How much torque do I need for a CNC machine?

Question

I have a handful of 31.2oz-in stepper motors (Mouser.com - Applied Motion: HT17-268D), and I was curious if they would be big enough to run a 3D printing/cutting/etching type (think RepRap) of machine. I had in mind to attach them via a simple gear to a screw-type drive to run the tool head back and forth.

• Maximum bed size would probably be ~1.5'³.
• Heaviest tool head would be something about half the weight of a Dremel tool.
• Hardest substances I would use it on would probably be hardwoods (with high speed cutter) and copper (for PCB etching).

How do I figure the amount of torque needed to drive the head, and would the motors that I already have be big enough to do the job?

Answer 1

I was hoping that someone else would answer this question with some sort of formula or rule of thumb that would apply to a much wider range of materials / cutting speeds / etc.

According to the RepRap website,

"It is recommended that you get approximately 13.7 N-cm (= 0.137 N-m or 1400 gf-cm or 19.4 ozf-in or 1.21 lbf-in) of holding torque (or more) for RepRap axis motors to avoid issues, although one stepper with less has been used successfully. For Wade's Geared Extruder (most widely used one as of 2012) it is suggested to use motor that is capable of creating a holding torque of at least 40 N-cm." -- http://reprap.org/wiki/StepperMotor#Holding_Torque

Since your motors have 31.2oz-in of holding torque -- significantly stronger than that recommendation -- the motors you already have should work fine for the axis motors of a RepRap-like machine.

Answer 2

Formulas do exist but when DIYing something like this there will be too much uncertainty in the required variables.

Here is an extremely basic way of going through the calculations:

You need to start with the required force delivered to the table at given speeds. Generally this will be the cutting force + way friction. Cutting speed/feed and force can be looked up from tables for given material/cutter combinations.

For high performance mills acceleration is a greater constraint than cutting force and should be used instead.

Way friction depends on what you are using for linear motion.

Once you know how much force you need to apply to the table at a given speed its pretty straightforward to figure out what motor you need. Put the numbers through screw reduction/efficiency and pick a motor/reduction combination based on the torque/speed charts.

Don't forget to use a 2-6x safety margin. Holding torque is not usable torque and should be approximately halved if operating torque is not given.

Answer 3

The torque you need on a CNC machine is dependant on: 1) How much force the mill is expected to exert on the material 2) The resistance of the linear bearings used to move the axes 3) The type/style of cnc mill(i.e mills using a moving gantry need more torque than a xy table) 4) The drive system (i.e. belt driven machines needs more torque than lead screw)

Ultimately the amount of torque depends on how much power (like watts power) you're pumping into your steppers. The rating on the steppers are based on their optimal; however you can over drive them. Also if you are running your machine slowly then you can escape with using less torque since there is less risk of stalling as opposed to a machine running at high speed.

Further as stepper motors speed up they loose torque, unless they are BEMF compensated.

Try with your steppers; it may work in which case you're just over thinking the problem and if it doesn't work then you have an idea of how much torque you will need.

Answer 4

Holding torque has nothing to do with driving the amount of torquelead screws. The holding torque rating is only for when the Axis is in stationary position holding. What you want it to maximize your stepper driver amperage with a stepper that has the same or a little less amp rating