GB802206A - Improvements relating to the automatic control of machines for shaping workpieces - Google Patents

Abstract

802,206. Programming for machine-tools. ELECTRIC & MUSICAL INDUSTRIES, Ltd. Oct. 15, 1954 [Oct. 28, 1953 ; Feb. 2, 1954], Nos. 29734/53 and 2997/54. Class 83 (3). A machine for automatically shaping a workpiece comprises cutting &c means 3 for traversing the workpiece, means 18 for sensing the shape of the work in the rear of the cutting- means, and means for controlling the position of the cutting-means during a succeeding traverse to produce a more nearly predetermined shape, in response to the output of the sensing means during a preceding traverse. In a machine for milling a cam 2 from a blank 1, the milling-cutter 3 is controlled by a servomechanism receiving its input from an electromagnetic pick-up command head 5 co-operating with an endless magnetic-tape 6. The tape is driven by sprockets 8, 9. Alternatively, the tape is provided with pulses serving as the magnetic equivalent of sprocket holes. The sprocket 9 serves to pull the tape past an inspection head 7, the two heads 5, 7 being positioned to co-operate with separate tracks 16, 17 on the tape. A probe 18 to the rear of the cutter transmits a signal through a mechanism 19, the output of which may be determined by the pressure on the probe 18 maintained constant by servo-mechanism moving a leadscrew. The voltage output from the mechanism 19 and from the inspection head 7 pass to a computer which subtracts the signals and records commands for the cutter 3 on a second command-track 22, through a head 21. The continuous tape completes a cycle while the work completes a rotation and the loop in the tape corresponds to the distance between the cutter 3 and the probe 18. In operation, the required radius of the cam 2 at each of a series of angular positions 13 recorded on the inspection-track 17. A series of commands for the cutter 3 is recorded on the command-track 16, taking into account the maximum practicable rate of cutting and the maximum ehange-ofrate of cutting. The first cut may thus be on the line 25. During the cut, the computer 20 computes the best value for the next cut by a differencing-circuit 26, a potential-divider 27, a transformer 28 for adding the fraction of the difference to the signal from the head 7, and a limiter 29 to limit the maximum cut. These commands are recorded on the second track 22 by the head 21, which is aligned with the head 7 if there is no delay in the computer 20. At the end of the first revolution of the work, a switch-operated mechanicaly, or by a pulse on the tape, causes the servo-mechanism to receive commands from the second-track through a second command-head. The commands on the track 22 are continuously erased by a head 24. The cycle of operations is then repeated using the track 22 until the computer signals that a predetermined tolerance has been reached and stops the machine for the finished and inspected cam to be removed. Instead of recording the command and inspection-signals continuously as an alternating-signal having an amplitude analogous to the corresponding dimensions, the signals for driving the servomechanism being amplified and rectified as required, the circuit may be modified to provide a bank of aligned heads, in place of each of the heads 5, 7, 21, co-operating with a binary digital-pulse-code. Information is then recorded only for discrete angular positions of the work, and digital-to-analogue signal-converters followed by an interpolating device are introduced between the bank of heads corresponding to head 5 and the servo-mechanism 4, and between the bank corresponding to head 7 and the computer 20. The converters and interpolator may be as described in Specification 796,993, [Group XXXVIII], and 796,994, [Group XXXVI]. The code is recorded on a continuous punched-tape, and the commands on two tracks on a continuous magnetic-tape, the tracks being used alternately. Because the lead-screw of a servo-mechanism may be of slightly non-uniform pitch or the tool may be worn, the command may not be carried out correctly by the tool. The signal from the sensing-head is therefore compared with the command being erased and any difference, which represents the inaccuracy of the machine in carrying out a command, is added or subtracted to the new command-track being recorded. In a modified machine, the roughcuts are taken by one tool and a second tool is moved to take the finishing cut by a signal from the computer. The tool and sensing-means may be moved instead of the work. Specification 796,995, [Group XXXVIII], also is referred to. Reference has been directed by the Comptroller to Specification 779,348.