A round or flat-head bolt can be made into a good rosebit or reamer for countersinking holes for screw heads.

Ill: Round and Square Heads of Bolts Shaped and Notched to Make Countersinks

In the illustration, Fig. 1 shows a reamer made of a round-head bolt, and Fig. 2, one made of a square-head bolt. The round-head makes the best reamer as more cutters can be filed in the surface and less work is required to file it into shape.