[Art Ransom]

I am contleplating using a harmonic drive for the A axis on my mill. I will be turning wood columns up to 24" by 12'. Since harmonic gearheads range in 30:1 to 200:1 the stepper or servo will have to run pretty fast. Anyone know the max sustained recomended speed for steppers or servos? Mill can be seen at www.turningaround.org

[RICHARD ZASTROW]

Art,
I'm dealing with the same problem on another project. The servo motor /drive supplier's engineering dept. tell me the servo/drive in question can reliably rotate from 0 rpm to its rated speed (1500 rpm) at it's rated constant duty torque. I have a hard time believing it, but I'm not an elect.eng. My next question was whether at low rpm's will there be cogging problems. They tell me a well tuned system (perm. magnet, brushless servo w/mating drive) will not have cogging problems. I know Danaher makes a "servo disc" that supposedly can rotate at less than 1 rev. per day without cogging!!! Used for telescopes, radar, tracking stations? You might enquire about this with more knowledgeable people than I and avoid the reduction mechanicals. Just a thought.

[DennisCNC]

How fast do need the column to rotate? Some stepper motor go up to 3k RPM but not much power at that speed, A/C servos can go up to 4K with some big power if its a big servo.

[Art Ransom]

Expect rotation in 20 RPM with large columns and 100 for small (6") ones. From what I understand of the servo disk is a harmonic drive. I know they are used in telescopes. Present design will use a 9:1 belt drive which I am pretty sure will work for under 12" with a 1200 oz stepper and give me enough knowledge for next step. Have 2 large rush jobs so it will be a week before I can back on the machine. Have all the electronics and axis shaft should be ready for pickup this week.

[RICHARD ZASTROW]

The Danaher ServoDisc is a flat servo motor, no gears. The magnets operate on the flat side of the disc, not on the periphery. There are other alternatives available as well. For example, low speed torque motors from Danaher, Yaskawa, INA etc. Some are available with hollow shafts so they can be attached directly to the machine drive shaft without alignment coupling problems.

[GVlcek]

Stepper motors max out at ~1500 RPM and have little or no RPM. Servo motors can spin up to 6000 RPM, depending on size, and have most of their torque.
Low speed cogging can be reduced on servo motor by either schewing the magnets (placing the magnets at an angle to "ramp" torque from each magnet) or most commonly, using servo drives that use sinusoidal commutation. The second is most readily applied with rotary motors however most linear motors use angled magnets. NOTE: A well tuned system is the best defense against cogging.
I do wonder why cogging will cause you a problem however, because you won't notice any problem through a gearbox with any sort of ratio. And through a harmonic drive with 200-300:1 ratio, you won't feel a thing.