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

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.