## 1 Introduction
In the case of quality control of components with elevated working parameters such as sliding rings for non-contact seals, the measurement of flatness deviation is particularly complex. It is extremely important for the functionality of the product and essential to perform. The use of special measuring equipment is a key element of the production process. Measurements of surface shape errors are used, among others, in the process of examining the fit of the surfaces that close injection molds. Measurements can be carried out using numerically controlled measuring machines, equipped with contact measuring probes. Thanks to such acquired data--measurement points--it is possible to modify the nominal CAD model by adding local deviations with the opposite sign to the measured value. The use, fully or partially automated, of 3D measuring techniques significantly accelerates the performance of such an examination. It is known that the use of equipment with various technical and metrological parameters significantly affects the final result. Interesting research issues include the impact of the technical and metrological parameters of equipment construction on the final result of the examination, and what impact the use of CNC control in the measuring device will have. In contact measurements, there is direct contact between the measuring probe and the element being measured. Two basic methods of contact measurements can be distinguished. The first method involves the use of a rigid measuring tip, and the second method utilizes a trigger probe. Rigid measuring tips require the participation of the operator because, after the probe comes into contact with the element being measured, they must send a signal to the device about the contact with the object. The measurement can be performed in a point-by-point manner or continuously (continuous scanning). During continuous measurement, the operator signals the start of the measurement with a switch, then moves the probe across the surface being examined, and the conclusion of the examination is signaled by pressing the switch again. Currently, the rigid measuring tip is rarely used in measuring machines. It is most often found in the equipment of measuring arms. Trigger probes are currently the basic equipment of stationary 3D measuring machines. In their case, the contact of the probe with the measured surface generates an impulse in the form of a discrete signal to the control devices. The use of these probes enables the automation of the measurement process. An undeniable advantage of contact probes is the accuracy of measurement. Unfortunately, the necessity of multiple contacts between the measuring probe and the measured element makes the measurement process slow, and in the case of measuring flexible elements, inaccurate or even impossible. Flatness, as defined in the PN-EN ISO 1101 standard [16], means the requirement that the plane being assessed is located between two parallel planes, where the flatness error is the distance between them. Determination of flatness consists of determining the mean plane and then identifying two parallel planes passing through the most distant points of the surface. The flatness deviation is the distance between these planes, taking into account the tolerances [17]. The study used two 3D measuring machines (one manual, one automatic) and a measuring arm. The measuring arm, although less accurate, is mobile and suitable for measuring large objects directly in production. The paper presents the problem of tolerance; in industrial sectors, tolerances do not have to be too strict, and less precise devices may be sufficient. In such cases, these devices enable a significant reduction in the cost, time and resources required to carry out measurements. The authors examined the issue of whether less precise devices can be accepted in industrial conditions. It was pointed out that many companies do not analyze in practice the risk resulting from the use of less precise devices, which makes these studies valuable.