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In product design, details often determine the success or failure of a product. Corner modification, a seemingly small design element, actually plays a crucial role. The most commonly used corner modifications are divided into filleting and chamfering.

Fillet and chamfer design is one of the essential skills for product designers. Compared to sharp corners, fillet and chamfer designs are more user-friendly, visually smoother, and more capable of showcasing the simplicity and beauty of a product. Moreover, the creative use of fillet and chamfer can make your product stand out.

Today, let’s talk about fillets and chamfers in product design, with plenty of fillet images attached at the end of the article.

What Are Fillets And Chamfers?

Filleting refers to connecting two curved surfaces of a part with a tangent circular surface. The standard fillet is usually represented by a radius value R to indicate its size.

Chamfering refers to connecting two curved surfaces of a part with an angled surface. The standard chamfer is usually represented by a distance value C to indicate its size, with the standard angle being 45°.

In product design, both of them are the process of cutting a beveled edge on the corners or edges of an object. This beveled transition can be either straight or curved, depending on the design requirements and functional purposes.

Functions of Chamfers and Fillets in Products

Safety:

Chamfers and fillets can reduce the sharpness of product edges, preventing users from getting injured during use. This is especially important in the design of handheld devices or children’s toys.

Aesthetics:

Proper chamfering or filleting can enhance the product’s appearance, making its lines smoother.

Ergonomics:

In the design of handheld devices or tools, chamfers or fillets can improve the grip, making the product easier to handle and enhancing the user experience.

Durability:

Chamfers and fillets help reduce stress concentration, thus decreasing the likelihood of material breakage in these areas and extending the product’s lifespan.

Simplifying Assembly:

When different parts of a product need to fit together, chamfers and fillets can act as guides, helping parts align correctly, simplifying the assembly process, and increasing production efficiency.

Consistency in Design Language:

Chamfers and fillets can be part of the design elements, creating a unified style with other features like rounded corners and curves, enhancing the product’s brand recognition.

Differences Between Fillets and Chamfers

Both fillets and chamfers can be used to dull sharp edges or create a gradual transition between two adjoining surfaces. In this respect, they are interchangeable. However, in many applications, there are distinct differences between fillets and chamfers.

1. Processing Methods

For standard machining, vertical inner and outer fillets can be easily processed, while inner chamfers cannot. Fillets require specific tools to achieve various radius sizes (it is good practice to have a fillet radius larger than the tool radius in the machining process). In contrast, chamfers do not need specific-sized tools; different-sized chamfers can be made by controlling the depth of cut.

Horizontal inner and outer fillets require more processing time (unless special fillet tools are used), while chamfering takes much less time.

It is important to note that for CNC machining, the processing time for fillets and chamfers is comparable.

2. Functionality

Both fillets and chamfers can reduce stress concentration and optimize flow. However, fillets provide a continuous transition, whereas chamfers provide a linear transition, leaving an edge after chamfering. Compared to chamfers, fillets offer lower stress concentration and less flow resistance, making them preferable in plastic parts unless the edges of the chamfer are further filleted.

However, if your design involves screw holes, locating holes, or locating pins, using fillets is not a good choice. Compared to fillets, chamfers with smaller, sharper edges can help move down the hole more smoothly, making assembly easier.

Additionally, chamfers can create tapered countersinks, which fillets cannot.

3. Aesthetics

From an aesthetic perspective, fillets are often preferred in industrial design due to their visually pleasing characteristics. Fillets are soft and comfortable, giving a sense of safety and closeness. In contrast, chamfers give a sharp, aggressive feeling. Consumer products rarely use large chamfers, but chamfers can create unique looks in certain details, adding the finishing touch.

Depending on the design requirements, you can use both fillets and chamfers on exterior edges. If appearance is not important, such as in metal machined parts, you can choose simple chamfers to dull sharp edges, reducing part cost.

On the other hand, if aesthetics are important and fillets are necessary, you must consider the radius size of the inner fillet. If the radius is too small, you may require more expensive and time-consuming processes like EDM or wire cutting. Larger radii are easier to process (larger tools can be used, reducing machining time and increasing machining depth).

Factors to Consider When Choosing Between Fillets and Chamfers

Structural engineers often find themselves uncertain when deciding whether to use fillets or chamfers in their designs. To avoid this confusion, the following points should be considered:

For Machined Parts

Processing Time and Cost

If the goal is simply to blunt sharp edges to prevent injury, chamfers are a better choice as they require less time and are more cost-effective. However, if CNC machining is used instead of traditional milling, there is no significant time difference between fillets and chamfers; the only time difference is due to tool changes.

For edges like these (concave fillets), it is especially challenging to manufacture using CNC machines because they require specialized tools such as ball-end mills, which are fragile and cut at slower speeds. This increases manufacturing costs, so it is best to avoid fillets on such edges. If fillets are necessary, they should be as large as possible.

Coating Coverage

Fillets are easier to cover with coatings than chamfers. This is because fillets transition smoothly and tangentially to adjacent surfaces, which allows parts to adhere to thicker, more uniform coatings compared to the edges of chamfers.

Stress Concentration

If the design goal is to reduce stress concentration, fillets are a better choice. Fillets distribute stress over a larger surface area compared to chamfers of the same size.

Holes or Pins

For screw holes, locating holes, or locating pins, chamfering the edges is a better choice.

For Plastic Parts

Plastic parts are molded differently from machined parts, and their design tends to favor fillets with chamfers used for specific purposes (such as aesthetic details or structural guidance). The benefits of fillets in plastic parts include:

Flowability

In the injection molding process, the flowability of molten resin is crucial. Fillets provide less flow resistance, and larger fillets facilitate filling. Sharp corners in the original design can cause air entrapment due to vortices during injection, leading to localized overheating and burnt appearance. Optimizing with larger radii ensures smooth flow of the plastic melt.

Strength of Plastic Parts

When molten polymer flows over sharp edges, it causes shearing, which breaks polymer chains. Shorter polymer chains result in lower average molecular weight, reducing the strength and performance of the plastic part. Adding even small radii helps with flow and prevents shearing.

Stress Concentration

Fatigue failure is caused by crack propagation, primarily at stress concentration points and on the surface of components. At locations where cross-sectional dimensions change abruptly, such as corners, sufficiently large fillets should be used. As shown in the related curve, the effective stress concentration factor decreases rapidly with increasing radius. Fillets help distribute stress over a larger surface, preventing rapid deformation or cracking of stressed parts. This is especially true for materials like polycarbonate, which are prone to stress cracking if molding conditions are improper or part structure is unreasonable.

Ease of Mold Processing and Increased Mold Strength

Designing fillets in plastic parts increases the strength of the mold since the corresponding mold parts will also have fillets. This prevents cracking due to stress concentration during quenching or use, thereby enhancing mold strength.

Reduced Electrode Wear Rate

In mold processing, for areas like dead corners (where tools cannot reach), electrical discharge machining (EDM) is used, requiring electrodes (commonly known as copper tools). Sharp corners and edges on electrodes wear faster during discharge than flat areas. As a result, the actual machined dead corners have small fillets (about 0.2), differing from the design. This must be considered to ensure the structure is not affected. If plastic parts have designed fillets at dead corners, electrode wear is reduced, and the final machined fillets closely match the design.

Principle of Fillet Design

1. Ensuring Uniform Wall Thickness at Corners

To maintain uniform wall thickness, the outer radius ? at corners should equal the inner radius ? plus the wall thickness ?. The minimum inner radius is recommended to be no less than 0.5 mm. For chamfers, to maintain absolute uniform wall thickness, the outer chamfer should be offset by the wall thickness ? to form the inner chamfer.

2. Facilitating Mold Processing with Fillet Design

Fillets at parting lines complicate mold configuration. At the parting line fillet, the cavity must be transferred to the other half of the mold, increasing machining costs and potentially creating parting lines that affect the appearance of the plastic part. Therefore, unless necessary, fillets at parting lines should be avoided.

In the following image, the original design’s parting surface with small fillets complicates processing and subsequent polishing. The optimized design removes the parting surface fillets.

Additionally, any time mold cavities and cores meet to form holes or other features in parts, these issues arise and should be considered in the design.

3. Avoiding Visual Defects in Plastic Parts with Fillet Design

For internal structural features like ribs, screw bosses, and snaps connected to main walls, fillets should be carefully considered. Fillets in these areas can cause localized thickening of the material, leading to surface sink marks. If fillets are necessary for strength, the material should first be reduced before filleting. For example, a typical method to reduce material in screw bosses is creating a “crater” shape.

4. Software Operations for Fillet Design

1). Adding Fillets Last in Structural Design

• Benefit 1: Improves the overall modeling speed in software.
• Benefit 2: Avoids the hassle of removing fillets during draft angle creation.

2). Avoid using a single command to create all fillets. Group fillets by type or uniform size for easier subsequent modifications.

3). Using “Complete Fillet” Flexibly

Gallery of Various Fillet & Chamfer Designs