Types of Self-Tapping Screws

Written by shane grey
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Types of Self-Tapping Screws
A long, self-tapping screw with a pan head. (screw image by Alison Bowden from Fotolia.com)

Self-tapping screws bore through and deform both plastic and metal materials. With the capability to cut material and create threads, self-tapping screws, also called "self-tappers," remove the steps of pre-drilling and tapping from many building and machining projects. With an understanding of the appearance and application of different types of self-tapping screws, the builder can choose the right one for each project.

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Thread-Forming Screws

The thread-forming type of self-tapping screw creates threads by deforming a substrate, typically soft metal or plastic. From a distance, the thread-forming screw's shank appears similar to a machine screw's shank; perfectly cylindrical with closely grouped threads. However, upon closer inspection, the thread-forming screw's shank slightly tapers toward the end. Despite its taper, the thread-forming screw's tip does not form a sharp point. Many thread-forming screws have a wedge-shaped incision at their tip. The sharpened edges of the incision begin to embed the screw's threads into the substrate and begin the thread-forming process. Depending on the relative hardness of the substrate material, thread-forming screws might require pre-drilling.

Thread-Cutting Screws

The thread-cutting screw creates threads by cutting and removing the substrate material, usually soft metal or plastic. The thread-cutting screw appears similar to a wood screw; its shank typically tapers to a sharp point. The extra-sharp, angled threads surrounding the thread-cutting screw's shank cut into the substrate and push waste material from the hole. During the cutting process, the screw's sharp threads create a corresponding set of threads in the substrate material. Construction professionals often refer to thread-cutting screws as self-drilling screws.

Self-Tapping Screw Heads

Construction professionals and machinists frequently classify thread-cutting and thread-forming screws according to their head type. Self-tapping screw heads vary according to two general characteristics: shape and drive. Shape refers to both the form and size of a self-tapping screw. Hexagonal heads are called "hex head" screws, flat-topped, tapered heads are called "countersunk" heads and rounded heads are called "round" or "pan" heads. Drive refers to the type of screwdriver bit used to twist the screw. Self-tapping screws are available as Phillips drive, slotted driver, star, square and hex drive.

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