An answer most designers might find difficult. ( Fillet Vs Chamfer)

One of the questions that caught my attention is what the difference between a fillet and chamfer is. Hearing the question at the first instance will very easily bring to the minds of everyone a one liner answer giving the geometric difference between both. They might even go further giving their common purpose of reducing stress concentration. Many people will not even consider it as a topic to reflect. But I find that the most basic of the things is the least understood and ignored. Yet the answers are hard to find and carry significance. Over the years of reading design related material I remember no article giving me the answers. So as usual I went to the best knowledge base of humans- the google to get the answer. But to my surprise I found that even Google does not have a consolidated material  about the topic. This was when I decided that I have found a topic I could help out people with.

In mechanical engineering a fillet is a rounding of the interior or exterior corner of a part. On the inlet fillets are concave while on exterior corners these are convex. Fillets are mainly used to reduce stress concentration compared to chamfers. These can easily distribut stress more uniformly.

An interior or exterior corner with an angle or type of bevel is called chamfers. The purpose of using a chamfer is that it keeps the edges from being damaged and makes non uniform edges look more uniform. Chamfers (45 deg to horizontal) are also used as bevelled edge of a hole ( usually to remove burr due to drilling operation) . It is usually used to allow the head of a screw or bolt to sit below the surface (i.e. Flush fit) rather than stick out. Larger chamfers (usually 60deg) are used as lead-in for a screw or a bolt. Chamfers are less effective compared to fillets in reducing stress concentration as they have sharp corners

The main points that help in deciding to choose a fillet or chamfer are the following:

1. When done manually one of the main factors that come in deciding which to apply is the machining time. A chamfer requires less machining time that a fillet radius.
2. When done on CNC both chamfer and fillet require the same time as only a tool change is required.
3. For fillets different radii of tools has to be stocked to create different radii, but a single tool can be used for creating different chamfers.
4. Higher machining time required translates into cost. Thus chamfers are less costly compared to fillets.
5. Industrial designers tend to prefer fillets compared to chamfers as these are considered to be visually pleasing.
6. One of the other reasons is that protective coating like paint are more uniformly distributed over a  fillet compared to chamfer. Thickness of coating is reduced on sharp corners of chamfers so coating is lost first on these spots. Fillets have no such issues due to uniform distribution of coating.
7. Since non uniform distribution of coating can lead to accelerated rusting this may be a disadvantage.
8. Fillet gives better stress flow (less resistance) compared to chamfers. Fillets generally give a lower stress concentration factor than chamfers .
9. Chamfers are more forgiving when fitting mating parts. i.e. even if there are inaccuracies in a chamfer mating parts might fit together. But if the radius of fillet changes it will be difficult to fit the mating parts

Below are some snips that I have included from one of the case studies I did using Ansys to study the effects of chamfers, and different radii of fillets on an engine valve. You will be able to find my complete report on my Linkedin profile.

 a) engine valve with no fillet or chamfer

 b) engine valve with  chamfer

 c)  engine valve with  fillet radius

Note: I have tried my best to cover all the information regarding fillets and chamfers. I am open to feedback and would like the experienced designers to help me improve the article. Please comment the change you would like to see in the article.