For my desktop mill CNC conversion, I gathered the following information:
My mill is a PM-30 clone, so I gathered information on what other hobbyists have used with their Precision Matthews PM-30MV CNC conversions.

Stepper Motor Sizing

According to Bruce from Heavy Metal CNC in the USA, for the X and Y-axis, 906 oz-in (6.39 N-m) will work well and for the Z-axis,  1200 oz-in (8.47 N-m) will work well.

Battle Resistance Outfitters has 640 oz-in (4.52 N-m) on the X/Y and 1200 oz-in (8.47 Nm) on the Z, and everything still works fine.

Torque Conversion Table
N-m  oz-in  Axis  Candidate
4 568
4.5 637 X, Y
4.6 652 X, Y
4.8 680 X, Y
5 710 X, Y
5.8 822 X, Y www.omc-stepperonline.com
6 852 X, Y
7 994 X, Y
8 1136 X, Y, Z
8.5 1204 X, Y, Z www.omc-stepperonline.com
9 1278 Z
10 1420 Z
11 1562
12 1704

Open vs Closed Loop

For me, the whole open-loop thing does not make sense on a milling machine where it is very likely that some unplanned forces may influence the coordinates. I think it is even more likely that without a degree or two in engineering,  that a DIY may have some of a flawed design. Therefore a motion system that has feedback/position encoder, may counter some of these flaws. Servo motors are the ultimate, but expensive. This is why I’m going the hybrid route, stepper motors with encoders, a closed-looped motion system.

Locally Available Products

3dprintingstore.co.za

Specifications

Motor Size: NEMA 34
Torque: 8.5Nm
No Load top speed: 2000 RPM
Full Load top speed: 1000 RPM
Encoder: Mounted to motor
Cables: 3-meter motor and encoder cables attached to the motor
Supply voltage: 18 – 70V
Micro stepping: Can be set between 200 and 51200 steps/rev
Motor Model Number: 86HSE8.5N-B32-3M
Driver Model Number: HBS86H

Price: R3,899.95 (inc VAT)

This may be a bit of an overkill for X and Y, but the next smaller model is 4.5NM, which may be too small. The price difference is R700.

It’s sized correctly for the Z-axis, but I would like a break on the Z-axis and this model is without a break.

DIY Geek now have 19% discount on the same motor and driver.

Nema 34 8.5N.m Closed Loop Stepper Motor + HBS860H Driver

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