Undercut Machining: A Complete Guide for CNC Machined Parts with Undercuts

undercut in machining

It is no longer news that technological innovation of CNC machines have helped make machining services easier. Not to mention that CNC machines are compatible with various metal and plastic materials to produce precise, quality parts.

However, machining trickier features likes undercut parts requires some key considerations and know-how to get the best result.

Hence, this guide contains all you need to know about undercutting machining and its applications for CNC machined parts.

What is Undercut Machining?

Undercut machining is a machining technique that involves creating recessed or sunken features in a machined part. Standard cutting tools used in most machining services, for example, end mills, cannot create undercuts. Besides, designing undercut parts in machining might seem challenging to achieve. Above all, this machining project aims to implement proper functionality to various machined parts.

Furthermore, manufacturing some precise undercut standards can incur more costs. This is because some of their surfaces are not directly accessible from overhead. Likewise, several undercuts exist where one workpiece part projects over another. As such, the overhang produces a gap that regular traditional tools cannot reach.

Thus, undercut machining requires specialized tools like slot cutters, lollipop cutters, or undercut end mills to make undercut parts.

What is a CNC Undercut?

An undercut in machining refers to a distinct type of recessed surface of machined parts. Undercuts are often depicted by one workpiece part extending over another.

undercut for CNC machined part

In CNC machining services, undercuts occur in different ways. Check the details below.

Undercut in CNC Turning

In the turning process, an undercut is a recess in diameter on the inside diameter of the machined part. A rule of thumb for undercuts in the turning technique is that it should have a length of at least 1.5 threads and a diameter of at least 0.38mm smaller than the minor diameter of the thread. Additionally, undercuts are often applied at the end of the threaded part of a screw or shaft. Thus, this provides clearance for the cutting tool.

Undercut in CNC Milling

In milling, the cutting tool gets mounted at the spindle. In some cases, undercut machining experts cut off part of the material from a direction where the feature is not seen from the angle of the spindle. However, this requires special tools to reach behind the visible material.

Special Tools for Undercut Machining

As mentioned earlier, undercut machining gets tricky most times. More so, the typical straight machining tools cannot access the gap created for most undercut parts. Thus, this machining process requires special tools. Take a look at these special tools below:

slot cutter and lollipop cutter

Slot Cutters and Lollipop Cutters

These unique tools have peculiar shapes that allow them to fabricate undercuts where traditional tools cannot. Slots and lollipop cutters have a wide end tip and narrow shaft.

But then, they differ in the shape of the end tip. You’ll find that the slot cutter has a flatter tip, while the lollipop cutter has a rounder end tip.

Furthermore, experts measure the distance between the outside tip and shaft edge in undercut machining. Thus, this helps determine the overhanging length to cut for CNC machined parts.

The rounded tip of lollipop cutters makes them suitable for machining curved undercut shapes. Aside from this, lollipop cutters get applied in edge bearing, deburring a workpiece. however, this undercut end mill offers multi-directional cutting making it more expensive.

Note that cutters with thinner necks help produce a more spherical undercut. This is due to the larger wrap angles that provide bigger clearance. In contrast, those with thicker necks have smaller wrap angles, thus providing less clearance.

Special Cuts in Undercut Parts Machining

Below are some of the different cuts you can carry out in undercut part machining:

T-slot in undercut

1. T-Slot

As the name implies, this cutting tool helps create T-shaped undercuts. T-slot tools comprise interlinked flat horizontal blades and a vertical shaft. These features allow the device to cut down into a workpiece and cut in either direction horizontally. As a result, it forms the T-shape. T-slot cutting tools are available in standard widths ranging from 3mm to 40mm in size.

dovetail in undercut

2. Dovetail

Dovetail cutting tools use angled blades to produce undercut parts. The resultant cuts have a trapezoid-like or dovetail shape. Further, the undercut part always has a narrow top and broad bottom.

Dovetail cutting tools have angled blades designed at standard angles of 45 and 60 degrees. However, other grades in the multiples of 10 degrees, ranging up to 120 degrees, are less employed.

3. One-Sided Undercutting

This unique cut utilizes a CNC undercut tool to make asymmetrical undercuts in machined parts. It cuts sideways while making further cuts that project from the previous undercuts.

Note that clearance is an essential factor to consider when using the one-side cutting technique. This ensures the CNC undercut tool works properly.

Tips for CNC Machined Parts with Undercuts

Below are some vital tips to aid your undercutting process for CNC machined parts:

1. Partner with Professionals

You will agree that partnering with experts will make your work easier. Hence, experienced individuals or companies machining undercuts will most likely have the necessary tools and expertise to help you in various ways. That way, working with experts will help you reduce the risk of errors to get suitable undercut parts.

undercut machining

2. Eliminate Undercuts If Possible

When staging the design for your machined parts, it would be best for you to avoid the undercuts wherever possible. This is because undercuts contribute to the complexity of your design. Besides, you may also need to get the special tools required, which can incur more costs. So, eliminating undercuts makes your work uncomplicated and saves you from more expenses.

3. Avoid the Need for a Custom Tool

You can also avoid the need for custom tools as they take time to manufacture. More so, these tools are for one-time use, and recurrent requirements for these tools can add more costs to your machining project. Thus, you can best avoid custom tools by designing your undercut features with standard dimensions.

4. Make Shallow Undercut

Keep in mind that there’s a limit to the depth of cutting tool blades. This is because most of these blades are often linked to the vertical shaft. Hence, ensure you only create the shallow undercut depths that your suitable cutting tool can process—moreover, the shallower your cut, the better.

Application of Undercuts Parts

Undercuts cannot always get avoided. Basically, manufacturers often apply undercuts in machining processes to give specific functionality to machine parts.

undercut part

Thus, undercut machining becomes necessary for parts that require the following:

  • Side holes: for consumer electronics
  • Interlock features: for easier assembly
  • Custom inserts: for unique design elements
  • Vertical threads: for many connectors, hoses, and other components
  • Barb fittings: for medical devices


Notably, some machining designs look easy to create, but are more difficult to implement during the machining process. So, challenging tasks like undercut in machining require quality machining services to get the best undercut parts.

WayKen Handles CNC Machined Parts with Undercuts for You

At WayKen, we have experienced professionals committed to handling all CNC machined parts with undercuts for you. By utilizing state-of-the-art professional tools, we use DFM to simplify complex undercutting tasks. Moreover, we assure quality, precise and affordable undercutting services every time

Do well to reach out to us today for one-on-one support service, and you will get a response within 12 hours.


Can undercuts be machined?

Yes, simple undercuts are often machined with special tools. Slot and lollipop cutters, also called undercutting end mills are often top choices for undercut machining. They have unique shapes and features that make them suitable for fabricating undercut parts.

What is the adverse effect of an undercut?

Generally, undercuts cause local stress concentration in metal parts. This adverse effect results from reducing the cross-sectional areas of metal parts from undercut depth.

Why should the mold have no undercuts?

To a large extent, molds should have no undercuts because they complicate the overall design and prevent part ejection. Thus, it’s best to avoid undercuts wherever possible.

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