A polygonal pin can make a 3D-printed press-fit into a round hole less sensitive to tiny clearance changes. In a 2015 Make: workshop example, Kurt Hamel replaced a round pin with an octagonal one for a PVC camera mount and reported that it fit on the first try. That result is a useful design idea—not a guarantee or a universal tolerance rule.
Why a conventional round pin can be hard to fit
With a round pin in a round hole, a small change in clearance can shift the fit from too loose to too tight. Hamel describes making at least six attempts to get the fit he wanted with an earlier design that used a chamfer and a slightly undersized pin. He also found that length, diameter, printer, material, and print orientation affected the result.
This is the practical challenge: a nominal CAD dimension does not by itself tell you how a particular printed part will mate. If you keep the round-on-round geometry, expect to test and adjust the dimensions for your printer and material.
How changing the pin shape changes the fit
Hamel’s alternative keeps the hole round but makes the pin polygonal. Its corners meet the round hole, concentrating contact at those points and allowing the pin and mating part to flex there. The idea is to make the fit more forgiving than a round pin whose circumference engages the hole all at once.
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For a 1.00-inch round hole, Hamel suggests a 1.00-inch cross-corner dimension for the polygonal pin. This is his design guidance for the geometry he describes, not a validated engineering standard or a dimension that will work for every printer, material, or application. The Make: article does not report controlled measurements of fit accuracy, strength, or service life.
Hamel’s octagonal pin example
Hamel designed a 3D-printed action-camera fitting for 3/4-inch PVC pipe and the larger diameter of 3/4-inch PVC fittings. He reports that his octagonal pin fit on the first try in that application. He also tried a spline-shaped cross section, but found no advantage over the octagon and said he would use octagons for both parts if redesigning the fitting.
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He preferred the octagon for practical reasons: it was easier to draw than the spline, its central hole added flexibility and helped establish concentricity, and it could be printed horizontally with a flat bottom and 45-degree angles. Those are advantages in his design and print orientation, not a claim that octagons are always the best shape.
Choosing between a round pin and a polygonal pin
| Design choice | What it may offer | What to keep in mind |
|---|---|---|
| Round pin in a round hole | Familiar geometry; you can tune the fit by adjusting clearance. | Hamel found the fit sensitive to small dimensional changes and needed at least six attempts on one early design. Results depend on the printer, material, dimensions, and orientation. |
| Polygonal pin in a round hole | Corner contact can add compliance and may widen the usable design window. Hamel found an octagon easier to draw than his spline option. | His first-try result was for one PVC camera-mount application. The article gives no controlled strength, fatigue, or accuracy comparison. |
Consider a polygonal pin when repeated clearance adjustments are inconvenient and the part’s geometry and loading make a flexible fit suitable. Retain a conventional round pin when that shape is necessary for the design; in that case, plan for test prints and dimensional adjustments rather than assuming a nominal clearance will transfer to your setup.
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Test the fit on your own printer
Hamel’s example shows a design approach, not a substitute for testing. A related Make: guide by Anna Kaziunas France, relaying advice from Kacie Hultgren, gives 0.2 mm as an example offset for a tight fit and 0.4 mm for a loose fit, while explicitly advising readers to test their particular model. Those example offsets are not universal press-fit settings and should not be treated as direct replacements for Hamel’s shaped-pin approach.
- Check the printed dimensions of the hole and pin rather than relying only on the CAD values.
- Keep the material and print orientation consistent while comparing test pieces; both can affect the result.
- Change one design variable at a time so you can tell which adjustment changed the fit.
- For a polygonal pin, check the cross-corner dimension and how the corners engage the hole. Treat Hamel’s matching-dimension suggestion as a starting point for his geometry, not a guaranteed fit.
Hamel’s 2015 account is a single maker-engineer’s example, and the related tolerance advice was published in 2013. Neither page establishes a population-wide success rate or a current standard for press-fit dimensions.
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Sources
- Kurt Hamel, “Tips for 3D Printing Press-Fit Parts,” Make:, July 22, 2015 (the page also displays July 23).
- Anna Kaziunas France, “Top Ten Tips: Designing Models For 3D Printing,” Make:, December 11, 2013.
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