On Sun, 24 Jan 2010 13:36:42 -0800, mac wrote:

My Garmin NUVI 500 for my bicycle uses a ball mount. The one that I
purchase from a third party is hollowed plastic and broke from
vibration. I adapted the broken piece to my light bar and it broke again
at the ball.
I toyed with several methods. One buy a ball turning tool, two find a
ball and mount it to a post. The ball was .670 inches and I didn't find
any one that size.
I decided to make a form tool for a section and move around the ball
shape. The critical section is smooth as the female move up to the major
diameter and that the major diameter size is maintained. The down side
also needs to be smooth for a short distance. I Machined a rectangle
piece of aluminum leaving the head 3/4 inches, in three direction. The
post section is 3/4 X 1/2 inches. Pictures are here.
http://www.billcotton.com/nuvi500.htm

Making a bunch of taper cuts to rough out the sphere, then filing it
'round' on the lathe may work well, too.

Your technique works, obviously, and would be the way to go if you need
to make a ton of parts. Probably makes a more precise ball, too, if you
get the contour of the tool correct. (In fact, this may be a good way to
make something like a Cox engine connecting rod).

--
www.wescottdesign.com