Hi Scott, Here's how I make the tangent posts round. I solder a
10-32 nut onto the end of a 2" piece of 1/4" diameter brass tubing.
The brass tube is then inserted into an electric drill. I then grab
the tangent flag with a pair of pliers that has tape wrapped around
the ends and push the rough cut tangent post into the spinning nut.
The sharp threads will then cut threads into the tangent post. Then
pull the post out of the nut and the sharp threads will rip the sides
off of the threaded post and produce a clean and very circular post. I
add a slight taper to the posts by lightly sanding with sandpaper.
This works well for key holes of 5/32". Here is a photo that better
explains the process.