Well, that's the idea anyhow. The last three days that I've spent frame building (which translates into the last two weeks), I haven't built much in the ways of frames. Instead, I've built what I hope will save me a great amount of time in the future: A front derailleur braze-on mount jig. That is, I built a thing that will allow me to (in a matter of minutes) braze on a front derailleur braze-on tab to a road bike seat tube.
Before I braze the front triangle of the frame together, I like to do the braze-ons. That way, any distortion made to the tubing happens before the frame is together. Because any distortion after the frame is together could cause misalignment, and add further stress preload to the tubes. But this makes it really hard to get the braze-ons exactly where I want them, as there is no other part of the frame to reference. Many braze-ons can be off by a millimeter or two, and not really affect anything. Just think of the travesty that would occur if your water bottle mounts were another millimeter further for you to reach whilst riding. Not to say that I don't spend entirely too much time making sure they're exactly where I want them. But if they were a tick higher or lower on the tube, I wouldn't exactly scrap the whole project. But if your fancy front derailleur needs to be one or two millimeters higher so you won't shift it straight into your chainrings, but can't because the mount is just a hair off... well that pretty much sucks. So normally, I spend about a half of a day measuring, re-measuring, checking, re-measuring, tacking, checking, and brazing to make sure that darn thing is exactly where I need it.
But now, once I make the BB miter on the seat tube, I hook up my modified bottom bracket shell into my frame jig, bolt on the derailleur braze-on tab, load up the seat tube (held in place on the top by my frame jig, and on the bottom by the braze-on tab itself and another bit of steel welded to the faux BB), and braze away. It should be dead on to both handle compact road cranks, and the typical 39-53 combo. If a customer does want to make it custom to handle something outside that range, I can use only one of the two tab mounting bolts, and go higher or lower. But no matter where I put it, the tab mounts square and flush to the seat tube, and only a few minutes to set up! Oh man, I can't even tell you how excited this makes me. And no, I won't build you one! (maybe one day when I have a full machine shop, and I can make something like this a little quicker)
Three days of hard work, and this odd little thing is what I ended up with.
A mitered out bit of extra down tube helps hold the seat tube in parallel.
Here it is in action, mounted up in the frame jig, just after I used it for the first time.
And after all that, there it is. Right exactly where I wanted it, and square to the BB shell. (when it is in there)
