Re: Renishaw scales giving 5.0 um to 0.2 um

[Jon Elson <jmelson-at-artsci.wustl.edu>]

Doug Fortune wrote:

> I am looking at a big old mill with average-worn
> acme screws (ballscrews maybe someday).
>
> I am assuming with high precision linear scales on the
> tables, then whether its acme or ballscrews shouldn't
> matter that much to the final achievable precision (if
> I gear down the  motors to be compatable with the
> encoder resolution).

This is mostly true for simple, 1 axis at a time positioning
work, but for any multi-axis moves, especially arcs, the
backlash will cause the table to jump every time the direction
is reversed.  So, when boring circular holes with an end mill,
you'll get lumpy star-shaped holes, with error equal to
twice the backlash, I think.  You most likely will get
severe hunting action, where the motor is constantly
rocking back and forth, overshooting one way and then
going back the other way.

Why this happens is that when the table position needs to
be moved just a hair the other way, the motor picks up
speed crossing the backlash, while the encoder doesn't
sense any movement.  Then, the motor takes up all the
backlash, and the Acme screw suddenly moves the table
too far before it can stop.  Then the process repeats in
the other direction.

>
> I'm looking at http://www.renishaw.com/encoder/encoder.html
> which has a gold plated "sticky" tape  available in
> lengths up to 70 meters, and various digital quadrature encoders
> (which all read the same tape, and therefore are
> field upgradable for increasing resolution just by
> swapping the reader module) variously giving approx:
>  -  5 um  .000 196"
>  -  1 um  .000 039"
>  - .5 um  .000 019 6"
>  - .2 um  .000 007 8"
>
> (see Modern Machine Shop  May 2000 page 282).
>
> I guess I'll have to wait until Monday to find out
> I can't afford their stuff!

Yup, you're not kidding!  The tape is $360 / meter, and the readout
head is about $500, I think.  I talked to them a couple months ago.

Jon