## 1 Introduction: State of the Art
### Surface Roughness in Milling  
Surface quality along with the dimensional and shape accuracy are often investigated by researchers studying machining. One of the essential products of machining (e.g., milling or brushing treatment) is the modification of the surface roughness [7]. Commonly, milling, drilling, or boring are not followed by any further machining. The technologist should thus ensure that the processes are both efficient and effective and that the surface roughness properties of the workpiece meet the functional requirements, be it for the tested Mg alloys or any other substrate. [PERSON] et al. investigated the changes in the surface roughness of Mg alloy AZ91 HP workpieces after milling in response to the modification of the cutting parameters (v\({}_{\text{c}}\), f\({}_{\text{z}}\), a\({}_{\text{p}}\)) and the geometry of the carbide cutting tool edge (\(\gamma=5^{\circ}\) and \(\gamma=30^{\circ}\)) [8]. The results from the measurements on the lateral face showed that surface roughness parameters decrease at increasing v\({}_{\text{c}}\), but rise with the increase in f\({}_{\text{z}}\). It was also shown that lower roughness parameter values were obtained when employing a cutter inclined at a \(5^{\circ}\) angle. The same material was used in the study of different machining processes reported by [PERSON] et al. [9]. Milling was shown to produce the highest quality surface: Ra = 0.124 \(\upmu\)m and Rz = 0.952 \(\upmu\)m; while the 180 non-woven tool the worst, Ra = 0.81 \(\upmu\)m and Rz = 3.82 \(\upmu\)m.  
The surface roughness of AZ91D alloy was analyzed by [PERSON] et al. [10]. In the study, the carbide cutter, micro-cooled with compressed air, was employed at a conventional range of speeds, maximum v\({}_{\text{c}}\) = 400 m/min. The tests concluded that Ra increases in response to rising v\({}_{\text{c}}\) and f\({}_{\text{z}}\). The lowest Ra = 0.5 \(\upmu\)m was produced at a lower cutting speed, v\({}_{\text{c}}\) = 50 m/min, and feed per tooth, f\({}_{\text{z}}\) = 0.2 mm/tooth. The roughness increased at higher cutting speeds. AZ91D alloy was also investigated by [PERSON] et al. [11], who obtained an exceptionally low surface roughness--Ra = 0.061-0.133 \(\upmu\)m--by machining at a small feed per tooth of f\({}_{\text{z}}\) = 0.03-0.09 mm/tooth and an axial depth of cut equal to a\({}_{\text{p}}\) = 0.2-0.3 mm. The lowest Ra of the workpiece surface was achieved at the highest cutting speed of v\({}_{\text{c}}\) = 900 m/min, effectively proving that milling may constitute the finishing operation (grinding or polishing are redundant) given that it is executed at high cutting speeds. [PERSON] et al. [12] applied solid carbide end mills with and without TiN and TiAlN coating to machining SiC and B4C-reinforced magnesium. The tests were executed in the range of spindle speeds from 1500 to 2500 rev/min (v\({}_{\text{c}}\) = 19-31 m/min) and the feed rate was in the range 1200-2000 mm/min. The results showed that the built-up cutting edge and feed per tooth change to the greatest extent the workpiece's surface roughness. The best quality of finish was provided by a TiN-coated tool, with an Ra equal to 0.35 \(\upmu\)m. The authors of a related study [13] aimed to determine the optimal combination of cutting parameters with respect to the surface roughness in the dry milling of AM60 magnesium alloy with a TiN-coated carbide cutter. Several important correlations emerged from the tests: Increasing the feed per tooth and the axial depth of cut increased the surface roughness, and secondly, at a higher spindle speed, the surface roughness was reduced.