GB945070A - Improvements in or relating to protractors - Google Patents

Abstract

945,070. Protractors. C. PIERCE, and W. W. ANDREWS. Sept. 17, 1962 [Aug.10, 1961], No. 28973/61. Heading G1X. A protractor comprises an outer ring 2 adapted to be rotatably received in a mount 1, an inner ring 3 supporting a screen and rotatably received in the outer ring, the screen having a plurality of radial lines of known angular spacing, means for locking the inner ring against rotation with respect to the outer ring, and a gauge 11 for indicating angular displacement of the outer ring 2 relative to the mount 1. The ring 3 is held in place in the ring 2 by a clip 5 and knobs 6 are provided for manual rotation of the ring 3. The locking means comprises a block 7, secured to the ring 2, provided with a clamping screw 8 for locking the ring 3. The block 7 has a shoulder 9 engaging the movable head 10 of the gauge 11, a tension spring 12 being provided to ensure engagement. The shape of the shoulder 9 is such as to provide a linear relationship between angular movement of the ring 2 and linear movement of the head 10. In use an image of a workpiece is projected on the screen 4, which is translucent, and the ring 3 is rotateduntil the 0 degrees reference line on the screen coincides with one side of the angle to be measured. The ring 3 is then clamped to ring 2 and the other side of the angle to be measured will then fall between two other radial lines on the screen. These lines are conveniently at 5‹ intervals and the angles are measured from the reference line in an anticlockwise direction. The gauge 11 is then adjusted to make the other side of the angle coincident with the lower value line of the two between which the side originally fell. The gauge is calibrated to read 0 to 5 degrees and thus the angle to be measured is given by adding the reading of the gauge 11 to the marked value of the appropriate radial line.