Manufacturing 28

0.015 to 0.030 in. is desirable.When grinding is used primarily for metal removal (called abrasive machining), infeeds are much higher, 0.020 to 0.040 in. being common. Continuous downfeed is often used, with rates up to 0.100 in./min being common. Grinding machines are available in which the workpiece is held in a chuck for grinding both external and internal cylindrical surfaces.Chucking-type external grinders are production-type machines for use in rapid grinding of relatively short parts, such as ball-bearing races. Both chucks and collets are used for holding the work, the means dictated by the shape of the workpiece and rapid loading and removal. In chucking-type internal grinding machines, the chuck-held workpiece revolves, and a relatively small, high-speed grinding wheel is rotated on a spindle arranged so that it can be reciprocated in and out of the workpiece. Infeed movement of the wheelhead is normal to the axis of rotation of the work (Figure 28-21). CENTERLESS GRINDING Centerless grinding makes it possible to grind both external and internal cylindrical surfaces without requiring the workpiece to be mounted between centers or in a chuck. This eliminates the requirement of center holes in some workpieces and the necessity for mounting the workpiece, thereby reducing the cycle time. The principle of centerless external grinding is illustrated in Figure 28-22. Two wheels are used.The larger one operates at regular grinding speeds and does the actual grinding. The smaller wheel is the regulating wheel. It is mounted at an angle to the plane of the grinding wheel. Revolving at a much slower surface speed—usually 50 to 200 ft/min—the regulating wheel controls the rotation and longitudinal motion of the workpiece and is a usually a plastic- or rubber-bonded wheel with a fairly wide face.

3. The mean spacing of active grains in the wheel surface (grain size and structure) 4. The properties of the grain (hardness, attrition, and friability) 5. The geometry of the cutting edges of the grains (rake angles and cutting-edge radius compared to depth of cut) 6. The process parameters (speeds, feeds, cutting fluids) and type of grinding (surface, or cylindrical) It is easy to see why grinding is a complex process, difficult to control. BONDING MATERIALS FOR GRINDING WHEELS Bonding material is a very important factor to be considered in selecting a grinding wheel. It determines the strength of the wheel, thus establishing the maximum operating speed. It determines the elastic behavior or deflection of the grits in the wheel during grinding.The wheel can be hard or rigid, or it can be flexible. Finally, the bond determines the force required to dislodge an abrasive particle from the wheel and thus plays a major role in the cutting action. Bond materials are formulated so that the ratio of bond wear matches the rate of wear of the abrasive grits. Bonding materials in common use are the following: 1. Vitrified bonds are composed of clays and other ceramic substances. The abrasive particles are mixed with the wet clays so that each grain is coated.Wheels are formed from the mix, usually by pressing, and then dried.They are then fired in a kiln, which results in the bonding material's becoming hard and strong, having properties similar to glass.Vitrified wheels are porous, strong, rigid, and unaffected by oils, water, or temperature over the ranges usually encountered in metal cutting. The operating speed range in most cases is 5500 to 6500 ft/min, but some wheels now operate at surface speeds up to 16,000 ft/min. 2. Resinoid, or phenolic resins, can be used. Because plastics can be compounded to have a wide range of properties, such wheels can be obtained to cover a variety of work conditions. They have, to a considerable extent, replaced shellac and rubber wheels. Composite materials are being used in rubber-bonded or resinoid-bonded wheels that are to have some degree of flexibility or are to receive considerable abuse and side loading. Various natural and synthetic fabrics and fibers, glass fibers, and nonferrous wire mesh are used for this purpose. 3. Silicate wheels use silicate of soda (waterglass) as the bond material. The wheels are formed and then baked at about 500°F for a day or more. Because they are more brittle and not so strong as vitrified wheels, the abrasive grains are released more readily. Consequently, they machine at lower surface temperatures than vitrified wheels and are useful in grinding tools when heat must be kept to a minimum. 4. Shellac-bonded wheels are made by mixing the abrasive grains with shellac in a heated mixture, pressing or rolling into the desired shapes, and baking for several hours at about 300°F. This type of bond is used primarily for strong, thin wheels having some elasticity. They tend to produce a high polish and thus have been used in grinding such parts as camshafts and mill rolls. 5. Rubber bonding is used to produce wheels that can operate at high speeds but must have a considerable degree of flexibility so as to resist side thrust. Rubber, sulfur, and other vulcanizing agents are mixed with the abrasive grains.The mixture is then rolled out into sheets of the desired thickness, and the wheels are cut from these sheets and vulcanized. Rubber-bonded wheels can be operated at speeds up to 16,000 ft/min. They are commonly used for snagging work in foundries and for thin cutoff wheels. 6. Superabrasive wheels are either electroplated (single layer of superabrasive plated to outside diameter of a steel blank) or a thin segmented drum of vitrified CBN surrounds a steel core.The steel core provides dimensional accuracy, and the replaceable segments provide durability, homogeneity, and repeatability while increasing wheel life. The latter type of wheels can use resin, metal, or vitrified bonding. Selection of bond grade and structure (also called abrasive concentration) is critical.

ABRASIVE GRAIN SIZE AND GEOMETRY To enhance the process capability of grinding, abrasive grains are sorted into sizes by mechanical sieving machines.The number of openings per linear inch in a sieve (or screen) through which most of the particles of a particular size can pass determines the grain size (Figure 28-3). A no. 24 grit would pass through a standard screen having 24 openings per inch but would not pass through one having 30 openings per inch. These numbers have since been specified in terms of millimeters and micrometers (see ANSI B74.12 for details). Commercial practice commonly designates grain sizes from 4 to 24, inclusive, as coarse; 30 to 60, inclusive, as medium; and 70 to 600, inclusive, as fine. Grains smaller than 220 are usually termed powders. Silicon carbide is obtainable in grit sizes ranging from 2 to 240 and aluminum oxide in sizes from 4 to 240. Superabrasive grit sizes normally range from 120 grit for CBN to 400 grit for diamond. Sizes from 240 to 600 are designated as flour sizes. These are used primarily for lapping, or in fine-honing stones for fine finishing tasks. The grain size is closely related to the surface finish and metal removal rate. In grinding wheels and belts, coarse grains cut faster while fine grains provide better finish, as shown in Figure 28-4. The grain diameter can be estimated from the screen number (S), which corresponds to the number of openings per inch.The mean diameter of the grain (g) is related to the screen number by g 0.7/S. Regardless of the size of the grain, only a small percentage (2 to 5%) of the surface of the grain is operative at any one time. That is, the depth of cut for an individual grain (the actual feed per grit) with respect to the grain diameter is very small.Thus the

ABRASIVE JET MACHINING One of the least expensive of the nontraditional processes is abrasive jet machining (AJM). AJM removes material by a focused jet of abrasives and is similar in many respects to AWC, with the exception that momentum is transferred to the abrasive particles by a jet of inert gas.Abrasive velocities on the order of 1000 ft/sec are possible with AJM.The small mass of the abrasive particles produces a microscale chipping action on the workpiece material.This makes AJM ideal for processing hard, brittle materials, including glass, silicon, tungsten, and ceramics. It is not compatible with soft, elastic materials. Key process parameters include working distance, abrasive flow rate, gas pressure, and abrasive type. Working distance and feed rate are controlled by hand. If necessary, a hard mask can be placed on the workpiece to control dimensions. Abrasives are typically smaller than those used in AWC. Abrasives are typically not recycled, since the abrasives are cheap and are used only on the order of several hundred grams per hour. To minimize particulate contamination of the work environment, a dust-collection hood should be used in concert with the AJM system. DESIGN CONSIDERATIONS IN GRINDING Almost any shape and size of work can be finished on modern grinding equipment, including flat surfaces, straight or tapered cylinders, irregular external and internal surfaces, cams, antifriction bearing races, threads, and gears. For example, the most accurate threads are formed from solid cylindrical blanks on special thread grinding machines. Gears that must operate without play are hardened and then finish ground to close tolerances.Two important design recommendations are to reduce the area to be ground and to keep all surfaces that are to be ground in the same or parallel planes (Figure 28-31). This is an example of design for manufacturing (DFM). Abrasive machining can remove scale as well as parent metal. Large allowances of material, needed to permit conventional metalcutting tools to cut below hard or abrasive inclusions, are not necessary for abrasive machining.An allowance of 0.015 in. is adequate, assuming, of course, that the part is not warped or out of round. This small allowance requirement results in savings in machining time, in material (often 60% less metal is removed), and in shipping of unfinished parts

Abrasive machining is a material removal process that involves the interaction of abrasive grits with the workpiece at high cutting speeds and shallow penetration depths. The chips that are formed resemble those formed by other machining processes. Unquestionably, abrasive machining is the oldest of the basic machining processes. Museums abound with examples of utensils, tools, and weapons that ancient peoples produced by rubbing hard stones against softer materials to abrade away unwanted portions, leaving desired shapes. For centuries, only natural abrasives were available for grinding, while other more modern basic machining processes were developed using superior cutting materials. However, the development of manufactured abrasives and a better fundamental understanding of the abrasive machining process have resulted in placing abrasive machining and its variations among the most important of all the basic machining processes. The results that can be obtained by abrasive machining range from the finest and smoothest surfaces produced by any machining process, in which very little material is removed, to rough, coarse surfaces that accompany high material removal rates. The abrasive particles may be (1) free; (2) mounted in resin on a belt (called coated product); or, most commonly (3) close packed into wheels or stones, with abrasive grits held together by bonding material (called bonded product or a grinding wheel). Figure 28-1 shows a surface grinding process using a grinding wheel. The depth of cut d is determined by the infeed and is usually very small, 0.002 to 0.005 in., so the arc of contact (and the chips) is small. The table reciprocates back and forth beneath the rotating wheel.The work feeds into the wheel in the cross-feed direction.After the work is clear of the wheel, the wheel is lowered and another pass is made, again removing a couple of thousandths of inches of metal.The metal removal process is basically the same in all abrasive machining processes but with important differences due to spacing of active grains (grains in contact with work) and the rigidity and degree of fixation of the grains. Table 28-1 summarizes the primary abrasive processes. The term abrasive machining applied to one particular form of the grinding process is unfortunate, because all these process are machining with abrasives. Compared to machining, abrasive machining processes have three unique characteristics. First, each cutting edge is very small, and many of these edges can cut simultaneously. When suitable machine tools are employed, very fine cuts are possible, and fine surfaces and close dimensional control can be obtained. Second, because extremely

For the electroplated wheels, nickel is used to attach a single layer of CBN (or diamond) to the OD of an accurately ground or turned steel blank. For the vitrified wheel, superabrasives are mixed with bonding media and molded (or preformed and sintered) into segments or a ring. The ring is mounted on a split steel body. Porosity is varied (to alter structure) by varying preform pressure or by using "pore-forming" additives to the bond material that are vaporized during the sintering cycle. The steel-cored segmented design can rotate at 40,000 sfpm (200 m/s) whereas a plain vitrified wheel may burst at 20,000 fpm. ABRASIVE MACHINING VERSUS CONVENTIONAL GRINDING VERSUS LOW-STRESS GRINDING The condition wherein very rapid metal removal can be achieved by grinding is the one to which some have applied the term abrasive machining. The metal removal rates are compared with, or exceed, those obtainable by milling or turning or broaching, and the size tolerances are comparable. It is obviously just a special type of grinding, using abrasive grains as cutting tools, as do all other types of abrasive machining. Abrasive grinding done in an aggressive way can produce sufficient localized plastic deformation and heat in the surface so as to develop tensile residual stresses, layers of overtempered martensite (in steels), and even microcracks, because this process is quite abusive. See Figure 28-11 for a discussion of residual stresses produced by varius surface grinding processes. Conventional grinding can be replaced by procedures that develop lower surface stresses when service failures due to fatigue or stress corrosion are possible. This is accomplished by employing softer grades of grinding wheels, reducing the grinding speeds and infeed rates, using chemically active cutting fluids (e.g., highly sulfurized oil or KNO2 in water), as outlined in the table of grinding conditions in Figure 28-11.These procedures may require the addition of a variable-speed drive to the grinding machine. Generally, only about 0.005 to 0.010 in. of surface stock needs to be finish ground in this way, as the depth of the surface damage due to conventional grinding or abusive grinding is 0.005 to 0.007 in. High-strength steels, high-temperature nickel, and cobalt-based alloys and titanium alloys are particularly sensitive to surface deformation and cracking problems from grinding. Other postprocessing processes, such as polishing, honing, and chemical milling plus peening, can be used to remove the deformed layers in critically stressed parts. It is strongly recommended, however, that testing programs be used along with service experience on critical parts before these procedures are employed in production.

Honing is a stock-removal process that uses fine abrasive stones to remove very small amounts of metal. Cutting speed is much lower than that of grinding.The process is used to size and finish bored holes, remove common errors left by boring (taper, waviness, and tool marks), or remove the tool marks left by grinding. The amount of metal removed is typically about 0.005 in. or less. Although honing is occasionally done by hand, as in finishing the face of a cutting tool, it usually is done with special equipment. Most honing is done on internal cylindrical surfaces, such as automobile cylinder walls. The honing stones are usually held in a honing head, with the stones being held against the work with controlled light pressure. The honing head is not guided externally but, instead, floats in the hole, being guided by the work surface (Figure 28-27).

In the casting and forging industries, the term often used for abrasive maching is snagging. Snagging is a type of rough manual grinding that is done to remove fins, gates, risers, and rough spots from castings or flash from forgings, preparatory to further machining.The primary objective is to remove substantial amounts of metal rapidly without much regard for accuracy,so this is a form of abrasive machining except that pedestal-type orswing grinders ordinarily are used. Portable electric or hand air grinders are also used for this purpose and for miscellaneous grinding in connection with welding. Grinding wheels lose their geometry during use.Truing restores the original shape. A single-point diamond tool can be used to true the wheel while fracturing abrasive grains to expose new grains and new cutting edges on worn, glazed grains (Figure 28-12). Truing can also be accomplished by grinding the grinding wheel with a controlled-path or powered rotary device using conventional abrasive wheels. The precision in generating a trued wheel surface by these methods is poorer than by the method described earlier. As the wheel is used,there is a tendency for the wheel to become loaded (metal chips become lodged in the cavities between the grains). Also, the grains dull or glaze (grits wear, flatten, and polish). Unless the wheel is cleaned and sharpened (or dressed), the wheel will not cut as well and will tend to plow and rub more. Figure 28-13 shows an arrangement for stick dressing a grinding wheel.The dulled grains cause the cutting forces on the grains to increase, ideally resulting in the grains' fracturing or being pulled out of the bond, thus providing a continuous exposure of sharp cutting edges. Such a continuous action ordinarily will not occur for light feeds and depths of cut. For heavier cuts, grinding wheels do become somewhat self-dressing,but the workpiece may become overheated and turn a bluish temper color (this is called burn) before the wheel reaches a

MOUNTED WHEELS AND POINTS Mounted wheels and points are small grinding wheels of various shapes that are permanently attached to metal shanks that can be inserted in the chucks of portable, high-speed electric or air motors. They are operated at speeds up to 100,000 rpm, depending on their diameters, and are used primarily for deburring and finishing in mold and die work. Several types are shown in Figure 28-25. COATED ABRASIVES Coated abrasives are being used increasingly in finishing both metal and nonmetal products. These are made by gluing abrasive grains onto a cloth or paper backing (Figure 28-26). Synthetic abrasives—aluminum oxide, silicon carbide, aluminum, zirconia, CBN, and diamond—are used most commonly, but some natural abrasives—sand, flint, garnet, and emery—also are employed.Various types of glues are utilized to attach the abrasive grains to the backing, usually compounded to allow the finished product to have some flexibility. Coated abrasives are available in sheets, rolls, endless belts, and disks of various sizes. Some of the available forms are shown in Figure 28-26. Although the cutting action of coated abrasives basically is the same as with grinding wheels, there is one major difference: they have little tendency to be self-sharpened when dull grains are pulled from the backing. Consequently, when the abrasive particles become dull or the belt loaded, the belt must be replaced. Finer grades result in finer first cuts but slower material removal rates (MRR).This versatile process is now widely used for rapid stock removal as well as fine surface finishi

The operation for which the abrasive wheel is intended will also influence the wheel shape and size. The major use categories are the following: 1. Cutting off: for slicing and slotting parts; use thin wheel, organic bond 2. Cylindrical between centers: grinding outside diameters of cylindrical workpieces 3. Cylindrical,centerless:grinding outside diameters with work rotated by regulating wheel 4. Internal cylindrical: grinding bores and large holes 5. Snagging:removing large amounts of metal without regard to surface finish or tolerances 6. Surface grinding: grinding flat workpieces 7. Tool grinding: for grinding cutting edges on tools such as drills, milling cutters, taps, reamers, and single-point high-speed-steel tools 8. Offhand grinding: work or the grinding tool is handheld In many cases, the classification of processes coincides with the classification of machines that do the process. Other factors that will influence the choice of wheel to be selected include the workpiece material, the amount of stock to be removed, the shape of the workpiece, and the accuracy and surface finish desired. Workpiece material has a great impact on choice of the wheel. Hard, high-strength metals (tool steels, alloy steels) are generally ground with aluminum oxide wheels or cubic boron nitride wheels. Silicon carbide and CBN are employed in grinding brittle materials (cast iron and ceramics) as well as softer, low-strength metals such as aluminum, brass, copper, and bronze. Diamonds have taken over the cutting of tungsten carbides, and CBN is used for precision grinding of tool and die steel, alloy steels, stainless steel, and other very hard materials.There are so many factors that affect the cutting action that there are no hardand-fast rules with regard to abrasive selection. Selection of grain size is determined by whether coarse or fine cutting and finish are desired. Coarse grains take larger depths of cut and cut more rapidly. Hard wheels with fine grains leave smaller tracks and therefore are usually selected for finishing cuts. If there is a tendency for the work material to load the wheel, larger grains with a more open structure may be used for finishing. BALANCING GRINDING WHEELS Because of the high rotation speeds involved, grinding wheels must never be used unless they are in good balance.A slight imbalance will produce vibrations that will cause waviness in the work surface. It may cause a wheel to break, with the probability of serious damage and injury. The wheel should be mounted with proper bushings so that it fits snugly on the spindle of the machine. Rings of blotting paper should be placed between the wheel and the flanges to ensure that the clamping pressure is evenly distributed. Most grinding wheels will run in good balance if they are mounted properly and trued. Most machines have provision for compensating for a small amount of wheel imbalance by attaching weights to one mounting flange. Some have provision for semiautomatic balancing with weights that are permanently attached to the machine spindle. SAFETY IN GRINDING Because the rotational speeds are quite high, and the strength of grinding wheels is usually much less than that of the materials being ground, serious accidents occur much too frequently in connection with the use of grinding wheels.Virtually all such accidents could be avoided and are due to one or a combination of four causes. First, grinding wheels are occasionally operated at unsafe and improper speeds.All grinding wheels are clearly marked with the maximum rpm value at which they should be rotated.They are all tested to considerably above the designated rpm and are safe at the specified speed unless abused.They should never, under any condition, be operated above the rated speed. Second, a very common form of abuse,frequently accidental,is dropping the wheel or striking it against a hard object. This can cause a crack (which may not be readily visible), resulting in subsequent failure of the wheel while rotating at high speed under load. If a wheel is dropped or struck against a hard object,it should be discarded and never used unless tested at above the rated speed in a properly designed test stand.A third common cause of grinding wheel failure is

The stones are given a complex motion so as to prevent a single grit from repeating its path over the work surface. Rotation is combined with an oscillatory axial motion. For external and flat surfaces, varying oscillatory motions are used. The length of the motions should be such that the stones extend beyond the work surface at the end. A cutting fluid is used in virtually all honing operations.The critical process parameters are rotational speed, Vr, oscillation speed, Vo, the length and position of stroke, and the honing stick pressure. Vc and the inclination angle are both products of Vo and Vr Virtually all honing is done with stones made by bonding together various fine artificial abrasives. Honing stones differ from grinding wheels in that additional materials, such as sulfur, resin, or wax, are often added to the bonding agent to modify the cutting action.The abrasive grains range in size from 80 to 600 grit.The stones are equally spaced about the periphery of the tool. Reference values for Vc and honing stick pressure, Ps, for various abrasives are shown in Figure 28-27. Single- and multiple-spindle honing machines are available in both horizontal and vertical types. Some are equipped with special sensitive measuring devices that collapse the honing head when the desired size has been reached. For honing single, small, internal cylindrical surfaces, a procedure is often used wherein the workpiece is manually held and reciprocated over a rotating hone. If the volume of work is sufficient, honing is a fairly inexpensive process. A complete honing cycle, including loading and unloading the work, is often less than one minute. Size control within 0.0003 in. is achieved routinely. ■ 28.7 SUPERFINISHING Superfinishing is a variation of honing that is typically used on flat surfaces.The process is: 1. Very light, controlled pressure, 10 to 40 psi 2. Rapid (over 400 cycles per minute), short strokes—less than 1 /4 in. 3. Stroke paths controlled so that a single grit never traverses the same path twice 4. Copious amounts of low-viscosity lubricant-coolant flooded over the work surface

The workpiece is held against the work-rest blade by the cutting forces exerted by the grinding wheel and rotates at approximately the same surface speed as that of the regulating wheel. This axial feed is calculated approximately by the equation (28-1) where F feed (mm/min or in./min) D diameter of the regulating wheel (mm or in.) N revolutions per minute of the regulating wheel angle of inclination of the regulating wheel Centerless grinding has several important advantages: 1. It is very rapid; infeed centerless grinding is almost continuous. 2. Very little skill is required of the operator. 3. It can often be made automatic (single-cycle automatic). 4. Where the cutting occurs, the work is fully supported by the work rest and the regulating wheel. This permits heavy cuts to be made. 5. Because there is no distortion of the workpiece, accurate size control is easily achieved. 6. Large grinding wheels can be used, thereby minimizing wheel wear. Thus centerless grinding is ideally suited to certain types of mass-production operations. The major disadvantages are as follows: 1. Special machines are required that can do no other type of work. 2. The work must be round—no flats, such as keyways, can be present. 3. Its use on work having more than one diameter or on curved parts is limited. 4. In grinding tubes, there is no guarantee that the OD and Internal Diameter (ID) are concentric. Special centerless grinding machines are available for grinding balls and tapered workpieces.The centerless grinding principle can also be applied to internal grinding, but the external surface of the cylinder must be finished accurately before the internal operation F = ND sin f dega-c28_756-789-hr 1/9/07 4:35 PM Page 776 SECTION 28.5 Grinding Machines 777 is started. However, it assures that the internal and external surfaces will be concentric. The operation is easily mechanized for many applications. SURFACE GRINDING MACHINES Surface grinding machines are used primarily to grind flat surfaces. However formed, irregular surfaces can be produced on some types of surface grinders by use of a formed wheel.There are four basic types of surface grinding machines, differing in the movement of their tables and the orientation of the grinding wheel spindles (Figure 28-23): 1. Horizontal spindle and reciprocating table 2. Vertical spindle and reciprocating table 3. Horizontal spindle and rotary table 4. Vertical spindle and rotary table The most common type of surface grinding machine has a reciprocating table and horizontal spindle (Figures 28-19). The table can be reciprocated longitudinally either by handwheel or by hydraulic power. The wheelhead is given transverse (cross-feed) motion at the end of each table motion, again either by handwheel or by hydraulic power feed. Both the longitudinal and transverse motions can be controlled by limit switches. Infeed or downfeed on such grinders is controlled by handwheels or automatically.The size of such machines is determined by the size of the surface that can be ground. In using such machines, the wheel should overtravel the work at both ends of the table reciprocation, so as to prevent the wheel from grinding in one spot while the table is being reversed. The transverse or cross-feed motion should be one-fourth to threefourths of the wheel width between each stroke

This procedure, illustrated in Figure 28-28, results in surfaces of very uniform, repeatable smoothness. Superfinishing is based on the phenomenon that a lubricant of a given viscosity will establish and maintain a separating, lubricating film between two mating surfaces if their roughness does not exceed a certain value and if a certain critical pressure, holding them apart, is not exceeded. Consequently, as the minute peaks on a surface are cut away by the honing stone, applied with a controlled pressure, a certain degree of smoothness is achieved. The lubricant establishes a continuous film between the stone and the workpiece and separates them so that no further cutting action occurs. Thus, with a given pressure, lubricant, and honing stone, each workpiece is honed to the same degree of smoothness. Superfinishing is applied to both cylindrical and plane surfaces. The amount of metal removed usually is less than 0.002 in., most of it being the peaks of the surface roughness. Copious amounts of lubricant-coolant maintain the work at a uniform temperature and wash away all abraded metal particles to prevent scratching. LAPPING Lapping is an abrasive surface finishing process wherein fine abrasive particles are charged (caused to become embedded) into a soft material, called a lap.The material of the lap may range from cloth to cast iron or copper, but it is always softer than the material to be finished, being only a holder for the hard abrasive particles. Lapping is applied to both metals and nonmetals. As the charged lap is rubbed against a surface, the abrasive particles in the surface of the lap remove small amounts of material from the surface to be machined.Thus the abrasive does the cutting, and the soft lap is not worn away because the abrasive particles become embedded in its surface instead of moving across it. This action always occurs when two materials rub together in the presence of a fine abrasive: the softer one forms a lap, and the harder one is abraded away. In lapping, the abrasive is usually carried between the lap and the work surface in some sort of a vehicle, such as grease, oil, or water. The abrasive particles are from 120 grit up to the finest powder sizes. As a result, only very small amounts of metal are removed, usually considerably less than 0.001 in. Because it is such a slow metal removing process, lapping is used only to remove scratch marks left by grinding or honing, or to obtain very flat or smooth surfaces, such as are required on gage blocks or for liquid tight seals where high pressures are involved. Materials of almost any hardness can be lapped. However, it is difficult to lap soft materials because the abrasive tends to become embedded. The most common lap material is fine-grained cast iron. Copper is used quite often and is the common material for lapping diamonds. For lapping hardened metals for metallographic examination, cloth laps are used. Lapping can be done either by hand or by special machines. In hand lapping, the lap is flat, similar to a surface plate. Grooves are usually cut across the surface of a lap to collect the excess abrasive and chips. The work is moved across the surface of the lap, using an irregular, rotary motion, and is turned frequently to obtain a uniform cutting action.

Vertical-spindle reciprocating-table surface grinders differ basically from those with horizontal spindles only in that their spindles are vertical and that the wheel diameter must exceed the width of the surface to be ground. Usually, no transverse motion of either the table or the wheelhead is provided. Such machines can produce very flat surfaces. Rotary-table surface grinders can have either vertical or horizontal spindles, but those with horizontal spindles are limited in the type of work they will accommodate and therefore are not used to a great extent. Vertical-spindle rotary-table surface grinders are primarily production-type machines. They frequently have two or more grinding heads, and therefore both rough grinding and finish grinding are accomplished in one rotation of the workpiece. The work can be held either on a magnetic chuck or in special fixtures attached to the table. By using special rotary feeding mechanisms, machines of this type often are made automatic. Parts are dumped on the rotary feeding table and fed automatically onto workholding devices and moved past the grinding wheels. After they pass the last grinding head, they are automatically unloaded. DISK-GRINDING MACHINES Disk grinders have relatively large side-mounted abrasive disks.The work is held against one side of the disk for grinding. Both single- and double-disk grinders are used; in the latter type the work is passed between the two disks and is ground on both sides simultaneously. On these machines, the work is always held and fed automatically. On small, single-disk grinders the work can be held and fed by hand while resting on a supporting table.Although manual disk grinding is not very precise, flat surfaces can be obtained quite rapidly with little or no tooling cost. On specialized, production-type machines, excellent accuracy can be obtained very economically. TOOL AND CUTTER GRINDERS Simple, single-point tools are often sharpened by hand on bench or pedestal grinders (off-hand grinding). More complex tools, such as milling cutters, reamers, hobs, and single-point tools for production-type operations require more sophisticated grinding machines, commonly called universal tool and cutter grinders. These machines are similar to small universal cylindrical center-type grinders, but they differ in four important respects: 1. The headstock is not motorized. 2. The headstock can be swiveled about a horizontal as well as a vertical axis. 3. The wheelhead can be raised and lowered and can be swiveled through at 360° rotation about a vertical axis. 4. All table motions are manual. No power feeds being provided. Specific rake and clearance angles must be created, often repeatedly, on a given tool or on duplicate tools.Tool and cutter grinders have a high degree of flexibility built into them so that the required relationships between the tool and the grinding wheel can be established for almost any type of tool. Although setting up such a grinder is quite complicated and requires a highly skilled worker, after the setup is made for a particular job, the actual grinding is accomplished rather easily. Figure 28-24 shows several typical setups on a tool and cutter grinder. Hand-ground cutting tools are not accurate enough for automated machining processes. Many numerically controlled (NC) machine tools have been sold on the premise that they can position work to very close tolerances—within 0.0001 to 0.0002 in.—only to have the initial workpieces produced by those machines out of tolerance by as much as 0.015 to 0.020 in. In most instances, the culprit was a poorly ground tool. For example, a twist drill with a point ground 0.005 in. off-center can "walk" as much as 0.015 in., thus causing poor hole location. Many companies are turning to computer numeric control (CNC) grinders to handle the regrinding of their cutting tools.A six-axis CNC grinder is capable of restoring the proper tool angles (rake and clearance), concentricity, cutting edges, and dimensional size.

In lapping machines for obtaining flat surfaces,workpieces are placed loosely in holders and are held against the rotating lap by means of floating heads. The holders, rotating slowly, move the workpieces in an irregular path.When two parallel surfaces are to be produced, two laps may be employed, one rotating below and the other above the workpieces. Various types of lapping machines are available for lapping round surfaces.A special type of centerless lapping machine is used for lapping small cylindrical parts, such as piston pins and ball-bearing races. Because the demand for surfaces having only a few micrometers of roughness on hardened materials has become quite common, the use of lapping has increased greatly. However, it is a very slow method of removing metal, obviously costly compared with other methods, and should not be specified unless such a surface is absolutely necessary. ■ 28.8 FREE ABRASIVES ULTRASONIC MACHINING Ultrasonic machining (USM), sometimes called ultrasonic impact grinding, employs an ultrasonically vibrating tool to impel the abrasives in a slurry at high velocity against the workpiece.The tool is fed into the part as it vibrates along an axis parallel to the tool feed at an amplitude on the order of several thousandths of an inch and a frequency of 20 kHz. As the tool is fed into the workpiece, a negative of the tool is machined into the workpiece. The cutting action is performed by the abrasives in the slurry, which is continuously flooded under the tool.The slurry is loaded up to 60% by weight with abrasive particles. Lighter abrasive loadings are used to facilitate the flow of the slurry for deep drilling (up to 2 in. deep). Boron carbide, aluminum oxide, and silicon carbide are the most commonly used abrasives in grit sizes ranging from 400 to 2000. The amplitude of the vibration should be set approximately to the size of the grit. The process can use shaped tools to cut virtually any material but is most effective on materials with hardnesses greater than RC 40, including brittle and nonconductive materials such as glass. Figure 28-29 shows a simple schematic of this process. USM uses piezoelectric or magnetostrictive transducers to impart high-frequency vibrations to the tool holder and tool.Abrasive particles in the slurry are accelerated to great speed by the vibrating tool. The tool materials are usually brass, carbide, m

\In lapping machines for obtaining flat surfaces,workpieces are placed loosely in holders and are held against the rotating lap by means of floating heads. The holders, rotating slowly, move the workpieces in an irregular path.When two parallel surfaces are to be produced, two laps may be employed, one rotating below and the other above the workpieces. Various types of lapping machines are available for lapping round surfaces.A special type of centerless lapping machine is used for lapping small cylindrical parts, such as piston pins and ball-bearing races. Because the demand for surfaces having only a few micrometers of roughness on hardened materials has become quite common, the use of lapping has increased greatly. However, it is a very slow method of removing metal, obviously costly compared with other methods, and should not be specified unless such a surface is absolutely necessary. ■ 28.8 FREE ABRASIVES ULTRASONIC MACHINING Ultrasonic machining (USM), sometimes called ultrasonic impact grinding, employs an ultrasonically vibrating tool to impel the abrasives in a slurry at high velocity against the workpiece.The tool is fed into the part as it vibrates along an axis parallel to the tool feed at an amplitude on the order of several thousandths of an inch and a frequency of 20 kHz. As the tool is fed into the workpiece, a negative of the tool is machined into the workpiece. The cutting action is performed by the abrasives in the slurry, which is continuously flooded under the tool.The slurry is loaded up to 60% by weight with abrasive particles. Lighter abrasive loadings are used to facilitate the flow of the slurry for deep drilling (up to 2 in. deep). Boron carbide, aluminum oxide, and silicon carbide are the most commonly used abrasives in grit sizes ranging from 400 to 2000. The amplitude of the vibration should be set approximately to the size of the grit. The process can use shaped tools to cut virtually any material but is most effective on materials with hardnesses greater than RC 40, including brittle and nonconductive materials such as glass. Figure 28-29 shows a simple schematic of this process. USM uses piezoelectric or magnetostrictive transducers to impart high-frequency vibrations to the tool holder and tool.Abrasive particles in the slurry are accelerated to great speed by the vibrating tool. The tool materials are usually brass, carbide, m

chips are small.As the grain diameter decreases, the number of active grains per unit area increases and the cuts become finer because grain size is the controlling factor for surface finish (roughness). Of course, the MRR also decreases. The grain shape is also important,because it determines the tool geometry—that is,the back rake angle and the clearance angle at the cutting edge of the grit (Figure 28-5). In the figure, is the clearance angle, is the wedge angle, and is the rake angle. The cavities between the grits provide space for the chips, as shown in Figure 28-6.The volume of the cavities must be greater than the volume of the chips generated during the cut. Obviously, there is no specific rake angle but rather a distribution of angles. Thus a grinding wheel can present to the surface rake angles in the range of 45° to 60° or greater. Grits with large negative rake angles or rounded cutting edges do not form chips but will rub or plow a groove in the surface (Figure 28-7). Thus abrasive machining is a

flanges will prevent the pieces of the wheel from flying and causing damage.Type 6, the straight cup, is used primarily for surface grinding but can also be used for certain types of offhand grinding. The flaring-cup type of wheel is used for tool grinding. Dish-type wheels are used for grinding tools and saws. Type 1, the straight grinding wheels, can be obtained with a variety of standard faces. Some of these are shown in Figure 28-17. The size of the wheel to be used is determined primarily by the spindle rpm values available on the grinding machine and the proper cutting speed for the wheel, as dictated by the type of bond. For most grinding operations the cutting speed is about 2500 to 6500 ft/min. Different types and grades of bond often justify considerable deviation from these speeds. For certain types of work using special wheels and machines, as in thread grinding and "abrasive machining," much higher speeds are used.

forces (and power), and reduced thermal effects while sacrificing wheel wear. Creep feed grinding eliminates preparatory operations such as milling or broaching, since profiles are ground into the solid workpiece. This can result in significant savings in unit part costs. Grinding machines that are used for precision work have certain important characteristics that permit them to produce parts having close dimensional tolerances.They are constructed very accurately, with heavy, rigid frames to ensure permanency of alignment. Rotating parts are accurately balanced to avoid vibration. Spindles are mounted in very accurate bearings, usually of the preloaded ball-bearing type. Controls are provided so that all movements that determine dimensions of the workpiece can be made with accuracy—usually to 0.001 or 0.00001 in. The abrasive dust that results from grinding must be prevented from entering between moving parts. All ways and bearings must be fully covered or protected by seals. If this is not done, the abrasive dust between moving parts becomes embedded in the softer of the two, causing it to act as lap and abrade the harder of the two surfaces, resulting in permanent loss of accuracy. These special characteristics add considerably to the cost of these machines and require that they be operated by trained personnel. Production-type grinders are more fully automated and have higher metal removal rates and excellent dimensional accuracy. Fine surface finish can be obtained very economically. CYLINDRICAL GRINDING Center-type cylindrical grinding is commonly used for producing external cylindrical surfaces. Figures 28-14 and Figure 28-21 show the basic principles and motions of this process. The grinding wheel revolves at an ordinary cutting speed, and the workpiece rotates on centers at a much slower speed, usually from 75 to 125 ft/min. The grinding wheel and the workpiece move in opposite directions at their point of contact.The depth of cut is determined by infeed of the wheel or workpiece. Because this motion also determines the finished diameter of the workpiece, accurate control of this movement is required. Provision is made to traverse the workpiece with the wheel, or the work can be reciprocated past the wheel. In very large grinders, the wheel is reciprocated because of the massiveness of the work. For form or plunge grinding, the detail of the wheel is maintained by periodic crush roll dressing. A plain center-type cylindrical grinder is shown in Figure 28-21. On this type the work is mounted between headstock and tailstock centers. Solid dead centers are always used in the tailstock, and provision is usually made so that the headstock center can be operated either dead or alive. High-precision work is usually ground with a dead headstock center, because this eliminates any possibility that the workpiece will run out of round due to any eccentricity in the headstock. The table assembly can be reciprocated, in most cases, by using a hydraulic drive. The speed can be varied, and the length of the movement can be controlled by means of adjustable trip dogs. Infeed is provided by movement of the wheelhead at right angles to the longitudinal axis of the table.The spindle is driven by an electric motor that is also mounted on the wheelhead. If the infeed movement is controlled manually by some type of vernier drive to provide control to 0.001 in. or less, the machine is usually equipped with digital readout equipment to show the exact size being produced. Most production-type grinders have automatic infeed with retraction when the desired size has been obtained. Such machines are usually equipped with an automatic diamond wheel-truing device that dresses the wheel and resets the measuring element before grinding is started on each piece. The longitudinal traverse should be about one-fourth to three-fourths of the wheel width for each revolution of the work. For light machines and fine finishes, it should be held to the smaller end of this range. The depth of cut (infeed) varies with the purpose of the grinding operation and the finish desired. When grinding is done to obtain accurate size, infeeds of 0.002 to 0.004 in. are commonly used for roughing cuts. For finishing, the infeed is reduced to 0.00025 to 0.0005 in. The design allowance for grinding should be from 0.005 to 0.010 in. on short parts and on parts that are not to be hardened. On long or large parts and on work that is to be hardened, a grinding allowance of from

fully dressed condition.A burned surface, the consequence of an oxide layer formation, results in the scrapping of several workpieces before parts of good quality are ground. Resin-bonded wheels can be trued by grinding with hard ceramics such as tungsten carbide. The procedure for truing and dressing a CBN wheel in a surface grinder might be as follows: Use 0.0002-in. downfeed per pass and cross feed slightly more than half the wheel thickness at moderate table speeds. The wheel speed is the same as the grinding speed. The grinding power will gradually increase, as the wheel is getting dull, while being trued. When the power exceeds normal power drawn during workpiece grinding, stop the truing operation. Dress the wheel face open using a J-grade stick, with abrasive one grit size smaller than CBN. Continue the truing. Repeat this cycle until the wheel is completely trued. Modern grinding machines are equipped so that the wheel can be dressed and/or trued continuously or intermittently while grinding continues.A common way to do this is by crush dressing (Figure 28-14). Crush dressing consists of forcing a hard roll (tungsten carbide or high speed steel) having the same contour as the part to be ground against the grinding wheel while it is revolving—usually quite slowly. A water-based coolant is used to flood the dressing zone at 5 to 10 gal/min. The crushing action fractures and dislodges some of the abrasive grains, exposing fresh sharp edges, allowing free cutting for faster infeed rates. This procedure is usually employed to produce and maintain a special contour to the abrasive wheel. This is also called wheel profiling. Crush dressing is a very rapid method of dressing grinding wheels, and because it fractures abrasive grains, it results in free cutting and somewhat cooler grinding.The resulting surfaces may be slightly rougher than when diamond dressing is used. ■ 28.4 GRINDING WHEEL IDENTIFICATION Most grinding wheels are identified by a standard marking system that has been established by the American National Standards Institute.This system is illustrated and explained in Figure 28-15. The first and last symbols in the marking are left to the discretion of the manufacturer. GRINDING WHEEL GEOMETRY The shape and size of the wheel are critical selection factors. Obviously, the shape must permit proper contact between the wheel and all of the surface that must be ground. Grinding wheel shapes have been standardized, and eight of the most commonly used types are shown in Figure 28-16. Types 1, 2, and 5 are used primarily for grinding external or internal cylindrical surfaces and for plain surface grinding.Type 2 can be mounted for grinding on either the periphery or the side of the wheel.Type 4 is used with tapered safety flanges so that if the wheel breaks during rough grinding, such as snagging, these

hard abrasive grits, including diamonds, are employed as cutting tool materials, very hard materials, such as hardened steel, glass, carbides, and ceramics, can readily be machined.As a result, the abrasive machining processes are not only important as manufacturing processes, they are indeed essential. Many of our modern products, such as modern machine tools, automobiles, space vehicles, and aircraft, could not be manufactured without these processes.Third, in grinding, you have no control over the actual tool geometry (rake angles, cutting edge radius) or all the cutting parameters (depth of cut). As a result of these parameters and variables, grinding is a complex process. To get a handle on the complexity,Table 28-2 presents the primary grinding parameters, grouped by their independence or dependence. Independent variables are those that are controllable (by the machine operator) while the dependent variables are the resultant effects of those inputs. Not listed in the table is workpiece hardness, which has a significant effect on all the resulting effects.Workpiece hardness will be an input factor but it is not usually controllable. ■ 28.2 ABRASIVES An abrasive is a hard material that can cut or abrade other substances. Natural abrasives have existed from the earliest times. For example, sandstone was used by ancient peoples to sharpen tools and weapons. Early grinding wheels were cut from slabs of sandstone, but because they were not uniform in structure throughout, they wore unevenly and did not produce consistent results. Emery, a mixture of alumina (Al2O3) and magnetite (Fe3O4), is another natural abrasive still in use today and is used on coated paper and cloth (emery paper). Corundum (natural Al2O3) and diamonds are other naturally occurring abrasive materials. Today, the only natural abrasives that have commercia

importance are quartz, sand, garnets, and diamonds. For example, quartz is used primarily in coated abrasives and in air blasting, but artificial abrasives are also making inroads in these applications.The development of artificial abrasives having known uniform properties has permitted abrasive processes to become precision manufacturing processes. Hardness, the ability to resist penetration, is the key property for an abrasive. Table 28-3 lists the primary abrasives and their approximate Knoop hardness (kg/mm2 ). The particles must be able to decompose at elevated temperatures. Two other properties are significant in abrasive grits—attrition and friability. Attrition refers to the abrasive wear action of the grits resulting in dulled edges, grit flattening, and wheel glazing. Friability refers to the fracture of the grits and is the opposite of toughness. In grinding, it is important that grits be able to fracture to expose new, sharp edges. Artificial abrasives date from 1891, when E. G. Acheson, while attempting to produce precious gems, discovered how to make silicon carbide (SiC). Silicon carbide is made by charging an electric furnace with silica sand, petroleum coke, salt, and sawdust. By passing large amounts of current through the charge, a temperature of over 4000°F is maintained for several hours, and a solid mass of silicon carbide crystals results.After the furnace has cooled, the mass of crystals is removed, crushed, and graded (sorted) into

improper use, such as grinding against the side of a wheel that was designed for grinding only on its periphery.The fourth and most common cause of injury from grinding is the absence of a proper safety guard over the wheel and/or over the eyes or face of the operator. The frequency with which operators will remove safety guards from grinding equipment or fail to use safety goggles or face shields is amazing and inexcusable. USE OF CUTTING FLUIDS IN GRINDING Because grinding involves cutting, the selection and use of a cutting fluid is governed by the basic principles discussed in Chapter 21. If a fluid is used, it should be applied in sufficient quantities and in a manner that will ensure that the chips are washed away, not trapped between the wheel and the work.This is of particular importance in grinding horizontal surfaces. In hardened steel, the use of a fluid can help to prevent fine microcracks that result from highly localized heating.The air scraper shown in Figure 28-18 permits the cutting fluid (lubricant) to get onto the face of the wheel. Metal air scrapers disrupt the airflow. Upper and lower nozzles cool the grinding zone, while a high-pressure scrubber helps deter loading of the wheel. Much snagging and off-hand grinding is done dry. On some types of material, dry grinding produces a better finish than can be obtained by wet grinding. Grinding fluids strongly influence the performance of CBN wheels. Straight, sulfurized, or sulfochlorinated oils can enhance performance considerably when used with straight oils. ■ 28.5 GRINDING MACHINES Grinding machines commonly are classified according to the type of surface they produce. Table 28-4 presents such a classification, with further subdivision to indicate characteristic features of different types of machines within each classification. Grinding on all machines is done in three ways. In the first, the depth of cut (dt ) is obtained by infeed—moving the

mixture of cutting, plowing, and rubbing, with the percentage of each being highly dependent on the geometry of the grit.As the grits are continuously abraded, fractured, or dislodged from the bond, new grits are exposed and the mixture of cutting, plowing, and rubbing is changing continuously.A high percentage of the energy used for rubbing and plowing goes into the workpiece, but when chips are found, 95 to 98% of the energy (the heat) goes into the chip. Figure 28-8 shows a scanning electron microscope (SEM) micrograph of a ground surface with a plowing track. In grinding, the chips are small but are formed by the same basic mechanism of compression and shear as discussed in Chapter 20 for regular metalcutting. Figure 28-9 shows steel chips from a grinding process at high magnification. They show the same structure as chips from other machining processes. Chips flying in the air from a grinding process often have sufficient heat energy to burn or melt in the atmosphere. Sparks observed during grinding steel with no cutting fluid are really burning chips, as shown in Figure 28-8. The feeds and depths of cut in grinding are small while the cutting speeds are high, resulting in high specific horsepower numbers. Because cutting is obviously more efficient than plowing or rubbing, grain fracture and grain pullout are natural phenomena

nozzle (design, orifice diameter, and material). Typical AWC systems operate under the following conditions: water pressures of 30,000 to 50,000 psi; water orifice diameters from 0.01 to 0.022 in.; and working distances of 0.02 to 0.06 in. Working distances are much smaller than in WJC to minimize the dispersion of the abrasive water jet prior to entering the material. Abrasive materials used include garnet, silica, silicon carbide, or aluminum oxide. Abrasive grit sizes range from 60 to 120 and abrasive flow rates from 0.5 to 3 lb/min. For many applications, the AWC tool is combined with a CNC controlled X-Y table, which permits contouring and surface engraving. AWC can be used to cut any material through the appropriate choice of the abrasive, waterjet pressure, and feed rate.Table 28-6 gives cutting speeds for various metals. The ability of the abrasive waterjet to cut through thick materials (up to 8 in.) is attributed to the reentrainment of abrasive particles in the jet by the workpiece material. AWC is particularly suited for composites because the cutting rates are reasonable and they do not delaminate the layered material. In particular, AWC is used in the airplane industry to cut carbon-fiber composite sections of the airplane after autoclavin

steel, or tool steel and will vary in tool wear depending on their hardness. Wear ratios (workpiece material removed versus tool material lost) from 1:1 (for tool steel) to 100:1 (for glass) are possible. Because of the high number of cyclic loads, the tool must be strong enough to resist fatigue failure. The cut will be oversize by about twice the size of the abrasive particles being used, and holes will be tapered, usually limiting the hole depth-to-diameter ratio to about 3:1. Surface roughness is controlled by the size of the abrasive particles (finer finish with smaller particles). Holes, slots, or shaped cavities can be readily eroded in any hard material—conductive or nonconductive, metallic, ceramic, or composite. Advantages of the process include that it is one of the few machining methods capable of machining glass. Also, it is the safest machining method. Skin is impervious to the process because of its ductility. High-pitched noise can be a problem due to secondary vibrations. In addition to machining, ultrasonic energy has also been employed for coining, lapping, deburring, and broaching. Plastics can be welded using ultrasonic energy. WATERJET CUTTING AND ABRASIVE WATERJET MACHINING Waterjet cutting (WJC), also known as waterjet machining or hydrodynamic machining, uses a high-velocity fluid jet impinging on the workpiece to perform a slitting operation (Figure 28-30).Water is ejected from a nozzle orifice at high pressure (up to 60,000 psi). The jet is typically 0.003 to 0.020 in. in diameter and exits the orifice at velocities up to 3000 ft/sec. Key process parameters include water pressure, orifice diameter, water flow rate, and working distance (distance between the workpiece and the nozzle). Nozzle materials include synthetic sapphire, due to its machinability and resistance to wear.Tool life on the order of several hundred hours is typical. Mechanisms for tool failure include chipping from contaminants or constriction due to mineral deposits. This emphasizes the need for high levels of filtration prior to pressure intensification. In the past, long-chain polymers were added to the water to make the jet more coherent (i.e., not come out of the jet dispersed). However, with proper nozzle design, a tight, coherent water jet may be produced without additives. The advantages of WJC include the ability to cut materials without burning or crushing the material being cut. Figure 28-30 shows a comparison (end view) of cutting corrugated boxboard with a mechanical knife and with WJC.The mechanism for material removal is simply the impinging pressure of the water exceeding the compressive strength of the material.This limits the materials that can be cut by the process to leather, plastics, and other soft nonmetals, which is the major disadvantage of the process. Alternative fluids (alcohol, glycerine, cooking oils) have been used in processing meats, baked goods, and frozen foods. Other disadvantages include that the process is noisy and requires operators to have hearing protection. The majority of the metalworking applications for waterjet cutting require the addition of abrasives. This process is known as abrasive waterjet cutting (AWC). A full range of materials, including metals, plastics, rubber, glass, ceramics, and composites, can be machined by AWC. Cutting feed rates vary from 20 in./min for acoustic tile to 50 in./min for epoxies and 500 in./min for paper products. Abrasives are added to the waterjet in a mixing chamber on the downstream side of the waterjet orifice. A single, central waterjet with side feeding of abrasives into a mixing chamber is shown in Figure 28-30. In the mixing chamber, the momentum of the water is transferred to the abrasive particles, and the water and particles are forced out through the AWC nozzle orifice, also called the mixing tube. This design can be made quite compact; however, it also experiences rapid wear in the mixing tube.An alternate configuration is to feed the abrasives from the center of the nozzle with a converging set of angled water jets imparting momentum to the abrasives. This nozzle design produces better mixing of the water and abrasives as well as increased nozzle life.The inside diameter of the mixing tube is normally from 0.04 to 0.125 in. in diameter.These tubes are normally made of carbide. Generally, the kerf of the cut is about 0.001 in. greater than the nozzle orifice. AWC requires control of additional process parameters over waterjet machining, including abrasive material (density, hardness, shape), abrasive size or grit, abrasive flow rate (pounds per minute), abrasive feed mechanism (pressurized or suction), and AWC

total downfeed or depth (d) is accomplished in a single pass (Figure 28-20).This is called creep feed grinding (CFG). (Table 28-5 compares CFG to conventional and high-speed grinding for CBN applications.) The CFG method, often done in the surface grinding mode, is markedly different from conventional surface grinding. The depth of cut is increased 1000 to 10,000 times, and the work feed ratio is decreased in the same proportion; hence the name creep feed grinding. The long arc of contact between the wheel and the work increases the cutting forces and the power required. Therefore, the machine tools to perform this type of grinding must be specially designed with high static and dynamic stability, stick-slip-free ways, adequate damping, increased horsepower, infinitely variable spindle speed, variable but extremely consistent table feed (especially in the low ranges), high-pressure cooling systems, integrated devices for dressing the grinding wheels, and specially designed (soft with open structure) grinding wheels. The process is mainly being applied to grinding deep slots with straight parallel sides or to grinding complex profiles in difficult-to-grind materials. The process is capable of producing extreme precision at relatively high metal removal rates. Because the process can operate at relatively low surface temperatures, the surface integrity of the metals being ground is good. However, in CFG, the grinding wheels must maintain their initial profile much longer, so continuous dressing is used that is form-truing and dressing the grinding wheel throughout the process rather than between cycles. Continuous crush dressing results in higher MRRs, improved dimensional accuracy and form tolerance, reduced grinding

used to keep the grains sharp. As the grains become dull, cutting forces increase, and there is an increased tendency for the grains to fracture or break free from the bonding material. ■ 28.3 GRINDING WHEEL STRUCTURE AND GRADE Grinding, wherein the abrasives are bonded together into a wheel, is the most common abrasive machining process. The performance of grinding wheels is greatly affected by the bonding material and the spatial arrangement of the particles' grits. The spacing of the abrasive particles with respect to each other is called structure. Close-packed grains have dense structure; open structure means widely spaced grains. Open-structure wheels have larger chip cavities but fewer cutting edges per unit area (Figure 28-10a). The fracturing of the grits is controlled by the bond strength, which is known as the grade. Thus, grade is a measure of how strongly the grains are held in the wheel. It is really dependent on two factors: the strength of the bonding materials and the amount of the bonding agent connecting the grains. The latter factor is illustrated in Figure 28-10b. Abrasive wheels are really porous. The grains are held together with "posts" of bonding material. If these posts are large in cross section, the force required to break a grain free from the wheel is greater than when the posts are small. If a high dislodging force is required, the bond is said to be hard. If only a small force is required, the bond is said to be soft. Wheels are commonly referred to as hard or soft, referring to the net strength of the bond, resulting from both the strength of the bonding material and its disposition between the grains. G RATIO The loss of grains from the wheel means that the wheel is changing size. The grinding ratio, or G ratio, is defined as the cubic inches of stock removed divided by the cubic inches of wheel lost. In conventional grinding, the G ratio is in the range 20:1 to 80:1.The G ratio is a measure of grinding production and reflects the amount of work a wheel can do during its useful life. As the wheel loses material, it must be reset or repositioned to maintain workpiece size. A typical vitrified grinding wheel will consist of 50 vol% abrasive particles, 10 vol% bond, and 40 vol% cavities; that is, the wheels have porosity. The manner in which the wheel performs is influenced by the following factors: 1. The mean force required to dislodge a grain from the surface (the grade of the wheel) 2. The cavity size and distribution of the porosity (the structure)

various desired sizes. As can be seen in Figure 28-2, the resulting grits, or grains, are irregular in shape, with cutting edges having every possible rake angle. Silicon carbide crystals are very hard (Knoop 2480), friable, and rather brittle. This limits their use. Silicon carbide is sold under the trade names Carborundum and Crystolon. Aluminum oxide (Al2O3) is the most widely used artificial abrasive.Also produced in an arc furnace from bauxite, iron filings, and small amounts of coke, it contains aluminum hydroxide, ferric oxide, silica, and other impurities.The mass of aluminum oxide that is formed is crushed, and the particles are graded to size. Common trade names for aluminum oxide abrasives are Alundum and Aloxite.Although aluminum oxide is softer (Knoop 2100) than silicon carbide, it is considerably tougher. Consequently, it is a better general-purpose abrasive. Diamonds are the hardest of all materials. Those that are used for abrasives are either natural, off-color stones (called garnets) that are not suitable for gems, or small, synthetic stones that are produced specifically for abrasive purposes. Manufactured stones appear to be somewhat more friable and thus tend to cut faster and cooler.They do not perform as satisfactorily in metal-bonded wheels. Diamond abrasive wheels are used extensively for sharpening carbide and ceramic cutting tools. Diamonds also are used for truing and dressing other types of abrasive wheels. Diamonds are usually used only when cheaper abrasives will not produce the desired results. Garnets are used primarily in the form of very finely crushed and graded powders for fine polishing. Cubic boron nitride (CBN) is not found in nature. It is produced by a combination of intensive heat and pressure in the presence of a catalyst. CBN is extremely hard, registering at 4700 on the Knoop scale. It is the second-hardest substance created by nature or manufactured and is often referred to, along with diamonds, as a superabrasive. Hardness, however, is not everything. CBN far surpasses diamond in the important characteristic of thermal resistance. At temperatures of 650°C, at which diamond may begin to revert to plain carbon dioxide, CBN continues to maintain its hardness and chemical integrity. When the temperature of 1400°C is reached, CBN changes from its cubic form to a hexagonal form and loses hardness. CBN can be used successfully in grinding iron, steel, alloys of iron, Ni-based alloys, and other materials. CBN works very effectively (long wheel life, high G ratio, good surface quality, no burn or chatter, low scrap rate, and overall increase in parts/shift) on hardened materials (Rc 50 or higher). It can also be used for soft steel in selected situations. CBN does well at conventional grinding speeds (6000 to 12,000 ft/min), resulting in lower total grinding in conventional equipment. CBN can also perform well at high grinding speeds (12,000 ft/min and higher) and will enhance the benefits from future machine tools. CBN can solve difficult-to-grind jobs, but it also generates cost benefits in many production grinding operations despite its higher cost. CBN is manufactured by the General Electric Company under the trade name of Borazon

wheel down into the work or the work up into the wheel (Figure 28-19). The desired surface is then produced by traversing the wheel across (cross feed) the workpiece, or vice versa (Figure 28-19). In the second method, known as plunge-cut grinding, the basic movement is of the wheel being fed radially into the work while the latter revolves on centers. It is similar to form cutting on a lathe; usually a formed grinding wheel is used (Figure 28-14). In the third method, the work is fed very slowly past the wheel and the

Manufacturing 28

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