What is a self-tapping screw?

What is a self-tapping fastener and how can it improve the way you work? Able to tap its own hole as it’s driven into the material – be it wood, plastic or metal – the cleverly-designed component can be used in a variety of different applications.

Why might you need a self-tapping fastener? In some cases, for example when using sheet metal, it isn’t possible to tap a hole as the material may be too thin. You may also require self-tapping fasteners if you can’t gain rear access to install a nut. In both these instances, self-tapping fasteners ensure you can get the job done smoothly and effectively. Understanding types and materials of which they are applied is vital - for example if you are using self-tapping screws for aluminium, they don't need pre-drilling and are easily removed without any damage caused to the metal. And, actually, they can be reused in the same material.

There’s a huge range of different types of self-tapping fasteners and each can be set apart by a designation which describes its point (tip) and threads. Here are the most common self-fasteners and how you might use them in your applications:

The Sheet Metal Screw

Also referred to as ‘Type A’ screws, the sheet metal variety was first utilised in sheet metal ventilation ducts. Different types evolved over time, however, including thread-forming, thread-cutting and thread-rolling.

How do you determine where your screw belongs in these? It’s down to the combination of point and thread style.

If the letter ‘B’ in a screw’s designation – for example, type AB or B – has spaced threads, while if the ‘B’ is absent (for instance, Type F), machine screw threads are present. Having said that, there are two exceptions, with Type A having spaced threads, and Type U (metallic drive screws) featuring spaced spiral threads. It’s worth knowing that while Type A is still available, it can be harder to come by.

It’s vital you consider hole size, as if it is too large, the screw will therefore be loose, and the hole might strip during assembly. The screw could also pull out under tension. If the hole is too small, on the other hand, a screw might break, due to the need to use a higher driving force.

Designed and manufactured to be used in thin gauge, malleable material, thread forming screws comprise coarse threads. A hole of the proper size is therefore required as, at the point of the fastener being installed, the hole will be made bigger when the material is pushed outward. The most common sheet metal screws are Types A and AB, both with a standard point, alongside Type B, with its blunt point.

The Self-Drilling Screw

Self-drilling screws and self-tapping fasteners aren’t the same. While self-tapping screws will tap a hole into a soft material, self-drilling screws are capable of drilling a hole into anything from sheet metal to wood.

This key difference is self-drilling screws have a tip like a drill bit, so they do not need a pilot hole. The fact that they can drill and tap their own hole into harder materials saves you using two tools, when this one component will do both jobs.

So, what's best when it comes to self-drilling screws? The drill point must drill completely through the material, prior to the point that the first thread starts threading into it. With several point styles available, a #2 point is typically used with light gauge materials, while a #3 is utilised for medium gauge, and a #4 and #5-point self-drilling screw is ideal for heavier materials.

Forced under pressure into the material you’re using, Type U metallic drive screws have spaced spiral threads and a blunt tip – and while this type of screw is considered permanent, others can be easily taken out.

The Winged Self-Tapper

With thin wings wider than the thread formed behind the self-drilling head, the winged self-tapper will cut a clearance hole in soft materials like wood or plastic. Be aware, though, that they can be destroyed by materials which are more robust, such as metal.

The Drywall Screw

Let’s look at one final design of self-tapping faster: the drywall screw. A common variety of screw, it’s available in sharp point and self-drilling styles, with the latter designed for steel studs and heavy gauge.

Self-countersinking, its unique, bugle head avoids tearing into the drywall paper surface and will also resist damaging the gypsum core.

Now for a few frequently-asked questions relating to self-tapping and/or self-drilling screws:

What's the difference between a self-tapping screw and a self-drilling screw?

This is a challenging question to answer and one which can and does puzzle many. Self-tapping screws tap their own thread, which means they require a pilot hole which is slightly smaller than the diameter of the screw. They work on the premise that if you drill a pilot hole into a soft material like wood, as you drive the tapping screw in, its threads will make the material secure by digging its way into it.

Ideal for a variety of materials, including metal, wood and brick, self-tapping screws might come with small notches on the thread. This helps cut the thread, but that not all tapping screws have a pointy tip. In contrast, some are flat or blunt.

Self-drilling screws, on the other hand, work as both a fastener and a drill bit. For that reason, they don’t require a pilot hole due to the drill bit end, and they were designed to secure thin pieces of metal together. Ideal for completing frame work on a car, for instance, they can also help you when affixing sheet metal to a metal frame.

Can you reuse a self-tapping screw in the same hole?

Often, thread-cutting screws are used when working with wood or metal – and one downside to using thread-cutting screws is that the threads could strip as the application is taken apart. This may prevent you reusing the fastener on a future project.

Do I need to drill a pilot hole for self-tapping screws?

Designed for drilling their own hole into softer materials like plastic and wood, sharp-tipped screws don’t require a pilot hole. If you’re using a thicker material, you may need more than one screw to drill through the surface, but you can use self-drilling or self-tapping screws to same labour and time.

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