May I ask what the function of the central hole in the shaft is?

The central hole is typically used as a manufacturing reference; all related machining operations make use of it for positioning, thereby ensuring a unified reference point that guarantees the positional accuracy between various machined surfaces (such as coaxiality). Specifically as follows: Function and structure of the central hole. The central hole serves as the positioning surface for shaft-shaped workpieces when they are mounted on centers. The 60° tapered hole of the central bore fits with the 60° tapered surface on the center tip ； The small round hole at the inner end ensures a tight fit between the tapered hole and the tip’s conical surface, and it also allows for the storage of a small amount of lubricant (butter). The common types of central holes are Type A and Type B. Type A center hole has only a 60° tapered hole. The 120° conical surface at the outer end of the Type B central hole is also known as the protective conical surface, which serves to protect the outer edge of the 60° conical hole from being damaged. Type A and Type B center holes are machined using the corresponding center drills on lathes or specialized machines. Before drilling the center hole in a machining center, the end face of the shaft should be turned flat to prevent the center drill from breaking. 1. The center hole is generally not used to control the axial dimensions, unless it can be ensured that the size of the center hole is very accurate. The maximum insertion depth is not controlled nor guaranteed; how can axial positioning be achieved? 2. The common center hole is used in grinders and lathes to machine shaft parts with steps. The part is positioned on the machine tool using a center pin, and its outer diameter as well as the end faces of the steps are machined; this ensures the coaxiality of the various outer diameters of the shaft and the perpendicularity of the end faces to those outer diameters. :victory: