rpm of stepper vs servo's

[Ed Williams]

How many rpm does hobby 200 oz steppers go. I have some servos that are rated at 190 oz but they are 3200 rpm. So I was wondering the rpm of steppers.
Thanks

[Karl_T]

That's the big advantage of servos. steppers lose torque as speed increases. Design your stepper system for maybe 200 RPM max. Use reduction gearing for more torque and design your sevo system for 3200 rpm max.

Karl

[Ed Williams]

Thanks
But what rpm do steppers go. I have a 5mm ball screw I'm wondering what gearing to use.
Thanks

[Karl_T]

The vendor of your stepper can give you a speed vs. torque curve. If you can estimate the torque to move your table at given speed, then you can calculate the gearing.

Or you can do what most everybody does, guess. Then re-belt if you need better. Keep in mind that all steppers rapidly lose torque at higher RPMs. I wouldn't even try a combination that calls for over 250 to 500 RPM.

Karl

[paulC]

Originally Posted by Karl_T The vendor of your stepper can give you a speed vs. torque curve. If you can estimate the torque to move your table at given speed, then you can calculate the gearing. Or you can do what most everybody does, guess. Then re-belt if you need better. Keep in mind that all steppers rapidly lose torque at higher RPMs. I wouldn't even try a combination that calls for over 250 to 500 RPM. Karl

Seems a little on the conservative side. There are torque/RPM charts that let you see the performance roll off. While the torque steadily decreases with rpm there is a serious fall off after around 900 rpm and this is what I would design to. Bearing in mind that I would not be cutting at these revs, just doing rapids. A lot depends on the type of machine you are designing. With a mill you would want max torque, where as with a router you want speed as well.
Paul

[ger21]

Originally Posted by paulC There are torque/RPM charts that let you see the performance roll off. While the torque steadily decreases with rpm there is a serious fall off after around 900 rpm and this is what I would design to.

The rpm where the torque drops off can be very different between motors. Some might drop off very fast at 500rpm. The rpm where the torque drops is also very dependant on the voltage supplied.

[Ed Williams]

What I'm concerned is gearing on the motor. I afraid that I will get the pulley too small on the motor and it will not have enough belt contact and will rip the teeth on the belt on torque load. On a 5mm screw what would you suggest the pulley sizes to be. Speed is not a big concern I will be milling mainly aluminum with a 1/8 or ¼ end mill bit. So I will usually be working in 6 to 20 ipm. This a graph for the motor except it is a 42v model. I was hoping to get by with a 36v power supply with large 87000uf cap.
Thanks