I got some more fea done tonight. The changes I made were to make a lot of my x and y axes solid sections instead of tubes. This helped a lot - the mounting surfaces for the x and y rails no longer deflect as much and the table doesn't end up tilted nearly as much. I also thickened up some of the z axis and added some more bracing, but this did not help much. In fact my stiffness under a z load went down, although my stiffness for an x load went from 153k to 200k lbf/in just looking at the z axis deflection. One of these days I will add the programming to automatically report the displacement of both the cutter and the vise and add them together. I also looked at stress / strain plots and will try adding some material in different locations based on that. The screen shots below are x loading for 1-3, displacement for 1-2, strain for 3, z loading for 4-5, displacement for 4, stress for 5.
I also realized I need to start thinking about a base for this thing. Currently I have the bottom of the x axis and the frame constrained vertically on some pads. But that is not really how it will function, so there will probably be a lot more deflection between the z axis and the x/y axis than I currently see. To stay within my height limit, this might require me to change things around a bit (again...) as it can only get about 6" taller than it is now and even that is pushing it - doesn't leave much room for a crane above it to do assembly. Thinking about this some more, I might be forced to move away from a gantry design if I want to keep my z travel. Then I'm getting into a more typical mill (which is fine if I can make it hit all my goals).
CNC mill build thread: http://www.cnczone.com/forums/vertical_mill_lathe_project_log/110305-gantry_mill.html