Tc 9-524 fundamentals of machine tools

Factors Affecting the Grade of Hardness
The factors described below will determine the proper grade
of hardness of the grinding wheel.
The harder the material, the softer the wheel.
The smaller the arc of contact, the harder the grade should
be. The arc of contact is the arc, measured along the
periphery of the wheel, that is in contact with the work at
any instance. It follows that the larger the grinding wheel,
the greater the arc of contact and, therefore, a softer wheel
can be used.
The higher the work speed with relation to the wheel
speed, the milder the grinding action and the harder the
grade should be.
The better the condition of the grinding machine and
spindle bearings, the softer the wheel can be.
Factors Affecting the Structure
The structure or spacing of the abrasive grains of wheel
depends upon the four factors described below.
The softer, tougher, and more ductile the material, the
wider the grain spacing.
The finer the finish desired,
grain spacing should be.
5-10
the closer, or more dense, the
Surfacing operations require open structure (wide grain
spacing).
Cylindrical grinding and tool and cutter grinding are best
performed with wheels of medium structure (medium
grain spacing).
Factors Affecting Bonding Material
The factors described below affect the selection of
material for the wheel desired.
Thin cutoff wheels and other wheels subject to
strains require resinoid, shellac, or rubber bonds.
bonding
bending
Solid wheels of very large diameters require a silicate
bond.
Vitrified wheels are usually best for speeds up to 6,500
SFPM and resinoid, shellac, or rubber wheels are best for
speeds above 6,500 SFPM.
Resinoid, shellac, or rubber bonds are generally best
where a high finish is required.
Selection
Refer to Table 5-1 in Appendix A for specific requirements
for typical grinding and materials (grinding wheel selection
and application).