Tight Tolerance CNC Machining: Datum Strategy That Works

Tight tolerance CNC machining succeeds or fails on datum strategy more often than machine capability. If your drawing sets ±0.025 mm true position but leaves datums ambiguous, the shop cannot reproduce inspection, and a capable process looks inconsistent. Define A, B, and C from functional mating features: a large flat face, a precise bore, or a ground edge that clamps without distortion.

For tight-tolerance CNC parts, call out datum targets where a full surface is not clampable. On a 6061-T6 housing, use three small pads on the primary face, two edges for secondary, and one bore for tertiary. This mirrors assembly and CMM probing. Avoid a thin flange as primary datum; clamping may move it 0.05 mm, especially in 7075 or Delrin. For flexible features, specify restrained or free-state condition and state the fixture.

True position and profile need a clear inspection method. For holes at ±0.05 mm, say whether to use CMM, vision, or pin gauge. Add basic dimensions and avoid mixing ISO 2768 general tolerances with critical GD&T without noting precedence. On stainless 303, 304, or 316, work hardening can shift bores after drilling, so leave a reaming allowance and verify after the part reaches 20 °C.

Cost comes from process control, not the tolerance number alone. Holding ±0.01 mm on a 200 mm aluminum plate can require multiple setups, temperature soak, and in-process probing, adding days and cost. Relax non-critical surfaces to Ra 3.2, reserve Ra 0.8 for sealing or bearing fits, and discuss lead time early. A datum plan matching function, fixturing, and metrology keeps tight tolerance CNC machining predictable.

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For more guidance on related work, see: Surface Finish Inspection: Callouts That Prevent Disputes.