Spot Welding

Questions and Answers

This question comes up frequently. Many look for a formula to set the squeeze and hold time for resistance welding.

Several articles have been written on this subject in this blog:

HOW DO YOU SET THE PROPERSQUEEZE TIME IN A RESISTANCE WEDING SCHEDULE?

IS THERE A FORMULA TO DETERMINE THE SQUEEZE TIME IN RESISTANCE WELDING?

IS SQUEEZE TIME IMPORTANT?

HOW WILL HOLD TIME CHANGE THE RESISTANCE WELD STRENGTH?

Resistance brazing is not spot welding so the electrode face geometry does not have to concentrate the heat into a small area. Frequently the electrode face is machined to conform to the surface that it is in contact with. Frequently the electrode will contact the entire work piece surface. In some cases the electrode faces is milled out. The work piece is nested into the face of the electrode in order to insure proper part placement and alignment. A small vacuum is sometimes pulled through the center of the electrode to hold the part in place before the electrode closes. This is very common for small part brazing.

The electrode material is frequently RWMA Class 2. It could be ETP copper and in many cases carbon electrodes are used.

Refractory faced electrodes2

When resistance brazing contact materials, refractory faced electrodes RWMA Class 11, 13 and 14 as well as Class 2 are commonly used.

Pressure, current and time (PCT) are the basic functions performed by the resistance welder. These functions are controlled or initiated by the weld controller. It initiates each step when told to start the weld process by input from the foot switch or automation PLC. The controller allows time for each step to operate and controls the current amplitude. In simplified form this is Squeeze, Weld and Hold. The squeeze sequence allows the pressure system to build up the force to contain the weld. The weld function is the actual current flow and is totally controlled by the weld controller. It regulates the amplitude and time of current flow. Hold is the period which allows the weld nugget to cool down and solidify under force. The controller regulates this time.

MFDC systems over the last twenty years have been the dominant new welding system. It has found usage in most new installations. That is not to say that AC systems do not have their place in the industry. They do and will continue to be used. Below is a comparison of the two systems pro & con:

Comparison of MFDC and AC 2

 

An inquiry came in suggesting that two forces are responsible for creating a resistance weld. Let’s examine this question. To do so we will look at two previously published articles in this blog:

“WHAT IS AC RESISTANCE WELDING”

AND

“WHAT IS THE DEFINITION OF RESISTANCE WELDING”

Per the definition of the Resistance Welding Manufacturers Alliance:

RESISTANCE WELDING IS THE JOINING OF METALS BY APPLYING

                                        PRESSURE
                                     AND PASSING
                                        CURRENT
                                FOR A LENGTH OF
                                           TIME
    THROUGH THE METAL AREA WHICH IS TO BE JOINED

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