Section 13. qxd

residual stresses,
the technique of 
gentle 
or
low-stress
grinding may be employed. This gen-
erally consists of removing a layer of about 0.010 in (0.25 mm) at depths
of cut of 0.0002 to 0.0005 in (0.005 to 0.013 mm) with wheel speeds that
are lower than the conventional 5,500 to 6,500 ft/min.
Truing and Dressing
In
truing,
a diamond supported in the end of a
soft steel rod held rigidly in the machine is passed over the face of the
wheel two or three times to remove just enough material to give the wheel
its true geometric shape. 
Dressing
is a more severe operation of removing
the dull or loaded surface of the wheel. Abrasive sticks or wheels or steel
star wheels are pressed against and moved over the wheel face.
Safety
If not stored, handled, and used properly, a grinding wheel can
be a very dangerous tool. Because of its mass and high rotational speed, a
grinding wheel has considerable energy and, if it fractures, it can cause
serious injury and even death to the operator or to personnel nearby. A
safety code B7.1 entitled “The Use, Care, and Protection of Abrasive
Wheels” is available from ANSI; other safety literature is available from
the Grinding Wheel Institute and from the National Safety Council.
The salient features of safety in the use of grinding wheels may be
listed as follows: Wheels should be stored and handled carefully; a
wheel that has been dropped should not be used. Before it is mounted
on the machine, a wheel should be visually inspected for possible
cracks; a simple “ring” test may be employed whereby the wheel is
tapped gently with a light nonmetallic implement and if it sounds
cracked, it should not be used. The wheel should be mounted properly
with the required blotters and flanges, and the mounting nut tightened
not excessively. The label on the wheel should be read carefully for
maximum operating speed and other instructions. An appropriate guard
should always be used with the machine, whether portable or stationary.
Newly mounted wheels should be allowed to run idle at the operating
speed for at least 1 min before grinding. The operator should always wear
safety glasses and should not stand directly in front of a grinding wheel
when a grinder is started. If a grinding fluid is used, it should be turned
off first before stopping the wheel to avoid creating an out-of-balance
condition. Because for each type of operation and workpiece material
there usually is a specific type of wheel recommended, the operator
must make sure that the appropriate wheel has been selected.