I now had a reasonably good mill (good enough for my purposes, light machining and not heavy industrial use,) I had to strap some motors onto it. The question remained What sort of servo motor, How much power, How much Torque, inch-pounds, Ounce-inches, Newton-Meters, Gram-Centimeters, Newton-Meters per Amp!!!  How much force will this produce, how much power will it draw from the power supply….Where do I start! Where does a beginner start… The internet!

Which components to use?

Another option was from keinginc, Their biggest servo could pump out 1125 oz-inches of torque, and would take up to 40 amps. The top speed was 3000 RPM, a little slower, but a little more torque, which is very important when accelerating from stop to rapid with a heavy machine.

Same standard “34” size frame. The prices were similar.

I found two websites that had motors I really liked. These were “www.kelinginc.net” and “www.homeshopcnc.com” I looked at motors and was initially confused by all of the numbers in the data sheets, with different units of measurement. How can you compare things measured in different ways?


Let me start by going through the two best choices that I had. The first option was a 850oz.in servo drive from “homeshop.” The motors looked great, had a nice cover over the sensitive encoders. The motor would take up to 38 amps, and produce a maximum torque of 850 ounce-inches at the maximum current (remember, for a DC motor, Torque is proportional to current). Max speed was 4200 RPM. Standard “34” size motor frame.


After looking at the available controllers for the servo drives, I reached the conclusion that the “Geckodrive” brand of servo controllers were going to be straight forward to use. The G320 was my final choice.


I realized that the Gecko drives could deliver a maximum current of 20Amps before their safety shutdown takes over!


All of these Torque specifications produced by the manufacturer were going

to be at maximum motor current, which a Geckodrive cannot deliver!!! It is beyond what a Gecko is designed to do. I looked at Rutex drives, which can deliver the full 40Amps, but they were beyond my budget although I am sure that they are a good drive. Time to re do the numbers...


Motor 1: 850 oz-inches at 38 Amps = 22.368 oz-Inch per amp, x 20Amps = 447.4 oz-inches.

Motor 2: 1125oz.Inches at 40Amps = 28.125 oz-Inch per amp, x 20Amps = 562.5 oz-inches, with the Geckodrive going flat out.


I decided to go for the second motor option after doing a force calculation.


To calculate the machine force a screw or thread can make, you need to know the motor torque and the pitch of the screw.


To convert Oz-Inches to Newton meters, Take the Oz-Inch number and divide it by 140.6

Example, 281.2 Oz-Inches / 140.6 = 2 Newton-Meters.


Force in Newtons  = 2 x PI x Torque (in Newton.Meters) ÷ (pitch of screw in meters)


For my leadscrews, 5mm pitch = 0.005 meters per full rotation of the screw. Torque in Newton.Meters = Oz.Inches ÷ 140.6, so in my case 562.5÷140.6 = 4 Newton.Meters

(2 x 3.1416 x 4.0) / 0.005 = 5026.5 Newtons of force!!!


To turn this into kilograms divide by 9.8; so 5026.5 / 9.8 = 512.9kg of force (1126 pounds.)


Is this enough torque and machine force? The answer turned out to be yes… Heaps!


If the above torque was not going to be enough when directly driving the lead screw with the motor, I could have geared it down, say 2:1 and gained more force, but at the expense of top speed.

Summary of what I wish someone could have told me when I was trying to decide which motor.


I learned that it is all about matching spindle power to lead-screw pitch and motor torque. With a given leadscrew pitch (the distance the nut advances with one revolution of the screw) torque is converted into linear force. There is a trade off you have to make between a machine with the highest rapid movement speed, and not much pushing force on the table, or a lower rapid speed and more machining force. With high pitch screws, say 20 mm per revolution, table movement speed is gained at the expense of machine force. 


After I did my research I did several calculations and determined that due to technical limitations (Parallel port pulse frequency 45kHz, quadrature encoder resolution 2000 cpr etc) I was able to spin my servo motors at a maximum of 1350 RPM, Even though they were designed to go to a max of 3000 RPM, I was not able to drive them that fast. With a 5mm screw pitch that combination gave me 5026 Newtons of force, and  rapid speeds of 6750 mm per minute (265 Inches per minute.) Resolution was 2.5 microns. This is what I ended up using in the final CNC conversion.


Two bad examples…

With a 20 mm pitch lead screw, still the same limitation of 1350 RPM, I would have lost machining force, I am down to 1256 Newtons of machine force, but I can reach rapid speeds of 27000 mm per minute, or 1062 inches per minute (in theory anyway) This would be a waste of speed, since most of the time the machine would be cutting slowly and then rapiding only a few hundred millimeters at a time between cuts, and it would not have enough travel to build up the momentum and reach this high top speed. I would have been left wanting more pushing power from the machine’s table. Resolution would be 10 microns.


At the other extreme end of the scale, another alternative was 2.5mm pitch lead screws, This would give me 10,053 Newtons of force, and a rapid speed of 3375 mm per minute or 132.9 inches per minute. 1.25 micron resolution. My goal was to achieve 200 inch per minute rapid speeds. With this example, even though the design in theory could produce 10,000 Newtons of force, the ball nuts were only rated to 3500N dynamic load for a 16mm ballscrew shaft.


I realized after many calculations and different scenarios that you won’t need high speeds with a small spindle because the cutters can’t chew hard enough into a big block of Alu or Steel to make the high speed cuts. High speeds are only required for rapid moves between cuts, and not during cuts when you will be cutting at say 200 mm per minute, or 8” in the old scale. Most of the time you will be wanting high force and high accuracy, not the speed.



Practically what does this mean… What motors should you buy?

If you are going to convert a small mill like a “mini-mill” a 23 frame motor with 200 to 400 oz. inches will be heaps! Any more and you will be wasting your money in big motors for nothing. This is because the spindle motor can’t deliver enough power to the cutting tool on a small mill to use all of the machine force that the powerful table screws and motors can deliver. I would aim for 1500N-2000N of machine force using the above calculations for motors and screw pitch.


For an X3 or similar, I would use 300 to 500 oz. inch motors, 23 or 34 frame motors. A bit more torque, but the spindles typically only have 1000Watts, and can’t push a cutter through any more material without getting bogged and snapping the cutting tool. You should still get reasonably good performance in Alu, and take very light cuts in stainless steel. I would aim for 2000N-3000N of machine force using the above calculations for motors and screw pitch.


For a large size mill drill like an RF45 copy they typically have 1500W or more of spindle power and can therefore chew a lot harder into a block of Alu, or reasonably hard into stainless steel . I would recommend 500 to 800 oz-inch  of torque from motors, but no less than 400, or you will find that the cutter isn’t going hard enough to stop chatter. Using small motors on a mill this size will cause cutters to chatter and the servo to stall causing the CNC to fault because of lost servo lock. Servo motors this big will be in a 34 size frame.

I would aim for 3000N-4000N of machine force using the above calculations for motors and screw pitch.


For a full size Bridgeport, I would recommend either using 500-800 oz-inch motors and gearing the motors 2:1 with a timing belt and pulley, or if you want direct drive, 1000 oz-inch or larger motors and a very good quality shaft coupling. I have not experimented with machinery as big as a Bridgeport to know precisely, but this is what my experience on smaller machines tells me.  Servo motors this big will be in a 34 size frame or some larger, and may require high current drivers, 40 Amps or more to get 1000+ oz-inches of torque. With such a big machine, I estimate you would require 8,000N to 10,000N or more to make really heavy cuts and have some power left over to ensure that your motors never stall. Otherwise the material will never be pushed into the cutter hard enough to make the big spindle motor do what it was designed to do - remove large amounts of material rapidly.


The above is a guide only, and remember- you will never get the full power out of your lead screws because of friction in bearings and the ballnut. You will never get all of the power from the table because of friction in the ways.


The friction can reduce your force by as much as 50% with poorly lubricated ways and cheap screws. Your situation will vary, but I am confident that my suggestions are well within the ball park.  I hope this helps someone, because I spent hours deliberating over which motors to use on my CNC.


Turning force

Text Box: Linear force

20mm pitch = 20mm linear movement per rotation

5mm pitch = 5mm linear movement per rotation