Reality in my brainstorming?

One of my current machines has exactly this XYYZZ configuration ( "Hawk" Heavy-duty dual-Z pseudo C-Beam ). I only have space for one large machine, so it has to cope with many different machining requirements, and so I took a 'punt' on the more complicated gantry design over using movable beds. I mill projects ranging from coffee tables made from 10" slabs of tree, through to wooden buttons, aluminium and brass plates, and milling circuit boards. I also have a 4th rotary axis that can quickly drop in and out, and also a secondary router spindle that I set horizontally to do milling of tenons on the end of long stock. My machine has a dynamic working envelope of 1320mm in X, 820mm in Y and 300mm in Z, and I have another 80mm of adjustment on Z by moving the spindle in the clamp (a 3kW water cooled spindle). The particular build concept I used could go bigger in Y and Z, but going more than 1500mm (1320mm working) in any one axis would get tricky, so a 1500x1500 nomimal X-Y run is very possible, with a 1320mmx1320mm actual milling footprint (and going more than about 450mm true travel in Z is likey to compromise rigidity).

I tried a machine design with a high-Z and a large removable bed previously so that I could gain extra clearance, but found that I spent much time resurfacing the spoil board to get it back into alignment with the gantry, as no matter how rigid I thought the bed was, it never quite went back in the same place in all 4 corners. The main issue I had though was when milling thick aluminium and brass plates, the Z would be close to its most extreme extent down onto the bed, and therefore the worst-case scenario for flexing and losing precision. For milling the tops of slabs of wood flat, or cutting bow-tie patterns into the slab, the Z was very close to the top, and that was fine. Going to the XYYZZ design gives the most rigidity when down at the minimum height (such as when milling ali or steel), with the least rigid configuration when the gantry is at the very top (when milling the tops of slabs or v-carving lettering).

For the 1500mm x 1500mm footprint, the build structure I used is plenty rigid enough, but going with any thinner beams would quickly run into rigidity issues with the heavy spindle I have, but if a much lighter spindle was used, then the X could probably be more light-weight. The structures for the Z drives on the sides though do need to be beefy to prevent flex. I have closed-loop Nema 34's driving the large machine, but have a smaller machine design I also tried that used Nema23's with no problem. I have found that the design I came up with is plenty rigid enough to mill metal fine with both the gantry either at the bottom, or at the top. That means I can do a brass or copper inlay into the top of a large slab, and then skim the brass off flat to the wood surface afterwards with no problems.

Now I have the machine, I cannot forsee ever going back to having the Z in a conventional configuration as having 300mm of true Z travel is a luxury I have quickly become used to, especially as it allows the option of the secondary horizontal spindle. The structure is far, far more complicated (and expensive) though than a conventional configuration, and for the simplest setup and operation, having a controller with 5 true axis outputs, and 6 if you want to have a rotary axis you can use as an indexer is a nice-to-have (I have a 7-output controller just-in-case). Having a 6-axis control box makes tramming and set-up a breeze, but if you ganged the two-y drives of a single-axis output, and similar for the two-z drives, a 3 (or 4 if you want independent A as well) output controller could definately be made to work, but tramming would be more tricky, but not much more of an effort than with a conventional XYZ or XYYZ type of configuration. For the X axis, there is only the weight of the spindle on it, so it does not take much to drive it. The two Z drives are lifting the X gantry and the spindle, and the Y drives are potentially moving a fair mass of metal around. The question regarding how big the motors need to be and therefore whether large external motor drives are needed pretty much comes down to how fast you want the machine to accelerate and what the top speed will be, with the requirements of moving the Y and Z being most dominant.

As Rick says, taking a machine like the Openbuilds 1515 with the double C-beam on X, but then spending much time looking at how a reconfigurable multi-height bed could be made, would certainly be far cheaper and simpler to build! Although a decent variable-height bed is not as rapidly reconfigurable, it could be very practical, especially if you also have a 'raised bed' option too if you want to mill metals. If you only remove a small section of the bed to put a rotary axis in, then that should be quite straight forward to re-align. The issue I had was that a 1500mm wide removable bed is not small and if thin, flexes well, so either needs to be rigid itself (such as a torsion box design), or needs a very rigid and flat structure to sit on at each of the different bed-height options.