## 2 Methodology and Materials  
### Methodology  
This study is part of a research project that includes different lightweight structural materials, machining processes, and cooling/lubrication systems [18; 19; 57; 58; 60]. The methodology used is similar to the one followed in previous works in which the bases that provided the initial knowledge for this work were established. In previous studies within the same project, and using a similar methodology to make the individual studies comparable, the influence factors on the turning of Mg-Al specimens were studied [19], the influence of the tool on the drilling of magnesium alloy parts was investigated [20], the best process parameters for re-drilling on titanium parts by dry machining were investigated to eliminate the use of lubricant/coolant fluids and thus make the process more sustainable and economical [18], and the parameters with the most influence on the re-drilling of hybrid multimaterial Mg-Al-Mg compounds with various re-drilling diameters were investigated [58,60]. This methodology is based on the Taguchi methodology and the Montgomery guidelines [59], to carry out a study in which the results can be compared with each other. The methodology includes the following stages: pre-experimental planning in which the problem under study is defined, the identification of factors of influence, levels and ranges, and the choice of the response variable. Subsequently, the selection of the experimental design, its performance, and the statistical analysis of the results obtained takes place. Finally, after the discussion and the analysis of the results, the conclusions are drawn. The problem under study is focused on repair and maintenance operations carried out by re-drilling on multimaterial or hybrid magnesium-based components, evaluating the cutting conditions and the sustainable cooling/lubrication techniques applied, and analyzing their influence on the results of the surface roughness obtained. Since it is aimed at repair and maintenance operations, a single value of 0.5 mm was chosen for the depth of cut factor, \(d\). Keeping the depth of cut at low values also helps to keep the cutting temperature low; being favorable to be far from the ignition temperatures of magnesium. The type of cooling system, \(C\), is a factor of great interest in the study of magnesium machining because of the special conditions that this material presents regarding ignition. For this reason, two levels were selected for the type of cooling, \(C\), factor, the first level is Cold Compressed Air (_CCA_) and the second level is dry machining. The cutting tool, which also has a great influence on the quality of the machining, was fixed on a 9 mm diameter high-performance twist drill. The test specimens consist of two magnesium plates with an aluminum plate in between. Chip evacuation was identified as a factor with potential influence, and, consequently, measurements of roughness were taken at the entrance and at the exit of each plate. The following factors were established as factors of potential influence: the plate on which the measures are taken, Location Regarding Insert, _LRI_, and the position where the measures are taken on each plate, Location Regarding Specim, _LRS_. The Location Regarding Insert factor has three levels depending on the plate _LRII_: magnesium plate Mg1, _LRI2_: aluminum plate Al2 and _LRI3_: magnesium plate Mg3. The Location Regarding Specim factor has two levels; depending on whether the measures are taken at the entrance of the hole, _LRS1_, or at the exit of the hole, _LRS2_. For the feed per \(\text{rev},f\) (_mm/rev_), and the spindle speed, \(N\) (_r-min-1_), both factors with potential influence, two levels were set based on the previous experience of researchers in the machining of this type of multimaterials, \(N\), 500/1200 and, \(f\), 0.05/0.1  
Table 1 summarizes the factors used and the levels chosen for each factor. The selected design was the product of a complete factorial \(2^{3}\) and a block of two factors (\(3\times 2\)).