Computational and Experimental Investigation of Conventional and Cryogenic Cooling in Milling Operations

In machining, high thermo-mechanical loads which limit tool life and efficiency are often managed using coolants. Appropriate coolant selection and well targeted application can transport significant thermal loads away from the cutting region, along with providing lubrication and aiding in chip evacuation. In this work, numerical and experimental studies of coolant applications contribute to both increasing fundamental understanding of coolant behaviours and supporting the design of improved cutting tools.

A multiphase conjugate heat transfer computational fluid dynamics (CFD) model is developed in OpenFOAM to study both the coolant coverage and tool temperatures during the application of an oil-water emulsion coolant in milling processes via through-tool channels. Utilising this model to investigate a standard tool design has shown the coolant jet is targeted near to the cutting edge, with coolant spreading out to cover the tool and workpiece surfaces to provide good coverage along the cutting edge. To support the development of improved tool designs, the CFD model has been further utilised in an exploration of the coolant channel design space and optimisation study. As well as providing new tool designs with minimised tool temperatures and maximised coolant coverage in this study of the design space, key physical features such as a shift in coolant spreading behaviour and the presence of designs with significant levels of splashing have been identified for the first time.

Cryogenic coolants provide a cleaner alternative to oil-water emulsion coolants in cutting operations. Liquid CO2 is one such coolant which has shown promise in experimental machining tests. To build towards the development of a CO2 CFD model which can support innovation of tool designs to maximise the benefits of this new coolant, fundamental experimental and modelling work is presented in this thesis. An experimental rig design has been developed to analyse heat transfer when using CO2 coolants in a simplified setting and this has been used to successfully validate a CFD model capturing the conjugate heat transfer when using liquid CO2 coolant.