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  1. #1
    Senior Member Sherfy's Avatar
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    Bottom Bracket Fixturing

    Looking for how people fixture their bottom bracket in their jigs, either on a flat plate or upright jig. I'm curious as to how you keep the shell axially aligned with whatever you're using to clamp the shell, either threaded rod or sliding 80 20.

    Any help is appreciated.

  2. #2
    Randomhead
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    I've always had something that referenced both the flat on the side of the shell and the threads. I recently found out that the minor diameter of BB shell threads varies quite a bit. If the shell faces are flat, the axial alignment of the shell will take care of itself if the clamps are flat. In reality, it's not easy.

    I built my first couple of frames in BBurg. And the one in my avatar for that matter

  3. #3
    Senior Member Sherfy's Avatar
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    Quote Originally Posted by unterhausen View Post
    I've always had something that referenced both the flat on the side of the shell and the threads. I recently found out that the minor diameter of BB shell threads varies quite a bit. If the shell faces are flat, the axial alignment of the shell will take care of itself if the clamps are flat. In reality, it's not easy.

    I built my first couple of frames in BBurg. And the one in my avatar for that matter
    Right on! Blacksburg is an awesome town and tech a great school. I've been meaning to find my way back there.

    I understand the flat part of the shell, that keeps things square, I guess my question is what do folks use on something like 3/4" rod other then the flat washer and wingnuts that I can visible in jig fixtures. Look through tons of jigs, most seem to have custom made pieces around the bottom bracket.

  4. #4
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    What about a tail pipe expander? It will hold the BB perfectly.

  5. #5
    Senior Member mpearson76's Avatar
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    Quote Originally Posted by ultraman6970 View Post
    What about a tail pipe expander? It will hold the BB perfectly.
    This thread is a bit old but I'm wondering if anyone's tried this. I'm trying to fix the shell in an 80/20 jig between two brackets holding a length of 1/2" threaded rod (I'll try to get a pic up here later, but I'm at the office now). Was debating whether to try to turn some cones as I did for the head and seat tubes, or try something like the tapered washers at Paragon Machine Works. Those washers are made for argon purging, which I don't need right now, but it seemed like a nice solution for my purpose. Only problem is that it will not fit snug on the threaded rod, so it doesn't really solve the problem of centering the shell.

    The other idea was to try something like this tailpipe expander, but I'd have to be able to get it to fit onto the threaded rod. I'm not sure what size those threads are, so I may just order it and see if I can get it to work. I'm guessing I could take the shaft out of the middle of it and thread the two cones and the expanding plates onto some threaded rod (either using a size that fits or drilling the cones to fit onto the 1/2" rod), then use nuts on either end to center and expand it. If this works it seems ideal, because it would hold it perfectly centered on the rod, and I'd have a bit of wiggle room to center it on the seat and down tube.

    Just curious if anyone has tried this or knows whether it will work. I think I'll go ahead and order that expander--with my Amazon Prime subscription, it's $12 shipped, so it seems like a pretty cheap experiment. I'll report the results.
    DFL>DNF

  6. #6
    Framebuilder
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    I doubt the tolerances for that expander are tight enough to make it hold the bb consistently straight.
    Just looked at your link, and for $11.99, I'm sure it won't be straight.
    I've seen cones offered by guys who make motorcycle jigs that are made to fit threaded rods. They are listed on ebay a lot.
    Also, you might want to contact Alex Meade http://www.alexmeade.com/Tools.htm and see what he can do for you.

  7. #7
    Senior Member mpearson76's Avatar
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    Quote Originally Posted by Live Wire View Post
    I doubt the tolerances for that expander are tight enough to make it hold the bb consistently straight.
    Just looked at your link, and for $11.99, I'm sure it won't be straight.
    I've seen cones offered by guys who make motorcycle jigs that are made to fit threaded rods. They are listed on ebay a lot.
    Also, you might want to contact Alex Meade http://www.alexmeade.com/Tools.htm and see what he can do for you.
    Thanks for the tip, I'll check it out.
    DFL>DNF

  8. #8
    Decrepit Member Scooper's Avatar
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    Joe Bringheli uses cones on bar stock with Allen set screws for securing the BB shell on his frame jig, and a machined surface round bar stock bolted to the table surface with a centered threaded rod onto which another round bar stock with a machined surface and centered female threads (matching the threaded rod) with a handle is screwed down to secure the BB shell on an alignment table. He sells the latter separately for those building their own alignment tables.

    Frame Jig:



    Alignment Table:

    - Stan

  9. #9
    Randomhead
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    I recently found out there are significant differences in the minor diameter of the threads on BB shells. I guess this is why many people use cones. I've always referenced off of the minor diameter on previous jigs I've made, and it never really was a problem before.

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