In metal fabrication, threads typically need to be tapped into a material piece by the use of special thread cutting tools, requiring distinct machining time and effort to complete before inserting the fastener itself. This process is called for when materials are robust, hardened, or heat-treated, as well as when thread profiles must be highly accurate or structural in nature. The time and cost of the entire thread cutting and fastening process are relatively high, as are the costs of replacing these special machining tools as they wear out over time.
Certain applications using soft, thin, or otherwise relatively workable materials can see reduced fastening time and cost by eliminating these thread cutting steps entirely. To do so, special fasteners called ‘Tapping Screws’ can be used. The term Tapping Screws describes a wide variety of related threaded fasteners that all share the feature of being able to imprint their own threads into the material in which they’re being installed. As a tapping screw is inserted into its host material, the oversized and aggressive external threads on the fastener body physically produce the threads that will retain the fastener once inserted.
The range of products available within the tapping screw family is quite large. Most folks familiar with general fasteners will recognize the two most common phrases used around tapping screws – Self-Tapping Screws and Self-Drilling Screws – but may not be aware of the nearly one dozen other categories, and dozens more individual styles available. To understand how tapping screws are grouped, let’s introduce the main categories next:
- Thread-Forming Screws – threads are formed by displacing material as the screw is being inserted, typically used in sheet metal and soft metal, plastic, and wood materials
- Thread-Rolling Screws – similar to thread-forming screws in that these fasteners displace material to imprint their thread pattern, thread-rolling screws use a more rounded thread profile instead of the high-angle pitch of a thread-forming screw
- Thread-Cutting Screws – threads are imprinted by removing base material while these screws are being inserted, physically cutting in their thread profile, and typically used in harder, denser and thicker materials
- Self-Drilling Screws – these screws are in fact a variant of any of the above types of tapping screws, with an added feature of a fluted bit-like tip that allows the screw to also drill its own hole as its being inserted. More on this below.
Tapping screws are available in practically all common materials, finishes, and SAE/Metric units, though are typically limited to smaller sizes such as #00 through #17 and M1 through M8. Anything larger generally requires separate machined threaded to be cut prior, in order to assure proper fit, tolerance, and strength.
Self-Tapping Screws vs Self-Drilling Screws
Two terms used in the Tapping Screw family – Self-Tapping and Self-Drilling – are often misapplied, and are worth a few words of clarification here.
- Self-Tapping Screws – any screw in the Tapping Screw family is a Self-Tapping screw, on the basis that the screw will imprint threads into a pre-drilled hole as the screw is being inserted
- Self-Drilling Screws – only a distinct portion of self-tapping screws can be self-drilling, which describes a fastener’s additional feature of having a fluted drilling tip at the leading end of the shaft, used to drill a hole in the base material during insertion
In common usage, people tend to interchange these terms, leading to a general sense of ambiguity around tapping screw categories. Further, another commonly interposed term is ‘sheet metal screw’, mostly used to describe self-tapping (or more accurately, thread-forming) style screws. (Your everyday sheet metal screw is technically labeled as a Type A Tapping Screw – picture below.)
Self-drilling screws are also commonly called ‘Teks screws’, after the name of the style’s most well-known manufacturer, Teks.
|Self-Tapping / Sheet Metal Screw (Type A)||Self-Drilling / Teks Screw|
Another important distinction between self-drilling and all other self-tapping screw types is the function of a pre-drilled pilot hole. For self-tapping screws, a very specific pilot hole must be drilled into the base material first, as the screw requires a very precise amount of clearance and free space in order to properly imprint its threads. If the hole is oversized, not enough material will remain for the thread to cut in deep enough to be secure. If the hole is undersized, too much material will restrict the screw from insertion and possibly lead to damage under increased force.
On the other hand, self-drilling screws do not require any pilot hole to be drilled, though some users may prefer to drill a pilot hole in order to better align the fastener and create space for excess material removal (in harder base materials). In these cases, the pilot hole size is not so critical, though should always be undersized to less than the diameter of the fastener shank, so that the threads have plenty of space to cut in.
Do you have a tapping screw application but are unsure of which specialty fastener might be best to use? E&T Fasteners offers expert support and product selection between the above options and more! Contact us to discuss your application, see our Plastic Fastener catalog here, or to see our entire fastener catalog, visit our product selection here.
Alternative Tapping Screw Designs
While Self-Tapping / Sheet Metal Screws and Self-Drilling / Teks Screws are the most common types used in the Tapping Screw family, there are other notable designs that fit into special applications. In no specific order, we’ll list out several next-most common types below. There are another handful of styles available as well – see ASME / ANSI Screw Thread Specifications (previously ASA B1.1) for more information.
|Type U / Hammer Driven Screw A type of thread-forming screw, the Type U hammer-driven helix screw is a permanent, tamper-resistant fastener that forms its own threads when inserted into a pre-drilled or pre-punched hole. Commonly used for nameplates, decorative sheet materials, and machinery components.|
|Type B Screw Type B screws feature large shanks and coarse threads. These thread-forming fasteners are used along with pre-drill pilot holes in thicker sheet metal, wood, and plastic applications. Commonly used for consumer electronics, metal-to-plastic covers and sheeting, automotive, and furniture components.|
|Type F Screw This thread-cutting fastener type is used for imprinting threads in harder materials, namely castings and forgings from ferrous and non-ferrous metals. A slot cut into the end of the fastener allows for flexure during insertion, as well as tip tension after installation to combat vibration loosening. Requiring a pre-drilled hole, common Type F screw applications include industrial equipment, commercial vehicle, and construction tooling.|
|Type BT Screws These special thread-cutting screws have partially fluted ends intended to remove fair quantities of softer material in order to properly cut in their thread profiles (with a pre-drilled pilot hole). Common applications include fastening of any long-depth plastic, wood, or cast/forged metal where heavy chip removal is called for given the depth of the hole needed.|
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