Step 6: Make the Jaws

For the jaws, I used two blocks of wood measuring approximately 3.5" by 1.5" by 5", a 4" hinge, and two lengths of 19mm steel tubing about 10" long. The blocks of wood were cut from an old pallet, and the steel tubing came from an old rotary washing line.

Although I could have run the wire through the tubing, I chose not to for reasons of simplicity, heat dissipation, and strength of the jaws (as extra holes would need to be drilled). Also I would like to leave open the option to use thicker wire at some point. I have seen a design which uses copper tubing for the jaws which then form conductors between the transformer and electrodes, however I thought the conducting area would probably not be enough. I used cross dowels to enable quick and easy removal of the electrodes for renewal and re-shaping.

Hinge blocks
Lay the two blocks of wood with the long edges together, lay the hinge on them with the pin along the edges which meet, and draw around it.

Using a chisel, cut a rebate just inside the line you drew on each block for the hinge. Slope the rebate off a bit towards the edge of the wood to allow for the slope up to the hinge pin. Check that the hinge fits correctly, and screw it into place.

Measure the centre line at right angles to the hinge pin, and mark the centre at the outer edge of each of the two blocks. Fold them together and check the centre marks meet. It isn't important that the marks are in the exact centre, but it is important that they are aligned. Use your square to mark lines across ends of the blocks at these marks, and then find the middle of each of these lines.

Remove the hinge and make a 19mm hole (or whatever size your tubing is) in the end of each block at the place you just marked. Go a couple of inches for each one - they do need to be about the same depth. The exact method you use depends on the tools you have available. I used a hole saw, first making a pilot hole with a wood boring bit. Drilling a pilot hole enabled me to check that the hole was square (to the face of the wood), as I was doing this using a hand-held drill. Apply the hole saw in stages, pulling it out frequently to remove sawdust. Break the core of the hole by inserting a drill bit into the centre hole and pushing it sideways to snap the wood across the grain.

Drill a pilot hole in each piece on the hinge side - this is where the screws will eventually go which lock the tubing in place, so they need to be partway down the depth of the big holes you made. Don't drill the tubing yet. I used a wood boring bit for these, with the tubing in place. This gave a nice shallow hole with a clear centre point, but not drilled through, precisely positioned as the drill doesn't wander. Offset the holes from each other along the pipe so the heads won't short the pipes (which will have the welding voltage across them)

Insert the two pieces of tubing into the holes as far as they will go, and fit the hinge back on.

Position the jaw assembly in place in front of the transformer, and mark where the electrodes will be. They both need to be exactly the same distance from the hinge, so use your square to mark both tubes together.

Make a second mark on each tube far enough from the first to allow for the thickness of the electrode, a gap of a few mm, and a cross-dowel, then a few mm again to allow enough steel to retain the dowel. Screw the retaining bolt into the dowel and hold it and the rod you intend to make the electrode out of, against the tube to check the position of this mark. This is where you will cut the tube to length. Don't cut it yet though in case you make a mistake or decide to change something.

Mark the position for a hole in the bottom wood block so it can go from the hinge-pin side to the tubing side without fouling the hinge screws, and also misses the mounting screws which will be underneath. The hole needs to be big enough for the wire to the bottom electrode to pass through (I used 13mm)

At this point, you might want to tidy up the ends of the wooden blocks so all is flush and nice.
Quick safety question. I have rewrapped the secondary of my mot with 2/0 with only a couple turns so I'm getting 2.5 v out. So at 12 amps in at 120 I have the potential of 575 amps out. So I'm just a tab bit nervous about holding the thing I want to spot weld....Even though I realize at 2.5 v it'd have a pretty hard time of poking me. You guys have had no problems?
<p>The secondary is isolated from the mains, so unless you did something wrong it can't hurt you. The current that flows is dependent on resistance - so even if you grabbed both electrodes firmly with your bare hands you wouldn't feel anything.</p>
Oh yeah that makes sense. Thanks!
I made this also, and I just cut one welding with a hacksaw and bent it open, stuck in a set of jumper cables, and super glued it back up, works like a charm!
<p>Interesting that you took the route of cutting a weld. I had thought about this but decided I didn't want to risk doing irreversible damage to the transformer. Brave you :)</p>
<p>i need technical specifications of the transformer</p>
<p>All I can tell you is that it's the transformer from an 800W microwave oven, and that the primary was wound with approximately 1 turn per volt. Everything else you need to know is in this instructable!</p>
<p>I am curious as to why most people are not using something like jumper cable or the like for secondaries.</p>
Tried it with expensive jumper cables, in about 10 seconds the outer plastic started to melt off xD
<p>Jump leads come in two types - cheap and expensive. The cheap ones tend to have very thick insulation to make them look better, so are not really a good type of wire to use, and with expensive ones probably the wire is too thick, and would you really want to cut one up anyway? Either way you would probably find it was simply too thick.</p>
<p>Is there any alternative for microwave transformer? how about other appliances do you have any idea?</p>
<p>Hi fburito</p><p>The only thing I can think of straight away is a &quot;site transformer&quot; - the ones that are used on building sites to provide safer power for the tools. They are pretty big and beefy. I see them on Ebay a lot for very little money, though they tend to be &quot;buyer collects&quot;. Of course they might not be available in your country. The next thing I can think of is a transformer from an arc welder - and it's already fairly close to what you want.</p><p>If you have access to some thin sheet steel you could of course cut out your own laminations. Would be a poor performer compared to proper transformer steel but it would do if you can't get anything else. I found this site to be quite informative: </p><p><a href="http://ludens.cl/Electron/trafos/trafos.html" rel="nofollow">http://ludens.cl/Electron/trafos/trafos.html</a></p><p>Good luck!</p>
Congratulations on an excellent piece of work. I&acute;ve been searching on the net for a MOT spot welder diy project. This one of yours is by far the best. Thank you. Keep up the good work. No need of a video step by step demonstration. Pictures, drawing, text, just right.
Wow - I'm deeply flattered! Thank you :D
taanks <br>
cool instructables!! but what about those chisels?? like them what is the brand?
may be a video instructional and step by step, how work well done thanks...
If I re-wind another transformer like this, I will certainly try to make a video of it.
Your ingenious solution to the secondary winding is terrific. An excellent design.
What a Great Instructable loads of pictures and well written with lots of information.....Thanks i've read lots of these as I want to make a spot welder,this is by far the best i've read..........If you want to make something out of the float ball from the cisten cock have a look at my 'Ball Mill' ...I have 4 ... I dont want to blow my own trumpet, but they are quick and easy to make and work really well......Thank for this again Phil..
I have no idea what I would mill though! Besides, it's a copper float and has great potential to make something *shiny* :o)
Very good work! Congratulations.

About This Instructable




Bio: Loving getting back into electronics as a hobby after a break of many years. Now I work as an EPOS engineer, so I spend my ... More »
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