## 2 Related Work
Their study concluded by demonstrating the robustness behind their segmentation technique when used under varying parameter conditions. In summary, the segmentation techniques discussed in the literature review are applied to improve the quality of the evaluated signal, such as detection and/or removal of transient and steady state portions, and process change point detection to differentiate between operational states. Comparatively, there is a lack of signal segmentation methods that are applied for localizing faults in a CNC linear axis. The crucial nature of the linear axis in the operation of the CNC machine is well established. In the recent comprehensive review by [25], a detailed discussion of the components found in the linear axis and the different efforts in advancing CBM in this area was provided. The review concluded by stating that application of CBM in the linear axis is a growing field of research. The importance of the linear axis is further highlighted in the legacy work by [26], where a comprehensive review of the control and design was explored to build an understanding on the linear axis. However, like any other subsystem in a CNC machine, the linear axis is not exempt from deterioration and developing failure faults (FFs) throughout its operational lifecycle. As stated by [27], when the components of a linear axis begin to wear, it may cause operational errors in positioning, straightness, and angular motions. Most recently, the research works in this field have focused in detecting localized wear, such as spalling, in the different components of the axis including the leadscrew, linear guide rail, and carrier block [21; 28; 29; 30; 31; 32; 33]. Despite these efforts, from a maintenance perspective, if localized damage has already developed in the linear axis component, it warrants the need to immediately be replaced. Therefore, it is crucial to take a step back and understand as to how the damage developed in the first place. From an industrial perspective, the root-cause of why localized damage occurs in the raceway and rolling components can be traced back to lack of lubrication, excessive loads, and dirty environment, but most importantly misalignment. According to the report by [34], misalignment represents one of the leading causes of premature wear in the components of the linear axis. When misalignment is present in the system, regardless of the type, it will result in uneven loading on the rolling elements and raceways. These uneven loadstypically will lead to wear in the form of pitting. As discussed in [35], there are two types of misalignments that need to be addressed when installation of the linear axis is being done, as illustrated in Figure 2.  
First, vertical misalignment refers to the offset relative to the base the linear axis component sits on. Secondly, the horizontal misalignment refers to the right or left offset relative to a reference linear guide. Both offsets must be below the tolerances suggested by the manufacturer to ensure no accelerated wear affects the system. Research on the effect and detection of misalignment in the linear axis has not received enough attention compared to other critical subsystems and components, as stated by [36]. Recent efforts in misalignment analysis and detection are outlined in Table 1.