Chapter 26 Abrasive Machining and Finishing Operations

ABRASIVE It is a small hard particle having sharp edges and an irregular shape e.g. sand sand paper and grinding wheels are used to sharpen knives, tools and give good dimensional accuracy and surface finish to products

FIGURE 26.1 Typical abrasive grains; note the angular shape with sharp edges. (a) A single, 80-mesh Al2O3 grit in a freshly dressed grinding wheel; (b) An 80/100 mesh diamond grit; diamond and cubic boron nitride grains can be manufactured in various geometries, including the “blocky” shape shown. Source: Courtesy of J. Badger.

FIGURE 26.2 A variety of bonded abrasives used in abrasive-machining processes. Source: Courtesy of Norton Company.

FIGURE 26.3 The types of workpieces and operations typical of grinding: (a) cylindrical surfaces; (b) conical surfaces; (c) fillets on a shaft; (d) helical profiles; (e) concave shape; (f) cutting off or slotting with thin wheels; and (g) internal grinding.

TABLE 26.1 Ranges of Knoop Hardness for Various Materials and Abrasives

Abrasives are harder than cutting tool materials TYPES OF ABRASIVES 1 Abrasives are harder than cutting tool materials TYPES OF ABRASIVES 1.conventional abrasives: Aluminum oxide (for carbon steel and ferrous alloys)and Silicon carbide (non ferrous metals, glass and marble) 2.super abrasives: diamond (ceramics and hardeneded steel) and cubic boron nitride (steels and cast iron) friability: ability of abrasive grains to fracture into smaller pieces. This gives abrasives their self sharpening characteristics essential in maintaining their sharpness during use

GRINDING WHEELS The abrasives grains are held together by a bonding material in order to achieve high material removal rates Marking of bonded abrasives 51 A 36 L 5 V 23 A: abrasive type AL 36 : grain size L grade from A (soft) till Z (hard) 5: dense from 1-…. V: bond type (R:rubber, S:silicate, V:vitrified 51 and 23: manufacturer

FIGURE 26.4 Schematic illustration of a physical model of a grinding wheel, showing its structure and its wear and fracture patterns.

FIGURE 26.5 Common types of grinding wheels made with conventional abrasives; note that each wheel has a specific grinding face; grinding on other surfaces is improper and unsafe.

FIGURE 26.6 Examples of superabrasive wheel configurations; the annular regions (rims) are superabrasive grinding surfaces, and the wheel itself (core) generally is made of metal or composites. The bonding materials for the superabrasives are (a), (d), and (e) resinoid, metal, or vitrified; (b) metal; (c) vitrified; and (f) resinoid.

THE GRINDING PROCESS Difference between action of an abrasive grain and that of a single-point cutting tool 1. Grains have irregular shapes 2. The radial position of a grain on the surface of the wheel varies, so not all grains are active during grinding 3. Surface speed of grinding wheels are higher

FIGURE 26.9 (a) Grinding chip being produced by a single abrasive grain; note the large negative rake angle of the grain. (b) Schematic illustration of chip formation by an abrasive grain with a wear flat; note the negative rake angle of the grain and the small shear angle. Source: (a) After M.E. Merchant.

FIGURE 26.11 Schematic illustration of the surface-grinding process, showing various process variables; the figure depicts conventional (up) grinding.

FIGURE 26.13 Chip formation and plowing of the workpiece surface by an abrasive grain.

FIGURE 26. 14 (a) Forms of grinding-wheel dressing FIGURE 26.14 (a) Forms of grinding-wheel dressing. (b) Shaping the grinding face of a wheel by dressing it by computer control; note that the diamond dressing tool is normal to the surface at the point of contact with the wheel. Source: Courtesy of Okuma Corporation. Printed with permission.

GRINDING OPERATIONS AND MACHINES Selection of a grinding process depends upon: 1. Shape of work piece 2. Its size 3. Ease of fixturing 4. Production rate required

TABLE 26.4 General Characteristics of Abrasive Machining Processes and Machines

SURFACE GRINDING work piece is flat grinding wheel is mounted on a horizontal spindle work piece is mounted on work table the table reciprocates longitudinally and is fed laterallyin the direction of the spindle axis

FIGURE 26.15 Schematic illustrations of various surface-grinding operations. (a) Traverse grinding with a horizontal-spindle surface grinder. (b) Plunge grinding with a horizontal-spindle surface grinder, producing a groove in the workpiece. (c) A vertical-spindle rotary-table grinder (also known as the Blanchard type).

FIGURE 26.16 Schematic illustration of a horizontal-spindle surface grinder.

FIGURE 26.17 (a) Rough grinding of steel balls on a vertical-spindle grinder; the balls are guided by a special rotary fixture. (b) Finish grinding of balls in a multiple-groove fixture; the balls are ground to within 0.013 mm (0.0005 in.) of their final size.

CYLINDRICAL GRINDING the work piece rotates and reciprocates along its axis in roll grinders, the grinding wheel reciprocates

FIGURE 26.18 Examples of various cylindrical-grinding operations: (a) traverse grinding; (b) plunge grinding; and (c) profile grinding. Source: Courtesy of Okuma Corporation. Printed with permission.

FIGURE 26.19 Plunge grinding of a workpiece on a cylindrical grinder with the wheel dressed to a stepped shape.

FIGURE 26.20 Schematic illustration of grinding a noncylindrical part on a cylindrical grinder with computer controls to produce the shape. The part rotation and the distance x between centers are varied and synchronized to grind the particular workpiece shape.

FIGURE 26.21 Thread grinding by (a) traverse and (b) plunge grinding.

FIGURE 26.22 Cycle patterns for a CNC precision grinder.

INTERNAL GRINDING a small wheel is used in grinding the inside diameter of a part the work piece is held in a rotating chuck the wheel rotates at a high speed(higher than 30000 r.p.m

FIGURE 26.23 Schematic illustrations of internal grinding operations: (a) traverse grinding; (b) plunge grinding; and (c) profile grinding.

FIGURE 26.24 Schematic illustrations of centerless grinding operations: (a) through-feed grinding, (b) plunge grinding, (c) and internal grinding; and (d) a computer numerical-control cylindrical-grinding machine. Source: Courtesy of Cincinnati Milacron, Inc.

FIGURE 26.25 (a) Schematic illustration of the creep-feed grinding process; note the large wheel depth of cut, d. (b) A shaped groove produced on a flat surface by creep-feed grinding in one pass; groove depth is typically on the order of a few mm. (c) An example of creep-feed grinding with a shaped wheel; this operation also can be performed by some of the processes described in Chapter 27. Source: Courtesy of Blohm, Inc.

FIGURE 26.26 (a) Schematic illustration of the ultrasonic machining process. (b) and (c) Types of parts made by this process; note the small size of the holes produced.

FINISHING OPERATIONS

FIGURE 26.27 Schematic illustration of the structure of a coated abrasive; sandpaper (developed in the 16th century) and emery cloth are common examples of coated abrasives.

BELT GRINDING

FIGURE 26.28 Turbine nozzle vane considered in Example 26.5.

WIRE BRUSHING work piece is held against a circular wire brush that rotates the tips of the wire produce longitudinal scatches on the surface of the work piece

HONING Used to improve the surface finish of holes The honing tool consists of aluminum oxide or silicon carbide bonded abrasive sticks called stones The stones can be adjusted radially to suit different hole sizes The tool has a reciprocating motion

FIGURE 26.29 Schematic illustration of a honing tool used to improve the surface finish of bored or ground holes.

FIGURE 26.30 Schematic illustrations of the superfinishing process for a cylindrical part. (a) Cylindrical microhoning. (b) Centerless microhoning.

LAPPING Used for finishing cylindrical, flat or curved surfaces The lap is relatively soft and porous, made of cast iron, leather or cloth Lapping is done under pressures depending upon the type and hardness of the work piece

FIGURE 26. 31 (a) Schematic illustration of the lapping process FIGURE 26.31 (a) Schematic illustration of the lapping process. (b) Production lapping on flat surfaces. (c) Production lapping on cylindrical surfaces.

POLISHING Produces a smooth , lustrous surface finish Softening and smearing of surface layers occur by frictional heating developed during polishing, and by very fine scale abrasive removal from work piece surface Polishing is done with discs or belts made of fabric or leather, coated with fine powders of aluminum oxide or diamond

FIGURE 26.32 Schematic illustration of the chemical–mechanical polishing process. This process is used widely in the manufacture of silicon wafers and integrated circuits and also is known as chemical–mechanical planarization; for other materials and applications, more carriers and more disks per carrier are used.

FIGURE 26.33 Schematic illustration of polishing of balls and rollers by magnetic fields. (a) Magnetic-float polishing of ceramic balls. (b) Magnetic-field-assisted polishing of rollers. Source: After R. Komanduri, M. Doc, and M. Fox.

DEBURRING OPERATIONS Burrs are thin edges that develop along edges of a work piece due to machining operations or shearing metal sheets Burrs can be detected with a finger, toothpick or cotton swab DISADVANTAGES OF BURRS 1. Interfere with the mechanical assembly of parts 2. Unsafe to personnel handling the parts 3. May reduce fatigue life of parts 4. May cause low bendability if the burr is on the tensile size

FIGURE 26.34 (a) Schematic illustration of abrasive-flow machining to deburr a turbine impeller; the arrows indicate movement of the abrasive media; note the special fixture, which is usually different for each part design. (b) Valve fittings treated by abrasive-flow machining to eliminate burrs and improve surface quality. Source: Courtesy of Kennametal Extrude Hone Corporation.

FIGURE 26.36 A deburring operation on a robot-held die-cast part for an outboard motor housing; the operation uses a grinding wheel. Abrasive belts (Fig. 26.28) or flexible abrasive radial-wheel brushes also can be used for such operations. Source: Courtesy of Acme Manufacturing Company.

FIGURE 26.37 Increase in the cost of machining and finishing a part as a function of the surface finish required; this is the main reason that the surface finish specified on parts should not be any finer than is necessary for the part to function properly.