Another great tip from the brilliant Emory Kimbrough:
Here’s a fast and easy way of finding the high point of a horizontal pipe, dowel, or other round object, using common tools.
First, clamp the cylinder horizontally into a drill press vise (or any other secure holder, such as a V-block, that you can slide around on your drill press table). Slide the clamped cylinder directly under the press’s chuck, using your best eyeball guess.
Next, place something sharp and pointy into the drill press chuck – the point in the center of a Forstner bit is ideal, but other bits will work, as will a sharp nail with the head cut off.
Now, place a metal ruler cross-ways on top of the cylinder, roughly balanced (see photo). Lower the drill press handle until your pointy object lightly presses the ruler against the top of the cylinder. Most likely, your eyeball guess was slightly off of perfect alignment, so the trapped ruler will lean left or right, since it’s being pressed into the cylinder slightly off of the peak. Just slide the clamped cylinder as needed to make the ruler level.
When the pointy thing holds the ruler level, the drill press chuck is now straight above the highest point on the cylinder. Clamp the vise to the table without moving it, and everything is now aligned to drill right through the center.
This method also works with spheres. Just check that the ruler doesn’t tilt right-or-left or front-or-back. You can also use a metal sheet to better see the tilt on both axes.
You can even do this by hand, without a drill press. Press the ruler gently against the cylinder or sphere with the tip of a pen or pencil, moving the point until the ruler levels. Now, carefully slide the ruler out of the way without moving the hand that’s holding the pencil or pen, and make a mark. No, it’s not as accurate as using the drill press, but it’s still better than guessing by eye.
If you need to draw a line along the crest of a cylinder instead of a single mark, just make two marks, as widely separated as possible, using this method, and draw a line connecting the two marks.