Hi
a picture of your actuator and the existing collet mechanism will help
Will get some photos Its leaver rotary motion of 70deg to open close a collet.
I'd use a 2 pole relay to power the motor to open and close the collet
with a limit switch to switch off the motor once the collet is closed and another when open
to save the G540 relay output for something else
I can't think of an easy way to use the step / direction signals to control a DC motor
unless you used a stepper motor to operate some switches
i didnt think of this. This could work, but it also isnt very simple its a motor to turn on a motor. Maybe belt reduction drive strait of a stepper motor to rotate the shaft of the collet closer. Would need to check the torque
or an electronic control circuit if you break into the printer cable to connect to the step / direction pins for an unused axis
(a C4 dual relay board is one possibility or may be C41 and PWM DC speed control if you need to control the motor speed
john