Originally Posted by Old Hammer Boy
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.