Originally Posted by
Kontact
Thank you, yes. The "original plan" called for using my granite kitchen counter as a reference surface with the drawn plan taped to it. So pins would have served like tack welds. But given how "sloppy" I'm going to have to make the DT lugs, I doubt pins alone would have preserved angles well enough.
Last year I learned some real lessons in how things can move around when I sweated a front sight with a spacer onto an antique gun barrel.
That's good to know you can get away with an all wood fixture.
Tubes moving when heat is applied is the reason a person has to have a strong strategy in order to have a frame end up in alignment. Beginners have the challenge of figuring out the best flame pattern to minimize distortion as well as trying to coordinate those flame movements. There are lots of opportunities to get it wrong. And they aren't necessary able to see what they are doing wrong and how to correct their mistakes.
A granite surface can make a great alignment reference. Here is my building philosophy to achieve good final alignment. And this is based on a typical beginner not wanting or able to buy or use expensive fixturing. i start by brazing the seat tube to the BB shell. I'll skip the explanation of how to do that. Next I'll miter and assemble the tubes to match the frame design. We would expect our home made holding system to keep the geometry within tolerance but not able to precisely hold alignment. Each lug has one spot on the 2 tubes it's holding. The frame is now taken out of the holding thing and aligned on a flat surface (like a granite kitchen counter). The 7 spots holding the tubes together allow for some movement to achieve alignment. I next put 2 more spots on the down tube lug's available centerline space (I'm only spotting the down tube lug so the rest of the frame can still have alignment movement after it is brazed). I check and adjust alignment again before brazing that lug. Of course it is common that their heating pattern will move tubes out of perfect alignment. Put the frame back on the alignment surface and adjust and put 2 spots on the top tube lug and realign again. The spots holding the frame allow for enough movement to put it right again. And braze that lug and so on.
I can think of other ways to braze a front triangle and have it end up aligned. I have found this method that i've outlined works well for my students (that often don't have prior experience) to be able to end up with a frame that is less than a millimeter out. Tube straightness is often worse than that. I'm leaving out a lot of details because my post is just to give you an idea of my building philosophy for beginners and not be a detailed description of what to do.
I'm explaining why pinning that will restrict alignment adjustment may work against a beginner whose flame movements are much more likely to distort a frame than a pro whose precise heat pattern minimizes distortion.