scale resolution, or RE:Re: Knuth DRO's from Singapore on EBAY - has anyone bought them?

Not to belabor a point, but the scale increments do not necessarily
correspond to scale resolution.
For less expensive scales this would be true, they have simple electronics
and digital outputs. Higher end scales, like the Heidenhain LS 603 have
quadrature analog outputs (sine/cosine). The readout head interpolates the
output to a much higher resolution than the scale pitch. This works quite
well if the outputs are pretty sinusoidal and the electronics fairly linear.

Accuracy is a separate issue from resolution.

About 20 years ago I made a special purpose scale. It used a 100 line/inch
grating and some custom optics and electronics. With some careful design
and experimentation, I found the conditions that gave a true sinusoid
output. The output was able to be interpolated accurately to 2 millionths
(of an inch), as determined by calibration with a HP interferometer. If I
could do it with a couple of days work, surely some people in the scale
industry can too?

There are many uses for interpolation of sine/cosine (fringe) outputs, in
the optical metrology world measurements are pushed to the one or two
nanometer level routinely these days. Heroic measures of fringe measurement
can take one to <0.1 nanometer.

Analog electronics is fussier than the digital stuff, though. The extra
effort required in the optics and electronics raises the cost of
interpolation, and this is a price sensitive business.

Elliot B.

Jon wrote:

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
>
>
> Ted Inoue wrote:
>
> >I've recently stumbled across the following EBAY ad:
> >item number 2570941080
> >
> >They have 2 and 3 axis, Knuth DRO units with glass scales, rated at
> >0.0001".
> >Looks like a decent unit and the prices can't be beat for the
> >features. $549 for 2 axis, $779 for 3 axis, plus $88 shipping
charges.
> >
> >
> Note that just because the readout DISPLAYS 4 digits to the right
> of the decimal point does not mean it actually has that resolution.
>
> People in the industry have told me than nobody makes actual
Imperial
> (Inch) scales since the late 1960s! So, all the scales are actually
metric,
> and they do a conversion to inches in the readout box. Unless you
pay
> a fortune, the best scales you can get are typically 5 uM, which
gives
> a resolution of roughly 0.0001969". That is about .0002", but not
> EXACTLY .0002". Therefore, if you fiddle with one for a bit, you
> will find there are certain readings you CAN'T get to! This drives
some
> Inch-bound machinist slightly batty.
>
> Jon



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