Spot Welding

Questions and Answers

The answer is yes. MFDC can be used to heat 10 mm bars.

In this application the equipment could be running nearly continuously. In the design of the equipment care must be taken to make sure the equipment is cooled adequately to insure it does not overheat.  Additionally the MFDC equipment must be sized properly.

Properly cooled and sized MFDC will handle this job nicely and perform well.

This is a classical application of Joules Law:

Joules Law

The heat is being generated by the resistance to the flow of current through the material.


Reference: RWMA, Resistance Welding Manual 4th Edition

Voids and cracks are not desirable in any resistance welding nuggets. In some material systems there is a tendency for cracks to form – such as Aluminum. In other materials oxides and surface impurities can end up in the nugget and voids or cracks form. Some cracks or voids form due to the volume expansion and contraction from liquid to solid state.

Depending upon the material and product and the industry there are specifications. Aircraft has specifications. Automotive has specifications. Individual companies have specifications.

Per AWS D8.1 Specification for Automotive Weld Quality – Resistance Spot Welding of Steel

Automotive specifies that cracks or voids lengths combined in the specified weld nugget area cannot exceed 25% of the nugget width. Therefor 1/3rd the nugget diameter is not acceptable in the automotive industry.

Per the definition of the Resistance Welding Manufacturers Alliance:
                                                AND PASSING
                                            FOR A LENGTH OF

Spot welding is one of the four resistance welding processes as defined by the Resistance Welding Manufacturing Alliance. The four processes are:

            RWMA CD containing 2 day program 390


There ideal answer is never. Given reality this is not likely. In normal resistance welding on coated material, a spot welding or seam welding wheel is going to react with the coating and wear at some accelerated rate. This will be faster than on bare stock. Dressing the welding surface will be necessary at some point. That point will be determined by the speed of the wear and the equipment being used.


Yes, changing the transformer turns ratio will change the current. A transformer with a tap switch works on this principal. The tap switch is wired to various points in the primary winding in order to vary the turns ratio. By changing the tap switch position the turns ratio is changed. One can choose the lowest voltage and resultant current in Tap 1 or the highest voltage and current output in the highest Tap. The number of taps and their range depend upon the transformer design.

Tap switch

Press Welder Transformer Tap Switch

Many weld transformers are manufactured with a tap switch. This tap switch gives one the ability to change the turns ratio as needed.  The net result is the output voltage changes up or down as the tap is changed.

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