On 1/15/2020 3:22 PM, Gene Heskett wrote:
I should probably interject here that with a good high-res encoder, it
won't care about the wobbly speed mechanics as long as it keeps on
turning under load. The z axis rigid tapping motion is 100% controlled
by the spindle position as it turns.  Since the Z is typically the
slowest axis, you will have maximum spindle speeds to deal with because
Z can't keep up. My G0704 can reverse directions much faster than Z, so
I have some hal trickery in the form of a limit3 to shape the reversal
decel/accel's into something resembling what the tv folks call a "sine
squared" signal, where the peak of the delta in speed matches the zero
speed and the z motor _can_ keep up because it has maximum torque at
zero speed.  This "shaping" of the reversal of the spindle has almost no
effect on the actual reversal times.

Hi rez in the encoder allows much higher Pgain in the spindle motors PID,
and with the speed being helped by the air drive if its wired up to the
high.on.low signals and fast enough in reaction times, rigid tapping
should not be a problem. Haveing some experience with split sheeves
needing several revs to adjust, I can see that being a problem, but it
would be an educational experiment to see how big a problem.
In addition to the split pulleys, my "mechanical" concerns are more along the lines of the heavy duty electrical contactors clunking every time the fixed speed, 3 phase AC spindle motor is called to switch back and forth; increased heat in the motor windings due to those reversals; and the air powered, mechanical spindle brake snapping on and off hundreds of times.  Yes it was very tempting, and yes, I'm sure some of my concerns could be mitigated, however I feel that this old mill (cira 1985) just isn't the right tool for the job.

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