Re: [CAD_CAM_EDM_DRO] Using Glass Scales instead of Rotary Encoders?

Bill Blackburn wrote:

>Hi Guys
>
>I am using Gecko controls and Servo Motors on my CNC Mill conversion and
>was wondering if you could use Glass type Scales (like the ones used for
>DRO's) instead of rotary type encoders like the US Digital type? I know
>allot of the high end CNC Mills use these glass scales for the feedback
>to the controllers. I wasn't sure if the Gecko units would work with
>the Glass Scales? I believe the glass scales produce a similar signal
>to the rotary encoders? I think this would give you a really tight set
>up, especially if the Ball Screws were direct drive to the servos.
>
>

The problem is that the typical DRO glass scales have very low
resolution, generally
approximating .0005" with 10 uM resolution = 0.0003937". If you work in
metric,
that will at least feel better, but it is still very low resolution for
CNC motion
control. My Bridgeport mill retrofit has shaft encoders with resolution
of .00005"
(10 x finer) on X and Y, and .000025" on Z. This allows very smooth
movement
at low speeds, and fine adjustment of dimensions. With a typical DRO
scale, you'd
have the servo hunting +/- 20 uM all the time, which will cause obvious
stair-steps
when milling out a round hole, for instance.

The high end CNC mills use interpolated analog scales that give similar
resolution in the
tens of uInches or sub uM range. Check the prices at Heidenhain and
Renishaw,
if you DARE!

Jon