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Self-Drilling Screws

Screws can provide rapid and effective means to fasten steel metal siding and roofing to framing members. Screws can also be used for connections in steel framing systems and roof trusses. 

Typically, the screws are broken down into two types are defined as “self-drilling” or “self-tapping” — and these types are not interchangeable. Technically, both of these types of screws will tap their threads, but the self-drilling screws are unique.

The tip of a self-drilling screw is shaped with a point and flute that resembles a drill bit. A notched area in the tip behaves as a reservoir to receive chips or filings as the material is carved away by the screw (when drilled). Thanks to this specialized tip, the self-drilling screw allows assemblers to skip the initial step of drilling a pilot hole. Self-Drilling Screws eliminate the need for separate drilling and tapping operations, helping to provide the user with a faster, more economical installation process. Self-Drilling Screws operate on the same basic principles as a drill bit or other cutting tool. Performance for these screws is determined by cutting speed, feed rate, depth of cut, and the working material being drilled into.

The self-drilling screws are externally threaded fasteners with the ability to drill their hole and form or tap their internal threads without deforming their thread. These screws are high-strength, one-piece installation fasteners and are used to drill and fasten rigid materials to 13mils (one thickness) or thinner. The sharp drill point will both drill a hole and form the mating threads in a single operation.

When choosing the proper fastener for cold-formed steel construction, two fundamental questions must be answered: What materials are being joined? And what is the total thickness of the material in the connection? When the application has been defined, it is then possible to choose fasteners with the appropriate point design, body diameter, length, head style, drive, thread type, and plating. Point types include self-piercing or self-drilling. 

The body diameter is specified by the nominal screw size. The length of the fastener is measured from the bearing surface of the fastener to the end of the point. The length of self-drilling screws may require special consideration since some designs have an unthreaded pilot section or reamer wings between the threads and the drill point. Common head styles include countersunk, flat, oval, wafer, truss, thin truss, hex washer head, pan, and round washer.

The cutting point feature of self-drilling screw tips is originated from the drill bit. The best geometry to use depends upon the properties of the material being drilled. The following geometries of drill-bit point angle, helix angle, and lip relief angle shall be taken into account for some commonly drilled materials. The recommended retail is dominated by the materials to be drilled. 

Thus, some key design features to consider for self-drilling screw fasteners are: the drill flute and point length of the self drilling screw tip. Both design features should be taken into account if only metal materials are to be fastened or drilled. The drill bit with winged reamer may be the feature for wood-to-metal connection. When selecting a self-drilling screw, consider the material thicknesses and types of materials to be joined. 

Drill flutes allow drilled material to exit the hole. Completely embedded flutes can no longer remove these chips, which contain approximately 80% of the heat created by the drilling process. A buildup of this material can cause the point to overheat and fail. Point length determines the material thickness which the screw can reliably penetrate. The unthreaded portion of the point (pilot section) must be able to completely drill through the material before the threads engage. If the threads engage before drilling is complete, the fastener can bind and break. The jacking may occur when some screws fasten thicker materials, such as wood to metal.

The solution against jacking may consider the requirements of the point length factor or pilot point. Point wings are used against thicker wood materials to metal connection. The wings enlarge the hole in the fastened material, allowing the threads to pass through without contacting the fastened material. The wings will break away in contact with the metal before the threads engage in the metal. 

The most common materials of self-drilling screws are carbon steel, stainless steel, or stainless bi-metal material. It depends on the application and intended use of fasteners. Self-drilling screws should offer a balanced combination of strength, durability, and resistance to rust/corrosion. Some self-drilling screws, however, made of carbon steel are with heat treatment, surface treatment and also coated with other metals like zinc for enhanced strength or resistance to rust/corrosion. The stainless bi-metal self-drilling screws are made of strengthened carbon steel drill points welded onto A2/A4 of ISO 3506 stainless steel shanks for the high quality of drilling capacity and resistance to corrosion. This bi-metal design enables the dual advantages of self-drilling through hard metal and corrosion resistance on the stainless shank when drilling, tapping, and fastening in one shot. 

As their name implies, self-drilling screws operate on the same principles as drill bits and other cutting tools. For any cutting tool, performance is governed by cutting speed, feed rate, depth of cut, and the work material itself. Then, the installation performance of self-drilling screws can be linked to the basic cutting tool parameters where suggested optimal parameter values are dependent on the nominal screw size of intended use. 

RPM is the speed at which the drive motor runs while the screw is installed with the power tool. This is often adjustable using a variable pull trigger or a different driver motor. The applied force is a measure of the user’s applied force as the screw is installed. More force is not necessarily better. Work-material hardness can be viewed as a material’s resistance to drilling or cutting. In most instances, the harder the work material, the more difficult it is to cut. Depending on the application, this may be out of the user’s control. Drill bits are cutting tools used to remove material to create holes, almost always of circular cross-section. Drill bits come in many sizes and shapes and can create different kinds of holes in many different materials. To create holes, drill bits are usually attached to a drill, which powers them to cut through the workpiece, typically by rotation. Drill bits come in standard sizes that can create holes with required screw tap sizes.

Self-drilling screws are susceptible to the same forces as drill bits. Cutting performance is impacted by the speed and depth of the cut made. Make sure that you select the correct sized screw for your application so you can avoid the drill head melting, especially if too much RPM is applied or snapping if too much pressure is applied. A simple rule of thumb is that small diameter screws can take higher RPMs but less force can be applied. Transversely the larger the diameter the lower the RPM but higher force can be applied.  The basic guideline to set a self-drilling screw would be to turn the screw at a moderate to slow speed through the entire process. Once the self-tap “drills” through the metal, the screw then begins to screw in. After only a few rotations the screw will be set.