The design, characterisation and application of an accelerated drill test for cutting tool development

Abstract

Metal cutting has been shown by other researchers, as well as within this work, to be a stochastic process with many complex behaviours and sources of variance which detrimentally affects the scatter in tool life results. Empirical testing is still the preferable method for cutting tool development especially in an industrial environment, as models are currently unable to replicate the complex interaction between cutting tool and workpiece as tool wear is the result of several mechanisms working simultaneously. The need to quantify differences in sample groups and make sound statistical inferences about their populations is crucial for any tooling manufacturer to manage the level of quality, as well as to continually improve their products. This is highlighted in the cutting tool market as end users are continually demanding longer tool lives, low variance and no early tool life failures, as large amounts of overhead costs are incurred in partially manufactured components, with sudden tool failure resulting in substantial costs to a business. Therefore, the objective of this research was to design and develop a destructive accelerated drill test which is robust, sensitive, rapid and low cost. It was found, that although sources of machining complexity and variance which affects the scatter in tool life data have been identified within the literature, a solution to deal with it has not yet been provided. Using a systems approach facilitated the management of the complex behaviours so a machining regime could be identified which offered a repeatable and mono-modal tool failure, a robust test, as well as, where possible, to minimise and empirically model the effect of machining variance on tool life scatter, a sensitive test. Annealed D2 cold work tool steel was able to offer a low tool life standard deviation and a mono-modal tool failure mode. In stark contrast P20 plastic mould steel showed a larger standard deviation and a bi-modal failure mode. This work also showed that for accelerated testing of HSS cutting tools, which are thermally sensitive, an abrasive wear test is preferable over a thermo-chemical wear type. Pre inspection of drill geometries for the rejection of Jobber drills outside of tolerance was found to be insignificant when distinguishing the difference between sample means. It was assumed that the drills outside of tolerance would be randomly distributed, along with their effect on tool life. It was important however to determine, that the drills outside of tolerance, would have a small effect on the standard deviation in tool life, relative to the sample size and relative to the sample means so statistically significant results could be determined. An extended Taylor’s tool life model was generated which modelled the effect of plate hardness between the range of 467HLD to 511HLD for annealed D2. This work showed that the life of uncoated M2 HSS 6.35mm Jobber drills is sensitive to small changes in plate hardness. At a cutting speed of 25m/min an increase in plate hardness from 467HLD to 492HLD, an increase of 5.3%, decreased drill life by 70.9%