Residual Stress
What are the main types of residual stress and how do they affect machined parts?
The two main categories are tensile stress, which pulls material apart and is more associated with cracking and fatigue risk, and compressive stress, which pushes material together and can improve surface fatigue resistance depending on the application. Machining typically leaves a tensile surface stress unless a controlled compressive operation is applied.
How is residual stress measured?
Common methods include hole-drilling, slotting, ring-core, and other strain-relief or diffraction-based methods that infer stress from measured strain rather than directly sensing stress.
Why do thin-walled aluminum parts distort so often after machining?
Machining often removes most of the original stock, exposing previously balanced internal stresses in a low-stiffness geometry. Once the restraining bulk material is gone, the remaining stresses rebalance and the thin wall moves, causing bow, warp, or size drift.
Residual stress is locked-in internal stress remaining in a part after the external load or thermal cause has been removed. In CNC machining, it is a self-equilibrating stress state from uneven plastic deformation, cutting heat, and clamping, leaving a finished part with a stress gradient that can cause dimensional movement after unclamping, secondary ops, or stress relief.
On the shop floor, residual stress behaves like a hidden load path. A part can measure perfectly at the machine and still shift after unclamping, during secondary operations, or after stress relief. This is common in thin-wall aluminum, stainless, and previously formed stock. Practical use means planning roughing and finishing sequences so stock removal stays balanced, choosing fixtures that do not over-constrain the workpiece, and allowing intermediate rest periods when stress relaxation is likely. If distortion-sensitive blanks are hot-rolled, cold-drawn, or welded, thermal stress relief before final finishing is often the difference between staying on tolerance and scrapping the job. The same mechanism appears in millwork and panel processing, where uneven material removal, moisture gradients, adhesive cure, and clamping pressure can leave panels that bow, cup, or spring back after machining. Recognizing residual stress keeps datums stable, protects thin sections, and prevents tolerance loss during later coating or heat-treat operations.
Thin-wall taper after roughing: Heavy asymmetric stock removal unloads one side faster than the other, bending the wall during machining and relaxing after unclamping, which leaves taper, bow, or out-of-flat condition.
Clamping distortion after release: Over-clamping deforms the part during machining, and once the jaws open the workpiece springs back, producing out-of-tolerance bores, lost parallelism, or warped datum surfaces.
Skipping stress relief before finish machining: Hot-rolled, cold-drawn, welded, or heavily roughed blanks release stored stress during final skim cuts or later heat exposure, causing unpredictable movement, chatter-sensitive finishing, and size drift after coating.