How can you assure a perfect 90 degree angle on both sides with a file? Yes, a file might bring down a high spot, but maintaining a perfect 90 degree angle requires the special tool. Go to Park Tool's site for an explanation on chasing and facing.

"Perfect 90 degree"???

If you mean, "how do you keep the shell faces parallel to each other?" Then yes you would have a point....doing it with a file will not achieve perfectly parallel faces, but in good hands the file will be able to put the faces close enough (in parallel) for any noticeable binding to be eliminated.

Actually, 90 degrees to the plane of the spindle. The shell faces may be parallel, but not at right angles to the spindle. But agreed that a file wouldn't do the job without a large degree of patience and checking. That is, if the facing was ever needed in the first place (which is rarely if ever is if a cartridge BB is to be used).

The ideal situation is to have the faces parallel to each other and at 90 degrees to the threads. Cartridge type bottom brackets would probably be more forgiving, but the chase and face process pretty much assures a good tight, creak free fit.

The ideal situation is to have the faces parallel to each other and at 90 degrees to the threads. Cartridge type bottom brackets would probably be more forgiving, but the chase and face process pretty much assures a good tight, creak free fit.