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Technical Section Phone: 516-328-3970 www.technocnc.com  7 In  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 machine’s 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  finer  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  better  suited  to  applications  requiring  a  high-end quality finish. Spindles Spindle Types There 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 measure of spindle accuracy. TIR measures how true the rotation of the shaft is. It is typically measured at a fixed distance  from  the  tip  of  the  spindle  shaft  and,  as  the measurement name indicates, determines how out-of- round the rotation is. Brush type spindle motors generally have TIR significantly 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 replaces it with another in a matter of seconds. The automatic tool change system, as the name implies, is done without any