IE 262
Abrasive Machining and Finishing Operations
Non-precision grinding The common forms are called, snagging and off-hand grinding. Both are done primarily to remove stock that can not be taken off as conveniently by other methods. The work is pressed hard against the wheel or vice versa. The accuracy and surface finish are of secondary importance.
Precision grinding Precision grinding is concerned with producing good surface finishes and accurate dimensions. 3 types of precision grinding exists –External cylindrical grinding –Internal cylindrical grinding
–Surface grinding
Bonded Abrasives Used in Abrasive-Machining Processes •
There ere are man any y situations in manufacturing where the processes described thus far cannot produce the required dimensional accuracy and surface finish for a part.
•
An ab abrasive iis s a small, ha hard particle having sharp edges and an irregular shape, unlike the cutting tools described earlier.
•
Abrasives are capable of removing small amounts of material from a surface through a cutting process that produces tiny chips.
A variety of bonded abrasives used in abrasive-machining processes.
Work Wo rkpi piec eces es an and d Ope Opera ratio tions ns Us Used ed in Grinding
With the use use of computer controlled machines, machines, abrasive processes processes now are capable capable of producing wide variety of workpiece geometries and very fine dimensional accuracy
Abrasives and Bonded Abrasives • Conv nven enti tion ona al Ab Abras asiives Aluminum Oxide (Al2O3) Silicon Carbide (SiC) • Superabrasives Cubic Boron nitride (CBN) Diamond These abrasives are much harder than conventional cutting tool materials. Characteristics of abrasives: 1) hardness, 2) friability Friability is defined as the ability of abrasive grains to fracture into smaller pieces. This property gives abrasives their self-sharpening characteristics.
Abrasive Workpiece Material Compatibility • As in selec selecti ting ng cut cutti ting ng too tooll mate materia rials ls for for mac machin hinin ing g parti particu cular lar workpiece materials, the affinity of an abrasive grain to the workpiece material is an important consideration. • Becau Because se of of its its chem chemic ical al aff affin init ity, y, diam diamon ond d cann cannot ot be be used used for for grinding steels because dimaond dissolves in iron at the high temperatures encountered in grinding. Aluminum Oxide: Oxide: Carbon steels, ferrous alloys, alloy steels Silicon Carbide: Carbide: nonferrous metals, cast irons Cubic Boron Nitride:Steels Nitride:Steels and cast irons above 50 HRc hardness Diamond:: Ceramics, cemented carbides Diamond
Grinding Wheel Model
Schematic illustration illustration of a physical physical model of a grinding wheel showing showing its structure structure and wear and fracture patterns. Because each abrasive grain typically removes only a very small amount of material at a time, high rates of material removal can be achieved only if a large number of these grains act together. This is done by bonded abrasives typicall in the form of a grinding wheel in which the abrasive grains are distributed and oriented randomly.
Grain size Important parameter in determining surface finish and material removal rate. Small grit sizes produce better finishes, larger grit sizes permit larger material removal rates. Also, harder materials need smaller grain sizes to cut effectively, while softer materials require larger grit size. Grain sizes used in grinding changes between 8-250, while 8 is very coarse, but 250 is very fine.
Grinding Wheels
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.
The Grinding Process •
Grin Grindi ding ng is a chip chip remo remova vall proc proces ess s that that uses an individual abrasive grain as the cutting tool. The major differences between grinding and single point cutting tool are:
The individual abrasive grains have irregular shapes and and are spaced randomly along the periphery of the wheel. The average rake angle of the grains is highly negative typically -60 deg therefore plastic deformations is higher Not all the grains are active because of the radial positions of the grains Surface speeds in grinding is very high
Chip Formation by Abrasive Grain
(a) Grinding chip being produced by a single abrasive grain: (A) chip, (B) workpiece, workpiece, (C) abrasive grain. grain. Note the large negative negative rake angle of the grain. grain. (b) Schematic illustration of chip chip formation formation by an an abrasive abrasive grain with a wear flat. Note th the e negative rake angle of the grain and the small shear angle.
Schematic illustration of the surface-grinding process, showing various process variables. The figure depicts conventional (up) grinding.
Undeformed chip length, Undeformed chip thickness, Grain force
⎛ v ∝⎜ ⎝ V
G
=
=
Dd
⎛ 4 v ⎞ ⎛ d ⎞ = ⎜ ⎟ ⎜ ⎟ ⎝ VCr ⎠ ⎝ D ⎠
⎞ ⎟(strength of the material) D ⎠ d
Temperature rise Grinding ratio,
t
l
∝
⎛ ⎞1/ 2 ⎜ ⎟ ⎝ ⎠
1/ 4 3 / 4 V D d v
Volume of material removed Volume of wheel wear
C :
the number of cutting points per unit area of the periphery of the wheel
r =ratio =ratio
of chip width to average undeformed chip thickness D=200 mm d=0.05 mm v =30 =30 m/min V=1800 m/min l=(200x0.5)^0.5=3.2
C =2 =2
per mm2 r=15
t= 0.006 mm
mm
Abrasive Grain Plowing Workpiece Surface The energy dissipated in producing a grinding chip consists of the energy required for the following actions: •Chip formation •Plowing •Friction The wear area continuosly rubs along the ground surface, dissipates energy and make grinding operation less effective. Chip formation and plowing of the workpiece surface by an abrasive grain.
Temperature • The The tem tempe pera ratu ture re rise rise in grin grindin ding g is is an an imp impor ortan tantt consideration because: It can adversely affect the surface properties including metallurgical changes The temperature rise can cause residual stresses on the workpiece. Because of the adverse effect of tensile residual stresses on fatigue strength process variables should be selected carefully
Grinding wheel wear • Attritious grai grain n wea earr • Grain fracture • Bond fracture
Grinding-Wheel Dressing (a) Forms Forms of of grindi grindingngwheel dressing. (b) Shaping the grinding face of a wheel by dressing it with computer control. Note that the diamond dressing tool is normal to the surface at point of contact with the wheel. Dressing is the process of conditioning worn grains on the surface of a grinding wheel by producing sharp new edges.
Various Surface-Grinding Operations
Schematic illustrations of various surface-grinding operations. (a) Traverse Traverse grinding grinding with a horizonta horizontal-spi l-spindle ndle surface surface grinder. grinder. (b) Plunge Plunge grinding grinding with a horizont horizontal-sp al-spindle indle surface surface grinder grinder.. (c) A vertical-spindle vertical-spindle rotary-table grinder (also known as as the Blanchard type.)
Horizontal-Spindle Surface Grinder
Schematic illustration of a horizontal-spindle horizontal-spindle surface grinder.
Cylindrical-Grinding Operations
Examples of various cylindrical-grinding cylindrical-grinding operations. (a) Traverse grinding, (b) plunge grinding, and (c) profile grinding.
Plunge Grinding on Cylindrical Grinder
Figure 26.17 Plunge grinding grinding of a workpiece on on a cylindrical cylindrical grinder with the wheel dressed to a stepped shape.
Internal Grinding
Schematic illustrations illustrations of internal internal grinding grinding operations: (a) traverse grinding, grinding, (b) plunge grinding, and (c) profile grinding.
Centerless Grinding Figure 26.22 Schematic illustration of centerless grinding operations: (a) through-feed grinding, (b) plunge grinding, (c) internal grinding, and (d) a computer numerical-control cylindrical-grinding machine. Source: Courtesy of Cincinnati Milacron, Inc.
Creep Feed Grinding
(a) Schematic illustration of the creep-feed grinding process. Note the large wheel depth-of-cut, d . (b) A shaped shaped groove produc produced ed on a flat surface surface by creep-gr creep-grindin inding g in one pass. pass. Groove depth is typically on the order of a few mm. (c) An example example of creep-fe creep-feed ed grindin grinding g with with a shaped shaped whee wheel. l. This operation operation also can be performed by some of the processes described in Chapter 27.
Honing
Schematic illustration of a honing tool used to improve the surface finish of bored or ground holes.
Lapping
Increase in Machining and Finishing Cost as a Function of Surface Finish Required
Figure 26.34 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 necessary for the part to function properly.
