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undercut machining

Undercut Machining: A Detailed Overview of the Process

Machining some designs can be more complex than others since the machinability of various features differs significantly. Undercut machining, compared to shallower cavities and larger features, is trickier to create in CNC machined parts.

Although it originated as a chemical machining technique involving using chemical etchants to remove material from a workpiece to form a recessed cavity beneath the surface, it is now a mechanical process that uses mechanical tools to cut out these specialized cavities in machined parts.

This guide explores the undercut machining process, its importance, and the steps for creating undercuts. Read on to the end to discover the common types of undercuts and expert tips for successful undercut machining in CNC machined parts!

What Is Undercut Machining

what is undercut machining
What Is Undercut Machining

Undercut machining involves creating a unique recessed surface in machined parts, which is difficult to achieve with traditional machining tools. Standard tools are not often suitable for the machining process since some of these surfaces are not particularly accessible from overhead. Machining experts frequently utilize smaller cutting tools or sophisticated CNC machines like multi-axis CNC machines to create undercuts.

Similarly, machinists may require more advanced CNC cutting tools, such as lollipops and slot cutters. These CNC tools and machines can overcome regular machining tools’ restrictions and provide the needed undercuts. Various undercut features are created where a part of the workpiece overhangs another. The overhang forms a gap inaccessible to traditional tools like three-axis mills and routers. Undercuts can be internal or external; internal undercuts (hidden within components like gear hubs) while external undercuts are like those found in molds.

Why Is Undercut Important in Machined Parts

cnc undercut machined part
CNC Undercut Machined Part

Incorporating machined parts with undercuts is vital in contemporary manufacturing because it helps achieve functional and design objectives. Although machinists often avoid undercuts in product designs due to their complexities, these recessed features are critical in providing secure locking mechanisms in joining components of an assembly without depending on fasteners.

More so, undercut helps reduce the weight of machined parts, particularly in industries such as aerospace requiring high performance and payload capacity. The undercut machining technique enables the creation of internal cavities without compromising structural integrity and reducing excess material. Product engineers add these undercut features in machined parts to implement the proper functionality of hydraulic systems. These recessed surfaces function as pathways for fluids, guiding the movement of liquid and greases through the created channels and spaces.

Stepwise Guide to Undercut Machining

stepwise guide to undercut machining
Stepwise Guide to Undercut Machining

The CNC undercutting machining process involves a series of meticulous steps to achieve the desired shape and depth. We’ll discuss a stepwise guide to the process below:

Step I: Evaluate the Geometry

The first step in undercut machining is analyzing the component’s geometry. Evaluate the profile to decide if it requires internal or external undercut. Note the required machining strategies and sequence of their application to achieve the preferred design.

Step II: Choose the Appropriate Cutting Tools

When it comes to machining undercut features, there is no one-size-fits-all tool. Each task requires a specific CNC undercut cutter. As a result, it is essential to choose a tool based on the material and the required profile and depth.

Step III: Set Up the CNC Machine

Prepare the CNC machine for the undercut machining process using CAD software to input the detailed design specifications. Mount the specialized spindle for undercut machining and clamp the material firmly onto the work table.

Step IV: Undercut Machining

The CNC machine handles the remaining task once you have attached the right cutting tool and clamped the workpiece. The machine follows the programmed instructions to cut material according to the defined path.

Step V: Quality Control

Inspection of the machined workpiece is critical to confirm the dimensions match the specified tolerances range. Performing quality control checks after machining undercuts helps ensure the recessed surface meets all design requirements and the machined component functions as expected.

Typical Undercuts in CNC Machining

v solt undercut
V-Solt Undercut

Undercut features could have varying profiles in different applications depending on the geometric profile. Here are the common types of undercuts in machining:

One-sided Undercut

One-sided undercuts is an asymmetrical recess aimed at one surface of a workpiece. They are specifically suited for a machined component requiring a precision groove on a particular side to accommodate specific assemblies, particularly retaining rings or seals.

Machinists often use a lollipop cutter to make one-sided undercuts since it has a single cutting side. The machinist clamps the lollipop cutter to a multi-axis CNC machine, which moves the tool around the workpiece’s periphery where the cut is needed. However, clearance is important during one-sided undercut machining to ensure the CNC tool works properly.

Dovetail Undercut

A dovetail undercut establishes a secure connection between two components. This feature has an angled blade design comprising wedges and recessed sections. The wedge assembly locks itself firmly in the recessed part, joining the two parts. The dovetail undercuts are a standard feature used in making different parts in the woodworking industry. The dovetail-cutting tools often feature slightly tapered edges with angles ranging from 45 ° and 60°.

T-slot Undercut

t slot undercut 1
T-slot Undercut

As the name implies, this recessed feature has a ‘T’ shaped fixture explicitly made to hold parts together. The bolt’s head enters the slot and can slide along the length, forming a secure connection. T-slot undercut machining involves making a slot with the standard end mill and using a specialized tool (T-slot cutter) to create the T-shape. The T-slotter possesses a vertical shaft known as a shank and a cutting blade vertical to the shank.

Tapered Undercut

A tapered undercut has an inclined or sloping surface that becomes narrow/taper from one side to the other. Machining tapered undercuts are ideal in applications demanding a tight, frictional fit between parts. A good example is mechanical assemblies or cases where a smoothly tapering surface aesthetics increases the final product’s value. Machinists use tapered end mill cutters to give the undercuts a gradual slope.

Spherical Undercut

Spherical undercuts feature a 3D curved surface similar to a sphere. These undercuts apply to parts that require rotary motion, including bearings or ball joints. Ball-nose end mills are suitable CNC tools for making these curved undercut shapes in machined parts. The rounded tip of these end mills facilitates efficient curved profile machining through a programmed CNC path.

Threaded Undercut

Threaded undercuts have internal threads like bolts and screws. Threaded undercuts are critical features in parts requiring screwing. Product engineers and machinists use specialized thread mills and taps to add threads to undercut parts. The thread mills form the thread path in a spiral motion suitable for external and internal threading.

O-ring Groove Undercut

The O-ring groove undercut is designed to house an O-ring, forming a tight connection between two components. This groove helps prevent leaks in different applications. Machinists achieve precise dimensions and groove placement with specialized O-ring groove cutters.

Relief Undercut

This undercut feature is often cut around bearings or shafts. The machinist cuts a small recess into workpieces to provide clearance or minimize stress concentration in parts. You can achieve these undercut profiles with standard undercut end mills or slotting cutters. The cutter removes metal as it moves along the predetermined path around the bearings or shafts, creating a precise groove. This process may require a series of passes to attain the preferred shape and depth.

Keyway Undercut

The specific purpose of key undercuts is to house a key that secures two mechanical parts together and prevents them from rotating separately. These undercuts are made in shafts or other rotary components that transfer torque, such as a gearing system. These shafts enhance mechanical engagement in machines.

Broaches or keyway cutters are typical CNC tools that machinists use to create these undercuts in shafts. A broach is a tool with multiple progressively larger teeth. It removes material through a linear motion to create a precise keyway or slot in a single pass. Conversely, a keyway cutter is mounted to a milling machine and rotates to cut material and make the slot.

Tools for Machining CNC Parts with Undercuts

undercut tools
Undercut Tools

Machining undercuts in CNC parts can be quite tricky. Hence, machinists utilize specialized tools to achieve complex undercutting tasks. Here are standard cutting tools used for machining CNC parts with undercuts:

  • Dovetail Cutters: They are custom tools with angled cutting edges for making dovetail undercut features. Thanks to its distinct placement of blades around the dovetail shape, they can cut the profile in one pass.
  • T-slot Cutters: T-slot cutters feature a straight cutting edge that helps create smooth slots in a single pass, cutting both the sides and bottom of the slots.
  • Lollipop Cutters: The lollipop cutters are specialized undercutting tools in the shape of a lollipop. This design allows it to create spherical bottom undercuts with a smooth finish.
  • Keyway Cutters: These CNC tools are suitable for creating undercut features with a keyway. Keyway cutters have straight cutting edges that can machine all sides in one pass. This custom tool is unique for its undercut depth coverage and smooth surface finish.
  • Undercutting End Mills: These are end mills with cutting edges at the sides and tip, designed to complete the undercutting task in a single pass. Undercutting end mills can create spherical undercuts and other geometries since they are available in varying configurations, like tapered and spherical.

Expert Tips for Machining Perfect Undercuts

cnc undercut machined part
CNC Undercut Machined Part

Undercut parts machining can be quite challenging and requires special skills and expertise. Here are helpful machining strategies for creating high-quality parts with undercuts:

Seek Professional Assistance

Partnering with professionals will help ease your undercut machining in workpieces. As such, experienced personnel or companies machining undercuts will possess the required tools and expertise to assist you. Hence, partnering with experts will mitigate the risk of errors and provide suitable undercut parts.

Employ Shallow Undercuts

Understand that there is a limit to the cutting tool blades’ depth since most of them are connected to the vertical shaft. Therefore, design your products with a shallow undercut depth that your compatible CNC tools can reach.

Choose the Appropriate CNC Machine and Avoid Custom Tools

Choosing a suitable cutting tool is the first and integral step in fabricating and ensuring your machined parts meet the intended requirements. Machining undercuts requires sophisticated machines like 5-axis CNC machines, which are more suitable for creating delicate cuts, including undercuts. While you may desire to personalize the project and use custom tools, it is crucial to remember custom projects are expensive because they require fabricating one-time-use tools before you begin the project.

If Possible, Eliminate Undercuts

When drafting your machined part’s design, you should try as much as possible to avoid undercuts because they make your design more complex. Also, you may need specialized tools to machine the recess, which can further increase machining costs. Therefore, your work becomes less complex and less costly when you eliminate undercuts


at machining your global partner for quality cnc machining services in china
AT-Machining Your Global Partner for Quality CNC Machining Services in China

Undercut machining is a specialized technique that creates complex structures in different applications, such as medical devices, aerospace components, and automotive parts. This machining process facilitates the machining of larger assemblies with complex, inaccessible features unattainable with standard tools. While these recessed features are tiresome to achieve, it is essential to understand everything about the process to achieve the desired results.

AT-Machining is a precision CNC machining expert that can handle complex parts requiring undercut milling and intricate geometries. Our highly skilled and experienced professionals leverage our cutting-edge CNC facilities, professional tools, and expertise to ensure your CNC machined parts meet design specifications. We offer other expert services such as 3D printing, sheet metal fabrication, die casting, extrusion, and injection molding. Submit your design file today for a free quote!

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