The Frame That Warped After Machining: Residual Stress, Not Bad Milling
An aluminum extrusion frame, 1.5 m long. We machined the mounting surfaces flat, checked them, and sent it to assembly. Two weeks later, the customer called: the mounting surfaces had twisted 0.3 mm. The linear rail no longer sat flat. The extrusion had warped after machining. The machinist said the frame was within spec when it left the mill. It was. The stress released later.
Why extrusions warp
An aluminum extrusion is produced by pushing heated aluminum through a die. The cooling after extrusion creates residual stresses, the skin cools faster than the core. When you machine the extrusion, you remove material from one side. The residual stress is no longer balanced. The frame twists toward the side you machined. It happens over hours to days, not during the machining itself.
This is normal. Every machined extrusion moves. The question is how much, and how to compensate.
How much movement to expect
| Material | Typical movement after machining | How to control |
|---|---|---|
| Aluminum extrusion (6061-T6) | 0.1-0.5 mm per meter | Stress relief before machining |
| Steel plate (hot rolled) | 0.2-1.0 mm per meter | Anneal before machining |
| Cast aluminum | 0.1-0.3 mm per meter | Aging (natural or artificial) |
| Machined plate (stress-relieved) | under 0.05 mm per meter | Buy pre-stress-relieved stock |
What we did
1. Stress relief before machining. We ordered the extrusion pre-stress-relieved (the supplier does a thermal treatment at 180 C for 2 hours). This costs about 15 percent extra but removes most of the residual stress. The warping dropped from 0.3 mm to 0.05 mm.
2. Rough machine, then finish machine. We rough-machined the frame (leave 0.5 mm stock), let it sit 48 hours, then finish-machined to final size. The rough cut releases the stress. The 48-hour wait lets the frame move. The finish cut captures the final geometry after the movement. This is the standard approach for precision parts.
3. Machine both sides symmetrically. We used to machine only the top surface. Now we machine the top and bottom equally (rough cut both sides, finish cut both sides). The stress stays balanced. The frame does not twist.
The flatness check after machining
After machining, we put the frame on a surface plate and check flatness with a dial indicator. But that check is at the machine. The real check is 48 hours later. We now re-check flatness after 2 days. If it moved more than 0.05 mm, we re-machine. The customer gets a frame that stays flat.
For welded frames
Welded frames warp even more (the weld shrinkage). The process: weld the frame, stress-relieve (anneal at 600 C for 1 hour, or vibratory stress relief), rough machine, wait, finish machine. This is what precision machine builders do. A welded frame that is only machined once will move.
The takeaway
A frame that warps after machining is not a machining error. It is residual stress releasing. You cannot eliminate it entirely, you can only manage it: stress-relieve the material, rough-machine first, wait, then finish-machine. The frame that twisted 0.3 mm was a raw extrusion, rough-machined once, shipped to assembly. The next frame was stress-relieved, rough-cut, waited, finish-cut. It stayed flat.
The machining rule: Precision surfaces on extrusions or weldments move after machining. Stress-relieve the stock. Rough machine, wait 48 hours, finish machine. Check flatness after the wait, not right off the mill. A frame that is flat when it leaves the machine is not flat two weeks later.
