Mounting the headstock to the lathe or mill
You may notice that the post onto which the headstock mounts is a loose fit where it projects from the lathe bed or column saddle. This is normal, and the diagram below will help you understand how it works.
FIGURE 7-Cross section of headstock showing locking screw.
The screw in the front center of the headstock has a cone point. The pivot pin has a tapered slot with a corresponding angle. When the screw is tightened, its angled face engages the groove and, because the pivot pin can not come up, it draws the headstock down into position, clamping it into place. If the pin were rigid, it could keep the headstock from pulling down squarely.