Another idea would be:
Acme screw rotated by a stepper.
A beam mounted on the moving piece. (there will have to be a slide here but not shown)
The beam can rotate CCW when it touches the tube.
A sensor (contact, optical etc) will sense when the beam touches the tube and starts to rotate.
"V" nest will hold the tube in proper location.
Pro:
- Simple
- Contact sensor does not have to be precise, only repeatable.
- No high tech sensor required.
Con:
*As drawn the beam requires the sensor to be more precise due to unequal distances from the center.
