Summary

Design
A very rigid frame employing 20x20mm & 20x40mm extruded aluminium. The design was inspired by the CR-10's which had a structurally sound frame resulting in a very stable printer (at relatively low cost).

By combining 20mm and 40mm extruded aluminium, you can reduces the number of corner supports required as the overlapping of the 20/40 extrusions can simply be bolted together. A bit more work tapping the ends but the resulting frame is very stable...

Use Existing Materials
The design takes advantage of 'open-build' materials (basically everything required for the build in one place. The pricing was reasonably but I did shop around and managed to save on some items...

V-Wheels
I also favor v-wheels over roller bearings for both quieter operation an better precision, after all, you can adjust v-wheels assemblies to eliminate misalignment which is not always possible with bearings construction.

Basically using v-wheels increase the build price quite a bit over roller bearings/rods but the other alternative such as Linear Motion Guide Rail were far more costly (at least at the time of this design).

Linear Motion Guide Rail
Recently I have searched and found linear rails considerably less expensive than I was lead to believe. I can't attest to their quality but if they are acceptable, this would make using them far more attractive. In fact, it might even reduce the cost and the complexity of the build...

BOM
I will include a spreadsheet (open office) detailing all materials and prices with links to where I bought them...

Current Build ~ 90 Complete
Mixed 20x20, 20x40 extrusion and OpenBuild Gantry plates with V-Wheels...
450mm Bed (450x450x8mm Cast Plate)...
Z-Axis motors on outside but will fit inside frame...
H-Bot Arrangement...
Print volume 450x450x450mm

Updates
Bed arrived and it's huge and much heavier than I had anticipated. Might have to rethink the Z axis design or fit more or larger Z motors but in the mean time I completed the frame and it's very solid, structurally very stable...

The NEMA 17 motors look tiny but should still be fine for the X and Y axis but what about the Z axis... just as well I bought those NEMA 23's...

The project has be sidelined as I'm busy with other projects but I will get back to it...
Mike