This article is from the Misc Bicycles FAQs, by various authors.
The solid plugs which had been attached at the top of the seat stay
needed to be drilled and tapped so that a binder bolt could be used.
The seat tube was affixed to the angular table of the drill press, which
was affixed to the press table. The frame is then leveled against the
press table. To determine where to drill, first a centering bit is used
next to the seat tube. With this, subtract half of the diameter of the
centering bit - 250 thousands of an inch which puts the center of the
drill right on the tube. Add back the lug width, the half the bolt
diameter, plus some extra thousands for clearance. The table was moved
back 200 thousands of an inch, so in essence, the table was only moved
50 thousands in towards the tube. To cut the hole involved many bits.
First was a start bit to get a hole started since the seat stays are at
an awkward angle. The bit is lowered slowly to get the best cut. Once
a hole is started, a bit of the inner diameter of the thread of the bolt
is used to cut through the seat stays, but not all the way through.
Once through the first stays, the bit is held in place by the hole while
starting into the second stay, which is also on an angle. Next a bit a
little larger than the outer diameter of the bolt is used to cut through
the first stay and slightly into the second stay. The bolt does not
need to be threaded in the first stay. Cutting just into the second
stay allows easier access for the threading bits. A bit of the outer
diameter of the bolt head is used to countersink the bolt into the first
stay. Next a threading bit is used to cut the thread. For this, Dave
just starts with the drill press and then does the rest by hand. The
bit is made to push metal scraps out the other end, but the hole wasn't
drilled all the way through. He would twist a little forward, then back
to clean the scrap, forward again, back again, etc. This bit had a
pointed end, so the last little bit wasn't threaded. So finally, a
bottoming threader was used, with a flat bottom, to finish the last
We're not done yet here, even after 7 bits. A slow needed to be cut for
the binder bolt to work with. Typically a hacksaw probably would have
been used, but Dave has a large, table saw like blade which fit into the
press which was used to cut the back of the seat tube with. Yet another
bit was used to bottom out this cut and provide a circular bottom.
Finally a file and sandpaper was used to deburr this and smooth off the
edges a bit. Whew!