Scrap Microwave Spot Welder

by gianteye, published

Scrap Microwave Spot Welder by gianteye Jul 1, 2015


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Ever see a microwave on the sidewalk waiting to be hauled off to the dump? Save that machine from the bin and make yourself a serious shop tool.

Spot welders are great for making sheet metal enclosures, building wire structures, attaching threaded fasteners permanently to metal forms, and tacking things together for more industrial welding later.


To construct this spot welder you'll need a junked microwave, 1/4" (nominal) Delrin, 1/8" (nominal) acrylic, some PLA printed brackets, and some hardware you can find on McMaster and Amazon.

I used:

  • 1 12"x20" sheet Delrin
  • 2 12"x20" sheet acrylic
  • 4 Handle brackets
  • 8 Vertex brackets
  • 8 Corner brackets
  • 38 10mm M5 BHCS
  • 8 20mm M5 BHCS
  • 6 25mm M5 BHCS
  • 4 M5 Nuts
  • 1 20mm M3 SHCS
  • 1 M3 Nut
  • 2 Copper lugs
  • 1 7"x1/4" Copper rod
  • 4 Rubber feet
  • Spade connectors
  • 12ga Wire
  • 2ga Wire

For a more detailed build log visit http://har.ms/blog/scrap-microwave-spot-welder and check out a video of the welder in action at https://youtu.be/450S4mBNAM8

These files are more for reference than a complete tutorial on making your own spot welder. If people are really into the design I'll revisit it with step-by-step instructions and diagrams. Yes, I know I'm mixing metric and imperial measurements, but this is because the products I'm naming end up being more searchable in certain units. If you're really that worried, type the measurement into Google for a conversion.

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really nice make... !

looked at the other youtube movies too, including the one from the king of random.. and started to make the spotwelder but i am VERY interested in either a schematic of the arduino expansion board or some more clearer pictures of it (from the top / side) so i can design the schematic myself.

Hi great job!
Please upload the complete toturial
First the file for laser cut will be appriciate
Thank you

For mine, I used a transformer from an old 1980's microwave, #00 awg wire, and a foot switch from Adafruit. With 3 windings, I get about 3.75V at 650amps (amperage estimate compared to others on YouTube).

For a descriptive video of how to remove the small windings, make one: https://www.youtube.com/watch?v=vrlvqib94xQ (which I noticed was also referenced by gianteye). I used a hammer, chisel and a Dremel cut-off wheel (with eye and respiratory protection). My transformer is slightly thinner than this one. Wiping the surfaces with acetone seconds before welding helps with adhesion.

THIS DEVICE CAN KILL AND BURN YOU!!! Touching the probes is not like an accident while changing a light switch....

I'm not trying to downplay any safety concerns. People should use caution and only approach this project if they already have a good understanding of electricity and high amperage systems. Any powerful tool for fabrication is also a powerful tool for hurting yourself. Even a drill press can kill if you put your head under it.

However, I think I can put your mind at ease somewhat. There is no 12 gauge wire beyond the transformer. The high amperage on the output side of the transformer can definitely generate a lot of heat. On the input side, not so much. From some quick napkin math, the wattage the welder draws could safely be carried on 14 gauge wire. If you're sincerely concerned that 12 gauge wire is a fire hazard, you might try inspecting the outlet you're plugging your welder into. Chances are the outlet itself is supplied by 12 gauge wire.

There is more of a danger of burning yourself from the hot metal post-weld than any shock you might get from the welder's electrodes. Your skin's low conductivity combined with the very low voltage at the probes means that the electricity is incredibly unlikely to find a path to ground through your body. You can find out more about the relationship between voltage, amperage, and electrical shocks here http://www.allaboutcircuits.com/textbook/direct-current/chpt-3/ohms-law-again/

I haven't heard about wiping surfaces with acetone for welding. This resistance welding guide from Miller (who make really good quality welders) is a great reference for getting the most out of your spot welder http://www.millerwelds.com/pdf/Resistance.pdf

I apologize, I said "you"...but I meant to say: "anyone wishing for maximum amperage to melt and fuse metal together". I also apologize for getting in too much of a hurry and missing the use of 12awg AND 2awg wire.

By death....3V at 600 amps...from one hand to the other, while potentially arcing through the heart can kill. I am aware of Ohm's Law, Watt's Law, and Murphy's Law (anything that can go wrong, will go wrong). My outlets have been tested and are safe. I do not know how many amps is drawn by the extension cord, but it does not throw the 20A breaker....and I was referring to the wire from the transformer to the electrodes.

Oxidation, oils, dirt, etc....increases the surface resistance and will affect the recreation of the crystalline structure of the bonded metals as the items cool. The bond between metal<-->metal, will be stronger than metal<-->debris<-->metal. (Source: Black Kohser DeGarmo's Materials Processes Manufacturing 10th (http://www.amazon.com/DeGarmos-Materials-Processes-Manufacturing-Black/dp/0470924675)...and knowledge from mechanical engineering class).

I just read part of the handbook for Resistance Spot Welding...paragraph 3-10: "All metals develop oxides which can be detrimental to resistance spot welding." ......"Surfaces to be joined by this process should be clean, free of oxides and chemical compounds, and have a smooth surface." There may be a better solution than acetone, however it is a multi-use product that works and is readily available in most regions of the world.

I compliment you on you documentation! And your SolidWorks seems to be well organized as well!! You may wish to upload this to GrabCAD as well?

Can you post the schematics and the wiring details?