Quite the timely thread. I wish everyone good fortune in their building.
I am in the design/parts collecting phase of a folding 4x5 project. My design as well includes dual focusing racks, and on paper I am quite pleased with how things are working out. (Many, many sheets of paper) I am seeing some different approaches to the rack and pinion design, and I wonder what others have done to achieve smooth and consistent focusing.
I may have dealt the whole idea a death-blow today by deciding to make my own ground glass and back. I have built pinhole cameras before, but nothing where focus is critical. I have measured a couple of film holders and seem to be getting just under 5mm depth between the rail and film. (4.87mm, to be exact) I am assuming that would be the depth of the ground glass as well.
BTW, I am doing my rough design by hand and then doing a more detailed 3D rendering using FreeCAD. This gives me a much better idea of how things will fit together.
I am hopeful, as well to make my own bellows, but I am not above blowing a bit of my budget on commercially made bellows if it will help finish the project.