Re: Digitizers, Renishaw

Alan M wrote:

>I would think the "star" has an insulating layer (pcb), with
>the "tips" of the star metal for contacts.<snip>
>a foam "spring" (non conductive) presses on the top of the star.

Alan,

Neither the 'star' or the spring need to be non-conductive.
The 'star' IS the contact between the closely spaced balls. If
the 'star' lifts, it BREAKS contact BETWEEN at least one pair of
balls!

I'm curious as to whether the 'star' has tapered arms(for self
centering) and/or a 'dish' shape(for the same effect).

I'm guessing the balls are ballbearings used for their low cost size
accuracy. I see a 1.5 inch diameter delrin rod pocketed(round) to a
depth of about 3/4 inch. Centered thru hole for the probe tip. 6 spot
drilled points for the balls to sit in, and through drilled for
connections. It would also be easy to make 3 copper arc
shaped 'segments' with spot drilled depressions for the balls to sit
upon/in. Then the delrin pocket would have 'deeper' pockets to fit
these arc-ed segments. Easier delrin program, easier assy.,easier
electrical connections, since any two of the copper plates comprise
the necessary 'switch'. Fabricate the 'star' from brass rod stock;
3 'arms', and one 'leg' with silver solder.(possibly CNC mill a jig
to get this done easily) Use pcb 'tinplate' solution on the end of
the arms of the starfor corrosion resistance and good conductivity.
If the "sticks out the bottom" (leg)rod is threaded to accept
standard probe tips, we're probably ahead in the long run.

Now turn a 'cap' from metal which includes a 'spigot' to hang the
thing in a collet or chuck... There are a myriad of ways to fasten
the two parts together. You may turn a 'tit' on the inside of the cap
to hold the spring in position.

Hope this helps.

Ballendo

P.S. Train of consciousness post, The star could also be a larger
central rod, threaded one end for probe tips, the other end turned
for a 'tit' for the spring. 3 holes drilled radially (120 degrees,
with tilt if necessary) for the arms of the star.