5-axis CNC machining center cutting a metal part under coolant spray Design accounts for just 5% to 7% of a product's total development budget, but the decisions made at that stage lock in 70% to 80% of eventual manufacturing cost. That gap is where most CNC headaches originate: a geometry that looks fine on screen but forces a machinist into slow tool paths, extra setups, or outright rework. None of the pitfalls below are exotic. They're the same five issues that show up, project after project, in DFM (Design for Manufacturing) reviews. Catch them in CAD and you save weeks of back-and-forth later. Here's what to watch for, and the numbers behind each fix. 1. Sharp Internal Corners (The Tool Radius Trap) It's an easy habit to bring over from sketching: a clean 90° inside wall. But CNC mills cut with round tools, and a round tool physically cannot produce a square internal corner. Push for one anyway and a shop either has to plunge with an undersized cutter (slow, chatter-prone) or flag the part back to you. Diagram comparing a sharp internal corner that cannot be machined against an optimized internal radius the tool can cut cleanly The fix: keep internal corner radii at R ≥ 1/3 of the cavity depth. A 12mm-deep pocket wants at least a 4mm radius. Go tighter than that and you're forcing tools with high length-to-diameter ratios, which is exactly the condition that causes chatter and premature wear. If a mating part genuinely needs a square internal corner, say for a snug press-fit, specify a T-bone or dog-bone relief cut in CAD rather than leaving the machinist to guess. 2. Over-Specifying Tight Tolerances Global tolerancing is a tempting shortcut. Set the whole model to ±0.005mm and you never have to think about it again. The shop, on the other hand, now has to hold aerospace-grade precision on every surface, including the ones nobody will ever measure. Tightening from standard shop tolerance (ISO 2768-m, roughly ±0.1mm) down to ±0.005mm doesn't add cost linearly

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