# Influence of Ball-End Milling Strategy on the Accuracy and Roughness of Free Form Surfaces
## 1 Introduction  
To remain competitive in the current market, the demand to produce increasingly complex high-quality products must be met using efficient production processes. Product variability has increased, and seriality has decreased, which complicates the standardization of processes and the reuse of known technological settings. There are freeform surfaces on an increasing number of engineering products, and computer numerical control (CNC) milling is considered the most efficient, productive, and flexible method for manufacturing these surfaces. The production of freeform surfaces is realized by CNC machines either directly or indirectly. In the first case, the part is made directly on the machine, and in the second case, a forming tool is used (mold, die, and pressing tool), followed by a CNC machine operation. Ball-end mills, owing to their shape, are widely used to finish freeform surfaces; the tool easily adapts to the machining of these types of parts. Developments in this area are intensive, as evidenced by reviews [1, 2]. The first characterizes the situation in the period 1997-2008, and the second analyses the results obtained in the ten years prior to 2021. Reference [1] focuses on three aspects of machining contours: toolpath generation, tool orientation identification, and tool geometry selection. There was noticeable progress in all three areas during the reviewed period, but the authorsalso identified the problem areas. They considered the computational and machining times to be the two areas requiring further improvements for efficient machining of freeform surfaces. According to the authors, it is necessary to pay more attention to quality when machining polyhedral, cloud-of-point, compound, and trimmed surfaces. They recommend measuring deviations to evaluate the quality of freeform surface machining. An upper and lower limit should be set. The first controls the maximum scallop height, while the second corresponds to gouging. The authors of a recent study [2] identified gaps for further research in the literature, for example, gaps were identified in the fields of freeform milling of difficult-to-cut materials, tool inclination, three-dimensional (3D) surface roughness, microhardness, and residual stresses. It is also necessary to study atypical milling tools and new types of coolants. The need for continuous improvement in generated toolpaths, simulations, and analytical tools is emphasized. Milling machines with three or five axes are typically used in the production of parts with free form surfaces. The decision to use a 3- or 5-axis CNC milling machine is not easily made. It determines the complexity and required accuracy of parts, operating costs of machines, productivity, and the required knowledge and experience of programmers and machine operators. To facilitate decision making, the authors of [3] propose a decision-making tool based on fuzzy systems. In general, the following applies to the use of 5-axis machines:  
* The production time is reduced by using one fixture and one origin (zero point) instead of several fixtures and zero-point settings for each fixture. * The accuracy of the parts is higher, in contrast to 3-axis milling, where each loosening and re-clamping has an adverse effect on accuracy. However, the use of 5-axis machines also has disadvantages:  
* The machine price and operating costs are higher than those of a 3-axis machine. * Staff with a higher level of education and experience are required, whether programmers or operators. * Programming and additional systems, that is, computer-aided manufacturing (CAM) systems and postprocessors, are more expensive, and the cost of training programmers is also incurred. [PERSON] et al. [4] point out the differences among free form surfaces made using 3-axis, (3 + 2)-axis, and 5-axis milling.