Emory Kimbrough: “There are numerous jigs to be found on the Internet for holding a cylindrical workpiece exactly vertical. You might, for example, want such a jig to drill down the axis of a dowel or a metal rod with a drill press. These jigs have to be made quite carefully, or the angle they’ll hold the stock at will be something other than vertical. Rather than spend time making one of these potentially finicky jigs, try this instead.
"Just trap the cylinder between two hand-screw clamps that are placed flat and at right angles to each other. One clamp prevents tilt in one vertical plane, and the other clamp prevents tilt in the other orthogonal vertical plane. The cylinder then has no freedom to do anything but point straight up.”