View Full Version : Recommended Stepper torque

04-16-2003, 12:39 PM
I'm working on plans for a 5'x9' (usable) router table. I'm having problems determine how much torque is "enough" for each axis. I'll be running mostly cast acrylic and wood, but want to be capable of running aluminum occasionally. It is unlikely that I would ever try to run steel. I would like to be able to hit pretty fast speeds in case I do some foam work (RC airplane wings and all).

The main need for the machine will be to cut out acrylic items for my saltwater aquarium business.

The design will use ball screws for each axis, .25 pitch for the X and .20 pitch for the Y/z.

I've been looking at things like the shopbot (305oz/in at 3.6:1 gear ratio), and others but see great differences in the motors used.

Can anyone give me any guidance here?


04-16-2003, 05:29 PM
Several of my metal working cnc retrofits are powered with 660in/oz servo motors. These machines are a typical 11 x 50" table, the size you'd use as a toolroom mill.

Depending on the amount of time you want to allow it to accelerate, it will go pretty fast, but typically the practical rate would be around 200 inches per minute on a .2" pitch ballscrew.

I still don't know what kind of real ACC/DEC curves you can set up with stepper drives, so I can't tell you what to expect there. But I think maybe you'll want to go bigger than what you were planning :)

04-16-2003, 07:25 PM
I did locate some 1242 oz-in steppers at http://www.clickautomation.com for very little more than the smaller ones, about $170 each. I'm starting to think that maybe I want/need servos instead though. Defiantly more research needed....

200 ipm would be pretty slow still for foam, but acceptable considering the tradeoffs.

Would going to 0.9 degree steppers and .5 pitch threads help high speed without sacrificing too much at the low end?



04-16-2003, 10:23 PM
Changing the pitch of your ballscrews is the same as selecting a 2:1 overdrive for your motor, so you will be cutting your effective motor torque in half.

I honestly don't know about stepper drives that much, but it seems to me that they are pretty cut and dried: the driver puts out a pulse and the motor had better follow it, or else you lose a step and your position is out the window.

A servo motor, whether Brushless AC or a conventional DC motor are the industry standard in reliability. The setup for servo motors has many configuration parameters that allow you to tune the motor to your system load and desired response. Even if the servos cost you a bit more, you are getting more. I think the drives are more nearly equal in price, so you'd only have to buy those once anyways :)

When running a high feedrate, you also need to be concerned with servo lag, just so you know. The motors respond quickly, but the higher your feedrate, the more time it takes them to accel/decel, and this is why special software is required to run these types of machines, because the software reads ahead in the code and anticipates sharp turns and begins to modify the feedrate early on, so that the motor does not overshoot (and possibly gouge) the corners.

I'd encourage you to look at CamsoftCorp software for your CNC controller. It has provisions (in the CNC Professional package) to help you control high feedrates. I have never used that part of the package, but I know its in there :)

The maximum speed of a DC servo is proportional to the voltage applied, so keep that in mind when you pick out power supplies. The rating of the motor does not guarantee the nameplate torque if you are low on available driver voltage.

04-17-2003, 01:10 AM
Great info, thanks! I'll be doing some more research (and pricing) on the servos. Sounds like I just jacked up my target price for the setup...again... :)

I'll take a look at CamsoftCorp too- I'm just getting in to trying to figure out what software I need...

Tons of options here...