Screws that have particularly sharp threads, which can carve into a material, are often referred to as self-drilling screws. However, there’s a distinction worth noting when working with these types of screws that can make a significant difference in their use and application.
Typically, the screws are categorized into two types that are defined as “self-drilling” or “self-tapping” — and these types are not interchangeable. Technically, both of these types of screws will tap their own threads (as most screws do to some degree), 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. This feature makes self-drilling screws a time-saving and cost-effective choice for certain applications.
In contrast, a self-tapping screw typically requires an initial pilot hole before the fastening process can begin, and can have a sharp, flat, or blunt tip. These types of screws are still incredibly common, with several styles and types available (and may also be referred to by their intended use — such as concrete, drywall, or wood screws). When choosing the correct type of screw, it’s important to first consider the application, including the thickness and hardness of the material.
Here’s a rule of thumb: All self-drilling screws are self-tapping, but not all self-tapping screws are self-drilling.
Essentially, a self-drilling screw is a self-tapping screw with the added feature of the drill point. With either type, the screws carve mated threads into the substrate for a tight fit. Both types of screws are typically made out of hard steel or stainless steel that has been treated to increase its hardness. A screw must be stronger and more durable than the material it will drill into to prevent failure of the joint or damage to the screw, material, or fastening tool.
These screws rarely require lock washers or other types of locking fasteners to prevent loosening.
Self-tapping and self-drilling screws are typically the fasteners of choice for construction, HVAC (heating, ventilation, and air conditioning), and other industries because of their ability to create a precise fit. They can be used to fasten several different materials, such as metal, aluminum, wood, drywall, and plastic. Self-tapping screws are also suitable for thin sheet metal (one or two layers) and masonry applications.
Like most fasteners, these screws are also available in a wide variety of sizes. Manufacturers often recommend an applied force and driver motor speeds when fastening a self-tapping or self-drilling screw, based on the screw size.
Bottom line: although all self-drilling screws are self-tapping screws, these two fasteners are not interchangeable. Mixing up the two can lead to errors and application failures.