What are thread cutting screws used for? You use thread cutting screws when you need to join two pieces together, but you can’t or don’t want to use tools to cut threads ahead of time to accommodate the screws.
Thread cutting screws are a special type of self tapping screw. People often use the words thread cutting, thread forming, and self tapping interchangeably. These classes of screws are very similar but they have subtle distinctions. Thread cutting screws create less stress on the workpiece than thread forming screws do. You can also get thread cutting screws for metal or plastic and wood.
What are threads and why would you want to cut them?
When you take apart a commercial product held together by screws, you’ll notice threads in the holes, like the threads you’ll find in a hex nut. Having threads in the hole helps the screw to hold it together and also accommodates disassembly and reassembly.
You can buy tools, called taps, to cut threads for screws. But you also have to have the right sized drill bit. Sometimes you can improvise with a drill bit that’s close, but for a professional quality fit, you need a set of numbered drill bits that match the taps. And while these are things a good hardware store will carry, they aren’t necessarily something you’ll find at the nearest big-box home improvement store. They are something Harbor Freight will carry, and Harbor Freight’s taps are fine for occasional use. But it’s an extra step. Eliminating extra steps in DIY projects is always a good idea because it also eliminates mistakes.
Thread cutting screws make everything more convenient. They are much more likely to let you get away with a pilot hole that’s too small. They save you the step of messing with a tap, because they cut the mating threads in your material as you drive them in.
You can buy them almost anywhere that sells any other type of screw. And when you use a thread cutting screw for wood, you’re much less likely to split the wood.
Thread cutting screws for wood
I was working with a friend on a project and he whipped out a box of unfamiliar screws. I noticed how little effort and driving torque it took to drive them into the wood compared to using regular wood screws. If you have a steady enough hand, you frequently don’t even need to drill a pilot hole. Just get them started, then drive. It wasn’t quite like having the finesse of a screwdriver and the power of an impact driver all at once, but it was close.
When drilling thin wood or close to the edge, you’ll want to drill a pilot hole to reduce the chances of splitting. But they split the wood much less frequently than regular screws. In my experience splits rarely happen unless I’m drilling into endgrain near an edge.
You can tell the difference from an ordinary wood screw by looking at the end. A regular wood screw has threads spiraling consistently all the way down to the tip. A thread cutting version has visible cutting edges near the tip that give the cutting action.
The brand I’m most familiar with is GRK Fasteners, but there are others. GRK’s thread cutting screws cost a couple of cents more per screw than comparable ordinary screws, but I find them worth it for the effort they save. I typically drill a shallow pilot hole, just deep enough to hold the screw in place, then drive the screw into place. I find this technique gets me a tighter, firmer grip than drilling a pilot hole the full depth of the screw.
Thread cutting vs thread forming screws
When you’re working with plastic or metal, you have more choices. Thread forming screws don’t require as deep of a hole because they don’t remove material, they just move it around. The downside is they introduce more stress. The thread cutting type have an advantage because they remove material, but since they remove material, you need a deeper hole. But they don’t produce as much stress, so you’re less likely to break the material.
The cutting surface on a thread forming screw is smaller and less pronounced. You have to look closely to notice the cutting surface that distinguish it from an ordinary machine screw. The thread cutting type has a pointier end that looks more aggressive.
The upside to a thread forming screw is that it won’t disturb existing threads if you take the item apart later. A thread cutting screw may cut across existing threads and leave you with a weakened hole. So for an item you expect to have to take apart for maintenance, a thread forming screw provides an advantage.
You can also remove the thread cutting screw afterward and replace it with a regular screw of the same thread pitch to avoid the problems of repeated disassembly and reassembly, but it’s an extra step. If you’re doing to do that, you might as well use a screw tap. The whole idea behind using a self tapping screw of any kind is to save steps.
One weird trick you’ll want to remember
When you reassemble something put together with any type of screw, here’s a trick to avoid damaging the threads. Start the screw by hand. Insert the screw, then turn the screw left while pressing down on the screw. Keep turning until you hear a click. That’s the sound of the screw dropping down into the threads. Then you can start turning right to tighten the screw the way our elders taught us. Turn a couple of turns by hand to start the screw, then you can use a drill or powered screwdriver to finish it if needed.
This trick works with any kind of screw, whether regular, thread cutting or thread forming, so it’s a good habit to get into.