The heat from brazing causes the tubes to expand and contract and as the heat is focused on one side at a time, this has the effect of pulling the tube one way, out of line.
Apparently, since the jig holds it all together, the tube will pull but be held in place and when the fillets cool, you'll have built tensions into the frame which can cause cracking or failures in the future.
Don't think any framebuilders will braze a full frame in a jig, just tack as M_V says.
(This is what I have read, I don't have enough experience to say either way)
Back in the early 90's when cheap lugged 501 frames were mass produced this was a common failure mode, e.g crack s between head tube lug and shifter boss.... Raleigh, falcon, Holdsworth etc brazed in jigs by factory workers
Interesting, I hadn't heard that before
That cheap Holdsworth I chopped up sprang like a fucking kangaroo when I cut through the seattube.
To be honest though, I don't think I've ever cut up a frame (and I've cut up a few) that didn't twist at least a bit once given the opportunity.
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