When and How to Use Step Drill Bits

Step drill bits are a great tool to have in your shop.  While many think that they are gimmicky, once you have one and use it you’ll understand why they are a popular drill bit for many trade professionals that drill holes through thin material such as sheet metal and acrylic.

A step drill bit (also known as a unibit) is a single conical shaped bit that allows you to drill a variety of hole sizes.  The tip or point of the bit will drill the smallest hole and the diameter increases as you drill deeper.

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What is a step drill bit used for?

Step bits are most commonly used to cut sheet metal but they are also often used on thin material such as plexiglass, plywood and laminate.

Have you ever tried to use a traditional twist bit to drill through thin metal?  It does not work well.  As soon as the bit penetrates the metal it catches a burr and pulls right through.  While regular twist bits are ok for very small holes, once your hole size is 10x the material thickness or larger this is where a step up bit will shine.  Step up bits have a straight flute which means that thin and flexible materials such as sheet metal and plexiglass won’t twist out of shape when the bit passes through them.  

Cutting holes in thin metal

Step drill bits are often used for drilling holes through sheet metal.  Since each step is typically 1/4″, step bits are often limited to use on 1/4″ thick sheet metal.  That said, if you are able to flip your part around you could also drill from the opposite side effectively doubling the thickness that you can cut through!

Step 1 – Set up for the cut

Before you begin cutting a hole in your piece of metal first confirm that the metal sheet is not thicker than the height of the steps on the bit.  As mentioned previously, most bits have 1/4″ steps but there are also bits available with larger steps.

If necessary, mark the step on your bit that you want to stop drilling at.  For example, if the bit has a smallest step of 1/4″ and largest step of 1-3/8″ and you want to cut a hole with a diameter that is 1″, mark the 1″ step on the bit so that you know where to stop.  If your hole will be the same diameter as the largest step on the bit you won’t need to make a mark on the bit.  You can just drill all the way through.

Step 2 – Add lubricating oil

When cutting metal it’s always best to use lubricating oil.  Add a drop of oil to the metal sheet where the bit will remove material.

Step 3 – Drill your hole

Slowly drill the hole to whatever depth is needed to achieve the correct hole diameter.

*Note that some step drill bits require the use of a punch before you begin drilling.

With a step drill bit you will get a clean and precise hole in thin sheet metal.

Step 5 – Optional – add a chamfer

You can easily add a nice chamfered edge as shown in the picture below simply by tapping the surface with the bevel of the next largest step.

Cutting holes in plastic sheets

Step drill bits are the perfect bit to use for cutting holes in acrylic/plexiglass sheets.  It’s the same process as cutting through metal except that you won’t need lubricating oil when drilling a hole in plastic.  If you’ve ever used traditional twist bits to cut plastic sheets you’re probably familiar with the two small cracks that almost always appear when the bit breaks through the other side of the material.  With a step up bit you’ll get a clean hole without any cracks!

I like to leave the plastic on my acrylic sheets while I cut them.

When you remove the protective plastic from each side of the acrylic sheet you’ll have a perfect hole!

Enlarging holes in sheet metal and plastic sheets

I find that one of the best uses for a step drill bit is when you need to enlarge an already existing hole in thin sheet metal or plastic.  The design of a step up bit allows the bit to remain centered even as you enlarge the existing hole.

Deburring holes

Another great use for step bits is deburring holes in sheet metal.  They will deburr all but the largest diameter holes that they are capable of drilling by just barely touching the next largest step.

Step drill bit vs conventional twist bit

Benefits of step drill bits:

1. One bit can drill various hole sizes without the need to change bits
2. Smoother and rounder holes
3. Short lengths makes maneuvering them in tight spaces easier

Cons of step drill bits:

1. They can wear out quickly
2. They are not easy to sharpen due to their unique shape
3. Cost – Some brands are very expensive!

Single step drill bit vs set of step drill bits

You may be wondering why if a single step drill bit can cut multiple holes sizes why would I need a set of them?  Well, there are really two reasons that you may want to go with a set like I purchased rather than a single bit.

1. Different size shanks ensure compatibility with various chuck sizes.
2. Using a smaller step bit on very thin material will save effort and prolong the life of the bit.

The set that I purchased was very inexpensive.  I’m a DIYer so these bits aren’t going to get a lot of use in my home.  They have worked perfectly when I need them but if you are using them a lot you may want to go with something that’s more heavy duty like a DEWALT, LENOX or Milwaukee.

Have you used step drill bits?  If so, let me know what you think about them in the comment section below!