A 14-inch ruling engine, whose operation with interferometer control of grating blank position between ruling strokes has been described previously, is now being operated with continuous blank advance and control. Displacement of the carriage holding the grating blank is measured in terms of the phase of a fringe system of constant inclination passing across a photoelectric pickup, which produces low-frequency ac whose phase is compared with that from a generator measuring the phase of motion of the ruling diamond. Synchronism between diamond and blank is maintained through corrections fed into a differential on the engine screw-worm by a balancing motor operated by phase differences. Grooves straight to one-tenth fringe, up to 9 in. long, can be produced by means of a cam-and-lever system which rectifies the otherwise simple harmonic motion of the diamond carriage. Continuous servo control results in improved elimination of screw errors and engine vibrations, and in simplified circuitry. The needed electronic and interferometric control systems have been found to function reliably over the long periods needed to rule large gratings.
A change-gear system permits passage of any desired fractional number of fringes per diamond stroke needed to produce from about 50 000 to 2000 grooves per inch. The signal-to-noise ratio obtained when green light from an Hg-198 tube is used to illuminate the carriage-translation interferometer is found to permit control over 10 in. of carriage motion, and stepping methods are being studied to permit multiplication of this distance. Plane gratings up to 8 in. in width of ruling and 5 in. in groove length have been produced, which show acceptably low ghost intensities despite original screw errors of more than 50 times the tolerance limit. Error-of-run and fanning appear to be under good control over distances of 8 in. Rapidly occurring random errors previously found have been traced to lateral motion of the diamond carriage and eliminated. Slow irregularities in groove position are ascribed to temperature variations, to which the engine has been found to be 10 times as sensitive as necessary; these are now being removed. Continuous oscilloscope records taken during ruling show that blank positioning is being controlled by servo interferometry to within 1/40 fringe or better, and indicate sources of disturbances whose removal should result in further improvement.
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