CNC Gantry Mill

So I am starting a new project, one that I have dreamed about for many years.  I've done a lot of work learning how to go about it, and followed a number of the more successful attempts at similar machines.  I've even had a few false starts, and intermediate attempts myself.

The problem statement is simple:

Make a machine that can carve fairly large parts out of softer materials like foam, fiberglass, and wood for the purpose of making streamlined vehicle bodies, and for artistic uses like architectural trim, signs, and sculptures. The machine must fit in my existing garage, and as an added bonus, I would like it to have some ability to work in softer metals, like aluminum.

Since I have a new streamliner project already taking shape for later in the year, I need to do all of this for a relatively modest budget for such a large machine.  So it needs to be accurate, but not break-the-bank, and take years-to-make accurate.

These desires led me to the following initial specifications:

1. Work Cube of 10ft x 4ft x 2 ft (X x Y x Z)
2.  Machining head can be manually tilted left and right to allow for maximum clearances for large parts.  Possibly convert to 4 or 5 axis in the future.
3.  Maximum height of the gantry must pass under 92.5 inches for a support beam in my garage
4.  Table height of 21 inches, so I don't completely breae my back moving large parts on and off, and so I can also store things under it.
5.  Table length of 12 ft to support the large parts and allow enough leverage on the gantry to handle the large 2 ft of Z travel.
6.  Have multiple Z mounting heights for the bridge, so that I can trade volume (for large foam parts) for rigidity (for not-as-large hardwood, and possibly aluminum parts).  Mounting heights are tentatively at 12, 18, and 25 inches, as in clearance heights under the bridge.
7.  I've settled on large section aluminum box tube (4 x 6 x .25) for the primary structure, since I was able to get a good value on it, and I like its sharp corners and good tolerances, and skate bearings riding on Cold Rolled Steel (CRS) for the ways.  Design must not preclude upgrading to linear bearings at a future date though.
8.  Since the X axis is so long, I'm going to use rack and pinion gearing for its actuation, but want to use ballscrews for the Y and Z axis.
9.  I've been very happy with Gecko drives over the years, and have 4x G201 stepper drivers already, so will use those with some large stepper motors to drive the thing.
10.  For starters, I will use my existing Porter Cable router head for the softer materials.  May make a nicer and heavier duty spindle for aluminum in the future or use one of the high speed liquid cooled things that have become popular.

My initial design is something like this:

The gantry itself becomes a bit of a problem, stiffness wise, when you go to such a large Z travel, so I have been doing studies to see how massive I really want it and have come up with something like this:

Since I am able to get a good value on 4x4x.25 aluminum box tube, I will be adding it liberally to the gantry to stiffen things up.

So anyway, this is my project, and I am going to attempt to track its progress here.  I hope someone finds it interesting and useful.

Ciao.