Gear Cutting Tools

Total
a – Initial edge wear
b – mechanical abrasion
c – built-up edge
d – oxidation
e – diffusion
e
Wear
d
b
c
a
Cutting speed/temperature
Causes of wear against temperature (according to Vieregge)
Hobbing has the additional phenomenon
of strong local variations
in stress upon the cutter teeth.
This is a consequence of the tooth
profile to be manufactured on the
workpiece arising only with successive
cuts of a number of cutter
teeth engaging in turn. The metal
removal capacity is provided principally
by the tooth tips, which
generate relatively large-volume
chips capable of sinking a corresponding
quantity of heat. By contrast,
much thinner chips are generated
in the region of the tooth
flanks of the hob; the particular engagement
conditions mean that
the effective relief angle is also relatively
small there, and the cut is
characterized by a comparatively
high frictional component which
generates heat. At the same time,
relatively thin, low-volume chips
with a low heat-sinking capacity
are generated. Consequently, a
correspondingly high quantity of
energy flows into the tool.
The resulting locally exaggerated
wear is compensated for by shifting.
Shifting produces a more even
tool stress distribution, with regard
both to the hob as a whole, and to
the individual cutter tooth. Both
the abrasive and the thermally
generated wear mechanisms are
distributed more evenly over the
tool.
During coarse shifting, in particular,
cutter regions temporarily uninvolved
in the machining process
have sufficient opportunity to cool
down.
The cutting conditions applicable
to hobbing are principally the
cutting speeds and the feeds.
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