Spot welder from scrap, part 3
As quick recap, I had the transformer with some hefty windings and copper clamps prepared in the Spot welder from scrap, part1. After that the metal frame with movable electrode and lever assembly was described in Spot welder from scrap, part 2. Now it’s time to plug the thing into the outlet, right? But how are we going to actually commutate it? The 6kW load is not something that may be turned on and off with just a push-button. For that purpose a properly sized relay/contactor must be used. I decided to go with solid state relay (SSR). SSR is basically a triac that switches your load and is controlled by a logical (5V) signal that may be safely handled by a small button/switch. For galvanic isolation between the power and the circuits an optocoupler, such as MOC3063 may be used. The SSR come nicely packaged as modules like that:
But those modules could be somewhat pricy. And buying pricy things did not exactly fit into the “from scrap” philosophy. But it does not cost much to buy a triac and an optocoupler. And here is the typical usecase schematic from the optocoupler’s datasheet:
Instead of the logical gate at the input we only need to provide a 5V source with a button. So I quickly assembled SSR from the mentioned components and leftover parts from my junk bin:
To house the electronics I took an enclosure from an old ATX PSU (those are fantastic source of components for junk projects). By making a square hole in its bottom I managed to fit it on top of the transformer like that:
A small zip-tied thingy on top right is a 5V power supply taken from an old phone charger. It provides power for the SSR signal circuit. I also took a liberty of repurposing the C13 power outlet connector for passing of the control signal. It is obviously not safe if used by a non-informed person, but hey, I’m the only one using it anyway.
The array of bolted connections are taps from sections of the primary winding. Moving terminals and jumpers around allows for adjustment of the transformer’s output voltage. Too small of a voltage and not enough current is passing through the workpiece to heat it up, to much of it and more power dissipates in the windings than the transformer may handle. In principle, the voltage must be adjusted for the kind of workpieces that are dealt with on the apparatus. But practically there is a more-or-less universal sweet-spot that, once found, may stay for most of the tasks.
To control the SSR I was in need of some kind of thing with a button that I can hold in a hand and push. So I milled one from a piece of wood and finished it with a lacquer:
And finally all the components are in place for the final assembly:
But hey, did you spot a difference? Now there is a nice finish on the square tubing of the frame.
What is that? Paint? — No, better! That is what’s called a phosphate conversion coating. And not even the common one that is, but rather a hot galvanically-assisted process by a very home-brew recipe. It produces a hard, thick, wear-resistant, heat-resistant, corrosion-resistant and electrically insulating layer on the surfaces of steel parts, and not only on outer surfaces, but inside the cavities as well! What’s not to love? This process definitely deserved a separate made-from-scrap article, so you might as well subscribe for my blog, not to miss the second part of it!
And now its a show-time! Let’s weld some metal strips together.
Ok, thicker ones?
How about a larger piece of steel rod?
Notice that I’ve switched electrode tips to steel ones in the last video. What was the reason? Because copper, as well as it conducts current, also conducts heat away from the part. And I wanted the part to be heated evenly. Why is that important? — By evenly heating steel part and then dropping int into water/oil it is possible to harden or otherwise heat-treat it. Heat-treated bushings make a great inserts for a drill guide like this one:
This drill-guide had a lot of use in assembling of the workshop’s custom furniture, such as described in Fume-hood DIY and jewelry workbench, for which another article is in preparation.
There were lots of other uses for the welder in my workshop but this article as already long enough. But I might gather some of additional materials for a small “after-show”.
That’s it for today. Stay tuned, spread likes, subscribe, you know it!
