Special Issue; Products for Industrial Machinery - NTN

NTN TECHNICAL REVIEW No.742006
New Product
Development of Aerostatic Bearing Spindle
for Precision Machine Tools
Teruyoshi HORIUCHI
Kazuyuki AONO
Hiroaki HIYOSHI
The main reasons for aerostatic bearing spindles in
precision machining are the increase in stiffness and reduction
of thermal displacement of the work. To meet such demands,
NTN has developed a new aerostatic bearing spindle for
precision machining by reviewing the bearing design, cooling
mechanism, and material selection. The results of the
performance evaluation test of the prototype air bearing
spindle are described here.
1. Introduction
As the demand for personal digital equipment,
including mobile phones and DVD recorders, has
increased, there has been a growing need to improve
productivity the machining precision dies for aspheric
lenses and other precision components. Ultraprecision
machine tools for machining very small
precision dies for these components often employ
precision aerostatic spindles that incorporate
hydrostatic air bearings. To improve the productivity of
these machine tools, their aerostatic bearing spindles
must be capable of both higher running speed and
greater precision.
NTN has long been providing high-precision
aerostatic bearings that are used on original recording
devices for optical disks, as well as on magnetic disk
inspection devices. By further advancing our
aerostatic bearing spindle technology, we have
successfully developed a high-speed, high-precision
aerostatic bearing product for precision machine tools.
2. Challenges for Aerostatic Bearing
Spindles for Machine Tools
Generally, an aerostatic bearing spindle is
comprised of two journal bearings that support the
rotary shaft in the radial direction, a pair of thrust
bearings that support the rotary shaft in the axial
direction, a driving AC servomotor, and a rotary
encoder for detecting the running speed of the
spindle, as illustrated in Fig. 1. The rotary shaft is
supported by the bearings without contacting them by
using the pressure of an air layer that is several
micrometers thick. The motor and the rotary encoder
are also non-contact types. Since the rotary shaft is
supported without contacting fixed members, the
frictional resistance on the rotary shaft is limited,
allowing the rotary motion of the rotary shaft to be
controlled very accurately.
In addition to the fundamental structure described
above, as with any other aerostatic bearing spindle for
precision machine tools, the spindle needs to
incorporate a bearing arrangement for high spindle
rigidity, a high-torque motor that provides the torque
necessary for the intended machining operation, and a
vacuum chuck mechanism to hold workpieces in
position with vacuum force. Also, measures against
thermal deformation resulting from heat generation on
the motor and bearings will be necessary in the form
of a cooling mechanism. Furthermore, since the
environments where the aerostatic bearing spindles
for precision machine tools are situated differ from the
environments in which aerostatic bearing spindles for
inspecting devices are used, a sealing mechanism
must be provided for the former in order to prevent
ingress of chips and coolant mist.
Product Engineering Department Precision Equipment Division
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