Originally Posted by
mklotz
One of the more awkward jobs to do accurately is marking the exact point over which a plumb bob hangs. Using the actual tip of the bob can damage it, especially if marking concrete, and is just, well, brutish.
One approach I've seen to this problem is a bob shaped like a horizontal +washer with four arrow points that point inward to the center of the annulus. After the bob comes to rest it's lowered carefully onto the stuff to be marked and a pencil mark made at the center of the four points. While it might work, it seems a bit gimmicky plus it wouldn't work well if the surface to be marked is angled.
IMO, a much better solution would be a plumb bob with interchangeable points. Unscrew the normal, sharp steel point and fit a thin, ultra fine point Sharpie. Then simply lowering the stabilized bob onto the surface would leave a precise mark.
Maybe I'm overthinking things - a machinist designing a push broom. Perhaps nobody needs to mark the sub-bob point that accurately.