Hi Peter
If you want to get it going again I suggest you take it to a local welding supply/service place, as I did with the one pictured. There'll be one in almost every city.?They told me a blade welder is stone-axe simple (just one transformer a few switches and springs, no electronics or timers). The technicians had all seen and worked on them. It shouldn't be hard to reconstruct yours.
This is how my one works, referring to the pictures attached:
Looking at the outside (photo 1), there are normally two cam-locking vices - one for each side of the blade to be welded.? They're made?from copper or bronze,?and each one is connected to one of the low voltage/high current outputs on the secondary side of the transformer. Cam-locks are a way to make a good low resistance connection to the blade ends.??
With the blade ends touching, when the current is turned on, the resistance of the steel is high enough to heat the blade ends to melting point.
To make the butt joint the white hot ends of the blade are pushed together.? Once it has moved far enough the current turned off.? The cooling metal solidifies and stops the ends from being pushed?together?any further.
To allow the blade ends to move toward each other one of the two vices slides on dovetails (photo 2), the other one is fixed.?
The 'Pressure'?knob?(top RHS photo 1) adjusts the pressure on a spring, pulling the moveable jaw toward the fixed jaw, via a cam and lever (photo 3). The more you wind the knob the more spring pressure is applied to the cut ends as they are butted together. When the ends melt the spring pulls the moveable vice toward the fixed one upsetting the weld, which stops the current from just blowing the ends off the blade (the vice must move freely with the spring pressure). There is a plunger-operated micro-switch bearing on the moving vice that cuts off the current when the vice has moved far enough (see it in photo 4, I removed to make the dovetails clear in photo 2). Then the vice will continue to move only for as long as the metal is molten.? The greater the spring pressure the further the vice can move in that time.? Wider and/or thicker saw blades use higher spring pressures.
The rest of the wiring allows different voltage taps on the primary side of the transformer?(the 2 in the centre of the strip of connections screwed to the transformer)?to pass lower secondary current (but still high enough to heat the joint) to anneal the weld and other connection to power the grinder - jv??
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