Re: stepper vs servo

Hi Jon, Mariss,

Is there a standard comparison (rule of thumb?) between servo torque
required for a machine, and stepper torque required? What would your
4.4 (in/lb) continuous and 15 in/lb (peak) servos translate to for
steppers? I'd think it would be more the 240 oz/in steppers!

What servos should I be looking for to drive my RF-31? I think I recall
650 oz/in being recommended.

Alan KM6VV


Jon Elson wrote:

>
> info.host@... wrote:
>
> > > Or, on the otherhand, don't overload the stepper and it will be just
> > > as reliable and accurate.
> > >
> > > Mariss
> >
> > When you see kits for upgrading Sherlines and the likes to CNC are the
> > motors (100 - 150oz/in) given as per their peak torque or their constant
> > torque? How much constant torque would it take to run a normal sized knee
> > mill like a Bridgeport series 1?
>
> Steppers do not have peak torque. because steppers need to waste
> power just to hold position, they normally run at the continuous rating
> point, and can't be 'boosted' to a higher current for a moment.
>
> Servos DON'T use much power just sitting, and therefore can be safely
> booted to higher current for a moment.
>
> But, steppers do have 'holding torque' and 'running torque'. The holding
> torque
> may be as much as twice the running torque, even at fairly low speed.
>
> I'm using 1/8 HP (continuous) servo motors on my Bridgeport Series 1,
> with 2.5:1 belt reduction ratio, and 5 TPI ballscrews. I calculate about
> 1000 Lbs linear force on the table with this setup.
>
> That motor works out to about 4.4 in/Lb continuous, and 15 in/Lb peak.
>
> Jon