Undercut in CNC Machining

An undercut is a recessed feature that extends beneath an overhanging section of a part. In CNC milling, the overhang blocks a standard straight end mill from reaching the recessed area directly from above.

A T-slot makes this easy to visualize. Viewed from the end, the slot has a narrow opening above a wider channel. The recessed areas beneath the projecting edges are undercuts. This shape can retain a matching nut while allowing it to slide along the slot, creating an adjustable fastening point.

What Matters in Design and Machining

For designers, an undercut can provide retention, guide a mating component, or create assembly clearance. The challenge is preserving that function while giving the machining tool enough room to work.

For machinists, access involves more than reaching the finished surface. The tool must enter the feature, remove material, and withdraw without hitting the part or its fixtures. A T-slot cutter, for example, has a cutting head wider than its neck, allowing it to cut beneath the projecting edges.

The farther a cutter must reach, the more demanding machining can become. A long tool extension or slender neck can bend or vibrate during cutting, affecting accuracy and surface finish. Lighter cuts, additional passes, or another setup may be necessary, increasing production time. Hidden surfaces can also require specially shaped measuring probes for inspection.

An undercut does not automatically require five-axis machining. The appropriate approach depends on the geometry and available access; additional machine movement does not remove every interference problem.

Details Designers Can Overlook

Keep the function, simplify the access. Before adding an undercut, identify what it must accomplish. Preserve features needed for retention or assembly. Where the recess serves no essential purpose, consider changing the opening or surrounding geometry so the surface is easier to machine.

Leave a route for the entire tool. A cutting head that fits inside the finished recess may still be unable to pass through its entrance. An open slot end or a suitable entry opening may provide access. Check clearance for the cutting head, neck, shaft, and holder throughout entry, cutting, and withdrawal—not just at the final position.

Choose geometry that suits available tooling. Discuss slot width, recess depth, angles, and corner radii with the machinist. Avoid unnecessarily deep, narrow features that require slender tools. Also check the material remaining in the overhanging edges, slot floor, and nearby walls.

Show hidden details clearly. Use a section view—a drawing that shows an imaginary cut through the part—to explain the recessed profile. Specify the dimensions that control fit and function, including relevant widths, depths, positions, angles, and radii. Keep the drawing consistent with the CAD model, avoid unnecessary tight tolerances, and confirm how critical hidden surfaces will be measured.