Internal Radius Design Tips


It seems contrary to the laws of nature to think that removing material from a piece of metal can make it stronger. Well, consider the humble internal radius.

What is an Internal Radius?

Where two perpendicular sides of a part meet, that corner can be surprisingly weak. An internal radius is made when an end mill creates a curve in that corner. The radius strengthens the corner, giving it better structural integrity, especially when you have thin geometries.

Designing Strong Curves out of Corners

Designing an internal radius is easy, but to do it in a way that speeds production requires some important specifications to get dimensions right. The math is pretty easy, though. The key is to use a standard fractional size for your curve. For example, when you want a 1/4 inch radius where the walls meet, we will use a 1/2 inch ball end mill to cut out the metal. Ball end mills have a rounded cutting surface, so using one leaves you with what looks like an arc in the material. You can see that in the example below, which first uses a square end mill, then switches to a ball end mill to make the radius.




Why Use Fractional Sizes for an Internal Radius?

If you want a radius that doesn’t follow the rule above, we can do it. The downside is that the multiple tool changes it requires will increase the number of steps to complete your part. Also, your curve will not look as good as using a single size of end mill.


Designing an internal radius using standard fractional sizes will give you the results you want in less time and with lower labor costs to you. The end result will be a stronger part with a more attractive surface finish.

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For more helpful machining tips, download our CNC Machining Design Guide.

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