Re: [CAD_CAM_EDM_DRO] Stepper/Servo sizing

wanliker@... wrote:

> Mariss,
> I am attempting to modify an EMCO Compact 5 CNC Lathe, the original motors
> are probably around 75 in/oz, from what I heard they work much better with
> about a 200 in/oz stepper. However I want to use the G320's with a servo,
> how do I size the servo? The lathe has ball screws and somewhere around a
> 2:1 reduction belt drive, very slow.

Well, it is a bit tricky, but there are shortcuts. The stepper only meets
the
holding torque spec at zero speed. As soon as it starts moving, the torque
drops, by 1000 RPM, most stepper/driver combinations have lost more
than 50% of their torque. Servos have both a continuous and a peak
torque rating. The servo only dissipates peak power during rapid
acceleration
of delivering high feed force during heavy cutting. Thus, it can keep cool,
and reserve its thermal mass for those momentary peaks. At rated
current, the servo delivers rated torque, regardless of speed - this is the
big advantage of servos - you get full torque right up to the maximum
speed, where the servo amp runs out of voltage.

> What is the relationship between steppers and servos when trying to size
> these things.

So, if a 100 oz-in stepper is recommended, a 50 oz-in continuous rated
servo with a 200 oz-in peak torque will almost surely outperform the stepper.

The only condition where this is not true is where the servo would be
required to deliver over 50 oz-in of continuous holding torque. Under
these conditions, the servo would overheat.

Anyway, servos are generally capable of moving MUCH faster than steppers,
2000 to 4000 RPM is no big problem for most smaller servo motors.
Ball screws and belt reduction are good for these applications, so you
are part of the way there.

The other part is the speed. Servo motors have a figure, called Kv,
that rates it in volts per thousand RPM. You want a motor that will
have somewhere around 25 V/KRPM. At 75 V, it will spin at 3000
RPM. A motor wound for 1 V/KRPM would limit the amp to 3-5 v for
maximum speed (depending on leadscrew pitch), thereby producing
very little power. A motor wound for 150 V/KRPM would also be
unsuitable for many servo amps, as you would only be able to go
500 motor RPM with a 75 V servo amp. That would come out to 250
RPM after the belt reduction, which would reduce your rapid feed rate.


Jon