So how do you keep a frame in alignment while building?

Normally a fixture is used to hold all the tubes in alignment, using the head tube, bottom bracket, top of seat tube and rear dropouts as reference points. At various points in the process, an alignment table is used to check and tweak the alignment. It is possible to use a flat surface and spacers as a reference to build a frame. My first frame was built with a straightedge, clamps and spacers. It rode fine for thousands of miles.

I only tack-braze the joins in the jig, then fully braze each join in turn in free space, checking the alignment on a flat table after each join. I try to use heat to keep joins in alignment so that minimum - if any - cold setting is necessary.

Scooper- Is there a particular part of the lug/joint you start with when you tack braze? Multi spots? A cross pattern, like when torquing something with 4 bolts? Thanks for any insight.

Also, Squeezebox check out PithyBikes channel on YouTube he has some nice easy to follow videos with excellent production about his process when frame building. https://www.youtube.com/channel/UCqQ...Cgviyj_E0VEy9Q

Regarding the brazing sequence, I use the process recommended by Reynolds Technology in its 531 Technical Advice data sheet, starting with the bottom bracket shell and working clockwise:

I usually tack braze each lug to tube join in four spots 90° apart while the frame is in the jig, then remove the assembly from the jig and fully braze the down tube-to-BB shell and seat tube-to-BB shell. I "pull" the down tube and seat tube into alignment using heat from the torch and checking alignment on my alignment table. Then I fully braze each successive join as described in the Reynolds procedure, checking and adjusting alignment as each join is brazed.

This method works for me, and ensures there is no cumulative stress built into the frame as often occurs if all joins are fully brazed in the jig.

Thanks wheelnut looks like an interesting set of videos. More than enough to satisfy my curiosity.

With TIG the jigs are often better, and the joints are often better. It's a different culture. With lugs, you can often get a dead nuts set up on a joint with a pass through of the tube. With TIG they are basically all butt joints, and ideally they would be perfect, which is easy to achieve with an industrial level set-up (or due care with a file). With TIG some welds will require a root pass, and a filler pass, others can just be tacked and welded. You have to be very tactical on how you tack the tubes to minimize distortion but build enough strength into the tack, but not make it unsightly when you weld through it on the run. I haven't done much welding recently, but I did a utility table where I had to weld some 1/4 wall tubing to a quarter inch plate, and I used a tiny stick unit, and made some small welds off 110 in the wall. I got a little overprottective and ended up bending the plate like a banana, I just never thought that would happen with the scale of the welder and the plates. The forces are considerable, even when they aren't matched to walls 1/10th the thickness. In fact bike fillets from TIG are way oversize, so the people that are good at this are running huge fillets on tiny thin walls, and turning out straight work.

Scooper's sequence still leaves a lot up to the skill of the operator. Guys that can do perfect lugs or brazing jobs, with minimal clean up and minimal distortion, that is lots of experience at play, and huge skill on display.