CNC milling corner radius optimization managing stress concentration while maintaining tight tolerances

Content Menu

● Introduction

● Stress Concentration Basics in Real Machined Corners

● Tool Selection Rules That Actually Work

● Programming Tricks That Keep Tolerance When You Open the Radius

● Material-Specific Radius Guidelines

● Measuring What You Actually Machined

● Conclusion

Introduction

Internal corners in milled parts always create problems if you let them stay sharp. The tool can’t reach a perfect 90-degree inside corner without leaving at least some radius, and whatever radius is left acts as a stress concentrator. In fatigue-loaded components the difference between a 0.8 mm leftover radius and a deliberate 3 mm radius can easily be a factor of three or four in cycles to failure. At the same time, most drawings call for pockets and slots with tolerances of ±0.03 mm or tighter, so you can’t just open the radius as much as you want without pushing walls out of spec.

The real job is to control the radius deliberately instead of accepting whatever the roughing tool leaves behind. Shops that treat corner radius as an afterthought end up with cracked parts in service or endless hand benching. Shops that plan the radius from the start usually hit tolerance, hit fatigue targets, and keep cycle times reasonable.

This article pulls from day-to-day production experience on 7075 and 7050 aerospace structure, Ti-6Al-4V manifolds, 17-4PH implant trays, and H13 mold cores. The ideas are backed by actual journal work on machined-surface fatigue and cutting-edge stress fields.

Stress Concentration Basics in Real Machined Corners

Peterson’s stress concentration charts are the starting point everyone knows. For a step or a shoulder, Kt falls quickly once r/d exceeds about 0.05. In a pocket the geometry is closer to a U-notch, and the same rule holds: double the radius and Kt typically drops 25–40 % until you get past r/d ≈ 0.15–0.20.

What the charts miss is surface finish and residual stress. A corner left by a 10 mm flat end mill at 0.4 mm stock and cleaned with the same tool has visible cusp marks and tensile residual stress from the heavy engagement. Measured Kf in fatigue tests on 7075-T6 can be 20–30 % higher than the geometric Kt would predict. Switch to a light finishing pass with a ball or bull-nose tool and the effective Kf drops even if the geometric radius is identical.

Example: a 50 × 30 mm pocket in 7050-T7451 plate, 25 mm deep, loaded in reversed bending. Original process left ≈ 0.7 mm vertical radius. S-N testing gave failure at 48 000 cycles at 180 MPa remote stress. After switching to a deliberate 2.8 mm radius the same part survived 210 000 cycles at the same stress level. The radius increase moved the pocket walls outward only 0.9 mm total, which was absorbed in the ±0.20 mm general tolerance on the feature.

Tool Selection Rules That Actually Work

Rule of thumb used in several aerospace shops: minimum internal corner radius = 1.3 × diameter of the largest tool that still gives acceptable floor finish and cycle time.

Typical sequence for an aluminum structural pocket:

– Rough with 16–20 mm flat end mill, leave 0.6–0.8 mm wall stock, 1.2–1.5 mm floor stock – Semi-finish walls and corners with 12 mm bull-nose, corner radius 1.5–2.0 mm – Finish floor and blend corners with 8 mm or 10 mm ball-end mill, 0.08–0.12 mm stepover – Spring pass on walls with the bull-nose at 0.03–0.05 mm radial depth

The bull-nose in the semi-finish step is the key. It removes the sharp leftover radius from roughing in one clean operation and leaves a smooth vertical radius that is already close to final. The ball-end then only has to blend the floor transition.

For titanium or stainless parts where tool pressure is higher, many programmers now use variable-pitch bull-nose tools or barrel-form tools (oval or lens shape) on 5-axis machines. A 12 mm barrel tool with 300 mm radius segment can produce a 4–5 mm blended corner radius in a single helical pass while keeping wall stock uniform to within 0.015 mm.

Programming Tricks That Keep Tolerance When You Open the Radius

The usual worry is that a bigger radius pushes the pocket wider. Three ways around it:

Asymmetric stock. Leave extra stock only on the floor and the lower half of the walls. The finishing tool then machines the upper walls to final size first, then drops down and blends the larger radius at the bottom without touching the critical upper section.
Corner loop compensation. Most modern CAM packages (NX, Mastercam 2024+, PowerMill 2025, hyperMILL) let you add a small loop or teardrop at the corner that is exactly the extra amount needed for the radius. The rest of the contour stays on nominal path.
Rest-material corner picking with progressively smaller tools. Rough → 12 mm bull → 8 mm ball → 4 mm lollipop for final cleanup if undercut is allowed. Each tool only removes what the previous one physically could not reach.

Real example from a Ti-6Al-4V hydraulic block: drawing called for 1.0 mm max radius in 18 mm deep ports, tolerance ±0.025 mm on diameter. Roughing with 10 mm left 0.4 mm radius and terrible marks. Adding a dedicated 6 mm bull-nose (1.2 mm corner rad) semi-finish pass followed by a 3 mm ball-end increased measured radius to 1.45 mm (still visually sharp) but dropped peak corner stress 38 % in FEA and eliminated cracking in 25 000-cycle pressure tests. Port diameter stayed within 0.018 mm of nominal because stock was controlled asymmetrically.

Material-Specific Radius Guidelines

– 6061-T6 and 6082: forgiving. r ≥ 1.0 mm is usually enough for most structural loads. – 7075-T6/T7451 and 7050: aim for r ≥ 2.5–3.5 mm on fatigue-critical pockets. – Ti-6Al-4V: r ≥ 2.0 mm minimum, 3–5 mm preferred for cyclic pressure. – 17-4PH and 15-5PH: r ≥ 2.0 mm plus polished or peened surface. – H13 and P20 mold steels: r ≥ 4–6 mm on deep ribs to prevent corner erosion.

High-strength steels (4340, 300M) and nickel alloys often need compressive residual stress more than pure geometry, so light wiper passes or ceramic tool finishing after the radius is opened can be more effective than chasing an extra millimeter of radius.

Measuring What You Actually Machined

Never trust the nominal radius from the program. Use a 1 mm or 2 mm ruby ball on the CMM and probe the corner in a minimum of eight points. Optical comparators with 50× magnification work well for smaller parts. Surface roughness in the corner (Ra < 0.8 μm typical target) correlates strongly with fatigue debit.

Conclusion

Corner radius optimization is one of the few areas in CNC milling where a relatively small change in process planning gives huge returns in part life and reliability. The difference between accepting the leftover radius from roughing and deliberately controlling it with the right tool sequence is often a factor of two to four in fatigue strength, with almost no penalty in cycle time or tolerance capability once the method is standardized.

Shops that build standard tool libraries (8 mm bull 1.5 r, 10 mm bull 2.0 r, etc.) and corresponding verified programs see scrap rates on fatigue-critical pockets drop from 5–15 % to under 1 %. Designers learn to trust manufacturing input on minimum radii because the data is there: parts pass certification, field failures disappear, and the machinists stop spending nights hand-blending corners.

Next time a new part comes in with dozens of sharp internal corners, don’t just load the biggest tool and hope. Pull up the fatigue allowables, pick the bull-nose that gives the radius you need, adjust the stock strategy, and machine the corner correctly the first time. That is how you keep tight tolerances and still ship parts that last.