timo888 

Is there a tool that can gauge whether drive-side and non-drive side threads on a BB shell have been tapped precisely and are on the same axis, or are misaligned?

Nessism

Bottom bracket threading tools do this. The tap for one side has a pin and the other side has a hole so both taps share the same axis.

guy153

So I guess the tool for checking it is to get one of those tools (which are not cheap) and see if it threads in easily on both sides.

timo888 

I was thinking of something inexpensive, a gauge of some kind, not a tool to alter the threads on the BB shell. Something like two solid BB cups with pinholes at the center, and a laser pointer.

timo888 

In a scaled-up factory operation, are BB shells tapped/threaded after they've been welded to the frame?

guy153

Good question. I should think unlikely. Someone building by hand would buy a pre-tapped shell and then chase the threads and face the shell after making the frame. I suspect that in mass-production they often just skip this last step. I use 40mm OD BB shells, which are therefore quite thick wall, and they don't usually need chasing.

A laser tool is an interesting idea. The problem is some light would escape from the far hole even if they weren't lined up. I guess you could make a couple of BB cups on a lathe with a centred hole say 10mm diameter (or modify some existing BB cups). Then machine a 10mm axle that was a tight push fit in the hole. Install the cups and check whether you can push the rod through. My guess is probably not. Then you could have a series of test rods that were incrementally looser fits and see what the tightest one was that would fit through to get an idea of the tolerance.

unterhausen

I would say that would cost about as much as a bb facer/tap to do it right. There probably are thread gauges out there, but $$$$. I'm pretty sure nobody is motivated to actually do this, so it would be a one-off.

I imagine it has been done this way, but I don't think it happens very often. It's much easier to do it ahead of time and relieve the center at the same time. It's fairly uncommon for production bikes to be faced after welding, it's pretty common to have a dip in the face back at the chainstays. In my mind this indicates they faced and threaded pre-weld. I will also note that they could bore a pressed bearing bb after they build the frame but it's pretty clear they don't. There are a lot more advantages of doing that than tapping the threads post-weld. They don't exactly worry about perpendicularity of the bb shell to the rest of the frame either, so that would complicate a post weld machining of the bb shell.

dsaul

Bottom bracket shells are generally pre-threaded and most likely single point threaded on a lathe, not tapped. The joining process causes the shell to shrink in the areas where other tubes are joined(seat tube/down tube and chainstays), so chasing the threads and facing the shell is necessary to correct any distortion that occurred. Without facing, the shell will be wider in the area where no tubes where joined to it and the faces will not be parallel to each other. The faces being parallel is more important for modern external bottom brackets, as the thread tolerance will allow the cup to shift off axis to match the face of the shell. The threading would have to be terribly off axis to cause a noticeable problem and there is not a ton of excess shell thickness to allow the threads to get too far off axis.

Toespeas

hmmmm maybe ask hambini LOLOL haven't seen his videos in a while tho , but if you skip through them he goes over how to make sure its round and true !

but if you could find a used bb and mark it dead center you could use some plastic maybe or something to see if the center holes line up , would take some invention skills !

Cynikal 

There are other BB facing tools that have a threaded insert (non-tapping). I know the Ice Toolz uses this method. Those inserts could offer a controlled point of alignment.

guy153

Yes I have the Cyclus tool which works like that. But IIRC the threaded insert is sort of double headed with the lefty-tighty and regular inserts back to back on the same bit of metal. So you can't screw it into both sides at the same time unfortunately.

Trakhak

Certainly in the 1970s and '80s, every Italian frame we sold in the shop where I worked required facing the head tube and facing and chasing the bottom bracket. The fork crown often needed a bit of cleanup, too.

Cynikal 

What I have is similar to the tool in this video. The insert might work for the OP needs.

unterhausen

I don't know what kind of tolerance the OP is looking for. To me, it's good enough to face and chase with a decent tool. If you had those cyclus cups, then checking the concentricity would not be too easy. Best way I can see is to pass a rod through the holes, so you are essentially back to the face/chase tool.

Now I want to see a video of Paragon making bb shells.

Cynikal 

I'm assuming he isn't looking to tap and face but to verify that it has been done correctly by someone else. Maybe the OP can clarify that.

dsaul

I think what most of us are saying is that this is a case where "close" is good enough and a tool to measure the level of precision is not really necessary. If it is not right, the only fix is to chase the threads with a proper tool anyway.

timo888 

Once it is inserted into the holes in the two pucks that get threaded into the BB shell, is there any slop in the metal rod?

timo888 

I actually discovered that precocious 5 year old just a few days ago. He has a page showing an intentionally exaggerated misalignment of a press-fit bearing, and I was wondering if similar (but slight) misalignment of a cartridge bearing was possible, which might cause the cup to gnaw away at the shell's threads over time, or the bearings to wear out prematurely, or just a creaky BB.

Cynikal 

No. They are loose enough to turn the tool so it's not highly accurate.

guy153

He has another video somewhere explaining that a threaded BB is always going to be slightly misaligned just because of the fact that it's threaded. In theory a press fit can be much better (although in practice often not)

It might be this one:

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unterhausen

Some of these process questions are why I wanted to see someone make a bb shell. My guess is that the cnc shells for welding/fillet brazing have all the internal cuts done in one operation. If that's true, the threads are pretty straight with each other. Cast lugged shells are probably done one side at a time. I'd be interested in seeing them done with a single point operation. I suppose they could cast extra material to hold them with.

I'm not sure I want to know what hambini is talking about.

guy153

I'd like to see that too. As dsaul says the threads are probably cut with a lathe. I think ideally you would cut both sides from one end without removing the shell from the chuck and flipping it around since when you remount it it won't be perfectly centred (unless you mess around with 4-jaw chucks). I assume they're CNCing things so that's probably what happens anyway. In a CNC machine the workpiece surely stays clamped in something throughout the whole operation and you want to do as much as you can in one program.

unterhausen

There is some youtube machinist that cuts threads that go both ways at once. Although the easiest way is probably a tool change to go upside-down and backwards

JohnDThompson 

The Campagnolo #724 pilot sleeve for the BB facing tool does this. It's not cheap, but is high precision. If the shell is not on-axis, you will not be able fully insert the sleeve.

JohnDThompson 

I'm not aware of any large-scale manufacturing that taps threads after the frame is assembled. At Trek at least, all the shells were threaded from the supplier (Nikko, Cinelli, Signicast) then chased with taps after the tubes were brazed in place.