Our CNC machine's spindle has a speed feedback encoder connected to it via a belt. The feedback rpm is displayed on the operator screen. When we first run the machine, everything works fine but after about 10 minutes ( about 5 parts later) the speed reading drops almost to zero rpm while the spindle actually is maintaining its idle speed (I say about 100 RPM). We checked the belt, setscrews, pulley, etc, which are all good. So, either the encoder or the cable is bad. I thought if the encoder or the cable was bad, it would show the problem right away, not after a while.