Spot Welding

Questions and Answers

Several sources are available to select the electrode material for spot welding. If the material to be spot welded it known you are ready to proceed. Two sources for information are:

AWS Standard C1.1 Recommended Practices for Resistance Welding

RWMA – Resistance Welding Manual 4th Edition

Stainless steel is a relatively strong resistive material. This means that in electrodes we will need strength but not necessarily a high conductor. The recommended electrode material for stainless is RWMA Class 3. This is the strongest of the copper alloy electrode materials but still has good electrical conductivity. The second choice would be RWMA Class 2 which is not as strong but has better conductivity.

The chart below shows electrode material selections for various spot weld material combinations.

Recommended Electrode Materials Guide

For additional information on this topic see article:


Reference: AWS C1.1 Recommended Practices for Resistance Welding
                  RWMA Resistance Welding Manual 4th Edition
                  Tuffaloy Products Catalog and Website
                  CMW Inc. Catalog

Pressure is one of the major components of the welding process Pressure, Current & Time (PCT). In most weld schedules pressure is called out as a force value. Pressure is this same value as force but applied to the surface area of the electrode contact face.

See the article - How are force and pressure related?

The relationship between pressure and the heat generated during resistance welding is well established. It is represented by the figure below.

Pressure vs heat14864

Increasing pressure will reduce the overall resistance and decrease the heat generated. Conversely reducing pressure will increase the resistance and increase the heat. The question asked was how does force affect the process. Pressure is that same force applied to a specific surface area (lbs/in2). Therefor increasing or decreasing force has the same effect as pressure.

To answer this inquiry we will start with a previously published question and expand upon it.


Per the definition of the Resistance Welding Manufacturers Alliance:

Definition of Resistance Welding

To expand upon this PRESSURE is provided by mechanical means of pneumatic or hydraulic cylinders, servos, cams and sometimes manually. Normally it is measured in pounds, kilos or equivalent. It is applied in sufficient amount to contain and control the weld to a specific desired area, before the current is applied. CURRENT is supplied by the control and transformer. The transformer does just that transforms relatively small current input from the buss to many thousand amperes needed for resistance welding at people safe low voltages. The control determines the amount of CURRENT and the TIME.

Heat is generated according to JOULES LAW.

Joules Law

The heat is generated by the current squared times the resistance multiplied by the time. It is the current flowing through the area to be joined for a length of time and the resistance at that joint which generates heat to make a joint. The force holds all of this together and forges the joint as it forms and cools down.

AC resistance welding would use AC current and transformers for the power in resistance welding. Most rocker arm and press welders have been manufactured with AC power supplies. The equipment can also be DC direct current. In the last thirty years MFDC mid frequency direct current is has become the go to equipment for automated robotic systems and many other applications.

There are many articles published in this forum about the advantages and disadvantages of the AC, DC and MFDC systems. One can find these by performing a search on the home page of this forum.

Reference: RWMA – Resistance Welding Manual 4th Edition



This question is getting into the area of physics and varies with the frequency of the input power.

The core is based upon flux density and can be calculated based upon physics calculations found in text books presented here.


Any further discussion is beyond the scope of this forum.
Contact a resistance welding transformer manufacturer for additional assistance.

The answer to this question is – it depends upon the geometry/design of the part, the nut being welded and safety. M6 weld nuts come in many shapes and sizes.

M6 Weld Nuts

               Assorted M6 Weld Nuts

Workpieces also vary in shape and size. Clearance for the electrodes will be determined by the geometry of the parts involved and the fixtures and tooling being used. There is no set value for spacing other than clearance to load the parts safely and not too much clearance to make the travel time too long for the electrodes to close.

After part design and clearance are determined what is left is safety and time. Time considerations are determined by the welding and loading equipment capability and the plant investment in tooling, automation and other factors.

Safety of personnel operating the equipment is a factor that must be considered in the design and layout of the equipment. Various safety codes could apply to the operation and determine pinch point, light curtain, lock out or other safety needs. A starting point to determine these needs could be ISO 12100 Safety of machinery — General principles for design — Risk assessment and risk reduction.

Automated or manual nut welding operations all require evaluations for safety and part clearance to determine the proper electrode opening. No two operations are alike therefor there is not a set answer to this question.

For economics and safety one would not open any more than is necessary to load the part. Safety must be built in to keep the operators hands out of the pinch points. This reduces electrode travel time and reduces room for an operator to be hurt.

Reference: ISO 12100 Safety of machinery — General principles for design — Risk assessment and risk reduction.

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