Measuring and Tolerances
CAD is exact. Printers are not. This chapter is about the gap between the two — and the numbers that bridge it.
Tolerance: how wrong you're allowed to be
A tolerance is the acceptable error on a dimension. When you model a 20 mm peg, the printer might produce 19.85 or 20.1 mm. Tolerance thinking means asking, for every mating surface: how much slop can this joint survive?
Two joints, two answers:
- Clearance fit (parts must move or assemble easily): design the hole bigger than the shaft, deliberately.
- Press/interference fit (parts must stay put): design the hole at or slightly below the shaft size, and let friction (or heat) do the work.
Clearance holes for screws
A screw needs a hole bigger than its threads. Starting points that work on most FDM printers:
| Screw | Screw Ø | Clearance hole Ø | Snug hole Ø |
|---|---|---|---|
| M3 | 3.0 mm | 3.4 mm | 3.2 mm |
| M4 | 4.0 mm | 4.5 mm | 4.2 mm |
| M5 | 5.0 mm | 5.5 mm | 5.2 mm |
"Clearance" = the screw drops through freely. "Snug" = it threads in with finger pressure and won't rattle. These are starting points, not laws — your printer, filament, and slicer settings move them by ±0.1–0.2 mm. The project in this guide uses 4.5 mm clearance holes for M4 screws, the middle of the road.
The printed-hole problem
Holes print smaller than modeled — typically 0.1–0.3 mm undersized. Molten plastic gets dragged inward as the nozzle circles, and the effect is worst on small holes. So:
- A modeled 4.5 mm hole often measures ~4.3 mm. For M4 clearance that's still fine — which is why the table above has margin built in.
- A modeled 3.0 mm hole for an M3 screw will likely not fit. Model it at 3.4 and thank the shrinkage.
- For precision fits (bearings, shafts), model slightly under and finish with a drill bit. A 8 mm drill through a modeled 7.8 mm hole gives you a true 8.00 mm bore — the cheapest precision in the hobby.
Designing mating parts
When two printed parts must fit together, don't model them line-to-line (shaft 10.0, hole 10.0). Give the joint room:
| Joint | Rule of thumb |
|---|---|
| Sliding (drawer, hinge) | hole = shaft + 0.3 mm |
| Easy assembly (lid on box) | hole = shaft + 0.4–0.5 mm |
| Snug (press with thumb) | hole = shaft + 0.1–0.2 mm |
| Permanent press | hole = shaft − 0.1 mm (or heat-set) |
And the golden rule: print a test coupon first. A 20-minute test print of just the joint beats a 6-hour print of the whole assembly that doesn't fit. Model a small block with your hole, a small peg with your shaft, print both, adjust once.
Shrinkage and elephant's foot
Two printer behaviors that eat tolerances:
- Shrinkage: plastics contract as they cool. PLA barely (~0.2%); ABS/ASA more (~0.5–1%). Large flat parts come out slightly small. Slicers have a shrinkage-compensation setting; for most PLA parts you can ignore it.
- Elephant's foot: the first layer squishes wider than modeled. If a part must sit flat and mate precisely at its base, add a 0.5 mm × 45° chamfer to the bottom edge, or enable the slicer's elephant's-foot compensation.
Measuring what you got
Close the loop: measure the print with calipers, compare to the model, and write the difference on the test coupon. After three or four parts you'll know your printer's habits — "my holes come out 0.2 under, my walls 0.1 over" — and your designs will start fitting the first time.
The one-line summary: model holes bigger than the math says, test the joint before the assembly, and let the caliper — not hope — decide.
Next: Designing for FDM