Poisson Ratio
Poisson’s ratio (ν) is the negative ratio of transverse strain to axial strain in a material under loading. It quantifies how a part narrows when stretched or bulges when compressed. In CNC machining, it predicts changes in width, wall thickness, or bore diameter under clamp, press-fit, or bending loads. For isotropic materials, this lateral-to-axial strain relationship is constant within the elastic range.
In CNC machining, Poisson’s ratio is critical for predicting press fits, thin-wall deflection, clamp distortion, and springback during setup or in-service loading. Compressing an aluminum sleeve in a press fit causes lateral expansion, altering interference and potentially cracking housings or altering fit class. FEA and CAM verification rely on this property, combined with elastic modulus, to model lateral movement under axial or fixturing loads, making ν essential in simulation material cards. Similarly, millwork and composite panel work see effects on panel compression, fastener bearing, and gasketed joints, as through-thickness loading induces in-plane expansion or contraction, affecting clearances and seal pressure. In tolerance planning, dimensions measured under clamp or assembly preload may differ from free-state size, with elastic strain consuming significant tolerance budget.
Is Poisson’s ratio a strength property?
No. It is an elastic deformation property, not a yield or tensile strength value; it describes strain coupling before the proportional limit is exceeded.
Why does Poisson’s ratio matter in tolerance stack-up?
Because a dimension measured after clamp load, press load, or assembly preload may not match the free-state machined size; the elastic change can consume a significant portion of a tight tolerance band.
Is Poisson’s ratio always constant for a material?
It is treated as constant only in the linear elastic, isotropic range. Outside that range, or in anisotropic materials like composites, directional behavior may differ from the simple isotropic model.