Patent GB-1202990-A (GB)
Title: Improvements relating to flash butt welding
IPC: B23K11/04
Filed: 1968-08-15
Abstract: 1,202,990. Flash-butt welding control systems. AMERICAN WELDING & MFG. CO. 15 Aug., 1968 [23 Oct., 1967], No. 39138/68. Heading H2H. A flash-butt welding system comprises means whereby the movement of the workpieces towards each other during various stages of the welding process is controlled in accordance with the volt drop between the surfaces of the workpieces. A flash-butt welding process comprises four stages, pre-flash; pre-heat; flash and upset, the movement of the workpieces during the pre-flash and pre-heat stages being controlled by the volt-drop between the workpieces. Prior to the beginning of a welding cycle the relative positions of the platens 12 will be at a predetermined distance from one another. When the welding cycle is initiated the platens 12 are moved towards each other by an hydraulic platen motion control 10 during which time an arc is formed between the workpieces during the pre-flash period to burn off any projections from the surfaces to be welded together. During the subsequent pre-heat portion of the welding cycle the arc heats the surfaces to a near molten state. When these two steps are completed the platens are moved together at a relatively high rate to effect the flashing portion of the welding cycle after which the workpieces are forced together in the final upset portion of the cycle to complete the weld. During the pre-flash and pre-heat portions the movement of the platens is under the control of signals from a circuit 18 by way of a servo amplifier 14 and the platen motion control 10. The pre-flash stage is controlled by a first reference signal V modified by the volt-drop between the workpieces, i.e. the die feed-back signal FB.V; the pre-heat stage being controlled by a second reference signal V similarly modified by the signal FB.V. Whilst the platens are moving together in these two portions of the welding cycle a pulse generator 20 generates a series of pulses which are counted by a digital counter 22. When a certain number of pulses have been counted the counter 22 sends a signal to the circuit 18 whereupon the circuit 18 switches from the first reference signal to the second reference signal thus ending the pre-flash period and beginning the pre-heat period. The pre-heat period is timed by a timer such as a delayed operation relay. When a sufficient time has elapsed, the circuit 22 causes the circuit 18 to apply a third control voltage to the servo-amplifier 14 which signal is derived from a parabolic function generator 26. Thus the motion towards each other of the platens during the flashing period is controlled by the function generator only. When a sufficient number of pulses have been counted indicating the end of the flashing period a second signal is sent from the circuit 22 whereby the platen motion control is then controlled by an upset control circuit 16 only to effect the final, upset, period of the welding cycle. This period can be terminated by a timer or manually by an operator after which a signal from the circuit 18 returns the platens to the positions from which a welding cycle begins. The movable platen is connected to the circuit 18 by a mechanical link 28 to ensure that the movable platen does not have to undergo a large abrupt movement at the beginning of a welding cycle.