I've been playing around with some CAD/CAM trying to get a few things designed for the mill that I can cut. I designed some small enclosures for the micro switches that I'm using for the limits (see first image). The switches are less than 3/4" in size and the mounting holes are too small to work with so I am going to try to cut these covers out of some stock Al which will also add some protection for the exposed wiring and terminals.
The other parts are for mounting the AC spindle motor to the top of the head. The idea is to have the top motor mount PL slide inside the two side brackets using an adjustment screw located at the rear for belt tension and 4 top-mounted fastening screws to lock it down.
I hooked the original spindle motor back up to the mill and wired it back together so I can cut these mounting brackets and plates needed to mount the AC motor that will be driving the spindle. I plan to use a 1.67:1 timing ratio which will give me about 5000+ RPM using a 20/12 pulley arrangement. I'll be using a 41 tooth 3/8" pitch 1/2" wide belt to drive them. I believe that 5k will be the absolute max that I would ever run this spindle (given the quality of the bearings, etc...) so that should be plenty. For anything faster than that I have another spindle solution that will take up to 1/4" bits.
I am going to sleep on this design for a couple of days I think before committing to cutting up my precious stock. It has a rather large footprint which may cause a problem later on when figuring out a powered drawbar solution. I wanted to allow enough range in the movement of the top plate so that I have some room to try different pulley ratios and belt sizes without having to scrap this design for another one should I decide against the 1.67:1 ratio at any point.