Fillet vs Chamfer: What’s the Difference in CNC Machining?

1. What is a Fillet in Engineering?

In engineering, a fillet means intentionally smoothing out the sharp edges or corners of a part, creating a smooth, rounded transition between two intersecting surfaces.

Fillets are typically produced using CNC fillet tools and can be either convex (outside curves) or concave (inside curves).

Simply put, a fillet replaces a sharp interior or exterior corner with a smooth, continuous arc.

(Image required: Diagram showing a sharp internal corner vs. a filleted internal corner, and a sharp external corner vs. a filleted external corner.)

1.1. Types of Fillets

There are two common ways to classify fillets in engineering and mechanical design:

By Location (The CNC Shop View)

This is the most practical way to talk about fillets because it tells the programmer where the tool needs to go.

Internal Fillet: This is the curve you find on an inside corner—for example, the bottom of a pocket where two walls meet. What is a rounded corner called is internal fillet.

External Fillet: This is the curve on an outside edge or corner. When it’s on an exterior edge, we often just call it a Round.

By Geometric Shape (The Designer’s View)

Concave Fillet: A curve that bows inward, like the interior of a bowl. (This is the same as an Internal Fillet.)

Convex Fillet: A curve that bulges outward, like a dome. (This is the same as an External Fillet/Round.)

An Internal Fillet is concave, and an External Fillet is convex. They are just two ways of describing the same fundamental geometry.

((Image required: A 3D model clearly marking and contrasting an Internal Fillet and an External Fillet/Round.)

CNC Terminology Tips:

While all the terms above are correct, here’s what to keep in mind when talking CNC shop:

• In the CNC world, we usually prefer Internal/External Fillet because it tells us exactly where we need to run the tool.
• Be aware that in some CAD software (like SolidWorks), the Fillet command is often used for the concave (internal) corner, and a separate Round command is used for the convex (external) corner. It can be confusing, but remember they are both a type of rounded corner!
• But some software, like SolidWorks, uses “Fillet” as a general term and offers both concave and convex options within the tool.
• To be absolutely clear and avoid errors, using the terms “Concave Fillet” and “Convex Fillet” is the safest bet, as they rely only on pure geometric shape.

1.2. Fillet Callouts

In modern engineering drawings and CNC manufacturing, fillets are primarily called out using the radius symbol (R), following key international standards such as ISO 129-1, ISO 8015, and ASME Y14.5.

ISO 8015 establishes fundamental rules for geometrical tolerancing, ensuring clarity and consistency.

ISO 129-1 defines the methods for dimensioning and tolerancing, including rules for radius callouts.

ASME Y14.5 is the core standard for dimensioning and tolerancing in the United States.

Following these standards ensures two things:

Clarity: There’s no room for guessing (no vague “sharp corner” notes!). The machinist knows the exact size needed.

Consistency: Every fillet on the drawing is communicated using the same, unified method.

Here are the most common ways you’ll see fillets documented on a drawing, moving from the most specific to the most general:

1.2.1. Direct Dimensioning

The most straightforward method: a dimension line points to the fillet, labeled with the radius symbol and value (e.g., R5 for a 5 mm radius).

Used for individual, critical, or varying fillet sizes.

(Image: Example of a radius dimension on a drawing)

1.2.2. Note-Based Callouts

Ideal when multiple fillets share the same size.

Local Note: A leader points to one fillet with a note like R3 TYP (where “TYP” means “typical,” indicating repeated features) or 4X R2 (meaning “four fillets, R2 each”).

General Note: Placed in the drawing’s notes section, e.g., BREAK ALL SHARP EDGES TO R0.5.

(Image: Example of local and general notes)

1.2.3. Limit-Based Callouts

Used when a fillet must not fall below a minimum size (e.g., for tooling or strength).

Format: R0.5 MIN (the radius must be at least 0.5 units).

(Image: Example of a minimum radius callout)

1.2.4. Model-Based Definition (MBD)

In advanced manufacturing today, the 3D CAD model is the true master blueprint. All the fillet sizes are already stored directly inside the 3D file.

The 2D drawing often serves only as a reference or a place to add critical notes.

Machinists use the 3D model to program the machine, making the digital feature itself the most accurate “callout.”

(Image: 3D model with embedded fillet definitions)

1.2.5. Implied by Linear Dimensions

If all corners of a part have the same fillet, the overall length and width dimensions may imply the fillet’s presence.

The manufacturer calculates the stock size based on these dimensions and the specified radius.

(Image: 3D

Key Considerations in Fillet Dimensioning

• Internal vs. External:The way we call out the R value is the same for both internal and external fillets. However, the designer must remember that a large external fillet might leave a very small, hard-to-clean corner (a “heel”) on the adjacent internal feature.
• ·Theoretical vs. Actual: If a fillet is absolutely critical to how a part fits or functions, its radius might be placed in a box (a Theoretical Exact Dimension). This means it  is the perfect size used as a baseline for other geometric tolerances.

1.3. How to Design Fillet Radius

How to Design Internal Fillet Radii

Designing an Internal Fillet isn’t just a random choice; it’s a careful balance between structural strength, manufacturing cost, and tool availability.

Here’s a friendly, step-by-step approach to get it right.

Step 1: Define the Function & Load

Start by asking the basics: What is this fillet meant to do? Is its main job to reduce stress concentration, make assembly easier, improve fluid flow, or is it more for safety and looks? Also, understand how the part will be loaded—is it a constant static load, a repeating fatigue load, or a sudden impact?

For features like shaft shoulders or bosses: Try to make the ratio of the fillet radius (r) to the section height (d) as large as you can. A great rule of thumb to start with is r/d > 0.1.

For internal corners (like where a wall meets a base): A good practice is to make the fillet radius at least 20-30% of the local wall thickness.

Step 2: Set the Initial Radius 

Now, let’s get practical and factor in how the part will be made.

General Wisdom: Brittle materials need more generous fillets. Remember, a very small radius does little to reduce stress.

In general, a larger radius means lower stress concentration and better fatigue life.

For CNC Machining (Your Go-To Rules):

Match Your Tool:

Your internal fillet radius must be at least as large as the cutter radius you plan to use (e.g., for a Ø4mm tool, your radius should be at least R2). To save time and money, always use standard tool sizes (like R0.5, R1, R1.5, R2, R3, R5). Designing a fillet that requires a custom tool is a recipe for added cost and delay.

Mind the Wall Thickness: 

For critical areas, a good guideline is to make the fillet radius (R) at least one-third (⅓) of the thinnest wall thickness (t) it connects. So, R ≥ t/3.

Common Range: 

For most CNC milled or turned parts, fillets between 1–6 mm are common (down to 0.5–1 mm for very small parts). Go larger for highly stressed areas.

For Sheet Metal Bending: 

The inner bend radius is typically close to the material thickness (R ≥ t).

Step 3: Verify and Refine the Design

Once you have a starting radius, model it in your CAD software. This is the time to double-check for any assembly conflicts, part interference, or to ensure fluid flow paths are smooth.

Step 4: Validate with FEA (If Needed)

For parts under significant stress, run a Finite Element Analysis (FEA). This will show you the real stress levels and allow you to fine-tune the fillet radius to meet all strength and fatigue requirements. 

Step 5: Finalize with Your Manufacturer

Have a chat with your machining partner! Confirm the standard tools they have available and agree on tolerances. Then, update your drawings clearly.

On your drawing, call out the fillet radius explicitly.

If the size is critical, specify a tolerance (e.g., R3 ±0.2) or a min/max range (e.g., R2 MIN / R4 MAX).

If there’s some flexibility, you can specify a range like R2–R4 or simply state “R2 MIN” to give the machinist some discretion.

How to Design External Fillet (Round)

External rounds are generally simpler.

Their main goals are safety and aesthetics, so the rules are far less restrictive. The R value here is just about the final shape.

Safety & Ergonomics: A small radius of R0.5 to R2.0 is often all you need to effectively break a sharp, dangerous edge.

Aesthetics & Feel: This is about product style. For instance, consumer electronics often use larger, palm-friendly radii for a premium feel.

Assembly: If the rounded edge needs to fit with another component, you’ll need to control its tolerance to ensure a proper fit.

1.4. How Fillets are Made in CNC Milling

In CNC milling, the way we create an internal fillet is quite different from how we create an external one (often called a “round”). Understanding this difference is key to designing parts that are both functional and efficient to produce.

Producing Internal Fillets (Concave Fillets)

Think of an internal fillet as the natural result of the tool’s shape. This is the most common method in CNC milling.

The cutting edge of the tool itself has a specific radius, and by running this tool along the edges of your part, it directly creates the fillet.

Tool Selection is Key

The most important rule to remember is this: The radius of your internal fillet is fundamentally limited by the radius of the cutting tool. You must choose an end mill with a radius that is equal to or smaller than your desired fillet radius: Tool Radius ≤ Fillet Radius

A Helpful Tip: Go Big for Speed

To maximize material removal and shorten machining time, a CNC programmer typically chooses the largest diameter tool that can still create the required fillet.

For example, to make an R5 fillet, they would ideally use a 10mm diameter end mill (which has a 5mm radius).

How It’s Programmed

The programmer uses circular interpolation commands (like G02 or G03) in the G-code to guide the tool’s center point around the corner. This precise path leaves behind the perfect radius in the material.

Producing External Fillets / Rounds

The Fastest Way: Use a Specialized Tool

The quickest and most precise method is usually to use a ball nose end mill or a dedicated corner-rounding tool.

These tools have a pre-ground profile that matches your desired external radius exactly. They can create a perfect, consistent round in a single, efficient pass along the edge.

The Flexible Way: Use a Standard Tool

It’s also possible to create an external round using a standard end mill.

However, this requires the programmer to define an exact and often complex tool path, guiding the side of the tool along the edge to “sculpt” the rounded shape.

While flexible, this approach is generally slower and more programming-intensive than using a dedicated corner-rounding tool.

1.5. How to Measure Fillets

After a part is machined, we need to check if that R value (radius) and its tolerance meet the drawing specifications. The method we use depends entirely on the required precision and the location of the fillet.

Measuring External Fillets / Rounds (The Outside Edge)

Checking an internal fillet is tricky because the measurement tool often has limited access.

1. Radius Gages (The Quick Check) 📏

What they are: These are sets of precisely cut metal blades, where each blade has a known, calibrated radius.

How it works: This is the quickest and simplest method. You simply place the metal blade against the machined fillet.

If there is little to no gap (no light shining through) between the blade and the part’s surface, the radius is a match!

The Catch: This is a Go/No-Go check. It confirms the nominal size, but it cannot give you a precise number for how far the actual radius deviates from the target.

2. CMM or Vision Systems (The High-Precision Check)

What they are: A Coordinate Measuring Machine (CMM) uses a probe that physically touches the part, or a Vision System uses a camera to scan it. This is the gold standard for high-tolerance parts.

How it works: The probe or camera traces (or scans) multiple points along the arc. Specialized CMM software then uses complex math to calculate the Best-Fit R Value and determine exactly how much it deviates from your design.

Measuring External Fillets / Rounds (The Outside Edge)

External fillets are easier to check because they are fully exposed and accessible.

1. Radius Gages (Quick and Easy)

Just like with internal fillets, you use the calibrated metal blades to quickly check the external curve against the specified R value.

2. Specialty Calipers

Some Vernier or Digital Calipers are equipped with curved jaws designed to sit flush against the fillet’s contact points. This allows the tool to provide a more direct and accurate measurement of the radius.

3. Optical Comparator / CMM (The Gold Standard)

Optical Comparator: The part’s silhouette is projected onto a screen and compared against a magnified template of the design. This lets you visually check the entire profile of the fillet.

CMM: Again, this provides the most detailed, computerized analysis of the R value and the overall profile, perfect for critical parts.

Caliper: A standard caliper can sometimes be used to estimate the radius by measuring the change in the edge profile, but this is often considered a quick estimate, not a precision measurement.

Key Takeaways of Measuring Fillets:

For a quick, practical check, reach for your radius gages. When you need precise data for quality control, the CMM is your best friend. And for a handy direct measurement of external rounds, radius caliper jaws are a great tool to have.

Internal vs. External Fillets: Key Differences in Design, Manufacturing, and Inspection