CNC Machining Undercuts: Design, Tooling and Cost Guide

CNC machining undercuts are recessed features that a standard straight tool cannot reach from a simple vertical direction. They appear in retaining-ring grooves, O-ring seats, dovetails, internal reliefs, connector features and parts that must capture or clear another component.

Undercuts can be machined accurately, but they require suitable tool access, cutter geometry and inspection planning. This guide explains common undercut types, design rules, manufacturing methods, tolerances, materials and cost drivers for CNC machined prototypes and low-volume parts.

What Is an Undercut in CNC Machining?

An undercut is a feature located behind an overhanging surface or inside a recess where the cutting edge must approach from the side, below or at a special angle. A normal end mill moving directly downward cannot always create it.

The feature may be external and visible or internal and hidden inside a bore or pocket. Its manufacturability depends on opening size, reach, depth, tool shank clearance and the number of machine orientations.

Why Undercuts Are Challenging

Common Applications

Undercuts are found in snap-fit interfaces, retaining-ring grooves, seal glands, wire clearances, bearing reliefs, bayonet features, T-slots, dovetails, internal threads, lathe relief grooves and components that need a hidden mechanical capture.

Some undercuts are essential to product function. Others result from CAD geometry that could be simplified. A design review should separate necessary features from shapes that add cost without improving performance.

External Undercuts

An external undercut is located on an outside face, shaft or protruding feature. It is often easier to see and inspect than an internal undercut.

External grooves can be produced by turning, side milling or specialized cutters. The surrounding geometry must still allow the tool body and holder to clear the part.

Internal Undercuts

Internal undercuts sit inside a bore, cavity or pocket. Tool reach and visibility are more restricted, and chips can remain trapped behind the feature.

Increase the access opening where possible and avoid placing a deep narrow undercut behind a small entrance.

One-Sided Undercuts

A one-sided undercut extends beneath one wall or ledge. A lollipop cutter, slotting cutter or angled setup may reach it.

Provide an open direction for the tool to enter and exit. Closed ends may require special toolpaths and leave an unavoidable radius.

Full-Circumference Grooves

A circular undercut can run around a shaft or inside a bore. CNC turning is often efficient for rotational parts, while circular interpolation or form tools can machine similar features on a mill.

Width, diameter, corner radii and axial location should reflect the mating seal, ring or relief function.

O-Ring Grooves

O-ring grooves require controlled width, depth, surface condition and edge geometry. The correct gland design depends on seal type, squeeze, stretch, pressure, motion and material.

Do not copy a generic groove without checking the seal supplier’s design data. Include sealing surface finish and inspection requirements on the drawing.

Retaining-Ring Grooves

Retaining-ring grooves locate a standard ring inside a bore or on a shaft. Groove geometry should match the selected ring standard and load direction.

The tool must fit the groove width while providing enough stiffness. Narrow deep grooves may require a fragile form tool and slower cutting.

Thread Relief Grooves

A relief groove provides space for a threading tool to exit and helps a mating component reach a shoulder. It is common near external or internal turned threads.

Specify a standard relief form when applicable. An excessively large relief can reduce shaft strength or interfere with sealing.

Dovetail Undercuts

Dovetails use angled flanks to guide or retain another part. They are machined with a dovetail cutter after an access slot or pocket has been opened.

Tool angle, tip width, neck diameter and reach influence the smallest possible feature. Sharp internal corners are not realistic at every transition.

T-Slots

A T-slot has a narrow entrance and wider lower cavity. It is generally produced in two steps: first the straight access slot, then the wider cross section with a T-slot cutter.

The opening must be large enough for the cutter neck, and the lower cavity must allow chip evacuation.

Lollipop-Cutter Features

A lollipop or spherical undercut cutter can machine contours behind a wall and may support five-axis toolpaths. It is useful for curved transitions and complex hidden surfaces.

The narrow neck limits rigidity. Avoid long reach and heavy engagement where a more accessible shape would work.

Keyways and Internal Slots

Internal keyways and slots may resemble undercuts when the tool cannot enter from the end. Broaching, slotting, wire EDM or special milling tools may be considered.

Process choice depends on length, width, corner condition, material and production quantity.

Back Counterbores and Spotfaces

A back counterbore creates a recessed bearing surface on the far side of a hole when the back face is not directly accessible. Specialized back-spotfacing tools can pass through the hole and deploy the cutting edge.

This process should be used only when the assembly needs the hidden seat and direct access cannot be provided.

Custom Form Undercuts

A custom profile may need a dedicated form cutter, multiple coordinated toolpaths or EDM. The full geometry must be defined in a section view or model-based annotation.

Custom tooling can be practical for repeated production but may be expensive for a small prototype quantity.

Standard Undercut Cutters

Standard tools are available in common slot widths, angles and diameters. Designing around them reduces lead time and replacement risk.

Ask the supplier whether a small dimension change can convert a custom feature to a stocked tool size.

Neck Diameter

The tool neck passes through the access opening while the larger cutting head removes material behind it. A small neck increases access but reduces stiffness.

Provide the largest access width the design can accept. A stronger neck can improve finish, accuracy and cycle time.

Cutting-Head Diameter

The cutting head must fit through or around the available approach path and still generate the required reach beneath the wall.

An excessively wide undercut relative to its entrance may be impossible with a one-piece tool and may require assembly redesign or another process.

Tool Reach

Reach is the distance from the accessible opening to the farthest cutting location. Longer reach increases deflection and vibration.

Move the undercut closer to an open edge, reduce wall thickness above it or provide a secondary access hole where function permits.

Tool-Holder Clearance

CAD models often show enough room for the cutting tip but ignore the shank and holder. The complete tool assembly must clear nearby walls, bosses and clamps.

A collision-free tool angle should be verified during DFM and CAM simulation.

Radial Undercut Depth

Radial depth controls how far the recess extends behind the visible wall. Greater depth requires a larger difference between cutter head and neck diameters.

Keep the depth to the functional minimum and use generous transition radii when possible.

Axial Width

Groove width must accommodate the mating component and manufacturing tolerance. A very narrow width may force a thin cutter, while a very wide groove may require multiple passes.

Standardize widths across the part when different features serve similar functions.

Corner Radii

Rotating cutters leave radii at internal transitions. A sharp inside corner usually requires EDM, broaching, a specially ground form tool or a design change.

Specify maximum or minimum radii only where the mating geometry requires them.

Entrance Chamfers

A chamfer can help guide a seal, ring or assembly into an undercut feature. It can also improve tool entry and remove sharp edges.

Control chamfer size near sealing grooves because an oversized edge break may reduce support or create a leak path.

Undercut Access Openings

The access opening must accommodate the tool neck, cutting head path, chips and inspection method. A small entrance connected to a large hidden cavity is costly and sometimes impossible.

Enlarge the opening, split the part or modify the mating design if the hidden volume greatly exceeds available access.

Open Versus Closed Ends

An open-ended groove allows the tool to enter and exit beyond the finished area. A closed-ended groove requires the tool to plunge, retract or turn within the feature.

Open ends generally simplify machining and chip removal. If containment is required, allow realistic end radii.

Tool Approach Direction

Place undercuts so they align with existing machining orientations when possible. A feature requiring a unique angle may add a setup or five-axis positioning operation.

The drawing should define orientation from functional datums rather than from an arbitrary CAD face.

Three-Axis Milling

Three-axis machines can produce many accessible undercuts with specialized cutters and suitable fixtures. The tool axis remains fixed during cutting.

Parts may need to be repositioned when features face several directions, which adds setup and datum-transfer risk.

3+2 Machining

In 3+2 machining, rotary axes position the part or tool at a fixed angle before a three-axis cutting operation. This can provide direct access to angled undercuts with fewer custom fixtures.

The approach is effective when the feature is reachable from a stable indexed orientation.

Simultaneous Five-Axis Machining

Five-axis motion can keep a lollipop or tapered tool oriented to complex hidden surfaces. It improves access but does not remove cutter-neck and holder-clearance limits.

Use simultaneous motion only when the geometry requires it. Simpler indexed operations may cost less.

CNC Turning

Rotational grooves and reliefs are often economical on a CNC lathe. A grooving tool moves radially or axially to create the undercut.

Tool reach, chip control and clearance inside small bores remain important, especially for deep internal grooves.

Mill-Turn Machining

Mill-turn equipment combines rotational and milling operations in one machine. It can reduce transfers for parts containing turned grooves plus off-axis holes or pockets.

The best machine choice depends on part family, volume and local supplier capability.

Electrical Discharge Machining

EDM can produce hard-to-reach shapes in electrically conductive materials. Wire EDM requires a continuous wire path, while sinker EDM uses a shaped electrode.

EDM is slower than conventional cutting for many features but can create sharp details, hard-material geometry and profiles that rotating tools cannot reach.

Broaching

Broaching uses a toothed tool to generate a slot or profile. It can be efficient for internal keyways and repeated shapes when the part allows a through path or suitable machine access.

Tooling cost and geometry make it less attractive for some one-off prototypes.

Splitting the Part

A hidden cavity can sometimes be converted into two directly accessible components that are later fastened, bonded, brazed or welded.

This may reduce machining difficulty but introduces alignment, joint strength, sealing and assembly considerations.

Using Standard Hardware

A retaining ring, insert, nut, clip or separate bracket may replace a complex machined undercut. Standard components can reduce machining risk and improve serviceability.

Compare total assembly cost rather than evaluating the machined part alone.

Aluminum Undercuts

Aluminum is generally machinable with sharp tools and good chip evacuation. Thin walls around an undercut can still deflect or distort.

Consider anodizing allowance, masking and edge protection around functional grooves.

Stainless Steel Undercuts

Stainless steel can create higher forces and heat. A slender undercut tool may rub or work-harden the surface if engagement is unstable.

Reduce reach, use suitable tooling and maintain a consistent cutting action.

Titanium Undercuts

Titanium requires careful heat and tool-wear control. Hidden recesses can make coolant delivery and chip evacuation difficult.

Early supplier review is valuable for narrow or deep features on high-value titanium components.

Plastic Undercuts

Engineering plastics can flex under side-cutting force and may retain burrs. Thermal expansion also affects measurement.

Use sharp tools, gentle clamping and sufficient wall support. Inserts or multi-part designs may be better for repeated mechanical capture.

Undercut Tolerances

Width, depth, diameter, position and profile may all affect function. Apply tight tolerances only to the dimensions that control retention, sealing or fit.

A hidden feature with a tight general tolerance can be expensive to machine and verify without improving assembly.

Groove Width Inspection

Accessible grooves can be measured with calipers, micrometers, pins, optical systems or dedicated gauges. Internal grooves may require specialty anvils or functional masters.

Agree on the inspection method before production if direct access is limited.

Groove Depth Inspection

Depth can be checked from a controlled datum using mechanical gauges, probe systems, optical methods or section-specific fixtures.

Define whether depth is radial, diametral or normal to an angled surface.

Profile Inspection

Dovetails, sealing glands and custom undercuts may require optical comparison, scanning, CMM probing or a functional gauge.

A complete profile tolerance can be clearer than multiple independent dimensions when the entire shape matters.

Surface Finish

Surface roughness can influence seals, sliding interfaces and fatigue-sensitive reliefs. Tool deflection and chip recutting can damage a hidden surface.

Specify roughness on functional faces only and state the measurement direction when relevant.

Deburring

Burrs are difficult to see and remove inside undercuts. Loose material can damage seals, block fluid systems or interfere with assembly.

Provide edge-access space and define acceptable edge breaks without requiring an impossible sharp corner.

Cleaning Hidden Recesses

Undercuts can trap coolant, chips, abrasive media and cleaning residue. Medical, optical and fluid applications may require controlled washing and inspection.

Include cleanliness and packaging requirements in the quotation package.

Common Undercut Defects

• Incorrect groove width or depth

• Tool chatter on the hidden surface

• Taper caused by cutter deflection

• Burrs remaining behind the entrance wall

• Incomplete corners or end radii

• Position error after an additional setup

• Surface scratches from trapped chips

• Coating buildup in a functional groove

How to Reduce Undercut Cost

• Remove nonfunctional undercuts from the design

• Use standard groove widths and cutter angles

• Increase the access opening and tool-neck clearance

• Reduce hidden reach and radial depth

• Allow realistic corner and end radii

• Align features with existing machining orientations

• Limit tight tolerances to functional dimensions

• Consider an assembly or standard hardware alternative

Prototype Versus Production

A prototype may be produced with a flexible toolpath, standard undercut cutter or extra setup. Repeated production may justify custom form tooling, dedicated fixtures or a redesigned near-net blank.

Discuss expected annual quantity so the process can scale without changing critical interfaces.

Information to Put on the Drawing

• Section views showing hidden geometry

• Groove width, depth, diameter and position

• Required corner and end radii

• Functional datum references

• Surface roughness on sealing or sliding faces

• Edge-break and burr requirements

• Surface finish, coating and masking

• Inspection method for critical hidden features

Information to Send for a Quote

• 3D CAD model and dimensioned drawing

• Material grade and condition

• Purpose of each critical undercut

• Mating ring, seal or component data

• Prototype and production quantities

• Tolerance and surface-finish requirements

• Cleaning, certification and inspection needs

• Assembly orientation and access restrictions

Questions to Ask a CNC Supplier

• Can a standard cutter produce the feature?

• Does the opening provide enough neck and holder clearance?

• Will an extra setup or five-axis motion be required?

• How will chips and burrs be removed?

• How will the hidden size and profile be inspected?

• Would a small geometry change reduce tooling risk?

• Is EDM, broaching or a split assembly more economical?

Frequently Asked Questions

Can every CNC undercut be machined?

No. The tool head, neck, shank and holder need a collision-free path. A large hidden cavity behind a very small opening may require a split design, EDM or another manufacturing method.

Are undercuts expensive?

They can be more expensive than directly accessible features because they require special cutters, slower passes, extra setups and additional inspection. Standardized, shallow and accessible undercuts are usually more economical.

What cutter is used for an undercut?

Common options include slotting, T-slot, dovetail, lollipop and grooving tools. The correct cutter depends on the profile, access opening, reach, material and machine orientation.

Can five-axis machining solve all access problems?

Five-axis motion improves approach options, but the physical cutter and holder must still fit. It cannot reach through an opening smaller than the required tool path.

How should an internal undercut be inspected?

Depending on access and tolerance, inspection may use specialty gauges, optical methods, CMM probes, scanning or a functional mating master. The method should be agreed before production.

Final Design Recommendations

Use an undercut only when it supports retention, sealing, clearance or another clear function. Make the opening and approach as generous as possible, minimize reach and design around standard cutters.

Provide section views, functional datums and a realistic inspection plan. Early DFM review can often preserve the product function while reducing special tooling, setups and scrap risk.

Request a CNC Undercut Machining Quote

Aluprototype provides precision CNC milling and turning for metal and engineering plastic prototypes and low-volume production. Our team can review undercut access, tool clearance, groove geometry, tolerances and inspection requirements before machining.

Explore our CNC machining capabilities, read the related CNC prototype machining guide, or request a quote with your CAD model, drawing and quantity.