View Full Version : home made spot welder

Alistair Hosie
01-07-2003, 03:11 AM
Has anyone made, or have knowledge of making a spot welder.I used to use a small one sometimes during the construction of orthodontic dental appliances it was foot controlled and bench mounted and was a real help in holding small pieces of metal together long enough to solder or weld.I have seen bigger bench mounted ones for making other objects lamp shades etc.I understand that a set of plans is available to make your own spot welder but wondered how difficult this would be anyone tried it please let me know thanks Alistair

01-07-2003, 12:00 PM
Check out AK-47.net in their Build it Yourself board archives. There were several designs one of which used a car battery for the power source.

01-07-2003, 04:19 PM
The Eastwood Automotive Tools Co. (www.eastwoodco.com) sells a small spot welding attachment for a conventional stick welder. $49.99, takes about 50 amps to operate. I have one, currently loaned out for the last three years. I recall using it and found it to be a good tool.

Perhaps it's time for me to reclaim that tool.

Uncle Dunc
01-07-2003, 08:46 PM
Here's two, one smooth and one rough. I did a search on Google for

"spot welder" (project OR projects)

and these two turned up in the first page. There were 843 total hits. I scrolled through four or five pages of results without seeing any more that looked relevant to your question, but they might be in there. http://bbs.homeshopmachinist.net//smile.gif


01-08-2003, 01:09 AM
You should make a field trip to Sweden and go shoping in the scrap yards. Or check local sals, theere is bound to be one show up sooner or later.

With the amount of copper in one, it is cheap er to buy one at a sale than to build one, but if you had a big welder power supply you could fake one. you can still fake one with your Mig, but it will not work very well for sheet metal. Mot enough jam. And if you were thinking of Aluminun - forget it, it takes many kiloamps to do a half assed job even on thin sheet.

01-08-2003, 02:05 AM
The stuff that Eastwood sells is a "spot" welder in that it welds a spot (but then you can do that with just about any welding technique). It is however not a resistive welding in the truest sense, since it doesn't use the resistance of the sheet metal to create the heat.

In order to resistive weld, you need lots and lots of current. Small portable spot welder are capable of deliver 1000-2000 Amps. P = I^2R. Because R is very very small for sheet metal (we're talking fractions of milliohms), you need lots of current to dissapate the required power (i.e. heat).

Another critical aspect of resistance welding is to apply pressure at the point of welding. This is best done using some type of clamping device. There are two reasons for this. The obvious is to press the two sheet metals together so that they will fuse when the temperature has been elevated to a melting point. The not so obvious is that if the contact is not good, the resistance will be higher, and although according to P=I^2/R, this may be good, the open circuit voltage on these welder is very low (around 1-2 volts) so I = V /R becomes the limit.

Trying to convert existing welder to a spot welder cannot be done effectively simply because of their inability to deliver the current.

Also, using car battery is very ill advised because it's difficult to switch such high current reliably. Most switches will arc weld instantly when you're trying to open a circuit that's conducting hundres of Amps. Not being able shut of the welder, you have a very dangerous situation where the battery may heat up and explode http://bbs.homeshopmachinist.net//frown.gif

The best way to build spot welder is to modify an existing transformer, which usually means replacing the second winding with a heavy gauge wire with perhaps 2 or 3 turns. The transform need to be large enough to handle the power, which should be at least 1000W.

Good luck.


Alistair Hosie
01-08-2003, 04:20 AM
Uncle Dunc I would say the spot welders shown in your reply look simple enough to make whether they would be effective I don't know

Dave (thrud)I dont intend to use it for aluminum but see were you are coming from as I always ask about aluminum(I guess you will all be glad when I finally use what little I have left of the that) http://bbs.homeshopmachinist.net//biggrin.gif http://bbs.homeshopmachinist.net//biggrin.gif http://bbs.homeshopmachinist.net//biggrin.gif

Albertwhat did you think of the ones shown by Uncle Dunc?

01-08-2003, 10:52 AM
I think both the of link provided by Uncle Dunc are very viable means of making a spot welder. I would have tried to use a heavier gauge copper wire since maximum power power transfer is achieved when winding resistance equals the sheet metal resistance. It's difficult measuring such low resistance, but you can use a AC volt meter to do the same. Meaure the open circuit voltage and then measure the voltage under load (ie. when welding). If the voltage under load is 1/2 that of the open circuit, you're achieving maximum power transfer.


01-08-2003, 01:34 PM
I work at a university and get old high voltage transformers and cut the high voltage winding off then put two raps around the core using several thin sheets of aluminum.The best one I made draws 50 amp at 220 volt will weld 1/16 sheet very well. I use ooo welding cable off the transformer and it gets hot in 10 to 20 welds


01-08-2003, 03:04 PM

If all you want to do is light stuff that first one in Dunc's link is a beautiful job. I would use bigger stranded copper cable for the secondary though, it looks on the dinky side - otherwise I am impressed. I might even have to build one of those myself...

Thanks Uncle Dunc for some inspiration!

01-08-2003, 05:51 PM
Alistair: A microwave oven transformer (large ones) are good for 1500 watts continous, would do 3000 watts short term with a fan.
No experience making a spot welder but I think, I would try to buy the "tongs", then disassemble a Microwave oven for the Xformer.

Every one I have used (and it has been several) has primary and secondary on seperate legs of the core. The secondary has the smaller wire. Cut the secondary off (it has maybe 10,000 (minus 1000 plus 5000 turns). I cut off with a wood chisel. Each had one volt per turn voltage out put. so two turns of your biggest welding cable, with short leads to the tongs, will give you two volts. you should get in excess of 1500 amps woth ease. THe primary at full rated load will draw about 15 amps- which can be supplied by most circuits in the USA. A series resistor just before the transformer, can be used if it too hot. Piece of cake buddy!!!! http://bbs.homeshopmachinist.net//smile.gif
BTW the link from dunc looks like it tells you every thing else excepting the transformer.

Say hi to your nice from all of us.


Alistair Hosie
01-08-2003, 07:09 PM
Thanks Steve you are as always very kind. She is out of hospital and so far doing better,thanks again my brother.
Thanks guys for the help with the spot welder question this is really fascinating to me . I think it would be a real worthwhile project as they are so expensive to buy.I just missed one at an auction several years ago don't come up too often so I would like to make my own.Alistair

Alistair Hosie
01-08-2003, 07:54 PM
Steve incidently would a motor from an old microwave make a good slow sharpening machine? similar to a Tormek.
I notice it goes lovely and slow when in operation but have never been able to tell if has any real torque as the door is always closed when in operation.
But this has been an idea of mine for a while to test the one I have when it breaks down.If you say it will work I may be tempted to help it on it's way http://bbs.homeshopmachinist.net//biggrin.gif http://bbs.homeshopmachinist.net//biggrin.gif http://bbs.homeshopmachinist.net//biggrin.gif just kidding dearest wife oops she heard me .Alistair

George Hodge
01-08-2003, 08:40 PM
Alistair,what did that dental spot welder look like? I bought a bunch of things at an auction of an old dentist and one item might be a spot welder or resistance soldering machine. I've never done anything with it.

01-08-2003, 10:05 PM
Alistair: I have never tried on of thoseturn tables.
I tell you :from experience, a phonograph turn table will not do it http://bbs.homeshopmachinist.net//smile.gif. Neither spring wound, 78 nor 33.3333. they slip or stall. Tried it as a kid with grindstone from a hand cranked grinder. http://bbs.homeshopmachinist.net//smile.gif

Uncle Dunc
01-08-2003, 10:08 PM
There was another fusion welder in HSM a long time ago. The guy had it set up as a fusion welder instead of a spot welder, but the electronics were all the same.

You might be able to find it in the index on the HSM home page. There was something wrong with the article. I couldn't figure out if they lost a page in the publish process or if the editing was sketchy, but there were references to a schematic which did not appear in the magazine. I never tried to build it, but I may yet, since I still need a bandsaw blade welder.

[This message has been edited by Uncle Dunc (edited 01-08-2003).]

Alistair Hosie
01-09-2003, 09:09 PM
George open this then on left hand side go to dental sector then open then on left side open welder then slightly to right open odontoweld or orthoweld and see if it resembles your machine let me know Alistair

Alistair Hosie
01-09-2003, 09:10 PM
george try again sorry forgot to download
this otherwise follow same instructions


regards Alistair

George Hodge
01-09-2003, 09:41 PM
So that's what that little rascal is used for!!! Alistair,mine is a Rocky Mountain Denta-Weld,Model 30 Made in Denver Colorado. Probably the Great-great grandfather-in-law of the one in the ad.It's about the size of half a loaf of sourdough bread. Brown Bakelite case,with the electrodes about the same as the pictured ones. What size of material will it work on? Is it mostly for gold,platinum and silver? I've never plugged this one in. Thanks Alistair !!!

01-09-2003, 10:10 PM
Found what I was looking for.
Alistair. In Model Engineer Workshop No.41
March April 1997 there is an article on building a bandsawblade welder along with an attachment that can convert it to a spot welder.If you want I can send you the article.

01-09-2003, 10:49 PM
www.hobartwelders.com/mboard/attachment.php?s=4425db932a0f9de9cfd787f4e9d0fab8&postid=4941 (http://www.hobartwelders.com/mboard/attachment.php?s=4425db932a0f9de9cfd787f4e9d0fab8&postid=4941)

01-10-2003, 01:44 AM
Forgot to mention that if you build one, to use pure copper for the electrodes - less chance of the tip welding (sticking) to the metal, and higher current transfer.

01-10-2003, 04:10 AM
Dave, an old timer once told be that the proper material for the tips is berylium copper.
On my old spot welder the tongs are copper and the tips are definately an alloy. I don't know what, but I assume berylium copper.

Alistair Hosie
01-10-2003, 10:41 AM
George we always used it purely on stainless steel but I am sure it would weld other things this is a spot welder as used by a dental technician for orthodontic appliances (braces in the states) maily to hold the whole thing together in order to silver solder but can be used to direct weld. Experiment with different settings on other material and see how you get on ver handy little tool also Paradise what did you use that one for looks like a heavy duty machine Alistair

01-10-2003, 11:01 AM
After having made several thousand spot welding tips, I can say that the preferred material is half hard copper. The dead soft stuff is just too soft to machine well.

01-10-2003, 02:01 PM
For spot welders, try to get a toroidal transformer, the ones that look like donuts.

They have lots of room for extra conductors of large size thru the middle, and they are generally more efficient than the "E-I" type.

You probably will NOT have to remove any windings to use a toroidal type. Messing with the existing windings of an EI core type is a pain, easy to damage the windings you want to keep.

Mark Hubler
01-21-2003, 12:59 AM
Here are two links on building your own spot welder:



By the way, I have seen some small spot welders (i.e. dental size) for sale on ebay.

Also Harbor Freight sells two sizes of spot welders: 120v 30amp 1.5 KVA and 220v 30amp 2.5 kva (these are made by JSC Kars (Armenmotors), Republic of Armenia) for about $200.

01-21-2003, 01:23 AM
A few weeks ago I got a 550 volt, 15 KVA transformer real cheap at an auction($5). I got if for the sole purpose of rewinding the secondary and building a resistance welder, some day. Has anyone tried attaching fins to the transformer(thermally conductive not electrically) and using fans to get a higher duty cycle? I know some spot welders use water cooling but that seems much too DANGEROUS for the amateur.