Understanding Tapping Screws
Sometimes it isn't possible to tap a hole because the material is too thin (like sheet metal), or you don't have rear access to install a nut. But you may need to disassemble the device later, so blind rivets won't work. That's one instance where tapping screws are ideal.
Tapping screws are considered externally threaded fasteners that "tap" their own mating threads in metallic and non-metallic (such as plastic) materials. Some screws require a hole (exact hole size is often important), while others drill their own hole ("self drilling"). Each type is identified by a designation that describes its threads and point (tip). The threads will either be spaced, similar to wood screws, or machine screw type. Point styles include "standard," sharp (a longer "standard" point), blunt (like a machine screw) and drill (approximating a drill bit).
Tapping screws, especially Type A, are referred to as "sheet metal screws" because one of their first applications was in ventilation ducts made of sheet metal. Over time, though, different types evolved. We'll cover the common groups: thread forming, thread cutting, thread rolling and self drilling. The combination of point and thread style determines to which group the screw belongs.
In general, if the letter "B" is in a screw's designation, such as Type AB or B, it has spaced threads. If the "B" is absent, as in Type F, the screw has machine screw threads. There are two exceptions: Although considered obsolete but still available today, Type A has spaced threads. And metallic drive screws, denoted Type U, have spaced spiral threads.
Hole size is important for all tapping screws except those capable of self drilling, which includes sharp point types. If the hole is too large, the screw will be loose, the hole could strip during assembly (you can't properly tighten the screw), or the screw could pull out under tension. If the hole is too small, higher driving force will be required possibly causing the screw to break, or the material may crack or split. Consequently, it's important to always drill or punch the correct size hole.
Thread forming screws have coarse threads and are designed to be used in thin gauge malleable material; a hole of the proper size is required. As the fastener is installed, the hole is enlarged by merely pushing the material outward. Consequently, burrs are common. Types A and AB, which have a standard point, and blunt point Type B, are common sheet metal screws; Types A and AB are usually considered interchangeable. Sizes typically range from #2 to 3/8.
If the metal is thicker—and harder—thread cutting screws are used. Cutting flutes at the tip of these screws act like a tap to create mating threads in the material as they are installed. Like taps, the correct size hole is required. And, also like taps, chips are created. If installed in a blind hole (the hole doesn't go through the material), the chips will collect in the bottom of the hole so make sure there is sufficient depth. If the screw is inserted into a through-hole, the chips will be deposited on the exit side. Be careful that metal chips do not create electrical shorts, contaminate lubricants, or somehow inhibit mechanical operation. Type F is a very popular thread cutting screw. Because these screws are essentially machine screws, their threads are more closely spaced than sheet metal screws. After a hole has been tapped by a thread cutting screw, it can be replaced by a machine screw of the same size (diameter and number of threads per inch). Sizes of 4-40 to 3/8-16 are common.
Self drilling screws, also called Teks®, have a tip that resembles a drill, and often have spaced threads, like sheet metal screws. In a single time-saving operation, these screws drill, tap and fasten. Do not use self drilling screws in blind holes (holes that do not pass through the material). Also, the drill point must drill completely through the material before the first thread begins to thread into the material. That ensures proper fastening because the material will be fully engaged by threads. Drill chips are created that affect electrical and mechanical equipment. Four different point styles—#2, #3, #4 and #5—are available. Generally speaking, a #2 point is used with light gauge materials, #3 for medium gauge, and #4 and #5 points with heavy gauge materials.
Type U metallic drive screws have spaced spiral threads and a blunt point. These screws are forced under pressure into the material. Drive screws are considered permanent whereas other tapping screws can be easily removed.
If you are working with low-density materials like plastic, particle board, Masonite® and wood, consider using fasteners with High-Low threads. The "high" thread is quite sharp, while the "low" thread is more conventional and about 1/2 the height of the "high" thread. These screws are easier to install (less driving torque is needed), thread stripping is reduced, pull-out strength is increased, and there is less chance of splitting or cracking the material.
One final design that is very common is the drywall screw. Available in sharp point and self drilling styles, the sharp point is used with wood and light gauge steel studs, and the self drilling style is designed for heavy gauge steel studs. The unique bugle head is self countersinking, resists tearing the drywall paper surface and is said not to damage the gypsum core.
You may also find these reference charts useful…
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