leadscrew conversion thoughts

[GEOFFREY]

Hi, this is my first post on this site (other than new members intro ) and am not quite sure where to post!!!

I have a very solid cnc router converted from a pcb drill router. the Z axis is very robust, using a vertical roller bearing slide on each side.

As the machine was not intended for the input of various Z depths it does not have a Z leadscrew, but the axis servo drives a horizontal shaft with a crank and conrod arrangement to drive the Z slide (Ihope that makes sense).

I intend to try to convert the Z axis to ballscrew. At present there is a 24mm hole from top to bottom of the Z slide which houses a spring to help lift the slide on the up stroke. I would like to fit a ball screw through the spring hole, but there is only about 2mm from the back ot the hole to the edge of the slide.

The ideal solution would be to find a ballnut that would fit into the 24mm hole. I think it would be possible to to bore out the top 50mm of the slide to accomodate a nut up to 28mm diameter if the nut flanges did not exceed 28mm. I think it would also be possible to mount the ballscrew offcentre (within the 24mm hole) to permit a larger diameter screw if neccessary.

As the Z stroke will only be about 50mm (total slide length is 205mm) would it be practical to use a fixed bearing at the bottom end (of the static section of the Z axis assembly), the ball nut fixed to the top of the slide, and do away with the top bearing (ie using the ball nut as the top bearing)? Would the Z axis with spindle be too heavy or would I need to incorporate a "helper" spring to lift the axis. Any thoughts would be much appreciated.
Regards, Geoff.