Make FDM walls at least 0.8 mm thick and 1.5 mm where you can: 2 and 4 walls on a 0.4 mm nozzle. Wall counts per slicer, resin minimums and walls vs infill.
Counterbore hole 3D print sizes: for M3, model 6.5 mm wide and 3.4 mm deep over a 3.4 mm hole. Tables for M2 to M8 and #4 to 1/4 in, and printing it face down.
3D print countersink sizes: 90° for metric and 82° for inch flat head screws, 0.2 mm over the head's sharp diameter. M3 socket: 6.92 mm over a 3.4 mm hole.
A 3D print living hinge needs a web 0.4 to 0.6 mm thick, two or three layers. In a fold test, PP passed 1 million folds and PLA about 160. Sizes and settings.
3D print snap fit design: undercut = 0.67 × strain × length² ÷ thickness. Strain limits for PLA, PETG and ABS, 0.20 mm clearance per side, and print the arm flat.
How to 3D print hinges: a bore 0.40 mm over the pin for an assembled hinge, 0.30 to 0.40 mm per side printed in place. Sizes for filament, M3 and snap-on pins.
A 608 bearing is 8 mm bore, 22 mm outside and 7 mm wide. What 608ZZ and 608-2RS mean, and the hole to print: 21.90 mm press fit, 22.20 mm slip fit.
A 3D print annular snap fit needs an undercut of strain times diameter: about 0.5 mm on a 25 mm PLA cap and 0.9 mm in PETG, with the ring printed standing up.
3D print bearing press fit: draw the bore 0.10 mm under the bearing's outside diameter, 21.90 mm for a 608. Bore sizes for 623 to 6002, 6800, LM8UU and bushings.
D-shaft coupler or knob to 3D print: model 5.20 mm with 4.70 mm across the flat for a 5 mm stepper shaft. Bore sizes for 6 mm, 5 mm and 1/4 in D-shafts.
3D print magnet hole size: model the hole 0.10 to 0.15 mm over the magnet for a press fit, 0.20 to 0.30 mm for glue, and 0.1 to 0.2 mm deeper. Sizes by magnet.
3D print nut trap sizes for M2 to M8 and US nuts: model the hex pocket 0.2 mm wider than the nut across flats (5.7 mm for M3) and 0.1 mm deeper.
3D print O-ring groove sizes: make it 70 to 80% of the cross-section deep for 20 to 30% squeeze. Face and radial seal tables, a worked example and print settings.
3D print press fit tolerance: model 0.05 mm of interference per side (0.10 mm on the diameter) in PLA, less in ABS and nylon. Test print and pressing steps.
Print-in-place clearance: 0.30 to 0.40 mm per side in XY on a 0.4 mm nozzle and two whole layers in Z, with values for hinges, gears, toys and bearings.
3D print snap fit box lid sizes: a 1.35 mm skirt 0.20 to 0.25 mm per side inside the wall and a 0.3 to 0.5 mm catch, plus an ESP32 case and PCB clips.