## 1 Introduction  
The subject of this paper includes quality control of aircraft casing covers for accessory drive train (ADT) gearboxes produced by way of the casting machining process. Due to the responsibility of these systems, all produced pieces are subject to inspection. A typical production quality control usually consists of in-process inspection of selected dimensional and shape-related characteristics by operators and in final inspection in a measurement laboratory using coordinate measuring machines (CMMs) [1]. The control includes, without limitation, measurement of parallelism and perpendicularity of surfaces, measurement of surface flatness, and measurement of dimensions of holes and their true position. The advantages of CMMs are widely known and are related mainly to high precision and capability to measure in an automatic manner, as well as the generation of measurement reports. The biggest disadvantage of their usage is the high cost of purchase and maintenance. Therefore, measurement laboratories are very often the bottleneck of the production system [2, 3]. This is due to the fact that a control cell equipped with a CMM usually supports multiple production lines. This also raises the problem of storage of manufactured products prior to measurement, as well as the involvement of technical resources and human resources in the in-plant transportation of products. Modern production systems in the aviation industry, and not only in this industry, are designed so that the human factor has the lowest possible impact on the quality of products [4]. This simultaneously enhances occupational health and safety [5]. It entails, however, the necessity to automatize as many operations as possible. Fully automated production systems realize the so-called closed door technology (CDT) [6]. CDT is characterized by maximizing machine functionality through proper task planning and eliminating human influence on the process [7, 8]. One of the ways in which this is implemented is the use of CNC machining centers to perform measuring operations. In recent years, probing packages with dedicated software to support them have become standard equipment in CNC machining centers. This made it possible to take basic measurements in the machining center and save the results as CNC system variables or measurement reports. It has also opened a new field for research and engineering works known as on-machine measurement [9; 10] or on-machine probing [11; 12; 13]. The core feature of this subject is that it allows reliable measurements of workpieces to be carried out during and after production using CNC machining centers [12; 13; 14]. More details on the use of CDT in the ADT casing production process are provided in [15], where a process executed in a standard manner and a CDT-based process are compared (Figure 1). That paper points out that some of the most difficult operations to automatize are related to ADT dimensional and shape-related inspection. This happens for at least two reasons. The first is the necessity to ensure adequate accuracy of a CNC machine, which should be used not only as a machine center but also as a measuring device used for quality control during manufacturing. This issue was analyzed and solved by the authors of [15]. The second reason covers variable conditions during measurements on a CNC machine with respect to measurements carried out using CMM. This variation is due to the different clamping conditions of the part on the CNC machine and on the CMM. Process requirements often impose the need to measure products in a measuring laboratory in an unclamped state (free support) approach, which is not used in practice in machining stations. A change in the clamping condition affects the release of stresses, and this causes the workpiece to deform. Thus, the measurement results of dimensional and shape-related characteristics on a CNC machine and on a CMM differ significantly.