• Blogue

Have you ever designed a part with a perfectly square pocket, only to find that the internal corners are difficult or expensive to machine?

This is a common design challenge in CNC machining.

The reason is simple: a rotating milling cutter is round, so it cannot produce a perfectly sharp internal 90° corner through conventional milling alone.

That small detail in your CAD model can affect tooling, machining time, and the final cost of your part.

1. Why can't CNC milling create a perfectly sharp internal corner?

CNC milling removes material using rotating cutting tools called end mills.

Because the cutter has a fixed diameter, it leaves a radius where two internal walls meet.

For example, a 6 mm diameter end mill has a 3 mm tool radius. It cannot produce an internal corner radius smaller than 3 mm through ordinary milling with that cutter.

If your design requires a smaller radius, the manufacturer may need a smaller tool or an alternative machining strategy.

2. Why can a small corner radius cost more?

A smaller internal radius often requires a smaller cutting tool. This can create several challenges:

• Longer machining time: Smaller tools may remove material more slowly.
• Reduced tool rigidity: Thin tools are more susceptible to deflection and vibration, especially in deep pockets.
• More complex machining: Additional finishing passes or tool changes may be needed.
• Greater manufacturing difficulty: Deep pockets with very small corner radii can be particularly challenging.

The result? A feature that looks minor on a drawing may add unnecessary cost if the tight radius is not functionally required.

3. How can you design internal corners for easier machining?

Here are three practical tips for engineers preparing parts for CNC machining.

Tip 1: Use a larger internal radius where possible

If the corner radius does not affect assembly or function, consider increasing it.

A larger radius may allow the use of a larger, more rigid cutter and help reduce machining time.

Tip 2: Consider pocket depth and corner radius together

A small radius in a shallow pocket may be straightforward to machine. The same radius at the bottom of a deep pocket can be much more difficult.

When reviewing your design, consider both the radius and the depth rather than treating them as separate requirements.

Tip 3: Check whether a sharp corner is actually necessary

If a square component needs to fit inside a milled pocket, you may be able to add a small chamfer to the mating component or introduce a corner relief in the pocket.

These options can provide the required clearance without forcing the entire feature to use a very small cutter.

Of course, the best solution depends on the assembly, loading, and functional requirements.

4. What if your design really needs a sharp internal corner?

Sometimes, a sharp internal corner is essential for assembly or function.

In that case, do not compromise the design just to make machining easier. Discuss the requirement with your manufacturing partner.

Depending on the geometry, material, and production volume, alternatives may include EDM (electrical discharge machining), a designed corner relief, or splitting the component into multiple parts.

Each option has different cost and manufacturing implications, so it is worth evaluating before finalizing the design.

The takeaway

Good CNC design is not just about creating the geometry you want. It is also about understanding how that geometry will be manufactured.

Before sending your next STEP file or engineering drawing for a CNC machining quote, check three things:

• Are the internal corner radii larger than they need to be?
• Are any small radii located at the bottom of deep pockets?
• Could a simple design adjustment achieve the same function at a lower manufacturing cost?

A small design review before production can help you avoid unnecessary machining complexity while keeping the part fit for purpose.

At AluCarbon Tech, we believe that discussing manufacturability early helps engineers make better decisions about custom CNC-machined parts.

When designing a CNC-machined pocket, do you usually specify the smallest possible corner radius, or the largest radius your assembly can accept?

Share your approach in the comments.

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