You might find this little tool useful:
This is a block with a series of vertical and horizontal holes from ⌀1 mm to ⌀3.6 mm (nominal) in 0.1-mm increments. I print this block with any new printer / new material at the layer thickness and speed I use most often and keep the blocks as references.
Then, when I want to print a part with a hole in it that will end up printed at the final diameter without any rework (or very close, but usually it ends up exactly right) I use the gauge block I printed with the printer and the material I intend to print the part out of to find out which nominal diameter hole I need to use in the model for the hole.
For instance, the block in the photo was printed on a Prusa Mk4 with PLA at 0.2 mm layer thickness. If I want a ⌀0.8-mm vertical hole with no interference in my final part in PLA out of that printer, I’ll need to model a ⌀1.2-mm hole. I know that because the shank of a 0.8-mm drillbit will slide freely in the vertical hole marked ⌀1.2 in the block.
Or if I want a hole that I can screw an M3 screw into without having to run a tap through it, the block tells me I would have to use a ⌀3.2-mm hole to fit a ⌀3-mm pin in freely, and I back off 0.4 mm to get a hole in which the screw thread will engage well but reasonably lightly, or 0.5 mm for a tighter engagement (but with a bit more risk with horizontal holes near an edge, because the layers might separate) so I know I’ll have to model a ⌀2.8 or ⌀2.7 hole in the model.
I use those gauge blocks all the time. They save me a lot of time on parts that I print often because I never have to ream or thread the holes: they come out the printer just right and ready to use.
And for anyone who winds up struggling with this when trying to gauge their own printer, make sure this option is enabled:
This is in Prusa/Slic3r and its derivatives. If this is disabled, the final outer wall perimeter can wind up being pushed out by some fraction of the width of the wall behind it, which will have the net effect of shrinking vertical holes in your model (and other critical clearances in the X/Y dimensions) by an unpredictable amount.
This won’t solve the fact that the filament is going to shrink a bit when cooling. Although you should already be scaling your models to account for this
Your slicer should also be able to compensate for this already.
PLA only shrinks about 0.3% which is negligible unless you are designing with super tight clearances. A 6mm hole, for instance, will be out 0.018mm which is probably scraping against the XY resolution limits of most consumer 3D printers anyhow.
Other materials can definitely shrink more. ABS is harder to manage than PLA, but for instance Nylon/PA’s shrink rate is comparatively immense – around 2%. The various engineering polymers that are filled with something like carbon or glass fibers actually tend to shrink less than their raw counterparts.
I wish I could print nylon, but the old oven I’m converting into a heated chamber is buried in the back of my garage.
