Re: rotary encoder versus glass scales

To this day very accurate machines use glass slides however these
machines usually fall into the toolmaking and measurement classes.
For most machining upto and including grinding precision direct
drives with encoders attached are used with recirculating ball
screws although you can also include a zero backlash gearbox into
the system and still maintain accuracy(Alpha gearboxes do one that
lasted us for 21 years and is still holding micron precision.

john

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "jcc3inc" <jccinc@o...>
wrote:

> Steven:
>
> Most CNC controllers these days use the signal from the motor

encoder

> as both position information AND rate feedback. They thus avoid
> having an added tach on the motor. A system is easier to stabilize

by

> using the motor encoder as a tach instead of getting data from the
> table. However the best *position* feedback comes directly from

the

> table position via the glass scale; this is the most accurate as

the

> lost motion as well as the *springiness* of the mounts, bearings,

etc

> are all included within the feedback loop. Therefore, if you want
> the best accuracy, the glass scale on the table is best. Also, if
> the provious user managed to operate with that configuration, it

must

> have worked! A previous respondent indicated that a tight
> recirculating ball preload did perform well on a PC drill.
>
> I think I'd give the table encoder a try!
>
> Regards,
> Jack C.
>
>
> > > I am in the process up gathering info and parts to upgrade an

old

> > > Hurco Bedmill to PC controls.I have ripped all the old
> electronics
> > > off the machine cept motors and limit switchs ..I have new

servo

> > > drivers in hand and just gotta get a coat of frsh paint on the
> > > machine and will be ready to wire something up....Cleaning on

the

> > > machine today I found that the X-axis motor's rotary encoder

had

> > been
> > > disconnected and somebody had installed a glass scale on

backside

> > of
> > > the bed for that axis.Reckon I should try to find a new rotary
> > > encoder or is the linear scale gonna be alright? I been

reading

> > about
> > > rotary optical encoders but havent a clue at the electrical
> > > differences between them and the linear scales..I want to

keep

> > this
> > > simple where I can and save the sweat for the worthy
> > > problems..Hehhehehheh
> > > Then another question if somebody wants to take a stab at

it

> is
> > > why did the old controller make use of a tach signal from the
> > > servomotor and these PC run conversions dont use that signal?--

Or

> > > maybe they do and I havent got to that part of the reading
> > > materials..This is my winter project and I got a lot to learn
> yet.I
> > > figure this isnt the last time I'll ask questions here.
> > > Thanks
> > > Steven