Tolerances for Welded and Machined Parts: ISO 13920 and ISO 2768 Explained
Use ISO 13920 for the general tolerances of a welded structure and ISO 2768 for machined features. ISO 13920 class B for lengths and class F for straightness and flatness suit most machine frames; ISO 2768-m suits most machined faces and bores. Specifying ISO 2768 for a whole weldment is the most common and most expensive mistake.
- ISO 13920Welded structures
- Class B / FCommon choice
- ISO 2768-mMachined features
- ISO 5817Weld quality level
- DatumsLink both
Why Welded Parts Need Their Own Tolerance Standard
Welding shrinks and distorts steel. A weldment cannot reach the tolerances of a machined part without machining, so ISO 13920 defines general tolerances for welded constructions: classes A, B, C and D for linear and angular dimensions, and classes E, F, G and H for straightness, flatness and parallelism. Class A and E are the tightest.
ISO 2768 defines general tolerances for machined parts: f (fine), m (medium), c (coarse) and v (very coarse) in part 1, and H, K, L for geometry in part 2. On a welded and machined part, both apply: ISO 13920 for the as-welded structure, ISO 2768 for the machined features.
ISO 13920 Linear Tolerances for Welded Structures (mm)
| Nominal size (mm) | Class A | Class B | Class C | Class D |
|---|---|---|---|---|
| over 30 to 120 | ±1 | ±2 | ±3 | ±4 |
| over 120 to 400 | ±1 | ±2 | ±4 | ±7 |
| over 400 to 1,000 | ±2 | ±3 | ±6 | ±9 |
| over 1,000 to 2,000 | ±3 | ±4 | ±8 | ±12 |
| over 2,000 to 4,000 | ±4 | ±6 | ±11 | ±16 |
| over 4,000 to 8,000 | ±5 | ±8 | ±14 | ±21 |
Extract for orientation. Use the full standard for drawings and contracts.
ISO 13920 Straightness, Flatness and Parallelism (mm)
| Nominal size (mm) | Class E | Class F | Class G | Class H |
|---|---|---|---|---|
| over 30 to 120 | 0.5 | 1 | 1.5 | 2.5 |
| over 120 to 400 | 1 | 1.5 | 3 | 5 |
| over 400 to 1,000 | 1.5 | 3 | 5.5 | 9 |
| over 1,000 to 2,000 | 2 | 4.5 | 9 | 14 |
| over 2,000 to 4,000 | 3 | 6 | 11 | 18 |
| over 4,000 to 8,000 | 4 | 8 | 16 | 26 |
Extract for orientation. Use the full standard for drawings and contracts.
ISO 2768-1 Linear Tolerances for Machined Features (mm)
| Nominal size (mm) | f (fine) | m (medium) | c (coarse) | v (very coarse) |
|---|---|---|---|---|
| over 6 to 30 | ±0.1 | ±0.2 | ±0.5 | ±1 |
| over 30 to 120 | ±0.15 | ±0.3 | ±0.8 | ±1.5 |
| over 120 to 400 | ±0.2 | ±0.5 | ±1.2 | ±2.5 |
| over 400 to 1,000 | ±0.3 | ±0.8 | ±2 | ±4 |
| over 1,000 to 2,000 | ±0.5 | ±1.2 | ±3 | ±6 |
| over 2,000 to 4,000 | – | ±2 | ±4 | ±8 |
How to Specify Tolerances on a Welded and Machined Drawing
State both standards in the title block
For example: “Welded construction ISO 13920-BF, machined features ISO 2768-mK”.
Mark machined faces clearly
Use machining symbols and show machining stock, so the fabricator knows which faces are finished after welding.
Put tight tolerances only where needed
Bearing seats, fits and mating faces get individual tolerances; everything else follows the general classes.
Define datums on functional features
Datums should be faces or bores that exist after machining and that the assembly really locates on.
Specify weld quality separately
Weld imperfections are governed by ISO 5817 quality levels B, C or D, not by dimensional tolerances.
Tolerance Mistakes That Cost Money
ISO 2768 for the whole weldment
Forces machining of faces that could stay as welded, or leads to rejected parts.
Class A everywhere
ISO 13920 class A needs heavy fixturing and straightening on large parts. Use it only where it matters.
No weld quality level
Without ISO 5817 level, weld inspection criteria are open to interpretation.
Measuring Welded and Machined Parts
We work to ISO 13920 and ISO 2768 every day and deliver dimensional reports with our parts. We do CMM measurement for machined features and inspect welds to ISO 5817. If a tolerance on your drawing will drive cost, we tell you on the quotation.
Read our guides on designing lower-cost welded frames and machining after welding, or see our quality page.
Frequently Asked Questions
What is ISO 13920?
ISO 13920 defines general tolerances for welded constructions: classes A to D for linear and angular dimensions and E to H for straightness, flatness and parallelism.
What does ISO 13920 BF mean on a drawing?
Class B for linear and angular dimensions and class F for straightness, flatness and parallelism. It is a common choice for machine frames.
Can I use ISO 2768 for welded parts?
Only for machined features. For the welded structure, ISO 13920 is the right standard; ISO 2768 tolerances on a whole weldment are often unachievable without machining.
Which standard defines weld quality?
ISO 5817 defines quality levels B, C and D for weld imperfections in steel. It is specified separately from dimensional tolerances.
Has ISO 13920 been replaced?
ISO 13920 is the standard most drawings refer to. Check the latest edition and any successor documents with your standards provider.
Not sure your tolerances fit the process?
Send the drawing. We check tolerances, machining stock and weld quality before we quote.