Dumb centering cone question.

I have cones on my jig while other designs seem to have cone on one side and flat disk on the other. I can take a bike out and put it back in the jig such that the position of the frame relative to the solid part of the jig is will repeat within less then 0.5mm when measuring the alignment of the rear drop outs. Basically right now I am using the jig as a poor man's alignment table. The bike comes out of the jig for welding and then goes back in for checking position. Like I said I get measurement repeatability of about +/- 0.5mm so for the point I am at the jig works just fine as a dual purpose device.

For now, I am using a C clamp to put a nice even pressure on the cones for clamping the frame. I am actually going to modify the jig to install a threaded rod to make this easier to manage.

My cones are steel and no damage to BB threads because they engage on the ring of the BB rather then on the threads.

https://febikes.files.wordpress.com/2011/02/sn851818.jpg?w=574

I used a rod w/ a flat register turned on it on the fixture side and a stepped washer on the outer side w/ a bolt though them. That doesn't work either. When welded, the top of the BB will shrink more. So until you're done welding and chase/face the BB its not square again. Quite the conundrum. My frame ended up w/ a VERY slightly tilted BB due to that. It never would sit even on the lip again. The seat tube cone was the only thing holding it square.

I think a rod w/ a DEEP step to just clear the ID of the threads would work best. Of course when the BB distorts oval it'll grab the rod Some sort of expanding arbor would be just about perfect - and act as a heatsink. Bonus.

So short of a super trick soln, I think cones are better than a flat surface. Just have to measure their offset more carefully. (A shell w/ slightly larger ID will sit a little deeper on a cone causing the BB to offset. Steep cones wont have much of a problem.)

smdubovsky said:

I used a rod w/ a flat register turned on it on the fixture side and a stepped washer on the outer side w/ a bolt though them. That doesn't work either. When welded, the top of the BB will shrink more. So until you're done welding and chase/face the BB its not square again. Quite the conundrum. My frame ended up w/ a VERY slightly tilted BB due to that. It never would sit even on the lip again. The seat tube cone was the only thing holding it square.

I think a rod w/ a DEEP step to just clear the ID of the threads would work best. Of course when the BB distorts oval it'll grab the rod Some sort of expanding arbor would be just about perfect - and act as a heatsink. Bonus.

So short of a super trick soln, I think cones are better than a flat surface. Just have to measure their offset more carefully. (A shell w/ slightly larger ID will sit a little deeper on a cone causing the BB to offset. Steep cones wont have much of a problem.)

Click to expand...

It's the same problem with a cone only worse. Anything that distorts the face will distort the threads and the threads start at the shell's ID chamfer. If the cone's taper is less than the shell's entry taper it will locate on the threads themselves and since threads are helical, you're guaranteed to have a misalignment.

What you're describing here is really a build/process problem. Every time you add heat to the BB shell it distorts. It distorts to the top when you weld the seat tube. It distorts to 2 o'clock when you weld the DT. It distorts to 9 o'clock when you weld the chainstays. It's NEVER flat after that first heating and it never will be again unless you machine face it. Hand facing can make it better but it still won't be that great; results are highly dependent on the operator and the tool being used.

What's the answer? Start with a blueprinted shell. Don't take the frame out of the fixture until you're done with the joining processes that affect the BB shell. Or at the least, don't chase your tail by constantly chasing and facing the BB shell to achieve some dubious result until after you're done joining tubes to the BB shell.

It's pretty obvious to me now regarding distortion. I got some ISCG tabs welded on first, which may sound like I'm doing things out of order.

This frame is going to be one of those carbon lugged bamboo frames, and I didn't want to weld the tabs on after the BB lug was done because I was concerned there might not be much real estate left (this is only my second frame, my previous BB lug took up a lot of volume around the ends of the poles), and didn't want to chance the heat making unknown cosmetic ugliness on the nearby carbon.

Anyway, back to the lathe for me, just going to turn some lipped cylinders that barely clear the inner thread diameter, and I guess it's going to be sort of a cut-and-check-and-repeat throwaway piece of work, since I don't have to worry about further warping.

I appreciate the insight.

Maybe the frame description sounds a little weird, trust me I've turned down the crazy dial a lot since my first set of ideas.