Re: [CAD_CAM_EDM_DRO] How bout this idea??

sspeed33317 wrote:

> I came up with this idea , using regular DC motors , not servomotors
> mounting linear encoders right to all the axes
>
> would eliminate the need for Servo motors , would eliminate
> the need for Zero lash ball screws ,would eliminate the need
> for expensive servo motors , wouldn't have to compensate for any
> backlash because it would be reading actual table movement .
>
> And the benefit of the ACME screws friction to help hold
> the table in position .
>
> And you could tie in for DRO use ,or tie into an existing DRO
> for CNC use if you already have one.
>
> Good idea??? Any body have input??? .
> Already in service??

Yup, great idea ... been using it for a few months now
(see my webpage where I sell this type of acme leadscrew
with an encoder nib on the end for mounting the encoder).

The only minor catch is that you still have to make sure
your motor-coupler-screw (or motor-gearing-screw) interface
has very little backlash (compared to the resolution of the
encoder mounted on the screw), otherwise a nasty oscillation
can be set up (I'm guessing that a few encoder counts of
backlash is OK).

Now your table will still appear to have backlash if your
table nut has backlash (this is a candidate for compensation
in software). Luckily I have found using long length nuts
(greater than 3:1 length:dia ratio) have under 1 thou backlash
(when new & unworn). I use a 6:1 ratio and there is no measurable
backlash.

Note that (expensive but worth it in some applications) ballscrews
have other desirable features beyond just the low/no backlash
ballnuts, so just because you can eliminate backlash doesn't mean
that ballscrews aren't the perfect answer (for some applications).


Doug Fortune
http://www.cncKITS.com