Undercut Machining Explained: A Detailed Overview of the Process

Undercut machining creates features that standard straight-access cutting tools cannot easily reach. It is a critical method in CNC manufacturing when a part needs hidden reliefs, sealing grooves, locking geometry, or assembly-critical clearances.

Many engineers add undercuts for a valid functional reason, but they often underestimate the effect on tooling, setup, inspection, and cost. Buyers face the same problem during sourcing. A part may look simple on the drawing, yet one undercut can change the entire machining strategy. 

In this guide, you will learn what undercut machining is, how it works, which tools are used, where it is applied, and when it makes sense to use or avoid it.

What Is Undercut Machining?

Undercut machining is the process of creating a recessed feature in an area that a standard straight cutting tool cannot reach directly. In CNC machining, this usually means the tool must cut behind a shoulder, beneath an overhang, or inside a restricted area.

Unlike standard slots or open pockets, undercuts require a different cutting approach because the feature sits in a partially hidden or limited-access position. This makes the machining process more complex than ordinary surface or profile cutting.

Why Undercuts Matter in Functional Part Design?

Undercuts matter because they help a part do its job more effectively. In many designs, a standard open feature cannot provide the shape or clearance needed for fit, sealing, or retention. An undercut makes that hidden geometry possible.

Improve Part Function

An undercut can improve part function by creating a local feature that supports retention, sealing, or controlled movement. A simple flat or open slot often cannot achieve the same result. In real part design, this means the undercut is there for a reason. It helps the component perform correctly under working conditions rather than serving as a purely cosmetic or secondary geometric detail.

Create Clearance

An undercut can create clearance where two parts need extra space to fit or move without interference. This matters when the overall geometry must stay unchanged, but one internal or side area needs material removed. By adding that local recess, designers can prevent contact, reduce assembly issues, and make sure adjacent features work together without forcing changes to the rest of the part.

Support Part Assembly

Undercuts often support assembly by making room for mating parts, engagement features, or installation paths that would otherwise be blocked. Without this added recess, a part may be difficult to assemble or may not seat correctly at all. In that sense, the undercut helps translate drawing geometry into real assembly function, which is why it often becomes necessary in precision mechanical parts design.

How Undercut Machining Works?

Undercut machining follows a controlled process because the cutting area is partially hidden or difficult to reach. In the CNC machining process, the shop must first understand the feature geometry, then choose the right tool, setup method, and cutting path. A small error in planning can affect access, accuracy, or surface quality, so each step matters before the machine starts cutting.

Undercut machining follows a controlled process because the cutting area is partially hidden or difficult to reach. In CNC machining, the shop must evaluate the geometry, select the right tool, prepare the setup, machine the feature, and then inspect the result. Each step affects the next one, so the process must be planned carefully from the beginning.

• Review geometry: The shop first checks the undercut location, depth, width, surrounding surfaces, and tool entry direction. This step helps define the machining approach and identifies any nearby walls or shoulders that may restrict tool movement.
• Choose tooling: Once the feature geometry is clear, the next step is selecting a cutter that matches the undercut shape and can reach the recessed area safely. The shop usually reviews the cutter profile, neck size, head diameter, and reach length before finalizing the tool.
• Set up the CNC machine: After tool selection, the part is mounted in a position that gives the cutter proper access to the undercut. The setup must keep the workpiece stable while leaving enough clearance for the tool to enter and cut the hidden feature.
• Cut the undercut: With the setup and toolpath ready, the machine removes material from the recessed area along a controlled cutting path. The cutter must follow the programmed geometry closely so the undercut reaches the required width, depth, and profile without damaging nearby surfaces.
• Check accuracy: After machining, the undercut is inspected to confirm that it matches the drawing requirements. The shop checks the size, depth, shape, and position of the feature to make sure it will perform correctly in the final part.

Common Types of Undercuts in Machined Parts

Undercuts appear in different forms depending on the machined part function, mating geometry, and machining access. Some create local relief, while others support sealing, locking, or guided engagement. In CNC machining, understanding the exact undercut type helps you evaluate tooling needs, manufacturing risk, and whether the feature is truly necessary in the design.

Internal vs. External Undercuts

Internal and external undercuts describe where the recessed feature sits on the part. An internal undercut appears inside a bore, cavity, or enclosed profile, while an external undercut appears on the outside surface near a shoulder, edge, or outer contour. This distinction matters because tool access, chip evacuation, and inspection difficulty change significantly depending on where the feature is located.

Internal undercuts are usually harder to machine because the tool must enter a confined space and cut without colliding with nearby walls. External undercuts are often easier to access and inspect, but they still require careful control to avoid damaging the surrounding geometry. 

T-Slot Undercuts

A T-slot undercut creates a hidden channel beneath a narrower top opening. The upper section remains relatively small, while the lower section extends wider underneath, forming a shape similar to the letter T. This geometry allows one part to slide into the opening and engage with the wider recessed area below.

Designers often use T-slot undercuts when a part needs guided movement, positioning, or a retained connection below the surface. They are also widely used in T-slot fixtures and clamping systems for securing workpieces, especially on machine tables and tooling bases. In practice, standard T-slot opening widths often fall within a range of about 5 mm to 35 mm, depending on the machine table standard and application.

Dovetail Undercuts

Dovetail undercuts feature angled sidewalls that form a trapezoidal profile. This geometry allows a mating part to slide into position while also creating a more secure mechanical connection than a straight slot. Common dovetail angles typically range from 45° to 60°, depending on the design standard, mating requirement, and intended holding strength.

These undercuts are common in parts that need alignment and retention at the same time. Designers use them in sliding interfaces, locating features, and mechanical assemblies where a simple open groove would not provide enough control or engagement. The angle is an important part of the feature because it affects how the mating component fits, slides, and stays locked in place.

Relief Undercuts

Relief undercuts remove material in a small localized area near a shoulder, corner, or transition. Their purpose is usually to create space so another feature can sit properly or so a surface can be finished cleanly without interference. Although the shape may look simple, it often supports a proper fit between mating parts.

These undercuts are widely used in practical machining because many part transitions cannot remain sharp or fully closed without affecting assembly. A relief undercut helps address this issue by providing the necessary clearance in a specific area while maintaining the rest of the geometry unchanged.

O-Ring Groove Undercuts

An O-ring groove undercut is a recessed channel designed to hold an O-ring seal. The groove provides the space needed for the seal to sit in position between mating parts and work correctly during assembly. Its geometry directly supports sealing performance in components that handle air, fluid, or pressure.

This type of undercut is common in parts where leakage control is important. The groove must match the seal application so the O-ring can compress correctly and remain stable in service. Because of that, the feature serves a clear functional purpose rather than acting as a general recess or clearance cut.

Threaded Undercuts

Threaded undercuts are small recessed features placed near the end of a threaded section. They create extra space for thread runout and allow the thread profile to terminate cleanly without interfering with the adjacent shoulder or surface. In standard thread systems such as ISO metric screw thread and Unified threads, the thread profile angle is typically 60°, while Whitworth and BSP threads use a 55° profile.

These undercuts are common in shafts, fittings, and threaded mechanical parts where proper thread engagement matters. Without this relief area, the thread may stop too abruptly, which can affect fit during assembly and reduce the functional quality of the threaded connection.

Keyway Undercuts

Keyway undercuts are recessed features used in or near keyways to support torque transmission and accurate positioning between connected parts. They help define the engagement area where a key fits between a shaft and a hub or another rotating component.

These undercuts are common in power transmission parts such as shafts, couplings, gears, and pulleys. In these applications, the feature enables the creation of the geometry required for secure mechanical engagement and stable rotational alignment.

Spherical Undercuts

Spherical undercuts have a rounded recessed form rather than a straight-sided or angled profile. This type of geometry creates a curved internal or side feature that supports smooth contact, localized clearance, or a specific mating shape in a confined area.

Parts that need a rounded engagement surface or a controlled curved recess may rely on this type of undercut. Compared with standard grooves or relief cuts, spherical undercuts are more specialized and are usually selected when function depends on a curved internal form.

One-Sided Undercuts

One-sided undercuts remove material from only one side of a feature instead of creating a fully symmetrical recess. This design works when clearance, retention, or access is needed in one direction while the opposite side must stay unchanged. It gives the part a localized recessed area without altering the full surrounding profile.

This type of undercut fits parts with asymmetric geometry or one-direction assembly requirements. Designers often use it when only one side needs adjustment for fit or function. It is especially useful in compact parts where space is limited, and the remaining material on the opposite side must be preserved.

Tapered Undercuts

Tapered undercuts use an angled or gradually changing profile rather than a straight wall. This creates a recessed feature with a controlled transition, which can support part engagement, localized clearance, or a specific fit condition inside the component. The shape changes progressively instead of maintaining one constant wall direction.

In some designs, a straight-sided recess cannot provide the needed contact or entry behavior. A tapered undercut offers a smoother geometric transition and can help a mating feature move into position more naturally during assembly. This makes it useful in parts where gradual fit, guidance, or controlled contact is important.

Overview Table of Common Undercut Types in CNC Machining

Undercut Type	Basic Shape	Typical Use
Internal Undercut	Inside a bore or cavity	Internal clearance or hidden functional features
External Undercut	On an outer surface	Outer relief and accessible engagement features
T-Slot Undercut	Wider below a narrow opening	Fixture tables and adjustable clamping systems
Dovetail Undercut	Angled trapezoidal recess	Sliding interfaces and retained joints
Relief Undercut	Small local recess	Shoulder clearance and interference reduction
O-Ring Groove Undercut	Seal groove	Static or dynamic sealing applications
Threaded Undercut	Relief at the thread end	Thread runout and clean assembly fit
Keyway Undercut	Recess near the key seat	Shafts, hubs, and torque transmission parts
Spherical Undercut	Rounded recess	Curved contact or special mating geometry
One-Sided Undercut	One-direction recess	Localized clearance in asymmetric parts
Tapered Undercut	Angled transition	Guided fit and gradual engagement

What are the Tools Used for Undercut Machining?

Undercut machining needs tools that can reach recessed areas and form geometry that standard straight cutters cannot produce. Each tool type matches a different feature shape, access direction, and cutting requirement. In CNC machining, the right tool is selected based on the undercut profile, the available space, and the location of the feature inside or outside the part.

T-Slot Cutters

T-slot cutters have a slim neck and a wider cutting head, so the tool can pass through a narrow opening and then cut material underneath it. The head extends beyond the neck, which gives the cutter its ability to form a hidden profile below the top surface. This type of tool is common when the machining path must stay narrow at the entrance but wider inside the feature.

Dovetail Cutters

Dovetail cutters carry angled cutting edges that generate sloped sidewalls rather than straight vertical faces. Their overall profile matches a trapezoidal form, and common cutter angles usually fall within a range of 45° to 60°. Because of that angled geometry, the cutter can shape an inclined recess with more controlled sidewall form than a standard slotting tool or straight-profile cutter.

Lollipop Cutters

Lollipop cutters feature a rounded cutting head with a narrower shank behind it. That combination gives the tool more freedom to reach into confined areas and work around edges that would block a conventional cutter. The spherical head also helps create smooth curved contact inside the recess, which makes this tool especially effective when the machined form is hidden, rounded, or difficult to approach directly.

Internal Grooving Tools

Internal grooving tools are long, narrow cutters with a compact cutting tip at the end. Their slim form allows them to enter bores or enclosed internal spaces where larger tools cannot fit. Once inside the part, the cutting tip removes material at a defined location without requiring open side access. This makes the tool especially suitable when the machining area is surrounded by internal walls.

Thread Relief Tools

Thread relief tools have a narrow cutting profile that works well in tight spaces near the end of a threaded section. Instead of shaping a broad open area, the tool cuts a small and controlled recess in a limited zone. Its compact geometry helps maintain precision close to shoulders and transitions, where the available space is small and the surrounding surfaces must remain clean and well defined.

Industrial Applications of Undercut Machining

Undercut machining appears in many industries because engineers often need local recessed geometry that supports fit, sealing, retention, or controlled assembly. Although the feature may look small on a drawing, it often serves a specific functional role in the finished product. In CNC manufacturing, the value of an undercut depends on how it helps the part perform inside a real assembly.

Aerospace Components

Aerospace components often include undercuts where assemblies require tight engagement, weight-conscious geometry, or controlled clearance in limited space. Designers may use these features in brackets, housings, couplings, and precision structural parts where local recesses support fit and mechanical function.

Because aerospace parts usually involve strict dimensional control and complex geometry, undercuts in this field are often tied to functional surfaces rather than decorative detail. The feature must support assembly performance while fitting within demanding engineering and manufacturing requirements.

Automotive Parts

Automotive parts use undercuts in components that need secure engagement, sealing support, or local relief within compact assemblies. These features can appear in transmission-related parts, housings, shafts, connectors, and other mechanical components where space is limited and part interaction is critical.

In automotive manufacturing, designers often balance function, repeatability, and production efficiency. That makes undercuts useful when a part needs a specific recessed form to improve fit or assembly without changing the overall size or layout of the component.

Medical Devices

Medical devices may include undercuts in precision parts that require compact geometry, controlled engagement, or specialized assembly features. These can appear in surgical instruments, device housings, connectors, and other components where local recessed geometry supports how the part fits or functions.

In this field, the feature often needs to work within a small and highly defined space. Designers may rely on undercuts when a standard open shape cannot provide the needed clearance or engagement condition inside a precise medical component.

Electronics

Electronics components sometimes use undercuts in housings, connectors, shielding parts, and support structures where the geometry must fit into a compact assembly. The feature can help create local clearance, guide part positioning, or support how one component sits inside another.

Because electronics products often combine tight packaging with complex internal layouts, even a small recessed feature can be important. An undercut may help preserve the outer form of the part while adding the internal or side geometry needed for function and assembly.

Tooling and Equipment Industry

Tooling and equipment parts often rely on undercuts in bases, holders, guides, clamping elements, and machine accessories. In these applications, the recessed feature may help create a retained interface, controlled movement path, or localized fit condition within the equipment structure.

This industry uses undercuts in a very practical way. Many tooling components need geometry that supports setup adjustment, part holding, or repeated mechanical engagement, and an undercut can provide that function without requiring a more complex overall part shape.

Marine Equipment

Marine equipment may use undercuts in sealing components, mechanical connectors, shaft-related parts, and structural fittings where secure assembly and local recesses are needed. These features help parts fit together correctly while supporting performance in assemblies exposed to demanding service conditions.

In marine applications, component geometry often needs to balance strength, fit, and sealing support within limited space. An undercut can help provide that local functional detail while allowing the surrounding part structure to remain stable and unchanged.

Key Design Considerations for Undercut Machining

Undercut machining needs careful design review before production begins. A feature that looks small on a drawing can still create major machining limits in CNC manufacturing, so engineers usually evaluate the geometry, tooling, material, and cost impact before finalizing the part.

Part Geometry

Part geometry is one of the first things to review because it determines whether the undercut can be machined efficiently and reliably. The depth, width, location, and surrounding walls all affect cutter access and tool movement. If the feature sits too close to a shoulder, inside a narrow cavity, or beneath a deep overhang, machining becomes much more difficult.

Designers should also look at how the undercut relates to the rest of the part. A feature that works well in an open area may become impractical inside a restricted section. In many cases, small changes in geometry, such as adding more clearance or reducing feature depth, can make the undercut much easier to machine.

Material Properties

Material properties directly influence how the undercut will be machined and how stable the cutting process will be. Different materials respond differently to cutting force, heat, chip formation, and tool wear, especially in recessed areas where chip evacuation is already more difficult.

For example, aluminum usually allows easier cutting and better chip removal, while stainless steel may increase heat and cutting resistance. Engineering plastics may also behave differently because they can deform more easily under clamping force or tool pressure. A workable undercut in one material may become more difficult in another, even if the geometry stays the same.

Production Cost

Production cost matters because undercuts often increase more than machining time alone. They can also add setup effort, special tooling needs, slower cutting conditions, and more inspection work. Even a small undercut may raise the total cost if it makes the part harder to machine or control.

Compared with casting, molding, or forming, CNC undercut machining is usually more flexible for prototypes and low-volume parts because it does not require dedicated tooling. However, in higher-volume production, those processes may offer lower unit cost if the undercut shape can be built directly into the part.

The Main Challenges of Undercut Machining

Undercut machining creates real manufacturing challenges because the cutting area is partly hidden and harder to reach than an open feature. In CNC production, the difficulty usually comes from limited access, lower tool rigidity, poor chip flow, and tighter control over the finished surface. Even a small undercut can become a high-risk feature if the geometry leaves too little room for stable cutting.

Limited Tool Access

Limited tool access is one of the biggest challenges in undercut machining. The cutter often needs to reach behind a shoulder, beneath an overhang, or inside a confined space where a standard straight approach will not work. This restricted entry path makes the feature harder to machine and reduces the number of practical tooling options.

When access is limited, the cutter must follow a more controlled path and work with less clearance around the cutting zone. That increases the chance of interference with nearby walls or surfaces. It also makes setup planning more important because the machine must position the part and tool accurately before the cut begins.

Tool Deflection and Chatter

Tool deflection and chatter become more likely when the cutter has to extend into a recessed area. In many undercut operations, the tool neck is long and the cutting head reaches into a hidden section, which reduces overall stiffness. Once rigidity drops, the cutter becomes more sensitive to vibration and side load during machining.

This affects both dimensional accuracy and surface consistency. A tool that deflects too much may leave the feature out of profile or produce uneven contact surfaces. Chatter can also damage the finish and shorten tool life, especially when the cut happens in a narrow area with limited support around the feature.

Chip Evacuation Issues

Chip evacuation is another common problem because undercuts are often machined in enclosed or partly enclosed spaces. Chips do not leave the cutting area as easily as they do in open milling, so they may build up around the cutter during machining. That can interrupt the cut and reduce stability.

Poor chip flow can also increase heat in the machining zone. When chips stay trapped near the feature, the tool may recut them instead of removing fresh material cleanly. This can affect surface quality and create more stress on the cutter, especially in deeper or less accessible recessed features.

Surface Finish Control

Surface finish control is more difficult in undercut machining because the tool works in a restricted area with reduced rigidity and limited visibility. Even when the overall feature size is correct, the finished surface may still show marks, inconsistency, or burr formation if the cutting conditions are not well controlled.

This matters because many undercut support fit, sealing, or part engagement. In those cases, the surface condition is part of the function, not just appearance. A poorly finished undercut may still exist dimensionally, but it may not perform as intended once the part enters assembly or service.

Best Practices for Reliable Undercut Machining

Reliable undercut machining depends on good design choices, proper tooling, and stable process planning. Because these features are harder to access and more sensitive to cutting conditions, small improvements in planning can make a big difference in quality and cost. In CNC production, the best results usually come from keeping the feature practical, machinable, and clearly matched to the real function of the part.

Select the Right Tool

Tool selection for an undercut should match both the feature shape and the real access condition around it. In undercut machining, the goal is not only to reach the hidden area but also to cut it with enough clearance, rigidity, and stability. Shops usually evaluate the feature from several practical angles before choosing the final tool.

The right tool for an undercut is usually selected based on these factors:

• Undercut Geometry: Use a lollipop cutter for curved internal undercuts, a T-slot cutter for T-shaped grooves, and a dovetail cutter for angled undercuts.
• Tool Access Direction: Use a lollipop cutter or dovetail cutter when side access is available. Use a 5-axis setup with a lollipop cutter when the feature needs tool tilt to avoid interference.
• Clearance Around The Feature: Choose a reduced-neck end mill or long-neck cutter when the cutting area is partially blocked and the tool shank needs extra clearance.
• Required Reach and Rigidity: Use a shorter lollipop cutter or short-reach T-slot cutter whenever possible. Move to a long-neck tool only when the undercut depth or surrounding geometry requires extra reach.
• Tolerance and Surface Finish Requirements: Use a lollipop cutter for smoother blended surfaces, a form tool for repeatable profile accuracy in production, and a finishing pass with a smaller undercut tool when the feature needs tighter control.

Keep Undercuts Shallow

Shallower undercuts are usually easier to machine, easier to control, and less likely to create instability during cutting. As depth increases, tool reach becomes more difficult, rigidity drops, and the hidden cutting area becomes harder to manage. A deeper feature can also increase vibration and make chip removal less efficient. Keeping the undercut only as deep as needed helps maintain a more stable cutting condition and reduces unnecessary machining risk.

Secure the Workpiece

Stable workholding is essential because undercut machining often involves side cutting, limited clearance, and concentrated cutting forces in a small area. If the part shifts or vibrates during machining, the feature may lose accuracy or develop poor surface quality. A secure setup gives the cutter more consistent cutting conditions and helps protect the surrounding geometry. It also reduces the chance of variation when the same undercut must be repeated across multiple parts.

Avoid Custom Tooling

Standard tooling is usually the better choice when the feature can be designed around commonly available cutter sizes and profiles. Custom tools may solve a geometry problem, but they also add cost, sourcing time, and process complexity. They can make repeat production less flexible and harder to standardize. In many cases, small changes in width, depth, or angle allow the feature to work with standard cutters and make the overall process easier to manage.

Use Effective Toolpaths

The toolpath should allow the cutter to enter, cut, and exit the feature in a controlled and predictable way. Because the machining area is partly hidden, the tool cannot rely on the same open movement used for simple profiles. A poor path may increase tool stress, create unstable cutting, or leave incomplete geometry. A well-planned toolpath helps maintain the intended profile, improves process stability, and reduces unnecessary cutter contact inside the recessed area.

Eliminate Undercuts When Possible

Not every undercut needs to remain in the final design. If the same function can be achieved with an open slot, a simpler relief, or a more accessible feature, the part often becomes easier to machine and inspect. This can reduce tooling difficulty, setup effort, and quote cost. Removing a non-critical undercut is often one of the most effective ways to improve manufacturability while preserving the essential function and assembly performance of the part.

When to Use an Undercut and When to Avoid It?

Choosing whether to use an undercut is an important design decision in CNC machining. Some undercuts are essential for fit, clearance, sealing, or assembly, while others only make the part harder to machine, inspect, and quote without adding real functional value.

Use It for Functional Features

An undercut should be used when the feature supports a real engineering or assembly requirement that cannot be achieved as effectively with a simpler open geometry. In CNC part design, the key question is whether the recessed feature improves how the part fits, seals, engages, or functions in the final assembly.

Typical cases where an undercut is worth keeping include:

• The part needs local clearance for a mating component
• The design requires a retained or locking feature
• A sealing groove must sit in a recessed area
• A thread needs proper runout or relief space
• The assembly depends on hidden engagement geometry
• A standard slot or open recess cannot achieve the same function

Avoid It in Non-Critical Areas

An undercut should usually be avoided when it does not contribute directly to part performance, assembly, or fit. If the feature only changes a non-critical area or exists because of legacy design habits, it may add machining difficulty without creating a real functional benefit.

Typical cases where an undercut should be removed or simplified include:

• The feature is only cosmetic or non-functional
• An open slot or simpler recess can achieve the same result
• The undercut adds no benefit to fit or assembly
• The geometry creates difficult tool access without clear value
• The feature increases cost but does not improve performance
• The design can be revised to a more machineable form

Conclusion

Undercut machining plays an important role in CNC manufacturing because it allows engineers to create recessed features that standard cutting methods cannot easily reach. From T-slot and dovetail forms to reliefs, sealing grooves, and thread-related features, each undercut type serves a specific purpose in part function, fit, and assembly. At the same time, undercutting also increases demands on tooling, setup, machining stability, and cost control. 

If you need support with undercut features in custom CNC parts, DZ Making can help. Our team provides CNC milling, turning, 5-axis machining, and engineering support for complex geometries across metal and plastic components. If you are reviewing a part design or looking for a reliable machining partner, feel free to contact us for manufacturability feedback and a quote.