Undercut Machining: Process, Types And Applications

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Undercut Machining: Process, Types And Applications

What do you do when the section you want to machine in a part is inaccessible through the regular machining methods? For complex parts such as dies and molds, undercut machining is an irreplaceable process. This method ensures that the optimal performance of such parts is possible.

In this article, we review the basics of undercut machining. A coverage of the operating principles, versions of the process, and its general relevance will be provided. This review aims to make undercut machining more understandable, and therefore, more applied in the industry.

What is Undercut Machining?

The undercut machining process is borrowed from the word ‘undercut’, which means a recessed surface. The undercut can take any shape, and can be located internally or externally on the work piece.

From this, it follows that undercut machining is the process of using a machining method to generate a recessed surface. This process uses a tool that is different from the common drilling or machining ones. To add to the specialized tool, the technique used is also non-standard. Several steps characterized what undercut machining is, as will be described in the next section.

How does Undercut Machining Works?

Here are the basic steps of undertaking an undercut machining work:

Step 1: Selection of the Appropriate Tool

If you already have the design, you want to select the appropriate tool. To go with the different types of undercut, there are many types of cutting tools. Make sure you have the correct end mill and other cutting tools to achieve the profile, depth, and other features of the undercut. The choice of the tool for undercutting will also be based on the type of material you are machining.

Step 2: Setting Up the Machine

The next step is the setting up of the CNC machine or whichever machine you are using to make the undercut. An important part of this step is the proper mounting of the work piece in readiness for machining.

Step 3: Programming the Machine

If you are using a CNC machine to machine the undercut, you will have to program the machine for that. It is important to analyze the undercut design for functionality before proceeding with this step. Remember to give an allowance for the tool.

Step 4: Machining the Undercut

Given the variety of undercuts, there is no standard tool or approach to machining an undercut. The process to get the machined parts with undercuts will depend on the specific CNC program.

Step 5: Inspecting the Finished Undercut

After machining the undercut, it is time to inspect the workpiece. You want to make sure that everything went according to the plan for tolerances, dimensions, and so forth.

Step 6: Post-Processing

The machined parts with undercuts may still be incomplete without post processing steps. The most common of these processes is deburring.

Types of Undercutting Tools

Undercut machining requires specialized tools. The tools have notable shapes so that they can reach normally unreachable parts. Here are five of the most common types of undercutting tools:

Undercutting End Mills

These have cutting edges at the tip and sides of the cutting tool. They are available in different configurations including spherical and tapered. This means that they can make spherical undercuts and other shapes. These undercutting tools are designed to complete the job in one pass.

Lollipop Cutters

The lollipop cutters are shaped like the lollipop. This specialized shape is to aid in creating spherical bottom undercuts. The design is effective in the creation of a smooth finish.

T-slot Cutters

As the name suggests, T-slot cutters are built for forming T-slots. T-slot cutters have straight cutting edges that ensure the creation of a smooth slot in one pass. It cuts both the bottom and sides of the slots.

Keyway Cutters

If you wanted to machine a keyway, this is the cutting tool to use. This type of tool has a straight cutting edge that is capable of machining all sides of the undercut in one pass. This coverage of the undercut depth and smooth surface results make this cutter special.

Dovetail Cutters

If you have ever wondered how dovetail undercut features are made, the answer is in this unique tool. The dovetail cutter features angled blades or cutting edges. The placement of the cutting edges around the dovetail shape allows for the cutting of the profile in a single pass.

Types of Undercutting Machining

Here are the most common types of machining undercuts:

T-slot Undercut

The T-slot undercut is achieved using a t-slot tool. This tool comprises a shaft with a blade perpendicular to it. With this arrangement, the tool can cut in two directions: The horizontal direction and directly into the work piece. The result is an undercut with a T-shape. You can get different ranges of undercuts of this nature by varying the tool width (typically between 3 and 35mm).

One-sided Undercut

In this type of undercut, only one side of the part is machined. You will see it in places on a workpiece where the user wants to place a retaining ring or seal. The tool used for one sided undercut also has a one sided cutting edge.

Dovetail Undercut

To achieve the dovetail undercut, you need a cutting tool that is appropriately shaped. The angled blade design is synonymous with this type of undercut. The goal of the machinist in dovetail undercutting is to get a dovetail-like feature. Popular angles for the cutting tool are 45° and 60°. However, you can get other angles with increments 10 degrees to a maximum of 120°.

Tapered Undercut

A tapered undercut is characterized by a slant surface. This feature is typically meant to facilitate a press fit. For this undercut feature that can also be decorative, the tool has a tapered edge.

Threaded Undercut

Some parts needed to be threaded to other parts. That’s where the threaded undercut comes in. The undercut incorporates internal threads that can attach to a screw or bolt. A unique threaded mill is used to attain these internal threads.

Spherical Undercut

You will notice a spherical undercut from a curved internal surface. As the name suggests, getting this interesting feature requires ball-nose-like end mills. There are plenty of those in the market, so you can get as many sizes as possible. A spherical undercut is necessary where parts join with spherical surfaces or where a bearing may be placed for rotary motion. The smoothness of this undercut is effective in the reduction of friction.

Keyway Undercut

This type of undercut applies where the goal is to place a key within a hole or on a shaft. Keys are necessary when you want to lock two mechanical parts to counter relative movement. The keyway undercut is done using a broacher or the keyway undercutting tool.

Relief Undercut

This is another possible undercut type, this time meant for clearance or stress relief. It is mostly associated with the interaction between bearings and shafts. A relied undercut is created on the shaft to reduce stress.

O-ring Groove Undercut

This type of undercut is designed to house the O-rings. It is effective in forming a tight seal between a pair of parts. With its sunken design, the O-ring groove undercut offers perfect placement for the rings.

Tips for Perfect Undercut Machining

For undercut machining process to be perfect, you want to adhere to a few best practices. Let’s go through the tips:

Avoid undercuts

This would be the first advice given. But is it entirely possible to avoid undercuts? Maybe not. The argument is that these cuts are tiresome to create. It might be more cost-effective to do away with them. If they are not necessary, you may want to replace undercut machining with standard cutting. But if you must include undercuts, read on for the next tips.

Stick to shallow cuts

When you decide to create undercuts, try to limit yourself to as shallow cuts as possible. Given that the undercutting end mills and other tools are typically connected to a vertical shaft, the possible depth is limited. Consider the size of the undercutting tools. You don’t want to exceed their reach.

Use Standard cutting tools

The urge to personalize the work and use custom tools is understandable. But remember that everything custom is expensive. Fortunately, there could be a way around this. You can get a custom tool, but use standard dimensions for the undercut parts machining.

Seek Expert Machining Help

In some instances, working with professional machinists for undercut machining projects is much better. A professional firm such as Aria Manufacturing is well-equipped for the best undercut machining services. No undercut machining task is too complex for them. Furthermore, they can optimize the designs for too complex undercutting tasks.

Application Of Undercuts Parts

Difficult as they may seem, machined parts with undercuts have many uses. Consider the following popular applications of undercut parts:


The electronics industry is one of the most popular destination for undercut parts. Many components, for instance, housings and connectors, feature undercuts. For connectors, the designers incorporate T-slot undercuts. Housings in the industry have relief undercut types.


Gears and shafts for automotive have keyways. These features can be created using undercuts. The undercuts are effective in preventing rotation in these components. Where fluids need to contained in some areas, O-ring grooves come in handy. Undercut machining process is very effective in creating the grooves.


The need for lightweight components with complex shapes in this industry requires specialized tools and machines. When undercuts are needed to reduce the weight of components, undercut machining comes in. While minimizing the weight, undercuts can also help protect wirings and other components. Grooves made by undercut machining process are popular for this.


Undercut machining can be challenging, particularly for someone who has little or no knowledge of CNC machining methods. The need for the special cutting tools and machines can also be a significant hindrance. However, it is a critical process for many industries, as we have indicated here.

We have covered the important elements of the undercut machining process, types, and applications. If you weren’t equipped with this information, now you are at a better place to handle these projects comfortably. In need of undercut machining services? Get in touch with our representative for the next steps.


Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.