Made another metal run yesterday, now I have the fundamental material for the gantry on hand.
Here's a cross section shot of the X axis gantry beam, it's a touch more involved than the SketchUp drawing I drafted in October. One point of concern I have here with the current loose mock-up is the linear bearing rail situated over the blue faced material will require a countersink channel on the inner angled face of the C channel wall to accept mounting hardware (unless I opt for threading the holes in the C channel, which is a design tactic I'm avoiding at the moment).
My main objective is to confirm the clearances I came up with on paper. The dimension of that nut pushed me into an adaptive frame of mind.
The bearings / Z axis car has a clearance of 0.2865 on paper, which adheres to my desire to keep roughly 0.25 clearance between moving parts. However the nut, suspended inside the channel as pictured, has 0.0335 inches of clearance. Initially my thought was to modify the nut to reduce the chance of interference, but there's also the matter of the bracket that ties the nut to the Z axis car. Not to mention that the nut itself is a point of wear, and will eventually need replacement.
I want to keep the spatial displacement of the axis' as small as possible, as off axis distance equates to magnification of slop. My current plan is to "elevate" the screw in the channel to position the nut so the collar rests on the inside face of the Z car, essentially putting 0.3730 of the mounting flange into the baseplate of the Z car. Since placing the flange on the outside edge of the car will reduce usable travel by roughly 0.3785 (widening the car), I plan to cut a slot in the baseplate of the car to accept the nut. This will remove one screw, the remaining three will tie into an L bracket milled into a horseshoe shape on one side.
By repositioning the screw the motor mount hardware will clear the 3 inch C channel, which will come in very handy should I need to R&R the NEMA23 motor. Not pictured here is the 0.5" thick "wall" that will cap off the X axis channels, provide the plane to mount the motor & bearing at far end of screw and tie into the legs that are not pictured, but are rendered in the drawing below.
So, visualize a 6x14" plate of 0.5" thick aluminum bracketing the ends (and obstructing the view of the cross section) and you've got it. I may have to remove some material from the car side of the upper branch of the 8 inch leg channel, but that will wait until I've got the gantry assembled and am mocking up the car.