Step Drill Bits

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A step drill bit is a drill bit that has the tip ground down to a different diameter. The transition between this ground diameter and the original diameter is either straight, to form a counterbore, or angled, to form a countersink. The advantage to this style is that both diameters have the same flute characteristics, which keeps the bit from clogging when drilling in softer materials, such as aluminum; in contrast, a drill bit with a slip-on collar does not have the same benefit. Most of these bits are custom-made for each application, which makes them more expensive.

A unibit (often called a step drill bit) is a roughly conical bit with a stairstep profile.[7] Due to its design, a single bit can be used for drilling a wide range of hole sizes. Some bits come to a point and are thus self-starting. The larger-size bits have blunt tips and are used for hole enlarging.

Unibits are commonly used on sheet metal[7] and in general construction. One drill bit can drill the entire range of holes necessary on a countertop, speeding up installation of fixtures. They are most commonly used on softer materials, such as plywood, ...
... particle board, drywall, acrylic, and laminate. They can be used on very thin sheet metal, but metals tend to cause premature bit wear and dulling.

Unibits are ideal for use in electrical work where thin steel, aluminum or plastic boxes and chassis are encountered. The short length of the unibit and ability to vary the diameter of the finished hole is an advantage in chassis or front panel work. The finished hole can often be made quite smooth and burr-free, especially in plastic.

An additional use of unibits is deburring holes left by other bits, as the sharp increase to the next step size allows the cutting edge to scrape burrs off the entry surface of the workpiece. However, the straight flute is poor at chip ejection, and can cause a burr to be formed on the exit side of the hole, more so than a spiral twist drill bit turning at high speed.

The unibit was invented by Harry C. Oakes and patented in 1973.[8] It was sold only by the Unibit Corporation in the 1980s until the patent expired, and was later sold by other companies. Unibit is a trademark of Irwin Industrial Tools.