GB1381291A - Measurement of pitch errors in screws - Google Patents

Abstract

1381291 Apparatus for measuring the pitch error in screws TRADE INDUSTRY SECRETARY OF STATE FOR 24 Jan 1972 [8 Jan 1971] 1126/71 Heading G1A In an apparatus for measuring the pitch error in screws two diffraction gratings are employed, a circular diffraction grating which may be rotated with the screw and a linear diffraction grating which may be held parallel to the screws axis. Each diffraction grating has a receiving head comprising a photo-cell and a further diffraction grating associated with it so that when the screw is rotated both reading heads move relative to their grating each producing cyclic output signals due to moire fringes. The gratings are designed and arranged so that if the screw is perfect the two output signals will be identical in frequency and phase so that any pitch error will produce a phase difference between the two signals which is measured by a phasemeter. In Fig.1, the screw 10 and its circular diffraction grating 22 are rotated by a motor 28 and its associated gears 20, 26 while a reading device 24a, 24c, b, d (not shown) comprising four equally spaced heads is held opposite the grating. A carriage 32 is moved to be substantially in step with the screw's helix by means of a further screw rod 58 driven by the motor 28 via gears 54, 56. The carriage 32 carries a mounting plate 31 attached to it by resilient ligaments 32 and 33, with a further reading head 34 adjacent a stationary linear diffraction grating 38, the plate 31 carries a flexible stylus 36 which engages the screw threads as shown and keeps the plate 31 precisely in step with the screw helix. The outputs of the two reading devices 24, 34 are fed to a phasemeter 42 which produces an output proportional to the phase difference and thus the pitch error of the screw. In a further similar arrangement Fig.2 (not shown) the reading head is attached directly to the carriage and a position responsive transducer is attached to the stylus. The outputs of the two reading devices are subtracted to give a signal representing the transmission error of the apparatus and this is subtracted from the position responsive signal to give as before a signal representing pitch error. Two further modifications are employed, Figs.3 and 4, Fig.4 (not shown), wherein the stylus is replaced by a non-contact making pneumatic probe 64 which is positioned to produce two jets of air 70, 72 at opposite flanks of the screw thread 84, 86 and a pressure sensitive transducer 82 may be employed to provide positions sensitive signals as before and, (Fig.4), to keep the probe in the centre of the thread.