IE 262

Abrasive Machining and Finishing Operations

Non-precision grindingThe 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 grindingPrecision 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

A variety of bonded abrasives used in abrasive-machining processes.

• There are many situations in manufacturing where the processes described thus far cannot produce the required dimensional accuracy and surface finish for a part.

• An abrasive is a small, 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.

Workpieces and Operations Used in Grinding

With the use of computer controlled machines, abrasive processes now are capable of producing wide variety of workpiece geometries and very fine dimensional accuracy

• Conventional AbrasivesAluminum Oxide (Al2O3)Silicon Carbide (SiC)• SuperabrasivesCubic Boron nitride (CBN)Diamond

Abrasives and Bonded Abrasives

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 selecting cutting tool materials for machining particular workpiece materials, the affinity of an abrasive grain to the workpiece material is an important consideration.

• Because of its chemical affinity, diamond cannot be used for grinding steels because dimaond dissolves in iron at the high temperatures encountered in grinding.

Aluminum Oxide: Carbon steels, ferrous alloys, alloy steelsSilicon Carbide: nonferrous metals, cast ironsCubic Boron Nitride:Steels and cast irons above 50 HRc hardnessDiamond: Ceramics, cemented carbides

Grinding Wheel Model

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

Because each abrasive grain typically removes only a very small amount of materialat 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 sizeImportant 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• Grinding is a chip removal process 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, (C) 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.

Undeformed chip length, l = Dd

Undeformed chip thickness, t = 4vVCr

⎛ ⎝ ⎜

⎞ ⎠ ⎟

dD

⎛ ⎝ ⎜

⎞ ⎠ ⎟

Grain force ∝ vV

dD

⎛

⎝ ⎜

⎞

⎠ ⎟ strength of the material( )

Temperature rise ∝ D1/4d3/4 Vv

⎛ ⎝ ⎜

⎞ ⎠ ⎟ 1/2

Grinding ratio, G = Volume of material removedVolume of wheel wear

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

C: the number of cutting points per unit area of the periphery of the wheel

D=200 mmd=0.05 mmv=30 m/minV=1800 m/min

l=(200x0.5)^0.5=3.2 mmC=2 per mm2 r=15

r=ratio of chip width to average undeformedchip thickness

t= 0.006 mm

Abrasive Grain Plowing Workpiece Surface

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

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 alongthe ground surface, dissipates energyand make grinding operation lesseffective.

Temperature• The temperature rise in grinding is an important

consideration because:

It can adversely affect the surface propertiesincluding metallurgical changes

The temperature rise can cause residual stresseson the workpiece. Because of the adverse effectof tensile residual stresses on fatigue strength process variables should be selected carefully

Grinding wheel wear

• Attritious grain wear• Grain fracture• Bond fracture

Grinding-Wheel Dressing(a) Forms of grinding-

wheel 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 grinding with a horizontal-spindle surface grinder.

(b) Plunge grinding with a horizontal-spindle surface grinder.

(c) A vertical-spindle rotary-table grinder (also known as the Blanchard type.)

Horizontal-Spindle Surface Grinder

Schematic illustration of a horizontal-spindle surface grinder.

Cylindrical-Grinding Operations

Examples of various cylindrical-grinding operations. (a) Traverse grinding, (b) plunge grinding, and (c) profile grinding.

Plunge Grinding on Cylindrical Grinder

Figure 26.17 Plunge grinding of a workpiece on a cylindrical grinder with the wheel dressed to a stepped shape.

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

Internal Grinding

Centerless Grinding

Figure 26.22 Schematic illustration of centerlessgrinding 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 groove produced on a flat surface by creep-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.

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.