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Making a better spot welder.... Answered

I am in the process of building a proper spot welder from scratch.
Proper more in terms of the electrical and electronics part but not so much in looks ;)

My problem now is to find useful info on what power levels are required for certain tasks.
I realise that welding thin sheet metal won't need as much time and amps as welding a 3mm stainless steel rod - but what is a "good" power level?
I watched a bunch of Youtube videos showing various approaches but for many it seems the producer had no clue about the difference between creating a short with burn marks and a weld...
Especially when it comes to creating battery packs with a capacitor bank as the main power provider you can clearly see the device burns holes but does not really create a welded spot.
On the other hand there are a few videos showing spot welder made from a MOT that seem to produce a proper melted and welded connection.

When I used a proper spot welder at work it had timing settings, power levels and even a feature to adjust how the current rises....
Not to mention a gauge that checks the pressure and only activates the welder once the set point is reached...

There is a ton of info out there that after a thausand words still tells you nothing you need to know :(
So is there anyone here who can shed some light on the actual process of spot welding in easy words for everyone to follow?
I am aiming for a max output of around 400A @ 1.5 -2.5V with an adjustable shunt in the transformer core to avoid oversaturating the core.
In a later stage I will add power control over the primary side but until then it is only time control, from a few ms to a max of 5sec if the damn controller arrives one day.

Big questions:
1. Is a power control really required or is it possible to cater from thin to thick just by using different timing settings?
2. Since a MOT is used for the power supply: Is it better to leave the shunts out to fully avoid saturation by adding an inductor in line with the primary or is it still better to adjust the shunts under load to get the maximum power possible?
3. Aluminium and other materials benefit from using AC but would be good to have a DC output too, if so then what materials really need DC?
4. All I could find is that copper is used for the electrodes, due to resistance and heat transfer - are there other options apart from using copper?
5. Tricky one: I would prefer to use the secondary winding as the new primary to avoid core saturation and to lower the load on the power outlet.
Where can I find very thin copper bar material that I can coil up and insulate as I would quite a few more turns to get at least 1.5V out of it?
Just don't like the idea of spending days rolling a copper bar thin enough....

For the advanced model at a much later stage:
Of course I would like to be able to use a proper power control instead of a motor dimmer or similar.
For obvious reasons an inverter microwave jumps to mind.
But after checking one I noticed one big problem: there are not really that many windings on the primary of the transformer at all!
Same way our modern switchmode power supplies only use a few turns these things do exactly the same.
After some quick and dirty initial tests I realised that even a single turn of thick wire already results in over 20V on the secondary.
Wasted a lot of wire and time making one coil with 5 turns less and one with 10 turns less but the system would not even start with it.
Seems these things need a fixed inductivity on the primary that matches the frequency used, in my case 36kHz.
Would love to overcome this problem so I can at least go down to a single turn to get under 5V on the output side as space is non existing on these inverters.
Can I cheat? Do I need to change the circuit to match the new primary coil? Am I thinking in the wrong direction altogether here?
And added bonus would be to be able to adjust the power from around 15% to 100%, so far the electronics don't allow anything below 45%.
Is it possible to drive these inverters in resonance? (Ok, off topic as I would like to use this for a beefy HV supply)


Last thoughts:
I know people already used Arduinos and Raspberries with displays and all but so far I have not found anything that shows how to do it properly.
Seems all that counts is to create connection one way or the other and to call it a spot weld even if it is just a burn hole from discharging a capacitor bank through a needle like electrode....
For obvious reasons I don't want to create just another spot welder that makes a professional pee himself laughing about it.
IMHO nothing beats personal experience with something but I don't really like wasting my time by trying what other people alread did a long time ago.
So if YOU already built a MOT based spot welder and used for more than a few spots I would love to hear from you!
Let me know what type you used, what problems or shortcomings you noticed or where you feel it just does not work out the way you expected it.
From simple things like always getting bad sparks or arcing, over how easy or hard it is to get consistand results to whatever really annoys you while using your homemade spot welder.
I hope that your feedback here will help me to write an Instructable on building a spot welder that does what you expect it to do, not once or twice, but everytime you use it.
Mechanics might vary the same way the electrode style does but the weld should always be a proper weld that won't tear apart ;)

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