Re: [CAD_CAM_EDM_DRO] Re: differential threads for extreme precision movement

john@... wrote:

> Doug,
> Drawbacks to this as far as I can see is that with these small steps
> you will have to have a very rapid computer to achieve anything like
> speed.

Well, those are extreme values. At the other end of the scale
are values like 2500 steps/inch (0.000 400"/step) using the basic
1800 step/rev motor/driver combo. Using a half step driver
gives a 2 thou step which is too course (so one would pick a
higher multiplier combo for use with a half step driver).

I used the extreme examples just to make a point.

> Secondly and I speak from experience here, that with differential
> threads any backlash will translate into a vast number of steps.
> Possibly too many for the software to handle.

Glad to hear someone is an expert! Actually, I can't see that
there will be backlash (normally we are using acme and ballscrews,
not threads). Whether the software can handle ( X TPI * n
inches) steps, well I hope everyone is using unsigned 4 byte
integer counters at least (giving 4+ billion total steps) - giving
335 inches in the 12,000,000 steps/inch example.

> > I've been playing with different mechanical leveraging ideas
> > to be used both for direct movement of the table and as a
> > way to increase the resolution of rotary encoders
> > HUGE number of steps per inch of advancement. For example,
> > using a 180 step/rev stepper with at 10 microstep driver like the
> > Geckodrive (ie 1800 steps/rev), then using a 56 TPI course and a
> > Metric 56.444 TPI (ie 0.45 mm/thread), then you'd get 12,801,539
> > turns per inch, or an advance ratio of 7.81E-8 inch per step
> (0.000 000 0781 ").

I did also want to point out that using really course and robust threads
like a 9.5 TPI and 10.0 TPI combination gives a multiplier of 190.000
(ie 342,000 steps/inch ie 0.000 002 923 inch/step).

As I am going to be using the Geckodrive, which can handle up to
200,000 steps/sec then my rapid would be 0.58"/sec which is the
only downfall. Changing to a less extreme multiplier results in
correspondingly faster rapids. It goes without saying that Celeron
chips are so cheap these days that buying a computer to drive the
machine adequately is a no-brainer.

Doug Fortune
Calgary

PS: I would like to hear in what application you used the differential
threads.