Converting stick welder to tack welder.

I need a tack welder for joining thin plates and electronic components. Like the tack weld you see in your NiCad battery packs.
I have no practical use for my 120v, 80-Amp stick welder so now I'll convert it into a tack welder.
I like to get some ideas on how to make a setup that will be safe and precise enough to do small electronic welds like the welds found on some relays. Does this sound possible, if not can you point me to a place to get a spot welding setup?
Thanks
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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 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. www.unitekmiyachi.com
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Clandestine wrote:

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.
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Clandestine wrote:

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
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Build your own resistance welder at home:
http://users.frii.com/katana/spotweld.html
Good Luck, please post results or casualties.
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Tim Zimmerman wrote:

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

How well did this work when YOU tried it? How did YOU keep from killing yourself on the high voltage winding? mike http://nm7u.tripod.com/homepage/welder.html
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mike wrote:

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
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Steve Taylor wrote:

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

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
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Steve Taylor wrote:

Thanks for the info. I was never successfull trying to switch the weld current. Welded a lot of contacts, but no battery tabs ;-) Typical CD system discharges more volts into a step-down transformer. mike

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mike wrote:

It would need to be a very good transformer, to handle high speed, high current transients like this - I doubt I could design one off the top of my head.
Steve
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You mean a starter solenoid?

Are these just safety buttons to prevent electrocution?

This CD welders (below) require 680,000 mirco F. What's the different about yours?
http://www.powerstream.com/spot-welder.htm
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Nick Huckaby wrote:

Energy proportional to the square of the voltage on the caps. Wonder how they get 1000A through their output connector? Wonder how they get 1000A through the small wires to their welding tweezers? My Unitek 125WS welder has half inch diameter output terminals and runs #2 wire to the head. Wonder what they use to switch the caps to the load? Would be interesting to see a graph of weld voltage and current vs time. mike
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I found a graph for a dual pulse CD welder here. http://religion.p5.org.uk/cdwelder.htm
I assume that most CD welders require about 680,000 mirco F. How's it possible that Steve can weld with only 2200uF? How does the power supply provide the necessary current to the capacitors?
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The size of the job would presumably determine how much current is required and thus set the minimum size of the capacitors. You can also vary the amount of charge held in a capacitor by changing the voltage. The Charge is Q and the Total Charge in a Capacitor is Q=CxV. Charge is also related to Current (I) by the formula Q=IxT so IxT=CxV and then I (current)= CxV/T. This means you can increase the current capability by increasing the size of the capacitors C and/or the Voltage V or shortening the discharge time T. Note that these are ideal equations and the resistance of the leads and workpiece also factors into what happens in real life. The voltage level also has to be within safety standards. However, you can't just increase the current by shortening the time it is applied and expect it to work. The high current has to be present long enough to heat and fuse the workpieces.
The above is also related to charging the capacitors. By increasing the time to charge the capacitors you can do it with relatively small amounts of current. In this type of welding done manually that is not hard to do because repostioning for the next weld etc gives seconds to charge the capacitor so the power supply does not have to deliver high currents.
Billh
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Nick Huckaby wrote:

You snipped the answer from my previous post. Doubling the voltage requires 1/4 the capacitance. 18V...400V, do the math.
You have at least two major problems. You have to store the energy somewhere. The voltage and the capacitance are the easy parts. Second, you have to deliver all that current to the load. You need extremely low resistance in the switch and all the path to the weld. The simple "fix" is to just up the energy to account for the relatively huge losses. Looks good on paper, but makes the process extremely sensitive to the weld resistance which depends on force, contamination, material, luck... Tweaking it to get a good weld ain't all that hard. Getting a dozen good welds in a row is not so easy.
My Unitek 125 switches 400 odd volts with an scr into a transformer. 400V in >> 7V out into .001 Ohm. It's not your ordinary transformer. I was surprised how small it is. IIRC about 2" on a side.
mike
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Nick Huckaby wrote:

...it cost about zero to build for a start !
Steve
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On Tue, 22 Feb 2005 12:48:03 GMT, "Tim Zimmerman"

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

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
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