On Sun, 24 Jan 2010 14:39:01 -0800, "anorton"
wrote:


"mac" wrote in message
...
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

It looks like this worked well enough for you.

In the optical lens industry, spheres and spherical sections are generated
using a cup-shaped grinding tool. The workpiece is rotated, the tool axis is
at an angle to the workpiece and spun more quickly. The edge of the cup is
aligned over the apex of the workpiece, and then the tool is moved into the
work parallel to the workpiece axis. Here is a rough diagram of how it
works:
http://www.britannica.com/EBchecked/...ave-and-convex
The radius generated will depend on the diameter of the tool and the angle.
It can make very precise spheres that are the ready for (very) fine grinding
and final polishing. Perhaps you can do this with a tool post grinder on a
metal lathe.

A long time ago in American Machinist, there was an article that
showed how to make a perfect ball on a Bridgeport (actually half a
ball). Tilt the head at 45 deg. Put in a fly tool with exactly the
radius you want, mount the part on a rotary table and start moving the
tool to the part very slowly, while turning the table. Said to make
near perfect hemispheres. In a small size, wouldn't take all that
long, I'd guess.

Karl Pearson