Yield Point
Why does yield matter in CNC workholding?
Clamp pressure and cutting forces must stay below the material's yield limit to avoid irreversible part distortion that breaks size, flatness, parallelism, or concentricity requirements.
How is yield commonly reported for metals?
It is reported as yield strength or yield stress, often in MPa, and for materials without a sharp yield event it is frequently defined at 0.2% plastic strain.
What shop sign suggests a part yielded during setup?
Persistent jaw imprints, relieved springback after unclamping, unexpected flatness loss, bore ovality, or a part that measures differently off-machine than on the fixture.
Yield point is the stress level at which a metal stops behaving elastically and begins plastic deformation, meaning it will not fully return to its original shape after the load is removed. For many modern materials, yield is defined by the 0.2% offset method rather than a sharp visible point.
In CNC machining, yield point dictates the boundary between recoverable flex and permanent damage. When a part is clamped, pressed, or machined, the material experiences stress; if that stress remains below yield, the part springs back to intended geometry after unclamping. If it exceeds yield, the deformation becomes permanent, as bow, taper, jaw marks, or out-of-round bores. This governs workholding choices: thin-wall aluminum, annealed steel, and slender stock need low-concentrated clamping force and larger contact areas to keep stress below yield. It also guides material selection - higher-yield steels resist denting and bending under service load, while lower-yield alloys machine more easily but distort from aggressive fixturing. Metrology is directly affected: elastic deflection is temporary and can be compensated, but once yield is crossed, the dimensional error is set and cannot be corrected by re-indicating alone. Recognizing the difference between sprung-back elastic movement and permanent set is essential for holding tolerances.
Over-clamping thin or soft stock: Excess vise or strap-clamp force can exceed the material's yield point, especially on aluminum plate, mild steel sheet, or long slender workpieces, leaving jaw marks, bow, or banana warp after release.
Aggressive presses, bends, or interference fits: If assembly force is too high during dowel insertion, bushing pressing, or edge-banding compression, the local stress can exceed yield and create crushed fibers, ovalized bores, or loosened fits after springback.
Mistaking elastic deflection for acceptable distortion: A part may look acceptable while clamped, but once removed, elastic recovery reveals true geometry error; if yield was crossed during machining, recovery is incomplete and the part stays permanently bent or twisted.