Re: [CAD_CAM_EDM_DRO] Glass scale problem

tig415 wrote:

>I have a bad spot in the center of one of my glass scales. The DRO
>just flutters and doesn't count for about .150". If I get up there
>with a laptop screen cleaning cloth and rub it on the glass it works
>for a while then starts acting up again. During the period that it
>does work it counts perfectly.
>I'm Kind of stumped on this one. I've removed the end caps and
>looked in with a flashlight but can't really see any debris. I'm
>thinking the swab leaves a film on the glass that makes it work then
>evaporates over time.
>

Well, it is hard to tell what is happening. First, check the system
that is used to
control the spacing between the scale and the read head. Cheap scales
use a piece
of teflon. When this wears down, the analyzer grating hits the scale
grating,
and they wear each other away. If this is happening, then it may
already be too
late.

If it uses tiny ball bearings that ride the glass, it should be OK. If
it uses bearings
that ride the aluminum housing, you may need to adjust the spacing, as
the aluminum
wears, too.

These scales are VERY sensitive to misalignment between the read head
and the
scale. In other words, it the head is allowed to twist at all as it
moves across the
scale, it could cause the problem you see. Usually there are some
springs or something
that constrains the head. But, a pull in the wrong direction by the arm
that moves the
head could cause it to twist in some areas.

If you haven't cleaned the read head as well as the scale, you should do
that. It is usually
pretty obvious what holds the head in place, if you are careful to bind
up the spring
or whatever, you can get the head out without much trouble. You can
then see if
there are any scratches or other damage to the gratings on it. (There
are usually
2 gratings, with one having its grating pitch shifted 90 degrees with
respect to the
other, so that when one is perfectly aligned with the scale's grating,
allowing the
maximum light to pass through, the other should be at 50 % of max
transmission.

> Can the glass scale be removed from the housing or is this suicide.
>
>

Very unlikely, on the scales I've been inside. Some have a fairly soft
glue to allow
differences in thermal expansion. But, I really think it would be very
hard to get it
out without breaking it.

Jon