A layer-height setting is not a dimensional tolerance. The printer, material, orientation, slicing and post-processing together determine how closely a printed feature matches its model.
Accuracy, repeatability and resolution
Accuracy concerns closeness to the target. Repeatability concerns how consistently the process reproduces a result. Resolution describes a discretization or minimum feature-related scale, depending on the technology. A finely layered part can still be consistently undersized.
Internal holes and external bosses may have different systematic errors. First-layer compression can enlarge a base edge; support removal can alter a mating face. Measure the actual functional feature instead of inferring its quality from a nominal nozzle or pixel size.
A clearance stack example
Suppose a printed pin is specified as 10.00 ±0.10 mm and its mating hole as 10.40 ±0.10 mm. Worst-case diametral clearance is 10.30 − 10.10 = 0.20 mm minimum and 10.50 − 9.90 = 0.60 mm maximum. Radial clearance is half those values for ideal concentric circular geometry.
Those tolerances are hypothetical design inputs, not a promise that a particular printer can hold them. The exercise shows how two uncertain dimensions combine. If measurements reveal greater variation or noncircular holes, the fit assessment must change.
Build a useful coupon
Include the actual feature sizes, orientations and wall thicknesses from the intended part. Print repeated specimens, measure them consistently after cooling or curing as appropriate, and test assembly under realistic conditions. Record material lot, orientation, settings and measurement method.
Use the results to distinguish a systematic offset from random variation. Compensating the model may correct a bias but cannot eliminate an unstable process. For critical fits, consider post-machining, inserts or another manufacturing method rather than requiring the print alone to do everything.
Specify the final state
State whether acceptance is before or after support removal, curing, coating or machining. Supplier capability figures are technology- and geometry-specific. Do not apply an SLA tolerance figure to FDM simply because both are called 3D printing.
Turn a clearance target into a test coupon
For an illustrative 10.00 mm pin, model a row of holes at 10.10, 10.20, 10.30, 10.40 and 10.50 mm. Label the nominal diametral gaps 0.10–0.50 mm on the coupon. These are trial gaps, not guaranteed sliding, press or snap fits. Include the same wall thickness and hole orientation as the real part; a vertical-hole coupon cannot establish horizontal-hole performance.
Separate bias from repeatability
Suppose three hypothetical 10.00 mm pins measure 9.90, 9.92 and 9.94 mm. Their average is 9.92 mm, giving a sample bias of −0.08 mm, while the observed range is 0.04 mm. If a test hole measures 10.24 mm, the observed diametral gaps for these pins are 0.34, 0.32 and 0.30 mm. Correcting the CAD offset may address the mean error; it does not remove the variation.
| Record | Why it changes interpretation |
|---|---|
| Printer, material lot and orientation | Makes the result reproducible rather than a universal gap rule |
| Hole diameter at two directions and heights | Reveals ovality or taper hidden by one measurement |
| Cooling, curing and support-removal state | Defines the condition in which the fit is accepted |
| Actual mating part and assembly force | Checks the function, not just one nominal dimension |
Three parts are useful for an initial check, not a process-capability claim. Choose a suitable sample plan and calibrated measurement method for production. Use the tolerance stack-up calculator to compare design limits; use measurements to establish whether the process can meet them. The distinction between accuracy, precision and tolerance follows Formlabs’ explanatory guide.
Content updated October 8, 2026. Sources and numerical examples are explained above; examples are not production test results. Editorial method · Report a correction.
References and verification
Original explanations and worked examples by MfgWorkbench. Sources below support the stated concepts or selected reference data; they do not certify every example or a production application.
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