Robyn Buck
Self-tapping screws are a type of threaded fastener that can be used to join a range of materials, including wood, plastic, metal, and even brick. They are useful when you can only access one side of the material, and they can be easily removed for disassembly and reassembly. Self-tapping screws are typically categorised as either thread forming or thread cutting. The size of the screw is important as it determines the thickness of the material it can be used on and ensures proper fastening. Hole size is also a critical factor, as a hole that is too large or too small can lead to issues such as loose screws or material cracking.
Characteristics and Values of Screw Tapping Sizes
| Characteristics | Values |
|---|---|
| Type | A, AB, B, F, U, Self-Drilling, Thread Forming, Thread Cutting, Thread Rolling |
| Point Styles | Standard, Sharp, Blunt, Drill |
| Hole Size | Depends on screw type; for Type A, sizes typically range from #2 to 3/8 |
| Nominal Screw Length | Varies, e.g., from 0.060 to 0.295 |
| Nominal Size or Basic Screw Diameter | Varies, e.g., from 0.071 to 0.191 |
| Penetration Gauging Depth | Varies, e.g., from 0.038 to 0.073 |
| Drill Point | Refers to the unthreaded section of the screw; should be considered when selecting screw length |
| Flute Length | Determines the thickness of the metal a self-drilling screw can be used on |
| Material | Metal, Plastic, Wood, Aluminium, Fibreglass |
What You'll Learn
Self-tapping screws vs self-drilling screws
Self-tapping screws and self-drilling screws are two types of screws used in metal building and general construction, as well as many other applications and industries. While they are often confused with each other, the two terms are not interchangeable.
Self-tapping screws are designed to tap their own hole in the desired material, meaning that no pilot hole needs to be drilled beforehand. They are used for materials such as wood, metal, and brick. However, they cannot drill through metal and do require a pre-drilled pilot hole. A pilot hole is created by using a drill bit that is slightly smaller than the screw, and the threads of the screw tap the metal or wood during installation. This tapping action keeps the material fastened together.
Self-drilling screws, sometimes known by the brand name Tek® Screws, are identified by their drill bit-shaped point. They are designed for various applications, such as fastening metal to metal and metal to wood. Self-drilling screws do not require a pilot hole, which speeds up installation time and lowers costs by eliminating the need for additional drill bits. The drill points are numbered #1 through #5, and they can drill through increasing thicknesses of steel as the number increases.
In summary, the main difference between self-drilling and self-tapping screws is that self-drilling screws can drill their own hole, while self-tapping screws cannot. All self-drilling screws are also self-tapping, but not all self-tapping screws are self-drilling. Self-tapping screws are suitable for materials with pre-drilled pilot holes, while self-drilling screws are ideal for drilling through light gauge metal sheeting without a pilot hole.
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Thread-forming screws
Thread-rolling screws are another type of self-tapping screw that create internal threads similar to how a thread-rolling tap would function. The terms "thread-rolling" and "thread-forming" are sometimes used interchangeably in the industry, which can cause confusion. However, thread-rolling screws like TAPTITE® are known for creating more clearly defined and cleaner internal threads than thread-forming screws.
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Thread-tapping screws
Tapping screws are a type of externally threaded fastener that can "tap" their own mating threads in both metallic and non-metallic materials such as plastic. They are ideal when it is not possible to tap a hole, for instance, when the material is too thin or access is limited.
There are different types of tapping screws, each identified by a designation that describes its threads and point (or tip). The threads can be spaced, similar to wood screws, or machine screw type. Point styles include "standard", "sharp", "blunt", and "drill".
Thread forming screws, for instance, have coarse threads and are used in thin, malleable materials. They require a hole of the proper size, and as the fastener is installed, the hole is enlarged by pushing the material outward. Types A, AB, and blunt point Type B are common sheet metal screws, with sizes typically ranging from #2 to 3/8.
Thread cutting screws, on the other hand, are used for thicker and harder metals. They have cutting flutes at their tips that create mating threads in the material as they are installed, requiring a correctly-sized hole. Type F is a popular example, with sizes ranging from 4-40 to 3/8-16.
Self-drilling screws, also known as Teks®, have a tip that resembles a drill and often have spaced threads. They can drill, tap, and fasten in a single operation, but they must not be used in blind holes.
Type U metallic drive screws have spaced spiral threads and a blunt point. They are considered permanent, unlike other tapping screws that can be easily removed.
When selecting a tapping screw, it is important to consider the material being fastened. For low-density materials like plastic, particle board, or wood, fasteners with High-Low threads are recommended. These screws have a sharp "high" thread and a more conventional "low" thread, reducing driving torque and the chance of splitting or cracking the material.
Additionally, the correct hole size is crucial for all tapping screws except self-drilling ones. An oversized hole can result in a loose screw or stripping during assembly, while an undersized hole can lead to increased driving force, potentially causing the screw or material to break.
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Self-drilling screw troubleshooting
Self-drilling screws are a form of self-tapping screws, but not all self-tapping screws are self-drilling. Self-tapping screws cannot drill through hard objects like metals and require a pilot hole for installation. Self-drilling screws, on the other hand, can drill through metal and do not require a pilot hole.
Selecting the Right Screw Type
When choosing a self-drilling screw, consider the material you will be working with. Different types of self-drilling screws are designed for specific materials such as wood, metal, or plastic. The size and length of the screw should also be considered, along with the desired level of torque or tightening force required.
Using the Correct Tools
Self-drilling screws are typically used with a drill or an electric screwdriver. While self-drilling screws do not require a pilot hole, creating one can help ensure that your screw is perfectly aligned with the hole. The pilot hole should be slightly smaller than the diameter of the screw to allow the screw's thread to catch effectively.
Avoiding Over-Tightening
Over-tightening can cause the threads of the screw to strip, reducing its holding power. This can make it difficult to remove the screw and may require the use of lock washers or epoxy to prevent rotation.
Reusing Self-Drilling Screws
Self-drilling screws can be reused unless they are damaged (e.g., bent or with a broken tip). However, they cannot be reused in the same hole. When reusing a self-drilling screw, first turn it anticlockwise until it drops into the already-formed thread to avoid causing damage.
Preventing Galvanic Corrosion
Galvanic corrosion occurs when two different metals corrode after being charged through electrical contact. To avoid this, use nylon washers to ensure that stainless steel fasteners do not come into direct contact with other metal panels.
Maintaining Stainless Steel Fasteners
Regular maintenance and cleaning are crucial to maintaining the durability and appearance of stainless steel fasteners. Extended exposure to water and contaminants can affect their longevity and aesthetics.
Choosing the Right Surface Finish
Over time, steel fasteners may develop cavities and roughness, leading to deposits on the screws that result in stains or a poor appearance. Opting for the right surface finish can help achieve better resistance to corrosion.
By following these troubleshooting tips and best practices, you can effectively use self-drilling screws and ensure a secure and reliable hold for your projects.
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Choosing the right self-tapping screw
Self-tapping screws are a versatile and convenient option for creating threads in a variety of materials. They are easy to install and can be used with a variety of tools, including power drills and screwdrivers. When choosing the right self-tapping screw, there are several factors to consider:
Material
Different types of self-tapping screws are designed for specific materials, such as wood, metal, or plastic. For example, self-tapping screws with a sharp cutting tip are suitable for use with harder materials like metal or hard plastics, while those with a tapered tip are more appropriate for softer materials like wood or soft plastics.
Size and Length
The size and length of the screw should be selected based on the specific application. Self-tapping screws come in a variety of sizes and lengths to accommodate different needs.
Torque
The desired level of torque, or tightening force, required for the application should be considered. Over-tightening can cause the screw to strip the threads, reducing its holding power.
Pilot Hole
It is important to ensure that the pilot hole, or the hole that the screw is inserted into, is the correct size and shape. The exact drill or punch hole size is crucial as it affects the security of the screw. If the hole is too large, the screw will become loose and not thread properly. Conversely, if the hole is too small, the screw may break or cause the material to split or crack.
Head Type
The screw head type should be chosen based on the specific application and the desired finish. Common head types include hex washer, modified truss, oval, pan, flathead, bugle, and pancake. For instance, the hex washer head is suitable for heavy-duty applications, while the flathead screw provides a clean finish with its flat top that sits below the surface.
Shank Size
The shank is the threaded shaft of the screw, and different sizes offer various qualities. Typically, the larger the number indicating the shank size, the larger the shaft diameter. It is important to select the appropriate shank size to ensure compatibility with the application.
Threads Per Inch (TPI)
The threads per inch (TPI) affect how effectively a screw works through a material. A lower TPI is generally more suitable for softwoods, while a higher TPI is better for metal and hardwoods. Choosing the right TPI ensures smoother fastening and prevents the screw from snapping due to excessive force.
Screw Point
The screw point influences how the screw interacts with the material during initial entry. Common types of screw points include needlepoint and drill point. Needlepoint screws feature sharp points for self-piercing in light sheet metal, while drill point screws have a blunter tip that facilitates easier insertion during initial impact.
Plating
The plating of the screw determines its durability and suitability for different environments. For indoor applications, zinc plating is standard, while hot-dipped and mechanical galvanized screws are recommended for outdoor use as they offer longer-lasting protection. Other plating options include ceramic and stainless steel, which are rust-resistant and ideal for outdoor use.
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Frequently asked questions
Screw tapping sizes refer to the designated sizes of screws that tap their own mating threads. These screws are used in metallic and non-metallic materials.
There are two main types of screw tapping: thread forming and thread cutting. Thread forming screws create a thread during insertion, while thread cutting screws act like a tap to create threads in the material as they are installed.
The size of the screw tapping depends on the material and the specific application. Factors to consider include the thickness of the material, the hardness of the material, and the required thread size.
Screw tapping screws are ideal when you only have access to one side of the material. They are also useful when disassembling and reassembling products, as they can be easily removed and reused.
Screw tapping screws are available in various head types, including pan head, flat head, and combination drive pan head. The choice of head type depends on the specific application and personal preference.
