At 11:43 PM 31/08/98 -0500, you wrote:
>>It depends on the axis that your looking at, and the type and date of
>>manufacture of the Lathe. If it is the tool bed i.e. The bit that moves
>>along the X axis, then a shaft encoder connected to the ACME thread is all
>>that's required to give that sort of measurement (With reference points
>>along the bed of course).
>
>Backlash would kill you with this setup.  All the CNC machine tools I've
>seen used optical encoders on the motors or ballscrews.  They could get
>away with this for two reasons--ballscrews have less backlash, generally,
>than an Acme screw and nut, and they can compensate for the residual
>backlash in software.  Ballscrews usually aren't practical for manual
>machine tools (too easy to backdrive).
>

I don't see how backlash is a problem. The encoder is mounted in the bed
(Not on the end of the ACME shaft i.e. The sensor moves with the tool), and
the equipment uses the reference points in the bed. The residual movement
(Backlash) is overcome by moving the tool (While cutting) in one direction only.
As for the ball screws, the biggest advantage is the near 0 force required
to move the screw in and out, backlash for these depends on the amount of
ejection or retraction (Fortunately this does not change much, and does
depend on the type of ball screw used ie. Fully encased or helical thread
type). Also using the ball screw removes the need to check the current
position against a reference point i.e X turns in or out = Y distance moved.
The method that I gave requires that the equipment updates its position to a
known reference. This makes the equipment cheeper to manufacture, but does
have reliability problems.
Also I may have confused the issue, as a optical encoder and shaft encoder
are considered the same in my industry. I used the term shaft encoder (Which
could be magnetic, optical or inductive) to separate it from an optical
position sensor.



Dennis