*3.1. SLD Analysis*

The SLDs of the samples were calculated before, during, and after the pocketing tests at different axial depths of cut (Figure 5). SLDs show the frontier line under which axial depth of cut and spindle speed values lead to a chatter-free milling. As expected, the maximum chatter-free depth of cut is lower when machining has finalized, because the rigidity of the part is lower. Also, bigger material removals lead to lower maximum depths, as the decrease of the lobes is more pronounced for the cases of machining taking place at *ap* = 1 mm and *ap* = 0.8 mm. This phenomenon is more obvious if only the critical depth of cut is analyzed, namely, the lower maximum axial depth of cut that guarantees chatter-free milling irrespectively of spindle speed (Figure 6).

**Figure 5.** Stability lobe diagrams before, during and after pocket milling. (**a**) TP10. (**b**) TP08. (**c**) TP04. (**d**) TP02.

**Figure 6.** Critical axial depth of cut for the machining of each sample for different material removals.

An important initial conclusion that can be derived from these SLDs is the difficulty to globally avoid chatter when a milling operation is performed in this kind of sample as they were set up. Even the machining conditions that according to the SLD initially avoid chatter (*ap* = 0.3 mm; 17,500 rpm) disappear because of material removal, which means that chatter would only be avoided at the beginning of the machining. In addition, the productivity would be low. Aiming to overcome these problems, a deeper analysis has been performed, studying separately each cutting pass of the machining path.
