Re: Do Stepper Motors need "Drag?"

On 5 Nov, tom.kay@... 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 the same set up but using EMC. Motors from Dan also.

> I have heard that steppers need some load, or "drag" resistance to
> operate happily, or they will tend to overshoot their steps. 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???
>
I suspect the overshoot can be delt with by adjusting the acceleration
parameters in the controller. I've done no more than this with mine and
it works well half step.

> 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).
>
I converted to ball screws (Rockford) right off for decreased backlash.
The decreased drag has caused no problems with CNC. However, my hands
still need some calibration, they are used to the acme lead screws and
tend to put too much torque into the ball screws under manual operation.

Hugh Currin
Klamath Falls, OR