Here are few pictures of what's already done. (this week) and how I made spindle housing.
I made few changes to my First spindle shaft design, and made it simpler, removed 7208 double bearings and placed single 6208 bearing and removed second lock nut M40x1.5, I don't know Why I placed two lock nuts, so now I will have only one and adjust 2 angular bearing preload with M50x1.5 nut. I decided to not make that High inertia shaft, because I will have to remake spindle housing to accept bigger 6309 bearing and that is a lot of work, I don't want to make. Now plan is to make this today's designed shaft it has low inertia 5630 kg/mm2 and see how it will cut steel with indexable cutter, and if it will make lot of vibrations, then make try and make inertia rings and double or triple inertia of shaft.
I noticed grammar mistake in topics description can moderator correct word "tapper" to "taper" .
I included picture of current milling machine Shaft, I disassembled the spindle head and lower roller bearing has this nasty tapper in ID, my first Idea was to use this original 40 taper shaft, and change bearings to angular contact, and make new spindle housing for direct belt drive, but that tapered bearing ID made it impossible. also I included picture of spindle housing and you can see bearing pocket is with holes, this because I ****ed up pocket by 0.15mm made it larger, and this is FIX I made with stick welder, now pocket is ok. it took 3 days to make that spindle housing, and I maxed out my 4kw cnc lathe capabilities, even grinded indexable insert TNMG corner radius from 0.8 to sharp to reduce chatter, and cut bearing pockets.