## 1 Introduction  
A needle roller bearing is one of the bearings employed in a variety of industrial applications where long fatigue life is one of the most important criteria [1]. Figure 1 shows a schematic of the needle roller bearing, which is mainly composed of an outer ring, cage and needle rollers. The quality of a needle roller strongly affects the fatigue life of the bearing in which the needle roller is employed [2, 3]. For the manufacturing of needle roller bearings having a long fatigue life, it is necessary to ensure the quality of needle rollers by evaluating their dimensional parameters, such as the diameter, out-of-roundness and so on [4, 5]. Figure 1: Needle roller bearing. The out-of-roundness of a cylindrical workpiece can be measured by the conventional rotary-scan method [6; 7; 8; 9]. For the precise evaluation of the out-of-roundness, error separation methods such as the multi-step method [10] and the multi-probe method [11; 12] have often been employed. Moreover, the V-block method can be employed for the waviness and cylindricity measurement of a cylindrical workpiece [13]. Meanwhile, in the out-of-roundness measurement, precise alignment of a target workpiece with respect to the rotational datum is one of the necessary steps before measurement. The alignment of a small target workpiece is not an easy task, even for the state-of-the-art roundness measuring instruments with automatic alignment functions; for example, a decrease in the workpiece length could affect the accuracy of the workpiece tilt adjustment. A decrease in the workpiece diameter could also make it difficult to carry out centering alignment in a roundness measuring instrument. An optical method has also been developed for the measurement of a small cylinder [14]. Although the method is capable of measuring cylindricity and/or roundness while allowing fast measurement with easy workpiece alignment, the measurable diameter was reported to be no less than 3 mm [14]. In responding to the background described above, an alternative method referred to as the stitching linear-scan method has been proposed [15; 16]. In the method, a series of arc-profiles of a small workpiece are obtained by a linear-scan surface form stylus profiler, which is often employed for surface form/roughness measurement [4]. For the positioning of a small workpiece, the unique workpiece-holding mechanism composed of a V-block and a round magnet jig having index marks has been developed [16]. Through the stitching process, the circumferential profile of a small workpiece can be reconstructed from the obtained series of arc-profiles for the evaluation of the workpiece cylindricity and/or roundness. The proposed method does not require a high-precision rotary table and is free from the influences of the workpiece eccentricity. Although the angular misalignments of the workpiece with respect to the scanning probe in the stylus profiler could degrade the accuracy of stitching operations, an experimental setup capable of compensating for the angular misalignments of a target workpiece has been developed to address the issue. Once the V-block is aligned with respect to the stylus of the commercial stylus profiler, only the circumferential misalignment of the workpiece will be the influence of the difference of user/operator on the measurement. On the other hand, the circumferential misalignment can be compensated through the circumferential stitching process--namely, a difference in the user/operator will not affect the results of the measurement. Experimental results have demonstrated that a workpiece having a diameter of 3 mm can successfully be evaluated by the proposed method [17]. The stitching linear-scan method is expected to have advantages with respect to the conventional rotary-scan method in the case where a workpiece diameter is smaller than 3 mm.