CNC Undercuts Design Guide: Types, Tool Access, and Cost-Saving Tips

Undercuts are common in machined parts that need locking features, sliding connections, retaining grooves, or clearance beneath a shoulder.

However, an undercut can be difficult to manufacture because a standard CNC end mill may not reach the required surface.

This CNC undercuts design guide explains the common feature types, how special cutters work, and what engineers can change to make parts easier and more economical to manufacture.

An undercut is a recessed feature that cannot be reached by a standard cutting tool from the normal approach direction.

Imagine a pocket with a lip extending over its inner wall. A conventional end mill can machine the exposed pocket, but it cannot easily cut beneath the lip.

Undercuts are used for several purposes:

1:Mechanical locking and sliding assemblies

2:T-slot mounting systems

3:Dovetail joints

4:O-ring sealing grooves

5:Thread reliefs and retaining-ring grooves

The important point is that an undercut is defined by restricted tool access, not simply by its shape.

Each type has different access requirements.

For example, a T-slot has a narrow opening but a wider cavity underneath. The cutting head must be able to enter through the opening and then remove material horizontally.

The cutting head may fit inside the feature, but the tool shank or holder may collide with the surrounding material.

That is why checking only the cutter diameter is not enough. The complete tool assembly must have a clear path into and out of the cutting area.

Special undercut cutters often have narrow necks or long reaches. This can reduce rigidity and increase the risk of vibration, poor surface finish, and tool breakage.

Hidden grooves can trap chips around the cutting area. This makes deeper undercuts harder to machine consistently and may increase tool wear.

Some undercut surfaces are difficult to inspect visually because they are hidden beneath an overhang.

For critical features, the supplier may need suitable probes, specialized gauges, or an appropriate CMM inspection strategy.

For a T-slot, the tool shank must pass through the entry slot without colliding with the walls.

For a dovetail, the cutter angle must match the intended profile.

For an internal groove, the cutting head, neck, and toolholder must all reach the feature without interference.

Whenever possible, use standard cutter sizes and angles rather than unusual dimensions that may require custom tooling.

An undercut should serve a function, such as locking a component, accommodating a seal, or allowing a mating part to seat properly.

If a recessed groove is hidden after assembly and provides no functional benefit, removing it may simplify machining without affecting the product.

Deeper undercuts usually require longer tool reach, which may reduce rigidity.

For preliminary design, some manufacturers use an undercut reach of around three times the cutting tool diameter as a conservative starting point. This is a rule of thumb, not a universal machining limit.

The appropriate reach depends on the cutter design, material, geometry, and machining conditions.

Sometimes the most economical solution is not to use a more complicated cutting tool. It is to change the design.

For example, imagine Mike, an automation engineer in the United States, designing an aluminum mounting block with an internal T-slot.

A conventional end mill can machine the top slot, but the wider lower cavity requires a T-slot cutter and careful tool-access planning.

If the application permits it, Mike could redesign the assembly using a simpler open slot and a separate T-shaped key secured with screws.

This approach can make both parts easier to machine using more conventional tools.

However, the redesigned assembly also needs proper alignment, fastening, and tolerance control. Adding another part is not automatically cheaper, especially when the assembly requires tight positioning or frequent adjustment.

For an actual product example of this geometry, see our Precision CNC Machined Aluminum Automation Guide Blocks with T-Slot Design.

These guide blocks are manufactured from aluminum billet and feature T-slots, guide slots, and mounting features for industrial automation applications. They illustrate why undercut geometry and tool access need to be considered together.

Undercuts can increase costs because they may require:

1:Special cutting tools

2:Additional setups

3:Slower machining conditions

4:More toolpath planning

5:Additional inspection

The most practical ways to reduce unnecessary cost are to simplify non-functional features, use standard tool sizes, and provide sufficient clearance for the selected cutter.

If a special undercut is required for the product to function, it should be retained. The goal is to make the necessary feature manufacturable, not to remove useful geometry simply to reduce machining time.

Our Custom CNC Machining Services support drawing-based components produced through CNC milling and multi-axis machining according to the part geometry and manufacturing requirements.

When requesting a quotation for an undercut feature, provide:

1:2D engineering drawing

2:3D CAD file

3:Material and grade

4:Undercut type and dimensions

5:Entry opening and required tool access

6:Critical tolerances

7:Surface finish requirements

8:Quantity

9:Inspection requirements

For internal or hidden undercuts, include a section view in the drawing when possible. This helps the supplier understand the complete geometry without having to infer it from an external view.

Successful CNC undercut design depends on more than the final shape. The cutter must be able to enter, reach the cutting surface, remove material, and withdraw without interference.

Keep undercuts functional, use practical dimensions, and check tool access before releasing the drawing.

If a feature is difficult to machine, consider whether a standard cutter, a different machining approach, or a two-piece assembly can meet the same functional requirement more efficiently.

Have a CNC component with T-slots, dovetail grooves, retaining grooves, or other undercut features?

Send your 2D drawing, 3D CAD file, material, quantity, and critical tolerances for review.