step drill bit, also known as a speed bit, or Unibit is a conical
shaped drill bit with a steps cut into the sides. Due to the design
of a step drill a single bit can be used for drilling a number of
different hole sizes. Some step drill bits come to a point and are
therefore self-starting. The larger-size bits have blunt tips and
are used for hole enlarging.
Step drill bits are most commonly used on softer materials - plywood,
particle board, drywall, acrylic, laminate, etc. They can be used
on very thin sheetmetal, but metals tend to cause premature drill
wear and dulling. A metal hole saw is
more appropriate for large-hole applications in thicker metals.