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Wallace
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I Tig them on brass plate LINK. You could use Mig but there would be a lot more material to grind off. Once welded the joint is really brittle and any flexing of the blade will snap it so anneal the joint before grinding and if it gets hot whilst grinding, anneal again before use. Annealing is the key to them lasting and trying not to get an undercut on the tooth side and back of the blade.
Wallace
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I used old blades to practice on and test the annealing process worked. I had one well used blade that started ticking as it went through the rollers and fully expected the joint to be cracking and was really pleased to find the blade cracking elsewhere in several places, the joint being perfect.
Dcal
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Excellent job @Wallace
I hadn’t seen that post before.
It looks like the splicing kit you used to get for magnetic tape.
I'm off to cobble something together and see how I get on.
Won't be as professional as yours but I hope I can make it work.
I get blades form Tuffsaws and one just broke at the weld and I can't be bothered sending it back to get it rewelded.
To be honest I'm not that impressed with them anyway as I can always feel / hear the weld going through the cut and that’s where they always start to loose teeth.
As a result I have lots of mostly sharp M42 blades with a dozen or so teeth stripped off.
It would be great if I could make a few usable blades out of them.
Wallace
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It is well worth trying a few welds on an old blade, the biggest problem is not flexing the blade after welding as the joint is like glass and breaks easily. I had a couple break when releasing the clamps and the blade returns to its natural state so I make sure to keep the welded section of blade flat and slide it across for annealing.
brightspark
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use them, in a fretsaw
Dcal
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Inspired by the thread @Wallace put a link to, I gave it a go.
I made a quick guide and clamp out of aluminium on the shaper.
I messed up cutting the relief for the weld with a milling cutter in the vertical drill but it won't affect how it works.
Tried it on some scrap blade to practice then gave it a go on a good blade with a broken weld.
I tigged it at 32amps in the end to stop blowing the edge using 0.8 mig wire for filler.
Ground it down with a cutting disc and heated it with a propane blow lamp to cherry red to temper it.
I then tried it out on some rail track.
Worked as well as a new blade.
I still hear and feel the weld going through the cut but it did the job.
We will see if it lasts, but I'm quite pleased for a first attempt.
I made a quick guide and clamp out of aluminium on the shaper.
I messed up cutting the relief for the weld with a milling cutter in the vertical drill but it won't affect how it works.
Tried it on some scrap blade to practice then gave it a go on a good blade with a broken weld.
I tigged it at 32amps in the end to stop blowing the edge using 0.8 mig wire for filler.
Ground it down with a cutting disc and heated it with a propane blow lamp to cherry red to temper it.
I then tried it out on some rail track.
Worked as well as a new blade.
I still hear and feel the weld going through the cut but it did the job.
We will see if it lasts, but I'm quite pleased for a first attempt.
Dcal
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Sorry, yes I know, just my bad writing, but it was late.
I do accept it's important to get these thing right so thanks for correcting.
How long should I keep the weld cherry red to fully anneal it?
It's that thin it cools as soon as you take the heat away.
Wallace
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I thought about the fast cooling when annealing, also how much gets annealed. Not saying this is THE way to do it but with the little butane pencil torch, I heat from underneath aiming to get the smallest section around 8-10mm cherry red then back the torch away slowly trying to keep the glow without heating too much of the blade. I should have taken more photos of the experimenting I did with old blades as I have forgotten most of it! 
I have been considering an electrical anneal with DC current between copper anvils but that's a bit exotic for the amount I need to make.
I have been considering an electrical anneal with DC current between copper anvils but that's a bit exotic for the amount I need to make.
Dcal
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I thought about the fast cooling when annealing, also how much gets annealed. Not saying this is THE way to do it but with the little butane pencil torch, I heat from underneath aiming to get the smallest section around 8-10mm cherry red then back the torch away slowly trying to keep the glow without heating too much of the blade. I should have taken more photos of the experimenting I did with old blades as I have forgotten most of it!
I did similar but with a blow torch and it heated a fair bit.
I was trying to give it time to let the carbon out.
Should I clamp some alloy each side to keep the rest of the blade cool?
I could make an alloy block to clamp the blade with just a slot for heating.
Might need more intense heat then (oxy acetylene?)
I'll keep an eye on the one I did and see if the teeth last in the heat affected zone.
It got a decent workout last night anyway.
Another thought, I've a spot welder.
Wonder if I could make up a couple of anvils for that?
Could start looking through the computer scrap for heat sinks for old CPU chips.
It probably wouldn't have enough grunt, but what the hell.
I had to anneal after spot welding you cant anneal after silver soldering as your blades would spring apart but the process of silver soldering means the blades are annealed automatically. the anvils were copper just a larger diameter at the tips. If you want to spot weld aluminum put bits of tin between the copper tips and the aluminum.
