Patent GB-629679-A (GB)
Title: Improvements in or relating to a straight edge
IPC: G01B7/34
Filed: 1946-02-15
Abstract: 629,679. Straight-edges. BROWN & SHARPE MANUFACTURING CO. Feb. 13, 1947, No. 4168. Convention date, Feb. 15, 1946. [Class 106 (ii)] [Also in Group XXXVIII] A straight-edge has combined therewith strain-sensitive means for gauging strains set up in the straight-edge by distortion thereof during gauging. This distortion may be due to sag or out-of-line displacement of the straight-edge when engaging a work-piece. The means for gauging strain may be of a mechanical, pneumatic, or electrical nature. A straight-edge 20, Fig. 1, for plain flat surfaces has two parallel trued edges 22, 24, strengthening-ribs 30, and lifting-handles 26, 28. Electrical gauge units 33, 34, each housing two strain gauges, the wiring for a Wheatstone bridge circuit, and a potentiometer unit 36 are mounted on the straight-edge. The gauge units 33, 34 are mounted equidistant from the longitudinal axis of the straight-edge and mid-way of the ends of the latter, and provide an indication of bending stresses set up in the straight-edge which is in proportion to the out-of-line displacement of the latter along its length. The unit 33 consists of two electrical strain gauges R31, R41, Fig. 9, comprising fine wire resistances in a paper bond cemented to a bar rigidly secured to the straight-edge and covered by a plate to exclude moisture &c. ; the unit 34 is identical, and contains gauges R51, R61. The unit 36 comprises a manually-adjustable potentiometer 38 and resistances R11, R29. An amplifier and indicator unit connected to the bridge circuit has sensitivity and zero adjustments. Current is supplied to the Wheatstone bridge from an A.C. supply through a rectifier. Part of the D.C. is regulated by a gaseous regulator tube to supply a 1000-cycle triode oscillator and the last amplifier valve of the amplifier circuit. The 1000-cycle oscillator supplies the Wheatstone bridge trough secondary T2,1 of the oscillator transformer, and also the rectifier RX through secondary T2,2 of the same oscillator transformer. Two'resistors R1, R19 and a potentiometer R2 form a balance for the bridge. Any unbalance voltage in the bridge due to strain in the straight-edge during gauging is fed to the potentiometer R3, which can be adjusted to vary the sensitivity of the circuit, and passed through an amplifier, transformer T3,1, the rectifier Rx and meter M. The reading of the meter M is proportional to the sag or out-of-line displacement of the straight-edge during gauging. Electric strain gauges can be applied to straight-edges of cruciform cross-section having four V-shaped edges for gauging V-slides of grinding-machines &c. In another form, several electrical strain gauges are arranged along the length of a straight-edge in predetermined positions where greater accuracy is essential. The mechanical strain gauge shown in Fig. 12 comprises a bar 110 pivoted at 112 to the straight-edge 108 and supported along its length by reeds 114, 116. Any distortion or sagging of the straight-edge during gauging will be reflected by the relative movement between the free end of the bar and the adjacent portion of the straight-edge, and this is indicated by a dial gauge 118 pivoted at 124 and adjustable by screw and stop means 126, 128. In a further modification, an adjustable nozzle, connected to a source of fluid under constant pressure, directs a jet of fluid against the free end of the bar 110. The flow from the nozzle is varied when the end of the bar moves relatively to the adjacent portion of the straight edge, and the resulting change of pressure in the pipe immediately behind the nozzle is noted on a pressure gauge. The pressure gauge and the dial gauge 118 are calibrated to register directly the out-of-line error in the straight-edge.