what is the difference between projection and spot welding? Whether projection weld and spot weld can be done in same machine? What exactly decides the welding practice is projection weld or spot weld?

-- D Ravindran (ravin.d@mailcity.com), March 12, 2002

Answers

A spot weld is made by concentrating electrical current through copper electrodes with a contact surface of about 1/4 inch diameter. This gives approximately 200,000 amps per square inch, and 10,000 amps is enough to raise the steel to its melting point.

Projection welding is done by concentrating the current with protrusions on one of the parts. The copper electrodes are still used to supply the current, but are usually larger, and longer lasting.

The Ohio Nut and Bolt company has a lot of on-line information about projection welding, and even offers some electrodes that look like they may fit on a regular spot welder's shanks.

The decision as to which process to use may depend on a number of things. I feel that spot welding is the most economical, but it is limited to sheet metal. Where fasteners are involved, projection welding must be used, as the contact area is too large to concentrate the current. I have seen projection welding used in sheet metal to control exact spot placement where close tolerances are involved. Where welds must be placed close together, they must be done all at once, and projection welding often is used this way. Welds made within 2 inches of a previous weld lose a lot of current through the shunt path of the previous weld, which has only 1/20th of the resistance of the new welding site. There are minimum spot spacing rules, based on material thickness, stackup and coating.

My experience is that projection welding is easier to maintain, and requires a lot less troubleshooting knowledge, although it does require careful attention to surfaces, fit-up and alignment.

-- David Bacon (dbacon@updatetechnology.com), April 20, 2002.