I'm converting my Supermax knee mill from its failing Anilam control to Mach3. I'm using an Intel Atom processor, ethernet SmoothStepper, Cncdrive DG2S16035 drives and AMT10X modular encoders. The encoders have user selectable resolution and will be mounted on the servo motors. I think  my math works out like this assuming the encoder set to 1000 and 200"/min rapids.
5 turns per inch lead screw x 1.5 motor to leadscrew ratio = 8 motor turns per inch x 200 inches per min = 1,500 motor turns per min x 4 pulses for quadrature output = 6,000 count per min
6,000 count per min x 1,000 encoder count = 6,000,000 pulses per min = 100,000 pulses per sec or 100 Hz.
First, is my math straight? Second, is 1000 count where I want to be or should I set the encoders to something less? 500, 512, 800 are also available settings.

I am not really clear what you are wanting but our numbers are similar Here is how I would calculate.

1000 count encoders will be 4000 as far as Mach is concerned so with a 1.5:1 reduction that would mean 6000 pulses per rev, you need 5 revs per inch so 30,000 steps per unit.
If your velocity is 200IPM then the frequency required would be (200 x 30,000)/60=100,000Hz or 100KHz.
The SmoothStepper will have no issues with that as it is capable of 40 times more, ie 4MHz. Your  CNC4PC breakout and possibly the drives  will be the limiting factor so you will have to see what their max frequency is, if however they are not capable of 100KHz then I would be surprised.


The higher the frequency you can have the better your machine should be.

Hood

Thanks Hood. I wasn't sure what the practical speed limits are. I take it the higher count the better up to the speed limit of the slowest component? I don't have the paper work in front of me but I believe the encoders will go over 2000 counts per turn.

Yes, if the hardware can handle it a higher count will give you better resolution. Also your servo drives will likely be able to respond quicker to position feedback and thus keep things tighter. Obviously that will depend on the update rate of the drives though.
Hood