Section 13. qxd

MACHINING AND GRINDING OF PLASTICS
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Section_13.qxd 10/05/06 10:32 Page 13-69

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MACHINING PROCESSES AND MACHINE TOOLS
the thicker the material the lower the number of teeth per unit length of
saw. Cutting speeds for thermoplastics range from 1,000 to 4,000 ft/min
(300 to 1,200 m/min) and for thermosetting plastics from about 3,000
to 5,500 ft/min (900 to 1,700 m/min), with the higher speeds for thin-
ner stock. High-carbon-steel blades are recommended. An air blast is
helpful in preventing the chips from sticking to the saw. Abrasive saws
operating at 3,500 to 6,000 ft/min (1,000 to 1,800 m/min) are also used
for cutting off bars and forms.
Plastics are tapped and threaded with standard tools. Ground M1,
M7, or M10 high-speed steel 
taps
with large polished flutes are recom-
mended. 
Tapping speeds
are usually from 25 to 50 ft/min (8 to 15
m/min); water serves as a good cutting fluid as it keeps the material brit-
tle and prevents sticking in the flutes. 
Thread cutting
is generally accom-
plished with tools similar to those used on brass.
Reaming
is best accomplished in production by using tools of the
expansion or adjustable type with relatively low speeds but high feeds.
Less material should be removed in reaming plastics than in reaming
other materials.
Polishing
and 
buffing
are done on many types of plastics. Polishing is
done with special compounds containing wax or a fine abrasive. Buffing
wheels for plastics should have loose stitching. Vinyl plastics can be
buffed and polished with fabric wheels of standard types, using light pres-
sures.
Thermoplastics and thermosets can be 
ground
with relative ease, usu-
ally by using silicon carbide wheels. As in machining, temperature rise
should be minimized.