Patent GB-682972-A (GB)
Title: Improvements relating to clinographs
IPC: B43L13/00
Filed: 1950-02-03
Abstract: 682,972. Measuring angles. THRING, L. G. P. Feb. 2, 1951 [Feb. 3, 1950], No. 2841/50. Class 97 (iii). In an adjustable set-square intended for use against a straight edge in which a generally triangular bearing-plate A, Fig. 4, has two straight sides D, L, at right-angles to one another and a third concave side shaped to receive slidably the circumferential edge of a part B in the form of a segment of a circle, scale markings being provided on the parts to indicate their relative angular position, the chord K of the segment subtends an angle of less than 60 degrees at the centre of the arc. The plate is formed with a tongue and the segment, whose tips are removed, with a groove, the plate and segment when in the correct relative positions providing a continuous hypotenuse. In the embodiment shown in Fig. 4, the chord K in one position of the segment forms substantially the whole of the hypotenuse of the right-angled triangle which has remaining angles of 30 and 60 degrees, the longer side D when produced being a tangent to the circle of which the arc of the segment forms a part, and the upper end of side L intersecting the circle at the point of contact of chord K. With side D contacting the straight edge, angles from 0 to 30 degrees may be read on a scale N, Fig. 6, provided on the segment, against an index P<SP>1</SP> on the bearing- plate and angles from 30-60 degrees by the same scale against a second index P<SP>2</SP> by moving the segment from position K<SP>1</SP> to position K<SP>2</SP>. Angles from 60 to 90 degrees are read in the reverse direction on scale N against index P<SP>1</SP> by moving the chord K to K<SP>1</SP>. The angle subtended at the centre of the circle by chord K is between 40 and 45 degrees, and when in its extreme positions the segment has about a quarter overlapping the plate. A scale P on the bearing-plate of sines of angles from 0 to 30 degrees is read against one end of scale N, verniers being provided. On the reverse side of the instrument the bearing-plate is graduated in sines of angles from 30 to 60 degrees and the segment those from 60 to 90 degrees. In a modification, Fig. 5, in which for a given radius of circle the instrument is larger, the side D produced forms a tangent to the circle but the short side M is such that when produced it does not intersect the segment when the latter is at 45 degrees to each side. On one side 0 to 45 degrees graduations on the plate are read against an index and vernier on the segment, a shoulder in the tongue of the plate engaging a corresponding step in the groove of the segment when the chord is at 45 degrees to the sides. On the reverse side a scale of sines of angles from 0 to 45 degrees is provided on the plate and those of 45 to 90 degrees on the segment, the latter scale being shifted through 1 degree, 15.6 minutes and the index line on the bearing- plate by the same amount so that the index lines correspond to definite markings on the sine scales. The tip of the upper end of the segment is cut at 45 degrees to the chord, and the latter subtends an angle of about 45 degrees at the centre of the circle. In a modification, magnets in the bearing-plate attract armatures in the segment or vice-versa and the minimum overlap is slightly reduced.