Complete guide
Weldment Tolerances
Welding moves metal. Every joint shrinks as it cools, so the part you machine is never exactly the part you drew. Tolerancing a weldment is the discipline of saying which condition each number applies to, and putting the tight ones only where they earn their place.
Two tolerance systems, one drawing
Get this right and most of the confusion in weldment quoting disappears.
The welded structure
Held to a general tolerance standard for fabrications, normally an ISO 13920 class. Classes A to D cover linear and angular dimensions, E to H cover straightness, flatness and parallelism. The permitted deviation scales with the size of the feature, so the same class allows far more on a 4 m member than on a 200 mm one.
Sensible default: class B with form class F for machine frames.
The machined features
Held to a normal drawing tolerance, from ISO 2768 or called out directly on the feature. This is where GD and T earns its place on a weldment: position on bores and bolt patterns, flatness on mounting faces, perpendicularity on uprights.
Sensible default: ISO 2768-m, with individual tolerances on the few features that matter.
Six rules that keep a weldment quotable
- Leave stock on every machined faceA common starting point is 3 to 6 mm on faces and 3 mm on the radius of a bore, and more on long or thin sections that are likely to move. Too little and the face cleans up in one corner only. Too much and the machining time doubles.
- Give the weldment and the machining separate tolerancesThe welded structure is normally held to an ISO 13920 class and the machined features to a normal drawing tolerance. Putting a machining tolerance on a welded dimension is the fastest way to a quote full of assumptions.
- Put the datums where the machine can reach themPick datum faces that will be machined and that a fixture can sit on. A datum on an as-welded surface moves with the distortion and every feature referenced to it moves with it.
- Say when flatness appliesFlatness and parallelism on a machine base are achieved after machining, not after welding, and they hold only if the part was stress relieved first. State the condition on the drawing so it is inspected the same way it was quoted.
- Decide about stress relief before you quote, not afterThermal stress relief for carbon steel typically runs at 580 to 650 degrees C with a soak of about one hour per 25 mm of thickness, plus controlled heating and cooling. It costs oven time and freight if the shop does not own an oven big enough.
- Name the inspection you wantVisual inspection of every weld is normal. Dye penetrant, magnetic particle, ultrasonic and radiographic testing each cost real money, so mark which welds get which method and at what rate rather than writing NDT on the title block.
Which standard governs what
Four class systems can apply to one drawing. They do different jobs.
| Standard | Scope | Classes | What to pick |
|---|---|---|---|
| ISO 13920 | General tolerances for welded constructions | A to D for linear and angular dimensions, E to H for straightness, flatness and parallelism | Class B with class F is a sensible default for machine frames and industrial weldments. Go to A and E only on a feature that genuinely needs it, because the shop has to fixture and check to that number on the welded structure rather than on a machined face. |
| ISO 2768 | General tolerances for machined features | f fine, m medium, c coarse, v very coarse, plus separate classes for geometry | Medium is the usual default for machined faces and bores on a weldment. Put individual tolerances on the few features that matter rather than tightening the general class. |
| ISO 5817 | Acceptance levels for weld imperfections | B stringent, C intermediate, D moderate | B on welds that carry primary or fatigue load, C on secondary structure, D on guarding and brackets. Applying B across a whole frame is the most common and most expensive drawing error in this niche. |
| EN 1090-2 | Execution of steel structures | EXC1 to EXC4 | Driven by the application and, in Europe, by regulation rather than preference. If your component is structural and placed on the European market, get the class confirmed for your product rather than assumed. |
We do not reproduce the limit tables from these standards. They are copyrighted, and a table transcribed badly is worse than no table because somebody builds a part from it. The reasoning is on the class selection guide and in our editorial standards.
Go deeper
- How Much Stock to Leave on a Weldment Before You Machine ItHow much machining allowance to leave on a welded frame, how to split weldment and machining tolerances on the drawing and where to put the datums.
- Putting Datums and Geometric Tolerances on a Weldment That MovesHow to apply datums, flatness, position and parallelism to a machined weldment, why welded and machined features need separate tolerance systems and where ISO 13920 fits.
- Which Weld Quality Level to Put on Your DrawingWhat ISO 5817 quality levels B, C and D actually accept, how the level changes welding cost and rework, and how to apply different levels to different joints.
- How to Read and Write the Weld Symbols on Your Own DrawingThe parts of an AWS A2.4 weld symbol, what the arrow side and other side mean, and the five things buyers most often leave off a weldment drawing.
- When a Welded Frame Needs Stress Relief, and When It Does NotThermal and vibratory stress relief for weldments: when residual stress will move your machined faces, what a PWHT cycle involves and where it belongs in the sequence.
Questions about tolerances
Can I put one general tolerance on a weldment drawing?
Not usefully. The welded structure and the machined features are produced by different processes with different capability, so they need separate tolerances. ISO 13920 covers general tolerances for welded constructions and a normal drawing tolerance such as ISO 2768 covers the machined features. One tight general note across both either prices the impossible or gets ignored.
How much machining allowance should I leave?
A common starting point is 3 to 6 mm on faces and about 3 mm on the radius of a bore, with more on long or thin sections likely to move. The right number depends on the part length, the weld volume and whether the part is stress relieved, so agree it with the shop rather than copying a default.
Should a datum sit on a welded surface or a machined one?
On a machined face wherever possible. An as-welded surface moves with the distortion, and every feature referenced to it moves with it. Pick datum faces that will be machined and that a fixture can physically sit on.
When is flatness measured, before or after the part relaxes?
That is exactly why the drawing must state the condition. Write flatness within 0.1 mm after stress relief and finish machining, so the part is inspected the same way it was quoted.
Drawing toleranced?
Send it and we will tell you whether a shop can actually hold it, then match it with ones that can. Reply within 24 business hours.