Step Drills for Sheet Metal

What it is

A step drill (also called a unibit or step bit) is a cone-shaped drill with a series of ground steps. Each step cuts one diameter, and the next step up cuts a larger one. A single tool can make a pilot hole, enlarge it through several sizes and deburr the edge. Step drills are made for thin material: sheet metal, electrical boxes and enclosures, panels, plastic and thin tube. They are the usual tool for punching conduit knockouts in the field when a knockout punch is not at hand.

Most step drills are high-speed steel, often with a cobalt grade or a coating such as TiN or TiAlN. Flutes are straight or spiral. A spiral flute clears chips better and cuts more smoothly; a single straight flute is simpler and easier to resharpen.

How to identify

Read the size range and step increment etched on the shank or body, such as "1/8 to 1/2 by 32nds" or "7/8 and 1-1/8." Many electrician bits are marked with the conduit trade size each step fits instead of the hole diameter. The step height (the flat length of each step) limits the material thickness you can cut cleanly. Shanks are round with three flats, or hex for quick-change drivers.

Conduit knockout steps

Hole sizes are the common nominal knockout sizes. Confirm the fitting maker's required hole before cutting a listed enclosure.

Key points

• Use step drills on material no thicker than one step height. In thicker stock the step below binds and the hole goes oversize or out of round.
• Run slower as the diameter grows. Speed should suit the largest step in use. See the SFM to RPM chart.
• Use cutting oil on steel and stainless. Dry cutting dulls the edges fast.
• Watch the marked line or count the steps so you stop at the right size. Some bits have a laser-etched ring at each step.
• A step drill leaves a small chamfer when you just touch the next step. This deburrs the hole.

Common mistakes

• Using a step drill on thick plate or solid bar.
• Running too fast, which blues and dulls the cutting edges.
• Drilling handheld at an angle, which makes an oval hole.
• Passing the wanted step and making the hole oversize.
• Expecting a step drill to hold close hole tolerance. Use a twist drill and reamer for that.

Governing standards: No single consensus standard governs step drill geometry. Follow the tool maker's data. Knockout hole sizes come from the enclosure and fitting makers.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.