What are the most popular and the most efficient ways to machine a fillet on an external edge of an object? If I were to use a CNC mill, what tools can I use for it, and how would the method change with the radius of the fillet?

I could imagine that internal fillets can be easily achieved by a ball-end to a mill, but I've never come across machining an external one.


2 Answers 2


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Depending on whether that's one-off or something to repeat, a properly shaped bit with "negative curve" will be the most efficient (single pass) and neat (no grooves from multiple passes) solution.

I don't have a photo of the tool in question, but the one in the middle has some similar properties - it was used to create undercuts (-30 degrees edges) and its side edge curves in a way similar to what you'd need to obtain. So, simply grind a bit to the correct radius and pass it once (or in enough approaches as not to break the bit).

Alternatively, if you either don't grind your own bits or are unwilling to use up a new blank for something you'll use once, use a round tip bit, and have the CAM software calculate tracks for the right curve, then make it pass enough times so that the lines will be possible to remove with sandpaper.

Note you don't need many bits for different curves. Make one of fairly large curve and route it along the right circle (radius = bit radius - fillet radius) in the Z-axis , and it will be much more graceful to smooth out than created with a round tip.

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  • $\begingroup$ These are both possible ways, but are fairly expensive in a production context, and not too common. $\endgroup$
    – Ethan48
    Commented Feb 5, 2015 at 15:59
  • $\begingroup$ @Ethan48: In production context you'll have a cast/die of the right shape... $\endgroup$
    – SF.
    Commented Feb 5, 2015 at 16:03

On a mass manufactured part, one solution is to cast the piece and shape the die (with an inside fillet) to form an outside fillet. Of course, if you want your casting to have an inside fillet, you'll need an outside fillet on your die.

Assuming you're not casting, and working with a commonly machinable material, external corners would typically be rounded over using a tool called a 'radius cutter' or more formally, a 'corner rounding end mill.'

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A particular cutter only cuts one radius, so if you have two fillet radii, you'd need a tool change. They are available in a range of sizes and made out of different materials with different coatings. High speed steel is probably the most common material.


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