Re: [CAD_CAM_EDM_DRO] Real-Time measurement

Hi Doug,

This may work in certain circumstances but I'm not sure you could get enough
accuracy to be sure the 'errors' detected are in the machine and not in the
measuring equipment. Errors in the pitch of the leadscrew at any point are
likely to be less than the errors in calculating the rotation of the
encoder - especially as there is some 'guesswork' involved in deciding the
effective diameter of the wire/pulley combination, the effect of the helical
path around the pulley, stretch in the wire, the effects of catenary, and
the precision of the mathematical conversion from linear to rotary motion.
Having said that, this method may be the nearest you can get to accurate
measurement in the home shop as anything better would need a travelling
microscope on an optical bench or, at least, a whole mess of accurate gauge
blocks!

Best wishes,
Ian
--

Ian W. Wright
Sheffield, UK

----- Original Message -----
From: "Doug Fortune" <pentam@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Sunday, April 14, 2002 12:05 AM
Subject: Re: [CAD_CAM_EDM_DRO] Real-Time measurement


> Lee Wenger wrote:
> >
> > I like tossing out an idea every now and then so I can get battered
>
> That happens to all of us!
>
> > Is there any (Cost Effective?) way to measure the current
> > location of the cutter?
>
> If there is, I'd like to know about it.
>
> Instead of real-time while cutting, I have an alternate scheme
> for mapping out the screws in 3D (with the spindle off and not
> cutting). You'd do this initially after building the machine,
> then perhaps every year or season depending on your requirements.
>
> Have a look in the files section, I had posted that a wire be
> attached to the spindle tip, and an encoder is used to sense the
> amount of wire that is reeled out to go to any reachable 3D location.
>
> In the drawing, I have the end of the wire fixed on the spindle,
> and reeling out from 0,0,0. However, I think it might be better to
> affix the end of the wire to 0,0,0 and reel it out from the spindle.
>
> These measurements can then be used to not only calibrate the
> leadscrews (by travelling down each axis independently and mapping
> the distance of the reeled out wire vs the theoretical distance you
> have traveled by emitting so many steps), but perhaps also to compensate
> for the non-orthoginal errors from imperfect construction of the machine.
>
> ie the distance reeled out is D = sqrt ( x*x + y*y + z*z) and if your
> D does not agree with your X,Y,&Z then something is off, and of course
> D must be correct (you might want to account for a slight excess reeled
> out due to the wire's catenary).
>
> In fact, I want to build a prismatic table (ie X & Y at maybe 60
> degrees instead of 90 degrees to each other) and the Z maybe off at
> a wild 15 degrees. After compensation, it should be able to machine
> a more-or-less perfect cube to single-step resolution, which should
> prove the theory and the software.
>
> To get back to your question, the Cost would be some wire, a pulley
> and a rotary encoder (and some programming time).
>
> Doug Fortune
> http://www.cncKITS.com NEW -> 1 KW power supply kits
>
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>
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