Self-tapping screws are usually drilled into pre-made holes (pilot holes) in metals, such as zinc, aluminum, and bronze and its alloys, as well as plastics and resin-impregnated plywood. They create threads in the material they are driven into. Self-tapping screws are one-piece fasteners—there is no nut—so they can be installed quickly and action is only required from one side of the joint. Mating threads made by the screw fits the screw’s threads closely, so no clearance is needed and the close fit keeps the screws tight despite vibrations.

Tapping screws are usually case-hardened and have tensile strengths of 100,000 psi or greater and relatively high ultimate torsional strengths.

There are three types of tapping screws: thread-forming, thread-cutting, and thread-forming.

Thread-forming screws displace or form the material next to the pilot hole so it flows around the screw’s threads. They are generally used in materials in which large internal stresses are desirable to increase resistance to loosening. The screw forms threads in the material that provide a fit with zero clearance since no material is removed.

Thread-cutting screws have cutting edges and chip cavities that remove material to create a mating thread. Cutting action keeps the required torque to a low level. These screws are used in materials where disruptive internal stresses are not wanted or where it takes too much driving torque to use thread-forming screws.

Thread-rolling screws are used on thicker-gauge metals, such as steel, brass, zinc, and aluminum. They are also the go-to alternative when thread-forming screws require high driving torques for installation or where it’s inconvenient or undesirable for thread-cutting screws to generate chips.
Thread-Forming Screw Type AB
The sharp point on this screw lets it work in pierced or punched pilot holes. It is used on sheet metal up to 18-gauge and resin-impregnated plywood. It can also be driven and inserted quickly.

Thread-Forming Screw Type B
This screw is designed for heavy-gauge sheet metal and nonferrous castings. It is also used in assembling easily deformed materials where the pilot hole is larger than the root diameter of the screw. It can be quickly driven.

Thread-Forming Screw Type C
This type of screw is used when a machine-screw thread is preferable to a spaced-thread form. It does not create chips when driven, but it may take extremely high driving torques due to the long thread engagement. It resists loosening by vibration and provides tighter clamping action than Type B thread-forming screws driven with equivalent torque.

Thread-Forming Screw Type U

This screw is for permanent joints in metals and plastics. It should not be used if the material is less than one screw diameter thick.

Thread-Forming Screw Type BP
This screw can be used to locate and align holes or pierce soft materials.

Thread-Cutting Screws Type D
This thread-cutting screw is suitable for low-strength metals and plastics, high-strength brittle metals, and rethreading clogged pre-tapped holes. It is an easy starting screw that requires less driving force than Type C screws and has longer length of thread engagement. And it creates more lamping force for a given torque than any other tapping screw.

Thread-Cutting Screws Type BT
Although similar to Type BF screws, it only has a one wide flute, which provides room for twisted, curly chips, so that binding or reaming of the hole is avoided.

Thread-Cutting Screws Type F
This screw is used on a wide range of materials. It drives quickly and resists vibration.

Thread-Cutting Screws Type G
Good for low-strength materials and requires less driving force than most other screws.

Thread-Cutting Screws Type T
This screw is similar to Type D, but has more chip clearance and cuts through material easier.

Thread-Cutting Screws Type BF
The cutting grooves in BF screws remove only a small amount of material and this maintains the maximum shear strength in the threaded hole’s wall. Wall thickness should be 1.5 times the screw’s major diameter. The screw reduces stripping in brittle plastics and die castings. It is suitable for long thread engagement, especially in blind holes. It drives more quickly than screws with finer threads.

Thread-Rolling Screw Type SF
This screw provides four-point contact with the materials, thus making it easier to insert the screw straight and with relatively little driving torque. It is recommended for both thin and heavy-gauge materials.

Thread-Rolling Screw Type SW
This screw is shaped to swage threads into tough metals including structural steel, zinc, and aluminum castings, and steel, brass and bronze forgings.

Thread-Rolling Screw Type TT
A trilobular cross sections gives this screw a bit of radial relief along its length as it deeply engages heavy materials with low driving torque.

Thread-Rolling Screw Type TR-3
This screw has a triangular section that makes it easier to start forming threads. It applies intermittent pressure as the screw is turned and threads are formed. It is designed for use on heavy materials. 

source: www.machinedesign.com