Sorry if I misread but some people do literally press separated AC's back together.
The Tormach spindle is a common spindle design used on low cost spindles. The DT bearing doubles your radial and axial capacity on the low end and is "backed up" with a lower capacity upper - all you have is belt lot at top and the opposite axial loading to the lowers.
I would not be surprised if the OEM pressed everything together with special tooling. This is all but impossible to duplicate in the field. He'res how I'd tune up the spindle if it were mine.
1 Carefully scotch brite the shaft and housing bores to get rid of "rash".THey should be clean and smooth when done.
2 tune up spindle until you get a transition fit of upper shaft journal so that you have very smooth and "perfect" slip fit of upper bearing to shaft. Bearing should not fall on, it should just slip on when carefully aligned. 600grit and red scotchbrite should do it..
2 Carefully scotch brite the shaft and housing bores to get rid of "rash". THe surfaces should be clean and smooth when done.
3 SHrink fit upper and lower bearings into place. Press to seat on OUTER RINGS ONLY is OK with good bushing/sleeve once the thing cools and freezes the bearing.
4 At this point, you should let the thing cool and stabilize a bit so the AC's dont fall out. BTW the 7000 series bearings are ALL A/C"s.
ONce the spindle hsg has stabilized, put it back in oven to fit the spindle shaft. Freeze the spindle overnight. Option, pack it in dry ice. When housing is near 220 f, and spindle is COLD froze, IT should drop cleanly into the lower bearing and PLUMET into the upper - which you already modified into a transition fit.
Setting preload: WIth all the bearings pressed at ALL postions, you have assuredly created a nightmare in assembling things without damaging one or more bearings. Besides, you end up with a indeterminate situation as to which bearings are loading and sharing loads.
BTW, DT bearings may or may not carry load equally due to tolerance stack ups. DU's are "matched to 0.0001" or better with respect to axial protrusion so load share is achieved.
What you've achieve by above is the capability to equally load share within the capability of the bearing set. ANd, you can accurately and properly set preload
Heres how. LIghtly oil the bearings. Make yourself a torque wrench out of a flat wrench and a fish scale. Call the oem to see if they have a recommended preload torque of the spindle when tight.
IT would have been a good idea to have recorded this before disassembly but improperly installed bearings precluded this from happening.
Anyway, slowly and carefully tighten the nut while checking rotating torque as you go. WHen you get to the prerecorded value or the OEM spec, stop. Don't go too far as too much STATIC preload can damage balls or cause over heat. IT is rotating torque you check NOT breakaway which varies all over hell with rolling element bearings
Use an industrial syringe to load inner and outer bearing with a light fill of grease- SNdok C is fine and affordable - put a small dab in front and behind each ball on the inner and outer raceway.
Option: you could probably slip fit both upper and lower bearings to spindle shaft. I did this to a 8000 rpm capable dyno shaft and it worked just fine. Made it much easier to rebuild than by having to press the hell out of everything.
If you do the above, make sure you "time" the bearings so you can get the grease into the "inner" piece.
DId the above to a "hobby spindle" of my own, Found some ABEC7 6206's and fitted them to a chinese drill press. Smoothest, quietest drill press spindle I ever heard. ABEC 7's in the motor made it nearly dead quite when running.
Heavy preload on spindle will make for higher stiffness and less spindle/bearing defelction under milling leads - probably should take more time to warm one up, too.
Lower preload makes for higher RPM potential and less heat build up in general. WIth reasonable and proper preload, a spindle operation temp of 180-200 would be considered normal. With any preload, you should warm up spindle at LOW RPM for several min prior to use.
THats how I"d redo a Tormach spindle if it were mine.