A small headless screw having a slot cut for a screw driver is called

Circular

The head of a thumb screw is circular in shape.

Woodruff key

A key that fits into a semicircular keyway cut in the shaft is woodruff key

60 Degree

The angle of I.S. thread (V-shaped) is 60 Degree

All the above

The reasons for rough and broken threads are
Thread cutting tool is blunt
The size of hole or diameter of the blank for bolt is not correct
The flutes of cutting tool are filled with chips