# Assessment of the Functional Properties of the Surfaces of Ductile Cast Iron Parts
## 2 Materials and Methods  
### Evaluation of the Machinability of Cast Iron by the Geometric Texture State of the Surface  
The occurrence of unstable conditions in the roughing and shaping milling of EN-GJS-600-3 ductile cast iron causes vibrations that adversely affect machinability (by limiting the range of free selection of the periodic cutting speed). However, for surface quality reasons, this effect can be neglected, as most often in roughing and shaping operations, surface roughness is not an important technological requirement. It only becomes important during finishing passes. Defining acceptance criteria for the realisation of stable machining conditions is very important. In addition to the CNC machine operator's subjective perception of stable or unstable machining conditions, there are a number of objective, i.e., quantitative evaluation methods. The values of surface profile indices are the most commonly used for this purpose. Modern surface metrology allows the definition of a very broad set of indices of the geometrical structure of a surface for a 2D or 3D profile related to its height, functionality or load-bearing capacity. In addition to indices characterising standard functional properties, so-called hybrid indices are increasingly used, characterising the service properties of the product surface and indices related to qualitative assessment, i.e., the type and direction of the produced texture. The description of the conditions for measuring the spatial state of the geometrical texture of the surface in the construction documentation is regulated, as shown in Figure 1.  
There is still no awareness among design engineers of the new possibilities related to the description of the technological and functional requirements of surfaces. This state of affairs is probably due to the fact that the method of determining the parameters used to describe the spatial state of the geometrical texture of a surface has been regulated by ISO standards relatively recently. Part of the standardisation work related to the ISO 25178 family of standards [44; 45; 46] (associated with measurement methodology) is still ongoing. Depending on the surface technology, its purpose, and the way it is used, the method of noting the functional requirements is very important. In construction documentation, the use of the roughness parameters \(Ra\), \(Rz\), \(Rt\), and \(Rmax\) has become most common for this reason. In recent years, one can see a similar analogy and a gradual spread of the use of parameters such as \(Sa\), \(Sq\), and \(Sz\) in the description of surface topography [32; 33; 34]. However, the above parameters say very little about the functionality of the surface. With a similar height of roughness, surfaces may have different load-bearing capacities and may be isotropic or anisotropic. In the area of a surface with similar \(Sa\) parameter values, there may be different numbers of valleys and peaks with different volumes. These valleys, peaks, and cracks can provide a point of attachment for lubricants. A suitable surface texture can increase the load-bearing capacity of the surface or determine the hydrophilic capacity of the finished product. Surface topographical analysis is very often used to investigate the degree of isotropy. Parts shaped by machining, where the roughness is influenced by the regular geometry of the tool and the kinematic conditions of the process, are generally characterised by anisotropic surfaces. In contrast, fine oscillating finishing operations such as honing and polishing, as well as abrasive blasting, lead to non-directional-isotropic surfaces with different height values of the geometric texture parameters of the surface. Often, the resulting surface of the finished product is the result of mapping another surface, such as a tool. This is the case when moulding castings in sand moulds or when shaping plastic moulded parts in high-precision pressure moulds.