A standard wheel marking system (Figure L-47) is used for the purpose of identifying five major factors in grinding wheel selection:
1. Type of abrasive
2. Grit size
A 60-J8V indicates the following:
Selection of grit depends on the amount of stock to be removed and the surface finish requirements. Usually, coarse grits are used for fast stock removal and on soft ductile materials. Fine grit is used for hard brittle materials.General usage calls for wheel grits ranging from 46 to 100.
Grade of hardness (Figure L-48) is a measure of the bond strength of the grinding wheel. The bond material holds the abrasive grains together in the wheel. The stronger the bond, the harder the wheel. Precision grinding wheels tend to be softer grades, because it is necessary to have dull abrasive grains pulled from the wheel as soon as they become dulled, to expose new sharp grains to the workpiece.
If this does not happen, the wheel will become glazed with dull abrasive. Cutting efficiency and surface finish will be poor. Later alphabet letters indicate harder grades. For example, F to G are soft, whereas R to Z are hard.
Fourth Symbol: Structure (A 60-J8V) Structure, or the spacing of the abrasive grains in the wheel (Figure L-49), is indicated by the numbers 1 (dense) to 15 (open). Structure provides chip clearance so that chips may be thrown from the wheel by centrifugal force or washed out
Figure L-48 Three sketches illustrating (from top down) a soft, a medium, and a hard wheel. This is the grade of the wheel. The white areas are voids with nothing but air, the black lines are the bond, and the others are the abrasive grain. The harder the wheel, the greater the proportion of bond and, usually, the smaller the voids. by the grinding fluid. If this does not happen, the wheel becomes loaded with workpiece particles (Figure L-50) and must be dressed. Note that in the illustrations, the work rest and spark guards have been removed to show the loaded and dressed wheel. The grinder must not be used with these safety features not installed.
In the machine shop, vitrified bonds are the most common. Vitrified wheels are used mostly for precision grinding. Vitrified superabrasive grinding wheels are becoming more common. Resinoid bond wheels are Figure L-49 Three similar sketches showing structure. From the
top down, dense, medium, and open structure or grain spacing.
The proportions of bond, grain, and voids in all three sketches are about the same.typically used in rough-grinding operations where some
flexibility is needed, such as snagging of castings in a foundry, with high wheel speeds and heavy stock removal.
Resinoid bonds are also used with superabrasives for carbide grinding on tool and cutter grinders.
Rubber-bonded grinding wheels are used in the finish grinding of bearing surfaces. Rubber is also the usual bonding material for regulating
wheels on centerless grinders.
Shellac still finds limited use in finish grinding the wear surfaces of cam shafts, although belted superfinishing methods are used more frequently for this task. Metal-bonded diamond wheels are also used for grinding hard nonmetallics such as ceramics and stone. Bonds also affect wheel speeds.
Vitrified wheels are rated up to 6500 sfpm. Resinoid wheels are rated to 16,000 sfpm or higher.