*3.4. Measurement Locations*

The measurements of the surface roughness were recorded using a Mitutoyo Surftest SJ 401 roughness tester (Mitutoyo America Corporation, Aurora, IL, USA) (Figure 5a). From Table 1, the measurement locations along with the specimen and the insert were used as influential factors (Figure 5b). These factors are *LRS* for the specimen and *LRI* for the insert. For the *LRS*, two values were chosen: *LRS1*, the roughness at the beginning of the hole, and *LRS2*, the roughness at the end of the hole along the feeding direction. Specifically, *LRS1* was recorded within the first 7 mm of the hole and *LRS2* was recorded within the last 7 mm of the hole (Figure 5c). For the *LRI*, three levels were established according to the drilling direction: *LRI1*, the roughness before the insert (first specimen), which is the roughness inside the holes of the first magnesium piece; *LRI2*, the roughness on the insert (second specimen), which is the roughness inside the holes of the titanium piece; and *LRI3*, the roughness after the insert, which is the roughness inside the holes of the second magnesium piece (third specimen).

**Figure 5.** (**a**) Mitutoyo Surftest SJ 401 roughness tester; (**b**) location of the measurements of *LRS* and *LRI* along the feeding direction, inside each hole; and (**c**) detail of the specific measurement locations in the holes of each specimen.
