Re: making a digitizer probe

Graham,

This had been discussed many times before. If you search the group
archive on digitiser, and digitizer; you will have lots to read.

There was an excellent rendered model of a very "buildable" digitiser
based on the renishaw patent (now expired) in our files section. I
looked for it recently, and could not find it.

Basically, you have a non-conductive center shaft, to which the probe
tip threads. Then, you have three radial arms, 120 degrees apart,
projecting at right angles to this center shaft. At the top of the
center shaft, there is a "tit" to keep a spring centered;more in a
moment.

Now the body of the probe is again non-conductive rod, with a
hollowed out portion near the top of a size to accomodate the three
arms, and a through hole for the center shaft (with the probe tip
hanging below). The three arms will sit on 6 ball bearings (which sit
in drilled recesses of the probe body), arranged in pairs to support
each arm; preferably with a 45 degree point of contact. A suitably
shaped pcb (with coutouts for the arms, and electrical traces
connecting the 6 balls such that they form a series circuit when the
arms are lying upoon them) makes contact with the balls AND serves to
hold them in place. Now close it up with a top body part which mounts
to your spindle, and remember to put that very light spring in before
closing it all up.<G>

The radial arms will tend to self center, and this design allows for
overtravel. It works in all three directions at once. Renishaw made
MILLIONS of dollars fgrom this design. It has one bad feature (well,
several, but one that bears mentioning). Which is, the force to
actuate is nopt the same in all directions, due to the arrangement of
the arms;i.e., if you are ON-axis with an arm it will
trigger "harder" one direction, and "easier" the opposite direction,
with varying degrees as the approach dir changes... BUT, the
activation POINT will be the same, which is what really matters.

You can look up digitising probe at the us patent office website and
see pictures of this design (If Tim or another moderator cannot find
the file that was in the group archive (I think it was Marios', or
Carlos' drawing)

Last year at Westec, I talked with a gentleman who tried to describe
his companies alternative to me (a better mousetrap, so to speak;
which soled the above problems). He was supposed to send me a
drawing, but never did. It was based on a disc and a cone shaped
recess in the body...

Hope this helps,

Ballendo

P.S. I have developed a design for this and would like to partner
with someone to put it on the market for hobbyists. Any interested
persons please contact me offlist. Also some other cnc projects.
Business experience is more important to me than cnc experience.

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Graham Stabler <eexgs@n...>"
<eexgs@n...> wrote:

>
> > I had envisioned chucking up a piece of steel in a nylon collet

and

> > running a wire to the probe. The other end to the part. then
> > whenever there is contact, the input changes state.
> >
> > Dave
>
> Good idea for conducting items, not so good for plastics etc.
>
> I suppose you would have to make sure the circuit would switch even
> with a mild contact, you wouldn't want to end up with lots of

dimples

> on the origional. I suppose this is also true for switched types
> making me think that my small but heavy slide might be a little too
> heavy and mark the surface I was trying to scan.
>
> What I need is a rod that will move precisely in and out of a small
> bushing. I guesss I could make that myself but perhaps there is
> something I can convert?
>
> I also saw some special transformers in a catalogue with a rod that
> extends out of them. They are used for posistion sensing and would
> be ideal, however the price was not.
>
> Cheers,
>
> Graham