Re: [CAD_CAM_EDM_DRO] Re: Digitizers, Renishaw

I hope you won't mind if I add a few comments to your post and maybe add a
few comments from other posts. I work for Renishaw's in the UK designing
some of the electronic interfaces for CMM probes. You will understand if I
don't reveal too much of the design much of which is or was covered by
patents. Much of the use of the probes is covered on the web site
http://www.renishaw.com which is due to be completly revamped soon. Many of
the user manuals will then be available for download.

>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 .

You are at least partly right but I think you will find it is much simpler
to calibrate if the stylus is on the same axis as the mounting screw.

> 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

Sounds like you have one of the big ones. I just measured a probe I have in
the workshop and it is only 13mm external diameter. The bigger ones are
normally used on the big machine tools and are designed to cope with long
stylus with large balls on the end. You may find that it is rather stiff
for digitising especially on soft materials.

>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 guy who had the 'great' idea just received a knighthood in the new
years honours list.

>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
>

There are two types of probe used for digitising. The simplest is the balls
and rollers probe as above. This is basically a switch where the contacts
of the switch are also the physical means of alligning the probe tip.
Variations of this use piezo sensing or strain gauges to separate the
contact sensing from the mechanical allignment. Other probes use optical
sensing, LVT's or LDT's (and probably others). All of these mechanisms
have a means of deflecting under pressure but returning very accurately to
the original trigger possition when the pressure is removed. The CMM will
need to be programmed with a rough track (possibly under hand control) to
start with and the trigger signal is used to initiate a reading of the
machines encoders.

The second type uses 3D linear bearings with weak spring centring. Each
axis has an encoder and there is no switch as such. The CMM follows a
programmed track across an object and as soon as a deflection in the
encoder readings is observed it moves the appropriate axis to return the
encoder to its orignal position. The CMM then reads its axis encoders and
notes the position. With this system the probe scales do not have to be
terribly accurate just reasonably linear. There are variations on this
system but it obviously requires a fair amount of programming in the CMM
controller.

There is a third type using a laser but you don't even want to know how
many noughts there are on the price for those. They are great for soft
fabrics. Try digitising a car seat with a normal probe!

Regards
Bill Phillips