Re: Low cost Servo Controller

Dean Franks wrote:

>
> Backlash is still a big problem, the system can deal with backlash on
> direction changes, but when you are moving a rotary cutter, the cutting
> action can create a force on the head at an angle to the feed vector. This
> means that if I change directions on the minor feed axis, I never know which
> side of the backlash the nut is loaded to (because I do not know if I am
> actually removing material or not). At this point, I am experimenting with
> current measurement on the spindle motor and taking into account the
> clockwise rotation of the tool. The problem with this approach is that I do
> not know if I am cutting with the full face of the cutter, or just one edge.
> I suspect that current measurement will not be effective, next try is strain
> gauges on both axes. I concern with strain gauges is the vibration levels
> when cutting with a 5hp router (wood).

Augghhh! Craziness!

> Does anyone know how low the actual backlash is on "zero backlash" ball nuts
> under load? Since they cannot be infinitely stiff, I assume that it is
> non-zero. Fortunately, about 1/200th of an inch is acceptable accuracy for
> most of what i plan to use the system for, sanding can erase a few sins.

It depends on the brand of ballnut, the grade, the accuracy of the screw, and
other parameters. On low accuracy, rolled ballscrews, the hard-preload nuts
can not be set too tight, as variations in screw pitch will cause extreme variations
in preload. Still, these nuts are usually capable of holding backlash below .001".
For high-precision ground screws, you can easily get backlash below .0001".
The backlash increases as the balls (and everything else) wears. The spring
preload systems are not as stiff, but will take up the slack as the balls wear.
For a lightly loaded system, they may be a better choice, although not too
many manufacturers make them this way, anymore.

Jon