Get Precision Square and Round Holes Every Time! GUIDE & Calibration Shape (Super Fast Print for Rapid Iterations)

WHY?!
I do a fair bit of massively complex functional prototyping.  I used to have to draw a 3.10mm hole to fit a 2.90mm screw if I wanted my 14mm square hole to be 14.00mm.  Now, I draw 2.90mm and get 2.85-2.95mm, and get my +/- 0.05mm 14mm hole too.  Why does this matter? Because as time goes on, printers will get better, so your designed in tolerances will render your model useless on your future printer.  If you want to send off your design to get machined, it'll render it useless there too.  Design it the way it is to be made, and tune your printer to hit those targets!  The hardware can do it!  All you need to do is tune your slicer settings.
Often, people dial in for square holes, and get mis-sized round holes, and vice versa. Save yourself a lot of headache and get both, at the same time, every time.

What you need:
The filament you use.
Calipers.  I am using $9 harbor freight calipers and as long as you don't drop them too hard, they do the job just fine.

Your printer.
I used Cura because it allows for more options in setting extrusion rates.  You can follow this with Prusaslicer, Superslicer, etc, but you will not be able to adjust your extrusion rates to the same level of granularity.

What it is for:
A 40*20*5 mm block with a 15*15 mm square hole, and a 15 mm round hole.  Use this print with some calipers to dial in extrusion rates.

Directions:

Update:
Skip down and see the bit about  using a 20x20x20 calibration cube to dial in line width.  You'll probably want to get that right first.  Then continue on.


If you have elephants foot, and your print is under 5mm tall, adjust your Z offset.  Once you are getting within your desired tolerances of 5.00mm tall, you can correct any remaining elephants foot by adjusting Initial Layer Horizontal Expansion rates.

If you see less than or more than 40*20 external dimensions, and you have modified your extruder stepper motor or stepper driver, then adjust your extruder current (this is a hardware fix, so only do this if you've replaced your stepper motors or drivers or modified their reference voltage), and then move on to adjusting your Horizontal Expansion rates.

Then once you have that, your square hole should be getting close, so now its time to adjust Hole Horizontal Expansion, checking both the round and square hole.

If you are on a Prusa, you should be able to get within 50 micron (0.05mm), like I can with my Anycubic i3 Mega (both when it was stock, and after modifications).  If you are on an Ender3 type printer, based on my knowledge, those aluminum extrusions seem like they add some wiggle and you may only be able to get within 100 microns (0.10mm) which is still great.  Please let me know in the comments what kinds of results you get and what printer you are on, as well as what modifications you have done.


Another tool I use along side this method:

I use the 20x20x20 calibration cube in Vase mode to dial in my line width to make sure I'm actually getting the layer width I expect.
https://www.printables.com/model/157798-20-mm-cube

Another option:
https://www.printables.com/model/81314-flow-calibration-cube

That is not the model I use, but I don't know which one I've been using, and a 20x20x20 cube is a 20x20x20 cube.

This is actually the step I'd start with.

Print Settings:
I printed these with one bottom layer, one wall, and two top layers.  I don't think you need to expend any more filament to it than that, at least at the temps PLA prints at.  My top layer is a bit imperfect but it gives a bit more information on the state of bridging.  I also identified some downsides to ironing by attempting that on only a two-layer thick top.

I printed on a 0.4mm nozzle
0.2mm layer height
0.4mm layer width (I always match layer width to nozzle size, you may want to do the same)

You may want to try this with each filament, and various temperatures if you have not yet identified differences in how each print, and what temperatures they prefer.
I tend to change the temperature depending on if I want a purely structural part (higher temps for better layer adhesion), or a more matte surface finish (lower temps).  If I'm printing faster, higher temps as well (someday I need to dry a CHT nozzle or a volcano, but all in time).

Notes & Updates:
NOTE: The terminology I'm using here is from Cura Slicer.  I've used and liked Prusaslicer, but I find I am better able to dial in my non-Prusa printer better on Cura, because I don't benefit from the effort Prusa puts in to making excellent default profiles for their printers (or so I'm told; I've yet to test that first-hand).  If you are using Prusaslicer or Superslicer, you won't have all of these options, but you can still follow along (mostly) if you switch to Expert Mode, and swap some terminology (which you'll have to look up because I don't remember them, sorry, but the documentation on Prusaslicer is great so you will find what you need).

NOTE 2: I've only been at this about 3 months.  I don't know everything and may even have some things completely wrong.  Feel free to yell at me in the comments (I won't take offense to constructive criticism).  If I can test your argument and find it valid, I can then correct this information to prevent passing along bad information any longer than necessary.  I don't have everything sorted.  I've not yet messed with flow rates on bridging, I just crank the fan and try to cool it as fast as possible (as of time of writing).  I do get great layer adhesion and consistent dimensional prints for functional parts once I have the printer dialed in (but I'm re-doing all my work at the moment, hence why I created this model), since I was unhappy with my extruder, and unhappy with the difficulties I had with flexible filament and long retractions causing me trouble, along with the massive headache of a big box around the hotend making working on it a real pain, so I swapped out the whole setup for a direct drive unit.  I'm hoping documenting this here can help someone else, but also give myself a point to reference back to down the road when I've been happily printing away dialed in, but then have to change something and try to recall how I got it dialed in in the first place.

NOTE 3: I would probably skip my .gcode file and just use the .stl in your own slicer with your own settings, and work it out from there.  My printer is weird, and modified, so doubly weird.  I only added it because it adds in some information like estimated print time.

01 Feb 2023: Added a PDF with instructions, to have on hand with the calibration model.