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How Do Self-Tapping Screws Work?

Self tapping screws with different sizes.

Entering the screw aisle at the hardware store can feel daunting. Dozens upon dozens of screw types in various materials, sizes, thread counts, slot heads, and more all stare at me, demanding that I know what it is I want without looking for hours.

I calmed that fear by learning more about the different screw types, like the self-tapping screw that works by creating the space the screw’s thread needs during installation.. Here’s some more of what I’ve learned about the self-tapping screw, its purpose, and how it’s different from other screws.

What is Tapping a Screw?

Mechanical hand tool set of screw tap.

A screw is a relatively small and thin rod with a thread of material running around its length. Tapping is a process that creates the reverse of the thread in the wood, metal, plastic, or other working medium. When you turn the screw into the hole, the threads sit inside the gaps made by tapping to anchor the screw in place.

A tap – the tool used for tapping – resembles a screw with a socket instead of a normal screw head. The socket connects to a tap wrench that allows the user to apply pressure and rotate the tap. As it turns, it carves out the thread while the notch allows debris to move out of the way.

Once the tap is made, a thread chaser can clear out the debris and touch up the interior thread. It’s just a bit smaller than a tap, so it doesn’t actually cut new threads for its intended size. A chaser might be used long after the tap is installed to clear up corrosion or other long-term damage, too.

Screws, taps, dies, thread chasers, and drill bits all have corresponding sizes and thread counts that work together to create a snug fit for an installed screw. The full chart is readily available from a number of places online, and there are magnetic versions that stick to the side of a toolbox so you don’t have to memorize it or pull out your smartphone in the middle of a task.

How Does a Self-tapping Screw Tap Itself?

Illustration of two different type of screw.

A self-tapping screw creates its own tap when screwed into a pilot hole by clearing away the bits of extra material. Aside from normal screw variations like size and thread count, self-tapping screws are divided into two categories: thread forming and thread cutting. 

A screw that forms threads compresses and shifts the surrounding material out of the way. The edges of the threads are left coarse so they don’t separate as much material on the way down. These will look the most like a “regular” screw. Because they rely on squishing the material, they don’t work as well on brittle substances that tend to crack when shifted.

A thread-cutting screw has notches similar to a tap plus a sharper edge on the threads, making it easier to tell them apart. Some compression will happen, but the space where the thread will go is sliced open as the screw is installed. The notch allows enough space for the debris to move along without getting in the way of the incoming thread.

What are the Pros and Cons of Self-tapping Screws?

A screwdriver and self tapping screw on the wood.

In projects that need the absolute best performance, a proper tap provides a more consistent and secure hold. Self-tapping screws provide that benefit with very few negatives. Still, they might not be the best choice for every project.

For an exceptional use case, surgeons are increasingly turning to self-tapping screws as a way to reduce the number of operating steps and risk to the patient. Although not all the findings directly translate to other fields, this has generated scientific study into the pros and cons of various screw types.

The only real downside compared to a normal screw is the frequently higher cost, but factors like the diameter, length, and even current market demand matter as much or more. Modern machining techniques make complex screw designs easier to manufacture than ever.

There is one minor downside that only matters in projects with extremely tight tolerances. When a tap creates the space for a screw’s thread, the material is either cut out or pushed into the surrounding layers. Any bits that are cut out will stick in the thread or move down as the tap makes room.

Since a normal tap comes back out, the extra material can be removed before the screw is inserted. For a self-tapping screw, the excess will be stuck within the hole. The impact is negligible in the vast majority of cases, but it’s worth noting as a minute negative.

Should I Use Self-tapping Screws on My Project?

For projects involving some metalworking, self-tapping sheet metal screws are convenient and worth the price. They are not truly necessary for other projects. You can sometimes use them in a pinch, but double-check with similar projects and ask around before you break something irreplaceable.

Typical home DIY projects involving wood or drywall don’t really need self-tapping screws, but they should work well enough on occasions when you don’t have any other screws on hand. Wood and drywall screws are readily available and usually better suited for the task, if you’re headed into the store anyway.

Self-tapping screws work for some plastic projects, but plastic has a wide range of characteristics depending on how it’s made. When deciding if you should use self-tapping screws on a plastic project, remember that the screws can cause cracking or other material damage.

Does a Self-tapping Screw Anchor Itself?

Self tapping screw on the side of the wall.

A self-tapping screw does “anchor” itself into a material, but it does not create its own anchor like one used for drywall installations. 

A drywall anchor provides extra support when there is empty space behind the drywall. As the screw is installed, the anchor bends back to the drywall to form a brace. Self-tapping screws don’t have this functionality, so they are not interchangeable.

Are Wood Screws Self-tapping Screws?

Wood screws are effectively self-tapping screws when used as intended, even though they aren’t technically the same style of screw. With just a pilot hole, they can push and cut the material to create space for its thread to cling. 

They are in a unique category of their own since they lack the cutting notches of the thread cutters and have sharper edges than the thread formers. This particular combination lowers the risk of wood splitting when driving in the screw.

Are Self-drilling Screws the Same as Self-tapping Screws?

Three self tapping screws with galvanize coating.

All self-drilling screws are also self-tapping screws, though not all self-tapping screws are self-drilling. The distinct drill bit at the end of the self-drilling screws sets them apart.

With a self-drilling screw, you don’t have to drill a pilot hole before installing it. Even with both a power drill and powered driver on hand, having to do both adds at least a few seconds for each screw between swapping tools and drilling the hole. Over time, doing all three steps at once adds up to an incredible amount of time saved.

Aside from the bit at the end, self-drilling screws tend to be made from harder composites. Since they need to drill into the medium, trying to screw it into something harder will dull the screw and potentially ruin the grip on the threads.