Technical SectionPhone: 516-328-3970www.technocnc.com7In contrast to steppers, servomotors have constant feedback from the optical encoder. This device sits on the back of the motor and keeps the controller informed of how far the motor has actually moved. This constant feedback is used to correct any discrepancy between a desired and an actual position. This automatic corrective action results in faster cuts (up to three times the throughput), and increased power (up to three times the torque) at high speeds. The closed-loop nature of the servo also ensures that stalling cannot occur unless there is an immovable object in the path. When such an obstacle is encountered, the closed-loop system would communicate to the machines controller to shut down rather than lose position.Servomotor resolution depends upon the encoder used.Typical encoders produce positional signals (or pulses) per revolution, and encoders range from 500 to 200,000 pulses per revolution. The more pulses there are, the fi ner the resolution capability of the motor. Servos can perform high-speed continuous motion much more reliably because of the constant feedback from the encoder, making them much bettersuited to applications requiring a high-end quality fi nish. SpindlesSpindle TypesThere are two broad categories of spindles used on CNC machines: brush types and brushless types. The brush type spindles, as the name implies, use commutating brushes to transfer current to the coils that cause the shaft to spin. The brushes tend to wear over time under normal use, and consequently wear out faster under heavier applications. Another drawback to brush type spindles is the noise associated with their commutating brushes. The main advantage, however, is that they are generally much lower in cost. In fact, in some CNC type applications, they are considered disposable.This low cost also tends to make these spindles less precise. TIR, or Total Indicator Runout, is an industry
measureof spindle accuracy. TIR measures how true the rotation of the shaft is. It is typically measured at a fi xed distance from the tip of the spindle shaft and, as the measurement name indicates, determines how out-of- roundtherotationis. Brushtype spindle motors generally have TIR signifi cantly greater than the brushless type spindles. In many applications, such as woodworking, this is not of concern.The other broad category of CNC spindles are brushless, also known as AC spindles. Brushless AC spindles do not have the maintenance issue of brushes wearing out or the noise associated with brush type spindles. Because AC spindles require less maintenance, have a better TIR rating, they are also a more expensive alternative to brush type spindles. The cost is usually justified when looking at the overall comparison between the two spindle types.A summary of the comparison of their features is shown in the table on the following page. In the category of AC spindle motors, there are 3 types: fixed collet spindles, manual quick change spindles, and automatic tool change spindles.The fixed collet spindle requires the collet to be changed with a manual operation that usually requires a couple of wrenches. This is a fairly simple process but takes a few minutes each time it is required. The manual quick change and automatic tool change spindles have an electro/pneumatic system that releases the current tool and replacesit with another in a matter of seconds. The automatic tool change system, as the name implies, is done without any