Sandvik Coromant Heat resistant super alloys - Manual (Page 62)

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Milling of HRSA materials
When milling super alloys, there are certain
process requirements which must be
observed.
• Milling of high-temperature alloys often
requires more rigid and powerful equip-
ment than the milling of carbon steels.
• Cutter accuracy in both radial and axial
directions is essential to maintain a con-
stant tooth load and a smooth operation,
and to prevent premature failure of indi-
vidual cutter teeth.
• Cutting edges must be sharp with an
optimised edge-rounding, to prevent chip
adherence at the point where the edge
exits the cut.
• The number of cutting teeth actually in
cut during the milling cycle must be as
high as possible. This will give good
productivity provided that the stability is
good enough.
• Cutting speeds for super alloys are gener-
ally low. Common practice is to employ
a fairly low cutting speed in combination
with a moderately high feed per tooth, to
produce a chip thickness not less than
0.1 mm which prevents work-hardening of
the material.
• Coolant should be applied in generous
quantities around the cutting edge when
the cutting speeds are low, in order to
reduce chip adhesion. Coolant supplied
through the machine tool spindle is
recommended for HRSA materials. High
pressure coolant (HPC) will give better
tool life. (no coolant for ceramic milling)
• The cutting edge geometry should always
be positive.
• For cutting depths below 5 mm, the
entering angle should be less than 45°.
In practice, a round, positive-rake insert
is recommended.
• If special design cutters are being consid-
ered for an application, it is essential to
allow sufficient space between each tooth
for effective chip evacuation around the
cutting edge.
• Flank wear around the cutting edge
should not exceed 0.2 mm for R390, 0.3
mm for round inserts in carbide, and 0.6
mm in ceramics. Otherwise the chance
of a catastrophic failure increases rap-
idly. Normal best practice is to index the
cutting edges at frequent intervals, to
ensure a reliable process.
• Down milling (climb milling) should be
used, to obtain the smallest chip thick-
ness where the edge exits cut and reduce
any chip adherence
• The machining of iron-based super alloys,
and solution treated, nickel-based alloys
(Inconel 625), usually machine easier
than nickel-based and cobalt-based super
alloys.