Newbie Stepper sizing options based on torque curves

# Thread: Stepper sizing options based on torque curves

1. ## Stepper sizing options based on torque curves

Hi,

I am delving into motor sizing calculations for my first DIY wood router.
My question is about the Z axis which will use a 5mm lead Ballscrew probably.

I am targeting a 5 meters/min feed rate (200 ipm) and less than 0.1mm resolution.
From my back of the envelope calculations I would need a minimal torque somewhere around 0.8Nm, which translates to 2.4Nm with safety margins.

Now there is the torque versus motor frequency that comes into consideration, in two ways. First because stepper torque is a decreasing function of frequency, and secondly because higher frequency adds to the required acceleration torque.

In direct drive 5 meters/min is 1000 RPM. At this speed the torque requirement I have is up to 1Nm (3Nm with safety margins).
Yet motor angular velocity could be reduced using a timing belt. For instance with a 1:3 gear ratio I get 333 motor RPM but at the cost of dividing the torque by 3, if I understand this well. However the required torque is now only 0.7Nm (2.1Nm with safety margins).

Also the general advice I have seen here and there is that around 15mm per motor rev is good practice, which is reached here with the 1:3 geared transmission.

I have made a quick compilation of torque curves gathered from "stepperonline.com", which is attached bellow (benchmarked with their own drivers). Based on the graph, choosing the right motor is not straightforward.

At bare minimum I could use the N23 3.1Nm at 3.5Amps running 330RPM with the 1:3 gear ratio. What happens in direct drive mode around 1000RPM is unknown because of missing data points.

The N34 4.5 Nm at 5.3 Amps would be fit in both speed ranges, as well as the N24 4.2Nm.

The disadvantage I see with the N23 is that I will never get much higher torque than 1Nm, even lowering my feed rate, and I will have to make the gear increase. However I would probably be able to significantly pass beyond the 5 meters/min feed rate target, witch is nice.

With the N34 at 4.5Nm there is a comfortable torque margin, it can be mounted direct drive and I can get tremendous torque at low RPM if needed. However powerful also means costly, and I am not sure that running a Ballscrew at 1000rpm is that good. It is also unsure that I could go much faster than my nominal feed rate (in direct drive at least).

How should I resolve this conundrum ?

Cheers,

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