Yes, it's a good strategy to lift the Z to increase stiffness.
Also for everyone following this thread, I looked into some alternate gantry designs. The search for 'better' largely depends on priorities--cost, labor, weight, simplicity, stiffness, preferred material and processes, etc. Weight is an issue because larger motors and drives are needed to drive a heavy gantry (or, it will be slower). So, here are 20 options; the most promising ones (IMO) are in bold:
Column J calculates the ratio of Stiffness divided by Weight--a measure of how efficient the cross-section is at being stiff. T-slot extrusions are not very efficient for a few reasons. Similarly, Column L is Stiffness/Cost -- 'bang for buck'.
Designs 3 and 4 are the twin 3060 ideas. Here is #4:
Design 5 adds another C-channel to make it a mostly closed section. It's stiff but quite heavy. Here's the cross-section:
Designs 6 and 7 use two 40x160 extrusions top and bottom for rail mounting, with alum sheet front and back to make a box tube, plus some channel bulkheads to stabilize the cross-section. Among the T-slot designs, it has the best ratios, but I don't know if the assembly would produce flat rail mounting surfaces. Here's the FEA and channel bulkheads:
Designs 8 and 9 are just solid barstock.
Designs 10-16 are regular alum tube, often with a diagonal sheet inside to stabilize the cross-section. Cost is much lower than T-slot extrusion, but these will generally have more warp in them, so the rail mounting surfaces would need to be machined or epoxied flat. (As I understand it, 8020/Misumi extrusion is generally straight enough to mount profile rail w/o leveling, or at least many people do that.) Of these:
- Design 12 is attractive because the thick walls don't need an internal diagonal sheet, yet it still performs well.
- Designs 15 and 16 give the highest stiffness and best stiffness/weight. As usual, big tubes rock! But these use internal diagonals or bulkheads, which I've done and can say is a fair bit of work. We used Design 15 on our 4x8 router -- our 8x8 tube came with about 1/8" of twist along its length, so leveling epoxy would likely be needed. This approach is probably overkill for a wood router, but would be a good choice for cutting aluminum.
I also tried some Baltic Birch Ply designs, Designs 17-20 . These perform very well in all categories: light, stiff, low $, reasonable labor. The cross-sections are similar to Design 15 with thicker walls. Well-sealed from moisture, they should be stable. A 12" jointer could flatten the front surface, if needed. The rails would need a stiffer surface (metal) to mount to, so something like a 1/4" alum bar or plate on the front would probably also be needed.
In general, you can see that to get the highest stiffness and the best stiffness/weight, aim for:
1) A big cross-section
2) As much material as possible to the far outside of the cross-section
3) Bulkheads or diagonals to stabilize the cross-section from 'collapse'. In wood, these are easy to do. In metal, they are doable but not easy. But, if the wall thickness is large enough, the cross-section is stable without bulkheads or diagonals. T-slot extrusion tends to have enough internal material to be pretty stable, although all that internal material makes it pretty inefficient for stiffness.
So, as with most design problems, there's no "right answer", but there are some interesting and good choices depending on priorities, budget, and capabilities.