OK bro I can only teach you the perimeter method is it OK with you? I used a perimeter method all the time on the rotary axis, I dont have a clue either on the 360 degree side calibration, if it`s OK with you im gonna proceed with the illustration
OK then, you need to measure your chuck diameter, let`s use it as a reference on turning into 360 degrees, whatever it`s diameter size convert it to perimeter using this >>> Online Conversion - Circle Solver Calculator after that mark your chuck with a pen, center your dummy bit on the spindle to that on your marked pen on the outside of the chuck, let`s just say perimeter = 212mm, go to your mach 3 calibration page then calibrate it just like normal XYZ movement.
circumference = perimeter
calibrate it using the circumference value as a full 360 degree, so if you point your dummy bit on your marked position on the chuck it should stop at your marked pen when you enter the circumference value, copy your speeds ans feeds on your Y axis to your A axis on the ports and pins to have a reference, if it turns over your marked point, measure the over turn in mm to that dummy bit position.
if the over turn let`s just say 26mm, add that number to the actual circumference, repeat calibration until the full 360 degree rotation of the A axis will stop @ your marked pen, you can do whatever you like on this method, ( custom TPI threads, gears, DOHC cam lobes, etc)
sometimes it over spins while calibrating, in this situation just input a minimal number for example 10mm, always zero your bit on your marked pen then enter 10mm on the calibration page, whatever measurement it travelled input it again to the calibration, then zero again your chuck to the dummy bit then enter the circumference until it fully stops on your marker when you input the perimeter value then you`re done.
Now on this method surface machining jobs are quite easy because you just draw it in a flat in a 3D environment and wrap the code with a G code wrapper, but on complex CAM designs like for example a whole car, you program the CAM first on a top then save the code, program CAM again on the bottom of the car then save the code, now it gets tricky open the top code in the notepad, add the command of the A axis (ex. 212 / 2 = 106) at the ending of the code then copy / paste the second code on it then save it as one file, once you input it on mach 3 the machine will do first on the top, once it`s finished it will rotate halfway then begin machining the bottom portion, it`s up to you if you divide the work piece by 2 or by 4, you just need to interconnect the codes while commanding the A axis on which to turn next then proceed with the next G-code set.
Find out the gear ratio for your rotary. Multiply that by the number of steps set on your stepper drive. Divide by 360. That gives you steps/degree.
To do it as KHOUJ describes, take the diameter of your part surface, multiply by pi (3.14159265) to get the circumference. Divide the gear ratio by the circumference, then multiply that by the number of steps set on your drive. That'll gives you steps per surface unit on the part.