Something Jim Rozen posted prompted me to post this. He uses a ball
end mill in the lathe tool post to cut thread reliefs. Well, router
bits can also be used for this. Carbide router bits are now fairly
inexpensive and come in a huge variety of shapes. So both convex and
concave radii can be cut along with many other contours. Since these
tools are made for wood they seem to leave a worse finish that cutters
made for metal. But the carbide router bits will even cut stainless
steel. So for the home shop, where money is often less plentiful than
time, the extra time it takes to sand the part to a better finish is
OK. Even though 1/4" shanks will work, the 1/2" shanks are really much
better.
Good luck!
Eric R Snow,
E T Precision Machine