Section 13. qxd

creep-feed grinding.
It uses equip-
ment such as reciprocating table or vertical-spindle rotary table surface
grinders with capacities up to 300 hp (220 kW). The normal stock
removal may range up to 
in (6.4 mm) with wheel speeds between
3,400 and 5,000 surface ft/min (1,000 and 1,500 m/min).
Finishing Operations
Polishing
is an operation by which scratches or tool marks or, in some
instances, rough surfaces left after forging, rolling, or similar operations
are removed. It is not a precision operation. The nature of the polishing
process has been debated for a long time. Two mechanisms appear to
play a role: One is fine-scale abrasion, and the other is softening of sur-
face layers. In addition to removal of material by the abrasive particles,
the high temperatures generated because of friction soften the asperities
of the surface of the workpiece, resulting in a smeared surface layer.
Furthermore, chemical reactions may also take place in polishing
whereby surface irregularities are removed by chemical attack.
Polishing is usually done in stages. The first stage is rough polishing,
using abrasive grain sizes of about 36 to 80, followed by a second stage,
using an abrasive size range of 80 to 120, a third stage of size 150 and
finer, etc., with a final stage of buffing. For the first two steps the pol-
ishing wheels are used dry. For finishing, the wheels are first worn
down a little and then coated with tallow, oil, beeswax, or similar sub-
stances. This step is partly polishing and partly buffing, as additional
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abrasive is often added in cake form with the grease. The cutting action
is freer, and the life of the wheel is prolonged by making the wheel sur-
face flexible. Buffing wheels are also used for the finishing step when
tallow, etc., containing coarse or fine abrasive grains is periodically
rubbed against the wheel.