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Creating Undercut Geometries with CNC Machining

Published on: 2023-07-28

Undercuts are unique features in a part’s geometry that are hidden or tucked beneath the surface, inaccessible from the top. These features, which include slots, grooves, or threads, often pose a significant challenge in machining due to their inaccessibility with standard tools. However, with Computer Numerical Control (CNC) machining, engineers have found ways to create these complex geometries.

In this article, we explore how to create undercut geometries using CNC machining and how Prolean’s CNC services can help you achieve precision and perfection.


What Are the Undercut Geometries?

When it comes to the world of CNC machining, the term ‘undercut’ refers to a recessed or tucked away feature of a part, which is typically out of the reach of a conventional tool moving along a top-down path. But before we delve into the types and applications of these undercuts, let’s understand why they are crucial to certain designs.

Undercut features are used for multiple reasons across industries. They often allow for additional functionality in mechanical parts, such as slots for a key in a gear or pulley system or internal grooves for snap-fit assemblies in plastic parts.

Undercuts can be broadly classified into three categories: dovetail, T-slot, and woodruff key seat undercuts.

  • Dovetail Undercuts: Reminiscent of a bird’s tail, these undercuts are inverted trapezoids in shape. The wider base of this trapezoid aids in keeping a mating part in place by providing a larger surface area for it to bind to the main part. Dovetail undercuts are commonly used in applications where strong mechanical binding without the need for additional fasteners is required.
  • T-slot Undercuts: These undercuts resemble a T in their cross-sectional view. The larger top section of the T allows for the passage and retraction of bolts, pins, or other connectors, which then secure in place within the narrower bottom section. This type of undercut is often seen in machine tool tables or component mounting systems.
  • Woodruff Key Seat Undercuts: Named after the specific cutter tool that creates them, these undercuts are semicircular and typically found on rotating components such as a shaft in a gear system. A woodruff key seats into this undercut, preventing relative motion between the shaft and the gear.

To understand which type of undercut is suitable for a specific application, a thorough evaluation of the design requirements and constraints is necessary.


The Challenges of Machining Undercuts

Machining undercuts presents unique challenges due to the inherent complexity of their geometry and the limitations of conventional tools. These hurdles can be broken down into several key points:

  1. Tool Accessibility: The primary challenge in machining undercuts lies in limiting conventional tool paths. Since most machining operations involve top-down tool movements, reaching into the recesses of undercuts becomes physically impossible with conventional end mills or drills.
  2. Design Constraints: Designing for undercuts requires a careful balance between the functional needs and the part’s manufacturability. While undercuts may enhance part functionality, they also increase manufacturing complexity and costs.
  3. Precision and Control: Undercuts demand high levels of precision and control during machining. Even the slightest deviation or error can damage the part of the need for rework, which increases time and cost.
  4. Tool Selection and Handling: Machining undercuts often requires specialized tools, such as lollipop cutters, T-slot cutters, or dovetail cutters. However, selecting and handling these tools requires expertise, as they must be precisely controlled to prevent part damage.

Despite these challenges, advancements in CNC machining have enabled the creation of undercuts with high accuracy and efficiency. With careful planning, correct tool selection, and expert machining, undercuts can be a viable part of many applications.


The Approach to Machining Undercuts

Despite the unique challenges undercuts present, skilled CNC machinists can create them with precision. The primary methods to machine these features are:

1. The Use of Special Tools

Conventional cutting tools are generally known to be unable to access the recessed features of undercuts, which require special cutting tools. These special tools, also known as T-slot, lollipop, and dovetail cutters, are designed to reach into these difficult-to-reach areas and make cutting easier. However, it is worth noting that using these tools may increase the cost of production due to the need for specialized tooling, which can be quite expensive.

Despite this added cost, specialized cutting tools are often necessary to achieve the desired results and create high-quality products that meet the needs of customers and end-users alike. By investing in specialized tooling and taking advantage of the latest advances in cutting technology, manufacturers can ensure that their products are of the highest quality and meet the most demanding requirements of their customers and the marketplace.

2. The Use of Multi-axis CNC Machines

Multi-axis machines, particularly 5-axis machines, are designed to provide high flexibility in machining processes. These machines can tilt and rotate the workpiece to allow for better access to the cutting tool, which can approach the undercut from multiple angles. This feature is particularly useful in the manufacturing of complex parts that require a high degree of precision and accuracy.

By eliminating the restriction of the top-down approach of traditional 3-axis machines, 5-axis machines provide greater freedom in machining operations, allowing for more complex geometries and shapes to be produced with ease. Additionally, multi-axis machines can significantly reduce the overall production time and costs associated with the manufacturing of parts, making it a highly efficient and cost-effective solution for modern manufacturing processes.

3. Electrical Discharge Machining (EDM)

EDM is used for undercuts that cannot be reached by even special tools or multi-axis machines. In this process, an electrode shaped to the mirror image of the desired undercut discharges electrical sparks to remove material from the workpiece. It is a non-contact method that can machine complex undercuts, but the process is slower and more expensive than traditional CNC machining.


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How Prolean’s CNC Machining Services Can Help?

Creating undercuts requires the right tools, the right machines, and a deep understanding of the manufacturing process, all of which are available with Prolean’s CNC machining services. Here’s how Prolean stands out:

  1. Experience in handling complex geometries: With years of experience, Prolean has an expert team that understands the complexities of machining undercuts. They can provide insights into the feasibility of your designs and suggest improvements if needed.
  2. Use of advanced CNC machines: Our facilities are equipped with state-of-the-art multi-axis CNC machines, which can produce complex geometries with high precision. These machines are capable of creating accurate undercuts, thereby providing customers with components that meet their exact specifications.
  3. Access to a variety of special tools: The company maintains a wide range of special tools required to create different undercut geometries. Regardless of whether you require a T-slot, dovetail, or woodruff key seat undercut, Prolean has the necessary tools to make your design a reality.
  4. Cost-effective services: Despite the complexity of creating undercuts, Prolean strives to offer cost-effective services. By efficiently managing the machining process, the company can keep costs down and deliver high-quality components.



Machining undercut geometries is challenging, requiring a deep understanding of CNC machining processes, access to specialized tools, and the right equipment. By considering key factors such as tool accessibility, design constraints, precision control, and tool selection, it is possible to create undercuts effectively and efficiently in CNC machining.

In the modern manufacturing, companies like Prolean have the expertise and technology to handle such complex geometries. Whether it’s providing advice on design feasibility or machining intricate undercuts, their CNC machining services have you covered.



What is an undercut in CNC machining?

An undercut is a recessed feature of a part that cannot be machined from the top down with conventional tools. They are commonly used in mechanical parts to increase functionality.

What are some examples of undercuts?

Examples of undercuts include dovetail undercuts, T-slot undercuts, and woodruff key seat undercuts. These features have various applications in mechanical parts.

Why is machining undercuts challenging?

Machining undercuts is challenging due to the inherent complexity of their geometry and the limitations of conventional tools. This requires special tools, multi-axis CNC machines, or EDM techniques.

How does Prolean help in machining undercuts?

Prolean helps in machining undercuts by offering expertise in handling complex geometries, using advanced CNC machines, providing various special tools, and offering cost-effective services.

What are the alternatives to CNC machining for creating undercuts?

When CNC machining may not be feasible for creating undercuts, techniques like EDM (Electrical Discharge Machining) can be used.


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