What is a step drill bit for and how do I use one?

A twist bit in thin sheet is a genuinely bad combination: the point breaks through before the lips have engaged, the bit snatches through, and you are left with a triangular hole and a torn burr. A step drill sidesteps the whole problem by never trying to cut a full circle at once. Only one shoulder is engaged at a time, and the material is thinner than a single step.

Safety

What you'll need

• Step drill bit covering the size you need
• Clamps or a vice
• Cutting oil or paste for metal
• Centre punch (optional)
• Eye protection

Step by step

1. Check the material is thin enough.A step bit only works where the material is no thicker than the height of one step — sheet, thin-wall tube and box sections. It will not go through solid stock, and forcing it burns the shoulder.
2. Clamp the work properly.Sheet metal that spins becomes a blade. Clamp it to a sacrificial timber backing, which also supports the material at the moment of breakthrough.
3. Punch the centre.A centre punch dimple stops the small leading step skating across a shiny surface. On a painted or coated panel, a strip of masking tape helps as well.
4. Run it slowly.Gear 1, low trigger. A step bit has a large cutting circumference and it burns instantly at speed. This is the single most common mistake with them.
5. Apply cutting oil in metal.A drop on the shoulder before you start and again as you go. In aluminium it stops the swarf welding to the edge; in steel it takes the heat away.
6. Watch the step, not the hole.The steps are your sizing method. Drill until the shoulder you want has just passed through, then stop. Reading the etched marking beforehand and knowing which step you are aiming for avoids overshooting.
7. Withdraw to clear swarf.The single flute holds very little. Back the bit out every few seconds, clear the ribbon away with pliers, and go back in. A packed flute burns the edge in seconds.
8. Let the next step deburr the hole.As the following shoulder passes it cleans the rim. If you have stopped at the top step, deburr the hole with a countersink or a larger twist bit turned by hand.

Tips

• Step bits are the standard tool for adding a cable entry to a metal back box or an enclosure, where a twist bit would distort the box.
• In plastic and acrylic they are excellent, because the gentle progression avoids the cracking that a twist bit causes. Go slower still and let it cool.
• A step bit with a straight flute is easier to control in thin sheet than a spiral one, which pulls itself in.

Common mistakes

• Using a step bit at drill-press speed — The cutting circumference is large, so surface speed at the shoulder is very high. It blues the edge and destroys the bit in one hole.
• Using it on solid material — More than one step engages at once, the load multiplies and the bit jams or snaps. It is a sheet tool.
• Holding sheet metal by hand — When the shoulder bites, the sheet spins with the bit. This is one of the most common workshop injuries there is.

Questions people ask