Patent GB-1215822-A (GB)
Title: Machine tool apparatus
Inventor: WATT GORDON JAMES
IPC: B23Q15/18
Filed: 1968-02-13
Abstract: 1,215,822. Automatic control systems for machine tools. SPERRY RAND CANADA Ltd. Feb.13, 1968 [Jan.27, 1967], No.7041/68. Heading G3R. [Also in Divisions B3 and F2] Automatic machine tool control apparatus for compensating for unwanted relative displacement between the tool and a workpiece when the two are in contact with each other comprises sensing means providing an electrical signal indicative of the displacement and an actuator mechanism responsive to the signal to compensate for the displacement, the mechanism including a pair of parallel spatially separated members having a pliable ligament extending therebetween and an electro-mechanical transducer disposed between the members.  In the described embodiment flexing of a cutting tool 11, due to variations in the radial force between its point 38 and the workpiece 54, is automatically compensated by a control arrangement comprising strain transducers 13, 14 which detect tool flexure in the direction of arrow 68 and supply an error signal to a transistorized control unit 62 whose output is fed to Piezo-electric stacks 17, 17<SP>1</SP> to cause a contraction and expansion of respective ones of these stacks and so produce a resulting restoring action opposing the flexure. The latter effect is achieved by a tool construction Fig.1 in which the stacks 17, 17<SP>1</SP> are compressed between the rear surface of a tool holder 32 and a spacer 30 by the engagement of a mounting fixture 35 with a support ring 31 connected to the holder 32 by flexible strips 33, 34. Each stack comprises alternate layers of flat polarized ceramic sections 19, 20, 21 and flat copper segments 22-25 bonded together by silver mesh and cement, alternate copper segments 22, 24 and 23, 25 being respectively connected to copper rods 26, 27 which receive the output signal from the control unit 62. The transducers 13, 14 are fixed to diametrically opposite sides of the tool shank 15 and comprise respective resistors and strain gauges connected to form a bridge circuit Fig.2 (not shown) whose output is supplied to the control unit 62. The latter unit consists of pre-amplifier input stage, a buffer stage and an emitter follower differential power amplifier output stage. After moving the tool 11 into position by transport mechanisms 58, 59, 60, final precision adjustment of the tool with respect to the work is made by adjustment of a bias control 61 in the unit 62, which adjustment causes contraction of one stack and expansion of the other to bring the cutting point 38 to its desired position. The Piezo-electric stacks may be replaced by thermal or magnetostrictive transducers and the strain transducer bridge circuit may be temperature compensated. Other modifications envisage the use of a single strain transducer or pairs of transducers placed to detect tool flexure in desired directions, e.g. two transducers in spaced quadrature or two pairs in mutually perpendicular planes. If the machining process is performed with a stationary workpiece and a rotating tool, the control signals between the transducers 13, 14, the stacks 17, 17<SP>1</SP> and the unit 62 are transmitted through slip rings, the preamplifier stage of the unit being mounted on the tool. The control arrangement compensates for tool flexure caused by other factors (e.g. incorrect feed rate, blunt tool) and may be made frequency responsive to prevent instability when the natural resonant frequency of the tool structure is within the bandwidth of the Piezo-electric stacks. To this end the unit 62 may include a lowpass filter network with a suitably chosen upper cut-off frequency. Fine positioning of the tool may be effected automatically using an integrator connected between the unit 62 and the stacks and coupled to the unit input prior to an operation. Application of the invention to grinding a crank-shaft is mentioned, the bending of the shaft being sensed and compensated by a tool rest member which is positioned against the shaft diametrically opposite the grinding wheel contact point and which includes Piezo-electric stacks operating in an identical instead of the differential longitudinal manner of those shown in Fig.3