The part changes between the weld shop and the mill
Every weldment I have ever machined arrived at the mill with a story. The fabricator welded it, it looked straight, they shipped it, and by the time it sat on my table it had pulled 1-2 mm somewhere. Welding is localized melting — the weld bead shrinks as it cools, and that shrinkage yanks the surrounding structure. On a frame or a base plate, that means the machined surfaces do not exist until you create them, and the raw part geometry is only a suggestion.
The first question on any weldment RFQ: what are the critical machined features, and what tolerance do they really need? That decides whether the job is a quick cleanup or a careful sequence of stress relief, setup, and finish machining.
Order of operations that actually works
- Stress relief (if specified): vibratory stress relief or thermal treatment after welding and before machining — this is where most of the movement gets removed
- Inspect the raw weldment: map where the surfaces are before you cut, so the machining stock is planned against reality
- Rough machine the critical faces, leaving 0.5-1.0 mm
- Re-check, re-clamp, and finish machine in the same setup that defines the datums
- If the weldment is large and the customer wants the datum faces stable, machine the mounting faces first, then everything else relative to them
The mistake I see most: treating the weldment like a solid block and trying to hold tight tolerances in one aggressive pass. Weldments need the relax step just like stressed plate does — often more, because the weld shrinkage is severe.
Stress relief: when it matters and when it is marketing
Vibratory stress relief is the practical default for medium weldments — it reduces residual stress enough for most machining. Full thermal stress relief (annealing) is stronger but costs more and can distort thin sections itself. My rule: if the drawing calls post-weld machining tolerances tighter than +-0.1 mm on features larger than 300 mm, get the stress relief done and say so in the RFQ. Without it, the weldment will keep moving during machining and the finish pass will chase a moving target.
Also worth knowing: weld shrinkage is predictable in direction. A long fillet weld on one side of a plate pulls that side concave. The fabricator can pre-set the part opposite to the expected pull — pre-bending — which is the cheapest distortion control there is. Ask your fabricator if the weldment was pre-set; it changes how much stock you need.
Datum strategy on weldments
Weldments do not have trustworthy raw surfaces, so the datums must be created, not assumed. The sequence I use: pick the largest machined face that other features reference — usually a mounting face — machine it flat first, then use it as the datum for everything else. The drawing should call out which surface is datum A and which is datum B, and the machinist machines A first, checks it, then builds the rest off it. If the drawing has no datum callout on a weldment, that is the first thing to fix.
And a practical note on setups: weldments are rarely square. Expect to indicate every corner. The machining stock should be generous — 2-3 mm on machined faces is normal for weldments, versus 0.5-1 mm for solid stock — because the raw part is not where the drawing thinks it is.
Realistic post-weld machining tolerances
| Feature | Realistic tolerance | What it takes |
|---|---|---|
| Mounting faces, flatness | 0.05-0.10 mm | Stress relief + finish pass on supported fixture |
| Bores in weldments | H7 | Fine bore after roughing, in final setup |
| Hole positions | +-0.10 mm typical, +-0.05 mm with care | Drilled in one setup from machined datums |
| Overall dimensions | +-0.5 mm | Standard for weldments without stress relief |
If a customer sends a weldment drawing with +-0.02 mm on features across a 500 mm frame, the honest answer is: this part cannot be welded to that tolerance, and machining it there means the weldment must be stress relieved, fully machined, and possibly straightened. That is a different price and a different schedule — better to know before the RFQ goes out.
The checklist to attach to your next weldment RFQ
- Specify stress relief (thermal or vibratory) and whether it is included in the quoted price
- Call out datums A/B/C explicitly — machined faces first, then features relative to them
- State the machining stock allowed on critical faces (2-3 mm for weldments is normal)
- Tell the shop the weld process and whether the fabricator pre-set the part — it affects distortion and stock
- For large weldments, consider machining in two setups with a re-check between — the part may move between setup 1 and setup 2
Frequently Asked Questions
Why do my weldments move after machining?
Welding leaves residual stress that keeps relaxing during machining. Stress relief after welding and a rough-relax-finish sequence absorbs most of the movement.
What tolerance can you machine a weldment to?
Mounting face flatness 0.05-0.10 mm, bores H7, hole positions +-0.10 mm typical — after stress relief and with proper datum strategy. Tighter than that needs special handling and costs more.
Do I need stress relief on my weldment?
If post-weld machined features are tighter than +-0.1 mm over 300 mm or more, yes. Without it the part keeps moving and the finish pass chases it.
Should I machine before or after welding?
For parts with critical fits, machine after welding — the weld shrinkage moves everything. Pre-machine only features that are not affected, or use pre-set/pre-bent weldments.