Tighter numbers feel safer on a drawing. Here is what each digit costs, which process holds which grade, and how to write tolerances that price fairly.
Tolerance is the single biggest lever on a machining quote — bigger than material, bigger than quantity. Yet it's usually the least-considered line on the drawing. This is the practical version, from a floor that runs everything from ±0.01 mm production work to ±0.002 mm mold cores.
General CNC milling and turning comfortably holds around ±0.01 mm — that covers most brackets, housings and enclosures. Push to ±0.003 mm and you're into five-axis territory: thermally stable machining centers, careful fixturing, and cut strategies that manage heat and deflection. At ±0.0025 mm you're on turn-mill cells doing complete machining in one setup. And ±0.002 mm — the classic mold-core grade — demands precision grinding and EDM finishing after the mill, not just better milling.
| Process | Typical economic tolerance | Typical work |
| 3/4-axis CNC milling | ±0.01 mm | Brackets, housings, plates, fixtures |
| 5-axis machining | ±0.003 mm | Complex monolithic parts, impellers, medical components |
| Turn-mill / turning | ±0.0025 mm | Shafts, fittings, rotary unions |
| Mold cores & cavities | ±0.002 mm | Injection mold inserts, precision cavities |
Going one grade tighter rarely adds a percentage — it adds a process step. A ±0.003 mm feature may force a finish pass on a more stable machine. A ±0.002 mm feature typically moves the part to grinding or wire EDM, adds stress-relief, and requires CMM verification with temperature control. Scrap tolerance shrinks, inspection time grows, and the yield curve steepens. None of that is the supplier padding the quote — it's the physics of holding geometry.
A row of ± dimensions on every feature tells the machinist nothing about what the part must do. Datums, true position, flatness and profile callouts let the shop hold what matters and relax what doesn't. A part with a tight true-position callout to one datum often machines faster and cheaper than the same part blanketed with ±0.01 — because the fixturing and inspection plan follow the function, not the grid.
A tolerance nobody measures is a rumor. At ±0.01 mm, calipers and micrometers still work. From ±0.003 mm down, you need CMM verification — and ideally the same machine class every time, so reports are comparable between shipments. We verify on three Zeiss CONTURA CMMs, first-article inspect 100% of runs, and ship the report with every order. If your supplier can't say which instrument verifies your tightest callout, assume it isn't being verified.
Call tight tolerances only on features with a function: bearing seats, seal faces, mating locators, optical mounts. Use GD&T with datums instead of stacked ±. State the inspection you expect — FAI plus CMM report per shipment — in the RFQ, not after the first nonconformance. And when a feature is critical, say why: a one-line note about the mating part often gets you a better process decision than any number.
Send a drawing — we'll quote the tolerances as written →
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