The Whats and Hows of Self-Tapping Screws

Need fasteners that are easy to install, can get into tight spaces, and don’t require specialist tools? Consider self-tapping and self-drilling screws. As the names suggest, these create threads on their own or drill into the workpiece without a pre-drilled pilot hole. This makes installation quick, simple and possible in materials of varying thickness and hardness.

Basics of Self-tapping and Self-drilling Screws

While both terms are used interchangeably, there’s a key difference between self-drilling and self-tapping screws. A self-tapping screw cuts its own thread but may need a pre-drilled or pre-punched hole to speed things up. A self-drilling screw, on the other hand, foregoes the need for pre-drilled holes, as it has an integrated drill point to do the job for you. The main difference then is that self-tapping screws can’t drill a hole, while self-drilling can, and also tap a thread.

Thread-forming and Thread-tapping Fasteners

These are the two basic types of self-tapping screws and differ in how they create threads into the material they’re inserted. Thread-forming or blunt-nosed screws change the material grain or structure, ensuring high holding strength without chips. This is largely down to the finer threading pitch extending from just under the screw head to the tip. The fasteners require a pre-drilled hole slightly smaller in diameter than the screw thread. Once inserted, they resist unloosening. Common uses are in plastics, electronics components and softer metals.

Thread-tapping (or thread-cutting) screws have pointed tips, creating chips in the material during installation. The specific thread and cutting profiles are designed to remove excess materials when tightening the fastener. They don’t need a pre-drilled hole, although existing holes can reduce installation times and the effort to get the fasteners to the correct torque. The pointed tip is handy in softer material, especially wood, and facilitates repetitive tasks, such as assembling and disassembling furniture. The piercing ability and high holding strength are a bonus in thicker metals and denser plastics.

Typical Uses

A variety of products can be fastened with self-tapping screws. This includes items made of a broad selection of plastics, soft and hardwood and metals. Some concrete and masonry fasteners are also sold in self-tapping varieties. Typical applications and industries include:

• Construction – the screws are frequently used to attach plasterboard, assemble metal framework, repair and installation of roofing, and applications that fasten metal to metal.
• Electronics and household appliances – computers, TVs, fridges and air conditioners are just some of the everyday items assembled with these fasteners.
• Automotive – the fasteners connect body parts, interior trim fittings, and electronics systems and instruments.
• Furniture – both thread-forming and thread-tapping screws are used in the assembly and repair of metal, wood and plastic furniture pieces.

The fasteners are favoured for quick and efficient installation, high strength, and appealing appearance with varying designs and finishes.

Choosing What’s Right for Your Application

Head Designs

When choosing the right fastener, observe the head type, size, material and finishes to ensure it works. A self-tapping screw can be optioned in varying head designs. For clean and flush finishes, and where the fastener needs to ‘disappear’ into the material, go with countersunk tappers. Alternatively, choose hex head screws for harder and thicker materials. These are tightened with wrenches or sockets to get the right torque. Pan and dome head screws also allow good holding and tightening strength, usually in woodworking applications, but also ensure easy removal. And if you need the best clamping force, choose flange-head screws. The wider head design prevents unloosening and is often used when attaching wood to metal, and metal to metal.

Sizing

Of course, you want screws in the correct size and one that matches the workpiece thickness. This can be stated in metric or imperial units, or often in standardised screw sizes, and refers to the thread width. The largest screws round out to more than 6mm or 1/4″ wide, but others are also several sizes to match your application. Additionally, pick the correct length. This can be anywhere from 6 to 200mm, or 1/4″ to 8″.

Materials and Finishes

Lastly, choose the screw material. Stainless steel offers the best strength, is resistant to metal fatigue, wear, heat and corrosion and is relatively inexpensive. Choose higher grades (316 stainless steel) for lasting performance. Mild or carbon steel is cheaper but must be treated to endure harsh conditions. Go with zinc-plated screws for indoor use, and hot-dip galvanised variants for screws used outdoors and in heavy-duty fastening with metal items.

How They’re Used

Keep all fasteners stored and sorted by size and type in toolboxes, trays or fastener bins. The first consideration when using self-tapping types is to choose a screw designed specifically for the material and job at hand. Once you’ve decided on design, head types, and size, time for installation. Use a wood marking pencil to define the area where the screw will be inserted. Pre-drilled pilot holes aren’t necessary for larger thread-tapping fasteners, but you may want one with blunt-tipped thread-forming screws, especially in plastic. Insert the screw with the appropriate screwdriver, drill or wrench, gently applying pressure as it turns clockwise and perpendicular to the surface of the workpiece. The screw should easily catch onto the material, more so with a pre-drilled hole. Tighten the fastener until snug, ensuring you don’t overdo it and damage or crack the material.