Increasing the accuracy of cylindrical spur gears manufactured with FDM technology by modifying the geometry

Additive manufacturing using fused deposition modeling (FDM) enables the rapid and cost-effective manufacture of polymer spur gears for prototyping and low-load functional applications; however, dimensional accuracy is often limited by thermally induced shrinkage. This study proposes a direct geometric pre-compensation approach to improve dimensional accuracy compared with conventional optimization of printing parameters. Cylindrical spur gears (z = 18, α = 20°, face width 7.0 mm, center bore diameter 15.0 mm) with modules ranging from 0.5 to 3.5 mm were modeled in Siemens NX using mathematically defined involute profiles, exported as STL files, and manufactured on an Ultimaker 3 printer using tough PLA under default printing parameters. Besides, two nozzle diameters (0.4 mm and 0.25 mm) were evaluated. Chordal tooth thickness was measured at the addendum, pitch, and base circles using a digital microscope with auxiliary reference markings to ensure accurate tooth indexing. Initial measurements revealed a systematic dimensional increase at the addendum circle and contraction at the pitch and base circles, with deviations increasing nonlinearly as the module decreased. Based on these deviations, the CAD geometry was modified and the tooth profile deviated from theoretical involute geometry. Following re-manufacturing, the measured chordal thickness values converged toward the nominal CAD dimensions for both nozzle sizes, effectively restoring the near-nominal functional involute geometry of the printed gears. The proposed method significantly reduced dimensional deviations, decreasing the maximum error from 0.22 mm to 0.06 mm for the 0.4 mm nozzle and from 0.14 mm to 0.05 mm for the 0.25 mm nozzle.