[analogman]

I am converting an RF45 using steppers (not servos, as I already own a control box with PS, G203Vs and BOB).

This mill will have a
- one shot oil system
- lapped ways
- head counterbalanced with gas struts
- 2005 ballscrews
- Geckos 6A max each
- 16A 50V powersupply

I am a hobbyist and do work every once in a while, so I dont need production feeds and speeds. Most of my work is in aluminum, so I figure:

Roughing cuts would be about 7.5IPM - screw RPM 37.5
Finish cuts 15IPM - screw RPM 75
Rapids 200IPM - screw RPM 1000

So...

Seems to me I wont be going over 1000RPM, so any old motor that has "enough" torque from 0-1000 should do fine.

Looking at various NEMA 34 motors 1200, 800 and 600 sizes they have about all the same torque at 1000RPM, but off course, the bigger motor has way more torque at the lower RPMs. Looking at a number of posts that say "bigger is not better" does not seem to apply.

So I am at a loss - what size of motors should I get? Any recommendation for parallel or series wiring?

[pippin88]

You generally want a stepper with the lowest impedance you can find.
Optimum Stepper Voltage = 32 x (square root of the inductance)
So inductance around 2.4mH for a 50V powersupply (I think I did the math right).

Some discussion here: http://www.cnczone.com/forums/steppe...selection.html

[CRO-WD40]

I am converting an RF45 using steppers (not servos, as I already own a control box with PS, G203Vs and BOB).

This mill will have a
- one shot oil system
- lapped ways
- head counterbalanced with gas struts
- 2005 ballscrews
- Geckos 6A max each
- 16A 50V powersupply

Im in a process of doing something similar, why did you select drivers and psu first and then you look for motors?
You should select the motors first and then size the rest of electronics to the motors.

I am a hobbyist and do work every once in a while, so I dont need production feeds and speeds. Most of my work is in aluminum, so I figure:

Roughing cuts would be about 7.5IPM - screw RPM 37.5
Finish cuts 15IPM - screw RPM 75
Rapids 200IPM - screw RPM 1000

So...

Seems to me I wont be going over 1000RPM, so any old motor that has "enough" torque from 0-1000 should do fine.

Looking at various NEMA 34 motors 1200, 800 and 600 sizes they have about all the same torque at 1000RPM, but off course, the bigger motor has way more torque at the lower RPMs. Looking at a number of posts that say "bigger is not better" does not seem to apply.

So I am at a loss - what size of motors should I get? Any recommendation for parallel or series wiring?

I don't know if im breaking any rules by posting a catalogue here. But this helped a lot in my calculations

http://www.moonsindustries.eu/index....tors&Itemid=54

It's not the exact torque curve of the motor youre buying, but similar motors have similar torque curves. Just compare winding resistance, max amps, inductance, motor size and weight.... both motors with similar characteristics will be in couple percent of each other in performance.

One of your requests is 200IPM rapids. With a 2005 ballscrew (Pitch=5mm) thats 1016 RPM all of these steppers have 200 steps/rev. Thats 3386 PPS on the motor. The motor with highest torque at those PPS , from this catalogue, is 34HD2403 , as pippin88 said, the one with lowest inpedence....

34HD2403 motor
Holding torque 8.8Nm
Rated current 5,6 A
Phase resistance 0,49ohm
Inductance 5,6mH

Most of the ebay 1200 oz inch steppers are of these or similar characteristics. But now you have a reference to check.
Also nice thing about those catalogues is that you know what torque you have at those rpm's .

Eyeballing it from the graph that would be 1.5Nm. Taking in account ballscrew efficiency of 0,9, you can produce 1696N of axial force to overcome inertia(acceleration) of what youre driving (X,Y,Z axis), and friction of the ways.

Same goes for desired rough cut and finish cut speeds.

for example at 7,5 IMP, thats 190,5 RPM, thats 635 PPS on the motor , eyeballing it from the graph again gives us 5,9 Nm , or 6672N of axial force to overcome inertia(acceleration) of what youre driving (X,Y,Z axis), and friction of the ways AND cutting forces.

Thats almost 4 times more... force to work with. Pretty much acceleration of machine parts at higher feeds is major problem for steppers.

And you have to take into account manufacturers BS factor....


there are also these low impedence 12Nm motors, but i couldnt find any torque curves for them. Most of the 12Nm steppers on ebay are 3.5A that dont have any usefull torque above 600-700 rpm.
4 Lead Wire Nema34 CNC Stepper Motor 154mm 1600 oz in 5A Stepping Motor | eBay

back to 1200 steppers , as pippin88 said , Optimum Stepper Voltage = 32 x (square root of the inductance) = 75,7 V.....

And then you size your drivers to that voltage and current, that motors need, and are manufactured for...

The higher the voltage, the flatter torque curve...which is nice.


does anybody have a torque curve for that 5A 12 Nm stepper?


sorry for a long post...

[analogman]

Im in a process of doing something similar, why did you select drivers and psu first and then you look for motors?

I did not select the drivers, PS, etc first. I already own already own them. They are from my CNC router. I am not going to re-purchase these parts.

So... the drivers and power supply are fixed variables. (50V, 7A per motor max). I need to make the steppers fit these parts.

[arizonavideo]

All you need for the X and Y is a 900 oz stepper which will have lower inducance.

The 6.1A 906oz from automation technologies is a good one.

NEMA34 Stepper Motor ? 906 oz in 6.1A Single Shaft (KL34H295-43-8A) | Automation Technology Inc