micro ball bearing

To decrease the outer diameter from 34mm to 20mm, I increased the screw size from M3x6 to M3x8 to allow the screw head and nut more clearance to the raceway. The outer wall is 2 lines thick and the inner wall is 3 lines thick, because the screw and nut can easily crush the inner raceway when over tightened on a 2 line wall. I chose an 8mm height to keep the thickness above and below the raceway the same as the outer wall.

A 6.2mm diameter raceway makes a very smooth spinning bearing. If you are more interested in a bearing with tighter tolerances than free spinning, I recommend reducing the raceway diameter to 6.1mm. There is room for one more ball in this bearing, but it spins faster with 5 balls than 6 balls.

For the smoothest bearing raceway, each of these pieces should be printed one at a time with the raceway facing up to avoid support material on the raceway. I also highly recommend using random seam position to avoid a seam along the raceway, but it my be contributing to my problem with stringing.

print settings:
printer = Prusa i3 MK3
nozzle = 0.4 mm

0.10mm DETAIL
Generic PLA
supports = everywhere
infill = 100%

layers and perimeters
layer height = 0.10 mm
vertical shell perimeters = 2
seam position = random
staggered inner seams = true

infill
fill density = 100%
fill pattern = concentric
top fill pattern = concentric
bottom fill pattern = concentric

support material
generate support material = true

filament
bed temp first layer = 65C
bed temp other layers = 65C

variable layer height
adaptive quality/speed = 0 (max quality)
  this varies the layer height from 0.250 to 0.070
smooth radius = (1-10) >> not used, no real benefit
 

I have included the stl files here, but you can also get a copy of the onshape model on my public onshape space. Onshape is free if your models are public!!!

https://cad.onshape.com/documents/97cf22dc823389064c09a4dd/w/efee174efa945cd21c240782/e/424a7108f4f3918c2034512f?renderMode=0&uiState=6817b03f28c93c651510363d