Sandvik Coromant Heat resistant super alloys - Manual (Page 65)

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Power/torque requirement
Basically, the power requirement varies with
the amount of metal to be removed, aver-
age chip thickness, cutter geometry and
speed. The greater the metal removal rate
(Q cm
3 /min) the higher the power require-
ment.
With spindle speeds for roughing much
lower than for less exotic materials, it puts
great importance on ensuring that power
and torque are available at low rpms (a
machine with insufficient torque and power
will give fluctuating chip thickness in turn
giving unstable performance).
The diagram below shows how a typical
machine power diagram looks with lower
power at low rpms.
Spindle coupling
Most HRSA alloys work-harden during
machining and have higher strength and
‘gumminess’ not typical in other materials.
Heavy duty machining equipment and an
ISO 50 taper (or equivalent) spindle is
recommended to minimise chatter and work-
hardening of the alloy ahead of the cutting.
Coolant
Unlike milling in most other materials,
coolant is always recommended to assist
in chip removal, control heat at the cutting
edge and prevent re-cutting of the chips.
High pressure coolant (70 bars) applied
through the spindle/tools and externally
has been seen to give good benefits com-
pared to low pressure.
Fixturing
The shape and fixturing of the component
is of great importance. Aerospace engine
components are often thin-walled and have
a lot of complex features which easily cre-
ate distortion and vibration.
This is often the reason why engine casings
have complex fixtures to reduce vibration
and support the workpiece in relation to the
direction of the cutting forces. Light cutting
tools with positive geometries can help to
ensure a safe, distortion-free machining
operation.
Stability
The condition and stability of the machine
has an effect on the quality of the surface
and can impair tool life. Excessive wear of
the spindle bearings or feed mechanism
can result in a poor surface structure.
As well as ensuring a stable machine
tool, other factors such as tool overhang,
Coromant Capto coupling, tuned adaptors
etc should be considered.
P
c
kW
Risk of power being too low
with the large diameters and
low cutting speed.
n
m/min