Re: [CAD_CAM_EDM_DRO] doubling resolution strategy # 2

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.