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Design for Manufacturing: The Hidden Costs of Compound Radii

Writer's picture: Tai KimTai Kim

At KT Tech, Inc., we believe that successful manufacturing begins at the design stage, long before any material is shaped. We pride ourselves on our CNC precision machining services, but we also aim to offer our customers a superior experience well before delivering any parts—we want to enable an efficient, cost-effective project overall. This commitment lies behind our comprehensive Design for Manufacturing (DFM) guidance.


We have previously discussed critical DFM principles, such as the effect of tight internal corners. Today, we’re exploring compound radii, a design feature that significantly impacts machining time, costs, and lead time. Find out why simpler alternatives can often achieve the same goals more efficiently.


What Is A Compound Radius?

In machining, a compound radius is a curved surface that has two or more different radii that transition into one another. By contrast, a simple radius follows one consistent curve in a two-dimensional plane. The picture below demonstrates the difference between a compound radius (left) and a simple radius (right).


compound radii

Compound radii can be essential for certain functional or aesthetic purposes such as ergonomic, user-facing parts or aerodynamic surfaces. In addition, many parts designed for injection molding or casting will require fixtures with compound radii. However, some designs incorporate this feature when simpler solutions would suffice.


Why Compound Radii Add to Part Costs

Machining a simple radius is relatively straightforward. Essentially, a form cutter can follow the edge one or more times and the feature is complete.


However, a compound radius requires multiple different complex cutting paths. In our California machine shop, we typically employ a ball end mill for these operations, but the tool must cover significantly more ground than when creating simple radii. As efficiency experts with a focus on on-time delivery, we always explore alternative approaches when possible.


Cost-Effective Alternatives to Compound Radii

Compound radii substantially drive up costs. The increased machine time can impact both production costs and lead time, especially in large production runs. The costs become even more significant if the product is manufactured with our 5-axis machining services.


While we’re fully capable of machining compound radii in applications that absolutely require them, we always suggest more cost-effective alternatives to customers when we receive their designs. Many customers utilize compound radii for sharp-edge breaking, but alternative approaches are just as effective in many use cases. 


Option 1: Add General Note For All Radii

Instead of including a compound radius, customers can add a general note to their designs requesting, for instance, “blend all sharp corners to 0.030-inch radius unless otherwise specified.” (30 thousandths is only a suggestion—numbers can differ depending on the part.) This callout would allow us to instead leverage more cost-effective processes such as hand deburring or tumbling.


Option 2: Utilize Chamfers Instead of Compound Radii

Chamfers were a recommendation in our DFM blog on thread design, and they’re a helpful design alternative in this case, as well. Small chamfers can be machined with a ball endmill in far fewer cutting paths than compound radii, reducing multi-axis machining cycle time and costs while eliminating the need for specialized cutters.


Collaborate With Us on Your Next Design

At KT Tech, our commitment to manufacturing efficiency drives us to continuously seek ways to optimize your parts’ production. Whether you’re looking to reduce lead times, minimize costs, or achieve specific design requirements, our expertise can guide your decision-making process. From rapid prototyping to assembly services, our shop is ready to tackle your next project.


Request a quote today to discover how our DFM expertise can benefit you.

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