.02 or 20 micron layer heights are rarely used or practical, .1 or 100micron is generally all you will ever need and most printers will do that. Know also that 20 micron prints take up to 10 times longer to print than 200 micron. That 1 hour print is now 10 hours. That 3 hour print is now 30 hours. etc...

As for hole sizes it is best to print slightly oversized, like 15.1mm for a 15mm hole and ream with a straight flute reamer if needed. Any sort of drive shaft should be supported by bearings, otherwise the friction will melt the plastic.