A buyer once sent us the same aluminum housing twice in one month. Both parts measured flat within 0.01 mm on the vendor's inspection report. Both arrived visibly bowed. This is not a rare story — it is one of the most common precision-machining failures, and it almost never shows up on the machine. Here is what actually causes it, and what to specify so it does not happen to you.
The question, stated precisely
Why does a part that measures perfect on the machining center end up out of tolerance after it is removed from the fixture, packed, or a week later?
The technical breakdown: it is almost always stress, not accuracy
When a CNC machine removes material, three things release energy at the same time:
- Residual stress inside the raw stock. Extruded, rolled or forged material carries locked-in stresses from its own manufacturing. Milling away one side of a stressed plate is like cutting one string on a pulled bow — the part moves to rebalance. Aluminum plates from the same batch can differ noticeably in this behavior.
- Cutting-induced stress. Heat and force at the cutting edge compress and stretch the surface layer. Deep, aggressive roughing in thin walls builds up more of it.
- Clamping force. A vise or fixture that pulls a flexible part flat will "help" it pass inspection while clamped. Release it, and the true shape reappears.
The machine itself is rarely the problem. A machine that holds 0.005 mm will happily produce a part that moves 0.05 mm the day after — because the geometry changed inside the material after cutting stopped.
The numbers: what to expect before you get burned
| Factor | Typical effect on flatness | Mitigation |
|---|---|---|
| Stress-relieved (T651/T7351) plate vs standard extrusion | Difference of 0.02–0.10 mm on a 200 mm span after heavy material removal | Specify stress-relieved temper up front |
| Roughing → release → finishing vs single-pass to final size | 0.03–0.15 mm improvement on thin-walled parts | Write rough-then-finish into the process |
| Roughing → release → finishing with an intermediate rest | Further 0.005–0.02 mm gain on long parts | Accept extra lead time |
| Deep roughing on thin walls (<1.5 mm wall) | Largest single contributor to post-machining warp | Control depth of cut, leave finishing stock |
| Final skim pass with light clamping | Recovers geometry the fixture was hiding | Insist inspection happens unclamped |
Two takeaway numbers worth remembering:
- A part is not "flat" until it is measured free of the fixture. Anything else measures the clamp, not the part.
- Symmetry matters. Removing equal amounts of material from both sides keeps the stress balance; machining everything from one side guarantees bending.
What to actually specify (the DFM answer)
- Ask for stress-relieved stock on parts with high material-removal ratio or walls under 2 mm. It costs more per kilogram and saves the whole batch.
- For critical flatness, require a rough → release → finish sequence, and say so on the drawing notes. Suppliers who already machine this way will not blink.
- Put the inspection condition on the drawing: "flatness measured in free state, unclamped." Without it, every party can be technically right and the part can still be wrong.
- For aerospace-grade alloys, an intermediate thermal treatment between roughing and finishing is the standard answer; for most industrial parts, a rest period plus symmetric machining is enough.
- Quote tolerances that match function. A spacer that rides inside a machined pocket does not need 0.01 mm flatness; an optical or sealing face does.
Conclusion
Post-machining movement is a materials-and-process question, not a spindle-accuracy question. The fix is specified by the buyer, executed by the supplier, and verified by unclamped inspection. Parts that arrive flat are the product of a sequence, not a lucky machine.
About the author
CHX Precision Metal is a CNC machining manufacturer in Shenzhen, China, operating since December 2013 across a 5,000 m² facility with 100+ staff and 220 CNC machines — including 100 five-axis machining centers, 50 CNC lathes and 70 Tsugami Swiss-type lathes. We hold ±0.01 mm standard tolerances (±0.005 mm with grinding) under ISO 9001:2015 and ISO 14001:2015, with 3–7 day prototyping and 7–15 day production, from a MOQ of 1 piece. If your last batch taught you what stress relief means, tell us about the part: https://chxmetal.com — or write to mc@chxmetal.com.
