On Wed, 20 Oct 2004, Mike Hord wrote:

> So am I then to understand that spot welding, in its simplest form,
> simply involves two electrodes on a metal object?  Say, a ground
> clamp in one corner of the sheet metal and a "wand" with which
> the desired spot weld points are tapped and PING! the metal is
> miraculously* joined?
>
> That sounds like it would be ridiculously easy to fabricate at home,
> but I suppose Lawrence already said that below...

Actually it's fairly hard. The point is to get a resistance that is *just* 
right between the pieces to be joined so that your power supply dissipates 
its power there and not somewhere else at least until the facing work 
pieces are red hot (by which time the heat increases their resistance and 
they start being willing to dissipate most of the power sent into the 
circuit as more heat). Actually achieving this is hard. For battery 
welding it is even harder because the battery contacts offer very little 
contact surface to make a low R contact on with the counter-electrode. 
Circuit resistances of 1-10 milliohms are usual, with up to 5V voltage for 
spot welding (means 500A - 5000A or more). Capacitor impulse welding 
requires that this current be switched nearly instantly, in addition to 
the above. 160,000 uF is a bit large imho, I had success using ~40J for 
capacitive impulse welding of tabs 0.4mm thick, which requires about 
100,000 uF and 25V. Additionally, using the right amount of force on the 
contacts is essential.

Peter
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