[dfaszer]

Hi all,

I am going to attempt to design and build a 5 axis CNC. I have the machine rough designed and am now working on modeling the fine details. I am working on trying to figure out the A/B axis stepper motor. I currently have the stepper motor transferring power thru a timing belt to my spindle holder. I chose the timing belt as apposed to direct drive so that I could potentially get a finer resolution.

I found this website that goes thru and helps you calculate what motor you need, but it performs the calculations based on screws transferring the power.
What size stepper motor do I-need.

I tried to follow the site, but I wan't sure what equations would pertain to my situation.

So here's what I got.
Weight of the spindle and the spindle mount : 2.75kg
The CG of the spindle and mount is roughly 27mm below the center of rotation (not sure if this is necessary).

I plan on only cutting wood, so based off the link above, the forces pushing against the spindle from the wood is about 5N. I don't know if this is for soft/medium/hard wood, so I figure a 10N force against the spindle would be more than enough to cover milling a hard wood.

The force based off the spindle and mount = F = m*a = 2.75kg * 9.81m/s^2 = 26.98N
Add the two forces together give 36.98N

The torque exerted on the spindle during a cutting pass would be = T = F * r
The radius from the center of rotation to the end of the spindle is .108m.

T = 36.98N * .108m = 3.99Nm

From the stepper motor to the spindle, I plan on having a gear reduction (is it still a gear reduction even though they are pulleys?) of at least 1:3. Probably more if I can find adequate pulleys. So with that gear reduction of 1:3, we have 3.99Nm / 3 = 1.33Nm.

I would like a factor of safety of 2, but would settle for less if I needed it. With the factor of safety of 2, the output torque (pullout torque I think is the technical term) of the motor will need to be 2.66Nm.

2.66Nm = ~27 kg-cm.


To me, my 2.66Nm doesn't seem right at all compared to the example in the link. The example in the link above has a required torque of .052Nm to move a 20kg item. The link above also uses static coefficients, which I don't. I especially think mine is wrong since I am having a hard time finding a motor that has more than even 20 kg-cm. I could go with more pulleys for a greater gear reduction, but then I might run into spacing problems (haven't really looked at this option yet), and by the time I buy the two extra pulleys, the shaft, two bearings, and belt, I'm guessing that I will have spent as much on the hardware as the difference in price for the motors.


Could someone look over my steps and tell me what I did wrong? Could you show me the equations so that I can apply them to my design? If for some reason I did solve this right, could you tell me where to find a motor that is that strong?


Thanks for everything,
-David

[ger21]

You don't use the center of rotation of the spindle (.108), you need to use the diameter of the pulley at the spindle, as that's where you'll be applying the torque.
And you also need to factor in the diameter of the motor pulley, as the larger the motor pulley, the more torque you'll need at the motor.

And you may also want to calculate the inertia of the rotating motor, and how much torque it will take to accelerate it at your desired rate.

[dfaszer]

So to get the torque, it would be my forces, 29.98N + 10N, then multiplied by the pulley diameter (~.054m) instead of the diameter of the spindle (.108m)? That would give me a torque of ~2.15 Nm. With the 1:3 reduction, safety factor, that will give me 14.68 kg-cm. Does this sound better?

How would I calculate the inertia of the rotating motor? Is this something that is a standard for NEMA motors that I can look up?

Thanks,
-David

[123CNC]

Perhaps these may help,

http://www.diequa.com/download/articles/inertia.pdf

Reliance - Formulas & Calculations

http://www.motioncontrolonline.org/f..._Selection.pdf

Each go into the why and how of motor to load inertia ratios.

If I understand your question, yes most all stepper and servo data sheets include rotor inertia in the specs.

You will have a hard time tuning your drives and or suffer poor performance if a large mismatch.

As you have mentioned using belts as part of your gear reduction, this compliant coupling is a very good reason to consider your inertia ratios.

[amina]

hey,
can someone with experience tell me are nema 24 stepper motors with holding toqrue of 220 N cm good for cnc . I got the project to make my own cnc to pass the exam. Machie isn't supposed to do any needy cutting, just drawing to show that all works fine, or maybe wooding

[Tkamsker]

I use 2 to 3 nm Motors in my machines Look at nanotec ,..


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