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