Special Issue; Products for Industrial Machinery - NTN

NTN TECHNICAL REVIEW No.742006
Rotary table
Feed-water inlet
Waterway for cooling
Rotary shaft
Water drain outlet
Workpiece
Air seal
Chuck surface
Air seal
Air thrust bearing
Motor
Rotary encoder
Air supply opening for air seal
Fig. 2 Structure of the air spindle which we developed
problem, the housing on the motor and bearing
sections is water-cooled and the associated
components are composed of materials with low
thermal-expansion in order to inhibit their thermal
deformation and the displacement of the chuck face.
Furthermore, air feed holes are provided in the bore
sides of the motor stators for feeding compressed air
to cool the motors.
For evaluation of the control measure against
thermal deformation, the aerostatic bearing spindle
was started from a standstill in a room temperature
environment and allowed to run constantly at 20,000
r/min, and displacement in the position of the
workpiece was measured. This measurement result is
graphically plotted in Fig. 4. Immediately after the start
of operation, the shaft developed an elongation of
5. Spindle Evaluation Test
A dummy workpiece simulating an actual workpiece
was mounted to the prototype spindle, and then the
spindle was subjected to an accuracy evaluation test.
Run-outs were measured at the locations shown in
Fig. 3 with a capacitance type non-contact
displacement gage. Jitter (variation in rotation
frequency) was determined by measuring variation in
the frequency of Z-phase signal (1 pulse/revolution) of
the rotary encoder with a pulse jitter counter. The
measurement result for non-repetitive run-out (NRRO)
is summarized in Table 2 and that for jitter (variation in
rotation frequency) is given in Table 3. As can be
understood from the data in these tables, NTN’s
aerostatic bearing spindle performed very well from
low speed to the maximum speed.
Shaft
elongation
Radial
run-out
Axial run-out
Dummy workpiece
Fig. 3 A measurement position
Table 2 A measurement result of NRRO
Running speed
r/min
1000
5000
10000
15000
20000
NRROnm
Radial
3.75
3.75
3.75
3.75
3.75
NRROnm
Axial
3.75
3.75
3.75
3.75
6.25
Running speed
r/min
Table 3 A measurement result of jitter (A dummy))
Jitter
nsec
Accuracy
%
500 213.0 180.1 190.9 205.5 195.0 0.00018
1800 82.1 72.7 68.5 84.3 67.6 0.00025
5400 33.5 28.2 27.9 31.8 26.5 0.00031
10000 20.8 21.2 17.4 21.2 18.8 0.00035
15000 14.1 12.2 11.5 13.6 14.3 0.00036
20000 9.3 10.3 10.9 10.8 10.2 0.00036
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