I would like to make a build but with 100mm extra on either the X or Y to accommodate the BigBox 200mm by 300mm heated bed. I read in another thread where it was suggested not increasing more than 50mm because of deflection. Then I thought perhaps deflection could be minimized by using 12mm shafts instead of 10mm. If it was done in the Y axis it would not add weight to the moving parts. Also I see that there are a few other metals available from Misumi that may be less prone to deflection than the "bearing steel" suggested in the BOM such as carbon steel. If so, maybe I could stay with the 10mm shafting. I am not an engineer. Can someone tell me if I my thinking has legs? Obviously the Z-axis plate would need modification.
I wouldn't recommend going with thicker shafts as that significantly increases weight and complexity/modifications required for the x and y axis. For the Z axis, that may be something you want to consider. If you extend the x axis to be 300mm wide, you should be able to use the original z axis design (as it wouldn't increase the cantilever distance which is the main issue with larger beds drooping). You probably would still want to redesign the z axis a bit to be more supportive and possibly space the linear rails further apart.
Also, the most effective way to expand this would be to expand the y axis, as a smaller x axis will minimize deflection. However this would look a bit odd and would require a full redo of the z axis since the bed would be 300mm overhanging, so this is not likely the best option.
I modified Eclips3D to accommodate the BigBox heated bed, X 300, Y 200mm (so the cantilever remains the same) and Z 300mm. I moved the Z rails further apart to reduce the overhangs at the sides and made my own customised bed support using 2020 extrusions and a lighter aluminium plate to support the 4 height adjustment supports of the bigbox. I'm starting to use it for architectural models hence the larger build requirement. I've just printed off the Alexander mask www.thingiverse.com/thing:32338, I was really pleased with the result.
Yeah I know it would look a little odd to expand the Y and keep the X at the original 200 but I think the "Form follows function" principal applies here. A friend of mine found a commercially made core-xy that is made just like that. Also I am thinking now of using the profile rails like the Eclips3d2. That might help solve some of the problems too. Archit, I like your solution to the Z support.