Avoid a Rejected Batch Three Weeks Later: ISO 2768 to GB/T Crosswalk
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Avoid a Rejected Batch Three Weeks Later: ISO 2768 to GB/T Crosswalk

2026-09-13 05:35:56

Avoid a Rejected Batch Three Weeks Later: ISO 2768 to GB/T Crosswalk

Engineer measuring an anonymous machined feature

Yes, ISO 2768 applies to any drawing that doesn’t call out an individual tolerance on a linear or angular dimension, and Chinese manufacturers work from equivalent standards, GB/T 1804 and GB/T 1184, that mirror it. Pick one of four classes (f, m, c, v) and print it on the title block now, or add a specific tolerance to any feature that actually matters. The shop, as an ISO-certified manufacturer, verifies both paths during quoting.

TL;DR:

  • Most Chinese manufacturers default to medium class tolerances when drawings reference ISO 2768 without specifying a class, which may not suffice for precision features.
  • Confirm whether the shop’s quality documents refer to ISO 2768 or GB/T 1804, and ensure they use the same edition year as your drawings to prevent miscommunication.
  • Use explicit tolerance class statements and specify individual tolerances on critical features to avoid ambiguous interpretations and costly rework.
  • Geometrical tolerances like flatness and perpendicularity generally require CMM verification, especially for safety-critical or dimension-sensitive parts.
  • Request documentation such as measurement reports or first-article inspection reports to confirm parts meet the specified general or individual tolerances.
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Table of Contents

What Does ISO 2768 Actually Cover?

ISO 2768 splits into two parts, and mixing them up is the fastest way to misread a drawing. ISO 2768-1:1989 specifies general tolerances for linear and angular dimensions that don’t carry their own individual tolerance callout. Part 2 handles geometrical tolerances, things like flatness, straightness, and perpendicularity, for features left otherwise unmarked.

Four classes cover the range of workshop accuracy, often described qualitatively as fine, medium, coarse, and very coarse tolerance levels, suitable for different functional requirements from precision-fit features to rough castings and weldments.

The whole point of general tolerancing is to declutter drawings. Instead of dimensioning every single edge and hole with its own plus/minus value, you state one class once, and it governs everything left unmarked. It was never meant to replace tolerancing on function-critical dimensions, bearing bores, sealing faces, anything that mates with another part still needs its own explicit callout. Worth flagging: ISO is publishing a revised Edition 2 of ISO 2768, expected in 2026, to replace the 1989 version, so confirm which edition your supplier’s quality system references before you lock a drawing.

What Does ISO 2768 Actually Cover? — overview diagram

How Do GB/T 1804 and GB/T 1184 Map to ISO 2768?

China doesn’t use ISO 2768 directly on most shop-floor documentation. It uses GB/T 1804, the national standard technically equivalent to ISO 2768-1, and GB/T 1184, which mirrors ISO 2768-2 for geometrical tolerances. The tables line up closely enough that engineers can treat them as interchangeable in practice, but “closely enough” isn’t the same as identical, and that gap is where miscommunication happens.

Before you send a drawing out, confirm a few things with the shop directly:

  • Which standard name appears on their internal quality documents, ISO 2768 or GB/T 1804
  • Which edition year they’re working from, since older shops sometimes still reference superseded tables
  • Whether their CMM programs and gauge calibration reference the same class definitions you intend

A one-line note on the drawing, “General tolerances per ISO 2768-m (or GB/T 1804-m),” removes almost all ambiguity. Ask for written confirmation in the RFQ that the supplier’s measurement basis matches what you specified. It costs nothing and prevents a rejected batch three weeks later.

How Do You Choose the Right Tolerance Class?

Class selection comes down to three questions: what does the feature do, does it mate with another part, and what process is making it? Skipping that assessment is how engineers end up either overpaying for unnecessary precision or shipping parts that fail in the field.

  1. Identify function first. A feature that locates, seals, or bears load needs individual tolerancing regardless of what general class you pick elsewhere on the print.
  2. Match the class to the process. Stamped or laser-cut sheet metal features typically land at ISO 2768-c unless a tight fit demands otherwise; CNC-machined features can comfortably hold f or m without much cost penalty.
  3. Test against real examples. A 10 mm shaft going into a rotating bearing fit almost always needs an individual tolerance, general class f alone won’t guarantee the fit. A 50 mm mounting plate with no mating surface can usually sit at m or c depending on the finish spec.

Tighter classes cost more, and not just in machining time. Fine-class parts often demand CMM verification instead of caliper checks, which adds inspection time and, on some processes, secondary finishing to hold flatness or perpendicularity. Reserve f-class and individual tolerances for the handful of dimensions where they earn their keep, and let the rest of the print ride on m or c.

Pro Tip: Before specifying a class, ask your shop what tolerance they hit consistently on that specific process without extra inspection steps. You’ll often find their comfortable default is tighter than ISO 2768-m, at no added cost.

What Belongs on the Drawing Before You Send It to China?

A drawing that leaves room for interpretation gets interpreted, usually not in your favor. Build the RFQ package around a fixed checklist rather than trusting memory:

  • Tolerance class stated explicitly (ISO 2768-m or GB/T 1804-m, for example), plus individual tolerances on every critical feature
  • Material callout and any required heat treatment
  • Surface finish specification, in Ra or an equivalent standard the shop actually uses
  • Datum references clearly marked, especially for parts with geometrical tolerance requirements
  • Units stated on the title block, millimeters versus inches has caused more scrapped parts than almost any other single oversight
  • A request for the standard edition the supplier’s QC team references, plus sample inspection reports or CMM output from prior jobs

For anything with real risk, tolerance-sensitive assemblies, first production runs, safety-relevant parts, request a first-article inspection before the full batch runs. WJ Prototypes’ own guidance on sourcing sheet metal in China walks through exactly this kind of coarse-tolerance risk management for bent and stamped parts.

Pro Tip: Attach a marked-up reference drawing showing exactly which dimensions carry individual tolerances versus which fall under the general class. Ambiguity on this single point causes more rework than any other drawing error.

How Should You Verify Parts Meet General Tolerances?

Height gauge checking an anonymous machined feature

Verification method should scale with what’s actually at stake, not default to the most expensive option every time. Calipers and micrometers handle basic linear checks fine for m and c class dimensions. Geometrical tolerances, flatness, perpendicularity, circular run-out, generally need a CMM or optical metrology to verify with confidence, especially against GB/T 1184’s geometrical requirements.

Match your inspection plan to risk and volume:

  • Low-risk, high-volume runs: AQL-based sampling is usually sufficient
  • First runs or safety-relevant parts: request 100% inspection or a documented FAI
  • Borderline dimensions: don’t reject automatically, assess function first, since general tolerances are meant to reflect customary shop accuracy rather than a hard pass/fail line unless the deviation genuinely impairs the part’s use

Document the disposition either way. A part slightly outside general tolerance but functionally fine should get a written concession note, not a silent pass.

Shop-Floor Lessons From Manufacturing in China

Most tolerance disputes trace back to two things: an undefined datum or an unstated unit system, not a disagreement over the standard itself. Shops working under GB/T 1804 default to medium class the moment a drawing goes quiet on the subject, which is fine until it isn’t. Balancing cost against precision means reserving tight classes for the features that actually need them and trusting general tolerances everywhere else. For deeper mechanics on sheet metal bending tolerances, WJ Prototypes’ K-factor calibration post is worth a read.

*— Nas

Get Your Drawings Reviewed Against ISO 2768 Before You Quote

Sending a drawing overseas without a tolerance class specified is how engineers end up paying for rework instead of parts. Incoming drawings are reviewed against ISO 2768 and GB/T 1804/1184 before production starts, flagging missing datums, undefined units, or ambiguous callouts before they become scrapped batches.

WJ Prototypes

As an ISO-certified manufacturer, CNC machining and sheet metal fabrication are run with documented process controls, meaning you can get measurement reports and first-article inspection documentation on request, not just a finished part and an invoice. That paper trail matters most on function-critical features where general tolerances aren’t enough. If you’re ready to move from spec sheet to production, get a quote on CNC machining and attach your drawing for a tolerance review as part of the process.

Where to Verify These Standards Directly

Cross-check tolerance callouts against the ISO 2768 listing, GB/T 1804, and GB/T 1184 directly, alongside WJ Prototypes’ tolerance guide for engineers.

Sources

FAQ

Does ISO 2768 Apply if My Drawing Doesn’t Mention It?

No. General tolerances only apply when the drawing explicitly references ISO 2768 (or GB/T 1804/1184) and states a class; without that callout, a Chinese shop has no standardized basis for unmarked dimensions.

What Tolerance Class Do Chinese Shops Default To?

Most default to medium (m) when a drawing specifies ISO 2768 without naming a class, since it reflects typical workshop accuracy under GB/T 1804.

Is GB/T 1804 the Same as ISO 2768-1?

GB/T 1804 is technically equivalent to ISO 2768-1, though engineers should confirm the specific edition year a supplier references, since table values can shift between revisions.

When Should I Use Individual Tolerances Instead of a General Class?

Use individual tolerances on any feature that mates with another part, bears load, or seals, shafts, bores, and sealing faces almost always need them regardless of the general class chosen elsewhere on the drawing.

Can WJ Prototypes Verify Parts Against ISO 2768 or GB/T Tolerances?

Inspection services including first-article inspection and CMM measurement reports for tolerance verification are offered by an ISO-certified manufacturer.