1. What is ISO 2768 and why does it matter?
ISO 2768 defines general tolerances — the dimensional and geometric accuracy that applies to all features on a drawing that do not have an explicit individual tolerance callout. Rather than tolerancing every single dimension, a designer specifies one class code in the title block, and the standard fills in all the gaps.
It comes in two parts:
- ISO 2768-1 — general tolerances for linear dimensions (lengths, diameters, radii) and angular dimensions
- ISO 2768-2 — general geometric tolerances (straightness, flatness, perpendicularity, symmetry, circular run-out)
Without an ISO 2768 callout, a drawing is technically incomplete: the fabricator has no defined accuracy requirement for un-toleranced features. In practice this leads to disputes — the designer assumes workshop practice, the fabricator applies their own judgment, and the inspection fails on something no one thought to specify.
2. ISO 2768-1: linear and angular tolerance classes
ISO 2768-1 defines four tolerance classes for linear and angular dimensions:
| Class | Symbol | Name | Intended use |
|---|---|---|---|
| f | fine | Fine | Precision machined components — turned, milled, ground parts where tight control is inherent in the process |
| m | medium | Medium | Standard machined parts — the default for most mechanical engineering drawings. Commonly used for offshore machined components. |
| c | coarse | Coarse | Parts produced by processes with lower inherent accuracy — welded fabrications, castings, hot-formed parts, heavy structural components |
| v | very coarse | Very coarse | Very rough processes only — use sparingly. Primary steel fabrication where dimensional accuracy is secondary to structural function. |
The classes apply to: external dimensions, internal dimensions, step dimensions, diameters, radii of curvature, chamfer heights, and angular dimensions. They do not apply to features that already have an individual tolerance callout — those always take precedence.
3. Actual tolerance values — the numbers
The tolerance values scale with the nominal dimension. Here are the linear dimension tolerances (±mm) for each class across the most relevant ranges for offshore fabricated parts:
| Nominal range (mm) | f (fine) ±mm | m (medium) ±mm | c (coarse) ±mm | v (very coarse) ±mm |
|---|---|---|---|---|
| 0.5 – 3 | 0.05 | 0.1 | 0.2 | — |
| 3 – 30 | 0.05 | 0.2 | 0.5 | 1.0 |
| 30 – 120 | 0.1 | 0.3 | 0.8 | 1.5 |
| 120 – 400 | 0.15 | 0.5 | 1.2 | 2.5 |
| 400 – 1000 | 0.2 | 0.8 | 2.0 | 4.0 |
| 1000 – 2000 | 0.3 | 1.2 | 3.0 | 6.0 |
| 2000 – 4000 | — | 2.0 | 4.0 | 8.0 |
For angular dimensions, tolerances are expressed in degrees and minutes:
| Shorter side length (mm) | f | m | c | v |
|---|---|---|---|---|
| up to 10 | ±1° | ±1° | ±1°30′ | ±3° |
| 10 – 50 | ±0°30′ | ±0°30′ | ±1° | ±2° |
| 50 – 120 | ±0°20′ | ±0°20′ | ±0°30′ | ±1° |
| 120 – 400 | ±0°10′ | ±0°10′ | ±0°15′ | ±0°30′ |
| above 400 | ±0°5′ | ±0°5′ | ±0°10′ | ±0°20′ |
4. ISO 2768-2: geometric tolerance classes
ISO 2768-2 covers geometric tolerances for features that do not carry individual GD&T callouts. It defines three classes:
| Class | Symbol | Intended use |
|---|---|---|
| H | H | Precision parts — tight geometric control required. Typical for machined components with functional geometric requirements (flatness of bearing surfaces, perpendicularity of bores). |
| K | K | Standard machined parts. The default companion to ISO 2768-1 class m. Good balance of geometric control and achievable tolerances for CNC machined parts. |
| L | L | Rough parts — welded fabrications, castings. Companion to ISO 2768-1 class c or v. Does not require tight geometric control from the fabrication process. |
ISO 2768-2 covers the following geometric characteristics for un-toleranced features:
- Straightness and flatness — applies to all surfaces and axes
- Perpendicularity — applies to edges and surfaces relative to a base
- Symmetry — applies to features constructed symmetrically
- Circular run-out — applies to rotating features (shafts, bores)
Note: ISO 2768-2 does not cover parallelism, angularity, position, cylindricity, profile of a line/surface, or total run-out. Those always require explicit GD&T feature control frames if control is required.
5. How to write the callout on a drawing
The ISO 2768 callout goes in the title block or general notes area. The format is:
ISO 2768-mKWhere m is the ISO 2768-1 linear/angular class and K is the ISO 2768-2 geometric class. The two are always specified together when both parts apply.
| Callout | Meaning | Typical use |
|---|---|---|
ISO 2768-fH | Fine linear + H geometric | Precision machined components |
ISO 2768-mK | Medium linear + K geometric | Standard machined offshore parts (most common) |
ISO 2768-cL | Coarse linear + L geometric | Welded fabrications, structural padeye plates |
ISO 2768-m | Medium linear only | When geometric tolerances are all explicitly called out, or ISO 2768-2 is not referenced |
6. Selecting the right class for offshore fabricated parts
The most common mistake is applying a single tolerance class to a drawing that contains both machined and fabricated features. The class should reflect the dominant fabrication process for the part — if a component is primarily a welded fabrication with some machined interfaces, specify class c or cL and add explicit individual tolerances for the machined interfaces.
Machined components (turned, milled, bored)
Use ISO 2768-mK as the default. This covers standard CNC machining tolerances and is achievable without special process controls. For high-precision fits (bearing housings, valve bores, hydraulic cylinder bores), add explicit individual tolerances per ISO 286 (limits and fits) for those features — do not rely on ISO 2768 for fit-critical interfaces.
Welded structural fabrications
Use ISO 2768-cL. Welding introduces distortion, residual stress, and inherent geometric variability that makes class m unachievable without post-weld machining or straightening. Class c matches typical workshop fabrication capability. Class L for geometric tolerances reflects the limited control of flatness and perpendicularity in a weld assembly.
For offshore structural padeye plates, brackets, and lifting pad assemblies: ISO 2768-cL is the correct class. The pin hole diameter, however, should always carry an explicit individual tolerance per DNV-ST-0378 Appendix E — typically ±0.2 mm or an H11 fit designation. ISO 2768 class c would give ±0.5 mm for a 40 mm hole, which may be acceptable for the plate but too loose for the pin engagement.
Castings and forgings
Start with ISO 2768-cL for the general form. Machined surfaces on castings/forgings should have explicit individual tolerances. For complex castings with tight dimensional requirements, consider referencing ISO 8062 (dimensional tolerances for castings) in addition to or instead of ISO 2768.
Sheet metal and plate cut parts
Laser-cut or plasma-cut plate: ISO 2768-m is typically achievable for laser, ISO 2768-c for plasma. Water jet cutting approaches class f for thin material. Flame-cut parts: class v is more realistic for cut edge accuracy. Always specify the cutting process in the drawing notes so the fabricator and inspector use the same expectation.
7. Common class combinations in offshore practice
| Component type | Recommended callout | Notes |
|---|---|---|
| Padeye plate (welded fabrication) | ISO 2768-cL | Add explicit ±0.2 mm on pin hole diameter per DNV-ST-0378 |
| Hydraulic cylinder body (machined) | ISO 2768-mK | Bore → explicit H7/f7 fit per ISO 286; rod → explicit h6 |
| Structural bracket / gusset (welded) | ISO 2768-cL | Bolt holes → add ±0.5 mm explicit if positional accuracy matters |
| Valve body (casting + machined) | ISO 2768-cL | Seat and bore → explicit ISO 286 fits; ISO 8062 for casting form |
| Subsea connector (precision machined) | ISO 2768-fH | Critical sealing/thread features → explicit individual tolerances |
| Accumulator shell (pressure vessel) | ISO 2768-mK | End cap bore and thread → explicit; outside diameter → individual callout for installation clearance |
8. How ISO 2768 interacts with explicitly dimensioned tolerances
ISO 2768 is a general tolerance standard — it applies only to dimensions that carry no individual tolerance. The hierarchy is:
- Individual tolerance callout on the dimension (e.g.
40 ±0.05or⌀40 H7) — always takes precedence. ISO 2768 does not apply to this feature. - GD&T feature control frame (e.g. flatness ⊡ 0.05) — takes precedence over ISO 2768-2 for that geometric characteristic on that feature.
- ISO 2768 general tolerance — applies to everything that has no individual callout. It is the fallback, not the primary specification.
ISO 2768 and EN 1090 (steel structure execution)
EN 1090-2 (execution of steel structures) defines execution classes (EXC1–EXC4) with their own tolerance requirements that may be tighter than ISO 2768-cL for structural members. When a drawing references both EN 1090 and ISO 2768, apply the following rule: EN 1090 governs structural and geometric deviations of the complete assembly; ISO 2768 governs individual component dimensions not addressed by EN 1090. Specify both on the drawing if both apply.
9. Drawing tolerance checklist
- ISO 2768 class stated in title block — both Part 1 (linear) and Part 2 (geometric) where applicable
- Class chosen to match the dominant fabrication process (machined → m/K; welded → c/L)
- Fit-critical interfaces have explicit individual tolerances per ISO 286 — not relying on ISO 2768
- Pin holes on padeyes have explicit individual tolerance (±0.2 mm or H11) per DNV-ST-0378
- Bore and rod diameters on hydraulic cylinders have explicit H/f or H/g fit designations
- GD&T feature control frames present for any safety-critical geometric requirement (perpendicularity, flatness of mating faces)
- ISO 2768 class not contradicted by EN 1090 tolerance requirements for the same part
- For castings: ISO 8062 referenced if dimensional accuracy of the cast form is critical
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