Difficulty getting good precision I have a reprap printer with 0.3mm nozzle. It prints quite well, I am really surprised with quality of all the surfaces and the general precision of the parts printed. BUT I HAVE a problem: when making (for example) a 10mm x 10mm x 10mm cube with a 2.8mm diameter hole from top to bottom (to fit a screw) after I print it gets a size of 3mm diameter.

I know this is related to extrusion width but cant the slicer software (I am using s3d) know that it is using a specific extrusion width and compensate for that in order to get the diamter right?

OBS: this printer is supposed to get 0.05mm precision.
It's a generally accepted fact that FDM/FFF printers will have deviations when it comes to holes and perimeters. Typically, holes print smaller than designed and external surfaces end up larger than designed.

In your case, it seems to be the opposite: the hole is too big. It could be that you're just printing too big overall. You might want to make sure that your printer is printing the 10mm X/Y dimension in you example correctly. If it's too big, part of it may be just the typical oversized perimeter, but some of it might be due to incorrect firmware X/Y 'steps/mm' or extrusion like you mentioned.

See also: "Are you printing undersized holes?"