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QUICK FACT GUIDES FOR MACHINISTS
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GUIDES |
STAINLESS STEEL MACHINING The
three characteristics of stainless steel that have the greatest influence on
machinability are its relatively high mechanical properties (including yield
strength), its high work-hardening rate, and its ductility, which explains the
material’s tendency to form a built-up edge during machining.
For example, the chips removed
in machining exert high pressures on the nose of the tool, and therefore tend to
weld fast, producing what machinists call a “bug.” This causes the tool to
run hot, slows down the job and interferes seriously with the finish.
The austenitic stainless steels
(300 Series) are not only troublesome because of “bugging” and chip
disposal, but they work harden so that the tool, in passing over the work, will
harden the surface and thus interfere with the next cut. The only remedy for
this is to reduce the speed, increase the cut somewhat, if possible, and keep
cutting. The tool must not be allowed to dwell on the work.
The best mechanical method for
chip control is to grind the tools with a fairly steep top rake, or lip angle.
Tools with a 5o to 10o angle will generate less heat and
be freer and cleaner cutting. Generous chip curlers or chip breakers are also a
decided advantage. It is also helpful to stone the top of the tool smooth as an
aid to skidding the chips. For “general purpose” drilling, twist drill
makers produce a drill for drilling stainless steel. It has a shorter flute and
overall length than regular drills and is therefore heavier and stronger. As
sold from stock this type of drills generally pointed with an included angle of
140o.
Where close tolerance and fine
finish are necessary, it is advisable to use a shave tool with a light cut and
fast speed. This tool should be sharply ground and stoned. Running at high speed
and taking a light cut (0.002/0.008”) produces an excellent finish and holds
to extremely close tolerance.
Sulfur-based cutting fluids have been recognized for years for their
ability to cool and prevent seizing. As a result, properly blended sulfur-base
fluids have become the standard cutting fluid for machining all types of
stainless steels. Here is a handy rule-of-thumb to use regarding the mixture, if
the chips are welding to the tool, keep adding sulfur-base oil; if the tools are
failing by rapid abrasion, add more paraffin-base oil.
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Copyright © 2002 QUICK FACT GUIDES FOR MACHINISTS
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