Time to read: 8 min

Fillets are one of those mysterious design features for which there seem to be no clearly defined rules. Either a part is entirely devoid of fillets, and most or all edges are well-defined, or the part’s designer decided to take the opposite route, and every single edge and corner is rounded with some size of fillet radius. Fillets are added to increase the strength (by reducing the stress concentration) of the edge of a part or improve the part’s look. 

There are two components of fillet engineering: designing fillet geometry and designing fillet machining. A fillet is created between the adjacent lines and faces in 2D and 3D CAD models, curving the surfaces between two lines or planes. In machining, a fillet occurs along the part’s edge and is most likely made with a CNC fillet tool such as a corner rounding end mill tool. This end mill cutter fillet machine tool is programmed to follow the edge of the part and it creates a high-quality fillet. 

Fillets are still useful and even increasingly relevant in the 2022 design world, especially when parts are destined for CNC machining, which will be the primary assumption in the following examples. This article will expose the cases in which fillets are not beneficial, optimal, and necessary (hint: corner fillets), so you can start tweaking your designs to be more cost-effective and more readily manufacturable.

Fillets vs Other Design Features

Fillets can be easily confused with other design features such as corner radiuses, chamfers, and bevels. All these features create a similar design whereby a sharp edge is somehow broken. A fillet is a sharp-edge-breaking feature that specifically adds a radius, either concave or convex, to the interior corner of a part. A fillet differs from chamfers and bevels because they don’t have a radius. A chamfer is typically a 45-degree angle added to the edge of a feature design. While a bevel is a slope from a horizontal or vertical edge. A fillet radius rounds inside corners and a corner radius rounds an external corner of the manufactured part.

Bevel & Chamfer

Non-Beneficial Uses of Fillets

Before discussing the optimal utilization of fillets, it is important to explore where usage of fillets is not beneficial to your overall part design. Unnecessarily adding on fillets could result in higher costs, without any added benefit. 

1. Don’t Design Fillets for 3D Printed Parts

Because 3D printing is an additive process, there’s no need to design a part assuming a tool will need to move around it and remove material, and a designer has much more freedom to utilize intricate and unusual geometries. Fillets are sometimes added for stress relief in areas of sharp geometry changes, but beyond that, there is little need for them. Pockets and internal features on printed parts can be angular or sharp-cornered, and you can even have cavities that are completely enclosed by surrounding material!

intricate 3D printed part

Also, keep in mind that if you’ll eventually be moving away from 3D printing towards another process, such as CNC machining, you must start planning for the limitations of that process early on, to save time and money down the road.

2. Don’t Design Fillets for Bottom Edges

Filleting the bottom edges of pockets, walls, blind holes, or boss features can be used to improve the aesthetics of a part or add strength to features (by reducing stress concentrations). However, fillets in these locations require the use of a ball endmill and will always make your part more expensive than square-bottomed features. This is because programming such a geometry usually requires 3D machining operations (which take longer to dial in). Also, ball endmills are by nature more fragile than their square counterparts and must machine at a much slower rate.

It turns out that modification of other geometric features, such as hole depth or proximity of the hole to other volume-removing features impacts the stress at the bottom of the hole or cavity more than a fillet at the bottom would. In addition, a design change to modify these features will be significantly more cost-effective than adding a complex fillet into the bottom of the cavity. For more information on how the stress concentration within a hole’s geometry is calculated, please see the equations here.

unnecessary fillets in a 3D part design

Optimal Utilization of Fillets

This next section will provide a few examples where fillets may come in handy, despite not being needed. 

1. CNC Fillets

Remember, however, that CNC fillets on a CNC machined part add programming and machine time— and therefore cost.  The advantages of fillet engineering will be specific for each part of your CAD design.  However, fillet edges are typically used to remove stress concentration at corners or edges. 

Fillets can also be created without the use of CNC machining manufacturing. Parts can be welded to create lap, corner, and T joints. Weld metal is placed in these corners to secure these joints, and a fillet weld is created at the corners. 

The experienced engineers at Fictiv know how to expertly utilize fillets to optimize the design of your CNC machined components, from simple to complex. They know how to use these fillets to distribute corner and edge stress onto the larger surfaces of each part, therefore, preventing deformation and failure. 

2. Cosmetic Face Edges

When designing a part with cosmetic faces, CNC filleting the edges of these areas can be a nice way to give your part the appearance that its faces blend seamlessly together, rather than transitioning harshly. Since cosmetic fillets are not going to provide mechanical or strength properties you should add cosmetic fillets after the rest of the geometry is determined. But these features should be used sparingly because they will impact the cost of the machined assembly.

3. Fillet Welds

While designing or manufacturing a part or assembly you may need to join two or more components via a weld joint. When designing the weld joint between two faces, you may choose to use a fillet weld. Fillet welds are welds that follow along the angle between two flat surfaces. 

Fillet Welds

Most welded joints will ultimately have some sort of radius to them, due to the nature of how the molten weld filler metal flows in the joint, due to capillary action. So, inadvertently, a weld joint may have a natural fillet. Some design engineers choose to specify that a weld joint is machined post-welding to create a more visually aesthetic smooth surface as well.  

4. Improved Handling and Safety

Adding fillets can prevent injuries from sharp edges if your parts will be handled frequently, especially if they are of metallic origin. It is standard practice for machinists to break all sharp edges anyway, so unless you desire perfectly radiused edges or your parts have ergonomic features with radiused areas, you may refrain from specifying a filleted radius to reduce costs.  Radiused parts are designed to eliminate sharp edges in the CNC machined parts. A part may be geometrically designed to eliminate these sharp edges and protect those who may be handling the parts in the future.

drawing of a handle with fillets for safety

Pin/Fastener Insertion

Getting a dowel pin to engage with a kit press-fit hole or a fastener to align with its female threaded mate can be tricky if the fit is tight. Usually, a small chamfer (read: bevel) is added around the edge of the hole to aid in insertion, although a fillet can also help if desired. A fillet hole can prevent the movement of a screw and bolt and it is the best design for pin insertion.  When a pin, screw, or bolt is needed, a chamfer is the better option. A chamfered corner, the preferred CAD design for screws, bolts, and pins, provides more easy insertion and is hidden from the stress that an external fillet endures as the chamfer corners are hidden within the part.  Chamfered corners do endure natural wear due to having sharper corners but are better suited when it comes to fitting mated parts together through screw, bolt, or pins. 

Part drawing with fillet on the hole and chamfer on the pin for easier assembly

Where Fillets are Necessary

This final section explores three cases in which fillets are required for a part to be optimally machined.

1. Internal Edges Between Vertical Walls

To cut via high-speed rotation, all CNC tooling is round and axially symmetric, so cutting a square corner between two vertical walls is impossible. Any edge where two vertical walls meet at an angle less than 180° requires fillet addition. This is the most common piece of DFM feedback we choose to give here at Fictiv about parts destined for CNC.

part drawing with internal edges filleted properly for CNC machining

2. Internal Edges Between Angled/Organic Surfaces

Like the first case in this section, edges between angled or organic surfaces with less than 180° between them also need fillets. If these edges aren’t perfectly vertical, they’ll be cut with a ball endmill, and the radius of that tool is the smallest fillet size that can be left between the surfaces.

3. Vertical Wall + Angled/Curved/Organic Surface

In a combination of the first and second cases, you’ll need to include fillets when a vertical wall on your part meets with an angled, curved, or organic surface below it. This one can be a bit tricky to reason at first, but if you picture a square or ball endmill cutting flush along a wall, you can visualize how there will always be material remaining between the wall and the surface below unless that surface is perfectly flat and normal to the tool.

Part with organic surfaces

Fictiv Standards

Now that you understand the general cases for and against fillet use, there are two main standards to adhere to if you’ll be producing parts through the Fictiv platform.

Minimum Fillet Size

The smallest milling tool our vendors stock by default is a 1/32” endmill (square and ball). This is just under 0.8mm in diameter, meaning the smallest fillet it can create is 0.4mm.

Fillet Size vs. Depth of Cut

Endmills come in lengths of standard multiples of their diameter, but there’s a limit to the obtainable length, due to tool vibration and chatter past a certain ratio. Material also plays a role here—it is much easier to cut a deep pocket into a plastic or cnc aluminum than into a harder material, such as steel. What this means for fillets is that they need to be a certain size, depending on how deep a cut is needed to make the feature on which they’re included. Fictiv’s max depth of cuts are as follows:

  • Steels: 5X tool diameter (10X fillet size)
  • Plastics/aluminum: 10X tool diameter (20X fillet size)

Overall, we recommend sticking to 3-5X tool diameter max, to the avoid sticker shock caused by excessive machine time.

drawing of a filleted pocket

Key Facts

  1. A fillet in manufacturing is the intentional rounding of a sharp edge or corner.
  2. A fillet is machined using a CNC fillet edge tool or a similar rounded tool which creates a convex or concave round at the intersection of two surfaces.
  3. A fillet is a rounded surface whereas a chamfer is a flat surface. Both are between the intersection of two surfaces. 
  4. A fillet provides a better stress concentration relief than a chamfer.
  5. Although fillets may add a negligible amount of increased cross-sectional surface area, their primary benefit is that they lower stress concentration in the area they are applied to.