Metrology


Measuring surface roughness in turned parts


Traditionally, cylinder bores surfaces in cars were like the Himalayas - lots of peaks and troughs. When piston rings went up and down, this would slice the tops of these off, so manufacturers made the bores tight to pre-empt this. Consequently, in the past motorists need to “run-in” their engine, limiting its speed for the first thousand miles or so. While the automotive world has moved on, this is an early example of how manufacturers recognised the value of surface characteristics. Here Mike John, technical director at industrial metrology specialist The Sempre Group explains the importance of surface roughness and how to get the most out of this data.


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urface roughness often dictates how one part interacts with another. For example, if a shaft is rotating inside a bearing, a rough surface is undesirable because it


causes excess friction. Meanwhile, a smooth and round surface ensures optimal performance by minimising resistance. If turned parts have the wrong surface characterisations, they could wear out, get bigger, smaller or rattle around. In the automotive sector, surface


texture is vital for anything that rotates in an engine. For example, camshafts will sit on a white metal bearing, a smooth object with a coating, and oil will produce a frictionless surface. However, if the surface roughness is poor, this will cause metal-to-metal contact. Consequently, the part will wear quicker, and irregularities in smoothness can produce nucleation sites where breaks and corrosion occur.


MeaSureMenT in aCTion On the shop floor, most engineers and manufacturers use roughness average (Ra) to measure change in process and understand micro- and macro-level geometric irregularities. The Ra will tell them if their tools are wearing out and producing different surface characteristics over time. Armed with this information, they can decide whether to make a tool change to remain within spec. Ra will not give the full picture, just an


average. It assigns a value to the deviation away from a median height - say, on an engine’s crankshaft - but says nothing about the direction of surface performance. Knowing what parameters will provide the required functionality of the surface is the first step. For instance, manufacturers can use plateau honing to create a metallurgically stable microstructure on the wall of a cylinder


August 2022 Instrumentation Monthly


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