On 2011-09-10, Lloyd E. Sponenburgh lloydspinsidemindspring.com wrote:
Ig, I'm having a thought that I can do a really bang-up job on the
precision of my lathe conversion with the most crappy ball screws on the
planet.

I have found some very affordable magnetic DRO scales and readers that
provide simple quadrature output (and a zero index) at TTL levels, and
can be had in both 2 micro-inch and 1 micro-inch resolution for very
little difference in price.

So, I'm thinking that instead of using the motor shaft encoders (which
are already there), I could use these scales to provide absolute
positioning accuracy down to .0002" in Z and .0001" in X, without much if
any changes in EMC's manner of looking at the data.

It seems to me that the "counts per revolution" datum could be used the
same with a linear scale as with a shaft encoder. Any errors in the
screws are simply going to be interpreted as speeding or slowing of the
CPR figure, rather than skewing the end position, so long as the motor
doesn't know or report what its rotational position is.

I think it might take a little code to get the motor to continue (or
stop) moving, unless I only use speed input on the drives, and not
positioning input.

What do you think of the idea of having absolute positioning information
in EMC, rather than the "dead reckoning" positioning provided by screws
of unknown accuracy? Do you know if anyone else has approached it this
way? (i.e. - taken a DRO, and used it for the positioning info?)

(Not just Ig... any other comments are welcome)

Lloyd, this will work just fine. It is a known approach. If your DRO
is absolute, you can also avoid homing.

Igor