Help select Servo type, conversion from steppers.

[JAZZCNC]

So I get fair good Rapids, but the system is tending to resonate espacially the Z-axis.

So how will servo's help this problem.!. . I don't understand your logic.? They will if anything make it worse as they spin much faster and don't like resonance which affects the tuning.

Then you have to consider the other things you'll need in place to make use of there extra speed and resolution. Namely the amount of pulses required to get full speed and make use of the encoders resolution. The parallel port isn't fast enough unless you make use of electronic gearing which defeats the main advantage of servos, resolution and even then it's stretched. So an external motion control device is required which gives much higher frequency's required to use servos correctly.

To be honest from what you describe then you don't require servos and steppers are more than good enough.
You'd be better off spending the money on stiffening up the frame and getting rid of the resonance. Because like any thing in life you only get out what you put in and if the machines weak then often upgrades just highlight weak areas more and no gain is felt. Often they can perform worse has the in-balance exagerates the weakness and high performance components are much less tolerant of sloppy engineering.!!