Test & measurement


The ins and outs of fastener


quality assurance T


here are numerous reasons a fastener may not meet the required specifications. Perhaps


there was a mix up during picking or labelling, and the fasteners have been delivered in an unmarked or incorrectly labelled bag. Maybe the wrong ISO


standard of part has been delivered, or the material is not tempered when it should have been. Many common errors are almost impossible to detect with the human eye, so what can we do to be sure our fasteners are the right ones for the job?


CHECK YOUR MEASUREMENTS The most common problem with fasteners is that they do not match the drawing — the dimensions, thread, head or length is incorrect. This is a relatively easy issue to pick up, taking a measurement using a vernier or a micrometer will identify around 90 per cent of problems. If needed, the next step would be to measure the component using a shadowgraph, which illuminates a shadow of the component that can be zoomed in on to identify any non-uniformities. Quality teams can also use thread gauges or a shadowgraph to check a fastener’s thread.


FINISHING Many fasteners are coated to improve their resistance to corrosion, temperature or friction. For example, powertrain fasteners in a vehicle are commonly coated with zinc to enable them to resist corrosion, heat and oil. Polytetrafluoroethylene (PTFE) coating is common in the aerospace and oil and gas industries as it is lightweight and provides high-corrosion resistance. Quality professionals should assess whether the coating thickness is uniform across the fastener and within stated tolerances — if it is not, the part could cause problems later on.


56 September 2020 Instrumentation Monthly


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