Homemade CNC 2 spindle lathe question

>> To All,
>> The control on one of my CNC lathes has failed to the point that
>> repair is more expensive that replacement. Even though it's going to
>> be expensive a new control opens up all sorts of possibilities. One
>> feature that would be really nice would be synchronised dual spindles.
>> This would allow work to be done on both ends of a part before it
>> comes out of the machine. I'm looking at various makes of software and
>> am leaning toward MACH 3. I'm thinking of mounting the spindle in the
>> tailstock and driving it in the Z axis with a ballscrew. But the real
>> problem is getting the rotation exactly the same as the main spindle.
>> One way I can think of is to use an encoder on the main spindle to
>> provide a signal to generate step and direction pulses. These pulses
>> in turn would be used with a servo amp from Gecko.

>
>        The problem that I see is that the spindle will be moving at a
>constant speed, while the tailstock spindle will be doing a series of
>stops and starts which will result in wear on the jaws of the tailstock
>chuck and on the workpiece harming its finish. During the time when
>both chucks are gripping, you will be subjecting the workpiece to
>torsion vibration too.
>
>        Note that a servo motor *can* run at a very stable speed, when
>controlled by a true servo amplifier and with tach generator feedback.
>You could probably even do what you need with an extra tach generator on
>the headstock spindle generating the command voltage to the true servo
>amp.
>
>        The Gecko servo driver, however, does not pay any attention to
>the tach generator on the servo motor (if it is present at all), but
>moves the motor in steps just like a stepper motor -- paying attention
>only to the encoder.
>
>        I would go with the tach generator on the headstock spindle, and
>a matching tach generator and servo amplifier on the tailstock spindle
>motor. Bring the chuck into position almost but not quite touching the
>workpiece in the headstock spindle, and slowly tweak the gain for the
>tach vs the command inputs on the servo amplifier until the two are
>moving at precisely the same speed. (One trick would be to cylinders of
>glass or quartz with bar patterns on them and a light inside. Then, no
>matter how fast the spindles are moving, if they are at the same speed,
>the light will be at a constant brightness. If they are out, there will
>be a pulsation in the light intensity, and the slower the pulsation the
>closer to the same speed you are.
>