Machining After Welding: How to Dimension a Weldment
Practical rules for engineers who design welded frames, bases and housings that are machined after welding: datums, machining allowance, tolerances to ISO 13920, stress relief and what to put on the drawing.
- ISO 13920Weld tolerances
- ISO 2768Machined features
- 3–5 mmTypical stock
- DatumsSet before welding
- CMMFinal check
Why Welded Parts Are Machined After Welding
Welding puts heat into steel and makes it shrink as it cools. A welded frame always moves a little: faces bow, holes shift, and long members twist. For most structural parts that is acceptable within ISO 13920. For mounting faces of gearboxes, linear guides or bearings it is not.
The answer is to weld first and machine second. Functional faces, bores and hole patterns are machined after welding, so their accuracy comes from the machine tool, not from the weld. This only works if the drawing is prepared for it.
Seven Rules for Weldments That Will Be Machined
Define datums
Choose a primary datum face that is machined first and all other features refer to. Show it clearly on the drawing.
Separate weld and machine tolerances
Use ISO 13920 for the welded structure and ISO 2768 or explicit tolerances for machined features.
Allow machining stock
Plan typically 3–5 mm per machined face on medium parts, more on large or thin-walled weldments.
Thicken machined pads
Make pads and bosses thick enough that the finished thickness is still sufficient after stock removal.
Keep tool access
Check that boring bars and milling heads can reach every machined feature after welding.
Specify stress relief
For tight tolerances on large or heavy weldments, specify stress relieving before finish machining.
Mark what is machined
Use machining symbols and notes, so the welder knows where stock is needed.
Which Tolerance Standard for Which Feature
| Feature | Standard | Typical class |
|---|---|---|
| Welded structure, linear dimensions and angles | ISO 13920 | Class A, B or C |
| Straightness, flatness, parallelism of welded parts | ISO 13920 | Class E, F or G |
| Machined features, general | ISO 2768-1 | f or m |
| Machined geometry, general | ISO 2768-2 | K or H |
| Bearing seats and fits | ISO 286 | e.g. H7, k6 |
| Weld quality | ISO 5817 | B or C |
What Goes Wrong Most Often
One tolerance for everything
Machining tolerances applied to the whole weldment make the part expensive and sometimes impossible.
No stock on functional faces
If the drawing shows final dimensions only, faces may be too thin after machining out distortion.
Datums on welded edges
A datum on an unmachined welded edge moves with every part and makes measurement unreliable.
Questions About Machining After Welding
How much machining allowance should I leave?
Typically 3 to 5 mm per face on medium weldments, more on large, long or thin-walled parts. Your fabricator can advise per part.
When is stress relief needed?
When tight tolerances must hold on large or heavily welded parts, or when the part will be machined in several stages. It reduces movement during and after machining.
Which tolerance standard applies to welded parts?
ISO 13920 for the welded structure; ISO 2768, ISO 286 or explicit tolerances for machined features.
Can you help prepare our drawing?
Yes. We review drawings before quoting and suggest datums, stock and tolerance changes that make the part cheaper or more reliable.
Send us your weldment drawing.
We review datums, stock and tolerances before quoting, and tell you what would make the part cheaper to build.