I have read the whole build and it brought back memory's as i have a similar mill, i hope Robin does not mind me adding some info of how i tackled the problems.

My biggest issue was the Z axis namely the play of the quill in the headstock. I also made three failed attempts at cnc'ing the Z

My own quill had just over .1mm slack which is abysmal, i had a quote to hard chrome the quill and grind it to size of £300 and thought there must be a better way.

I cut through the front of the headstock with an angle grinder also at the top of the headstock to allow the quill bore to become flexible in diameter.

So now the quill was too tight, so i have adjusting screws that allow me to open the bore for the quill this allow me to get a beautiful fit of the quill in the headstock tight enough that there is no play but free enough that there are no issues with lost steps.

To CNC the Z i based my design on a Bridgeport mill (Getting the Nut As Close To The Quill As Possible) which i have also, by machining a slot in the quill i have mounted a custom housing that takes the ball nut it is made in steel and by using two strips of steel in the headstock i can take all the wobble out of the quill.

In fact i have the quill rotated 180 degrees in the bore as that was where it seemed to slide as easy as possible so now the the teeth for the gear are at the front.

It has two steel cheeks attached to the headstock and looks very similar to Robin's in that respect.

I do not suffer from flex in the column and my belief it that the flex in Robin's machine in fact comes from the poor fit of the quill.

Although clamping the quill will minimise problems it will not eliminate it completely.

Phil