I built a frame many years ago, it was done as a TIG practice at my first job (they sold welding gases, MG Industries). It was terrible. I had a fab'd jig but things didn't work like I expected.
Terry's idea for a jig seems a lot more solid than I what I hobbled together which was wood blocks I drilled through and cut in half and then screwed to a plywood table. The only way I would attempt a frame without a jig is with lugs personally, although a track frame is probably not nearly as critical. You can focus on the weld/braze without worrying about how the tube align and are held in place. You at least need some sort of fixture for that I'd think.
But with a full jig holding all the tubes you can make sure things are straight. This was the basic problem with my frame, the rear wheel was off axis to the front, so it tracked odd and drifted left. Not to mention the blow through spots... If I was to try again it would 100% be a lugged steel that was brazed.
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