What tolerance means in FDM printing
Tolerance is the acceptable range of variation from the intended dimension. A part designed to be 50mm long will not come out of an FDM printer at exactly 50.000mm — it will come out somewhere within a range of that target. How tight or loose that range is depends on the machine, the material, the print settings, the part geometry, and the orientation of the print.
For most FDM production, the practical working tolerance is approximately ±0.5% of the dimension, with a lower limit of about ±0.5mm regardless of how small the feature is. That means a 10mm feature should come out between 9.5mm and 10.5mm, and a 100mm feature should come out between 99.5mm and 100.5mm.
What affects dimensional accuracy
Several variables influence how close a printed part comes to the intended dimensions:
- Material shrinkage. Thermoplastics contract as they cool. Different materials shrink at different rates — PETG behaves differently than PLA, and TPU behaves differently than both. Slicers apply shrinkage compensation but it is never perfect.
- Print orientation. The Z-axis (layer stacking direction) is generally less accurate than the X and Y axes. Parts with critical dimensions in the Z direction may need orientation review.
- Layer height. Coarser layers print faster but with less Z-axis resolution. Finer layers improve surface detail and Z accuracy but add print time.
- Part geometry. Overhangs, bridging, thin walls, and complex curves all introduce opportunities for dimensional variation. Simple geometry holds tolerance better than complex geometry.
- Infill and wall count. More perimeter walls and higher infill improve dimensional stability and reduce warping on large flat parts.
Where tolerance matters most
Not every dimension on a part is equally critical. Knowing which ones are helps frame the quote request correctly.
Press fits and snap fits are the most tolerance-sensitive applications. If you are designing a peg that needs to press into a hole, a 0.5mm variation can mean the difference between a tight fit and a sloppy one. These applications almost always require a test sample before a full production run.
Clearance fits — where two parts pass near each other or need to slide — are more forgiving. A 0.5mm gap built into the design usually absorbs the tolerance variation without issue.
Mounting holes and bolt patterns in typical ranges (M3 through M8) generally print well without adjustment. Holes in FDM tend to print slightly undersized due to filament deposition, so adding 0.1–0.2mm to hole diameters in the model is common practice.
Cosmetic and non-functional dimensions — overall length and width on a housing, for example — rarely need tight tolerance. ±1mm on an enclosure lid is completely acceptable for most practical uses.
How to design for better tolerance
A few design habits lead to significantly better outcomes in FDM production:
- Add 0.1–0.2mm to hole diameters to account for FDM hole undersizing.
- Design clearance fits with at least 0.3–0.5mm of gap per side.
- Identify which dimensions are critical and note them in the quote request — they influence orientation decisions.
- For press-fit features, request a test sample at the start. One sample run costs far less than a failed batch.
- Keep critical features in the X/Y plane rather than the Z direction where possible.
When to request a test sample first
Test samples make sense whenever a part has mating geometry — two parts that fit together, a peg in a hole, a snap clip, a hinge, or any assembly that requires parts to interact dimensionally. The cost of one or two test samples is small relative to the cost of discovering a fit problem on a larger batch.
Include "test sample requested before full run" in the quote request and it will be priced as a separate line item. Once the sample confirms fit and finish, the full run is scheduled with the same settings.
What to include in the quote request
To get the most accurate production result, include in the quote request: which dimensions are critical, whether any features are press or clearance fits, surface expectations for mating faces, overall part dimensions for build feasibility review, and whether a test sample is needed before the full run. The more the review team knows about how the part works, the better the production decisions.