in a vertical boring machine. Irregular-shaped parts, such as crankshafts, may require the use of counter-weighting to achieve dynamic balance for vibration-free turning.
For both turning and boring, the rotation speed, feed, and depth of cut determine the rate of material removal and resulting surface quality. Feed rate for most applications falls between 0.005 and 0.020 in./rev (0.13 and 0.51 mm/rev). Finishing cuts have a significantly lower feed rate (e.g., 0.001 in./rev, 0.03 mm/rev), and roughing cuts are made at a significantly higher feed rate (e.g., 0.25 in./rev, 6.35 mm/rev). Boring is not limited by the L/D ratio of a hole — this ratio can be as great as 50 if the tool bar and workpiece are adequately supported.
Planing and Shaping
Description and Applications. Planing is a widely used process for producing flat, straight surfaces on large workpieces. A variety of contour operations and slots can be generated by use of special attachments. It is often possible to machine a few parts quicker by planing than by any other method. Shaping is a process for machining flat and contour surfaces, including grooves and slots.
Principle of Operation. Planers develop cutting action from straight-line reciprocating motion between one or more single-point tools and the workpiece; the work is reciprocated longitudinally while the tools are fed sideways into the work. Planer tables are reciprocated by either mechanical or hydraulic drives, with mechanical drives predominating.
Shapers use a single-point tool that is supported by a ram which reciprocates the tool in a linear motion against the workpiece. The workpiece rests on a flat bed and the cutting tool is driven toward it in small increments by ram strokes. Shapers are available with mechanical and hydraulic drives, with mechanical drives predominating.
Machine Tool and Machining Parameters. Planing and shaping are rugged machining operations during which the workpiece is subjected to significant cutting forces. These operations require high clamping forces to secure the workpiece to the machine bed.
In general, it is advisable to plane steel with as heavy a feed and as high a speed as possible to promote good chip-formation conditions so that chip breakers are not needed. Carbide cutters allow cutting speed to be increased from 225 to 300 surface feet per minute (sfm) (70 to 90 m/min). For best results, uniform cutting speed and feed are maintained throughout the entire stoke.
In general, speeds are related to workpiece material characteristics and associated machinability. Feeds are influenced by the workpiece machinability, but also by ram speed, depth of cut, and required dimensional accuracy and surface finish. Common practice in shaping is to make roughing cuts at as high a feed and slow a speed as practical, and make finishing cuts at a low feed rate and high speed. For low carbon steel, a typical speed for a roughing cut is 50 sfm (15 m/min), while for a finishing cut it is 80 sfm (25 m/min) using a conventional cutting tool. Similarly for aluminum, a roughing cut of 150 sfm (45 m/min) is typically followed by a finishing cut of 200 sfm (60 m/min).
There is a practical lower bound on minimum feed rate. Feed rates that are too low will cause the tool to chatter; feed rates less than 0.005 in. (0.125 mm) are seldom used in shaping. Similarly, shallow cuts (less than 0.015 in., 0.38 mm) will cause chatter during shaping.
Key System Components. Planers are available in a wide range of sizes. Tools are available in a variety of configurations for undercutting, slotting, and straight planing of either horizontal or vertical surfaces.
Shapers are available in a large variety of sizes, ranging from small models with a maximum stroke length of less than 6 in. (150 mm) to large machines with a maximum stroke of 36 in. (914 mm). On each machine, the length of stroke can be varied from its maximum to slightly less than 1 in. (25 mm) for the largest machine, and to 1/8 in. (3.2 mm) for the smallest machine.
Capabilities and Process Limitations. Planing is a precision process in which flatness can be held within 0.0005 in. (0.013 mm) total indicated runout (TIR) on workpieces up to 4 ft2 (0.4 m2). Although planing is most widely used for machining large areas, it is also used for machining smaller parts, although 12 in. is about the minimum distance for a planing stoke. Size of the workpiece that can be planed is limited by the capacity of the planing equipment.
Shaping is a versatile process in which setup time is short and relatively inexpensive tools can be used. Under good conditions, a shaper can machine a square surface of 18 in. on a side (0.2 m2) to a flatness within 0.001 in. (0.025 mm); under optimum conditions this can be improved to 0.0005 in. (0.013 mm). The size of the workpiece that can be shaped is limited by the length of the stoke, which is usually about 36 in. (914 mm). Shaping should be considered for machining flat surfaces in these instances:
· Required flatness cannot be achieved by another method.
· Production quantity is insufficient to justify the tooling costs of milling or broaching.
Planing and shaping are interrupted cutting processes, and are comparatively inefficient means of metal removal; for example, shaping costs five times that of milling, exclusive of the tooling and setup costs.
Milling
Description and Applications. Milling is a versatile, efficient process for metal removal. It is used to generate planar and contour surfaces through the action of rotating multiple-tooth cutters. Surfaces having almost any orientation can be machined because both the workpiece and cutter can move in more than one direction at the same time.
Principle of Operation. Cutters with multiple cutting edges rotate in a spindle. The machining process is interrupted as the teeth of the milling cutter alternately engage and disengage from the workpiece.
Key System Components. Most milling is done in machines designed for milling. Milling can also be done by any machine tool that can rigidly hold and rotate a cutter while feeding a workpiece into the cutter. Milling machines are usually classified in terms of their appearance: knee-and-column, bed-type, planar-type, and special purpose. The knee-and-column configuration is the simplest milling machine design. The workpiece is fixed to a bed on the knee and the tool spindle is mounted on a column. For very large workpieces, gantry or bridge-type milling machines are used. Machines having two columns can provide greater stability to the cutting spindle(s). Special-purpose machines are modifications of the three basic models.
The usual power range for knee-and-column machines is 1 to 50 hp (0.75 to 37 kW). Bed-type machines are available in a wide range of sizes, up to 300 hp (225 kW). Planar-type machines are available from 30 to 100 hp (22 to 75 kW).
There is a wide variety of milling cutters, using the full range of cutting tool materials; there are three basic constructions:
· Solid — Made from a single piece of HSS or carbide; cutters can be tipped with a harder material; teeth can designed for specific cutting conditions; low initial cost.
· Inserted blade — Usually made from HSS, carbide, or cast alloy; individual blades can be replaced as they wear out, saving replacement cost; ideal for close-tolerance finishing.
· Indexable insert — Cutter inserts are made from carbide, coated carbide, ceramic, or ultrahard material such as diamond; each insert has one or more cutting edges; as inserts wear, they are repositioned to expose new cutting surface or indexed to bring another cutting insert on line. These inserts, widely used in computer-controlled machines due to their performance and flexibility, can produce a rougher surface than the other tool constructions and require somewhat higher cutting forces to remove metal.
Machine Tool and Machining Parameters. The angular relationships of the cutting edge greatly influence cutting efficiency, analogous to single-point cutting tools. A milling cutter should have enough teeth to ensure uninterrupted contact with the workpiece, yet not so many so as to provide too little space between the teeth to make chip removal difficult.
Milling speed varies greatly depending on workpiece material composition, speed, feed, tool material, tool design, and cutting fluid. Speeds as low as 20 sfm (6.1 m/min) are employed for milling low machinability alloys, while speeds as high as 20,000 sfm (6100 m/min) have been reported for milling aluminum. If the setup is sufficiently rigid, carbide or carbide-tipped cutters can be operated three to ten times faster than HSS cutters; top speed is usually constrained by onset of tool chatter.
For highest efficiency in removing metal while minimizing chatter conditions, the feed per tooth should be as high as possible. The optimum feed rate is influenced by a number of factors: type of cutter, number of teeth on the cutter, cutter material, workpiece machinability, depth of cut, width of cut, speed, rigidity of the setup, and machine power. The surface finish obtainable by milling can be quite good. A finish of 125 µin. (3.2 µm) can be readily achieved under normal circumstances with HSS mills, and finishes of 63 µin. (1.6 µm) are common if carbide tools are used. With careful selection of cutters and stringent control of process conditions, a finish of 10 min (0.25 mm) can be produced.
Process Limitations. The initial cost of a milling machine is considerably greater than that of a planar or a shaper that can machine workpieces of similar size to similar finishes. Milling tools usually cost up to 50 times as much as tools for planers and shapers, and the setup time is usually longer. However, milling is far more efficient in removing material, and milling machines are commonly highly automated. Therefore, milling is preferred for production operations.
Grinding is often preferred to milling when the amount of metal to be removed is small and the dimensional accuracy and surface finish are critical. Milling and grinding are frequently used in combination.
Broaching
Description and Applications. Broaching is a precision machining process. It is very efficient since both roughing cuts and finishing cuts are made during a single pass of the broach tool to produce a smooth surface, and further finishing is usually not necessary. Consequently, close tolerances can be readily achieved at a reasonable cost for high rates of production.
Broaches are expensive multitoothed cutting tools. Thus, the process is usually employed for low or high production when broaching is the only practical method to produce the required dimensional tolerance and surface quality. An example of the latter case is the dovetail slots in jet engine turbine disks.
Principle of operation. Broaching is a machining process similar to planing. A broach is essentially a tapered bar into which teeth are cut, with the finishing teeth engaging last on the end with the larger diameter. A single broach has teeth for rough cutting, semifinishing, and finishing. Broaching involves pushing or pulling a broach in a single pass through a hole or across a surface. As the broach moves along the workpiece, cutting is gradual as each successive tooth engages the workpiece, removing a small amount of material. Overall machining forces are much greater than that of other machining methods, and consequently broaching is considered to be the most severe of all machining operations.
Key System Components. Broaching machines are categorized as horizontal or vertical, depending on the direction of broach travel. Industry usage is almost evenly divided between these two categories. The selection of machine type depends heavily on the configuration of the workpiece and available space in the factory, considering both floor space and vertical clearance requirements.
Broaches can be categorized by the method through which they are actuated (push or pull), by type of cut (internal or external), and by the construction of the broach body.
Machine Tool and Machining Parameters. Length and depth of cut have the most influence on determining the required broaching tool length. For internal cutting operations, as the cut length increases, more chip storage capacity must be provided between the cutting edges for the same amount of tooth advance. Cutting fluids are useful in preventing the work metal from adhering to the broach, and thus result in higher-quality surface finishes and increased broach life.
The primary consideration in the selection of optimum broaching speed is the trade-off between speed and wear rate. In general, steels are broached at 10 to 30 sfm (3 to 9 m/min); the harder the steel, the slower the broach speed.
Capabilities and Process Limitations. Broaching can maintain tight tolerances during long production runs since metal-cutting operations are distributed among the different roughing and finishing teeth. Also, broach teeth can be repeatedly sharpened, allowing cutting efficiency and accuracy to be maintained.
Broaching is an extremely fast, precise machining operation. It is applicable to many workpiece materials over a wide range of machinability, can be accomplished in seconds, is readily automated, and can easily be done manually. For example, for low-carbon steels, tolerances of 0.002 in. (0.05 mm) can be readily attained with a surface finish of 60 µin. (1.55 µm); if desired, tighter tolerances and surface finishes of 30 Liin. (0.8 Lim) are possible without much additional effort. For difficult-to-machine super-alloys, tolerances of 0.001 in. (0.025 mm) and surface finishes of 30 µin. (0.8 µm) are commonly achieved in production.
Broaching is rarely used for removing large amounts of material since the power required would be excessive. It is almost always more effective to use another machining method to remove the bulk of material and use broaching for finishing.
Since a broach moves forward in a straight line, all surface elements along the broach line must be parallel to the direction of travel. Consequently, the entire surface of a tapered hole cannot be broached. Also, cutting is done sequentially with the finishing teeth engaging last. Therefore, a blind hole can be broached only if a sufficiently long recess is provided to permit full travel of the broach.
The direction of travel of the broach cannot realistically be changed during a broaching stroke, except for rotating the tool. Thus, surfaces having compound curves cannot be broached in a single operation. On external surfaces, it is impossible to broach to a shoulder that is perpendicular to the direction of broach movement.
Grinding
Description and Applications. Grinding, or abrasive machining, refers to processes for removing material in the form of small chips by the mechanical action of irregularly shaped abrasive grains that are held in place by a bonding material on a moving wheel or abrasive belt. In surface-finishing operations (e.g., lapping and honing) these grains are suspended in a slurry and then are embedded in a roll-on or reference surface to form the cutting tool. Although the methods of abrasion may vary, grinding and surface-finishing processes are used in manufacturing when the accuracy of workpiece dimensions and surface requirements are stringent and the material is too hard for conventional machining.
Grinding is also used in cutoff work and cleaning of rough surfaces, and some methods offer high material-removal rates suitable for shaping, an area in which milling traditionally has been used.
Grinding is applied mainly in metalworking because abrasive grains are harder than any metal and can shape the toughest of alloys. In addition, grinding wheels are available for machining plastics, glass, ceramics, and stone. Conventional precision metal and ceramic components and ultraprecision electronic and optical components are produced using grinding.
Mechanics of Grinding. Three types of energy are involved in grinding. Rubbing energy is expended when the grains (cutting edges) of the grinding wheel wear down. As they wear, they cut less and produce increasing friction, which consumes power but removes less material. Plowing energy is used when the abrasive does not remove all of the material but rather plows some of it aside plastically, leaving a groove behind. Chip-formation energy is consumed in removing material from the workpiece as the sharp abrasive grain cuts away the material (or chip) and pushes it ahead until the chip leaves the wheel.
The grinding wheel experiences attritious wear as the abrasive grains develop wear flats from rubbing on the workpiece, or when grains break free from the bond material. Attritious wear gives rise to rubbing energy resulting from friction, and thus can lead to thermal damage as power consumption increases without an increase in material removal rate. The wheel can wear through fracture, predominating at relatively high in-feed rates. In this case, pieces of the abrasive grain break free and expose a new, sharp surface.
Materials can be classified as either easy to grind or difficult to grind. For easy-to-grind materials, most of the power consumption becomes invested in chip formation; thus, rubbing and plowing energy are minimal. Difficult-to-grind materials involve considerable rubbing and plowing energy since the force required to remove chips is comparatively high.
Types of grinding. In surface grinding, the grinding wheel traverses back and forth across the work-piece. Grinding can take place by using either the periphery or side face of the wheel. The table holding the part may also reciprocate. Surface grinding is done most commonly on flat surfaces and surfaces with shapes formed of parallel lines, e.g., slots.
Creep-feed grinding is a form of surface grinding in which the wheel feeds into the workpiece at a low rate (0.4 to 40 in./min, 10 to 1000 mm/min) while grinding at a large depth of cut (0.04 to 0.4 in., 1 to 10 mm, or deeper). A large amount of material can be removed with one pass of the wheel, compared with conventional surface grinding in which the wheel makes many quick passes over the workpiece at slight depths of cut. This process is limited by the large amount of heat generated at the grinding arc which can result in thermal damage (grinding "burn"). Application of coolant is critical in creep-feed operations. CBN wheels, with their good heat transfer property, can also reduce the severity of burn.
Cylindrical grinding produces round workpieces, such as bearing rings, although some machines can also grind tapered parts. The workpiece is mounted to a spindle and rotates as the wheel grinds it. The workpiece spindle has it own drive motor so that the speed of rotation can be selected. Both inner surfaces (internal cylindrical grinding) and outer (external cylindrical grinding) can be worked, although usually the same machine cannot do both.
There are three variants for external grinding:
· Plain grinding — the wheel carriage is brought to the workpiece on its spindle and in-feeds until the desired dimensions are reached.
· Traverse grinding — the rotating workpiece is mounted on a table that reciprocates under the wheel; the grinding wheel is stationary except for its downward feed into the workpiece.
· Plunge grinding — the table with the rotating workpiece is locked while the wheel moves into the workpiece until the desired dimensions are attained.
Centerless grinding is a form of cylindrical grinding. In this method workpieces are not held in a centering chuck but instead rotate freely between a support, regulating wheel, and the grinding wheel. The force of the rotating grinding wheel holds the workpiece against the support. The supports are usually stationary and so a flow of lubricant is required to reduce friction between workpiece and support.
Abrasive belt machines use a flexible fabric coated with an abrasive stretched between two rollers, one of which is driven by a motor. Usually, the abrasive coating is aluminum oxide for steels and bronzes and silicon carbide for hard or brittle materials. In the metal industries, common use of such machines is for dry grinding of metal burrs and flash and polishing of surfaces. Grinding fluids enhance chip removal and provide cooling and lubrication, which results in better cutting action and longer belt life.
Honing, lapping, and polishing use abrasives to improve the accuracy of the form of a workpiece or the surface finish beyond the capabilities of grinding.
· Honing is a low-surface-speed operation, usually performed on an internal, cylindrical surface but possible also on external ones. Stock is removed by the shearing action of abrasive grains: a simultaneous rotary and reciprocating motion of fixed abrasive in the form of a stone or stick. Finishes range from under 1 to 50 µin. (0.025 to 1.3 µm). The development of CBN has revolutionized the honing process because this material easily outperforms conventional abrasives such as aluminum oxide, lasting up to 100 times longer.
· Superfinishing, like honing, uses fixed abrasives in the form of a stone. Unlike honing, which has a helical motion inside a bore, superfinishing uses high-speed, axial reciprocation combined with slow rotation of the outside diameter of the cylindrical component being processed. The geometry produced by a previous operation generally is not improved.
· Lapping is a fine-finishing, abrasive machining process used to obtain superior finish and dimensional accuracy. Lapping is unlike other finishing processes because some of the abrasive is loose rather than bonded. In general, lapping occurs when abrasive grains in a liquid vehicle (called a slurry) are guided across a workpiece by means of a rotating plate.
· Polishing uses free abrasive, as in lapping, but requires a soft support unlike the relatively hard support used in lapping. The total depth of cut during polishing can be as little as nanometers where chemical interactions will play a stronger role than mechanical or physical interactions. When the depth of cut is greater than 1 µin. (0.025 µm), the interactions are usually of a mechanical nature. Many industrial components, especially electronic and optical, required highly polished surfaces.
Key System Components. A grinding wheel consists of thousands of small, hard grains of abrasive material held on the surface in a matrix of bond material. The bond material is matched to the characteristics of the grain to retain the grain sufficiently to maximize its use before shedding it.
The structure of the wheel formed by specific types of grains and bonds determines its characteristics. The grains are spaced apart depending on the cutting required. Widely spaced grains (open structure) cut aggressively, which is useful for hard materials or high rates of material removal, but which tends to produce coarse finishes. Closely packed grains (dense structure) make fine and precise cuts for finish grinding.
Grain spacing is also important for temporary storage of chips of material removed from the workpiece. An open structure is best for storing chips between the grains, which are then released after wheel rotation moves the grains away from the workpiece. An open structure also permits more coolant to enter the spaces to dissipate heat.
The bonding material is important to grinding performance. This material is weaker than the cutting grains so that ideally, during grinding, the grain is shed from the wheel surface when it becomes dull, exposing new sharp grains. For instance, wheels with friable abrasives that fracture to expose new, sharp grains must retain the grains longer to maximize the use of the abrasive; these wheels use stronger bonding materials.
Grinding wheels must be resharpened on occasion. Dressing, not always required, sharpens the grains before grinding. Truing operations ensure the wheel conforms to the required cutting shape and will rotate concentrically to its spindle.
Because grinding wheels have relatively high mass and high operating speed, they must be precisely balanced. Imbalance causes vibrations that reduce the quality of the workpiece, hasten the wear of the spindle and bearings of the machine, affect other devices mounted on the grinder, and possibly transmit vibration from the grinder through the shop floor to other machines. Mounting of the wheel on the spindle and subsequent wheel wear can degrade the balance of the rotating system. Wheels are balanced by moving counterweights on balancing flanges. Some machines have an automatic balancer that shifts internal counterbalance masses.
Coolants are usually sprayed on the grinding zone to cool the wheel and workpiece, lubricate the surface to reduce grinding power, and flush away the chips. Excessive heat can damage both the wheel and workpiece by inducing undesirable physical changes in materials, such as metallurgical phase changes or residual stresses, or softening of the bond material in the grinding wheel. Coolant application is especially important in creep-feed grinding where the wheel-to-workpiece contact arc is long, heat generation is high, and the chips produced and abrasive lost from the wheel must be flushed away.
Selecting the type of coolant system depends on many factors, including the grinding wheel speed, material removal rate, depth of cut, and wheel/workpiece materials. The type of fluids used in this system requires consideration of both physical and environmental issues. Use of oil fluids can favor the formation of preferred residual stress patterns and better surface finish, and these oils can be recycled for long periods. However, oils present health risks, potential for groundwater contamination, and fire risks (especially with high-sparking superalloys). Water-based fluids offer far fewer environmental problems. Disadvantages of water-based fluids lie in their limited life expectancy of 3 to 12 months. Also, the relatively low viscosity of a water-based fluid at high velocity promotes a dispersed jet which reduces cooling capacity.
Capabilities and Process Limitations. Surface grinding can be a cheaper, faster, and more precise method than milling and planing operations. For profiled shapes, the grinding wheel can be dressed with less cost and inconvenience than changing milling setups for different parts. Grinding can be used as a high-stock-removal process; for example, creep-feed grinding has a depth of cut more typical of milling operations (0.1 in., 2.54 mm, and deeper). Creep-feed grinding is used for machining materials that are too difficult to work by other machining methods.
High-speed grinding can be extremely efficient. CBN abrasive allows high rates of material removal because CBN transfers heat away from the grinding zone due to its relatively high thermal conductivity, and CBN does not react with steel.
Considerable effort has been expended on modeling and testing the thermal limitations of grinding. Nearly all models depend on a fundamental model which depends on sliding contact theory. All models confirm the following guidelines for grinding with conventional abrasives when burn is a limitation: decrease wheel speed, increase workpiece speed, use softer-grade wheels.
Grinding operations can be limited by two types of vibration:
· Forced vibration. Typical causes are out-of-balance wheels, nonuniform wheels, couplings, belts, noise from hydraulic systems, bearing noise, and forces transmitted from the floor to the grinding machine.
· Self-excited vibration. Typical causes are wheel wear, workpiece surface error regeneration, wheel loading, and wear flats. Typical solutions include softer grinding wheels, flexible (for dampening) wheel structures, and stiffer machine structures.