CNC Undercuts: What They Are, Why They’re Expensive, and How to Design Them Out
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.
Why Undercut Features Increase CNC Machining Cost
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.