CNC machine

[Amal Ghodbane]

Hi everyone,

I was wondering what parameters should be kept in memory while choosing the motors to a CNC machine.

And why stepper motors are the most used in this type of machine

[Jeff]

Choosing the right motors is actually a little tricky.  You need to match the motor's speed and torque to the requirements of your machine, and also the motor's voltage and current requirements to whatever driver you are using.  This requires some calculations.  Most people use stepper motors because they are cheap and the drivers for them are cheap.  But then they hear about problems with losing steps, so they buy the biggest motors and drivers they can afford. The primary spec listed for stepper motors is the holding torque (the amount torque a stepper motor can resist, when standing still).  Unfortunately, the holding torque has little relation to the actual torque delivered by the motor when running at speed.  You really need to look at the full speed-torque curve of the motor.  (But not all motor manufacturers publish the speed-torque curves - you have to ask them for it!)

The other big point of confusion is how to compare servo motors to stepper motors.  People looking to replace a 250 oz-in (holding torque) stepper motor on a machine with a servo think they need a 250 oz-in servo motor.  Firstly, the amount of torque reliably delivered by a 250 oz-in stepper running at speed might only be 50 oz in.  Furthermore, servo motors like to run 3x to 4x faster than stepper motors, so you really want to add maybe a 3:1 gear or belt reduction if using a servo.  This would reduce the required motor torque to more like 20 oz-in - a far cry from the 250 oz-in spec'd for a stepper.