Q&A OF THE WEEK: “How Do I Cut Threads in Steel for a Bolt?”

Q: What’s the best way to make threads in steel for small bolts? I know there are tools for this, but I’ve never used them.

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A: The name of the tool used for cutting threads inside a hole is a “tap”. You drill a hole of the correct diameter, then thread the tap into the hole. It takes more force to turn the tap than it does to turn a bolt of the same size, but that’s because the sharp edges of the tap are actually cutting threads inside the hole. Once these threads are cut, you’re ready to thread in a bolt. That’s one of my sets of taps in the image below.

Sets of taps are sold at most hardware stores and they often come with companion tools called “dies”. These cut threads on the outside of a rod, effectively turning that rod into a piece of threaded rod. I work with metal a fair amount in my home workshop, and for every 10 times I might use a tap I only use a die once. That’s because if you want threads on the outside of something, you can always grab a bolt or piece of threaded rod. Cutting threads in a hole for a bolt is a much more frequent operation. The last two times I cut threads was for grease fittings in a couple of metal projects I made with moving parts.

All threaded fasteners have numbers that refer to the diameter and thread spacing. For Imperial fasteners, this is expressed as 1/4-20 for one of the most common small bolts/nuts. The “1/4″ refers to 1/4” diameter, and the “-20” refers to twenty threads per inch. In the metric world threaded fasteners are also rated in diamter, but also a pitch number. For instance, a 6mm x 1.0 bolt is 6mm in diameter and the thread pattern is describe by the 1.0 pitch figure.

All sets of taps include a chart that shows the exact size of drill bit needed for a given size of tap. As you’d imagine, the size of hole is critical for the tap to work well. Also, important is the use of cutting oil to lubricate the tap and keep it cutting cleanly and lasting a long time.

DEWALT is one company that sells a drill bit that’s also a tap. That’s what you see above. You drill and tap all in one operation using an electric drill. It works really well for small bolts and you can see it in action on video below. I cover a few different topics in this video, but the drill/tap combination is found at 6:17. You can scroll ahead to that spot if you want.

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– Steve Maxwell