Hi Potatomill

Some good work there using analysis to explore the design. Some comments -

Although aluminium is 3 times lighter than steel it is also 3 times less stiff. Using steel in the base assembly, where the extra weight is of no consequence and is actually very beneficial, might negate the need to add the extra triangular bracing. Especially if you can go for high wall thickness.

Don't forget that the stiffness you are aiming for (10-20 N/um) is for the WHOLE machine, from tool tip to bedplate. If you design each sub-assembly to be 10-20 N/um you will get somewhat less stiffness in total because they will behave like a set of springs in series. In other words each sub-assembly needs to be significantly stiffer than 10-20 N/um so that when added together the total stiffness is in the 10-20 N/um range.

The Z axis is always a challenge with this style of machine. Try 20-30mm thick aluminium plate and see what that does. I suspect it will not give what you want especially at 450mm long. So another option is to use a piece of RHS (steel) with say width similar to the current plate width, depth 80-100mm, and length 450mm. Then house the spindle inside with cut outs or access holes to the spindle as required.

I think the limiting factor of the design, if you really are trying to get 10-20 N/um will be the torsional stiffness of the gantry as seen by the tool tip (i.e. Z axis and gantry modelled together with load applied at tool tip).

Good luck with it all . . .