Knowledge About Fillets In CNC Machining

Newly processed metal parts may end up with sharper edges than the end user expected, unless the design makes these sharp edges smooth. Fillets and chamfers are usually added to solid models to make them look good and eliminate sharp edges and corners. Many engineers easily use these features in their CAD models to improve functionality and enhance design. To create an edge break, the design should incorporate sloped or beveled edges and rounded corners called fillets.

What Is A Fillet?

In mechanical engineering, machinists refer to rounded corners as circular parts designed inside or outside edges.

They are three types of fillet mechanics: chamfered fillet, concave fillet and convex fillet. On the inside, the fillets are concave, while on the outside, they are convex. Engineers use fillets to reduce stress on parts. Therefore, the fillet helps to distribute the stress on the larger surface and prevent the stress part from deforming rapidly.

When the design needs to eliminate sharp edges and low stress concentration, fillet engineering is the best choice for machinists.

A fillet is a circular edge or corner of a part. Fillets help to reduce stress and distribute it over a larger surface, thereby improving load-carrying capacity and helping to prevent rapid deformation of pressure parts. The concave fillet is located inside the part, while the convex fillet is located outside. For example, when a vertical wall meets an inclined surface within a part, a fillet can also create a transition between features.

Fillets And Other Design Features

Fillets are easily confused with other design features such as fillet radius, chamfer, and bevel. All these features create a similar design in which sharp edges are broken in some way. Fillet is a sharp edge fracture feature that adds a concave or convex radius to the inner corner of a part. Fillets are different from chamfers and bevels because they have no radius. The chamfer is usually a 45 degree angle added to the design edge of the feature. And the slope is a slope of a horizontal or vertical edge. The fillet radius circulates the inner corner of the manufactured part, and the fillet radius circulates the outer corner of the manufactured part.

Non-Beneficial Uses Of Fillets

1. Do not design fillets for 3D printed parts

Since 3D printing is an additive process, there is no need to assume that the tool needs to move around the part and remove material to design the part, and the designer can more freely use complex and unusual geometry. Fillets are sometimes added in areas where the geometry changes dramatically to eliminate stress, but they are rarely needed. The pockets and internal features on the printing unit can be angled or pointed, and you can even have a cavity completely surrounded by the surrounding material!

2. Do not design fillets for the bottom edge

Rounding the bottom edge of a groove, wall, blind hole, or boss feature can be used to improve the aesthetics of the part or increase the strength of the feature (by reducing stress concentration). However, rounded corners at these locations require the use of ball end mills and will always make your part more expensive than the bottom features. This is because programming such geometry usually requires a 3D machining operation (requires a longer time to dial in). In addition, ball end mills are inherently more fragile than square end mills and must be processed at a slower speed.

It has been proved that modifications to other geometric features, such as the depth of the hole or the proximity of the hole to other volume removal features, have a greater impact on the stress at the bottom of the hole or cavity than the fillet at the bottom. In addition, design changes that modify these features will be more cost-effective than adding complex fillets at the bottom of the cavity.


Optimal Utilization Of Fillets

Edge of part

To avoid accidents during handling and inspection, it is safer to break all edges of the part design. However, chamfering is not always the best choice for part edges. Sharp edges may cause injury when handling. Therefore, in this case, you may need to select a fillet.

When designing parts with finishing, it is a good way for CNC to fillet the edges of these areas, so that the appearance of your parts can be seamlessly integrated without rough transition. Since cosmetic fillets do not provide mechanical or strength properties, you should add cosmetic fillets after determining the remaining geometry. However, these functions should be used with caution as they affect the cost of machined components.

Internal edges between vertical walls

To cut by high-speed rotation, all CNC tools are circular and axisymmetric, so it is impossible to cut a square angle between two vertical walls. Any edge where two vertical walls intersect at an angle less than 180 ° needs to be rounded.

Internal edges between inclined / organic surfaces

As in the first case in this section, rounded corners are also required for edges less than 180 ° between angled or organic surfaces. If these edges are not completely vertical, they will be cut with a ball end mill, and the radius of the tool is the minimum fillet size that can be left between the surfaces.

Vertical wall + bevel / curved surface / organic surface

In the combination of the first and second cases, when the vertical wall on your part meets the inclined, curved or organic surface below it, you need to include fillets.


Fillets in manufacturing are intended to round sharp edges or corners.

The fillet is processed using a CNC fillet edge tool or similar fillet tool, which forms a convex or concave circle at the intersection of two surfaces.

A fillet is a circular surface, and a chamfer is a flat surface. Both are between the intersections of the two surfaces.

Fillets provide better stress concentration relief than chamfers.

Although fillets may increase a negligible cross-sectional surface area, their main advantage is that they reduce the stress concentration in the area to which they are applied.

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