Re: [CAD_CAM_EDM_DRO] Re: PIC Based DRO

beer@... wrote:

> Exactly, a shaft mounted encoder cannot measure backlash - or wear -
> or
> misadjusted gibs - or lack or parallelism in the ways - or racking of
> the table - or any of a number of other sources of positional error.
>
> An encoder placed on the leadscrew IS a DRO, but what it is Digitally
> Reading Out is NOT the POSITION of the table. What IS being read out,
>
> even with the best of leadscrews, is the number of turns of the crank
> and an APPROXIMATE table position. I guess that's better than just
> counting the turns manually, but it hardly seems worth the trouble.

With acme leadscrews, that is quite true. with good anti-backlash
ballscrews,
however, it can be quite accurate. Trouble can develop without your
being aware
of it, so it pays to keep an eye on backlash, but you can machine quite
accurate
parts with good ballscrews and shaft encoders. In fact, a high-accuracy
ballscrew
and shaft encoders may be cheaper than a medium accuracy linear encoder
and
medium accuracy ballscrew. Linear encoders get very expensive when
resolution
increases beyond .0005".

Many high quality CNC machining centers use ballscrews and shaft
encoders on the
standard models, and have a linear encoder option for higher accuracy.
The linear
encoder option runs from about $10,000 on small machines to several
$100,000
on the big ones.

Jon