Screws that form their own threads cause for a lot of confusion. Here we delve into self-drilling screws, self-tapping screws, thread-cutting screws, and thread-forming screws.
Self-Drilling Screws
Self-Drilling Screws drill their own hole and make their own threads. They actually look like a drill bit on their end. Many people refer to them as a tek screw. These screws work well in both wood and metal alike. Be sure the material you drill into is softer than the screws. This shouldn't be a problem since self-drilling screws are usually hardened steel. Self-Drilling screws remain a popular choice because one doesn’t need to make a pilot hole so installation is a one-step process.
Self-Tapping Screws
Self-Tapping Screws (sometimes called a sheet metal screw or a Parker screw) form threads in the material into which they are installed. To date, most people refer to them as sheet metal screws since they are ideal for securing thin sheet metal onto wood or metal frames.
They create a snug fit, but they allow for the assembly created can be taken apart and put back together again. Self-tapping screws' installation requires a pilot hole, but because of this one does not need a lot of torque to install them. Tapping screws come in either coarse or fine threads.
The cutting edges of a self-tapping screw displace the material within the pilot hole. Some types of self-tapping screws dispel the material (thread- forming screws) and some actually move the material around inside the pilot hole (thread-cutting screws). The latter excel in assemblies where you do not want vibration to cause loosening. Read below for further explanation.
Thread-Cutting Screws
Thread-Cutting Screws are a type of self-tapping screw that dispel the material into which they are tapping. This screw requires a pilot hole and works well in wood and metal. Thread Cutting Screws can further be categorized by their point.
Type 1
Type 1 cutting screws work best for harder metals. They utilize a standard machine screw thread and blunt point and perform their thread cutting function in metals of any thickness. Type 1 screws may be used again and again. Because these screws possess almost twice the strength of ordinary machine screws much smaller screws can be used. When driven into untapped holes (drilled or punched) they stay snug in their self-cut, perfectly mated threads. This is a great screw for electronics.
Type 23
Type 23 cutting screws work best in softer metals like zinc and aluminum. They also work well in plastic. They have a wider thread-cutting slot so the material they remove while cutting threads comes out clean. Type 23 screws come in stainless steel, Monel and brass. Also known as Type T, Type 23 screws have machine screw threads with a blunt point and tapered entering edges, with one or more cutting edges and chip removal indentations. In this sense they are similar to Type F screws. The cutting edge on the point, however, is broader and deeper than that of the Type F.
Type 25
Type 25 cutting screws are specifically designed for for plastic. Also known as Type BT, Type 25 screws have spaced, incomplete tapered threads with a blunt point and tapered entering edges. They work best in plastic, asbestos compositions, and other composites. Type 25 screws are typically hardened and possess a design that cuts so the material won’t form clogs and less torque is needed for installation. This prevents brittle plastic fracturing during the install process.
Type F
Type F cutting screws offer the most secure fit in vibrating parts and perform well with most materials.
Refer to this detailed chart on the above mentioned screw point styles plus all others.
Thread-Forming Screws
Thread-Forming Screws (aka Thread-Rolling Screws) name the second type of self-tapping screw that actually moves the material around inside the pilot hole. This way the fastener assembly achieves a really snug fit. No additional lock washers or Loctite® are needed in situations normally requiring them to remain secure in vibrating assemblies. These screws do require some extra torque to get them in. They have a trilobular cross-section, and form threads in pre-existing holes by pushing material outward during installation. When installed in sheet metal, thread rolling screws often require an extruded hole; the extrusion forms a lead-in and provides extra length for improved retention. Thread rolling screws are popular for applications where the loose chips formed by thread cutting processes must be avoided.