Can CNC Machine Undercuts?

Yes, in most cases, CNC machining undercuts is possible. Many buyers send drawings that include a hidden groove, an internal snap-fit lip, or a T-slot, only to receive a flat "can't do" or a quote two or three times higher than expected. This article explains how undercut machining actually works, when the feature becomes difficult or expensive, and the design choices that keep costs reasonable.

An undercut is any recessed feature whose surfaces cannot be reached by a standard end mill coming from a single direction. The tool can enter the opening, yet the overhanging material blocks a straight path and prevents the cutting edges from contacting the hidden surfaces. In simple terms, the cutter goes in but cannot reach sideways or underneath without special geometry or extra setups.

Common examples already appearing on many drawings include:

O-ring grooves on internal walls or shafts

Dovetail slots for sliding or locking joints

T-slots for fixtures and modular tables

Internal snap-fit lips or retaining features

Side holes or pockets that widen inside the part

These features cause concern because standard tooling cannot produce them, extra setups raise the risk of tolerance stack-up, and custom cutters or multi-axis machines add cost. Understanding undercut machining early prevents most of those surprises.

CNC shops machine undercuts every day with proven methods. Here is how it is done in practice.

Special undercut tools
A T-slot cutter has a narrow shank with a wider disc-shaped cutting head. It first enters through a pre-machined slot or hole, then cuts laterally to form the wider undercut. Lollipop cutters have a spherical or rounded tip on a reduced neck and work well for curved or one-sided recesses. Standard T-slot widths typically range from about 3 mm to 40 mm; designing to these sizes avoids custom tooling.

Dovetail cutters for angled undercuts
Dovetail cutters produce the classic trapezoidal profile used for locking slides and precision joints. Standard angles are 45° and 60°. Matching the design to a commercially available angle keeps both lead time and cost low. Non-standard angles usually require a custom-ground tool.

5-axis CNC for undercut parts
When the undercut is deep, internal, or oriented at complex angles, a 5-axis machine tilts either the tool or the workpiece. This gives the cutter access that a 3-axis setup cannot achieve in a single clamping. Many complex undercut parts become practical once the part or tool can be oriented freely.

Multi-setup machining or turn-mill centers
Simple undercuts can often be finished on a 3-axis mill by re-fixturing the part so the feature faces the spindle. Turn-mill machines handle undercuts on rotational parts-such as shaft relief grooves or internal O-ring lands-in one setup. When milling reaches its limit, wire or sinker EDM remains a reliable backup for the most inaccessible features.

Knowing how to machine undercut features depends on geometry, access, and volume. A 3-axis CNC machine can cut many undercuts when the right special tools and entry path exist; deeper or multi-sided features benefit from 5-axis capability.

Three situations push cost and difficulty higher:

The undercut is very deep or narrow relative to the opening, so only long, fragile tools can reach.

Entry clearance is insufficient for even a reduced-shank cutter.

The feature requires a non-standard profile that forces a custom tool.

In these cases the comparison between CNC vs 3D printing for undercuts becomes practical.

Additive processes ignore tool-access limits, making them attractive for one-off complex internal channels or lattices. For functional metal parts that need strength, tight tolerances, and good surface finish, CNC machining undercuts remains the preferred route once the design is optimized.

Following established undercut design guidelines dramatically reduces both risk and price:

Specify standard tool sizes (T-slot widths in whole millimeters or common inch fractions; dovetail angles of 45° or 60°).

Keep undercut depth reasonable relative to width and tool reach (ideally within 3–4× the cutting diameter for rigidity).

Provide clear entry for the tool shank-slot or hole width must exceed shank diameter plus clearance.

Add internal corner radii that match the tool tip radius; sharp internal corners are expensive or impossible.

Leave adequate clearance between the undercut wall and any opposing surface (a common rule is at least four times the undercut depth).

Submit drawings early for a design-for-manufacturability review so potential issues are caught before tooling is ordered.

These simple choices directly influence how much undercut machining cost you will face. The more standard and accessible the feature, the closer the price stays to ordinary milling.

The challenge
A customer needed 120 aluminum fixture plates containing internal T-slots and one-sided retaining lips. The undercuts sat deep inside pockets, and the original drawings specified non-standard slot widths. Another shop had quoted a 22-day lead time and a unit price nearly three times higher than typical milling work.

The approach
We adjusted the design to standard T-slot cutter sizes and widened the entry slots by 0.8 mm for tool clearance. Parts were machined on a 5-axis platform so the undercuts could be reached with short, rigid tools after a single primary setup. Critical dimensions were held to ±0.02 mm and verified with a CMM plus custom gauges.

The result
First-article approval arrived in 9 days. The full batch of 120 pieces shipped in 14 days total, with zero tool breakage and consistent quality. The final unit price came in 38 % lower than the earlier non-optimized quote. The customer now uses the same process for repeat orders.

Before placing an order, ask these questions:

Do you have 5-axis equipment (or turn-mill capability) in-house?

Have you produced similar undercut parts-can you share photos or inspection reports?

Will you perform a free design-for-manufacturability review on the drawings?

Can you supply a sample or first-article before full production?

What inspection methods and reports do you provide for undercut features?

A capable CNC undercut parts manufacturer will answer these questions clearly and offer practical suggestions rather than simply declining the job. Look for a custom CNC machined parts factory that treats undercuts as a routine capability rather than an exception. For higher volumes, confirm they can scale to wholesale CNC machined parts while maintaining the same process control.