A Mini spot welder from scrap

[skyline1]

Hi Pat

Until I started building this one I had only used a spot welder very occasionally myself and it was many years ago so I was on a learning curve myself.

This is only designed for thin sheet the maximum I have used it on is 0.5mm (each piece) which is around 20 thou' but I think it would be able to do a little more say around 1.6mm overall (63 thou') It would probably struggle at much more than that.

You don't need to bring the jaws together simply lift the top jaw against the spring and slide the components in the gap. You then just gently let it drop against the bottom one and you're ready to go, use gloves though as it may be quite hot. Check the components are lined up as you want and readjust if needed. Then as the song goes "push the button" .

You could use a spring loaded pedal if you wish, this is simply how I did it. Experimentation is the name of the game with this sort of thing. A foot switch to "fire" the welder would be a good idea though so you could use both hands on the job. You could wire it across the weld pushbutton on the controller and put a socket in the back. A momentary action guitar footswitch would be ideal and they come with a suitable plug fitted (1/4" Jack). Very good idea to turn the welder off unless you are actually welding with it lest you accidentally step on the pedal.

Yes you could make the electrodes shorter for things like tubes as you describe. I think the best way to do it would be to make one longer than the other thus bringing one of the weld arms closer to the circumference of the tube and enabling it to fit inside easier. Have a look on Ebay for spot welder arms for inspiration on this, prices are pretty scary though that's why I had a bash at building one.

Regards Mark

 

[skyline1]

Hi All

I've had a little trouble uploading the drawings but Austin is looking into it for me meanwhile should anyone want them PM me and I'll send them to you.

Back to the build








Having built the timer board it was tested and worked fine so I hooked it up to the welder (only 4 wires, +12V to PSU yellow, 0V to PSU black, + and - to the relay). The board will operate from quite a high voltage range but 12Volt relays are easily obtainable (and SSRs).

"Live" testing then commenced and this is where I hit a slight snag, the timer worked great for thicker material but on very thin stuff was a bit inconsistent. (Like my experiments making wacky bits of "modern art" out of old tin cans, which was great fun, but back to the plot).

The problem was that the pulse from the trigger pushbutton needs to be shorter than the timer delay or it "holds on" until you release the button. On really thin stuff like tin cans. (which is only a few thou' thick) the weld time is so short that it's less than the time it takes to press and release the button.

A little research was needed, luckily a search for "NE555 timer circuits" yielded hundred of sites devoted to the "little blighters" including this one http://www.doctronics.co.uk/555.htm which I found very interesting. It also has breadboard layouts and calculators for working out the timing component values for anyone who fancies having a play with them.

It turns out that what was needed was a "Pulse Differentiator" (What a mouthful) which despite it's grand name actually consists of only 4 components 2 resistors a diode and a small capacitor. As one of the components is already on the board (R4) the other 3 were mounted on a small piece of stripboard which can be seen in the photos next to the main board.

I decided that I also needed a manual override and this is where S1 on the diagram comes in. This is a single pole double throw toggle switch with a centre off position. It is spring loaded back to off on one side (the manual side) and latches on the other. (the auto side) I thought that this might be a difficult and expensive component to obtain but not so I obtained mine for Maplin in the U.K.http://www.maplin.co.uk/sub-miniature-toggle-switches-2341 Part No FH02C R.S. and Farnell also do them. In other parts of the world they should be stocked by most Electronic Component suppliers. Failing this try your local motor spares shop they are widely used in motor cars for auxilliary switches.

In operation when the timer energises it turns on TR1 and grounds the negative pole of the relay thus energising it so to pull it in manually simply bypass the transistor to 0Volts, this is how S1 works. It has the added advantage that the timer also runs although it doesn't actually do anything which can be handy to set the weld time. If this is not needed the controller will work perfectly well without it.

With everything now working I had quite a "birdsnest" of wires and bits of electronics next to the welder, (and several tin can "sculptures") so it was time to mount it up properly.

I had, in my vast collection of electro-mechanical junk (yes I am a hoarder, I don't see rubbish, I see projects). Some little project boxes. R.S. part No 508 - 605 if still available. These were not new as they had no front or back panels and had had some holes drilled in the bottom. To cut a long story short front and back panels were made from thin galvanised steel. (something else I had quite a bit of) and in went the various switches and LEDs Mostly either from the timer Kit or salvaged from the old server case. A gland in the back for the cable and the weld time pot. I will make a label for the front panel shortly to tidy things up a bit. (Scarcity of round tuits).

The two circuit boards were mounted into the case along with the chocolate block connector and the timer was wired up with 16/0.2 wire, the connection diagram above should be fairly easy to follow. I used twisted pairs mostly for neatness (well sort of, it's still a bit messy). Quick way to do it BTW is put one end in the vice or put a weight on it then wind 'em together with a pistol drill.

The controller circuit can be adapted any way you want and could be built in but it should be in a metal box for shielding (555s are pretty tolerant though).

Well there you are spot welder pretty much complete. just one more post about the cable connections and the "round tuits" (When I get one). And hopefully a video of it in action I haven't forgotten, Gus.

Regards Mark

 

[skyline1]

Hi all

I hate Stainless Steel, It fights you !:rant:

Sorry to start with a bit of a rant but I've been breaking up the inside shell of the old microwave to recover the stainless steel and guess what, it's spot welded together.

Now I am breaking spot welds rather than creating them I am beginning to realise what a monster I've created. Spot welds even tiny ones are very strong indeed especially in Stainless. It is taking quite a lot of drilling and grinding to " Resolve" this into some flat sheets I can use.

Final bits of the build.

View attachment Spot welder cable termination.pdf

In a previous "life" I ran a small P.A. Company doing concert sound and the like. As a result I had a quantiy of 3 pair screened audio Cable which proved ideal for the welder. The Connection diagram is above. Its the same old story slight overkill but it was lying around doing nothing.

It is probably not strictly necessary to go to to these lengths and only 5 wires are needed as the 2 0Volt connections go to the same point on the PSU but I had a 6 Core cable so I used all of them.

It is probably possible to use plain unshielded cable although I haven't tried it and as we are talking about low voltages and currents here a piece of computer network cable would probably be suitable. Or indeed anything flexible and rated at 12Volts or above that can carry 75mA.

Further "Round Tuits"

During the course of this post many good ideas have been suggested which I shall take up in due course

I need to finish and paint the casing and make a front panel (not sure what of yet plywood possibly.)

I shall reroute the cables from the transformer to the welding heads (shortest possible route).

I will do a label for the controller with all the names of the various controls and a scale for the weld timer.

I will probably be reworking the controller to put it all on one P.C.B. as I can see uses for it elsewhere.

Following Lew's comments about SSRs I am designing an SCR (Triac) based
power controller to vary the welding current with an Enable/Disable Input from the controller and an overheat protection function.

As well as the advantages previously discussed it has 2 other I think, quite important ones.

It will energise the transformer in a much more controlled manner and can be to an extent "soft started" reducing switch on surge.

The Transformer as it stands has a magnetic shunt fitted which I mentioned right at the start of this build (it is the metal piece between the two windings). The purpose of this is to limit the output of of the transformer. Which is why Microwave transformers ere quite well suited to this sort of job. In the microwave it also has another function but it is a bit technical and needn't concern us is this application.

The effect of removing it would normally be that the transformer would draw as much current as it wanted (too much) resulting is blown fuses, tripped circuit breakers, and very likely a burnt out transformer, (It wouldn't do the relay or the wiring much good either).

But with a means of effectively reducing the input voltage by phase control (the same way a dimmer or a speed controller works) I think It would be possible to extract more current from the transformer than it's original design allows In short bursts of course and the supply wiring would need to be beefed up a bit (you can't get out what you don't put in)

There are problems to be overcome with this idea and this is taking me more into the realms of commercial units and away from my scavenge and repurpose theme but is worth future experimentation.

Well that just about wraps up the spot welder from scrap project I hope it has proved interesting and given some food for thought.

Regards Mark