Internal Corner Radii in CNC Milling: Geometry and Tool Access

A rotating end mill sweeps a circular envelope. When it mills the vertical walls of an enclosed pocket, that geometry leaves a rounded internal corner. A perfectly sharp corner in a CAD model is therefore not a complete instruction for an ordinary end-milling operation. Xometry explains the clearance needed as a cutter follows an internal corner. Corner-path explanation

Separate three different radii

Giving all three the name “fillet” can conceal different functional and manufacturing requirements. Identify each intended feature on the model or drawing rather than assuming one radius instruction controls them all.

Radius and depth belong in the same review

Protolabs’ original transcript describes how small pocket corners may require progressively smaller tools. It recommends discussing actual limits with the supplier. A radius should therefore be considered with the depth and the space available for the tool, rather than selected from a universal minimum-size table. Original CNC design transcript

The transcript is an application explanation, not a standard. It also contains numerical rules outside this page’s scope; those rules have not been adopted here.

Preserve the mating function

For a hypothetical rectangular insert seated in a pocket, first identify whether its corner must touch the pocket corner or whether the seating faces establish position. As an editorial design-review exercise, consider a permitted blend, clearance relief or a change to the mating component. Each option changes geometry and requires design approval. It is not an automatic instruction to add relief.

The finished pocket corner and the path followed by the cutter centre are different geometries. Xometry illustrates a rounded cutter-centre path and a sharp path transition. A part-radius value alone does not describe the complete cutter path. Cutter-path illustration

Related geometry concepts

The milling and turning introduction explains why a rotating cutter and its feed path need to be distinguished. For the separate question of unwanted material at an edge, see burrs and edge conditions.

Limits and useful connections

There is no single recommended radius for all materials, depths, tools and mating requirements. A supplier must review the actual feature and route; alternative processes are a separate feasibility discussion. State the required radius and any permitted changes in the RFQ package rather than relying on unspoken default modifications.