This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
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.