# Preassembly Cryogenic Drilling in Carbon Fiber Sandwich Sheets with Inner Foam Core
## 5 Conclusions  
This work presents a study on cryogenic machining in carbon-fiber-reinforced polymer sandwich sheets with an inner foamed polyvinyl chloride core. The outcomes are of interest because these materials can be used in ships sailing in very cold waters and can also be applied to the forming of components for their manufacturing under the selected cutting conditions (drill bit rotation speeds between 2000 and 6000 rpm and feed rates between 200 and 600 mm/min). In these cutting conditions, the inlet and outlet diameters achieved very low deviations, complying with strict tolerances according to the ISO standard. This allows us to avoid additional preassembly operations and is thus important in maintenance operations. Good results have been found for other materials such as GFRP or CFRP, but this sandwich material is lighter.  
Figure 7: Inlet diameters with maximum and minimum differences regarding the nominal diameter: (**a**) Test 4; (**b**) Test 6; (**c**) Test 7; (**d**) Test 8.  
A contraction occurred in the outlet diameters with respect to the inlet diameters in laminates of both core thicknesses. This phenomenon was identified by the developed predictive model for outlet diameters. The proposed model and those developed for inlet diameters and thrust forces have a high coefficient of determination and provide adequate predictions. The optimal cutting conditions have been identified and experimentally contrasted, being located in the range of 2100-3100 rpm for drill bit rotation speeds and 200-320 mm/min for feed rates.  
The conclusions are limited to the range of cutting conditions studied, as is usual in this type of analysis. The study of delamination and surface roughness for this material at cryogenic temperatures in the range of cutting conditions estimated as optimal, as well as the results of other types of drill bits, are presented as immediate lines of research.  
Conceptualization, R.D. and M.M.M.; methodology, R.D., M.M.M. and N.R.-P.; validation, R.D. and M.M.M.; formal analysis, R.D. and M.M.M.; investigation, R.D., M.M.M., N.R.-P. and R.A.-F.; resources, R.D.; writing--original draft preparation, R.D., N.R.-P. and R.A.-F.; writing--review and editing, R.D., M.M.M., N.R.-P. and R.A.-F.; supervision, R.D.; project administration, R.D.; funding acquisition, R.D. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Spanish Ministry of Science and Innovation, projects RTI2018-102215-B-I00 and PID2022-1433290A-I00, and the College of Industrial Engineers of UNED, project 2023-FISII-UNED-05.  
Not applicable.  
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.  
The authors would like to thank the College of Industrial Engineers of UNED for supporting them.  
The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.