CNC Machining Tolerances: A Practical Guide for Engineers
Every RFQ we receive includes the word "precision." But precision isn't free — and more importantly, precision isn't always necessary. Understanding what each tolerance band actually costs is one of the fastest ways to reduce your machining budget without hurting your part.
What "Standard Tolerance" Means
Unless you specify otherwise, most machine shops — including us — work to ISO 2768-medium (or the equivalent ±0.1 mm rule for small features). For most brackets, housings, spacers and enclosures, this is entirely sufficient. A part that "fits" at ±0.1 mm covers the vast majority of industrial hardware.
The Tolerance Cost Curve
Tolerances don't get more expensive linearly — they get more expensive exponentially:
| Tolerance | Relative Cost | Typical Use |
|---|---|---|
| ±0.1 mm (standard) | 1× | General geometry, non-critical fits |
| ±0.05 mm | 1.3–1.5× | Sliding fits, dowel locations |
| ±0.01 mm | 2–3× | Bearing seats, press fits, precision shafts |
| ±0.005 mm and below | 4–10× | Gauges, optics, high-end tooling |
Why? Tight tolerances mean slower feeds, temperature-controlled measurement, dedicated finishing passes, CMM verification, and a higher scrap rate. The machinist's time is what you're really buying.
Three Rules for Smarter Tolerances
1. Tolerance only what mates.
If a surface doesn't touch another part, it usually doesn't need a callout. Put tight tolerances only on functional interfaces: bearing bores, shaft diameters, sealing faces, locating datums.
2. Watch the datum chain.
A position tolerance of ±0.01 mm measured from a datum that itself is ±0.1 mm is a trap. Always anchor critical dimensions to stable, machinable datums — ideally in one setup.
3. Ask before you tighten.
Sometimes a fit that looks tight can be solved with a cheaper feature: a slot instead of a precise hole pattern, or a clearance designed around a tolerance zone. Good manufacturing partners suggest these — we do it free with every quote.
How We Handle It
At Scanvex, standard quotes assume ISO 2768-medium. Critical dimensions flagged in your drawing are verified on our CMM and reported in a dimensional inspection sheet with every shipment — at no extra cost for batch orders.
Not sure which of your dimensions matter? Upload the drawing and ask — the DFM review is free.
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