One of the crucial issues in precision machining is maintaining a constant temperature of the positional stages and the tool holders. At the tolerance levels required for fabricating high frequency waveguide structures, temperature changes of a few degrees can result in considerable errors. In this respect, the high speed air bearing spindles offer another advantage compared to conventional electric motor driven spindles which exhibit temperature increases of several tens of degrees during operation. The temperature rise in the air bearing spindles is only a couple of degrees centigrade after half an hour of operation, after which it remains stable. In order to understand the temperature stability of the room while operating the machine, temperature was monitored at eight different locations in and around the micro NC machine and careful records were kept.
After repeated experiments machining with and without temperature
control, it was found that a closed loop temperature control system
significantly improved machining accuracy and repeatability. A heater
on the metal blocks that house the spindles (seen on top of the
spindles in Figure 2) forms part of the control loop. Before
commencing actual machining the machine is usually run for ten to
twenty minutes to reach temperature equilibrium. Several fans and
blowers are run around the machine to provide adequate circulation of
air. At present, although the temperatures of the positional stages
and workpiece are monitored, they are not controlled. The closed loop
system maintains the temperature of the spindles to within half a
degree of the temperature set point, 
C, which is a few
degrees above ambient.