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CNC Machining Glossary
Internal Corner Radius in CNC Machining
An internal corner radius is the size of the rounded curve at an inside corner of a part. In CNC milling, the term commonly describes the corner where two sidewalls meet inside a pocket or recess. Look down into a rectangular milled pocket, and you will usually see rounded corners rather than perfectly square ones.
The curved transition is an internal fillet; its radius tells you how large that curve is.
An end mill has a circular cutting profile, so it leaves material in the corners of a pocket. In conventional pocket milling, a 6 mm diameter cutter has a theoretical minimum internal corner radius of 3 mm. The same cutter can produce a larger corner radius by following a curved path.
What Matters in Design and Machining
For machinists, the specified radius limits the diameter of the cutter used to finish the corner. A tight radius may require a small tool, even when a larger cutter removes most of the pocket material.
Pocket depth makes this more important. A small cutter extending deep into a part is more likely to bend or vibrate under cutting forces. Maintaining accuracy and a consistent surface finish may therefore require lighter cuts and additional passes, increasing machining time and cost.
Details Designers Can Overlook
Leave room for the cutter to turn. Where the design allows, make the corner radius larger than the radius of the intended finishing tool. This gives the cutter room to follow a curved path through the corner and helps avoid abrupt increases in cutting load. The theoretical minimum is not always the most economical choice.
Consider radius and depth together. A small radius in a shallow pocket may be manageable; the same radius in a deep pocket can be much harder to machine. Avoid combining deep pockets with unnecessarily tight corners.
Check the assembled fit. A square-cornered component may not seat inside a pocket with rounded corners. Round or chamfer the mating component's corners, or provide suitable corner relief in the pocket to remove the interference.
Specify wall and floor radii separately. A corner between two sidewalls is different from the curved junction between a wall and the pocket floor. These features depend on different aspects of the cutter's shape and may require different machining strategies.
A well-chosen internal corner radius gives mating parts the clearance they need while allowing the machinist to use a stable, efficient cutting process.