Rapid Calibration and Correction of Comparator Screws and the Photographic Production of Scales

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Abstract
The automatic recording and computing comparator previously described has been adapted to the production of uniform and nonuniform scales. A photographic plate is clamped to the covered comparator carriage, where it is exposed to diminished images of a slit illuminated by an argon lamp which is made to flash periodically under control of the wave-length dial of the comparator. A new automatic comparator with 22-inch travel has been constructed, and the periodic errors of its screw have been reduced to less than 1 micron in amplitude by means of a new type of compensator operated by a three-dimensional rotating cam. Scales up to 20 inches long have been produced photographically in ten minutes with this instrument, those with lines 0.01 mm wide and 2 mm long spaced at 0.1 mm intervals being found convenient for screw calibration. With such a scale the recording attachment will determine the correction curve for a screw in a few minutes. The correction cam can readily be shaped to offset the error found, and fairly cheap screws can thus be made to serve for precise measurements. A satisfactory method of mounting comparator screws on ball bearings has been devised, and this feature, together with the use of a half-nut and ball-bearing rollers under the comparator carriage, reduces to one-third the torque required to operate the comparator, thus increasing its life, ease of operation, and precision.

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