Okay, I was going to try to line out the answers to a lot of the above questions but I started getting bogged down trying to organize it all. So I will boil it down to how to scale the printer.
I designed mine for a 200mm build diameter. The height is a separate issue so I will focus just on the xy scaling for now. My shoulders have a 37.5mm offset. My arms are 187.5-200mm long. My hub offset is 50mm. I list all three because you need to scale their combined length. You can scale all of them equally or just make sure you get the right combined length. Your screws are placed 1.5 build diameters apart so 300mm in my case.
Example: If you want want a 400mm build diameter, then you can just double all of the lengths above.
When you scale I would also scale your bolts, bearings, and cross section of members. (Bolts and bearings probably don't need to be scaled as much but if you go big enough you should really consider up sizing.) I use 25mm x 25mm cross sections.
Example: A 2X machine should have 50mm x 50mm cross sections.
Now for the sad part. You loose a lot of height when you arms stick down. My design wasted a significant portion of the screw. You can mount the stepper up higher and have a smooth shaft extender to maximize the usage of your screw but I don't do that in favor of simplicity. I have 24" screws and only get 10" of z travel. The 14" that I lose will scale roughly with the build diameter. So a 16" build diameter will have me sacrifice about 28" of the screw. (These are very rough ways of estimating a scale up. Drawing out your plans is the only sure way.)
And of course you need to pick the right size for your screw. Quite a few posts back I discuss the appropriate size of the screws.
@A2: Why do you want to use 4 bearings per joint? I only use 2. Do the extra 2 gain you something?
I designed mine for a 200mm build diameter. The height is a separate issue so I will focus just on the xy scaling for now. My shoulders have a 37.5mm offset. My arms are 187.5-200mm long. My hub offset is 50mm. I list all three because you need to scale their combined length. You can scale all of them equally or just make sure you get the right combined length. Your screws are placed 1.5 build diameters apart so 300mm in my case.
Example: If you want want a 400mm build diameter, then you can just double all of the lengths above.
When you scale I would also scale your bolts, bearings, and cross section of members. (Bolts and bearings probably don't need to be scaled as much but if you go big enough you should really consider up sizing.) I use 25mm x 25mm cross sections.
Example: A 2X machine should have 50mm x 50mm cross sections.
Now for the sad part. You loose a lot of height when you arms stick down. My design wasted a significant portion of the screw. You can mount the stepper up higher and have a smooth shaft extender to maximize the usage of your screw but I don't do that in favor of simplicity. I have 24" screws and only get 10" of z travel. The 14" that I lose will scale roughly with the build diameter. So a 16" build diameter will have me sacrifice about 28" of the screw. (These are very rough ways of estimating a scale up. Drawing out your plans is the only sure way.)
And of course you need to pick the right size for your screw. Quite a few posts back I discuss the appropriate size of the screws.
@A2: Why do you want to use 4 bearings per joint? I only use 2. Do the extra 2 gain you something?