The Home-Brew Spot Welder

5 Bears Home

A spot welder is a necessity for the production of a homemade gas turbine, unless you can accurately and easily TIG weld a LOT of very fine stainless steel, on the order of 0.5 mm. Commercially produced spot welders have several things wrong with them... first, they are rather expensive, typically $250-$500 U.S. Secondly, the welding tips, where the actual fusion occurs, are relatively thick, and difficult to squeak into tight crevices like the bottom of a combustion chamber. They are also mostly hand-held devices, which would make the spot-welding of small parts challenging. So I guess if I was welding up a bed frame, a commercial welder would suit, but not a 2" dia. combustion chamber.

Thus, the need for a custom spot welder. Desireable attributes are a high quality, repeatable weld, ease of use, and low cost.

All of the parts for this welder (except the transformer) are available from MSC. The transformer must be scrounged from an electronics/industrial surplus outlet. Look for a BIG 115VAC transformer (700 Va or better) with the secondary windings on the OUTSIDE. Listed below are MSC parts numbers, and current prices.
Timer Relay 54023122 $58.26
Octal Socket 54038666 $7.40
Foot Switch 54097191 $17.10
Solid State Relay 54050372 $56.06
Transformer Surplus  
Schematic diagram. For simplicity, I depict the Timer control relay as a box with the necessary pin numbers depicted. Use at least 16 guage stranded copper and crimp or solder suitable terminal connections. The octal socket is required for the timer.

Please, please don't attempt something like this unless you truly understand the hazards involved. You must be adept at basic wiring, and must understand how a transformer operates. There are lethal voltages involved!

An overall view of the completed welder. Plans were "created" on the spot, using knowledge gleaned from the internet on such a project. The basic concept is to find a big 115VAC transformer which has the secondary windings as the outer layer. The secondary is carefully (but tediously) removed, and replaced with enough coils of #4 copper cable to create a secondary voltage of ~4 VAC. a DeStaco clamp is adapted for the upper arm, while the lower arm is fixed, but insulated.

There are two relays... This one is a control relay, c/w microprocessor, variable timing, and variable logic. This little jewel costs $60, and is worth every penny for the excellent functionality it delivers. A foot switch activates the selected cycle. I am using the one shot logic which simply energizes the entire welder for the chosen interval, then shuts it down. This produces excellent and repeatable welds in thin sheet steel.

A second relay, not shown, controls the amperage flow through the transformer primary, which in turn energizes the secondary and produces the very high welding current.

A closer view of the modified transformer. After the secondary was removed, there was enough room for 5 turns of #4 copper, insulated cable. You can use welding cable, or you can cannibalize the cable from a set of automobile battery jumpers. Be sure it is heavy #4, and flexible enough to wrap around the transformer.

I got very lucky... the same surplus store I found the transformer at (Bill Williams Tool, Ft. Worth), had a supply of extra-deluxe surplus aerospace copper cable. This stuff was made for aircraft wiring, and cost me all of $1/foot.

The small black box on top is a fuse box for a 20 amp prmary fuse.

The arms are pinching a little SS test sandwich prior to applying current. I originally got cute and tried a tungsten electrode in the top arm rather than the brass one shown... not a good idea. The tungsten sputtered and welded itself to the steel. The arms are 1/2" dia brass, and the tip is turned to a 90 degree cone from 5/16" brass.

And now for the not-so-funny tale of woe... I completed the welder wiring at roughly 10:30 A.M. one Saturday. I anxiously inserted the first test piece, and applied the current. NOTHING!! I began a painstaking trouble shooting which took hours - I rechecked all the wiring, checked the secondary voltage (O.K. at 4VAC), checked everything. Arrrgh! About 4 hours later, I scared the heck out of myself when I closed the energized pincers without a sample, generating a hot spark. Wow, it works. Suspicious, now, I checked the "samples" I was using, only to find there was thin film of transparent plastic on the stainless steel which insulated the whole thing!!!!! It is always the simple things which bite us in the butt.

In use, the clamp arms and the cable heat up, but not frighteningly so. This does limit the duty cycle... after roughly 10 to 15 welds, I found it was best to allow the unit to cool for a couple of minutes.

The weld quality is excellent. Destructive testing pulls a nugget off of one sheet, and requires a surprising force even with .010 stainless. The entire cycle takes only one second. Neat stuff!