Thanks for all the good advice I've taken it on board wherever I can.

Unfortunately due to the space available on a Bridgeport series 1 head there is insufficient room to mount a BK bearing and pulley at the bottom of the head so the ballscrew will have to be in compression rather than tension. In addition as its a milling machine rather than a router the axial loads are much higher so I'm a bit worried about skipping the floating bearing.

There isn't enough room as I propose to mount the ballscrew only 25mm from the quill sheath rather than the usual conversions which are 75mm from the quill as there will be less side loading and less chance of buckling. This is also relevant regarding the coupling to the quill which in the Bridgeport is a single screw (3/8 fine IIRC) (reduced leverage)

Yes I do need to do some calculation rather than supposition and the Hiwin site was a mine of useful info I just need to shut myself away for a couple of hours :) .

So in conclusion I'm still undecided! Its awkward to fit a floating bearing in the limited space available and as I'm already machining the maximum amount of non stress bearing metal from the Bridgeport head casting I don't want to machine more off to provide a bed extension to fit a standard BF bearing block.

In the best of bodgers tradition I'll probably do the calculations and unless they are too bad try the single support and if it doesn't work make up a custom floating bearing support.

Wish me luck.

Cheers