Friday, April 27, 2007

A view from the bottom.

The cups worked. I also used my normal Harris white flux and tried to be a bit more patient with the heat. On the second of the 2 frames, with more BB warpage and therefor more gap in the joint, the silver flowed very easily. If I did this type of joint again, I would rethink a few things like designing an intentionally smaller radius on the yoke to create a larger gap with the shell.

And with the bike....

New cups installed on the frame fixture.

My solution to the heat control issue was to make some new BB fixturing cups. My standard setup is a stainless insert that is a light press fit into the BB shell. This slides on a 1" dia post on the fixture. The insert makes near full contact on the inside of the shell. These new cups only hold the outer .200 of the shell on each side. This will allow the center of the shell to reach brazing temp with less heat. I made these out of 6/4 titanium. Why? Because Ti is a crappy conductor. I wanted a material that would help contain the heat in the frame. ( I couldn't afford ceramic)

Well that didn't go as planned. It seems my fit between the yoke and the BB shell was a little too perfect. There wasn't enough gap to pull the silver into the joint. I also had heat control issues. I tried using black flux since I was anticipating some overheated spots. The BB post of the fixture acts as a massive heat sink and prohibited me from getting the shell hot enough.

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