[matsci1]

Greetings All,

I am building my first CNC machine and I have a question about what size of stepper motor I should be using. This machine is to have a working area of 15 X 15 centimetres and be used for machining wooden patterns for metal casting or wooden “Tiles” for decorative purposes.

I intend to build the frame and slides out of ITEM aluminium profile and use a Dremil for the milling tool.

This machine is to be for light work and also so that I can get experience at how all the components work together before moving on to building something bigger.

Stepper motors: They seem to be rated in “Holding Torque” with the cost rising in proportion to this unit of measurment. I see one offered for 29,95 € ( I am in Germany) with a 0.25 nM holding torque

Emis Schrittmotor E547-52500 12 V/DC Halte-Moment 0.25 Nm Phasen-Strom (max.) 0.7 A Wellen-Ø 5 mm im Conrad Online Shop

Is something like this enough to make light cuts in wood and move a fixture holding a Dremil up and down?

[James Newton]

Yeah, holding torque means very little. It only speaks to static (non-moving) loads. Everything in CNC is dynamic; moving loads so what you need to look at is the weight you wish to move or the torque required to move it, and how quickly you wish that motion to be. This is the measure of the power required which can be estimated in Watts and then translated into what amperage and voltage your motors and drive must deliver.

This page: techref.massmind.org/techref/io/steppers.htm#Estimating Has some "rule" of thumb calcs that may help translate the weight and speed you are hoping for into drive wattage, and explains how to translate that into voltage and amperage which will then allow you to pick out a suitable power supply, driver, and motor.

The motor you are looking at is very small with only 12 Watts of drive power. It will spin at a maximum of 600 RPM in unipolar mode (less in bipolar) and could move a 6lb load at about 1000 IPM.

If cost is an issue, you might want to look for used motors or scavenge old office equipment for free motors. There is a list with some good sources at:
techref.massmind.org/techref/io/stepper/linistep/motors.htm

[matsci1]

Thank you for the information James. This makes much more sense now.

About the heaviest object this machine would be moving is the Dremil on the Z-Axis. Lets say the whole Z-Axis weighs 5 Kilograms (11lbs.)…which it does not. I would say that it has a travel distance of 5 centimeters max but for calculation lets say 10cm (3.93in). If the motor moved this 11lbs a distance of 3.93in. in one second that would be (11lbs)(236.2IPM)/531 = 4.9 Watts.

I am surprised at how small a value that is.

The X and Y axis have to deal with the force of cutting. The material will mostly be only wood and I intend on only light cuts at that so I do not think it will be nearing a force of 11lbs.

So now I am looking at Amps and Volts on the motors and not Holding Torque…I did not really like that unit anyway.

I see here a motor that is listed at 0-40 VDC and 1 Amp. This is then 0 to 40 Watts.

Trinamic QMot Schrittmotor 1,8° QSH4218-35-10-027 0 - 40 V/DC Halte-Moment 0.27 Nm Phasen-Strom (max.) 1 A Wellen-Ø 5 ... im Conrad Online Shop

and one below it for the same 0-40 VDC at 1 amp but a greater Holding Torque...(0.49Nm vs 0.27Nm)...There is this holding torque again.

Trinamic QMot Schrittmotor 1,8° QSH4218-51-10-049 0 - 40 V/DC Halte-Moment 0.49 Nm Phasen-Strom (max.) 1 A Wellen-Ø 5 ... im Conrad Online Shop


I take it that with motors like these a person can choose the voltage that his system will run on…so if I choose 12 volts I get 12 watts max out of the motor.

Back to this holding torque…If I were to guess at it, the motor with the greater holding torque will not have as hard of a time developing the same amount of power as the one with small holding torque…but the same amount of power can be developed by both motors. Is this correct?

[James Newton]

Keep in mind that those are "rule of thumb" and not perfect. For example the formula based on weight and speed doesn't account for friction in the lead screw. The torque based calculation is more accurate, but you can't measure the torque until you build the machine! I recommend you build it first, and the accurate measurements will come. LOL...

Anyway, include a healthy error margin.

Actually the holding torque can be unexpected depending on the mH rating of the motor. More mH = more holding torque, but a lower top speed. See the speed calculator a bit farther down on the page.