Patent GB-812427-A (GB)
Title: A grinding process and a grinding machine for providing a rounded edge on the periphery of a plate or disc
IPC: B24B9/10
Filed: 1956-03-13
Abstract: 812,427. Grinding. EBAUCHES S.A. March 13, 1956 [April 30, 1955], No. 7770/56. Class 60. [Also in Groups XXXII and XXXVII] A rounded edge is produced on the periphery of a glass or other plate 6, Fig. 3, by relative advancing movement between such periphery and a grinding disc 55 having a working surface complementary to the rounded edge to be produced, and during this operation the grinding disc is oscillated about an axis passing through the centre of curvature of the concave profile of the grinding disc at its point of operation on the plate so that the angle between the planes of the grinding disc and plate changes continuously. The plate 6, Fig. 3, to be ground and a pattern plate 23 are mounted in a vacuum chuck for rotation about a horizontal axis, and the grinding disc 55 and a feeler disc 26 for contacting the pattern plate are mounted in a head 5 at the end of an arm which can be swung about a horizontal axis to lower or raise the head in relation to the plates 6, 23. The plate 6, Fig. 2, is held by vacuum on a table 8 carried at the end of a motor-driven shaft 9 enclosed by a tube 7, and a pattern plate 23 of a shape corresponding to that of the plate 6 is screwed to a support 13 which is connected to the table by a pin 13<SP>1</SP> and initially pressed towards it by springs 14; in this position of the plate 23, the plate 6 is supported by members 25 on the plate 23 while it is being adjusted. At the start of the operation, a vacuum pump at the driven end of the shaft 9 applies vacuum through the interior of the shaft to the underside of the plate 6, and through channels 19 and the holes 17 housing the springs 14 so as to withdraw the support 13 and plate 23 from the table. Packing rings 10, 11, 18 are provided between the table and plate 6 and the table and support 13. At the start of the grinding operation, the arm carrying the grinding head 5 is lowered so that the grinding disc 55 and feeler disc 26 are brought into correct position relatively to the plates 6, 23. The grinding disc 55, Figs. 3 and 4, is mounted on the shaft 56 of a motor adjustable by a screw 59 along a guide 57 in a casing 58 in the head 5. The casing has two arched guides 62 running between guide rollers 63, 64, 65 on the head and is connected by a crank rod 66 with a motor-driven disc 69 whereby it and the disc 55 are oscillated about the centre of the arched guides which is also the centre of the concave profile of the grinding disc at its point of operation on the plate 6. When the arm with the head 5 is lowered, the feeler disc 26 first engages the edge of the pattern plate 23 and serves to bring the head slowly to rest. For this purpose, the shaft 27 of the disc 26 is rotatably mounted in a member 28 rotatably mounted on a shaft 29 in a cradle 30 which is rotatably mounted in bearings 31 for swinging movement about an axis disposed at right angles to the shaft 29. Initially, a spring- pressed plunger 34 co-operating with a dash pot 39 at its remote end swings the cradle 30 about its axis against the action of a return spring 32 so that its upper end is spaced from a stop 302 on a screw 301. When, however, the disc 26 in the lowering movement of the head 5 engages the edge of the pattern plate 23, the cradle 30 is swung back so as to push the plunger 34 rearwardly and, by the action of the dashpot 39, gradually brings the head to a halt with the cradle engaging the stop 302. The head 5 is arranged to be adjusted about the axis of the plunger 34 by mounting the part 5<SP>1</SP> in bearings 42, 43, in the free end of the arm 4 and fitting on it a disc 45 connected by a crank 46 to the piston of an hydraulic cylinder also mounted in the arm. The control valve 50 of this cylinder is so actuated by a feeler 51 co-acting with a projection 52 on the member 28 that, as the latter is displaced on its ahaft 29 to either side of its mean position, fluid is admitted to one side or the other of the cylinder and so adjusts the position of the head 5; a spring-pressed finger 54 acts on the projection 52 opposite to the feeler 51 so as to compensate for the pressure of the control valve. As the disc 26 engages the rotating pattern plate 23, the member 28 therefore swings about the shaft 29 so as to maintain the plane containing the shafts 27, 29 perpendicular to the tangent to the plate at its point of contact with the disc, and these movements of the member 28 act upon the control valve 50 to adjust the position of the head 5 and maintain the grinding pressure of the disc 55 always perpendicular to the tangent to the plate 6 at the working point. The table 8 is driven at a variable speed so that the power consumed by the motor driving the grinding disc is kept contsant. For this purpose, a transformer 74, having a divided primary 73, 75 connected as shown in Fig. 9 in a three-phase system feeding the grinding motor 71, has its secondary 78 connected with the windings 83, 86 of a control device 84 connected to the control grid of one or more thyratrons in the circuit of the motor driving the table 8. The primary windings 73, 75 are so designed that, at a predetermined speed of the motor 71, no voltage is induced in the secondary winding 78 and the control device then applies such a voltage to the thyratrons that the driving motor of the table 8 operates at medium speed. Rise or fall of the power consumed by the motor 71 results in one or other of the primary windings overbalancing the effect of the other and so adjusting the control device that the speed of the table motor is correspondingly decreased or increased. At those places where it is not desired to grind the edge of a plate, the pattern plate is shaped to lift the grinding head and disc away from the plate 6, with the result that the speed of the grinding disc is reduced and that of the plate correspondingly increased.