Re: Newbie question: Motor sizing

Hi,

A good place to start is to ask "how fast do I want it to go and how
much "push" do I need at that speed?" Use IPM (inches / min) and lbs
for "push".

(1) Watts = IPM times Lbs / 531

Use your answer (Watts) to size your motor.

0 to 100W, use a size 23 step motor.
100 to 200W, use a size 34 step motor.
200W or more, think about servos instead of steppers.
-------------------------------------------------

(2) Steppers have lots of torque at low speeds and very little torque
at high speeds. This is a good thing. Work is done at low speeds
where you need a lot of torque; high speeds are usually "rapids"
where little torque is needed.

As a rule of thumb, steppers are pretty much out of torque (rapids)
at:

Size 42, 300 to 600 RPM, 800 in-oz or more low-speed torque
Size 34, 600 to 1,200 RPM, 150 to 450 in-oz low-speed torque
Size 23, 1,200 to 2,400 RPM, 50 to 150 in-oz low-speed torque
Size 17, 2,400 to 4,800 RPM, less than 50 in-oz torque
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Now figure the torque you need from the motor.

(3) in-oz = 2.55 times Lbs of "push" / TPI times "eff"

If you know your leadscrew efficiency (as low as 30% for an ACME
thread to as high as 95% for a ball-screw), make "eff" that value
(0.30 to 0.95). That will be what you need from the motor.
-------------------------------------------------

(4) Now figure the speed you will get for a "rapid"

IPM = RPM / TPI

Use the guidelines from (2) for your estimate. You will probably have
more speed than calculated because the values in (2) are conservative.
-------------------------------------------------

(5) Let's go thru an example of the above using what you have (and
many guesses on my part).

Assume the following:

TPI = 5 turns per inch
Lbs = 500 lbs of push needed
IPM = 60 inches per minute max for your "work" speed
eff = 75% (0.75) for your leadscrew.

From (1), W = 60 times 500 / 531 = 57 Watts, so consider a size 23 or
34 motor.

From (3), in-oz = 2.55 times 500 / 5 times 0.75 = 340 in-oz, so a
size 23 is out of the competition. Use a size 34 double or triple
stack (in-oz > 300).

Finally, from (2) and (4), your "rapid" will be IPM = 1,200 / 5 or
240 IPM or better. That will take you from end to end on your major
axis (32") in 8 seconds or less.

Hope that helps, hope I did the math right.

Mariss

--- In CAD_CAM_EDM_DRO@y..., "rawen2" <rawen2@w...> wrote:
> Hi Group. Just joined a few days ago. Read 99% of the postings
from
> May. Whew. I'm already 200% more knowledgable. I'd like to build
a
> 3 axis CNC using a Jet JMD-18 benchtop or similar. It has an
approx
> 9.5" x 32" table. I see that Microkinetics on their "CNC Express"
> puts 470 oz/in motors on the X & Y axes with ballscrews. At first
> I'd like to run with the standard acme screws and was wondering
what
> size (torque) steppers do I need? I'd like it to be middle of the
> road on speed and capable of machining materials from wood to
steel.
> I assume it will get faster after eventually converting to
> ballscrews. Thanks.