Let it be known that mitering, or cutting curves into the ends of tubes so that the end can press flush against another curved surface, by hand is not easy. The title of the skill alone points to at least one flaw in my assumptions. We aren't using any power tools. I thought that, since there was a power mill with what looks like roughly a 30mm bit attached, we'd drill the rough shape of our miter and polish it by hand. No such luck. Thus, we miter by hand with a gamut of files. It's plenty fun, but definitely a perfectionist's nightmare.
First, a bit about the tubes. Our first bit of mitering was cutting the seat tube to intersect the bottom bracket shell and the down tube. Koichi first explained the necessity for precision and the differences between lugless (fillet-brazed) and lugged bottom bracket shells. One of the risks that one runs with any joint is failure, and doubly so with a lugged bottom bracket shell. Not only can the joint shear or crack, but the seat tube bond can come undone and the whole tube slide down into the bottom bracket shell, causing chaos. With a closed cylindrical bottom bracket shell used in lugless frame construction, the tube can still come unbonded and cause chaos, but the tube won't ever break through the shell and jam up your bottom bracket. For this additional reason, the necessity for clean and strong brazing is very important for a lugged bottom bracket shell; however, no matter what style of frame, you want your joints to hold up forever and ever and failure can be calamitous.
Since the down tube and top tube come so close at the bottom bracket shell, an additional miter must be cut into one of the two tubes. In theory, either tube could be cut again and the same bonds could be achieved. However, to at least begin to address the safety concern above, Koichi suggests that the seat tube get the second miter. That way, the lip of the down tube that enters the bottom bracket lug can offer some support to the downward force of the seat tube. Does that make sense?
Also, in our case the seat tube is single-butted steel. That means that one side of the tube has a thicker end of a given length than the other; the inner diameter is smaller at one end than the other, and at some point along the tube there is a transition. The thicker end is meant to be joined to the bottom bracket shell, and the thinner end to clamp down around your seat post after being cut to fit. However, the length of that butted end can be shortened for a more flexible - and slightly lighter weight - bottom bracket cluster. Thus, for some lighter riders, that end would get trimmed down to make the bike a bit less stiff and more comfortable, but for a heavier rider such adjustments may make the bike feel inefficient and flexy. More things to consider during the build!
So, after a few brazed towers this morning, we got into mitering. There are some pretty neat programs and web applications out there that offer custom printable templates for different miters. Our first miter for the seat tube bottom bracket shell is a ø28.6mm tube to intersect a theoretical ø33.0mm tube (roughly the inner diameter of the bottom bracket shell) at 90º orientation, like a T. We trimmed our templates and taped them around the tubes, and used a permanent marker to trace the curvature. Clamped in the vice and adjusted to level, we began filing down the flat surfaces until the shape roughly followed our marker line. Then, we fine-tuned the cut with sand paper and light file strokes until the miter was flush against a surrogate ø33.0mm scrap tube that was also perfectly level. Being our first time, it took about two hours before our work was passable. (Koichi's demonstration cut took him about 15-20 minutes, just for reference.)
Next, we worked on the down tube - bottom bracket shell miter, which was basically a duplicate of the seat tube work we had just done. (The only difference was the inside of the tube. The down tube is double butted, rather than single butted. Both ends of the tube get thicker away from the middle.) The work went much faster this time, delayed only by me slicing open my finger on a sharp edge when my file slipped. A couple paper towels and some hydrogen peroxide later I was back to work.
Once these tubes were both cut to fit, it was time to miter the seat tube again where the seat tube makes room for the down tube. This time, we used the lugged bottom bracket shell as a reference for the curvature needed, and began filing again. Koichi made a point of adjusting our vices so that the tube was angled to match the angle at which it enters the bottom bracket shell. This simple adjustment meant that we could keep our filing motion exactly the same as that which we used to file perpendicularly to the tubes. Straight, horizontal, slow and steady movements made for consistent miters. The most difficult part about this step was accurately tracing the inner edge of the bottom bracket shell onto the seat tube, and having that line actually work for us when filed down. Mine was way off, and took a while to make workable.
Finally, we cut the top tube to fit against the seat tube, at the appropriate angle to match your seat angle. Mine bicycle will have a 73º seat angle and a horizontal top tube, so I had to miter the top tube accordingly. It takes a lot longer to work at an angle like that, because (a) you're filing more metal, and (b) the metal smears more as you file, adding to the friction and basically making you file some bits over and over again. You do what you can to keep it to a minimum by filing the inner and outer surfaces of the tubes as often as you think of it.
Eight hours, four miters, three tubes, and one bloody finger later, and I've got something that resembles a bike frame.
THIS IS SO COOL!
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