Critique required on y-axis design.

[Spedley]

I've put a lot of thought into the force and travel requirements of a CNC machine and what I have come up with differs from the usual design. My machine is mostly based on milling softwood at very high speed with a clean planed finish. Cost is very much an issue but quality within tolerance is more important.

Diagram is of cross section of Y-axis including X-Axis rail. Requirements are X-axis 1000mm, Y-axis 1600mm, Z-axis 20-50mm (Yes, Y-Axis is larger than X-Axis!)

Quick diagram key:
Dark grey: X axis rail, supported above the bed and approximately level with cutting surface.
Green: Carriages
Yellow: Aluminium plate - the width of the carriage to support the Blue C-sections.
Blue: Aluminium C-section providing the basic structure of the Y-axis. Dotted lines represent possible I-Beam shape, however I have not been able to locate suitable parts.
Light Grey: Y-axis rails, four times cost but they are a structural item and of minimum size.
Orange: Aluminium plate to support Z-Axis. Structurally screwed to Y-Axis carriages.
Central Dotted region: Z-Axis and spindle attached to orange plate
Holes in Orange Plate: Proposed ball screws for Y-axis.

Goals
The idea is that the single carriage on the X-axis maximises travel.
The blue C-sections (approx 6" high) act as a single very large beam when combined with the rigid orange plate (which moves on the Y-axis with the spindle)
The usual 30mm travel of the Z-axis creates very little torque although it can travel more depending upon design.
The Y-axis rails add structural support to the blue C-Section.
The blue C-sections will overhang beyond the X-Axis' allowing for stepper control beyond axis and full travel.
The ends of the blue C-Section will be structurally capped with a larger 'Orange Plate'

Notes
I'm not certain if the blue C-Sections would benefit from being I-Beams as shown by the doted lines. There is little effected on the design, however I cannot find any I-beams made for non architectural purposes (which are massively over engineered for a CNC machine).
The orange plate should prevent almost all torque twisting the blue C-sections.
The Y-axis supported rails should add significant strength to the design.
The X-axis carriage should be fine with a short Z-axis but may need to be doubled with further travel.
The blue C-sections should be of minimal thickness, the strength coming from the light-grey rails and the Orange Plate.

Sorry for the bitmap drawing but I have not yet found a CAD program which I find intuitive and creative to use.

I very much require input and criticism about this design before I progress to the next stage - anythng will help because I often overlook the obvious! :)