Spotwelding for home shop

A cheap transformer type arc welder can be used to make a spot welder for steels. A capacitive discharge spotwelder is needed for nonferrous metals. The basic requirements for a spot welder are (1) heat, (2) pressure, and (3) time. These three things are individually adjusted to produce the required weld relative to need, metal thickness, and metal type, whether stainless, or carbon steel.

The steps of operation required are as follows: place metal parts to be welded between tips. Apply proper clamping pressure. Fire proper electrical amperage through the tips for a prescribed time, then shut off. Release clamping pressure of tips. The proper transformer size or welder is used for electrical supply to flow through the welding tips, which can be made from Mallory 3 copper, available in stick form from a spot welding supply house. The tips can be solid for short-duty home shop use, or hollow water cooled for more rigorous welding duty.

In basic form, the welder is the power supply. An electrical contactor of proper amperage rating is used to control electrical power to the SUPPLY, or PRIMARY side of the welding transformer, as a smaller contactor can be used, relative to trying to control the secondary side. A pair of welding tips must be arranged, and electrically isolated from each other, to apply weld pressure to the pieces being welded. An analogy would be to place your right and left hand index fingers together tip to tip, and pushing them towards each other with force.

This can be accomplished by attaching tips to either a mechanical pincher device, or an air cylinder, in order to pinch the tips together. If the welder is to be mechanically operated, a compression spring can be provided to work as an overtravel device, so that after the weld tips are brought together, an electrical limit switch can fire the weld contactor after the proper pressure is applied. Once the contactor is fired, the engagement time is controlled by an adjustable timer, to control the weld time. Remember, heat, pressure, time?

If an air cylinder is used to apply pressure to the tips, a pressure switch reading the pressure applied to the air cylinder can be used to fire the contactor after the proper clamping pressure is applied to the tips, and pressure rises in the cylinder. A timer is once again used to determine how long the contactor stays engaged. If one studies the individual steps involved in a basic spot weld as I have described here, it's easy to see that the mechanical application of pressure to the welding tips is the "hard" part of constructing your own spot welder, whether it is to be portable or bench mounted.

This information is repeated as an answer in Willy3486's thread link below. If you have spotwelding experience to share, either in constructing or using a unit, tip selections, etc, please do so.

http://www.garagejournal.com/forum/showthread.php?t=137079
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I have the HF spot welder. I'm not impressed. It does ok, but the contact points are fairly small and I'm reluctant to make them bigger for fear of drawing to much current through and cooking it.

On a water cooled spot welder, the tips are hollow, and may have various types of specialty metals applied to the working end of the tip, as sometimes, a special resistance may be required at the contact area. The machine may have a tapered female holder for the tip, similar to a morse taper, to hold and seal the tip in place. A small diameter tube with an angled end protrudes into the hollow of the tip, and water is flowed down this tube, and back out alongside the tube to be recirculated and cooled, or dumped to waste. Welding tips can be customized or simply refreshed on a metal lathe as they become mushed during use, which will create substandard welds. The welding tips I deal with at work vary in working diameter from 3/32" to 1/4" in diameter. We have several custom shop built units being used to spot weld fairly small parts, along with purchased units in use.

Ascinder said:

I have the HF spot welder. I'm not impressed. It does ok, but the contact points are fairly small and I'm reluctant to make them bigger for fear of drawing to much current through and cooking it.

Click to expand...

You could try reading your primary input amperage with an Amprobe, and adjusting the size of your welding tip to keep the current at an acceptable level to protect the transformer. Is the input current marked on the unit?