Re: [CAD_CAM_EDM_DRO] doubling resolution strategy # 2

Hello Ian,
Are You able to lend a set of verification blocks of quality 00?
Try those. And use them at 20 to 23 Degree Celsius.
Most digital verniers are not accurate at all. People trust them too much.
A new dial caliper is more accurate.
Not only resolution and Screw quality are responsible for exact positioning.
Take in account bed ways, Screw rest bearings ,nuts and table furfaces.
Many shops trust their DRO and forget that the rest is sloopy.

Good Luck
Sven Peter



Ian Wright wrote:

> Hi,
>
> This is something I have been fighting with for the last couple of days. How
> DO you check the accuracy of either cnc or dro systems to the accuracies we
> seem to be capable of?
>
> I have just got one slide of my scratch built mill working under power (the
> rest of the metalwork is currently in the workshop being finished off). I
> decided I should try to verify that the slide actually moved correct
> distances and that I have EMC set up correctly. I am using a metric set-up
> which will, theoretically, give me a resolution to 1/1200th mm. (400 steps
> driving through a 3:1 reduction to a 1mm ptich leadscrew). Initially, I set
> up the .ini file as seemed appropriate and, using my digital vernier, zeroed
> it the distance between the carriage and the end block. Then I commanded a
> move of 100.000mm and re-measured - it gave me a reading of 100.02mm. So, I
> sent it back to 0 and rechecked - it read 0.00 - good. I did this a couple
> of times and it all seemed Ok. Now, I thought, I wonder how accurate it is
> on smaller moves? So I fitted a metric dial gauge to the end block and moved
> the carriage to just touch it. Then I commanded a move of 1.000mm - the dial
> moved to 0.900mm exactly!! Sent it back to 0 - it now read 0, sent it to
> 1.000mm, it read 0.9mm - damn! This was getting frustrating. So, I decided
> I'd try the digital vernier again and fixed one leg to the end block and
> clamped the other to the carriage - now that must be accurate! Sent the
> carriage to 100.000mm - the vernier now read 99.75mm - back to 0 - it read
> 0. Sent it to 50.000mm - it read 49.68! still perfectly repeatable back and
> forth. This is silly. The next trial will be a mechanical vernier height
> gauge which I can remove the column from but I still don't know whether I'll
> trust it.
> So, how do you work out the accuracy of a screw thread well enough to be
> able to make a good correction table? And how can you be sure your home-made
> dro is giving an accurate reading?
>
> Ian
> --
> Ian W. Wright
> Sheffield UK
> www.iw63.freeserve.co.uk
> ----- Original Message -----
> From: "Doug Fortune" <pentam@...>
> To: <CAD_CAM_EDM_DRO@yahoogroups.com>
> Sent: 09 March 2001 04:07
> Subject: [CAD_CAM_EDM_DRO] doubling resolution strategy # 2
>
> > arcstarter@... wrote:
> >
> > > Doug Fortune <pentam@...> wrote Thu Mar 8, 2001 1:28am:
> > > >Here today, I present another way to double (or more) the
> >
> > >At these resolutions, it will become increasingly desirable for all of us
> > >to invent some kind of super-precise calibration method, in order to
> > >correct for screw (and encoder scale) inaccuraccies in software.
>
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