Re: Digitizers, Renishaw

Ballendo,

The tips of the star are indeed conductive, but I'm thinking the three
"arm" contacts need to be insulated from each other. 3 pole, single
throw. Connect the switches in series. Otherwise, if sw 1 connects to
sw 3, sw 2 can open, and not be seen.

I'm with you on the delrin body, and ball bearings, and depressions for
the balls. the shape of the "star" may indeed be angled. I'm thinking
of the "break water" cement pyramids, 4 legs, three of which are always
down, one up. In our case turn it all upside down, the single arm now
is thin and long, the other three angle up, and contact the three pairs
of balls. But you've already imagined that also, right (the mind
thing)! ;>)

Alan KM6VV


ballendo@... wrote:

>
> 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.