Re: [CAD_CAM_EDM_DRO] DRO intermitent fails

Dear Jon,
your answer gives me very detailed search directions,
although my post was very inexact. That means you are
pretty deep involved in the matter.
My respect!

It is a Mitutoyo glass scale of an elder style (I have no idea
of the model) The scale is mounted at an angle.
45 - 60 deg? I can't remember well because I was too excited.
=> Yesterday wasn't my best day!
It doesn't happen too often that I crash equipment I have to fix.
(+ the telephone running hot)

The reader's basement is linked to the reader head by a
connecting rod that is spring loaded and has on the ends
two brass balls. These balls sit in spherical nests which are
supposed to keep the balls tight as long as the spring load
is strong enough. Well, I see the nests pretty worn compared to
the ones of another broken reader head I could compare with.

The reader head itself is a pair of pcboards that is equipped
with 5 roller bearings. The upper one has 3 (kind of) LED's
the lower one has a glass surface with 3 gratings
and below are 3 light sensors. I guess that the central
one is the main and count grating and the side ones are
25% offset of a stepping pass.(90 degree talking in a
sine wave comparison) These ones probably only give the direction
for counting.

I will clean up once again thoroughly and try somehow to tighten
the connecting link.

Thank you very much.
Sven Peter





Jon Elson wrote:

>
> Sven Peter wrote:
>
> > Hello friends,
> > Can give me somebody a hint:
> > Today I finished the reconstruction of a #2 TOS- mill.(checoslowakia)
> > Aligning the Mitsubishi-DRO-ruler I over traveled it.
> > First I got no readouts. After opening it and cleaning the ruler with
> > contact cleaner it worked only in the test by hand.
> > Later I had to readjust the spring and then it started to work
> > in one direction but sometimes in the other direction it fails to
> > count the travel. I suspect that there is still some oil on the
> > glass ruler or the reader doesn't travel right.
> > The amazing thing is that the readings do not fail on the same spot
> > and not always and there is an area of 25 cm where the ruler works
> > perfect.
> >
> > I saw that the nests where the balls of the connecting rod sit, are
> > worn.
> > In the first moment I thought it could be fixed with light center point
> > hits in order to close the nests. (or some Epoxy)
>
> There are so many designs for these linear readout spars, I'm not
> sure what variation you have. But, assuming it is a glass scale
> optical encoder, they often use a scheme where the readout head's
> analyzer grating is separated from the main scale by little strips
> of teflon. If these get messed up, the analyzer grating can rock
> when the direction changes, instead of sliding straight both ways.
> Also, if the analyzer grating is allowed to twist, instead of sliding
> straight parallel to the scale, it will throw off the quadrature
> 90 degree relationship between the A and B signals.
>
> The tricks to get the wire, bail, link or whatever that drives the read
> head back and forth without bowing (which effectively causes
> backlash) are many, and these usually take the brunt of overtravel
> accidents. When messed up, they can cause all sorts of alignment
> errors between the analyzer and scale gratings.
>
> I haven't run across this ball link scheme, I can kind of guess what
> they are trying to do, and it sounds better than the music wire
> designs I've seen in Teledyne/Gurley spars, for instance.
>
> Anyway, if there is any rocking or twisting of the analyzer grating
> when you change direction, that needs to be fixed.
>
> Jon
>
>