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Frequently asked Questions- Motors

Are servo motor driven machines better then stepper motor driven ones?

Depends on the criteria used. A servo motor excels at accelerating an inertial load, a stepper motor excels at low speed constant force operation. What does this mean for a CNC router? A servo driven machine can move point to point very quickly but can have following errors while actually cutting the part. Conversely, a stepper motor driven one will typically take longer to reach the start point, but will have better accuracy while cutting out the part. What's more important: getting there fast or cutting an accurrate part. You decide.

It is a fallacy that stepper motors have no feedback. A state of the art microstepping drive does measure and control the winding currents for precise positioning and smooth motion.

Many lower end machine use DC brush servo motors which output far less torque (measured in oz-inch units) than a square type of stepper. Sometimes inferior lower resolution  feedback encoders are used to save on the cost of the DC brush motors. This can result in either a less rigid motor or rougher performance under cutting load. Some of these drives use non-digital analog circuits (analog tachometer feedback) that can drift in performance characteristics over time.

Stepper motors are actually a form of brushless motors and are more closeley related to their bigger cousins: the AC brushless motors often used on highest end CNC machines. Both motors have no brushes to wear out or cause sparks, and have the windings located on the outer stator of the housing for maximum heat dissipation. 

If driven correctly stepper motors can outperform many smaller DC brush motors, are as stiff and smooth at low to moderate speeds as an AC brushless motor, and are far more cost effective than any other CNC drive system.

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