Hi William,
Found this today good read. Your 70N/um is way up there but as usual aim high....Peter

https://books.google.com.au/books?id...N%2Fum&f=false

you do need to include bearings : 1) you have to be able to assemble the assemblies 2) bearings take up real estate and their packaging size may drive other dims c) you need to think about tramming the spindle. How are you going to do that? These things get sorted at assemble level. Parts are always very stiff, assemblies are 30% to 50% efficient in stiffness at least.