Section 13. qxd

Superfinishing
is a honing process. Formed honing stones bear against
the workpiece previously finished to 0.0005 in (0.013 mm) or at the
most to 0.001 in (0.025 mm) by a very light pressure which gradually
increases to several pounds per square inch (1 lb/in
2
0.0069 MPa) of
stone area in proportion to the development of the increased area of
contact between the workpiece and stone. The workpiece or tool rotates
and where possible is reciprocated slowly over the surface which may be
finished in a matter of 20 s to a surface quality of 1 to 3 
m
in (0.025 to
0.075 
m
m). Superfinishing is applied to many types of workpieces such
as crankshaft pins and bearings, cylinder bores, pistons, valve stems,
cams, and other metallic moving parts.
Deburring
involves removing burrs (thin ridges, generally triangular,
resulting from operations such as punching and blanking of sheet metals,
and from machining and drilling) along the edges of a workpiece.
Several deburring operations are available, the most common ones
being manual filing, wire brushing, using abrasives (emery paper, belts,
abrasive blasting), and vibratory and barrel finishing. Deburring opera-
tions can also be carried out using programmable robots.
MACHINING AND GRINDING OF PLASTICS
The low strength of thermoplastics permits high cutting speeds and
feeds, but the low heat conductivity and greater resilience require
increased reliefs and less rake in order to avoid undersize cutting. Hard
and sharp tools should be used. Plastics are usually abrasive and cause
the tools to wear or become dull rapidly. Dull tools generate heat and
cause the tools to cut to shallow depths. The depth of cut should be
small. When high production justifies the cost, diamond turning and
boring tools are used. Diamond tools maintain sharp cutting edges and
produce an excellent machined surface. They are particularly advanta-
geous when a more abrasive plastic such as reinforced plastic is
machined.
A cutting fluid, such as a small blast of air or a stream of water,
improves the 
turning
and cutting of plastics as it prevents the heating of
the tool and causes the chips to remain brittle and to break rather than
become sticky and gummy. A zero or slightly negative back rake and a
relief angle of 8 to 12
should be used. For thermoplastics cutting
speeds generally range from 250 to 400 ft/min (75 to 120 m/min) and
for thermosetting plastics from 400 to 1,000 ft/min (120 to 300 m/min).
Recommended tool materials are M2 and T5 high-speed steels and
C2 carbide.
In
milling
plastics, speeds of 400 to 1,000 ft/min (120 to 300 m/min)
should be used, with angles similar to those on a single-point tool. From
0 to 10
negative rake may be used. Good results have been obtained by
hobbing plastic gears with carbide-tipped hobs. Recommended tool
materials are M2 and M7 high-speed steels and C2 carbide.
In 
drilling,
speeds range from 150 to 400 ft/min (45 to 120 m/min),
and the recommended drill geometry is given in Table 13.4.6. Tool
materials are M1, M7, and M10 high-speed steel. Usually the drill cuts
undersize; drills 0.002 to 0.003 in (0.05 to 0.075 mm) oversize should
be used.
In
sawing
plastics, either precision or buttress tooth forms may be
used, with a pitch ranging from 3 to 14 teeth/in (1.2 to 5.5 teeth/cm),