What Is a Step Drill Bit?
A step drill bit — also called a unibit or cone drill — is a conical cutting tool with multiple stepped diameters ground into a single body. Each step increases the hole diameter by a fixed increment, allowing you to drill several hole sizes with one bit.
Unlike twist drill bits that cut a single diameter, step drills progressively enlarge the hole as you feed deeper. This makes them ideal for thin materials where a standard twist drill would grab and tear.
When to Use a Step Drill Bit
Step drills excel in specific applications where twist drills struggle:
Sheet metal — Steel, aluminum, and stainless sheet up to 3mm thick. The stepped geometry prevents the bit from grabbing and producing ragged exit holes.
Electrical panels — Cutting conduit knockouts and cable entry holes in switchgear and junction boxes.
HVAC ductwork — Clean holes in galvanized sheet for fittings and fasteners.
Automotive bodywork — Enlarging existing holes or drilling new ones in body panels without distortion.
Plastic and fiberglass — The gradual cutting action prevents cracking in brittle materials.
How to Choose the Right Step Drill
Step Count and Size Range
Step drills typically come in 9 to 13 steps, covering ranges like 4–12mm, 4–20mm, or 4–32mm. Pick the range that covers your most common hole sizes. A 4–20mm bit handles most general sheet metal work.
Material Grade
M2 HSS — Good for mild steel, aluminum, and general-purpose use. The most cost-effective option.
M35 HSS (5% cobalt) — Better heat resistance for stainless steel and harder sheet metals. Worth the premium if you regularly drill stainless.
Coating
TiN (Titanium Nitride) — The gold-colored coating reduces friction and extends life by 3–5× compared to uncoated bits. Best all-around choice.
TiAlN (Titanium Aluminum Nitride) — Maximum heat resistance for high-production environments and stainless steel.
Shank Type
Most step drills use a 3-flat shank (also called a tri-flat) that prevents spinning in the chuck. Hex shanks are also common for use with impact drivers on job sites.
Tips for Getting the Best Results
Use low RPM — Step drills work best at 500–1,500 RPM depending on material. Too fast generates excess heat and dulls the cutting edges.
Light pressure — Let the bit cut at its own pace. Forcing it causes chatter and uneven steps.
Cutting fluid — Always use cutting oil on steel and stainless. A spray lubricant works for aluminum.
Mark your target step — Wrap tape around the bit at your desired step to avoid over-drilling.