Efficacy of Cutting Fluids: Advancements in Machining Friction Testing Methods for Lubricant Performance Evaluation

This EngD thesis addresses the existing limitations and challenges in cutting fluid research by proposing a state-of-the-art fluid testing methodology and conducting a comprehensive analysis of frictional properties in a machining environment. The complexity and extreme conditions encountered at the tool-chip interface have posed significant challenges in accurately measuring and comparing results across studies. This research aims to bridge the gap between industrial accessibility and core academic understanding by utilising a purpose-made machining condition tribometer for a more controlled and accurate assessment of cutting fluid performance.

The research methodology consists of three main stages: milling, orthogonal cutting, and tribometer testing. The milling stage establishes a high-quality dataset of a full-scale, high-complexity cutting process, which serves as a benchmark for subsequent analysis. Special attention is given to controlling errors within the system, and standard metrics are employed to evaluate the quality of these indicators as measures of fluid performance. Additionally, a new methodology of high-frequency analysis of friction is conducted to explore the feasibility of experimentally assessing frictional properties of cutting fluids.

The parameters and conditions derived from the milling trials are replicated in orthogonal cutting to investigate the effect of fluid entrainment into the cutting zone. Non-destructive testing techniques, specifically X-ray diffraction (XRD), are employed to analyse the cut surface and optimise surface homogeneity across different cutting parameters. The insights gained from this stage inform the setup of the open-loop tribometer system.

In the tribometer testing stage, a novel tribometer system is utilised to replicate the extreme conditions observed in cutting tests and to evaluate the lubrication effect in a sliding system compared to the stick-slip zone encountered in machining. This tribometer system also enables a comparison of frictional responses between conventional pin-on-disk bench-top tests and machining-like contact. Moreover, both closed-loop and open-loop states are examined to assess the influence of a nascent surface on frictional properties. Frictional mapping across a range of contact parameters demonstrates the usefulness of the test for analytical simulations.

The overall novelty of this study lies in the introduction of new engineering methodologies and analyses for evaluating the frictional properties of cutting fluids in a machining environment. The thesis presents a comprehensive analysis of the entire range of machining tribology testing capabilities, employing the same materials and conditions for thorough comparisons. Specific contributions include the development of state-of-the-art sliding length milling theory, the investigation of tool measurement frequency on tool wear trials, and the calculation of instantaneous coefficients of friction using high-frequency force data.

Furthermore, this study determines the effect of lubricity on continuous cutting processes, optimises parameters for subsurface damage in cutting fluid testing, and utilises non-destructive X-ray diffraction (XRD) measurements for surface analysis. The effectiveness of conventional tribometers in testing cutting fluids is evaluated, and comparisons are made between different system complexities and contact conditions.