Measuring on the adaptor from the Z screw to the quill, near the quill, I have well over 0.5mm of flex - thats a bad figure as it equates to backlash at the tool, so doing nothing here is not an option.

So, looking at the options -

Power the quill via the left side of the pinion shaft, this has slop in it so will need some clever way of preloading the quill upwards - this could lead to rapid rack/pinion wear as it will need enough preload to counter cutting forces from pulling the tool downwards.

Power the knee - this is an option but will cost a lot as it needs a ball-screw, servo, drive plus full tear-down to fit it all.

Alter what i have got - looking at the picture below, is there any harm in removing the web of metal circled?? It would mean i can get a bigger connecting arm in there with much more contact below the bolt to resist the down-forces, without losing any Z travel.

Click image for larger version.  Name: dIMG_1616.jpg  Views: 1213  Size: 1.12 MB  ID: 19503