Fillet Machining: Tips and Techniques for Precision Cuts

An Overview of Fillet

A fillet is a rounded edge created in the interior or exterior corner of connecting surfaces to eliminate sharp edges likely to cause damage or injury. It is added between the adjacent lines and faces in 2D and 3D CAD designs, turning the surface between two lines or planes into a curve. 

Fillets are typically soldered, welded, or brazed in joints in mechanical parts manufacturing. Fillet geometry is a line of convex function on an exterior corner and a line of concave function on an interior corner. Fillets are a critical feature employed on points and lines of expected high-stress concentrated edges to lower stress concentration factors in mechanical parts. Fillets significantly enhance mechanical parts’ strength and load-bearing capacity by ensuring uniform stress distribution over a larger area.

The rounding of an interior corner is referred to as a fillet radii. It is often used in load-bearing mechanical parts to enhance strength and quality. Hence, fillet radius is a typical standard allowance in a casting design. Product designers can achieve effective stress relief in mechanical parts with increased radius fillets. However, it is essential to consider radius size when employing a round fillet because a big radius might cause design complications.

An Overview of Chamfer

The chamfered edge is an angled or sloped corner often used to keep the edge of mechanical parts safe from damage and achieve a more uniform appearance. Unlike a fillet with smooth edges, a chamfered edge is sharp and straight. They are mainly used for low-stress concentration since they can deform the workpiece.

However, it is not an ideal option for high-stress concentration edges. However, it would be best to understand the chamfer edge vs. bevel edge for any design and how chamfer in AutoCAD functions to make comparisons and insightful decisions. While a bevel is a sloped edge on the top, a chamfer has a beveled edge at the bottom of two connecting surfaces.

There are two major classes of chamfer in mechanical engineering including:

• Chamfer with 45-degree employed for cutting burr in the drilling operation. It allows a bolt to sit flush below the surface instead of protruding.
• Chamfer with 60 degrees has sharp corners and is ineffective in reducing stress concentration. These angled edges are used as a lead-in for fasteners such as bolts and screws. Chamfering is a crucial post-processing process for mechanical parts to make bevels, furrows, or grooves.

Reasons Why Fillet Machining Is Important

Here are some of the instances of how fillets optimize the designs of mechanical parts:

Pin/Fastener Insertion

It can be tedious to achieve tight fits between a fastener and its female threaded material or engage a dowel pin with a kit press-fit hole. A small fillet is typically added around the hole’s edges to allow easy insertion, although a chamfer is sometimes used as an alternative. Product designers use a filleted hole circumference to prevent screw and bolt movement, even though it is the ideal design option for pin insertion.

Cosmetic Face Edge

CNC fillet machining edges of mechanical parts help enhance the part with cosmetic faces that blend smoothly. You might add cosmetic fillets after deciding the rest of the geometry since they don’t affect the mechanical or strength properties. However, it could be best to use these features carefully since they can considerably increase the cost of the machined assembly.

Improve Part’s Handling and Safety

Designing mechanical parts with fillets helps avoid injuries from sharp edges if it is a product to be handled frequently, especially those made from metals. Breaking all sharp edges is standard practice for machinists unless perfectly radiused edges or ergonomic features with radiused areas are a vital requirement of the part design. The primary purpose of radiuses is to eliminate sharp edges in CNC machined parts. In some cases, you may geometrically design parts to remove these sharp edges and ensure the safety of end users who will handle the items.

Where Is Fillet Machining Important?

Here are the expected cases when you can employ fillet engineering in your process:

Internal Edges between Planar Faces

Since most cutting tools are round and axially symmetric, cutting a square corner between two vertical walls at high-speed rotation may be impossible. Hence, you must add a fillet to any edge where two vertical walls connect at an angle of less than 180°. Our DFM feedback for CNC parts includes the addition of such fillets. More importantly, note that not all tooling for CNC mills is axisymmetric, and most CNC lathe tooling is the same.

Vertical Wall and Angled or Curved Surface

Fillet design is necessary when machining a vertical wall meeting with an aligned, curved, or organic surface beneath it. For instance, suppose you are cutting a flush along a wall with a square or ball end mill. Unless the surface is evenly flat and normal to the tool, the material will always remain between the wall and the surface below.

Internal Edges between Organic/Angled Surfaces

Edges between organic or angled surfaces with less than 180 degrees between them also need fillets. You will need a ball end mill to cut internal edges that aren’t perfectly vertical, and the tool’s radius is the smallest fillet size that can be left between the surfaces.

When Do You Need to Avoid Fillets in Parts?

Although fillets are essential features that improve overall part design, the unnecessary addition of fillets could cause an increase in costs without added advantages. Here are instances of when you should avoid using fillets:

For Bottom Edges

Adding fillets to the bottom edges of blind holes, pockets, or walls can improve a part’s aesthetics and strengthen features by lowering stress concentration. However, you will need a ball endmill that will make your part costlier than square-bottomed features because it requires 3D machining operations, which take longer to dial into a program such as geometry. Moreover, ball endmills are fragile compared to square-bottomed mills and have a slower material removal rate.

Furthermore, modifying other geometric features like proximity or depth of the hole to other volume-removing features influences the stress at the bottom of the hole more than the fillet at the bottom. Similarly, a design change to alter these features is significantly more cost-effective than machining a complex fillet at the cavity’s bottom

For 3D Printed Parts

3D printing is an additive technique that allows manufacturers to create parts with intricate designs and unusual geometries. Hence, designing a part that will require a machine tool to move around it to remove material is unnecessary. In some cases, designers add fillets to designs of parts to relieve stress in areas of sharp geometry changes. However, there is little need for these machined rounded edges. Pockets and internal features on 3D printed parts can be sharp, cornered, or angular, and you can also achieve cavities that are fully enclosed by surrounding material.

However, you must account for the limitations of other processes, such as CNC machining, if you decide to move from 3D printing (additive manufacturing) to save money and time in the long run

Fillet vs. Chamfer in Machining

Although fillet and chamfer can be easily confused with each other since they are design features between two surfaces and both suitable for outside edges, they possess distinct geometric attributes. While a fillet eliminates or smooths out sharp edges and corners, a chamfer doesn’t. It is a linear transition between surfaces at a 45-degree angle.

• Fillet machining can be tedious since maintaining the accuracy of fillet radius is a major challenge, especially in intricate designs. Despite the vast advantages of manufacturing fillet edges, they also require extra machining time and effort than chamfered edges. As such, it can affect the overall cost of machining.
• Fillets are curved edges that facilitate better stress flow on a large radius, preventing deformation. Hence, machinists employ fillets for exterior parts since less stress flow in chamfers can lead to material deformation.
• Although chamfers are simpler to machine, they are not ideal for parts exposed to repetitive stresses or high loads. Most especially, the sharp edge of a chamfer may be unsuitable or unsafe for end users in particular applications.
• Filleted edges are safer to use since sharp edges are absent. Meanwhile, machinists must use chamfers carefully due to their sharp edges.
• Fillet engineering requires specialized tools to achieve varying fillet radii. Conversely, chamfers don’t need tools of any specific size to produce various chamfer radii because one size tool will be suitable for the process.
• Fillets are better suited when it is necessary to coat a machined part. However, the sharp edges of the chamfer can easily cause rapid deformation of the material. More notably, fillets prevent accelerated rusting of material since they allow uniform distribution of paints and coatings.

Application of Fillet and Chamfer

Typical applications of fillet and chamfer edges include:

• Sheet Fabrication: Fillets are typical design features that ease sharp corners and edges of various metal sheets when producing components like panels, cabinets, compartments, and panels.
• Welding: Both edges are applicable in welding metal materials for filet and chamfered corner joints, arc joints, edge joints, L joints, and T joints.
• Construction: Manufacturers use fillet edges to assemble structural metals, while chamfered corners are best used for assembling trusses, shafts, columns, and beams.
• Piping and Ductwork: Manufacturers in the HVAC industries use fillets for ductwork rather than chamfer because of their heat resistance and thick layers. However, fillets and chamfers are suitable for plumbing applications such as joining and fitting pipes.
• Manufacturing: Fillets and chamfered corners help manufacture automobile parts, tooling, and machinery components.
• Carpentry: Fillet machining helps make furniture edges like bed edges, chair edges, handles, and table tops because of the curved corner’s concave and convex structures. Conversely, chamfers are better suited for joining vertical walls or wood panels like tables, chairs, cabinets, and wardrobes.

Cost-Effective Tips for Fillets and Chamfer Machining

Despite the simplicity of design and aesthetic appeal of chamfers and fillets, they can increase the machining time and costs of machined parts. So, how can you save production costs when machining fillets and chamfers? Here are other considerations for making chamfer and fillet machining cost-effective:

• Consider Part Production Volume: The cost per part will be significantly high when you produce only a few parts. Therefore, adding a fillet or chamfer design won’t be cost-effective. However, mass production runs prove cost effective. Therefore, adding a fillet or chamfer design would be cost-effective.
• Avoid Tight Tolerances: Expert machinists advise against tighter tolerance in parts with fillet or chamfer edges. Adding any of these design features will increase the cost per part since tighter tolerances demand frequent fillet and chamfer inspections.

Conclusion

Fillets and chamfers are essential design edges that help minimize stress concentration by distributing forces across larger surfaces. Both fillet and chamfer edges contribute to the extended lifespan of mechanical parts, providing improved aesthetics and safety. With this guide’s succinct knowledge, you can make informed decisions about chamfer and fillet machining.

AT-Machining can render professional assistance in choosing and specifying the appropriate radius for your fillet and chamfers. Our engineers and technicians can help you optimize the design of your CNC machined parts regardless of their complexities. Submit your CAD models for fast and reliable quotes!