Tips For CNC Machined Parts With Undercuts Machining

When machining parts, some designs are easier to produce than others, so the machinability of different features is an important consideration to keep in mind. Undercut is a feature that standard end mills cannot achieve. For this reason, the design of parts with undercut is often difficult, and the processing of such parts with difficult characteristics requires some key considerations and expertise to obtain the best results.

What Is Undercut Machining?

Undercut machining is a machining technology, which involves creating a depression or depression feature in a machined part. Standard cutting tools (such as end mills) used in most machining services cannot produce undercuts. In addition, it seems difficult to design undercut parts in machining. Most importantly, the machining project is designed to achieve the proper function for various machined parts. In addition, manufacturing some precise undercut standards will incur more costs. This is because some of their surfaces are not directly accessible from overhead. Many undercuts occur where one part of the workpiece protrudes from the other. Overhang creates gaps that cannot be reached by traditional tools such as routers and three-axis milling machines.

Special Tools For Undercut Machining

As mentioned earlier, undercutting can get tricky in most situations. What’s more, typical straight machining tools do not have access to the gaps created for most undercut parts. Simple undercuts often require tools called slot cutters and lollipop cutters, the latter also known as undercut end mills. These tools have unique shapes that allow them to make undercuts that traditional machining tools cannot.

undercut for machining

Slot Cutters And Lollipop Cutters

Both grooving and lollipop knives have narrow shafts with wider ends. In slot knives, the tip is flatter, while in lollipop knives, the tip is rounded, perfect for cutting curved shapes. The distance between the outside of the tip and the edge of the shaft is how far the machinist can cut under the overhanging surface. Lollipop knives in particular can also be used for deburring and edge breaking any part of the workpiece, including the underside.

Lollipop cutters are available in a variety of wrap angles, such as 220 and 300 degrees. A cutter with a thinner neck allows for a larger wrap angle and a more spherical cutting end, providing greater clearance. Knives with thicker necks have smaller wrap angles, so they provide less clearance but more rigidity and stability.

The rounded tip of the lollipop cutter makes it suitable for machining curved undercut shapes. In addition to this, lollipop cutters are also applied to edge bearings to deburr workpieces. However, this undercut end mill offers multi-directional cutting and is therefore more expensive.

Slotting knives and lollipop knives are handy tools, especially for simple undercuts. However, one disadvantage of them is that most computer-aided manufacturing (CAM) software does not contain the algorithms needed to support them. It is possible to use these tools with CAM software, but doing so often requires clumsy workarounds, such as using manual G-code or manually operating a milling machine.

Special Cuts In Undercut Parts Machining

Here are some unique cuts you can perform when the product needs undercut. The two main types of undercut are T-shaped groove and dovetail groove, and sometimes unilateral undercut is required.


As the name implies, this cutting tool helps to create a T-shaped undercut. The T-groove tool comprises a horizontal blade and a vertical shaft connected to each other. These features allow the machine to cut into the workpiece and cut horizontally in either direction. As a result, it forms a T-shape. The standard width range of T-groove tools is 3 mm to 40 mm.


Dovetail cutting tools use angled blades to produce undercut parts. The resulting cut has a trapezoidal or dovetail shape. In addition, the undercut portion always has a narrow top and a wide bottom.

Dovetail cutting tools have standard angles of 45 degrees and 60 degrees to choose from – other angles are also possible, usually in multiples of 10 degrees and up to 120 degrees, although these are less common.

One-Sided Undercut

The difference between one-sided undercut and T-groove and dovetail groove cutting is that the former will produce symmetrical shapes, while unilateral undercut will not. Unilateral undercut an additional cut that is cut from the side using a special tool and protrudes from the original undercut, just like the protruding part on a standard puzzle.

If you are designing an interior wall with an undercut on one side, you must keep the gap in mind. Keeping the main width four times the undercut depth will help ensure that the knife has enough clearance to work properly.

Undercut Machining Skills

1. Cooperation with professionals

You will agree that working with experts will make your work easier. Therefore, experienced individuals or companies are likely to have the necessary tools and expertise to help you in various ways. In this way, working with experts will help you reduce the risk of errors to obtain the right undercut parts.

2. Eliminate Undercuts If Possible

When designing your machined parts, it is best to avoid undercutting as much as possible. This is because undercutting increases the complexity of the design. In addition, you may need to get the special tools you need, which may incur more costs. Therefore, eliminating undercuts makes your work easier and saves you more money.

3. Avoid the need for custom tools

During the design of parts, undercut shall be avoided as much as possible. Undercutting is difficult to use and requires special tools to make. Therefore, it would be more cost-effective and efficient not to include them in the design. An example of a design that does not require undercutting is to design a workpiece with a tool space so that standard machining can be used from the back. This avoids difficult undercutting from the front.

4. Make shallow undercut

Remember that there is a limit to the depth of the cutting tool insert. This is because most of these blades are usually connected to the vertical axis. Therefore, make sure that you only create the shallow undercut depth that is suitable for your cutting tool – in addition, the shallower your cutting is, the better.

A deeper design may also compromise the stability of the cutting tool because the vertical axis will be longer. This may cause an increase in tool vibration, thereby reducing tool accuracy. In addition, the tool may be damaged. Therefore, it is important to create a design that is deep enough for most undercut tools.

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