Fillet vs. Chamfer: Which Design Choice Wins for Your Project?

Deciding between Fillet vs. Chamfer? Learn the structural, cost, and manufacturing differences to optimize your CNC and sheet metal designs.

Choosing between a rounded edge or a sloped one might seem like a minor aesthetic decision, but in the world of precision manufacturing, it’s a choice that impacts your budget and part durability. If you’ve ever stared at a CAD model wondering which edge break to use, you’re in the right place. In this guide, I’ll break down the Fillet vs. Chamfer debate so you can design smarter and manufacture faster.

At Baosheng Industry, we see hundreds of designs weekly where a simple change from a fillet to a chamfer—or vice versa—saved the client thousands in tooling costs. I’m going to share the mechanical pros and cons of each, along with the “hidden” costs that most designers overlook.

Understanding the Anatomy: Curved vs. Angled

Before we dive into the “why,” let’s clarify the “what.” A fillet is a rounded corner or edge, usually designed to reduce stress concentrations or for safety. A chamfer is a sloped or angled edge, typically at 45 degrees, used to ease assembly or provide a clean, decorative finish.

While they both “break” sharp edges, they interact with cutting tools and stress loads in completely different ways.

When Stress Distribution is Your Top Priority

If your part is going into a high-stress environment—like an engine mount or a structural bracket—the fillet is almost always the winner.

Sharp corners act as “stress risers.” When a load is applied, the stress concentrates at that sharp point, leading to cracks and eventual fatigue failure. A fillet distributes that stress over a larger surface area (the radius). In engineering terms, a larger radius equals a lower stress concentration factor. For load-bearing internal corners, always lean toward a fillet.

The Hidden Cost of the Fillet

Here is the industry secret: Fillets are often more expensive to machine than chamfers.

In CNC milling, a fillet on an external edge requires a specialized radius cutter or a complex 3D surfacing toolpath with a ball-end mill. This increases machine cycle time significantly. If you specify a 5mm fillet but the shop only has a 4mm radius tool, they have to “interpolate” the curve, which drives up the price.

Pro Tip: If the edge is purely for safety (so the operator doesn’t get cut), use a chamfer. It’s faster to cut with a standard countersink or chamfer tool.

Why Chamfers Win in Assembly and Alignment

If you are designing parts that need to fit together—like a pin going into a hole—the chamfer is your best friend.

The angled slope of a chamfer acts as a “lead-in” feature. It guides the mating part into position, making manual or robotic assembly much smoother. In sheet metal fabrication, chamfers are also preferred for exterior corners because they are less prone to “deforming” during the bending process compared to complex radii.

Material Matters: Aluminum vs. Hardened Steel

Your material choice should influence the Fillet vs. Chamfer decision.

• Aluminum:Soft and easy to mill. You can get away with fillets here without a massive cost penalty.
• Stainless Steel/Titanium:These materials are “work-hardened.” Every second the tool spends rubbing against the surface to create a radius increases tool wear. For these tough metals, keep your design simple with chamfers whenever possible to save on tool replacement costs.

Design for Manufacturing (DFM) Summary

To keep your production lean, follow these three rules:

• Use Fillets for internal cornersto increase part strength and prevent tool breakage (since CNC tools are round anyway).
• Use Chamfers for external edgesto reduce machining time and simplify assembly.
• Standardize your sizes.Don’t use a 3.1mm radius and a 3.5mm radius on the same part. Pick one and stick to it so the machinist doesn’t have to swap tools.

Final Thoughts on Edge Selection

The Fillet vs. Chamfer decision isn’t just about looks; it’s about balancing mechanical integrity with manufacturing efficiency. By understanding how these edges are actually cut in a shop, you can create designs that are not only functional but also cost-effective to produce at scale.