Facing of BB housing

Basically, you are correct here.

The BB has threads in it but the brazing process will often distort it.

After building the frame you use a set of taps that have a hole through the center of them. There is
a sturdy rod that goes through this hole to control the taps, so that they stay 'lined up'. This
process 'chases' the threads, so that they are in line with each other.

You then 'face' the BB perpendicular to the threads.

--
Lewis.

.........................

*******************************

"Sergio SERVADIO" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

The two faces of the BB housing must obviously be parallel. What garantees parallelism of
the cups?

I have never watched a builder actually making a frameset, so the following might be quite naive.

In conventional frames, the housing is cut from a tube, drilled to

accomodate

Quoted message said:

the heads of the pipes, brazed and finally tapped. The tap being run, independently, from both
sides. So, no matter how well, when and no matter how carefully one does face the BB housing, its
macroscopic precision must be traced back to the drilling and brazing sessions, when the
cylindrical geometry was to be done and/or ensured.

Is all this correnct?

Sewrgio

Perpendicularity. -tom

"Sergio SERVADIO" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

The two faces of the BB housing must obviously be parallel. What garantees parallelism of
the cups?

In article <[email hidden]>,
[email hidden] says...

Quoted message said:

The two faces of the BB housing must obviously be parallel. What garantees parallelism of the cups?
I have never watched a builder actually making a frameset, so the following might be quite naive.
In conventional frames, the housing is cut from a tube, drilled to accomodate the heads of the
pipes, brazed and finally tapped. The tap being run, independently, from both sides. So, no matter
how well, when and no matter how carefully one does face the BB housing, its macroscopic precision
must be traced back to the drilling and brazing sessions, when the cylindrical geometry was to be
done and/or ensured. Is all this correnct?

BB taps come as a set. They mate into each other as you tap the BB so that keeps them going in
straight. Go to parktool.comBTS 1.shtmlOpen ↗ for a good picture and description of how
this works.
------------
Alex

Sergio SERVADIO said:

The two faces of the BB housing must obviously be parallel. What garantees parallelism of
the cups?

I have never watched a builder actually making a frameset, so the following might be quite naive.

In conventional frames, the housing is cut from a tube, drilled to accomodate the heads of the
pipes, brazed and finally tapped. The tap being run, independently, from both sides. So, no matter
how well, when and no matter how carefully one does face the BB housing, its macroscopic precision
must be traced back to the drilling and brazing sessions, when the cylindrical geometry was to be
done and/or ensured.

It doesn't matter so much these days, at least for the LH side. Shimano cartridge BBs, used on most
new bikes, only bear against the face of the shell on one side.

But BB shells are bought ready-made (good ones are usually investment cast) so your description is
incorrect. They have threads before the frame is built.

Sergio SERVADIO said:

The two faces of the BB housing must obviously be parallel. What garantees parallelism of
the cups?

I have never watched a builder actually making a frameset, so the following might be quite naive.

In conventional frames, the housing is cut from a tube, drilled to accomodate the heads of the
pipes, brazed and finally tapped. The tap being run, independently, from both sides.

Here's where you're wrong. The taps are not run independently from each side, but through a common
pilot sleeve that ensures they remain co-axial. Once you have co-axial threads then you insert
another threaded sleeve to guide the facing cutter. This ensures that the faces are milled
perpendicular to the axis of the threads.

In most cases, the BB shell comes pre-tapped, and the builder only chases the threads after brazing
in the tubes, and then uses the facing guide sleeve and cutter to mill the faces.

One of the reasons French and Italian threading (these use the same thread on both sides, as opposed
to English, which uses a left-hand thread on the fixed cup side) were popular is that you could use
a single tap, inserted from one side and cut all the way through to the other side without really
needing the pilot sleeve. Since one tap in one operation cut both sides, they had to be coaxial when
you finished.

--

-John ([email hidden])

Bottom bracket shells are threaded at manufacture before they become part of a frame. A tap is
normally run through again after the frame has been made to clean up any brazing material and fix
any distortion introduced by the brazing/welding. The taps used at this stage aren't independent.
The left and right taps are on a shaft that ensures that they remain coaxial, and you start both
taps simultaneously to ensure that they are correctly aligned in the frame. See
parktool.comBTS 1.shtmlOpen ↗ for a photo of the Park tool and
parktool.comhowfix bbtap.shtmlOpen ↗ for a description of how to use it.

Nick

"Sergio SERVADIO" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

The two faces of the BB housing must obviously be parallel. What garantees parallelism of
the cups?

I have never watched a builder actually making a frameset, so the following might be quite naive.

In conventional frames, the housing is cut from a tube, drilled to

accomodate

Quoted message said:

the heads of the pipes, brazed and finally tapped. The tap being run, independently, from both
sides. So, no matter how well, when and no matter how carefully one does face the BB housing, its
macroscopic precision must be traced back to the drilling and brazing sessions, when the
cylindrical geometry was to be done and/or ensured.

Sergio SERVADIO said:

The two faces of the BB housing must obviously be parallel. What garantees parallelism of
the cups?

I have never watched a builder actually making a frameset, so the following might be quite naive.

In conventional frames, the housing is cut from a tube, drilled to accomodate the heads of the
pipes, brazed and finally tapped. The tap being run, independently, from both sides. So, no matter
how well, when and no matter how carefully one does face the BB housing, its macroscopic precision
must be traced back to the drilling and brazing sessions, when the cylindrical geometry was to be
done and/or ensured.

I wrote about that a few days ago.

Ideally the two sides are threaded simultaneously on one axis with mating taps.

Then a set of pilots is screwed into that thread axis. The facing mill references exactly on the
thread axis when cutting the faces exactly perpendicular.

Then all frame alignment is taken from the faces of the BB
- simply apply a straightedge along the side and measure against the main tubes. Main tubes then are
all in one plane, head and seat tubes are parallel and the rear end is aligned from the plane of
the main tubes.

YOu are correct in that all is derived from that initial thread axis through the BB.

--
Andrew Muzi www.yellowjersey.org Open every day since 1 April, 1971