*7.2. Explanatory Data*

Whilst understanding the physics of a CO2 MWF strategy does not remove the necessity of machining trials, it does increase the probability that the resources used during those trials are wisely allocated. Knowledge of the science informs an engineer as to which variables would be prudent to manipulate in order to drive improved performance outcomes, and as such, data that explains the fundamental physics of a process are crucial. In the context of CO2 MWFs, knowledge of the thermo/fluid dynamics of the coolant or the tribology of the cut allows a machinist or project engineer to make on-the-spot adjustments based on feedback received during the machining operation. As an example, knowledge of

the machinability data informs an engineer that CO2 MWF strategies perform poorly in the context of machining Inconel (Section 6); however, knowledge of the phenomena is needed to explain why that is the case.

Undoubtedly, both machinability and explanatory data are crucial to encourage the uptake of CO2 MWF strategies; however, it is clear that the majority of the research outlined in this review has been focused upon developing a broad machinability data set, with much less emphasis on understanding the science of the process. This is a reasonable strategy during the genesis of a new technology, as it explores whether a technology has the scope to be of value; this, in turn, paves the way for significant research commitment in the future. Nonetheless, as the prospective benefits of CO2 MWF strategies are made clear (in this review and elsewhere), it becomes apparent that explanatory experimentation should become the focus of future research efforts, not least to allow for more direct and effective machinability research going forward.

To summarise, this review was produced with the outlook of assessing the operational implications of cryogenic cooling, as well as the practicalities of employing cryogenic MWFs. The review also served to assess the sustainability of CO2 MWF strategies by considering the social, operational, and economic implications of conventional MWFs and contrasting that against CO2 cooling. Further, this work begins to highlight the contexts within which cryogenic coolants are most suitably applied in lieu of the current industrial standard of conventional emulsion-based cooling. In this sense, a compelling case for cryogenic-assisted machining is made and, as such, it is shown that there is clearly a significant motivation for future research, and undoubtedly a great deal of remaining scope for novelty within the field.

**Author Contributions:** Conceptualization, L.P. and N.T.; methodology, T.S.; investigation, L.P.; writing—original draft preparation, L.P.; writing—review and editing, N.T and T.S.; visualization, N.T.; supervision, N.T. and T.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the EPSRC Industrial Doctorate Centre in Machining Science (EP/L016257/1).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Conflicts of Interest:** The authors would like to declare no conflict of interest.

#### **References**

