Re: Do Stepper Motors need "Drag?"

Hello Tom Kay,

I finished my 3 axis Enco Mill/Drill last winter using a 3/4" screw
and Zero-backlash plastic nut from Kerk Motion products on all 3
axis. My current project for a GEM500 will use 3/4" ball screws. So
far my experience with the mill/drill conversion is so-so. Using the
plastic nut system I did, allows the use of the mill in "manual
mode", as the screws won't backdrive the motors. Negative is the
effiency of the screws when used with motors, as they take more
torque to overcome friction than ballscrews, subtracting from the
force available for machining. I used Sanyo-Denki 720oz/in steppers,
direct drive for all 3 axis. One last thought on the plastic nut
system I used - the nut has a dynamic load capacity of 350lbs, while
the ballscrews I am shopping for have a minimum of 1300lbs capacity.
I would recommend ball screws over plastic nut systems.

Allan

--- 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. 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???
>
> 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).
>
> Thanks all, Tom Kay.