I joined the Sonopill team with the task of managing the development of a capsule called “Magnocap.” Starting the process from scratch, a significant portion of my work has been on the mechanical design – something I thoroughly enjoy. It is rewarding to conceive an idea, sketch it, discuss it with colleagues, model it, fabricate it and then hold it in your hands, or better yet, see it in action!

I have a running joke with a few of my colleagues that “you can never have enough fillets!” Maybe I do have a slightly exaggerated fondness for the unassuming fillet, but I think there is more to them than a smooth finish… [For those unfamiliar with CAD modelling, a fillet is feature that rounds-off a corner – yes, it’s that simple!] So, apart from giving a more rounded appearance to a part, are there any other reasons to justify my ‘appreciation’ of fillets? Here are some of the main, perhaps surprising benefits:

Firstly, there are ergonomic (human interaction) advantages: When filleting, sharp corners are removed and so give the part a smooth feel. This may not be so important for some parts, but for others, sharp corners can damage surrounding objects and the people handling them. Ergonomics can extend further to the assembly process – specifically the inserting or joining together of parts. Filleting edges of parts that must be inserted into one another makes it easier to align the parts and “push-fit” them together. The fillet essentially acts like a funnel, gradually aligning the parts as the one is inserted into the other.

The second advantage is in manufacturability; can the part you have just designed be manufactured with your chosen method? Some of the more common methods we use are 3D printing, machining and moulding or casting. Filleting sharp corners can improve the outcome of these by reducing the feature detail (or resolution) of the intricate part. In both 3D printing and machining, a tool must trace the outline of all the features of the part to create it. So, if there is a sharp angle or very thin feature, then this detail may be lost as the tool cannot enter into or trace around that particular area.

A third benefit is the potential to improve the robustness (strength) of a part. Taking a small rod attached to a flat plate as an example: when you bend the rod there will be a high stress concentration at the sharp transition/edge where the rod joins the plate. A high stress concentration can lead to the material failing. Adding a fillet to the interface between the rod and plate, in this case, adds a small amount of material around the base of the rod and smooths out the previously sharp transition between them. The added material, and its rounded shape, reduce the stress by distributing it over a large area. Result: stronger rod connection with very little change in the design.

And finally, fillets can ensure a more efficient use of space in devices with multiple internal components. Chances are, if you have a square-shaped void in a part and you insert a square-shaped component into it, there will be some wasted/empty space – particularly around the corners. This is because it is very uncommon to have perfectly sized, perfectly shaped parts – the designer may have added fillets to ease manufacture, or maybe there are just manufacturing inaccuracies in the parts. Either way, adding fillets can help to make sure the parts fit together with little wasted space by removing those sharp corners (fine features). An additional benefit is that the reclaimed space can increase the surface area of the part which, if glue is being used, can improve the resulting adhesion strength! These sorts of considerations are particularly important on small-scale device like the capsules we develop, as the parts can be tiny and the space very limited.

I bet you didn’t realise something so simple could be so useful!

First published: 23 January 2018

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