Drilling a large hole in thin sheet metal with a standard twist drill bit is difficult. The bit tends to catch, tearing the metal and leaving a ragged, triangular hole. A step drill bit, often called a Unibit, is shaped like a cone with stepped ridges. Each step is ground to an exact diameter (such as 1/4, 3/8, or 1/2 inch) with a single flat flute that cuts clean, perfectly round holes.
As you press down, the bit cuts the first small hole, then the next step chamfers and enlarges it. This allows you to drill a wide range of hole sizes with a single bit. Step bits are ideal for drilling firewall holes for wiring harnesses or modifying body panels. Run the drill at low speeds and use cutting oil to prevent the bit from overheating and dulling.