A large proportion of modern centrifugal impellers are machined from solid forgings rather than made from cast metal. The CNC milling process offers options to manufacturers to minimise manufacturing costs whilst also enhancing performance of the impeller. Efficient manufacturing can result in cutter tool marks and paths and associated roughness remaining on the hub and blade surfaces of impellers as a result of minimising passes and maximising the cut. The goal of manufacturers is to allow these marks to be as deep as possible to minimise machining costs but without any negative effects in performance and possibly even enhancing it. There are existing modelling methods that predict the influence of roughness on compressor performance using the definition of an equivalent sand grain roughness. The purpose of this study is to relate the performance directly to the tool mark characteristics that are by-products of machining, namely cusp height, cutter path roughness and orientation of the cutter path relative to the local flow velocity, to review the current modelling techniques for predicting the influence of surface condition on compressor performance and to show the scope for optimisation of manufacturing and performance considerations.

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