Spot Welding
Questions and Answers
Generally projection welding electrodes are large flat surfaces. Sometimes holes are incorporated for locators (nut welding). In crosswire welding the lower electrode may be a single long bar of copper alloy or refractory material spanning many welds. The electrode delivers force and amperage through a relatively large surface area into the part being welded. Secondarily they provide cooling of the part.
The part by design concentrates the current and force to create heat and forging at a concentrated location. Projection welding electrode life is long when compared to spot welding electrodes. The wear is frequently mechanical abrasion. RWMA Class 2 is the first choice for electrode material in projection welding. If mechanical wear and tear is a little heavy and Class 2 has to be faced too often, try RWMA Class 3. It is mechanically harder and more resistive to wear than Class 2. If there is a lot of heat and excessive wear turn to RWMA Class 11, refractory metal electrodes may be the best choice. Nut welding sometimes is a heat generator and the Class 11 refractory electrodes are successfully used. Electrically Insulated locator pins are employed when needed.
Reference: AWS C1.1 Recommended Practices for Resistance Welding
AWS J1.3 Specification for Materials Used In Resistance Welding Electrodes and Tooling
AWS J1.2 Guide to Installation and Maintenance of Resistance Welding Machines
It will be assumed that a 1000 volt primary supply is connected to the primary of a resistance welding transformer. The question is should this produce amperage on the secondary? Assuming that the control is functioning, wiring is correct and the transformer is good the answer is yes. There should be voltage and current at the transformer secondary terminals. The amount depends upon the equipment.
This means that many items need to be verified to insure that all is correct. If all is correct and there is no voltage and current present at the secondary, back up the circuit and test.
Is there voltage at the primary?
Is the transformer grounded properly? If the transformer is failing to ground it should have tripped a breaker. In either case investigate the transformer.
Is there a properly sized breaker on the system?
If the transformer is good, move up a level.
Then back up another lever head to the control etc. Check all connections and fuses. The control could be failing and not activating the SCR. A component has failed or is wired improperly somewhere. Search till it is found.
A conductor, fuse, component has failed somewhere in the system. A 1000 volt connection will flow if connected and the components are functioning properly.
Reference: RWMA - Resistance Welding Manual 4th Edition
AWS - J1.2 Guide to Installation and Maintenance of Resistance Welding Machines
Automated lines are very common in today’s factory. In the resistance welding world this means the parts are automatically brought to a work station. A robot with a weld gun is activated and makes a series of welds. It pulls back and waits or dresses the electrodes at an automated dressing station during the part index time. Sometimes these dresses happen after each part, sometimes after several parts. The goal is that the electrodes are generally dressed frequently but lightly and kept fresh for a long life.
These automated dressing systems are incorporated into the weld systems. Often the dressers are built and designed by the system integrator. In some cases the integrator incorporates dressers manufactured by others into their systems.
PEUMATIC HANDHELD DRESSER - NOT AUTOMATED
In either case automated equipment systems are outside the area of this forum.
To find information on this subject contact your local equipment or system supplier or resistance welding distributor.
RESOURCE: RWMA - Resistance Welding Manufacturing Alliance has a listing of member companies who could be of assistance in this endeavor.
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.
For additional information on this topic see article:
HOW DO I SELECT THE PROPER ELECTRODE MATERIAL FOR SPOT WELDING?
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.
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.
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