Internal Corner Radius in CNC Milling: How to Choose R for Cost and Machinability

An end mill sweeps a circular path, so two milled walls meet with a finite internal radius. The useful DFM question is therefore which radius preserves assembly function while allowing a sufficiently stiff cutter. Leaving the CAD corner mathematically sharp does not remove the radius; it merely transfers the decision to manufacturing. Specifying a practical minimum radius can reduce tool changes, corner cleanup, cycle time, and quotation uncertainty.

A small radius near the top of a shallow pocket may be routine, while the same radius at the bottom of a deep pocket can require a long-neck tool or a small holder. Greater overhang reduces stiffness and raises deflection risk. Designers should separate vertical corner radius from the wall-to-floor radius when their functions differ, and avoid applying one small radius to an entire pocket unnecessarily.

Possible design solutions include a dog-bone or T-bone relief, a chamfer or radius on the mating part, staged milling, or EDM where geometry and material permit. Wire EDM requires conductive material and wire access and cannot create a closed blind pocket. Every relief or process change modifies geometry, stiffness, sealing, stress, or cost, so it needs design approval.

A robust process removes most stock with a larger tool, distributes finish allowance, then uses a smaller tool only where necessary. Rest-machining data must reflect the previous cutter and remaining stock; otherwise the cleanup tool may meet an unexpected load in the corner. Constant-engagement or adaptive paths reduce load variation but cannot remove the geometric limit of cutter radius.

If the radius is only tool relief, avoid unnecessarily tight tolerance. A functional radius should identify its controlled area, tolerance, finish, and datum relationship. Profile instruments, optical systems, a suitably configured CMM, or functional gages may be used depending on size and access. Passing radius alone does not prove the adjacent walls or floor satisfy position or orientation.

Provide controlled 2D and STEP files, pocket depth, wall thickness, minimum or acceptable radius range, mating-part geometry, relief permission, tolerance, finish, material, quantity, and post-processing. Identify areas where EDM, start holes, or tool witness marks are not allowed. VietTech reviews milling, workholding, and inspection for each drawing rather than assuming every small radius is practical.

• 2D and STEP at the same revision
• Minimum or acceptable R range
• Pocket depth, floor radius, and wall thickness
• Mating part and allowed relief
• Tolerance, finish, datums, and inspection
• Material, quantity, treatment, and delivery

This guide is cross-checked against Sandvik Coromant and Seco Tools guidance on pocket milling, engagement, overhang, and stable toolpaths. Manufacturer recommendations must still be matched to the exact cutter, material, machine, and setup. The guide does not replace stress analysis, product standards, or cutting trials; safety-critical geometry changes require design-owner approval.