Bridgeport Manual to CNC conversion...

[JAZZCNC]

Ok well in theory there's no difference in requiremnets between Servo and stepper for connecting to PC. However in practise there is specification difference.

Servo's require much higher Pulse frequency and much cleaner signals. Now your average £5 Bob doesn't cut it and the Parallel port is just doesn't cut the mustard.

The speed and resolution you get from Servo is dependant on the encoder fitted. The encoder also determines the pulse frequency required.
For instance most Servo come fitted with 2500 count encoders which are quadrature encoders so this number is *4 so 10,000 pulses required for one revolution at full speed. If 3000Rpm that's 30,000,000 pulses in 60s. Break this down to seconds and that's 500,000 pulse per second or 500Khz required to achieve full motor RPM.
Now PP is only 25Khz or at realistic best 45khz 45,000 pulses. At best this means you'll only get 270rpm out of your 3000rpm motor.

Now most servo drives provide a pulse multiplier so one pulse becomes 2, 4 etc depending on amount set. This however comes at the cost of Encoder resolution which gets reduced by the same amount. IE: 1:2 Multiplier 10,000 = 5000ppr 1:4 2500ppr etc.
Not problem your thinking I don't need all that resolution anyway but it's not just resolution that suffers, so does smoothness of the motor and really the whole point of servos is for there superior Resolution and smoothness along with linear torque.

So bottom line is if using servos then really you need an external motion control card that gives you required pulses of good quality.

On more positive note there are other advantages to servos than just resolution and linear torque. 3000Rpm often is too fast so you'll use ratio which increases torque so smaller cheaper servo can be used.
Torque is linear and the rated servo torque is when at full motor speed unlike steppers which is when at stand still and drops quickly when rpm rise. Large steppers drop very quickly and not much use after 900-1000rpm.
Servo drives are powered directly from mains so NO PSU required.

If I was converting a mill then I'd use servos because they are much stronger than Steppers and smoother in operation. Torque is linear so much stronger when your pushing hard and deep.