Hi will, a good point. To mill with the quill right up would mean that the knee drive becomes a prime factor in Z axis movement and so I would contemplate gearing the stepper down a lot, for more precise incremental movement, as you only want to move to get depth of cut increases once the cutter is near the job, as opposed to X axis drive which really has to cover some ground, and requires a free running slide set-up.
In the end, retro fitting drives to an existing machine, not intended for the new purpose, is always a compromise instead of starting off with the requirements for milling and drilling and designing to these.
Take a Bridgeport for example, A very large and heavy machine, but I don't think it would be practical to drive the knee up and down as opposed to using the quill to half it's out travel for milling.
As far as quill lock is concerned, the ball leadscrew will hold it in position if it's braked and mounted firmly enough.
CNC milling is usually taking small cuts at a fast travel rate and the ploughing cuts normally asscociated with manual milling arent suited to CNC due to the weight involved.
When I worked in UK we had a bank of Cincinnatti vertical mills. The heads on these were/are massive, but they have a huge counterweight that allows the head to be moved with ease with a small hand wheel and power Z axis drive.
My ideal mill for CNC would have a table that moved as freely as a cylindrical grinder's, and they run on balls.
Second the X and Y slides would be firmly attached to the base, no knee, and the head, no quill, would slide on linear ball bearings.
Cast iron or steel fabrication throughout, no alluminium.
Last but not least a variable frequency drive for the main head spindle and side high speed spindle.
I would go for linear bearing slides throughout and ball screws on all axis.
Small cuts and many of them as opposed to ploughing, and spray mist cooling with total enclosed cabinet with extractor.
Table size would be about 30" X 8"
Z axis travel would be 12".
To enable a tool changer the main spindle taper would be 30 Int, with pneumatic draw bar.
Tool changer would be a swinging arm 180 deg type with vertical drum magazine and 10 tool stations.
I think alluminium could be used for the base and column and probably the head.
Ian.