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How to Design Better Fillets for Mass Production 3D Printing

Fillets are an essential feature in 3D CR parts, offering numerous benefits when used correctly. However, it's important to avoid fillets on the bottom layer of a part. The reason for this is that filleting the bottom can create a near-horizontal first few layers, leading to overhangs that either require support or risk deformation. This happens because the first layer is laid down flat, and subsequent layers extend outwards, creating a gradual recovery that is not ideal.


If fillets are necessary on the bottom, it's advisable to avoid standard circular fillets, where each edge is tangent to the adjacent edge. Instead, consider a custom fillet design that curves in slightly, maintaining aesthetics without the overhang issue. This approach allows for easier layering and results in a cleaner, more precise part.



This modification can alter the geometry, leading to a larger contact area and a slightly sharper edge than usual. But it effectively prevents a rough, unsightly bottom layer. Technically, the maximum overhang in a 3D printed part should not exceed 30°. This can be achieved by designing a custom fillet for rotational items or when cropping. Even with a 30° angle, there's a risk of deformation or a 'ring wobble' effect on the part's bottom as the layers build up.


A shallower angle is preferable, and ideally, using a chamfer ensures each layer is a consistent step up from the previous one. Aim for a minimum of 30° to create a viable structure.


In practical applications, such as in switch covers or similar items, designers often opt for a standard circular fillet, resulting in an undesirable appearance at the lower corners. By employing a slightly reduced bevel, the same aesthetic can be achieved without the flaws associated with standard fillets.


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Example of a Good Fillet Design on a Switch Cover
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Example of a Bad Fillet Design on a Switch Cover

This design choice ensures the piece looks refined, is easier to mass-produce, and avoids printing imperfections.

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