Re: Digitizers, Renishaw

Hi Ian,

Good observations. "Pairs" at 120 deg, not all evenly spaced. The
pointed ends of the star rest on the balls, or the angled faces of a 3D
star? I guess if the points had sufficient angle, they would only go
down so far (above 1/2 level of balls). "Down turned arms"? like an
umbrella?

At any rate, a Schmitt trigger (74C14 ?) gate would be needed to
"debounce" the "switches". Yeah, I haven't even tried to calculate what
deflection would be needed.

And to reply to Fred's comment on kinematics, on a PC, one has to
generate EACH step anyway, so as long as the G61 switch is processed
(sets a software flag to decelerate, and save cord's), the axis can take
a little time to slow down. I don't think we're going to go very fast
anyway! Might take a L-O-N-G time to digitize something! (small
Sherline mind at work).

This is GOOD!

Alan KM6VV

Ian Wright wrote:

>
> Hi Alan,
>
> The way I read it is that the balls are in pairs set into some kind of
> insulating medium (resin?) to probably about half their depth. They are set
> as 3 sets of 2 at 120 degree angular spacing around a circle and the balls
> with wide spaces between are electrically connected together (except 2 of
> them. This makes 3 sets of contacts in series. The 'star' has downturned
> arms with pointed ends which rest on the pairs of balls which are close
> together and so make a continuous closed circuit. Now, any movement of the
> 'star' will break contact with one or more of the balls and so provide a '0'
> signal. Having the balls and the star with pointed arms will make the star
> self centring but I wonder if the star would also need some other restraint
> to stop it moving around if it is deflected too much - probably not. The
> sensitivity of the whole thing will depend on the ball circle diameter and
> its relationship to the length of the measuring arm.....or will it? My brain
> is not in gear enought to cope with difficult concepts like this at
> midnight!!
>
> Ian
> --
> Ian W. Wright
> Sheffield UK
> www.iw63.freeserve.co.uk
> ----- Original Message -----
> From: "Alan Marconett KM6VV" <KM6VV@...>
> To: <CAD_CAM_EDM_DRO@egroups.com>
> Sent: 04 January 2001 23:55
> Subject: [CAD_CAM_EDM_DRO] Re: Digitizers, Renishaw
>
> > Gordon,
> >
> > I would think the "star" has an insulating layer (pcb), with the "tips"
> > of the star metal for contacts. The three "pairs" of balls are wired in
> > series. That should do it, but 6 4mm balls on a 30mm ring circle don't
> > have 2mm spacing (is my math ok?), so we'll have to work on that. a foam
> > "spring" (non conductive) presses on the top of the star.
> >
> > Patented? Probably so. But as I understand it, we can make a model of
> > it for our own use. We just couldn't SELL them without a license
> > (someone can correct me if I'm wrong).
> >
> > This would be good for the "auto origin" hardware we were talking about
> > also!
> >
> > DEFIANTLY not 1000's of $$ to make!
> >
> > Alan KM6VV
> >
> > Smoke wrote:
> > >
> > > The only thing I can't visualize from the description Peter provided, is
> how
> > > the actual electrical circuit is constructed.
> > >
> > > A skecth would be real helpful here. I think this system could be built
> for
> > > a whole lot less than $1000. Is it patented?
> > >
> > > Smoke
> > >
> > > >That's just the kind of information we're looking for! And it doesn't
> > > >sound too hard to make! It would be useful to get a better idea what
> > > >the "star" looks like, but I just drew the balls and star up in Vector,
> > > >and with a few tries, we should be able to get a star that works (good
> > > >contact angles). Pictures would help! Do you have any? Also, do you
> > > >have a URL for CNCDIGIT?
> > > >
> > > >Thanks for the contribution!
> > > >
> > > >Alan KM6VV
> > > >
> > > >
> > > >
> > > >PMeinhard@... wrote:
> > > >>
> > > >> Hi
> > > >> I´m digitizing on a little stepper-driven mill using a Renishaw. (got
> a
> > > >> broken one from the company I worke for and repaird it). The
> innerparts a
> > > >> quite simple and there ist no need for high accuracy . So I think
> it´s no
> > > big
> > > >> thing to built one. Basic thing: The Probing Arm has to be able to
> move
> > > in
> > > >> every direction to avoid breaking . They manage it that way: 6 little
> > > >> Steel-ball (4mm) are fixed in a circle of 30mm diameter. always two
> with
> > > gap
> > > >> of 2mm between them , half buried in electric isolating material. on
> > > them
> > > >> lies a 3-arm-Star of 4mm matrial making an electrical connection
> while it
> > > is
> > > >> pressed with a soft spring on them . This is now forming an
> electrical
> > > >> circuit . Lifting an arm of the star would break the circuit. In the
> > > center
> > > >> of the star ist an arm leading straight down through a much bigger
> hole
> > > in
> > > >> the center of the isolating material ,60mm long with a little ball
> on
> > > the
> > > >> end. The star is always centering itself, drawing the ball in each
> > > direktion
> > > >> makes one star-arm lift off the balls and breaking the circuit. And
> that
> > > >> simple thing costs about 1000$- but the idea is great.
> > > >> The accuracy is about 0.005 mm.
> > > >> ( the dimensions are as kept them in my mind and might vary a little)
> > > >> The Software I use is simple but good, CNCDIGIT. cost about 150$( as
> far
> > > as I
> > > >> remember) remeber
> > > >> Regards
> > > >> Peter
> > > >
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