If you’ve ever requested a CNC quote and gotten back a note about an undercut or a flat rejection, this article is for you.
Undercuts are one of the most common design features that either significantly drive up part cost or make a part un-machineable. They’re not always obvious when you’re designing in CAD, but they’re a hard reality once your file hits the shop floor. At SendCutSend, our CNC machining service can handle most complex geometries across 3-axis, 3+2-axis, and 5-axis setups, but undercuts still have real physical limits, and understanding them upfront will save you time, money, and redesign cycles.
What Is an Undercut in CNC Machining?
A standard end mill cuts from the top down. It spins, plunges, and moves laterally but it always approaches the workpiece from above. Any surface geometry that hides from that approach direction is an undercut.
Think of it this way: hold a flashlight directly above your part and point it straight down. Any surface that stays in shadow is an undercut. The tool can’t reach those areas for the same reason the light can’t.
Common examples include dovetail slots, T-slots, grooves cut into sidewalls, and the underside of any lip or ledge. Basically anything where material hangs over the cutting path.
As shown above, getting a standard slot requires nothing more than a regular end mill. Getting the undercut version of that slot requires a specialized keyway mill that can reach laterally under the material. Same-looking feature, completely different tooling story.
Why Undercut Features Increase CNC Machining Cost
The tooling alone is the first problem. You can’t cut an undercut with a standard end mill, which means you’re now into specialty territory such as lollipop cutters, T-slot cutters, keyseat cutters. These tools are less common, more expensive to source, and slower to run.
On top of that, the job requires having the exact cutter geometry needed for your specific feature. If that cutter isn’t on the shelf, it means a custom order or a redesign on your part.
Undercut Depth Limits: Why Tool Reach Is the Real Constraint
An undercut tool works by mounting a horizontal cutting edge on a vertical shaft. The cutting edge does the work, but the shaft is the limiting factor. Every millimeter of depth you need is another millimeter of shaft that has to clear the surrounding geometry.
There’s no universal rule for maximum undercut depth. It depends entirely on the tool and the part. But the relationship is simple: the deeper the undercut, the fewer tools can physically reach it.
It gets worse when the undercut sits at the far wall of a deep pocket. In that case, the tool shaft will collide with the near wall before the cutter ever reaches the feature. At that point, the undercut isn’t just expensive, it’s physically unreachable. No tool, no workaround, no part.
This is physics, not opinion. The geometry of the tool and the geometry of the part have to agree and they often don’t. Designing an undercut at the base of a deep, narrow pocket is one of the fastest ways to get an un-machineable quote back.
How to Avoid Undercuts: Design Alternatives That Actually Work
If your drawing has an undercut on it, you’ve got three real paths forward. Here they are, roughly in order of cost and complexity.
1. Design it out
This is almost always the right answer. Most undercuts exist because no one stopped to think about how the part actually gets made. The functional requirement, the thing the undercut was supposed to accomplish, can almost always be met a different way. Nine times out of ten, the undercut wasn’t the point; it was just the first geometry someone sketched.
Before you move on to the other options, spend ten minutes asking whether the feature is doing something your design actually needs. If it isn’t, remove it.
2. Split the part
If the undercut is critical and you can’t design around it, consider splitting the part at the center of the feature. That gives you two conventional pieces with no undercuts, since you now have room to approach the feature from the open side. Machine both halves, then fasten or bond them together.
You’re trading one hard part for two easy ones plus an assembly operation. Depending on geometry and volume, this is often dramatically cheaper than trying to machine the undercut directly.
3. 5-axis machining
More approach angles mean the tool can reach surfaces a 3-axis setup never could. It’s the brute-force option. You’re paying for the machine time and the programming, and there are still physical limits the geometry can hit.
The best option is almost always the first one. The cheapest undercut is the one that isn’t there.
4. Stacked parts
This is a play on 2, splitting the part. Instead of splitting it vertically, you can split the part “horizontally” at the top of the undercut, and use a stacked part to overhang. This can be a simple 2D cut part (Waterjet, laser, etcetera) or another CNC part.
This has the benefit of being removable, which oftentimes enables designs at the assembly level that would be impossible (think captive hardware) otherwise.
Key Takeaways: Designing CNC Parts Without Undercuts
An undercut is any surface the tool can’t reach from its standard approach direction. It requires specialty tooling, extra setups, and a machinist who has to get creative. The deeper it goes, the fewer tools can get there. Past a certain point, the geometry becomes physically unreachable.
Before you upload your file to SendCutSend, take one more pass at your drawing and ask: is this undercut doing something my design actually needs, or is it just the first shape I sketched?
In most cases, a small design change now is worth far more than a re-quote or a scrapped part later.
