Patent GB-1293759-A (GB)
Title: Machine tool control systems
Inventor: SHILLAM NORMAN FREDERICK
IPC: B23Q15/00
Filed: 1968-12-03
Abstract: 1293759 Measuring force electrically DEFENCE SECRETARY OF STATE FOR 4 Feb 1970 [3 Dec 1968] 57434/68 Heading G1N [Also in Division G3] In a control system for a machine tool, e.g. a lathe, wherein a cutting tool and a workpiece are moved relatively to one another, a transducer (e.g. semi-conductor strain gauge in a bridge circuit) senses stress on the tool in reaction to the feed motion of the tool (e.g. parallel to axis of rotation of workpiece) and control means responsive to a signal from the transducer adjusts the feed-rate of the tool so as to maintain a predetermined feed force on the tool substantially independent of fluctuations in the cutting speed and force produced in reaction to the cutting motion. The system further includes a feedback loop arrangement for controlling the cutting speed to maintain a predetermined cutting tool temperature and responsive to a thermo-electric voltage generated between the cutting tool and the workpiece. When the tool 21, Fig. 4, engages a workpiece 16 in a chuck driven by lathe spindle 15, both thermal e.m.f. (cutting speed) and tool force (tool feed-rate) signals will be developed and both control systems will switch to closed loop operation providing the values of those signals exceed the predetermined levels of respective threshold circuits 124 and 6c. Thus before the tool engages and when being repositioned (and under some fault conditions) the spindle speed will be relatively low being dependent upon the setting of a potentiometer 110 while tool force is determined by a potentiometer 8. Under 'outer' closed loop control the spindle speed and hence cutting speed are under the control of a potentiometer 100 and the tool force under control of a potentiometer 1. TOOL FEEDRATE CONTROL A saddle 18 carrying a tool holder 20 is arranged to be driven by a feed shaft 12 to engage the tool with the workpiece; the shaft being driven by a servo 11 via a gear-box 13. As the tool is moved parallel to the axis of rotation of and in engagement with the workpiece a compressive stress is applied to one side of the toolholder and is sensed by strain gauge 23 in a bridge circuit Fig.2 (not shown). When the threshold circuit 6c receives peak value signals above a predetermined value from the transducer 23 via units 26, 9 and peak follower 51, the circuit switches to its second state in which it enables AND gate 6a and inhibits AND gate 6b. Thereby the servo 11 control is modified by an outer feedback loop comprising strain gauge 23, detector 26 amplifiers 9, 3, peak follower 52, circuit 54 AND gate 6a, protection circuit 10 and servo 11. The amplifier 3 output is an error signal which corresponds to the difference between the actual value force signal in line 4 and desired value signal from potentiometer 1 so that the servo 11 is operated to reduce the error to zero. Warning of tool chatter is given by Neon lamp 57. CUTTING SPEED CONTROL A Ward Leonard system 109 drives the lathe spindle 15 in accordance with the setting of a potentiometer 110 when a tool temperature threshold circuit 124 enables an AND gate 108b and inhibits an AND gate 108a. This occurs when the tool is disengaged from the workpiece and no thermo-electric voltage V TW  is generated between the tool edge and the workpiece 16. When cutting, the thermo-electric voltage signal is fed back via an outer loop including an amplifier 115, peak follower 119, R123, amplifier 103. The output of amplifier 103 is an error signal which corresponds to the difference between the voltage representing the actual temperature and that representing desired temperature as set up on potentiometer 100. When the feedback voltage exceeds a predetermined level the threshold circuit enables AND gate 108a and inhibits AND gate 108b so that the Ward Leonard system is controlled by the output of amplifier 103 to alter the lathe spindle speed and hence cutting speed and so reduce the error to zero. The thermo-electric voltage V TW  is divided into two parts V SR  across R 113  which is connected via a brush and slipring 111 to the workpiece, and V TH  across R 117  which is connected to the tool holder. The holder being insulated from the machine frame and earth. These voltages are combined in the amplifier 115 to give a feedback voltage V TW . SWARF CONTROL Accumulation of swarf about the tool edge 21 and workpiece tend to produce attenuation of the thermal e.m.f. In the present arrangement short circuit by swarf between the workpiece and machine frame short circuits R113 thereby increasing the thermo-electric current and voltage V TH  across R 117 . Swarf threshold 116 compares V TH  with a reference voltage set up on a pot 129 and when V TH  exceeds the reference it switches a bistable unit 125 to its "1" state whereat a lamp 126 is lit to attract the lathe operator. At the same time an override signal is sent to threshold circuit 124 switching 124 to its first state so that the spindle speed is governed by the pot 110 regardless of the other input signals to circuit 124. When the spindle speed is such that there is abnormally low or no output from tacho-generator 14, and AND gate 10a is inhibited so that the tool feed is stopped, the servo 11 input being earthed via R10b. The threshold circuits may comprise Schmidt triggers.