Mountain Bike Reviews Forum banner
1 - 18 of 18 Posts

·
will rant for food
Joined
·
4,005 Posts
Discussion Starter · #1 ·
Aspiring hobbyist here, and this concern might not even matter, because I'm building my own jig out of 80/20, so it's not the most precise thing in the world...

I got some centering cones machined for me on the cheap. Is using cones to secure the bottom bracket shell a bad idea? I imagine now that the start of the shell threads would cause the alignment to be slightly off.

Not worried about damaging the threads as the cones are aluminum and the BB is steel.
 

·
Registered
Joined
·
861 Posts
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
 

·
Registered
Joined
·
861 Posts
I am not sure what you mean by "cock" the BB. I am concerned that that clamping forces I am using is putting stress on the edges of the BB shell and agree that flat areas clamping on the shell would be better for alignment checks while the cone works well for checks that involve reference to the center of the BB.
 

·
Non Dual Bliss
Joined
·
6,240 Posts
Drew Diller said:
I think he meant damaging the threads.
No, what he means is that unless you put a machine taper/chamfer on the ID of the BB shell your cones are going to follow the thread entry which means your BB will be held at an angle and not perpendicular to the center-line of the frame. I've yet to see a BB shell from any of the suppliers that was chamfered again after the threads were cut. You can do it yourself easy enough right up until you attach a tube to the shell, then distortion sets in and you have to interpolate the chamfer which makes it several magnitudes harder to do for most builders.
 

·
Registered
Joined
·
323 Posts
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.)
 

·
Registered
Joined
·
861 Posts
I tried last night and found that I got better results with the cone vs. a flat surface but in my case I am not working with very precise stuff and I am happy with measurement techniques that give me +/- 0.5mm so we are not talking aerospace.

My bikes will have 2.35" tires and be ridden on the dirt.

For cones I recommend steel because with a steel BB and aluminum cone you are asking for wear issues.
 

·
Non Dual Bliss
Joined
·
6,240 Posts
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.)
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.
 

·
will rant for food
Joined
·
4,005 Posts
Discussion Starter · #11 ·
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.
 

·
Registered
Joined
·
493 Posts


There is a third option. I use a muffler pipe expanding mandrel that I found at an auto parts store. You have to remove the rubber o rings before welding but it works better then cones or flat plates.

Tim
 

·
Registered
Joined
·
323 Posts
smdubovsky said:
Some sort of expanding arbor would be just about perfect - and act as a heatsink. Bonus.
TimT said:
I use a muffler pipe expanding mandrel
Brilliant! I even have one of those somewhere. FWIW, They don't work very well on muffler pipes:) One should be able to find a spring from a seal thats about the right size to replace the o-rings.
 

·
Registered
Joined
·
493 Posts
Yep I found out that cones can catch the lead of the thread on BB shells and if not careful cock in the jig and often not every BB shell has the same ID. (ever have a BB jiggle on your post?) So with a post mount it often doesn't repeat. So when I made Jigature 3.0 I saw the mandrel and had this idea.....

Tim
 

·
Non Dual Bliss
Joined
·
6,240 Posts
We used internal mandrels for awhile & still have some sets laying around. They come with their own set of problems and they add a lot of expense without any performance when dealing with the great variety of BB widths and sizes today.

To solve your o-ring problem, get yourself a spring winder and the appropriate gauge wire. Spring winders make it easy to whip out a spring clip for that bad boy on a lathe or mill.
 

·
Registered
Joined
·
1,640 Posts
Find an old Mavic bb cutter that puts a 45 degree camfer of the inside of the shell, while still allowing use on the threads. Then make cones to match?

Or make/have made a bb post with parallel faces!

-Schmitty-
 

·
peace, love, happiness
Joined
·
403 Posts
The mandrel, you can't deny, is the best solution by far because of heat distortion issues.

If you want no heat distortion issues and perfect alignment, then get into carbon.
 

·
will rant for food
Joined
·
4,005 Posts
Discussion Starter · #18 ·
I'm going to alter my jig to include a mandrel after having a circus trying to square things up with my existing fixture.

I've read on here that you generally screw up your first three frames. Well, I'm on my first real one and I managed to put my BB shell 2.5mm to the non-drive side - squared up as it may be - because I wasn't thinking straight while accomodating the width of some ISCG tabs.

Awesome.
 
1 - 18 of 18 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top