Re: [CAD_CAM_EDM_DRO] glass scale alignment

Denis Casserly wrote:

>Hello,
>I have a pair of ' Metric 01 MM ' glass scales and I can't get the reading
>head aligned properly with the glass scale so that it will output a reliable
>signal. I can move the head along the scale by hand and it seems to read
>accurately but I can't find that sweet spot when I put it back on to it's
>bracket. The bracket doesn't appear to be bent or loose. Would anyone know
>how to set up the reader head alignment.
>
>

If this is the typical glass scale encased in the aluminum extrusion,
then the external
alignment is generally isolated from the internal alignment by some kind
of wire
or metal strip. This allows slight misalignments between the spar and
the machine
slides without putting large forces on the read head. The read heads
ride on teflon
gliders or tiny ball bearings. Some cheap scales are actually designed
to self-destruct
after a certain amount of linear travel. The teflon sliders wear down
until the
read head scrapes the grating off the glass scale. Keeping the insides
of the scale
clean greatly extends the life of the unit.

The analyzer grating in the read head needs to be VERY close to the
scale, on the side with
the grating on it. Some scales can be assembled backwards with the
analyzer on the
plain side, and the light source on the side of the scale with the
grating. This won't
work. Most of the medium-resolution scales are not that critical on
alignment, at least
to just produce quadrature signals.

You don't give us much info to work with. No make or model, not even a
description
of how the read head is kept aligned with the scale. I've never seen a
good scale
for machine tool applications where the read head was simply mounted to
one part
of the machine and the scale mounted on the other. Accu-Rite, Sony,
Mitutoyo, Teledyne/
Gurley, etc. all use some scheme where the read head rides on something
within the
spar, and a link to the machine drives the read head back and forth.
This scheme
always allows a tiny bit of hysteresis, but if the link is stiff and
straight, it allows the
machine to not be quite perfectly aligned or even travel in a straight
line without
possibly ramming the head into the scale, and yet have the two move
together in
the machine's axis of travel.

Jon