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.

Illustration from The Boy Mechanic, Book 2: 1000 Things for Boys to Do

Round and Square Heads of Bolts Shaped and Notched to Make Countersinks (Fig. 1, Fig. 2)

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