Undercuts in CNC machined parts—features such as T-slots, dovetails, O-ring grooves, or internal snap-fits—cannot be reached by a standard end mill from a conventional direction. Poorly designed undercuts often lead to higher costs, longer lead times, and special tooling requirements. Following proven design-for-manufacturability (DFM) best practices helps engineers create functional parts that are easier and more economical to machine.
1. Eliminate unnecessary undercuts whenever possible
The most effective strategy is to avoid undercuts entirely if they are not functionally required. Many undercuts can be replaced by redesigning the part: split it into two components that assemble with fasteners, convert an internal feature into an external profile, or use an open groove instead of a closed undercut. Removing non-essential undercuts typically reduces machining cost by 30–50% and simplifies inspection.
2. Ensure clear tool access and adequate clearance
A cutting tool must first enter through an existing opening (slot, hole, or edge) and then move laterally. Make the entry width larger than the tool shank diameter and add 0.5–1 mm clearance. For one-sided undercuts, keep the primary opening width at least four times the undercut depth so the tool has room to maneuver without collision.
3. Keep undercuts shallow and use standard dimensions
Limit undercut depth to roughly 2–3 times the cutter diameter (up to 3–4 times for standard T-slot cutters) to maintain tool rigidity and surface finish. Prefer common tool sizes: T-slot widths of 3–12.7 mm, dovetail angles of 45° or 60°, and internal corner radii of at least 0.3–0.8 mm. Matching standard cutters eliminates the need for expensive custom tools.
4. Avoid tight tolerances on non-critical surfaces and consider material
Apply strict tolerances only where function demands them. Soft materials machine more easily; hard alloys increase tool wear and may require slower feeds. When geometry remains challenging, evaluate 5-axis machining, part splitting, or secondary operations early in the design stage.
By applying these CNC undercut design best practices—prioritizing accessibility, standard tooling, shallow features, and functional necessity—engineers can significantly improve manufacturability, reduce cost, and accelerate production. Review undercut features with your CNC supplier before finalizing drawings to ensure a smooth path from design to finished part.