Ive been trying to get my mind around building a CNC machine, and looking at hardward and software, i'm not seeing where the measurement comes in. if( and i plan to) use MACH2 or if i can afford to buy it, MACH3, where does the position feedback come into it? it looks like there is not much I/O on the parallel port, so is positioning all done by step counting and math conversion based on the set-up and pitch of the leadscrews? if so, then i dont need any encoders or linear scales...right? or is there a way to integrate them?
Mach2 and Mach3 are both open loop. They send out step pulses and relies on the hardware to move the machine accordingly. If you have servo-motors the encoder-feedback goes back the servo drive and the positionloop is closed by the drive. It is then up to the drives to let Mach3 know if, for some reason, the following error is exceeded. This is done via a digital fault signal back to Mach3. If you have step-motors then no encoders are necessary.
With that said it is still possible to connect either rotary or linear encoders to Mach3 and have Mach3 display the position. You can then use the machine as a manual machine and Mach3 would act as an ordinary DRO. You can add a second LPT-port and set pins 2-9 of that port to inputs, that would give you enough inputs for a couple of encoders.
But again, Mach3 runs open loop. The position loop is closed outside of the software.