Patent GB-1142566-A (GB)
Title: Control system for grinding machines
IPC: B24B21/00
Filed: 1966-06-14
Abstract: 1,142,566. Grinding. CARBORUNDUM CO. 14 June, 1966 [17 June, 1965], No. 26484/66. Heading B3D. [Also in Division G3] A method of maintaining substantially constant grinding pressure between an electric motor driven abrasive element and a workpiece comprises conveying the workpiece between the abrasive element and a back-up roller, sensing motor current, comparing said motor current with a reference signal to obtain a resultant signal, and adjusting the grinding pressure in response to said resultant signal by adjusting the spacing between said abrasive element and said back-up roller to maintain the grinding pressure substantially constant. An abrasive belt 14 is driven by an electric motor 11. Workpieces 16 are fed by a conveyer belt 19 mounted on a work-table 15 vertically adjustable by screw jacks 20 driven by a reversible motor 21. A back-up roller 23 mounted in the work-table 15 is actuated by an hydraulic cylinder 25, the depth of metal removed from the surface of a workpiece depending primarily on the force imposed by the roller. The flow of hydraulic fluid in conduits 26, 27 connected to the cylinder is controlled by an hydraulic servo control 28 controlled by a control circuit 29, a control signal in which is responsive to the current in the motor 11. In the control circuit the primary winding 101 of a transformer 102 is connected in series with one of the 3-phase connections to the motor 11; the potential produced by the secondary winding 103 is dropped across an adjustable resistor 107. The primary winding 105 of a transformer 104 is connected across two of the 3-phase connections to the motor 11; the potential developed across the secondary winding 106 is dropped across a reactor 108, having an adjustable tap 110 for adjustment of the reference level of the signal. With proper adjustment, the system responds only to changes in grinding pressure. An inductor 111 receives a certain proportion of the outputs of the transformers 102, 104 and mixes these to develop a combined E.M.F. across the inductor 111. Currents set up by the potential dropped across the inductor 111 are rectified by rectifier 112, the output of which is smoothed by a filter network and flows in a circuit 116 including the control winding 120 of an electrically controlled servo valve 121. The primary winding 124 of a transformer 123 is connected to a source of A.C. potential. The potential developed across the secondary winding 125 is rectified through rectifier 126, smoothed by a filter network and develops a constant potential across Zener diode 130 to develop a current through an adjustable resistor 131 and a control winding 122 of the servo valve 121. To ensure that the signals in control windings 120, 122 bring the servo valve to the correct control condition, despite friction or viscosity, a small A.C. signal is fed into the circuit 116 through a transformer 132a fed from a tapped reactor 133a connected across the secondary winding 125 of the transformer 123. Hydraulic fluid is passed by a constant flow pump 134 to an adjustable relay valve 137. A by-pass return conduit 138 leads from the valve 137 to a valve 139 to return the fluid to the tank 132b; the valve 139 is set to open when the pressure is slightly above atmospheric to prevent fluid draining from the system when it is not operating. The relief valve 137 is manually adjusted to allow the pressure to build up above that required for grinding. A conduit 140 connects the relief valve 137 with the servo valve 121 and includes a needle valve 141 and a filter 142. By bleeding a controlled flow of fluid through the conduit 140 back into the tank 132b, the hydraulic servo control system causes the relief valve 137 to regulate the pressure at the cylinder 25. Fluid from the relief valve 137 passes to a reversing valve 143; when solenoid 144 is energized, valve 143 is moved to a position in which fluid is supplied to urge the back-up roller 23 toward the abrasive belt 14; when solenoid 144 is deenergized, spring 145 moves valve 143 to a position in which the back-up roller 23 is lowered. The conduits 26, 27 between the valve 143 and cylinder 25 are provided with means for permitting free flow to the cylinder but restricted flow from the cylinder. The servo valve has a spool 153 connected to an armature 150 pivoted at 151 and acted on by the control windings 120, 122. When the grinding motor current is greater than the preset reference value the spool is moved to the left allowing fluid to flow from the conduit 140 to the tank 132b; the degree of displacement of the spool depends on the extent of the unbalance of current in the control windings 120, 122. A stationary back-up roller may be used, the grinding head being movable to maintain constant grinding pressure or both the back-up roller and grinding head may be movable but in opposite directions. The system may be used for conditioning strip steel or expensive metals, e.g. alloys of rare metals, and also for weld skipping.