What are residual stress and distortion?

Residual stress is mechanical stress present in a workpiece with no external force acting on it. Tensile and compressive stresses balance each other within the material. Remove material and that balance changes, so the workpiece deforms until a new balance is reached. That deformation is distortion. It often only shows once the part comes out of the setup, because the workholding holds it in shape until then.

What do residual stress and distortion depend on?

Residual stress arises from rolling, drawing, forging, casting, welding, hardening and quenching, and in a thin surface layer from machining itself. In precipitation-hardened aluminium such as 6082-T6 the largest stress comes from the quench after solution treatment; T651 plate is stretched afterwards to reduce it, and cast, stress-relieved plate usually has even less. How far a part distorts depends on the stress distribution in the stock, how much material is removed and where, the stiffness of the final shape and the machining sequence. Thin, long and asymmetric parts, and parts where most of the block is machined away, are the most sensitive.

How do residual stress and distortion work in practice?

The usual measures are roughing with an even allowance on all sides, then releasing the part so it can deform freely, and only then finishing to size with low clamping force. That does not remove the stress, but it lets the distortion happen before the final size is cut. For steel and weldments, stress relieving between roughing and finishing can lower the stress itself; for precipitation-hardened aluminium, heat treatment is usually not an option and stress-relieved or cast plate is the better choice. These measures can add setups, a subcontracted step or a thicker block of stock. An estimate that prices only the final shape misses those steps.

What are residual stress and distortion often confused with?

Distortion from residual stress is not clamping distortion: in clamping distortion the workholding pushes the part elastically out of shape; it is machined in that state and springs back into a different shape once released. Residual stress sits in the material itself; both show only after unclamping. Thermal distortion from heat during machining is something else again: it depends on temperature and changes as the part cools. Nor does it point to a machine error: the program can be right and the part still bent.

How do you see residual stress and distortion in Blake?

Where the part is stress relieved between roughing and finishing, that appears in the Blake estimate as a separate subcontract line, with the price taken from your subcontract library; without a price the line stays open. The extra setups around the stress relief are yours to enter and accept per route. More on estimating.