This article is part of the DFM for CNC Machining: Design Rules and Tolerance Checklist on CNX Precision.
Internal corner radius machining is a basic rule that separates good CNC designs from expensive ones. Every milling tool is round, so it cannot cut a sharp inside corner. The radius left in the corner must match the tool, and the tool size controls speed, finish, and cost. This guide explains how to choose internal corner radius values that machine well.
Sharp internal corners look clean in a CAD model. In the real workshop, they force small cutters, slow passes, and long cycle times. In internal corner radius machining, a simple radius change in the model often saves hours of machining and hundreds of dollars per part.
Why Internal Corner Radius Machining Needs a Radius
A standard end mill is cylindrical. Its cutting edge sweeps a circle, and the smallest radius it can produce in an internal corner equals the tool radius. A sharp 90-degree corner would require a square tool, which does not exist in conventional milling.
Designers often request sharp internal corners without knowing the cost. The machinist must then use a tiny cutter, remove material slowly, or switch to EDM. All three options add time and money. A small radius change in the model avoids the entire problem.
The rule also applies to slots and grooves. A slot machined with a 4 mm end mill has rounded ends at a 2 mm radius. If the design needs square slot ends, the machinist must switch tools or add a separate operation. Therefore, think about the cutter size before you place tight features.
For this reason, internal corner radius machining is a core topic in any DFM review. It is one of the first details our engineers check when they open a new model. While internal corner radius is a small feature, its effect on cost is huge.
Internal Corner Radius Machining and Tool Size
The corner radius in a machined pocket is equal to the cutter radius, not the cutter diameter. For example, a 6 mm end mill leaves a 3 mm corner radius. If you need a 1 mm radius, the machinist must use a 2 mm cutter.
Small cutters are slower. They remove less material per pass, so cycle time climbs. They also break more easily, especially in steel and titanium. Therefore, the largest possible corner radius is almost always the cheapest option.
Here is a practical guide for internal corner radius machining in CNC milling:
| Corner Radius | Tool Diameter | Max Pocket Depth | Typical Use |
|---|---|---|---|
| 0.5 mm | 1 mm | 2-3 mm | Fine features, electronics |
| 1 mm | 2 mm | 5-8 mm | General pockets |
| 2 mm | 4 mm | 10-15 mm | Structural pockets |
| 3 mm | 6 mm | 15-25 mm | Large housings |
| 6 mm | 12 mm | 30-50 mm | Deep, high-volume pockets |
The depth column matters. A longer tool deflects more, so a deep pocket with a tiny corner radius is very difficult to hold in tolerance. Match the radius to the depth, and the machining process stays stable.
Recommended Internal Corner Radius Machining Values
Good internal corner radius machining starts with these rules:
- Set a minimum internal corner radius of 0.5 mm, or 0.020 in, on machined parts.
- Prefer 1 mm or 2 mm radii when the design allows.
- Keep the radius at least one-third of the pocket depth.
- Avoid radii below 0.25 mm unless the feature is very shallow.
- Use the same radius everywhere in the part to reduce tool changes.
For example, a 15 mm deep pocket should have at least a 5 mm internal corner radius. In practice, most designers use 3 mm or 6 mm because these match standard tool sizes. Standard sizes keep tooling simple and quotes low.
For deep pockets, consider a two-tool strategy. A large cutter clears the main cavity, and a smaller cutter finishes the corner. This keeps cycle time low and still gives the required radius.
In internal corner radius machining, surface finish also improves with larger radii. A 1 mm radius cut with a 2 mm tool leaves a good finish in aluminum. In steel, the same radius needs a slow feed, so a 3 mm radius often finishes faster and cleaner. When you use the same radius across a part, the machinist programs one tool path and one cutter.
Sharp Corners and Alternative Processes
Sometimes a sharp internal corner is truly required. A hydraulic manifold, for instance, may need a square pocket for a seal. In that case, the options are EDM, broaching, or an assembled design.
Wire EDM can cut a square corner with a small radius limited only by the wire diameter. Sinker EDM uses a shaped electrode and can reproduce a near-sharp corner. Both methods are accurate, but they add a separate operation and extra cost. EDM costs more per hour than milling, so reserve it for features that truly need it. In internal corner radius machining, remember that EDM and broaching are slower than milling.
Another option is to split the part. Machine the pocket in two halves, each with an open corner, then join them. This approach keeps milling fast and still gives a visually sharp internal edge.
Broaching is another option for straight slots with sharp corners. A broach tool pushes through the feature and cuts the final shape in one pass. This works well for keyways and simple profiles.
Frequently Asked Questions
What is the smallest internal corner radius for CNC machining?
Most CNC shops can hold 0.25 mm, but 0.5 mm is a safer minimum. Below 0.5 mm, the cutter is small and fragile, and the surface finish suffers. For production parts, CNX Precision recommends a minimum of 0.5 mm.
Does the corner radius affect the price of a part?
Yes, and the effect is large. A small radius forces a small cutter, more passes, and longer cycle time. Increasing the radius from 0.5 mm to 3 mm can cut machining time dramatically and lower the unit cost.
Can a 90-degree internal corner be machined?
A true 90-degree corner cannot be milled with a standard round cutter. You can achieve a near-sharp corner with EDM, or you can redesign the pocket with a radius. The best choice depends on the function and the budget.
Internal corner radius machining is not a cosmetic detail. It drives tool size, cycle time, surface finish, and cost. When you send a model to CNX Precision, our DFM team will flag every tight corner and suggest a practical radius. Start with a 1 mm radius, and your parts will machine faster and cost less.
For related information, see our guide to cnc machining service and 5-axis cnc machining and cnc milling vs turning and cnc machining tolerances.