Enlarging a hole in wood or other material requires a lot of guesswork to start the bit in the exact center.
woodworking technique shop tip

Insert a scrap of dowel the diameter of the existing hole and slightly recessed from the surface—about 116 ". Fit a drill bit the same size as the hole into the recess and tap the end of the shank just hard enough to mark the dowel center. Use this mark to start drilling the larger hole.
—Ed Good, Nordland, Wash.