A fine-spot welder is one of the few equipment where building yourself is cheaper than buying. There are already published a lot of DIY spot welders, this one has 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.
See more here:http://www.avdweb.nl/tech-tips/spot-welder.html