Patent GB-801846-A (GB)
Title: Improvements relating to tracing systems for automatic profile or contour machining
Inventor: STOKES JOHN ARTHUR
IPC: B23B3/28
Filed: 1955-06-24
Abstract: 801,846. Electric copying machines. BRITISH THOMSON-HOUSTON CO., Ltd. June 8, 1956 [June 24, 1955], No. 18357/55. Class 40 (1). In a contour or profile tracing system in which a tracing head arranged for relative movement along two usually mutually perpendicular axes with respect to a model of an article, carries a deflectable stylus which in operation engages the model surface and traces along a contour or profile defined thereon, deflections of the stylus from a position of zero deflection towards which it is at all times biased being arranged to produce corresponding signals by which a servo mechanism including respective drive motors for the two axes is caused to control the relative movement along these axes so as to tend to cancel the stylus deflection, with the result that the tracing head is constrained to describe substantially the same course as is traced by the stylus, i.e. to follow the contour or profile, one or each of the drive motors is arranged to drive an actuating device for displacement between limit positions so spaced that the actuating device will be driven from one to the other in substantially the time taken for backlash to be absorbed, following reversal of the motor concerned, the actuating device being effective when adjacent at least one of the limit positions during such displacement to initiate a temporary overriding control action on the relative driving speeds of the motors of the system such as to reduce undue stylus deflection or the effects thereof. When the tracing head controls relative movement between a forming tool and a workpiece the backlash in the mechanical drive from the motors introduces errors in the deflection signals which prevent the workpiece from being a true reproduction of the model since when the stylus reaches a position on a curved profile at which one of the drive motors must reverse its motion the backlash must be absorbed before the tracing head responds to the change in direction and the stylus accordingly becomes unduly deflected; this results in a relatively high rate of motion after the backlash is taken up which leads to over-correction of the position of the tracing head in relation to the stylus and thus hunting is initiated. The overriding control action initiated by the actuating device may take one of three forms or any combination of these namely an increase of the speed of the motor near the beginning of the backlash period followed by braking of the accelerated motor to a crawling speed near the end of the period, a reduction in the motor speed only to a crawling speed near the end of the period or a reduction in the speed of driving affected by the other motor during the backlash period. As shown, the actuating device comprises a rotatable disc 1 coupled through a friction coupling 2 to a shaft 3 which is driven from the drive motor in question. The friction coupling 2 consists of a flange member 4 fast with a shaft 3 and having a friction lining interposed between it and the back of the disc 1, the latter being urged towards the member 4 by means of a spring 6. A thrust bearing 7 permits rotation of the shaft 3 with respect to the disc 1 when slipping occurs at the coupling. The disc 1 carries two projecting fingers 8, 8<SP>1</SP> which can be secured at any desired position in an annular groove 9 by means of screws 10. Two switches 11, 11<SP>1</SP> have their operating members 12 in the paths of the fingers 8, 8<SP>1</SP>. The shaft 3 is driven by the motor through suitable gearing so that in slightly less than the backlash period the disc 1 is driven through the friction coupling 2 from a position in which one of the fingers is in engagement with one of the operating members 12 to a position in which the other finger engages the other operating member 12; the arc through which the disc is driven is adjusted by varying the spacing of the fingers in their groove 9. The switches are arranged to control the motor; thus if the motor is to be reduced to crawling speed towards the end of the backlash period the switch which is operated near the end of the period initiates a temporary braking action on the motor which is terminated after a short interval by a time delay relay. If the motor is to be accelerated at the beginning of the backlash period the other switch can be arranged to initiate this. The switches can alternatively be arranged to reduce the speed of the other drive motor. Instead of employing switches the disc can be arranged to initiate graded control action at appropriate angular positions of the disc either electrostatically or electromagnetically.