GB608944A - Improved device for scribing ellipses - Google Patents

Abstract

608,944. Ellipsographs. ROBINS, T. H. Feb. 6, 1946, No. 3792/45. [Class 97 (iii)] In a device of the kind wherein two legs are guided along two lines at right angles and the ellipse is traced out by a marking leg in line with the others, one end leg is used for scribing, and one of the legs is so constructed or mounted that it can move axially in a direction to allow the scribing point to be brought into contact with the drawing surface. The guides are provided in or on the upper surface of a guideplate which can be pivoted on the drawing surface at or near the intersection of the guides. The instrument shown in Fig. 1 comprises a pair of compasses, one arm 15 of which is pivoted at 20 to a bracket 21 having a guiding leg X and an arm 23 sliding in a groove in the bracket, and adjustably fixed by the screw 24, and having a second guiding leg Y. The pencil Z is moved into contact with the paper by movement of the arm 15 about the pivot 20. Instruments are also described where the compass arms are adjusted by a threaded pin having threads of opposite hand, or where a pair of adjustable bows is pivoted to the arm 15 in place of the bracket 21. Alternatively, a beam compass is used having three legs, the position of at least two of which is adjustable along the beam, and the effectivelength of one of the legs is self-adjusting by a spring-loaded telescopic device to enable the scribing point to move into contact with the paper. The guide member of Fig. 5 has a pin 79 by which it may be pivoted to the drawing-sheet so that the ellipse may be completed at the ends by turning the guide member about the pin. The pointed ends 80, 82 are for location with the major and minor axes respectively. The guide member of Fig. 7 has a carriage 89 sliding in an undercut slot 90, the leg 7 being placed in the opening 88 in the carriage. The slot 87 for the leg X is continued beyond the slot 90 a distance equal to the amount by which the opening 88 is offset. By this means the scribing instrument can describe half the ellipse, the guide member being held fixed by the pin 86. For scribing the other half, the guide member is rotated through 180 degrees about a pin inserted through the hole 85. In all cases, one guide groove is appreciably wider than the other, and each has a cross-section semicircular at the bottom and widening towards the top. The legs differ in diameter so that they cannot be wrongly placed in the grooves.