Added steel channel to X-axis c-beam

[wiremonkey]

Ah yes, I did take measurements the preferred way. For each measurement, I simply plunged the router down onto the scale, the same way for all measurements taken. Sorry I wasn't clear about that. I will also take some deflection at the collet while applying lateral force in both the X and Y directions.
Yes, from what I'm learning, closed structure for beam is best. Presently I'm thinking about using a 2" x 5", 3/16" thick steel beam (8.15lbs per foot) with 1/2" steel end caps welded on, for mounting to the gantries. The way I see it, since the worst of the deflection is clearly from twisting in the middle of the X beam, I'll start there. I can upgrade the rest in stages, as money allows. The idea is that I can reuse the steel beam in whatever configuration I end up with. If I go thicker on the steel tube walls I think it gets too heavy for my design.
I may go with linear bearings, but I'm also considering v-track and v bearings. The the prices are $300 and $200 respectively, in the long run, not much of a difference.
Again, thank you for all of the feedback! I'll post a part 3 video soon in which I discuss some of things we're talking about here and consider options. On another note, I'm seriously considering leaving this CNC machine as is and saving my pennies for a mini mill for metal work and leaving the larger machine for wood and softer materials. The CNC, as is, is quite adequate for such purposes. Taig, I found them today, makes some incredible tiny mills capable of milling mild steel for short money, and you can order them CNC ready, just hook up your steppers and off you go. Have a look: http://www.taigtools.com/mmill.html
Pip, pip!