# Surface Quality Evaluation in the Milling Process Using a Ball Nose End Mill
## 1 Introduction  
Nowadays, freeform shapes are some of the most difficult surfaces to produce. They are present in various areas of production, such as the automotive industry, the aerospace industry, or the production of dies, molds, and many others, where the shape corresponds to the shape of the future product [1; 2; 3]. In this group of future products, we can find different shapes comprising convex, concave, or flat surfaces. In some cases, these surfaces can be described mathematically. Each part must be manufactured to meet the recommended quality and dimensions, so the requirement for the correct selection of milling strategy is justified. To support this requirement, it is necessary to know the effectiveness and influence of strategies in milling free-form shapes [4; 5; 6]. The most frequent use of freeform surfaces is in plastic moldings. Molds, whose shape corresponds to the future product, consist mainly of free-form shapes. To achieve the required mold shape, the milling process represents the main production operation. To achieve this, CNC machines are used, which can produce parts in three or five axes, where the NC program is generated by the CAM system [7]. For the best machining of these complex shapes, different CAM systems are used, where the user can choose the appropriate strategy to match the specific toolpath in accordance with the geometry of the part. The main disadvantage of the CAM system is that the simulation process does not provide the microsurface texture after machining, related to the cutting edge of the tool. Proper selection of the free-form milling strategy canimprove the surface roughness [8]. When programming with CAM systems, strategies such as zigzag, radial, raster, or spiral curves are most useful for milling freeform surfaces [9; 10]. Many researchers describe the effect of tool path strategies on roughness, but only a few studies address the effect of toolpath strategies on surface topography [11; 12]. Currently, many CAD/CAM systems incorporate different strategies to select various milling shapes and to achieve recommended shapes and dimensions [13]. Therefore, it is very important to select the most appropriate strategy considering the relationship between best roughness, higher dimension accuracy, and effective time production. Cutting tools with ball milling cutters are used in various areas of production. They are most commonly used in mold-making, automotive, aerospace, and other industries. In these areas, it is important to achieve the desired shape of the future part, which can include a variety of shapes ranging from convex and concave curves to planar surfaces or variously shaped complex surfaces. All of these surfaces must be produced based on production requirements in terms of accuracy, dimensions, and other factors. Of the three basic machining operations, such as roughing, semi-finishing, and finishing operations, ball-end milling tools are used the most in finishing operations. It is very important to keep in mind that the tool-surface contact relationship is different from conventional milling. One of the main characteristics of shaped surface machining is that the contact between the tool and the workpiece is constantly changing. In addition, machining through the center of the cutting tool can negatively affect the surface quality [14; 15]. The contact between the tool and the surface is different when milling free-form surfaces compared to milling simple shapes. In the contact position, the cutting speed varies from the programmed value. These are the areas where contact is made between the tool and the machined surface. In the first region, the tool axis is parallel to the machined surface, and in the second region, the material is cut through the center of the tool, and the position of the tool axis is almost perpendicular to the machined surface [16; 17].