Converting stick welder to tack welder.

Those "tack welds" are created by resistance heating between the parts. There is no arc involved. It takes considerably more power (watts) to weld by resistance than by arc. They are also done faster than most arc welds. To put this in perspective those electrical components required about 1,000 Amps in =BD second. Usually the current is turned on/off by an SCR (or similar switch). I have never heard of converting an arc welding power supply into a resistance welding power supply. Keep in mind these critical factors during this type of welding

FORCE - you must "pinch" pieces together (approximately 500 lbs) POWER - you need a high, controlled amount of electricity TIME - You need to regulate the power flow within 1 cycle (1/60 second).

Unitek-Miyachi makes small resistance welders.

Build your own resistance welder at home:

Good Luck, please post results or casualties.

I can vouch for the Unitek-Miyachi system, they work well and the price is usually decent. They're step-pulsed, capacitive-discharge systems, and are as far removed from arc welding as swimming is from bob-sledding...

I worked for 4 years at a battery "wholesaler" my main job was to crack dead packs open and "re-cell" them, and then glue them back together. generally cheaper for the customer than buying a new pack, and 9 times out of 10 they had more capacity. If you're looking into this kind of stuff, let me know and I'll give you my former boss's contact info and he'll be able to point you to our connection on the west coast where we got our welder from.

find your self dead microwave oven ( high power line), use the heater tap on the transformer.

Check the rec.crafts.metalworking news group. Lot of info there on things like this (and a lot of other DIY stuff).

Likely a capacitive discharge to deliver the high current.

Or like said a RC that drives an SCR as a switch of a storage Cap.

Martin

I believe you are following the wrong path here. You need little voltage, but a LOT of current.

I made a very good spot welder for batteries and similar tasks from an ex microwave oven transformer, the biggest I could find. Hack off the HV secondary, thousands of turns of very fine wire, then I rewound it with just 3 turns of wire, but I packed in as much 8g and 12g wire as would fit, and paralleled all the turns.

The secondary is controlled by a SSR (Croydom CSD2410) pulsed by a simple 555 timer circuit. It can vary from about 75-300 mSec I can also switch in one of 3 wirewound resistors in the secondary to give me fine control.

The electrodes must be made to suit your exact application, and some trial and error can be expected. I first used copper and brass, but now get much better results from a proper spot welding electrode machined to suit my application. It was not cheap, about $11 for a

3/8" rod about 3 inches long, but it gives very good results. It had a trade name like "Elkalloy" IIRC.

For optimum results, it is also important to control the electrode pressure, but I find I can achieve satisfactory results by hand.

Barry Lennox

Tim, The reason your stick welder is not good for spot (what you call tack) welding is because the voltage is too high and the current too low. I experimented with a microwave oven transformer and was able to get 400 amps at 3 volts. This is done by removing the high voltage secondary windings and replacing them with a few windings of heavy wire or even copper bars. See other replies for links etc. for building your own. ERS

How well did this work when YOU tried it? How did YOU keep from killing yourself on the high voltage winding? mike

When I did it, I drilled the HT winding out. Brutal, but fast. To be honest, the idea didn't work for me at all, and I built a miniature capacitor discharge welder that DID do the job - a modest bank of old PC power supplies yielded enough capacitors to hold "useful" amounts of energy.

Steve

Post some details on voltage, capacitance, how'd you switch it? electrode construction? mike

Hi. Oddly enough, you can do spot welding with an arc welder. I tried it, and all I got was the typical mess that you would guess. Burned up spots with no strength.

But, just because I cannot do it does not mean that it can't be done. There are plans on Ebay:

385664469 This auction is for a set of completely illustrated plans to build a spot weldi ng/cutting gun that works with your arc welder for less than $50.00.

I have not tried this out, so please buy the plans and report back to the group.

Or, you could try Eastwood's version, which is very similar, but uses carbon electrodes. Also, please report back to the group after trying.

Recently, I tried to do a blind spot weld with 1/8" steel. It worked just great. So, the problem may be power, control, and excessive heat, which the above solutions allude to. Note that this is arc welding, not resistance welding with a low voltage rewound microwave oven transformer.

With only 3 volts, the resistance of the metal and any 'dirt' best be doing zero ohms... Not much punch through voltage.

Martin

"Martin H. Eastburn" wrote

How about 12V? Would two car batteries work?

Repeatability is a BIG issue with this. A CD system tries to deliver fixed energy. That's less dependent in path resistance.

Sure, if you had some way to turn them on/off quickly. Be sure to use a heavy metal box to contain the battery explosion if something goes wrong.

FYI Here's the voltage waveform for a Unitek 125 intoa .001 Ohm load .

Hi Mike,

We needed to weld some exotic metals, that required CD welding. Our welder was built in a glove box, The electrode construction was similar to your, we modified a toggle clamp to do the job with 1/16" diameter tips.

The cap- bank was around 2200uF (10 x 220uF 400V reservoir caps) Energy supply was a large variable O/P PSU, large because thats what we have around. Drive was 0-400V. Welding occured at around 40V.

Discharge was effected by a very large old automobile relay , with contacts bigger than US pennies (around 1" - like the old UK pennies)

Job was pinched in the jaws of the spotter, then the hands had to operate two buttons simultaneously to activate the spot.

Yes, I'd have preferred to use a huge ignitron, or a hockey-puck thyristor, but we didn't have time - this was a two day oh-god-we-have-to-do-this-yesterday kind of thing.

We just about managed to weld molybdenum foil ~0.2mm thick, with it.

Steve

I'd be nervous calling it a 0.001 ohm load - but ok.

I think the connectors are exceeding that - two clamped down with bolts and the two on spring loaded clamps.

I'd measure the Tr fro 10 to 90% point :-)

Thanks for the waveform and idea.

Martin

You're being too picky. The manufacturer publishes a specified waveform for their device under controlled conditions. Gives you some idea of what you're up against welding battery tabs. mike