This article is from the Misc Bicycles FAQs, by various authors.
Up until this point, no cleanup had been done on the top of the seat
tube, other than reaming it and finishing off the binder bolt assembly.
During assembly, about two inches are left above the top of the lug
since it didn't need to be an exact cut. During the main triangle
brazing, Dave had added a bunch of extra brass around the front of the
lug which could be finished off into a point later. It was time to
finish off the top of the seat tube. It began with a trip to the belt
sander. Ron did most of the work with the belt sander - removing much
of the extra top tube and beginning the point on the front. It takes
some practice to figure out how to hold the tube (frame) next to the
sander and at what angle to get the desired rounding of the tube - and I
didn't have it yet. That done, one more power tool. I used the hand-
held belt sander to sand down the points on either side of the gap on
the back of the seat tube. The front would be a point, while the rear
would be a smooth sloping curve from the top of the lug down into the
gap along the curve of the seat stay.
Power tools out of the way, I had a lot of filing and sanding to go. It
was suggested that I get the front point finished and level first, then
I could look over the front towards the back to see if the back was
level. Two things were involved with the front point - making sure it
was centered with the point of the lug on the top of the seat tube and
making sure the slope of each side of the point was similar. I pretty
much used a large half-round file to get this done. After awhile I
would start to second guess myself so I stopped and later when I looked,
it looked level and straight.
To finish off the back, it was a lot of filing since I removed quite a
bit of metal to get the contour of the back of the seat tube to follow
that of the seatstay. Then a bunch of hand sanding to get smooth,
similar curves. After more than an hour's work, I was finished - it
looked really nice!