Controls & Transformers
Questions and Answers
Squeeze is the first segment of the weld schedule. It’s function is to apply force on the work pieces to bring them together. This pressure under the face of the electrode in turn makes good electrical contact and controls and contains the resultant weld nugget during the weld and hold portion of the weld cycle. If current is applied prematurely before the proper force has been reached these functions will be compromised and expulsion and poor nuggets should be expected.
Squeeze time is the first sequence of a weld schedule. It is designed to allow the electrodes to close and build up full force on the parts before current is applied. If squeeze time is too long it does not hurt the welding operation but it does increase the cost of the total operation by increasing the total time of the weld schedule and thus fewer parts per shift.
Spot welding can decrease in weld size as you weld across the part. One cause for this is increasing magnetic material in the throat of the welder as you move across the part. Most parts are low carbon steel which is a magnetic material. In AC welding applications, if after each weld more material is moved into the throat of the machine, the weld current will decrease.
METAL IN THE THROAT CAUSES IMPEDANCE TO INCREASE
Cracks appearing in the parent material around the weld nugget can be caused by several issues. They are:
High Weld Force – can fracture the parent material with excessive forging action.
Insufficient Cooling – Over heating can cause excessive stresses from expansion and contraction along with the forging action.
Metallurgy of Material Welded – Some materials become brittle at the temperatures experienced in resistance welding.
Surface expulsion is caused when the part surface heats up. So anything which would cause this, could be a reason. The appearance is similar to interface expulsion. In this case we are looking for conditions which cause the contact surface heat to be high.
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