Re: Do Stepper Motors need "Drag?"

Thanks Steve, great feedback. Oh, another quick thought; would
leadscrews and plastic nuts be OK for the average hobbiest who is not
earning money with his benchtop mill? Or is the wear factor enough of
a pain/expense to go right to ballscrews, in your opinion?

Cheers, Tom.

--- In CAD_CAM_EDM_DRO@y..., stevesng@n... wrote:
> --- In CAD_CAM_EDM_DRO@y..., tom.kay@n... wrote:
> > I am converting a mill to cnc with Dan Mauch's 5 amp boards and
his
> > 521 oz-in stepper motors. I plan on using a 2:1 reduction from
the
> > motors to the screw. Software, probably Master5 from Art Fenerty.
> >
> > I have heard that steppers need some load, or "drag" resistance
to
> > operate happily, or they will tend to overshoot their steps.
>
> Stepper motors and the mechanics they drive form a spring-mass
> system which will have a resonant frequency. If the resonance
> is too great, the stepper can actually move far enough to jump
> from its selected "step" into the next major step with the same
> electrical phase. Drag or friction acts as a sort of shock
> absorber, dissipating the excess energy before it causes trouble.
> Steppers will often behave much better attached to a real load
> than they do running un-loaded. This can be due to the friction
> losses, but may also be due to the increased mass changing the
> resonant frequency to a region where is causes less trouble. Many
> people have had satisfactory results using half-step drives on
> milling machines. Problems usually arise with limits to the maximum
> speed before the system starts to loose steps. Many other factors
> including smoothness of pulse streams from the controller and the
> voltage and current settings of the driver will also affect speed.
>
> > This may
> > be less of a problem with Mariss's 10 microstepping gecko drives,
> but
> > the boards I have only do full or half step. Someone actually
told
> me
> > to install some little friction brakes on my x and y axes, so
that
> > the motors don't overshoot. This person mentioned that it would
be
> > less necessary if I used a pulley/belt reduction, presumably
> because
> > this adds some friction, or because losing a step isn't is big a
> > problem???
>
> Microstepping drives will cause significantly fewer problems with
> resonance in steppers. A bit more improvement is also possible
> by employing "electronic damping" such as the Gecko drives use or
> other electronic tricks to excite the resonance less. Pulley/belt
> systems will often result in fewer resonance problems because the
> belt is slightly elastic. Pulley/belt systems also offer the benefit
> of being able to change the drive ratio and making shaft alignment
> less of an issue.
>
> > And related, would be a choice between ballscrews or leadscrews
> with
> > anti-backlash plastic nuts. I know ballscrews are about 95
percent
> > efficient (they won't even stay put unless locked), and
leadscrews
> > are closer to 50 percent, but has anyone got a suggestion there?
> The
> > leadscrews would certainly provide a built-in drag force, if
that's
> > needed (and not pure bunk).
>
> Plastic anti-backlash nuts would provide the "benefit of drag" and
> can have backlash performance similar to ball screws, but they are
> best used in light load situations. They might go well for
engraving
> or circuit board milling, but would likely not have enough spring
> tension in the anti-backlash setup to carry the loads seen in a
> milling machine. They also wear out faster under load.
>
> > Thanks all, Tom Kay.
>
> Overall the choices you mentioned; 2:1 belt drive ratio, half-
> stepping,
> and the Master5 software sound like good choices. Ball screws are a
> lot of trouble, but worth the effort. More advanced drivers can be
> added to your system later if needed.
>
> Good luck,
> Steve Stallings