Hi Charlie,
OK, I've made a few assumptions for dimensions and compared different Z axis options for deflection. The deflections are
artificially small because the rest of the machine is grounded, but you can compare the numbers with each other to see the trends.
1. Shows the conventional layout (carraiges on Y axis plate, rails on Z plate) using 20mm aluminium plate at full extension. This gives 10um.
2. Shows the same conventional layout as 1 but at almost minimum Z extension. This gives 0.2um. This is 20x stiffer just by raising the job so that cutting is done with the Z axis near the home switch.
3. Shows the unconventional layout (carraiges on Z plate, rail on Y plate) using 20mm aluminium plate. This is drawn at an arbitrary extension because with this design all extensions have the same stiffness. This gives 10um. You can see that it is the same as 1. But if you need to do a deep cut, or hold a fine tolerance etc. then you cannot raise the Z to increase the stiffness. So, onto your question about recovery . . .
4. Shows the unconventional layout using 30mm aluminium. This has reduced the deflection to 3um. 3x better than 20mm aluminium, but still nowhere near the best conventional result (2).
5. Shows the unconventional layout but using 20mm steel for the Z plate. This has a deflection of 3.5um, so about the same as the 30mm aluminium plate. This is also nowhere near the best conventional result (2).
So, if all of your cutting is done near full extension, and your machine is stiff enough to cut well at that extension, then it does not matter if you choose conventional or unconventional (and both with 20mm aluminium plate) as they are the same.
But chances are you might not get the finish you want at full extension as you are cutting at quite a distance from the support bearing, and so with the conventional layout you can raise the job in Z and try again. You can stiffen the axis quite a bit by doing this. With the unconventional layout this makes NO difference (in terms of Z plate bending) so you are stuck with it. You can upgrade the Z plate from 20mm to 30mm aluminium and stiffen it a bit more, or go for 20mm steel (works out about the same), and gain an improvement. But you cannot get close to the stiffness improvement of the conventional layout raised close to the home switch.
I can't tell you which option will be stiff enough for you, but I can say that once you have made the machine if you run into problems and have gone the unconventional route it is difficult to make a significant improvement. Hope this helps.
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