# Thread: What size stepper motor do I-need.

1. Originally Posted by masinecc
How can i calculate what motor size i need for my gantry if i use rack and gear on both sides. This example is for ballscrews. Looking forward for your answer.
Backwards. Choose the RP size, gearing so that you have as a result normal rapid speeds with acceptable resolution. Then figure how many RPM you need in reality so you have mostly that in mind. Then you go back to the gearing ratio and gantry weight and from there you decide Nema 23 or 34.

Obviously as the RPM is the important part here cause most possible it will be well geared, then you finish with low impedance motors driven by high voltage.

And knowing that nema 23 motor driven at 70VDC moves a gantry max 50-60kg with gear 1:1 without problem at 10m/min via ballscrew, you can figure the motor you need for your case.

2. Nobody works it out, they just assume that the biggest motor they can find has to be the best option. Then they buy a set of 3 because that saves even more thinking and they end up with a Z axis that could raise the Titanic and is so heavy the gantry has to be made from RSJ

3. Originally Posted by Robin Hewitt
Nobody works it out, they just assume that the biggest motor they can find has to be the best option. Then they buy a set of 3 because that saves even more thinking and they end up with a Z axis that could raise the Titanic and is so heavy the gantry has to be made from RSJ

4. Hi Sir,
I am making a small scale CNC. I just want to ask that how are you calculating the power in the end four examples of this article?

Regards,
Zaryab Shahid

5. Thanks Irving for a great tutorial

6. Hi guys,

Can you please check if i am using this motorcalcs the right way?

(40kg moving gantry X+Z on hiwin type rails using 2* c7 type 2010 1500 mm ballscrews)
Really unsure about cutting forces (5N) and gantry weight distribution

Lets say i put this info in the sheet.
http://cnc4you.co.uk/resources/Stepp...1-03%204Nm.pdf

INPUTS OUTPUTS Version 1.2 - 30 Jan 2010
Units Metric

Load Calculations

Mass of Load 40 kg Load inertia 1,013E-04 kgm^2
Friction co-efficient 0,1 Load torque 8,937E-02 Nm
Cutting forces 5 N

Leadscrew calculations

Screw pitch 10 mm Screw mass 3,676 kg
Screw dia 20 mm Screw inertia 1,850E-04 kgm^2
Screw length 1500 mm
Screw minor dia 16 mm
Screw efficiency 80% %
Screw Fixing Fixed-supported Critical Speed 1066 rpm

Max linear speed 2500 mm/min Motor revs 250 rpm
for: Cutting
Total Inertial load 2,890E-04 kgm^2
(inc rotor inertia)
Proposed Motor Acceleration torque 0,504 Nm
Rated current 4 A
Winding Inductance 3 mH Total Torque 0,594 Nm
Holding Torque 3 Nm at 833 steps/sec
Detent Torque 0,04 Nm
Rotor Inertia 275 gcm^2
Ideal Voltage 55 V
Motor Usage Esitmated Motor
Supply Voltage 68 V Corner Speed 1082 rpm
Set Phase Current 4 A Estimated Torque
(at required speed) 2,081 Nm
The motor & operating conditions chosen have a good
margin and should be OK at the required speed
(subject to driver circuitry)

So what i am looking for is the following.
My design is 1500 mm of travel on the X-Axis.
Gantry weight about 50-70kg
There will be 2 ballscrews.

I am planning on using a timing belt reduction 1:2 ballscrew:stepper

I am not sure how to input a reduction in the sheet.
Filling out 2005 type screw makes the screw rev by a factor of 2
Filling out 2010 type screw makes the motor spin to slow so torque is off by a factor??

Will 2500mm/min speed be plenty fast for a 2.2 kw chinese R20 spindle using carbide bits on aluminium?
What would be a material that needs a higher speed rates over 2500mm/min???
Planning on machining hard/soft wood, aluminum, plastics.

(i have no idea what speeds and feeds are in the resonable range )
I tried filling in some diameters/types cutters and number of flutes in a speed and feed calculator for wood and alu

Looked like 2500mm/min is max any one would need in the hobby range, am i right here?

I am not sure if i can just reduce the gantry weight to 50% if i plan on using 2 steppers. (slaved X axis setup)

Please point me in the right direction.
Any help is welcome.

7. That spreadsheet is great, and incorporates a lot of theory. Two problems, really (and I don't want to take anything away from the work that went into it):

- it takes a bit of interpretation in some cases, as you have found. Two ballscrews, belt drive with step-down, etc. Not impossible to work around, but it's not obvious how at first glance.

- actually, there are only a few answers available although the spreadsheet results can help you tweak them.

I went through pretty much exactly the same process a couple of years ago. My machine is close to yours in size - 1500 long axis, two ballscrews, gantry around 50kg (guess). For motors, in reality, the choice is Nema 23 or 34. The larger motor has high inductance and a lot of inertia. When you do the sums, it gives poor performance. A friend recently swapped the 34 motor on a machine he bought second-hand for a 23 and significantly improved performance. So the answer is Nema 23. Make sure you get a low-inductance motor, and 3Nm or 4Nm will work fine. I use two 3Nm motors as 4N were not available widely at that time but I would probably go 4Nm today for the sake of a couple of quid. It is important to drive them properly, hence talk of 68V. Why this funny number? Because it's a safe value to use with a decent digital stepper driver (EM806 or AM882).

Speeds - I went through the same "what's a good speed" thinking. I'm using 2005 ballscrews which reach their critical speed at about the corner speed of the steppers (900-1000 RPM). That gives me 5000mm/min. Actually, I think I'm running at about 4500 at the moment to avoid any possibility of stalling one motor (which is also why you want modern digital drives with stall detection). This is not as fast as the machine could use although it works well. However, I seem to be doing quite a lot of fine cutting with a lot of 2d/3d detail and acceleration is more important than max speed in this case. Your 2010 with 1-1 belt drive would run faster than mine at the expense of resolution (but not enough to worry about).

Some thoughts to get you started, and i'm sure that others will be along with different views! However, my remarks are based on a similar machine that is up and running, not just theory.

8. ## The Following User Says Thank You to Neale For This Useful Post:

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