on what depends layer height

Hello Im beginner repraper ,
Im planning to build a 3d printer.
I want to know on what layer height depends, on: nozzle ,Threaded rod,hot end,motor or etc
I want to achieve 20 micron layer height

thanks in advance

All of the above.

At those heights any print will take many many hours or days or even a week or two. Your hotend and extruder need to be of really high quality and calibrated to perfection (forget cheaper clones) speeds needs to be in the 20 to 30mm per sec, your built plate needs to be pefectly flat and perfectly leveled.

You need to go easy first, learn all the aspects of a print with 0.3, 0.2 and 0.1 in that order. Once you have done a dozen perfect prints at 0.1 then go crazy and lowered more.

50 microns its already insane, can't imagine 20.

Why in gods name would you want to print 20 microns!
Even at .1mm prints take FOREVER and its much harder to print.

All I can say is good luck...

A regular ziplock sandwich baggie plastic film is about 50 microns thick. Even if you got your bed flat enough, and the obscene print time wasn't an issue, I would have serious questions whether you can physically extrude ABS, PLA, or any other common material in that thin of a layer consistently.

If you want low layer height for looks or for dimensional accuracy, that isn't the best way to do either. I'd stick with 0.1 if I were you.