GB531568A - Improvements in and relating to indicating comparators - Google Patents

Abstract

531,568. Linear-dimension gauges. POLLARD, A. F. C. July 26, 1939, No. 21700. [Class 106 (ii)] A measuring head for a comparator comprises a body, a glass optical flat, and a rolling ball or its equivalent so arranged as to cause interference fringes with monochromatic light the number of which fringes changes with change of dimension. A short length 1 of rectangularsection bar has formed at one end a plane 2 sloping at a specified angle to the upper surface 3 of the bar, the " meet " being perpendicular to the length of the bar. The " reference plane " 3 and the plane 2 are lapped to an optical flat finish. A glass optical flat 4 is placed on the plane 3 and overlaps it to form a V-space with the surface 2 in which is located an accurate ball of steel, quartz &c. the point of contact with the plate 4 being such that a " median line " perpendicular to the square edge of bar 1 will bisect that edge. When the ball contacting with surfaces 2, 4 is moved to tilt the plate 4 at a small angle to the " reference plane '' 3 equally spaced straight interference fringes;under monochromatic light will be formed between surfaces 3, 4 square to the " median line." The number of fringes is a measure of the small angle and any movement of the ball will change the angle and the number of fringes. The centre of the ball is kept in the median plane by guide plates. In a modified form two pieces of steel 1, 2 Fig. 3a, 3b are formed with sloping lapped surfaces 4, 5 and secured together so that the ball 15 rolls along a V-groove. A stirrup 16 prevents the ball falling out when the apparatus is not in use. The blocks 1, 2 may be fixed to the bottom of a channel piece 6 between two co-planar optical plates 7, 7<SP>1</SP> corresponding to frame 3. They may be fixed by screws but preferably are fixed kinematically so that the slope of the V-groove may be delicately adjusted To this end the lower contingent edges of blocks 1, 2 are bevelled to form a V-groove where they abut and a screw 11 pulls the blocks down upon the ends of three adjusting screws 8, 10 tapped into the part 6. The screw 8 contacts with the end of the groove and screws 10 with holes in the under surfaces of the blocks 1, 2. By turning the screw 8 the axis of the V-groove may be tilted. A single block with a sloping surface may be fixed to the channel 6, the sides of the channel serving to " centre " the ball. A part ball may be fixed to the end of block 1 by a flat spring so that the centre moves along a line perpendicular to the spring and inclined to the reference plane at the required angle. The apparatus is shown as applied to an inside gauge having a fixed anvil 6 which may be screwed into the block. For other than vertical holes the optical flat is held in place by spring means.