On Fri, 7 May 2010 07:29:29 -0700, "Bob La Londe"
wrote:

So what's the proscribed method? I expect I need a pretty consistent radius
bottom. What's the best way. Make a rough semi V lathe bit, and then make
a decent round one to finish?

Use a rectangular groove.

Lots of information (11 meg download) in
"The Sealman's O-Ring Design Handbook"
http://www.physics.harvard.edu/servi...g_handbook.pdf