**4. Conclusions**

In this study, the impact of tool geometry and operative conditions on the hole drilling of thermoplastic-based fibre metal laminates was investigated. Cutting load, delamination and internal roughness were assessed. Drill geometry shows very significant influence on all measured machinability indicators. The inclusion of grooves on the primary cutting edges of the drill revealed neither an appropriate technique toward efficient chip control nor delamination mitigation. In contrast, the usage of a double-point angle drill stands out as an effective method toward improved hole-making in fibre metal laminates, with minimal delamination. Back support was shown to be an adequate alternative method for minimizing delamination, especially when in conjunction with smaller feed rates. Fracture energy was successfully estimated using asymmetric double cantilever beam tests, enabling the calculation of critical force thresholds for delamination occurrence, which adequately predicts the delamination with some exceptions duly justified from known model limitations.

**Author Contributions:** Conceptualization, A.M.P.d.J. and A.T.M.; methodology, F.M. and F.G.A.S.; software, F.M. and F.G.A.S.; validation, F.M. and F.G.A.S.; formal analysis, P.A.R.R., A.M.P.d.J. and A.T.M.; investigation, F.M., F.G.A.S. and T.E.F.S.; resources, A.M.P.d.J. and A.T.M.; data curation, F.M. and T.E.F.S.; writing—original draft preparation, F.M. and T.E.F.S.; writing—review and editing, T.E.F.S., P.A.R.R. and A.M.P.d.J.; visualization, F.G.A.S. and A.T.M.; supervision, F.G.A.S. and A.M.P.d.J.; project administration, A.M.P.d.J. and A.T.M.; funding acquisition, A.M.P.d.J. and A.T.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work has been conducted under the scope of MAMTool (PTDC/EME-EME/31895/2017) and AddStrength (PTDC/EME-EME/31307/2017) projects, funded by Programa Operacional Competitividade e Internacionalização, and Programa Operacional Regional de Lisboa funded by FEDER and National Funds (FCT). This work was supported by Add.Additive project (POCI-01-0247-FEDER-024533) funded by ANI under the scope of Programa Operacional Competitividade e Internacionalização and funded by FEDER and National Funds (FCT).

**Institutional Review Board Statement:** Not applicable

**Informed Consent Statement:** Not applicable

**Data Availability Statement:** Not applicable.

**Acknowledgments:** Palbit S.A. is acknowledged for providing necessary tools for experimental cutting tests. INEGI is also acknowledged for providing the materials used in this study.

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