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CNC Machining Design Tip: Optimizing Internal Corner Radii

Picture of Dean McClements
Written by
Aaron Lichtig
Updated by
 2 min read
Published September 29, 2020
Updated August 13, 2024

It's not just standard size tooling that will help you get internal corners right.

part rendering with internal corner radii

One of the ways to save on CNC machining costs is to leverage standard size tooling.

Anytime a manufacturer can use off-the-shelf tools (vs. custom), your parts will be less expensive to manufacture. This is particularly true for tapped holes and threading, where using standard tool sizes helps save on costs.

But for inside corner radii, it may actually be better to use a non-standard radius. This is because an endmill needs clearance to turn and continue milling while tracing the internal corner radii.

For example, if a part features a radius of 0.25″, and a standard endmill is also 0.25″, then tool would need to hammer the corner, come to a complete stop, pivot 90 degrees, and then resume cutting. This method not only slows down machining speed (adding costs), but also causes vibration, which creates chatter marks.

Not optimized: cutter path comes to a sharp corner
Optimized: center line of cutter path has a radius

Not optimized: cutter path comes to a sharp corner

As a best practice, we recommend modeling your design with at least 0.02″ – 0.05″ added to internal corner radii, or in this example, a dimension of 0.27″ – 0.30″. This allows the cutter to turn slightly, without coming to a complete stop—like the above diagram on the right.

The result will be not only cost-savings on your project, but also improved CNC parts.

For more cost-saving tips, check out our blog post Top 10 Ways to Save on CNC Machining, or visit our site for an instant CNC quote.

FREE CNC Machining Design Guide

Picture of Dean McClements
Dean McClements
Dean McClements is a B.Eng Honors graduate in Mechanical Engineering with over two decades of experience in the manufacturing industry. His professional journey includes significant roles at leading companies such as Caterpillar, Autodesk, Collins Aerospace, and Hyster-Yale, where he developed a deep understanding of engineering processes and innovations.

Read more articles by Dean McClements

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