Re: Do Stepper Motors need "Drag?"

--- 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