Hey, no problem on the numbers. I'm trying to give various people who read these responses some insight into the thinking as some have indicated they'd like to understand more about how to go about the calculations. IMHO better to have them there and for people to get their heads around them as and when than not to have them.

Intuitively they are too close IMHO to give much torsional stiffness, tho its the plate rather than the box section that'll distort. For the sort of cutting forces you'll encounter the box section will twist a miniscule fraction of a degree but the ali plate will bend about .2mm so at the tool tip that'll be considerably more...

I'd want to consider double the spacing at least...

Not sure I understand whats wrong with your current setup?