Patent GB-1012871-A (GB)
Title: Apparatus for defining the position of a movable machine part
IPC: G01B11/00
Filed: 1962-10-23
Abstract: 1,012,871. Machine tool details. SOC. GENEVOISE D'INSTRUMENTS DE PHYSIQUE. Oct. 23, 1962 [Nov. 24, 1961], No. 4004l/62 Heading B3B. [Also in Division G1] Apparatus for defining the location of a part of a machine tool or measuring machine includes two standard scale carrying rules 29, 30 moving with the part, means for amplifying the scale on the rule 29 to accurately define the position of the part, means for amplifying the scale on the rule 30 with a smaller magnification to give a rough definition of the location, the amplifying means for the scale 29 having an optical system adapted to move the image of a scale subdivision towards a fixed point at the centre of the field of vision produced by the optical system, and means for measuring the shifting of the image required to make it register with the fixed point. In the measuring machine shown in Fig. 2, an article to be measured is placed on a table 12, to either side of which are fitted two headstocks 13, 14 having feelers 20, 21 respectively, which are urged against the part to be measured. The headstocks and table 12 may be set in any desired position and the feeler 20 is carried by the body 13b of the headstock 13, the body being supported by elastic blades on a support 13a. The body 13b carries the highly accurate rule 29 and the rule 30 which is transparent. The feeler 20 is urged against the part to be measured by an adjustable blade 22 acting on a rod 23. The body 13b may be moved back and forth by a linkwork (not shown), the return movement being damped by an hydrodynamic brake 26, 27. The rule 30 as it moves with the body 13b moves past a fixed optical system which is adapted to project an image of part of the scale on the rule 30 on to a translucent screen 36. This optical system comprises a light source 33, lens 34, 38, mirrors 39, a glass block 40, lens 41 and mirror 42. The rule 29 has sub-divisions, e.g. 1 mm. and a pencil of light is projected on to this rule from a fixed light source 44 through a slit 46, glass blocks 47, 48, mirror 49, vibrating light deflector 50, and lens 51. Light reflected from the rule 29 is deflected by a prism 52 and mirror 53 on to a photo-cell 55. The deflector 50 is in the form of a glass blade surrounded by a coil (not shown), fed with an A.C. voltage and disposed on elastic suspension means between the poles of a permanent magnet. The pencil of light from the source 44 is thus made to oscillate on the rule 29, the amplitude of the oscillations being greater than the width of a graduation mark on the rule but less than the distance between successive marks. When the oscillating pencil of light passes over a sub-division line on the rule 29, the amount of light reflected to the cell 55 will be subjected to a sudden variation which produces an electric pulse in the cell. These pulses are transformed by means, not shown, into rectangular current pulses alternately positive and negative, the duration of which depend on the time interval between two pulses from the cell 55. The rectangular current pulses are fed to a milliammeter and when the centre of oscillation of the light pencil is coincident with a sub-division on the rule 29, the hand of the milliammeter will remain stationary along the centre line of its dial. The adjustment of the pencil of light to obtain this condition is achieved by angular movement of the glass block 47 about a pin 61 caused by rotation of a hand wheel 74. The hand wheel shaft 70 carries a drum 73 and the shaft 70 drives a shaft 65 having a similar drum 68 through a coupling 71. The shaft 70 may be arranged to rotate a hundred times during one rotation of the shaft 65 and the position of the shafts and thus indirectly the position of the scale 29 may be indicated by markings on the drums 68 and 73. As the glass block 47 is moved, the glass block 40 is also correspondingly moved about a pin 60 to alter the position of the part of the rule 30 projected on to the screen 36. As the relation between angular rotation of the glass block 47 and linear movement of the centre of oscillation of the light pencil on the rule 29 is not exactly linear, a correction is applied by angular movement of the glass block 48 about a pin 76 controlled by a suitably shaped cam 79 mounted on the pin 61. Specification 686,274 is referred to.