Fillets, chamfers and corners that print without support

Fillets, chamfers and corner geometry sit at the intersection of two separate FDM problems: they affect whether support is needed at all, and they affect how the very first layers of the print behave on the bed. Get the bottom edge wrong and you get elephant foot. Get an internal corner wrong and you get a stress concentration that cracks under load. Both are cheap to fix in CAD and expensive to fix after the part is printed.

Bottom edge chamfers versus fillets, and the elephant foot effect

Elephant foot is the slight outward bulge that appears at the very bottom of a printed part, where the weight of everything above compresses the first few layers before they have fully cooled, and where the nozzle's first layer squish deliberately over-extrudes to guarantee bed adhesion. On a part with tight bottom dimensions or a press fit that starts right at the base, that bulge is enough to throw off the fit.

The standard countermeasure is a small chamfer or fillet at the very bottom edge of the part, breaking the sharp corner between the vertical wall and the bed into a sloped or rounded transition. A chamfer of around 0.4 to 0.6 mm at 45 degrees, or an equivalent small radius, gives the first layer squish somewhere to go without pushing the nominal wall outward. Chamfers are simpler to model and slightly more forgiving to slice reliably; fillets look better on a visible edge and avoid the sharp witness line a chamfer leaves. Either works for elephant foot control, the choice is mostly aesthetic and modelling convenience.

Internal corner radii and stress concentration

A sharp internal corner, the inside of an L-shaped bracket or the base of a rib for example, is a stress concentration point regardless of manufacturing process, but FDM makes it worse because the layer lines running through that region are already a plane of weakness. Cracks in printed parts start at sharp internal corners far more often than anywhere else on the part.

The fix is a generous internal fillet, typically 0.5 to 2 mm depending on part scale, wherever two walls meet at an internal angle under load. This is free in CAD and effectively free in print time, since a small radius adds negligible material. It is one of the highest-value, lowest-cost changes in any FDM design review.

External corner rounding

External corners are less of a strength issue and more of a print quality and handling issue. Sharp external corners are prone to slight lifting from the bed if adhesion is marginal, since they cool and shrink from two directions at once, and they are the first place a part chips if dropped or knocked in handling. A small external radius, or at minimum a light chamfer, reduces both risks without changing the part's function in most designs.

Lead-in chamfers for assembly and inserts

Beyond print quality, chamfers do real assembly work. A lead-in chamfer at the entrance of a hole for a heat-set insert centres the insert before the soldering iron tip pushes it in, reducing the chance of it going in crooked. A chamfer at the entrance of a shaft bore makes hand assembly of a pin or bearing forgiving instead of requiring the two parts to be perfectly aligned on first contact. These chamfers are typically 0.3 to 0.5 mm at 45 degrees and cost nothing to add.

Which fillets print cleanly without support

Not every fillet is free of support concerns. A fillet on a vertical edge, running parallel to the build direction, prints exactly like the wall it rounds and needs no support. A fillet that transitions a horizontal top surface into a vertical wall, essentially a rounded external corner seen from above, is also generally self-supporting because the overhang angle it creates stays shallow through the transition.

The problem case is a large-radius fillet on the underside of an overhanging feature, where the radius itself creates a steep, curving overhang at its lowest point. Here the same 45 degree logic from the overhang guide applies to the local tangent angle of the curve, not just to the feature as a whole, so a very generous underside fillet can quietly reintroduce the exact support requirement you were trying to design away.