Check out this How To Video, on making A Professional Grade Spot Welder Made out of a Microwave Transformer. There's more info, in the links below and check out the related links too...

Don

DIY resistance fine-spot welder

<br>

Video Link...
https://www.youtube.com/watch?v=HdyLPcTVVFI

5,735

Published on Nov 2, 2013

A fine-spot welder is one of the few equipment where building yourself is cheaper than buying.
See more here: http://www.avdweb.nl/tech-tips/spot-w...

These are some unique features:
It can be used in 2 welding applications: opposed and series configuration.
The construction is kept very simple.
Accurate electrode force adjustment.
It has a solid electrode holder, made of a radiator earthing clamp.
An Arduino microcontroller is used to set the weld time accurately.
Creates a double pulse which improves clamping.
The current can be reduced for welding sensitive parts.