Scrap Microwave Spot Welder

This project involves AC power, high amperage, and high temperatures. Although this project is simple in principle replicating it on your own offers a lot of opportunities to hurt yourself. Proceed with caution.

Microwaves are treasure troves of useful electronic components. They’ve usually got some nice microswitches, a big transformer, a magnetron, and some smaller transformers and rectifiers to drive the display. I found a decent sized microwave hanging out on the street and transformed it into a shop tool I’d been wanting for a while – a spot welder.

Why Spot Welders are Useful


Spot welders are handy to have around the shop. They can tack together wire for quick brazing, and permanently weld sheet metal for durable enclosures. If you want a thorough guide on what you can do with a spot welder, Dan Gelbart has all the answers. I’ve been looking to up my prototyping game and have more freedom to build custom components when off the shelf parts won’t suit. Unfortunately my workshop is in Brooklyn and space is at a premium. I spent a lot of time fabricating structures out of steel wire in school and have found that it’s a good replacement for bent sheet metal and structural framing if you play your cards right. Wire is easy to store in a small shop, doesn’t take much equipment to manipulate, and can hold good tolerances in various dimensions as its behavior is very predictable. You can still find the manual I follow for a lot of my techniques on Amazon.

I’m not going to go into exhaustive detail on how I fabricated this spot welder. A lot of it is just frosting on top of other peoples’ tutorials. However, I am eager to share what I feel are some useful additions you might consider including in your own version. This particular machine was based off of designs by the King of Random, Albert van Dalen, and Olli Niemitalo.

The Welder Power System


The main steps in turning a microwave into a spot welder are cracking open the microwave, pulling apart the transformer and rewinding it with thick cable, fabricating your enclosure and arms, and creating a timing circuit. It’s important to remember that you’re turning a high voltage and low amperage supply (standard AC) into a high amperage low voltage system via the rewound transformer. This means that resistance is going to become a dominant force everywhere beyond the transformer. This means beefy connections, thick copper cable, and an understanding that even conductive metals like copper have some resistance and that will cause heat no matter how you work the problem. Using a spot welder continuously, especially with thin cables carrying the current out to the electrodes, will create a lot of heat and likely melt your cable’s insulation.

I used this battery cable for rewinding the transformer. I wish I’d had a cold chisel around the shop to get the secondary coil out of the transformer block, but time and hammering eventually freed it. I went through a few different versions of cables I had lying around the shop (including some uselessly wimpy dollar store car jumper cables) to get something that could stand up to high cycle times without overheating. I ended up biting the bullet and purchasing these. Along with some copper lugs (which came with flathead set screws, not socket set screws as pictured… they need to be switched out eventually) and cut rods, I had the welder power system built.

The Timing Circuit


The timing circuit plays an important role in keeping welds consistent. I based mine off a rotary potentiometer plugged into an Arduino for convenience’s sake. The key is to test your weld at different cycle times on the machine with some scraps from the same gauge sheet or wire you’re using for your project and try to break the weld. When you end up with a weld that’s difficult to break but doesn’t create a deep sink in the metal, you’re right on the money.

Contrary to popular belief I don’t do much work with electronics. On most projects I prefer to buy a battle hardened boxed solution rather than rolling my own. This time I thought I’d get some practice in and see if I could design a timing circuit with components I had around the shop.

I stole the 20VDC transformer and rectifier off of the microwave’s display circuit and soldered it onto and Arduino Protoshield. I had a foot pedal for a small sewing machine hanging around the house that was really just a switch with a mini audio jack on one end. I connected the foot switch’s female jack in parallel to the onboard switch on the protoshield for testing the system without the pedal, and soldered an LED to one of the digital pins for a status light. I had a spare Power Switch Tail hanging around the shop and it was a simple matter to tap the AC lines coming in from it to power the transformer. Putting a voltage divider between the transformer and the Arduino gave me a nice steady 10VDC. The best part is the Arduino just hangs out on the tail like a remora so if I need it for another project the microcontroller can just hang out where it is without interfering with anything except a digit or two after the decimal place on my power bill.

You can find the code below. It was only slightly modified from a program by Zmo.

The Enclosure

Screen Shot 2015-07-01 at 3.09.59 PMI use a laser cutter as my go-to prototyping tool. I’ve found that most things can be made with 1/4″ Delrin (usually .26″ or 6.6mm), 1/8″ acrylic (usually 1.1″ or 2.78mm), and off the shelf fasteners. That, plus some printed brackets can get just about anything fabbed. One of the advantages in prototyping this way is that measurement errors or structural problems can be corrected without too much fuss. On the first rev of the design the arms weren’t beefy enough to handle the force of clamping pieces to be welded without slipping and misaligning. Fortunately all I had to do was cut some thicker beefier arms.

Holes in Delrin can be piloted on the laser and then tapped with a drill and will yield a really strong threaded fastening. If there’s one thing I love it’s reducing the part count on a design, especially when they’re fiddly parts on the inside of an enclosure that could easy fall out and rattle around when you’re servicing a machine. I haven’t found a good rule of thumb for offsetting the hole sizes to compensate for kerf, but it’s easy to laser cut a test of a few different hole sizes into a scrap of plastic and then tap them to test fits. I’ve found that 4.3mm holes for M5 fasteners, which are the most common threaded fastener size I keep on hand, work well in 6.6mm Delrin.

I wanted a simple trigger action for clamping pieces for rapid fire welding, so came up with a mechanism recessed into the handle. The first rev had an interesting cam action, where if it was fully engaged the jaws would stay up until you pulled the trigger back down. I though this was a pretty useful feature so refined it a bit for the final design.

The most unintuitive part of the design process was just how stiff the 2ga wire I used for the secondary transformer winding was. Initially I thought I’d be able to bend it around to fasten as I pleased and save some room in the enclosure. It turns out it was pretty intractable so I ended up measuring where the ends naturally rested without too much tension and put the fasteners there instead.

Wrapping Up


When all the bits were cut, printed, and tapped I screwed everything together. I used spade connectors for the illuminated safety switch on the back and even managed to salvage the microwave’s original power cord (though with a new plug found at a dollar store).

If there’s enough demand I’ll clean up the source files a bit and publish a complete tutorial. In the meantime you can download the relevant files on Thingiverse.