Here - ETMM-Online

Technology
Synchronous Motors and Shaft Cooling Are Among
Spindle System Innovations That Support Micromilling
Steady progress in the miniaturization
of industrial parts has made micromilling
an ever more important manufacturing
process. Medical devices, optical
component housings, and EDM
tooling are typical micromilling applications.
To meet demanding performance
expectations requires use of the
right spindles. Thus, a new generation
of micromilling spindles offered by
Fischer AG Präzisionsspindeln sets
improved market benchmarks for reliability,
speed and precision.
Synchronous Motors
Whether a synchronous motor is to be
preferred for an ultra-high-precision
application depends on the outcome
of an accurate analysis of specific application
requirements. Both asynchronous
and synchronous motors are
very robust. However, improper parameterization
of the converter system
driving the spindle can cause a fatal
error. Consideration of the total
Axial growth of spindles with asynchronous and synchronous motors compared.
mechatronic system (spindle including
motor, plus converter) is crucial for
ensuring that micromilling cutting requirements
are met.
Asynchronous motors offer high
temperature resistance, but their permanent
magnets demagnetize at operating
temperatures above 150°C. This
critically stresses the rotor band. Synchronous
motors’ higher power density
is a major advantage; having a
greater centre bore results in better
static stiffness and spindle dynamics.
Disadvantages of synchronous motors
18 European Tool & Mould Making ■ September 2009
have been their smaller constantpower
range and the mandatory inclusion
of an encoder system. However, a
major breakthrough by the Fischer
Precise Group enables synchronous
motors to be driven without an encoder.
Thus, their benefits are available
for micromilling applications.
Owing to lower rotor temperatures,
which lessens thermal axial growth of
the spindle shaft, spindles with synchronous
motors achieve better toolcentre-point
repeatability. While the
influence of mechanical forces on
axial growth remains constant regardless
of the speed chosen, synchronous
motors’ lower rotor temperatures do
have a beneficial impact.
Tests performed on a grease-lubricated
100-mm-OD HSK-E25 spindle
with a top speed of 50,000 rpm compared
an asynchronous and a synchro-
The Fischer shaft cooling system.
nous motor in terms of their influence
on spindle growth (see graph at left).
Using a synchronous motor decreased
spindle rotor temperature by about 20
K, which resulted in turn in 14 μm less
axial growth, or a reduction of 26%.
Considering only thermal expansion
(subtracting mechanical growth from
total axial growth), synchronousmotor-originated
spindle growth is
only half that caused by an asynchronous
motor.
Shaft Cooling
The temperature behaviour of spindles
matters a great deal in micromilling
applications. Not only the temperature
but the saturation time has to be minimized.
Here, Fischer’s shaft cooling
system (image at top) sets a standard.
The complete shaft, including rotor
and bearing inner rings, is cooled with
water flowing through a series of channels
and supplied via a special threechannel
rotary joint that can also be
configured as a regular rotary joint for
sending coolant through the centre of
the shaft to the tool tip.
When such major heat sources as
the rotor and bearings are cooled
directly, tests show, up to 2 kW in losses
from the spindle shaft can be eliminated.
Consequently, spindle warm-up
time is reduced by 80%, and less heat
migrates into the machine.
Fischer AG Präzisionsspindeln
HERZOGENBUCHSEE, SWITZERLAND
www.etmm.info/2009/09/029