Stress Relief
Stress relief is a controlled process used to reduce residual stress locked into a workpiece from rolling, welding, casting, forming, rough machining, or heavy clamping. In CNC machining, it is typically performed after roughing and before final finishing, so the part stabilizes and is less likely to move, warp, or crack during later machining or service.
On the shop floor, stress relief is scheduled after roughing and before finishing: bulk material removal happens first, then the part is stabilized before tight-tolerance cuts. For steels, thermal stress relief runs below the transformation range, commonly 1,000–1,200°F (538–649°C) with slow cooling; aluminum uses lower-temperature thermal cycles or controlled aging depending on alloy condition. This prevents parts from opening up after unclamping, reduces taper changes after pocketing, and keeps flatness, parallelism, and true position within tolerance during finish passes. Large welded frames or structures that cannot tolerate furnace heat are treated with vibratory stress relief (VSR) to redistribute stress without thermal distortion. Polymer work such as POM uses low-temperature annealing before or after machining to reduce molded-in stress and improve dimensional stability. Machinists also machine symmetrically, alternate opposing sides, and avoid excessive clamping pressure so stress release stays balanced and inspection reflects the true part condition.
Is stress relief the same as annealing?
Not always. Thermal stress relief is a controlled heating process below the transformation temperature, intended to reduce residual stress without significantly changing hardness or tensile strength. Annealing may also alter hardness or microstructure depending on alloy and cycle, so the two processes serve different metallurgical goals.
When should stress relief be done in CNC machining?
It is commonly performed after roughing and before final finishing. This lets the part move and stabilize before the last precision cuts are made, preventing size drift, out-of-flat conditions, and shifted hole positions after unclamping.
When is vibratory stress relief (VSR) used instead of thermal relief?
VSR is used when a part is too large, too sensitive to heat, or at risk of distortion from furnace treatment. It is especially practical for welded structures and frames where full furnace heat could change geometry.