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Thanks for explaining ,amazing method! And amazing patience!
I was thinking about this problem. You could 3D print a "lug", which goes around the tube and lets you mark the cutting line directly on the tube with a sharpie. These "lugs" could have feet, so that one on each end of the tube puts the mitres perfectly in phase.
If you would like to try this I'm happy to design and print some for you, I'm curious if it works.
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RabiCycle
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Probably a bit wasteful for single use. Every frame will have different angles and tube diameters. You can print off paper templates to wrap around the tubes that do this job fine albeit without the clocking
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Ah nice yeah its a good idea, ive seen similar things that people have done to mitre tubes for roll cages and things. I think the challenge with a frame is that the angles vary so much for each frame that you would need to print a new pair each time for each tube, as @PhilDAS said you can get mitre templates out of bikecad. However itd be good if you had a batch of a few tubes with all the same fit.
PhilDAS
Biggles567
Thanks :)
Yeah at the moment i mitre everything by hand. I did buy a tube notcher just before i left london but didn't manage to get it properly set up to use, and its still in london. I would like to figure out another way as by hand can be a bit tedious sometimes, especially with tig because you need really good fit up. My technique for main tubes is fairly simple, i measure the shortest distance along the mitres using the tool in the photo. Then I cut the short butt mitre and mark from that the shortest distance, then i can file/cut to that. The cutout for the downtube/seatube i mostly eyeball. The biggest challenge is keeping things in phase, so that the mitres are aligned rotationally to each other.
Chainstays are fairly easy, i weld them to the dropouts first then i can cut them to fit, they're quite easy to eyeball and the shape of the mitre is very standard so not too bad. Seatstays are a bit awkward but you just have to take it slowly and continually sense check where you are cutting, otherwise its really easy to file the wrong way and scrap one.
Usually the downtube is the hardest part for me, it just takes a lot of patience to slowly creep up on the right cut, its particularly difficult with large diameter tubes like the track frame i'm building now, which is a 44mm life tube. Im not sure why the larger tubes are hard but they seem more sensitive to being out of phase. A big tube like this is also difficult because its larger than the bb shell, which is 38mm. So i offset the cut in the downtube so that he difference in diameter is all left above the bottom bracket, this then gets removed from the downtube by the cutout for the seat tube