• Machine comes down and goes up before touching parts
• Machine comes down on parts, then stops and stays without welding
• Machine comes down on the welds, but does not go back up

When a resistance welding machine starts, but then does not complete the cycle, there is ussually an I/O signal missing back to the controller/timer. There are several Inputs that could cause this to happen. The problem is even more complex to troubleshoot when a PLC or other controls are interfaced with the RW unit or controller. We'll go over just the RW timer/controller.

First, if it comes down and then goes back up it could be one of two things. Either the squeeze-weld-hold cycle was too short (or the cylinder too slow) and the unit thinks it made a weld without going down all the way or the latch for the welding (foot pedal switch, palm buttons etc.) was released accidentally, causing the machine to go back up for possible safety reasons due to initiation release prior to completion. On some controllers an improper amount of weld cycles could cause a weak weld.

You may also experience the machine coming down and stopping or coming down and "locking up". This is most often a problem with a water flow switch, pressure switch, zero-cross-over-detection, temperature switch for the TX or the SCR, etc. In this case the error codes in the unit should lead you to the correct wires to check. If there is PLC redundancy then the issue is even more complex. Most of the time it's a pressure switch. Basically, the input needs to be there when the timer wants to see it. Sometimes if the input is not there then the unit will timeout and stop in the down position.

Sometimes timers/controllers are set up for checking things after the weld has been made. This is a quality issue or possibly more PLC I/O. Often seperate weld checkers will "pull out" an Input signal to lock up what might be a bad weld. There might also be interal current control montoring.If this is the case, the machine will stay down to signal a bad part, even though it may actually be a good weld. This is a weld checker problem and depends upon how it is interfaced with the robot, machine, or controller/timer.