"Can you hold ±0.1 mm?"
The honest answer: it depends on the process, the geometry, the material and how the part is post-processed. What follows is the detail behind that answer, including the numbers we quote and the design rules that make a printed part fit the first time.
Nominal tolerances by process
| Process | Typical tolerance | Best case (tuned and post-processed) |
|---|---|---|
| FDM (filament) | ±0.3 mm or ±0.5% | ±0.15 mm |
| SLA (resin) | ±0.1 mm or ±0.2% | ±0.05 mm |
| SLS (nylon) | ±0.3 mm or ±0.3% | ±0.15 mm |
| CNC (for reference) | ±0.05 mm | ±0.01 mm |
Whichever number is larger wins: the percentage dominates on large parts, the absolute figure on small ones. On a 200 mm FDM bracket, ±0.5% is ±1 mm — which is why long parts get designed with clearance, not with hope.
What drives the variation
- Thermal shrinkage. ABS shrinks roughly 1.5% as it cools, PLA around 0.3%, and nylon powder shrinks non-uniformly across a build. We compensate with scale factors per material, but compensation is an average, not a guarantee.
- Warping. Long flat parts pull up at the corners. Orientation, brim and chamfered corners cost less than fixing a warped part later.
- Layer adhesion and Z accuracy. Vertical resolution follows layer height, so a 0.2 mm layer height quantises every height in the part to 0.2 mm steps.
- Support contact. Down-facing surfaces touching support come out rougher and slightly oversized. If a surface is critical, we orient it upward or machine it afterwards.
- Position in the build. In SLS, parts near the edge of the powder bed cool differently from parts in the middle. On a fifty-part batch, that shows up as spread.
- Moisture. Nylon absorbs water from the air and grows slightly. Precision nylon parts are dried, then measured after conditioning rather than straight out of the machine.
- Machine calibration. A drifting bed, a worn nozzle or a tired resin tank costs accuracy quickly. This is the boring one that matters most.
Where tolerance actually matters on a car part
| Part type | Tolerance needed | What we'd run |
|---|---|---|
| Press-fit bushings and inserts | ±0.05–0.1 mm | SLA, or SLS then reamed |
| Snap-fit trim clips | ±0.2 mm | SLS nylon or FDM |
| Aesthetic trim panels and vents | ±0.3 mm | FDM or SLS |
| Bezels with visible gaps | ±0.2 mm on the visible edge | SLA or finished FDM |
| Threaded features | n/a — see below | Heat-set brass inserts |
| Shaft-and-bore assemblies | 0.2–0.3 mm designed clearance | SLS nylon |
Two rules cover most of it:
Design clearance, don't chase interference. Printing does not hold interference fits reliably. If two printed parts must slide, give them 0.2–0.3 mm of clearance and let the tolerance land inside it.
Don't print threads under load. Printed threads strip. We print an undersized boss and heat-set a brass insert, which turns a weak feature into the strongest part of the assembly.
Designing around the process
- Holes print undersize. Plan for reaming or drilling on anything that has to locate precisely. A nominal 5 mm hole in FDM commonly measures 4.7–4.9 mm.
- Chamfer the first layer. A 0.5 mm chamfer at the bed kills elephant's foot — the slight bulge on the bottom edge that stops flat parts from sitting flush.
- Give snap fits a radius. Sharp internal corners are where printed clips break. A 0.5–1 mm fillet roughly doubles the fatigue life of a hook.
- Wall thickness has a floor. Below about 1 mm in FDM and 0.7 mm in SLS, parts get fragile and dimensionally unpredictable.
- Mark the critical dimensions. If three dimensions out of forty matter, say which three. It changes the orientation, the process and sometimes the price.
How we hit tighter specs
For parts where tolerance is critical, we run a three-step approach:
- Print oversize on critical surfaces, leaving machining stock.
- Post-machine those surfaces to spec — drill, ream, face-mill.
- Measure every unit against the drawing before it ships, with the readings recorded for production batches.
It is slower and more expensive than a raw print. It also gets automotive-grade accuracy out of SLS nylon and SLA resin, which is usually cheaper than the alternative of machining the entire part from stock.
How to specify a part so the quote is right
Send us:
- the STEP file (not just an STL — STL throws away the exact geometry),
- a drawing marking the critical dimensions and their tolerances,
- the mating part or its dimensions,
- the working environment: temperature, sun exposure, chemicals, load,
- the quantity, now and later.
That's enough to answer honestly, including the cases where the answer is "this should be machined, not printed."
Need a tight-tolerance part?
Tell us the critical dimensions and why they matter. We'll pick the process and tell you plainly whether we can hold it. Start a project, or read more about our 3D printing service.
If the part exists but the CAD doesn't, 3D scanning is the first step. If you're still choosing a process, FDM vs SLA vs SLS covers the trade-offs, and 3D printing materials 101 covers what to print it in.
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