How to Design Undercuts for CNC Machining

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CNC machining is renowned for its precision in creating complex parts from a solid block of material. However, its subtractive nature presents a unique challenge: undercuts. An undercut is a recessed feature that cannot be accessed directly by a standard vertical tool. While often necessary for part functionality, poor undercut design can drastically increase cost, complexity, and lead time. Mastering their design is key to optimizing your parts for manufacturing.


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Understanding the Challenge

Standard CNC tools cut from above. An undercut’s geometry, typically an internal groove or a recess behind a protrusion, is hidden from this topdown approach. Machining these features requires specialized tools, like Tslot cutters or lollipop (undercut) end mills. These tools have a smaller cutting diameter than their shank, allowing them to reach under obstructions. However, they are less rigid and more prone to deflection and breakage, which limits the achievable depths and tolerances.

Key Design Principles for Effective Undercuts

1. Optimize the Undercut Dimensions: The most critical rule is to design the undercut to match a standard tool size. The depth of the undercut should not exceed the tool's cutting diameter. A common guideline is to keep the depth to 24 times the tool diameter for aluminum, and less for harder materials. Designing for a standard, readily available tool (e.g., a 3mm or 1/8" undercut mill) avoids the expense and delay of custom tooling.

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2. Provide Adequate Clearance: Ensure there is sufficient clearance above and around the undercut for the tool’s shank to pass without collision. The shank diameter is always larger than the cutting head, so the design must account for this to avoid gouging adjacent walls.

3. Consider TSlots and Dovetails: For features like slots for mounting, a Tslot is a classic undercut application. Similarly, dovetails are used for interlocking parts. When designing these, ensure the neck (the narrow opening) is wide enough for the tool to enter and that the geometry allows for proper tool pathing without interference.



4. Utilize OpenSide Designs: Whenever possible, design an undercut that is open on one or both sides. This allows a standard end mill to profile the feature from the side, completely eliminating the need for a special tool, reducing machining time, and improving accuracy.

5. Alternative Manufacturing Methods: For complex internal undercuts or highvolume production, consider if the part could be split into two components that are assembled later. This can often be more costeffective than machining a single, highly complex piece. For certain plastic parts, injection molding might be a more suitable alternative.

Partner with a CNC Expert

Successfully integrating undercuts requires a partnership with a skilled manufacturer. At our onestop CNC machining service, our engineers specialize in Design for Manufacturability (DFM). We analyze your designs and provide actionable feedback to optimize undercuts for strength, functionality, and costefficiency. By designing with manufacturing in mind, you unlock the full potential of CNC technology, resulting in superior parts, faster turnaround, and a stronger bottom line for your business.