Why the part was flat on the machine and bowed on the bench
A large titanium plate comes off the machine measuring perfectly. It is unclamped, sits overnight, and by morning it has 0.4 mm of bow across its length. Nobody made a mistake. The stress that was locked into the raw material has redistributed itself.
This is the dominant failure mode on large, thin, or heavily machined parts in exotic alloys, and it is almost entirely controlled by process sequence rather than by cutting accuracy.
Where the stress comes from
Rolling and forging. Plate is squeezed between rolls that deform the surface more than the core. The result is a plate in equilibrium with itself: compressive stress near the surfaces, tensile in the middle. Remove material from one face and the balance breaks.
Quenching. The surface cools and contracts first, the core follows. Every heat-treated bar carries a stress gradient from that sequence.
Extrusion and drawing, which leave long-range residual stress along the axis.
Machining itself. Cutting introduces its own stressed layer. Heavy passes in titanium and nickel alloys leave a work-hardened, stressed skin that pulls on the part — which is another reason light finishing passes matter for reasons beyond surface finish.
Welding, which is a locally applied thermal cycle and the most concentrated stress source of all.
The stress is invisible and the part is stable until you unbalance it. Machining is exactly the act of unbalancing it.
Who suffers
Roughly in order of risk:
- Large thin plates and frames, especially machined mostly from one side
- Long slender parts — shafts, rails, beams
- Parts where more than about 70 % of the starting material is removed
- Weldments machined after welding
- Anything heat treated between operations
Titanium is worse than steel for two reasons: its lower elastic modulus means the same residual stress produces about twice the deflection, and hot-rolled titanium plate tends to carry high residual stress to begin with. The stiffness problem is the same one behind thin-wall titanium parts.
The sequence that works
1. Stress relieve the raw material before machining. For titanium, typically 480–595 °C for one to four hours, then slow cool. For steels, below the tempering temperature so hardness is unaffected. This is cheap and it is the step most often skipped.
2. Rough with stock remaining. Leave 1–3 mm on all faces depending on size. Rough symmetrically — take material off both sides in roughly equal amounts rather than hogging one face.
3. Stress relieve again after roughing. This is the important one. Roughing releases the bulk of the locked-in stress and introduces its own. Relieving here lets the part move while there is still stock to correct it.
4. Let it move, then re-datum. After the second relief, re-fixture and re-indicate. The part has changed shape; the finishing operation must reference where it is now, not where it was.
5. Finish with light, balanced passes. Symmetric where possible, so the finishing operation does not itself unbalance the part.
6. Release from the fixture and re-measure. Final inspection in free state, not clamped.
Each relief cycle adds days and cost. For a small, chunky part it is unnecessary. For a 600 mm titanium frame with 80 % material removal, skipping it means scrapping parts.
Fixturing that does not lie to you
Clamping can force a bowed part flat, let you machine it flat, and then release it back into a bow — with the machined features now in the wrong place relative to each other.
The fix is to clamp lightly and support fully: many low-force clamps rather than few high-force ones, support under the part along its length rather than at the ends, vacuum where geometry allows, and indicating the part in its natural state rather than pulling it down to the table.
The alloys with special rules
Duplex and super duplex must not be given a conventional stress relief. The 650–950 °C range precipitates sigma phase and destroys toughness and corrosion resistance — see duplex stainless machining. Distortion in duplex has to be managed by sequence and fixturing alone.
Precipitation-hardening alloys change dimension during aging by a predictable percentage, which is a different problem from stress relief and needs its own allowance — 17-4 PH conditions and Inconel 718.
Tool steels distort during the hardening quench, which is the main argument for hard machining — tool steel machining.
What to put on the drawing
Say whether flatness and profile apply in the free state or under a defined restraint. This single line prevents most distortion disputes, because a part can genuinely satisfy one and fail the other.
If the sequence matters to you, specify it: “stress relieve after rough machining, per AMS 2801” is a quotable instruction. If you would rather leave the route to the supplier, say that too — but then accept that different suppliers will price different routes, which is one more reason quotes spread.
Send the model and the flatness requirement together — on large parts those two things decide the price more than the tolerance table does.