gantry mill

i will be starting to build a cnc gantry mill some time this summer. right now i am finishing up a retrofit on a 40 x 20 Mitek cnc mill. i have picked up a new job that requires 26 inches by 50 inch travels and i don't have room for a bed mill or machining center. (not enough ceiling height) i plan on building the frame from steel tubing, using linear guideways, ball screws and ac servos. i already have 2 knee mills retrofitted with Mach 3 software and rutex drives. one is a Bridgeport V2E3 and the other a Hurco KMB1. For the z axis, i plan on using the head off of a bridgeport cnc knee mill with a VFD drive. this should allow around 5000 rpm on the high side. the material to be machined is 7050 aluminum. i am planning for travels of 75 inches by 52 inches with 5 inches in z from the bridgeport head. this allows me to stagger the parts and run 3 in one setup.
is anyone else running aluminum on their gantry mill? i am looking for feedback and suggestions before i get started because experience is the best teacher.
how about the new hardware from Ajax that works with Mach3? i also would like to try the ac servos from Mach Motion because of the plug and play simplicity.

thanks
keith

Here is my mill design from rectangular tubing -- 120x60mm mostly.
Travels are 400mm for X any Y, 200mm for Z. 4-5-6 axis are design choices, don't need them all in real machine :)
For your envelope I would stretch machine sideways, so the table moves/overhangs are shorter.
Or you might consider to put X-rails under the table, and have carriages bolted to the frame.
If you choose to design from steel tubing, consider not to do welding on parts which are intended to be plain :) Use bolts instead.
There is also a great site here -- http://oneoceankayaks.com/madvac/machine_frame.htm
This is then you prefer complexity of moving gantry...
For the heavy bridgeport head, you might want to have Z rails on sideway vertical columns moving
whole Y-tube with the head attached to moving plate... as here http://www.cnczone.com/forums/showthread.php?t=91552
but consider using 2 motors for Z :) as here http://www.cnczone.com/forums/showpo...65&postcount=3