Rough times

Researchers at the US National Institute of Standards and Technology (NIST) have developed a novel technique for measuring the roughness of surfaces that is casting doubt on the accuracy of current procedures.

Surface roughness is a key issue for auto manufacturers and other industries that use sheet metal, one that goes far beyond simple cosmetics. Faint striations and other marks that appear when metal is shaped can indicate residual stresses that can cause the part to fail later on. They also lead to extra wear and early retirement for the expensive stamping dies used to form sheet metal into car bumpers and other body parts.

Measures of surface roughness feed into models that predict friction and the metal’s 'springback' - the amount it will unbend after being stamped. Springback has to be known and controlled to build accurate dies for complex metal shapes.

Conventionally, roughness is measured with a profilometer, an instrument with a probe that is tracked in a line across the test surface to record the peaks and valleys. The process is repeated several times at intervals across the test surface, and the results are averaged into a 'roughness' figure.