Homemade Countersink for Wood
A round or flat-head bolt can be made into a good rosebit or reamer for countersinking holes for screw heads. 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.
Round and Square Heads of Bolts Shaped and Notched to Make Countersinks (Fig. 1, Fig. 2)