stepper RPM?

[JTH]

How do you determine a stepper motors RPM? Those details don't seem to be published.

I'm building a CNC plasma cutter and want some pretty rapid travel speeds. I'll need to know the approximate expected motor RPM so I can select the correct ball screw.

[ger21]

You need to see a torque curve to know how much torque the stepper has at a given rpm. Most chinese made steppers don't come with torque curves. Then you need to calculate how much force is required to move your machine at your required speed. Then pick a motor that fits your application. Most stepper manufacturers have sizing applications available at their websites.

[desertdriller]

Isnt the RPM controlled by the software like Mach3?

[ger21]

Yes, but I think he's looking for a MAX rpm. Correct me if I'm wrong.

[mcpltd]

As said, you need to get the torque curves, but the torque curves are only accurate if you are going to be using the same driver at the same voltage and resolution as the torque curves are done at.
All manufactures of stepper motors will have torque curves, including the Chinese manufacture (95% of stepper motors are manufactured or part manufactured in china)
Manufactures may not publish them though. (Worried that a competitor may use them to copy their motors)

[kreutz]

Ger21 is right.

Isn't the RPM controlled by the software like Mach3?

One thing is what you could ask from your motor (software driven pulse rate)and another what it is going to deliver (will your motor keep up that rate? will it stall? will it skip steps?).

That is why you should chose your motors based on speed, acceleration and load requirements.

[massajamesb]

I am getting roughly 6-8 revs/sec at 28 volts, from a Kelinginc 425 ozin stepper, and a Powermax II stepper, both running on Gecko 201's.
Yes, the speed of the motors will be controlled by Mach or equivalent, but the maximum RPM of a stepper is not a relative measurement.
The reason I say that is because just because a step motor is "advertised" to top out at the 1000 RPM mark, that does not mean it will have ANY useful torque at all at that speed.
Steppers develop their torque at lower speeds, and quickly lose torque after a certain speed, making them useless at high rpms.
If you are running spur gear and gear rack (for example) and have a 20 degree pitch, 20 degree pressure angle spur gear, with 20 teeth, then you will move 3.14 inches for every revolution of the motor. If you even have 100 RPM's from your motor, then you will move 314 in/min.
Problem with that is that you will have no torque.
Gear down the steppers to at least a 1:3 ratio, and you will have plenty of torque, and still have your high speed ability.
Steppers will spin faster when you apply higher and higher voltages, within reason.
If I have 6-8 Revs/second now (360-480 RPM) at 28 volts, imagine what twice that would do. At 8 revs/sec, I am moving at almost 400 IPM, way beyond the useful range of what I am doing.
I suggest you look into the steppers available through www.kelinginc.com or www.xylotex.com, as well as many other places. Almost any modern stepper will probably suit you, unless you need ultra high speeds and a lot of torque at the same time, inwhich I would direct you to look into servo motors.
Just my 2 cents.
I didn't want to go on a rambling tangent, but I thought I would clear this up for you. Your desired speed is well within the range of most Nema 23 motors even after gear reduction.

Edit: I just now noticed you mentioned using ballscrews. That may be your limiting factor, depending on the TPI of the ballscrew. I suggest you look into spur gear and gear rack. It will cost MUCH less, and ballscrews will (IMHO) wind up being wasted accuracy. Plasma is not a precision burning process, and ballscrews, if nothing else, will have to be cleaned constantly.

Look up Tom Caudle (Torchhead on here) and ask him. He has Ballscrews on his plasma, running servos. Ask him his opinion as he knows much more about ballscrews than I.