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AUTOMATIC HIGH ACCURACY COMPARATOR 
General Arrangement 
As with the plate previewer, the AHAC is built around an available measur- 
ing machine. 
Specification requirements call for an overall measuring accuracy of £0. 5 
microns. It is not possible to meet these requirements unless the straight- 
ness of carriage action, in both X and Y directions, is well below the above 
stated tolerances. Both carriage actions of the basic machine are stated 
to deviate from the true straight line no more than +1-2 microns. These 
specifications are not good enough and it will probably be necessary to re- 
finish the roller ways to achieve straightness of carriage motion in the 
order of +.1 or .2 microns such that the inherent accuracy of the metering 
can be fully utilized and specifications met. 
The Link Division has developed finishing techniques which enable the 
routine production of parts associated with the Fringecount Micrometer to 
flatness (or straightness) to tolerances approaching . 025 microns. It is 
expected that modification of these techniques can be utilized for refinish- 
ing the machine ways to achieve the required straightness of motion. Link 
has also developed absolute measurement techniques which will enable a 
much closer matching of the carriage rollers. The carriage travel straight- 
ness and orthogonality measurement system proposed, and discussed later, 
would also be used during possible refinishing. 
Drive System 
The actuating system to be obtained for the AHAC machine was selected 
to avoid the well-known difficulties encountered in precision machines 
equipped with conventional drive systems. 
Specification requirements demand that the X-Y table motion have essen- 
tially zero backflash, zero windup, extreme stiffness, high sensitivity, 
wide input speed capabilities, and extremely long life expectancies within 
minimum adjustment. The above requirements cannot be completely ful- 
filled by hydraulic, hydraulic-pneumatic, preloaded nut lead screws, or 
preloaded ball-nut lead screws. Drive motion must be extremely smooth 
and uniform without acceleration variations or jerkiness, because lack of 
drive smoothness can result in instantaneous velocities exceeding the 
maximum counting rate of the metering system, thus resulting in miscounts 
and erroneous measurements. 
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