Internal Fillet Radius: Why Go Bigger Than the Tool?

Internal fillet radius changes how the cutter behaves in every inside corner, and a few thousandths of an inch make a real difference.

Why Do Machinists Ask for Internal Fillets Slightly Larger Than the End Mill Size?

Because a corner radius equal to the tool radius makes the end mill wrap the corner, and engagement jumps toward a full slot. Make the radius a little bigger and the tool follows a real arc instead.

The question came from an r/Machinists thread where a student said someone from the college machine shop had complained about a part with every internal fillet set to a nominal ball end mill size. The advice was to model them about 0.01 in bigger. The student had always cut them at tool size without chatter.

What happens at an exact-size corner

At an exact-size corner the tool briefly wraps the corner and has to change direction sharply.

One reply summed it up: you can cut nominal fillets straight in, but cutting on two sides of the tool at once raises tool load and causes deflection that can throw dimensions off, so you have to slow down. Another described the fix. Program a larger radius and the tool moves in an arc, keeping engagement close to what it was on the wall.

What a slightly bigger internal fillet radius buys

You don't have to slow down as much, and the corner finish matches the walls. Rules of thumb from suppliers:

• A bit larger than the cutter. Protolabs Network (Hubs) suggests going slightly above the recommended corner radius, for example by 1 mm, so the tool cuts on a circular path. PCBWay's corner radius guide uses 130% of the cutter radius as a practical guideline, which works out to about R6.5 for a 10 mm end mill.
• About one-third of the cavity depth or larger. That's Hubs' rule for vertical corners, so a stiff enough tool reaches the bottom.
• When the exact size is fine. One reply said nominal fillets are easier on quick, simple parts or mostly manual work. Off-nominal radii pay off on CNC, where the toolpath can take advantage of them.

When you really need a sharp corner

If a square part has to seat in a pocket, add a dogbone or T-bone relief in the corner instead of shrinking the radius, as Hubs' sharp-corner guide explains. A truly sharp inside corner needs a process other than milling.

For deep pockets, see avoiding re-cut chips in deep pockets and what as-milled surface finish to expect.

At DigiForge

Our CNC service covers 3-, 4- and 5-axis milling and turning, with ISO 2768-m as the listed standard tolerance, and the quote checks internal corner radii automatically. The same page lists a Sodick VN600Q wire EDM for features milling can't reach. If a corner has to be sharp, note it on the drawing and ask us when you quote.