The practice of designing printed holes slightly oversized (typically +0.2–0.5 mm diameter) to compensate for FDM over-extrusion and material expansion that shrinks holes below nominal.
FDM holes print smaller than designed due to: (1) extrusion pressure expanding outward, (2) filament shrinkage on cooling, (3) rounding of circular perimeters by the toolpath planner. Effect is most pronounced for small diameters (< 10 mm). Compensation is machine- and material-specific.
Rule of thumb: add 0.2 mm (0.1 mm per side) to hole diameter for a loose fit; add 0.4 mm for a clearance fit with a M3 bolt. Measure a calibration print with calipers. For threaded holes: design to tap size and tap after printing, or use heat-set inserts. Horizontal holes are rounder than vertical holes in FDM.
Over-compensating creates excessive clearance. Compensation changes with layer height, print speed, and temperature. Never apply a blanket compensation across all holes: measure and compensate per printer, per material, per diameter range.
Related terms: Tolerance, Clearance Fit, Press Fit, Dimensional Accuracy
| Field | engineering |
|---|---|
| Also called | printed hole compensation, hole tolerance FDM |
| Source | EDi Brain: rules-core |
Engineer, author of The Big Book of 3D Printing and additive manufacturing expert