How to Design CNC Parts with Undercuts Without Increasing Cost

Undercuts have a reputation. Mention one to a machinist and you’ll usually get a slight wince, because undercuts are where an otherwise straightforward part suddenly needs special tooling, extra setups, or a five-axis machine. They’re the feature most likely to turn a cheap quote into an expensive one – not because they’re impossible, but because designers spec them without knowing what it takes to cut them.

Here’s the good news, though: undercuts don’t have to be expensive. Most of the cost comes from *non-standard* undercuts – an oddball groove width that needs a custom tool, a radius no stock cutter matches, a geometry that forces an extra setup that a small tweak would’ve avoided. Design undercuts with the tools in mind and they often add almost nothing to the price. This guide explains what undercuts actually are, the tools that cut them, and the specific design moves that let you keep the feature you need without paying a premium for it.

Quick answer: An undercut is a recessed feature that a standard straight-down cutting tool can’t reach — such as an internal groove, an O-ring channel, or a cavity wider beneath its opening. Undercuts are machined with specialty tools like T-slot, lollipop (undercut), and dovetail cutters, or with multi-axis machining. They add cost, but standardizing widths and radii to match stock tools keeps that cost minimal.

What an Undercut Actually Is

Strip away the jargon and an undercut is simply a feature a tool can’t reach by plunging straight down from the top. A normal end mill approaches vertically and cuts what’s directly below it. An undercut hides from that approach – it’s tucked under an overhang, recessed into a side wall, or wider at the bottom than at its opening.

The classic examples make it concrete. An O-ring groove machined into the inside of a bore. A T-shaped slot that’s wider below the surface than the opening that leads to it. A dovetail channel with angled walls. A relief groove at the base of a shoulder so a mating part seats flush. In every case, the geometry curls back under itself, and a straight tool coming from above simply can’t get to the whole feature. That’s the defining trait – not the shape, but the inaccessibility.

The Tools That Cut Undercuts

You beat an undercut with a tool shaped to reach around the obstruction, or by tilting the part so the feature becomes accessible. Here are the workhorses.

Each specialty cutter is essentially a tool that cuts *sideways* or *around* rather than straight down. The catch is that many come in fixed, standard geometries — a T-slot cutter makes a specific slot width, a dovetail cutter cuts a specific angle. Design to those standard sizes and the shop uses a stock tool. Design to something in between and you’re paying for a custom cutter. For complex or organic undercuts that no standard cutter suits, Mecanizado CNC de 5 ejes is often the cleaner answer — it simply rotates the part so the undercut faces the tool, turning an “impossible” feature into an ordinary cut.

Why Undercuts Add Cost (and When They Don’t)

Undercuts drive cost through three channels, and understanding them tells you exactly where to design smart.

• Specialty tooling. Non-standard dimensions force custom tools, which cost money and lead time. Standard dimensions use stock tools and add almost nothing.
• Extra setups. An undercut on a face the tool can’t reach in the current orientation means unclamping, refixturing, and re-zeroing — each setup adds labor and a small accuracy risk.
• Slower cutting. Undercut tools often have thin, delicate necks (a lollipop cutter especially), so they run at conservative feeds to avoid snapping. That means more machine time.

Here’s the same idea as a quick reference — the same undercut can be cheap or costly depending entirely on how it’s designed.

The key insight: it’s rarely the undercut itself that’s expensive – it’s the *non-standard* undercut. A groove sized to a stock cutter, positioned so it’s reachable in an existing setup, can be nearly free. The same one, sized oddly and buried on a fifth face, can double a quote. Getting your model right also matters here, since an ambiguous file hides undercut intent from the programmer – which is why the best file format for CNC machining is worth getting right before you send it.

Design Rules to Keep Undercut Cost Down

These are the habits that let you keep the feature and lose the premium.

1. Standardize stock tool sizes. Size grooves, slots, and dovetails to common cutter dimensions. Ask your shop for its standard undercut tooling and design to it – this single move avoids most custom-tool costs.
2. Match radii to stock cutters. Just like internal corners, undercut radii should match a standard tool rather than forcing a special grind.
3. Keep undercuts shallow and accessible. A shallow undercut lets a stubbier, stiffer tool do the work fast. Deep undercuts need long, thin necks that run slow.
4. Group features on reachable faces. Position undercuts so they’re accessible in setups the part already needs, avoiding an extra fixturing operation just for one groove.
5. Ask whether you need the undercut at all. Sometimes a chamfer, a split part, or a small redesign delivers the same function without any undercut. Always worth checking before you commit.
6. Loosen tolerances on the undercut. Thin-necked tools deflect, so a tight-toleranced undercut is doubly hard. Ease the tolerance and the shop cuts it faster.

These flow directly from broader design rules for CNC milling parts — and when an undercut is genuinely tricky, it’s worth talking it through before finalizing the model. Our team offers engineering support exactly for these judgment calls, and complex undercut work often routes through our Servicio de mecanizado CNC where multi-axis capability is on hand.

A Quick Word on Tolerances and Undercuts

Because undercut tools tend to be small and thin-necked, they deflect more than a stout end mill, which makes tight tolerances on undercut features genuinely harder to hold. If you tolerate an O-ring groove or a dovetail as tightly as a critical bore, you’re forcing slow, careful passes and more inspection. Reserve tight tolerances for the surfaces that actually seal or mate, and let the rest breathe – the same discipline from our CNC machining tolerances guide applies with extra force here. An undercut that’s tolerance sensibly is cheap; one that’s over-specified is not.

Preguntas frecuentes

What is an undercut in CNC machining?

An undercut is a recessed feature that a standard straight-down cutting tool can’t reach – such as an internal O-ring groove, a T-slot that’s wider below the surface, or a dovetail channel with angled walls. The geometry curls back under an overhang or side wall, so it needs a specialty tool or multi-axis machining to cut.

How are undercuts machined?

Undercuts are cut with tools shaped to reach sideways or around an obstruction – T-slot cutters, lollipop (undercut) cutters, dovetail cutters, woodruff cutters, and internal grooving tools on a lathe. Alternatively, 5-axis machining tilts the part so the undercut faces the tool, letting a standard cutter reach it without special tooling.

Do undercuts make CNC parts more expensive?

They can, but mainly when they’re non-standard. Custom dimensions force special tooling, awkward positions force extra setups, and thin-necked tools cut slowly. An undercut sized to a stock cutter and placed on an accessible face adds very little cost — it’s the oddly sized, hard-to-reach undercut that drives the price up.

What is a lollipop cutter used for?

A lollipop cutter – also called an undercut or spherical cutter – has a ball-shaped head on a thin neck. The ball reaches under overhangs and into recesses to cut spherical undercuts and internal reliefs that a normal end mill can’t access. Its thin neck is delicate, so it runs at conservative feeds to avoid breaking.

How can I design undercuts to minimize cost?

Standardize undercut widths, angles, and radii to common stock tools, keep them shallow and accessible, and position them on faces the part already presents to the tool so no extra setup is needed. Loosen tolerances on undercut features, and check whether a chamfer or minor redesign could eliminate the undercut entirely.

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About the author

Lewei Precision Engineering Team  is a manufacturing writer and CNC process specialist who has reviewed thousands of part drawings for manufacturability. He focuses on design-for-manufacturing guidance that helps engineers keep the features they need while avoiding the cost traps hidden in tooling and setup – turning shop-floor know-how into rules a designer can use before cutting the first chip.