Re: Max stepper motor speed - rpm

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Scott Riddle" <sariddle@l...>
wrote:

> I am looking at setting up a CNC router and I am starting to size
> components like the ball screw and its pitch along with the stepper
> motor speed and torque needed. One question I have is how to determine
> the max rpm you can get out of a specific stepper. There are basically
> 3 parts, the power supply, driver, and motor. Assume that the power
> supply is sufficient how do you calculate the max rpm capability from
> the driver? Say I start with a Gecko drive that has a freq=200kHz and
> it is a 10 micro step drive and the motor os 200 steps per rev. Would
> it be 200,000/(200*10)= 100 steps per second. Times it by 60 to get
> 6,000 RPM. So the drive is capable. Now is the motor. Would you just
> have to look at the speed torque curves and see for the desired torque
> what speed you can run at? Is there some way to determine this if you
> do not know the Mfg of the motor?
>
> My main reason for this question is that I am thinking of buying a
> stepper that puts out around 600 Oz-in torque and direct couple it to
> my .200" pitch ball screw. This would give me good speed capabilities
> when routing on wood. I am trying to avoid to have to add the extra
> complexity of a belt system.
>
> Thanks
> Scott

Scott: First let me apologize for Rowlands tactless appoach to
helping out a new person in a fantastic hobby. The Max RPM of a
stepper is controlled by several factors. Steppers "jump" from one
pole to the next. Microstepping tends to smooth that jump factor out
but there is still a finite time to charge a coil and cause the motor
to rotate then discharge it and have it charge the next coil, etc.
What you find in practicality is most steppers are limited to 1000 to
1500 RPM and only then if you smack them with sufficient voltage to
shorten the coil charge time. The other factor to consider is that
the motor torque curve will drop off with higher RPM. That means the
you can count on the RPM for non-cutting rapids but not to run a 1/2"
bit through oak (;-). Most routing is not done at 200IPM and you will
find that a machine with 120 to 150 IPM max speeds will serve you for
most wood routing unless you want to invest in a large ($$$$)
commercial spindle.

Given the constraints listed and your desire to use a .2 pitch
leadscrew (and direct connection) the math works out like this:

target RPM of motor = 600 RPM = 10 RPS
.2 in /rev yields 2in/sec = 120IPM

To get that with microstepping, you need 2000 pulses per revolution or
20,000 per second to hit 10 RPS. Most of the PC based controllers
will do that easily for three or four axis.

The 200,000 max input pulses on a gecko is a number that is "there" to
be observed but few pulse systems can output that frequency of pulses.
(and your motors couldn't spin that fast anyway)

You can probably push the rapids on up to as high as 200IPM but you
start to approach the physical limits of the motors, the "whip limit"
of the leadscrews (a function of length and diameter at a given RPM)
and the ability of the software to get enough pulses to the driver
without using step multiplication or an external pulse card.

Keep in mind, I speak in generalities. I have seen claims where
steppers are being driven to 3000 RPM but those are on optimized,
tuned, matched systems.

Depending on the size table you plan on building you might find that
600 oz-in motors are overkill. A lot of good screw driven router
tables have been built with 250 to 350 oz-in motors. The screw gives
you about a 6:1 torque multiplication so 600oz-in turns into 3600oz-in.

Let us know what you plan on cutting and the work area you are needing
and we can comment in more detail about the mechanical and electronic
needs.

Thanks for your post and your participation in the form.

Tom Caudle