DIY Spot Welder from Re-Purposed Microwave Oven Transformer

[Mark Presling]

Some years ago I found that the lithium battery pack for my Milwaukee drill had died and I was considering repacking the battery myself. I soon realised that the terminals on the replacement lithium batteries had to be spot welded instead of soft soldered. That got me interested in making my own spotwelder from the ubiquitous microwave oven transformer. When I started to research the rewinding of the transformer I realised that this would be the easy part! Making the associated clamps, electrodes, electrode arms and chassis was going to be the real challenge. Most of the designs that I had seen on YouTube were cobbled together from bits of plywood and scrap and although they worked I wanted the finished machine to look, well, finished! I was just starting out in home metal casting and I had a 3D printer, a laser cutter and a mill and lathe so I figured I would like to engineer a proper spot welder which was capable of welding sheet metals and wire. I soon forgot about welding battery tabs. During my career as a technology teacher I had used a small portable spotwelder made by Aston here in Australia. It was a great little machine with lots of accessories including a foot pedal and a variety of electrodes and arms. It was heavy but could be taken to the job rather than being bolted to the bench. I based my design on the Aston welder but had to work from memory because by the time I got around to this project I was retired and no longer had access to the original machine.
The design that I came up with worked right from the get go and despite some upgrades that were less than successful, the latest iteration works reliably and is capable of welding material up to 2mm thick. I recently made some stainless steel filter frames for my two rainwater tanks and it was a joy to be able to spot weld the flat sheet parts together rather than relying on pop rivets which invariable rot out after a few years.
The playlist for this welder is on YouTube and it includes the engineering, the design considerations, the electrode design and a failed attempt to make a better quality secondary winding for the transformer. However you should at least watch the last episode in the series which shows the latest iteration of the secondary winding which has turned out to be the best.
If you are interested in the design, I put all the 3D models for the parts and the circuit diagram on Thingiverse here: https://www.thingiverse.com/thing:2368142
Feel free to use those resources and share them as you see fit.
Regards,
Mark Preslinghttps://www.youtube.com/playlist?lis...oaUxIzfH_B-6Kp

[DIYer]

Thanks Mark Presling! We've added your Spot Welder to our Welding category,
as well as to your builder page: Mark Presling's Homemade Tools. Your receipt:

Spot Welder by Mark Presling

tags: spot welder

[Scotsman Hosie]

...However, you should at least watch the last episode in the series, which shows the latest iteration of the secondary winding which has turned out to be the best.

That is an impressive bit of engineering – culminating in an excellent build. I would suggest the thing that has eluded your understanding, of the failed single coil secondary, is two-fold.

1st, stranded wire will always carry more current, than the same sized solid copper. This is due to the "surface effect" of electrical current. It travels on the surface of the wire. More conductors, many times more 'surface' than a same-sized solid conductor.

The 2nd would seem to be not making a complete loop, through the actual magnetic field of the transformer. But, rather, making a U-shaped – in-and-back-out – with your solid copper secondary [almost a] coil.

Regardless, you've completely redeemed yourself, with the latest iteration and performance of your latest build. For build quality, a tool in a class by itself.

As an addendum, hydroelectric generators are often wound with hollow tubes. the design feature making use of both the inside and outside surfaces of the conductors, for maximum current-handling.

[Mark Presling]

Scotsman Hosie,
I was keen to do some more development of the laminated secondary coil and I had a lot more strips of copper that I could have added but at the end of the day the final version with the 00B&S wire turned out to be suitable for what I wanted. That's interesting regarding the surface effect. The original spotwelder made by Aston had used copper shim as the laminations in the secondary. The shim would have only been a few thou thick. I had a assumed this was to make it more flexible where it joined directly to the moving electrode arm. Unfortunately I couldn't dismantle it to see if it was formed in a loop rather than just a U shape.
Regards,
Mark

[DIYSwede]

Just 2 late cents: For this application, stranded wire doesn't offer much else than ease in winding the coils,
as the "Skin Effect" gets more pronounced at (much) higher frequencies.
One example is the AM ferrite rod antenna (@ ca 1 MHz) from the radios way back, with their Litz wire coils.
For the skin effect to be problematic at 60 Hz, the conductor diameter has to be big, for example like in a big hydro-electric generator.

"At 60 Hz in copper, the skin depth is about 8.5 mm. At high frequencies the skin depth becomes much smaller.
Increased AC resistance due to the skin effect can be mitigated by using specially woven litz wire.
Because the interior of a large conductor carries so little of the current,
tubular conductors such as pipe can be used to save weight and cost."

https://en.wikipedia.org/wiki/Skin_effect