GB615950A - Improvements in mathematical drawing instruments - Google Patents

Abstract

615,950. Draughting-machines. LIBER, H. S. Aug. 21, 1946, Nos. 24990 and 24991. Convention dates, Feb. 17, 1945, and Aug. 2, 1945. [Class 97 (iii)] An orientable head for a draughting machine in which an automatically engaging bolt enables the support for the rulers to be locked in one of a number of predetermined positions and wherein the bolt can be withdrawn to enable the support to be rotated to and subsequently locked by a brake in any other position is characterised by the provision of a single member for controlling the operation of the bolt and braking mechanism. The ring 1, Fig. 1, fixed to the linkage arms of the instrument carries a support 4 rotatably mounted on the pivot 3 having extensions 5 to which the drawing instruments are secured. A case 6 is secured to the support 4 and its upper part forms a knob 7 by which the support is rotated relatively to the ring 1. On the casing 6, adjacent to knob 7, is rotatably mounted a second knob 8 which drives a sleeve 11 by means of claws 9 engaging notches in the upper peripheral face of the sleeve. Sleeve 11 rotates on a fixed tube 12 inside which slides a bolt 13 whose lower end can project into perforations 14 at predetermined spacing in the ring 1. Pins 15, 16 pass through the bolt 14 and elongated slots in the tube 12 and engage with cams formed on the upper and lower faces of the sleeve 11. In the position shown in Fig. 6 bolt 14 is engaging with one of the predetermined perforations in ring J thus locking the ring to the support. On rotation of the knob 8 pin 15 is lifted on the inclined faces 19, 19a of the cam surface of sleeve 11 and the bolt is withdrawn from its engagement. A skirt 24 on the sleeve 11 is so shaped that until the pin 15 reaches the upper end of the inclines 19, 19a the skirt is engaged by a pawl 26 to which is attached a spring 27, the arrangement being such that on rotation of the operating knob 7, with the pin 15 in that position, until the bolt is over the next perforation of the ring 1, the pawl will turn the sleeve back to its original position so that the bolt engages in the perforation. Further rotation of the control knob 8 brings the pin 15 onto the flat portions 20, 20a of the cam and the pawl 26 can no longer engage with the skirt 24, so that the operating knob can be rotated to move the ring and support relatively into any desired position. As the control knob 8 rotates it drives, through gearing 27a, a pinion 28 in which is threaded a rod 29 carrying a brake shoe 30 with a heel 31, which is adapted to engage with the peripheral face of the ring 1 when knob 8 has been rotated sufficiently for pin 15 to be at the end of its travel on the flat portions 20, 20a of the sleeve 11. By the operation of the knob 8 in the reverse direction from this locking position, the successive operations are carried out in the reverse order by the co-operation of pin 16 with the lower cam surfaces of sleeve 11. A modification is described in which rotation of the control knob drives a bolt and a sleeve, the latter having inclined surfaces at its lower end engaging with inclined slopes on the support so that the sleeve is raised as it is rotated, and a similar movement is transmitted to the bolt by projections therefrom which engage with the sleeve and the knob. In this embodiment the brake is operated by means of an eccentric cam carried on the sleeve. If desired, the operating knob 7 may be shaped as a handle. Means are also described for adjusting the relative position of the ring and the linkage arms of the machine by either a micrometer screw or an eccentric knob.