Printing tiny things

[printbus]

I agree with Roxy that there's another problem if the large case was wrong. I'd solve that problem first.

Did you adjust your steps per mm to get the 20mm cube dimension right? If so, go back to the original setting. X, Y and Z steps per mm are defined by the design of the hardware and generally shouldn't be adjusted through a calibration. When the steps per mm are tweaked to get the 20mm (or any other single calibration print exactly right), you can be left with a printer that can *only* print the calibration dimension correctly, with everything else off by some percentage. Search for prusa calculator if you don't know or want to double check the steps per mm settings for those axes.

Configuring the slicer to print external perimeters first might help a bit on the holes. That will cause the exposed wall of the hole to be printed first, and other perimeter walls then added around that. Otherwise like Roxy said the exposed wall will be pushed towards the center of the hole a bit more when it prints against the perimeters that are already there.

I've found printing holes on the bottom of things using a heated bed and PLA filament is especially troublesome since the PLA material stays soft sometimes for several layers of a print. Adding new layers on top of still soft layers will rarely lead to good prints. Adding a good print cooling system for PLA helps "freeze" the filament quicker and help it retain it's shape.

Finally, it may be specific to openSCAD, but search on polyhole to see one design approach of improving resulting sizes of small holes. Printing PLA and the polyhole library function in openSCAD, I now do pretty good in having things like M3 to M5 holes come out right, using standard tap & clearance dimension tables for proper dimensions on clearance holes, holes for self-threading, etc. But I still have an assortment of deburring tools, small files, and drill bits to clean up anything that needs some final adjustment.

EDIT: A bit of a correction. In double-checking the hole dimensioning I used in my refinement of the greg's wade extruder suite, I see in that openSCAD design I did add 0.2 to 0.3mm to what a mechanical craftsman would use for a clearance hole. Still not bad for a fudge. In the openSCAD source for the refined greg's wade, I include a table in the contents showing what standard hole sizing would be. I copy that table into many of my openSCAD designs as a handy reference. In that file, I also provide the option to generate a plate for testing the hole sizes before bothering with a full formal print of the parts. I wish more people offered that in their designs.