Stepper Motor Holding Torque

[matsci1]

Greetings,

I am designing the Z axis for a CNC router.
The "Router" will be a Dremel model 300 that weighs 450 grams. The Z axis carrier will add about another 500 grams or so....I have only built it in a CAD program and not out of material yet. So the Motor will need to move a weight of about 2 to 2.2 pounds in the Z direction.

The stepper motors are rated in a unit of "Holding Torque". The defination that I found on the internet of Holding Torque is " The amount of torque required for a motor to remain in a fixed position."

I am not sure how to calculate how much "Holding Torque" a motor would need to have in order to comfortably move my 2 pound Z-Axis around and keep it steady.

Any information as to how to determine the Holding Torque in this situation would be appreciated.

thank you

[ger21]

I am not sure how to calculate how much "Holding Torque" a motor would need to have in order to comfortably move my 2 pound Z-Axis around

Holding torque is when the stepper isn't turning, so no amount of "holding torque" will move your axis.

How are you moving the Z axis (screw?) and how fast do you want to move it. You need to determine how much force you need at a given speed, and calculate how much torque you need to provide that force. Then you need to look at torque curves of motors, to find one to give the needed torque at the given rpm.

But in reality, with a screw drive, any 200 oz stepper should be able to move a 2lb Z axis with no trouble at all.

[Al_The_Man]

Any information as to how to determine the Holding Torque in this situation would be appreciated.

thank you

Holding (stationary) torque of a stepper is the torque achieved when the rated DC voltage is applied, in this case the rated current will also flow, and this will also equal the maximum torque capability of the stepper.
In order to achieve the same torque when in motion, stepper drives use either a PWM or the less efficient series resistor method of control.
Al.

[matsci1]

Thank you for the reply,

a 200 oz stepper. Is 200 the output in some unit...torque is foot pounds or Newton Meters.

I plan on mostly using it on wood or plastic...perhaps Aluminium or a soft alloy like brass. As far as the speed goes....I would like it to move about....oh...so fast..or so. I do not see the Z axis having a travel of over 50mm and it will spend alot of time being stationary as compared to the x and y axes. Most of the wood has a thickness of between 18 and 24mm. So a cutting depth of about 10mm to 12mm would be an expected range. It does not really need to be a speedster to cover that distance in a reasonable amount of time.

[ger21]

200 oz-in = 1.4Nm

[matsci1]

Thank you for the answer.

My next question is how does one determine the correct Holding Torque ( and thus the power of the motor) for a given design. How did you arrive at the figure of 200 oz.?

[port_huon]

Check out this post in a previous thread. It contains a spreadsheet for calculating motor torque etc.