In mechanical design and manufacturing, fillets often sit in a gray area. Some parts are entirely devoid of fillets, with every edge sharply defined, while others feature rounded corners on nearly every possible edge.
Fillets are more than just cosmetic details — they can significantly improve mechanical performance by reducing stress concentration, preventing cracks, and enhancing ergonomics. Cependant, unnecessary fillets can increase CNC programming time, machining cost, and even complicate assembly.
Fillet engineering has two main components:
Fillet Geometry Design – Creating the rounded transition between two surfaces in a CAD model.
Fillet Machining – Producing the fillet in the physical part using CNC tools, most commonly a corner rounding end mill or ball end mill.
Dans ce guide, we’ll explore when fillets are essential, when they should be avoided, and how to design them for maximum performance and manufacturability.
Fillets vs. Other Edge Treatments
Fillets are sometimes confused with other design features such as chamfers, corner radii, and bevels. While they all modify sharp edges, they differ in geometry and function:
Filet – A smooth, curved transition between two surfaces, defined by a radius. Peut être interne (concave) ou externe (convexe).
Corner Radius – Specifically rounds an externe edge of a part.
Chanfreiner – A flat, coupée inclinée (often 45°) that removes a sharp corner.
Biseau – A sloped surface extending from a horizontal or vertical edge, not necessarily at 45°.
Key distinction: Fillets have a true radius curve, while chamfers and bevels do not. Chamfers are often preferred for assembly features (par exemple., screw or pin insertion), while fillets are better at reducing stress concentrations.
When Fillets Should Be Avoided
Not every part benefits from fillets. Adding them in the wrong places can increase cost without improving functionality.
3D Printed Parts
In additive manufacturing, there’s no tool needing clearance to cut around features, so sharp internal corners are entirely feasible. Fillets may only be necessary in 3D-printed designs for stress relief in high-load areas or for aesthetic purposes.
Cependant, if you plan to transition from 3D printing to CNC machining later, incorporate manufacturable fillet sizes from the start to avoid redesign costs.
Bottom Edges of Cavities and Holes
Filleting the bottom edges of pockets, murs, or blind holes requires 3D machining paths and ball endmills, lequel:
Slow down material removal compared to flat endmills.
Increase tool wear and breakage risk.
Require longer setup and programming time.
Dans de nombreux cas, stress at the bottom of holes can be reduced more effectively by changing hole depth, wall thickness, or feature placement — far cheaper than adding complex bottom fillets.
CNC Machining Considerations for Fillets
Minimum Fillet Size Is Tool-Limited
CNC cutters are round, and they can only cut curves as small as their own radius.
The smallest standard tool most shops keep in stock is 1/32" (~0.8 mm diameter), which means the smallest fillet radius they can make without special tools is à propos 0.4 mm.
If you design a fillet smaller than that, the shop may need to order a custom cutter — and that means longer lead times and higher costs.
Conseil: Stick to common cutter sizes to keep costs low and avoid delays.
Depth of Cut Affects Fillet Size
The deeper a tool has to reach, the more it will flex (deflect) and vibrate. This limits how deep a small-diameter cutter can go without breaking or leaving a poor finish.
Here’s a rule of thumb most machinists follow:
| Matériel | Max Cutting Depth | Equivalent Fillet Size Limit |
| Acier | 5× tool diameter | 10× fillet radius |
| Aluminum/Plastics | 10× tool diameter | 20× fillet radius |
Exemple: If you use a 1 mm radius fillet cutter in aluminum, you can cut about 20 mm deep max before you run into trouble.
Bigger Radii Are Usually Easier
Machinists love larger fillets because they let them use stronger, bigger cutters that remove material faster and last longer.
A tiny radius may look sleek in CAD, but it could double machining time in reality.
Avoid Unnecessary Cosmetic Fillets
If the fillet is purely for looks (cosmetic), ask yourself: is it worth the extra machine time?
Parfois, just breaking the edge with a quick chamfer or a small radius done manually after machining is faster and cheaper.
Match Fillet Sizes to Standard Tools
Common fillet radii — like 1 mm, 2 mm, 3 mm, 6 mm — match up with standard cutters.
Odd sizes like 2.37 mm mean the machinist will have to interpolate (mill it in multiple passes), which takes more time.
Consider Alternative Methods
If you just need a rounded edge for strength (like in welded joints), you might not need to CNC machine it at all.
Fillet welding can create a rounded joint after the parts are assembled, skipping the extra machining altogether.
Optional Use Cases for Fillets
While not always necessary, fillets can improve certain aspects of part design.
Cosmetic Face Edges
A small radius can make surface transitions appear seamless and improve perceived quality. Use these only after finalizing functional geometry, as they increase machining time.
Safety and Ergonomics
Parts with sharp edges can cause injury during handling, especially in metal components. Machinists typically “break” edges by default, but if you need a precise radius for comfort (par exemple., poignées ergonomiques), specify it clearly in the drawing.
Assembly Assistance
Fillets can guide pins, arbres, or fasteners into place, mais chamfers are generally preferred for mating parts because they provide better lead-in without adding significant cost.
When Fillets Are Necessary
Inside Corners Between Two Vertical Walls
CNC cutting tools are round, so they can’t make a perfectly sharp 90° inside corner.
If your design has two vertical walls meeting in an inside corner, you have to add a fillet that matches (or is bigger than) the radius of the cutting tool.
Pourquoi? Without the fillet, the tool would leave extra material in the corner, and your part wouldn’t fit with its mating parts.
Exemple: Think of a square pocket in a metal plate — the corners will always be slightly rounded because the milling cutter is round.
Internal Edges Between Angled or Curved Surfaces
When two angled or curved surfaces meet, especially in organic or freeform shapes, they’re usually machined with a ball endmill.
The smallest fillet you can have there will be the radius of that ball endmill.
Pourquoi? If you try to make the curve tighter than the tool radius, the cutter simply can’t reach into the corner without leaving uncut material.
Exemple: On a car engine part with flowing, sculpted surfaces, all the internal transitions between curves will have built-in fillets based on tool size.
Where a Vertical Wall Meets a Sloped or Curved Surface
This one’s a bit trickier to visualize — imagine a tall vertical wall, and at its base, instead of a flat floor, there’s a slope or a curve.
If you try to cut the wall and the curve without a fillet, the cutter will leave a small strip of uncut material right at the junction.
Adding a fillet here ensures a smooth transition and avoids leftover material.
Exemple: In a mold cavity for a plastic part, the transition from a vertical side to a curved base almost always has a fillet so the cutter can move freely.
Engineering Benefits of Fillets
They Reduce Stress Buildup
Sharp corners act like stress magnets — when a part is under load, the force concentrates at that sharp edge, which can lead to cracks.
A fillet spreads that force out smoothly, so the part can handle more load without breaking.
Exemple: Think about bending a piece of metal — it almost always cracks at the sharp bend, not in the middle. A smooth curve fixes that problem.
They Help Parts Last Longer
In parts that see repeated loading and unloading (like machine arms or brackets), sharp corners can cause fatigue cracks over time.
Fillets reduce this “fatigue effect,” meaning your parts last more cycles before wearing out.
Exemple: Aircraft components almost always have fillets in high-stress areas to prevent fatigue failures.
They Make Parts Safer to Handle
If your part will be touched or assembled by people, sharp corners can cut hands or snag clothing.
A fillet removes that sharp edge and makes the part more comfortable and safe to work with.
Exemple: Handles, couvertures, and control panels often have generous fillets so they feel smooth in your hand.
They Can Improve Flow
In designs where liquids or air move around a corner, sharp edges can cause turbulence or block the flow.
A fillet allows the fluid to change direction more smoothly, Amélioration de l'efficacité.
Exemple: In piping systems, internal fillets reduce pressure drop and improve fluid movement.
They Look Better
A smooth rounded edge often gives a product a more “finished” and professional look.
While this is more about aesthetics than function, it can be important for consumer-facing products.
Exemple: The rounded corners on your smartphone aren’t just for comfort — they also make the device look sleek.
They Can Make Machining Easier
En usinage CNC, some sharp internal corners simply can’t be cut because tools are round.
By adding a fillet, you’re designing the part in a way that matches the tool’s shape, which can make machining faster and cheaper.
Key Facts About Fillets
A Fillet is Just a Rounded Edge
À la base, a fillet is simply rounding off a sharp corner where two surfaces meet.
It can be à l'intérieur a corner (concave) ou outside on an edge (convexe).
Exemple: The inside curve where the blade meets the handle on a kitchen knife is an internal fillet.
CNC Tools Make Fillets with Special Cutters
Most CNC fillets are made using a corner rounding tool ou un ball endmill.
These tools are shaped to naturally leave a smooth curve as they cut, so you get a consistent radius every time.
Fillets Are Curved, Chamfers Are Flat
It’s easy to mix them up, but the difference is simple: difference in details of fillet and chamfer
Filet = curve
Chanfreiner = flat angle
If you run your finger over the edge, a fillet feels smooth and rounded, while a chamfer feels like a straight slope.
Fillets Are Better for Stress Relief
When a part is under pressure or bending, sharp corners can create “hot spots” of stress that can crack over time.
Fillets smooth out the transition and spread that stress over a larger area, making the part stronger.
Exemple: This is why airplane window corners are rounded — sharp ones would crack under air pressure cycles.
They Don’t Add Much Material, But They Add a Lot of Strength
Adding a fillet barely changes the cross-sectional area of the part, but it can drastically reduce stress concentration.
That means you can improve strength and durability without making the part heavier or bulkier
They Can Save or Cost You Money
If a fillet matches a standard tool size, it can actually make machining easier.
But if it’s an odd size or in a tricky location, it can increase machining time and cost.
The trick is knowing où to put them and how big to make them.
Conclusion
Fillets in Usinage CNC are powerful design features — but only when used strategically. Par compréhension when they are required, when they can be skipped, and how to size them appropriately, you can reduce machining costs, improve part performance, and simplify manufacturing.
En cas de doute, consult with your machining partner early in the design process. Well-planned fillets can be the difference between a costly, overcomplicated part and an optimized, manufacturable design.
