[pgruendema]

Hi H.O:
I am using Reliance 690 servos for the X and Y axes of a B'port mill. I have measured the watts used by these motors when making 1/2" deep cuts in mild steel, using almost the entire diameter of the 3/4" roughing end mill. The power used is tens of watts, something like 40 or 50 watts. This is small portion of the power available with these 480 watt motors. They are rated something around 1,200 inch-ozs. While they look like they are the right size for a standard B'port, they are in fact huge.

While i had the power meter (Brand Electronics E-1850) on there i confirmed that the motors pull zero watts when they are on location. That's what servos do, they only draw power when there is a difference between commanded and sensed position. Position is sensed by the encoder and the circuit that spends its time counting up and counting down. For your planning purposes, 10 counts off location brings on modest power and 40 counts off location brings on significant power to restore position.

You mentioned 2000 line encoders which are the same as 8000 count per revolution. This allows extremely fine position control, finer than is needed for milling but might be ok for a cnc jig grinder. And your PC controller can output perhaps 30,000- 50,000 steps / second so you might be stuck with a machine that positions very precisely but has a top speed of ten inches/ minute.

Generally the USB port is by design not a real-time device. The parallel port is. To get real time pulses out of the USB port the pulses are generally stored in an external box which then parses them out on a real time basis. Extra complexity but it can do so faster than the parallel port.

If you are using 2:1 belt and pulley step down then you might want to look at 1,000 or even 500 line encoders, depending on the pitch of your lead screws. Do not set up for too many pulses/ inch! 20,000 - 40,000 per inch is fine for general purpose milling.

Pete