Ball bearing is very important for 3D Printer, which make x axis, y axis, z axis move smoothly. Normally ABEC-5 is good enough for 3D Printing. Oil as lubrication inside the bearing also protecting the bearing over spin.
U604ZZ Ball Bearing, this is a popular size of Skate bearing or any application
that uses this size 4mm x 13mm x 4mm. Bearing is made of Chrome Steel,
also bearings are Oil lubricated to be smooth.