Material, machine and your own numbers
Here's the quiet truth the whole section has been building toward: nobody can tell you the exact clearance your parts need. Not the datasheet, not the slicer preset, not the person on the forum with the same printer. The real dimension a feature prints at is the product of four things acting together — the material, the machine (its motion, its flow calibration), the nozzle (diameter and how a bead lays down), and the dryness of the filament — and that combination is unique to your bench. What you can do is measure your own small set of numbers, once, per material and nozzle, and reuse them across every design you draw afterwards. That's not a compromise. It's the only honest way to make parametric fits that actually fit.
Why presets can't do it for you
A slicer preset gets flow roughly right for a material family. It knows nothing about the extra tenths a wet spool oozes, the way your particular extruder over-extrudes the first perimeter, or how your 0.4 mm nozzle rounds an inside corner differently than someone else's. Two printers running the identical file and identical material produce holes that differ by a few tenths of a millimetre — and a few tenths is the entire difference between a peg that presses home and one that won't start. The preset can't measure your machine; only you can.
Establish your numbers with coupons
The method is small and it pays for itself the first day. Print a test coupon that sweeps a fit across a range — a row of holes stepping from tight to loose around a target diameter, or a pin-and-socket ladder. Try the real mating part in each, note which step gives the feel you want (press, slide, free), and that gap is your number for that fit, in that material, on that nozzle. Write it down. From then on you don't guess clearances — you type in a value you've proven. Validate before you print covers the coupon loop in full, and Tolerances and fits shows how to wire that proven gap into a model as a parameter.
| Fit type | PLA | PETG |
|---|---|---|
| Press fit (won't move) | −0.05 mm | −0.10 mm |
| Location fit (taps home) | +0.10 mm | +0.15 mm |
| Sliding fit (moves freely) | +0.20 mm | +0.30 mm |
| Clearance hole (bolt passes) | +0.30 mm | +0.40 mm |
The numbers above are a shape to copy, not values to trust — yours will differ, and that's the whole point. What matters is that once each cell is measured, you stop re-deriving clearances on every project and just reuse the table.
Know when the table is void
A fit table is only valid for the exact conditions it was measured under. It's invalid the moment any of the four inputs change: a different material (each shrinks and spreads its own way), a different nozzle (a 0.6 mm bead lays down nothing like a 0.4 mm one), or damp filament (moisture makes plastic ooze and bloat, quietly closing every hole). Change any one of those and re-run a quick coupon before you trust an old number.
Material, machine and your measured numbers together decide the part that comes off the bed. Once it's off the bed, though, it's rarely finished — smoothing, joining, painting and post-processing are a craft of their own, and that's where the next topic area on finishing a printed part begins.