"Dude...what a tool!"

*** UPDATE 6/11/08: I have created 3D models of the new version of this jig (described near the end) ***

The next phase of this project is to make a fixture for brazing brake mounts to a frame/fork.

I got the idea from the excellent Nicola Cycles blog (thanks for the link, dude!). This summer they hosted a free framebuilding workshop near NYC that must have been a blast.
In a batch of photos from around that time, I found this little gem:


Money shot:


This is a clever design that was likely pretty quick to machine. I started playing around with how to improve on it, with the help of a mechanical engineering student and some machinist friends.
Perhaps most importantly (for me), this design will work for both standard cantilever brake bosses/pivots and custom-made centerpull bosses/pivots.

I/we eventually decided to implement the following additional features:
- Make the dummy axle self-centering, by copying the clever Anvil dummy axle design, minus the tricky-to-machine (but oh-so-sweet) "D"-shaped ends that keep the dropouts parallel:

Don and his dad do a great job on these...they are little works of machinists' art.

- The Nicola design appears to permit the fixturing of brake pivots that are out of plane with the axle (and thus the rim). One solution is to machine a flat on the top of the "spine" bar that runs up the middle of the fork, and use a set screw to set the blocks in plane with one another. (n.b. Any error in the angle of the posts would likely be easy to correct when setting up the brake pads. I'm just anal like that.)

- Because of equipment limitations (no slitting saw), we replaced the pinch bolts on the pivot holder with another set screw and a milled flat.

- Add the ability to use this tool as a brake/fender bridge. This is achieved by tapping 2 holes for the set screws mentioned above, and leaving open the option of using a long M6 bolt to hold a bridge in place. The tricky part is that the aluminum might get too hot and deform or even melt.

- Change all materials (except the blocks, which remain aluminum) to 303 stainless, which is easier to work than other kinds of stainless, though it's still a pain to machine compared to mild steel. But I got some 303 scrap in the right sizes for free.

Thus I drafted and scanned the following:




A few notes on drafting:
- These were made for a friend who I trust as a machinist and who with whom I've discussed the design, so I'm letting him choose a few dimensions, and violating a few other rules of drafting. In theory, the drafter makes all the decisions about sizes and shapes, and the machinist makes all the decisions about methods. I've crossed those lines in a few places.
- Dashed lines indicate hidden features or edges.
- "Phantom" lines (short-short-long) indicate centerlines and imaginary features. Gear teeth are often replaced with phantom lines, as are threads, because drawing them would be repetitive and wouldn't add anything.