
Selecting the right screw size can be a tedious task, but it is an important one. The right screw size ensures the screw can efficiently hold or anchor materials together. The most important factors in screw selection are length and diameter. To determine the right length, it is essential to first understand how manufacturers label fasteners. In the United States, for example, fasteners are typically categorized with two numbers and a nominal length. The nominal length of the screw does not include the head. The general rule of thumb is that the screw should enter at least half the thickness of the bottom material. To determine the right diameter, it is important to understand the difference between the major and minor diameter. The major diameter is the diameter of the fastener to the end of the thread, while the minor diameter is the diameter of the inner shaft of the screw, excluding the threads.
Characteristics | Values |
---|---|
Length | Should be three times the thickness of the material it will be driven into |
Should enter at least half the thickness of the bottom material | |
Should be long enough to hold the two boards together securely | |
Should be at least 1 to 1 1/2 inches of thread into the receiving board | |
For metal-to-wood connections, there is a minimum embedment required | |
Diameter/Width | Depends on the width of the material and the weight it will have to support |
Should be thick enough to grab onto the two boards | |
Should be thick enough to efficiently hold or anchor | |
Thread Spacing/Pitch | Should be factored into the total load capability |
Should be between 35-40 threads per inch | |
Should be less than 1 mm | |
Type | Sheet metal screws, drywall screws, wood screws |
Indoor or outdoor use | |
Material | Zinc, silicon-coated bronze, stainless steel |
What You'll Learn
Screw length
Another rule of thumb is that the screw length should be three times the thickness of the material it will be driven into. For instance, if you are screwing into a 2x4, the screw should be at least 3/4" long.
When selecting the screw length, it is also important to consider the direction of the forces that will be applied to the fixing. If the force is lateral or pushing along the joint of the two materials being fixed, a thicker screw should be used.
The screw length should be long enough to securely fasten the materials together without going all the way through and sticking out the other side. The length will depend on the forces involved: the greater the force, the longer the screw that will be required.
For metal-to-wood applications, there is a minimum embedment required for the screw to perform to its specifications. When the fastener is fully seated against the metal panel, there should be a minimum of three fully developed threads into the wood substrate. This ensures that the screw can withstand the forces applied and securely hold the materials together.
It is also important to consider the number of screws that will be used. Using more screws will provide a stronger fixing, especially if there are limitations on the length of the screw that can be used.
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Screw head
The head of a screw is an important consideration when selecting the right screw for your project. The head type or shape of a screw can be flat, round, or oval, but there are other options available, too. The type of head you choose will depend on the project and the effect you want to achieve. Here are some of the most common types of screw heads:
Flat Head
Flat-head screws have a flat top and a conical underside. This type of screw head sits flush with the surface of the material it is screwed into, resulting in a smooth surface. Flat-head screws are a good choice when you want the screw to be inconspicuous or when you need a smooth surface.
Oval Head
Oval-head screws have a conical underside, like flat-head screws, but they have a decorative top that protrudes slightly above the surface of the material. Oval-head screws are a good choice when you want the screws to be visible and add a decorative touch to your project.
Round Head
Round-head screws are flat on the underside of the head and have a domed top that sits above the surface of the material. Round-head screws are a good choice when you want the screws to be visible but don't want a completely smooth surface. They can also be a good choice for projects where the screws need to be easily accessible for removal or adjustment.
Phillips Head
Phillips-head screws are commonly used and are suitable for a wide range of applications. They have a cross-shaped recess in the head that allows for more precise control when driving the screw. Phillips-head screws are a good choice for projects that require a secure fit and a neat appearance.
Square Head
Square-head screws have a square recess in the head and are designed to keep your drill bits in place, preventing slipping and stripping. They provide a strong and secure hold and are often used in power drilling applications.
Torx/Star Head
Torx or star-head screws have a star-shaped recess in the head and are designed to provide a strong and secure hold. They are commonly used in applications where a high level of torque is required and are often used in automotive and industrial settings.
When selecting a screw head, it is important to consider the material you are working with, the depth of the material, and the weight the screw will support. The screw head should also be compatible with the type of drill or screwdriver you are using.
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Screw thickness
The tightening of the screw during application also needs to be factored into the total load capability. It is recommended to ensure that the pre-load is at least twice the weight of any external loads to reduce risk. Check the packaging to confirm load capabilities before application.
When choosing a screw length, it is important to remember that the nail should always be three times as long as the thickness of the material it will be driven into.
The numbers on screws (#6, #8, etc.) refer to the diameter of the screw, with higher numbers indicating larger diameters. For instance, a #6 screw is designed for small crafts, jewellery boxes, attaching hinges, etc., while a #10 screw is designed for general construction, heavy-duty furniture, outdoor projects, etc.
The general rule of thumb is that the screw should enter at least half the thickness of the bottom material. For instance, a screw entering 3/4" into a 2 x 4.
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Screw material
The material of a screw is an important consideration when selecting the right screw for your project. The type of screw material you should use depends on factors such as the type of project, the materials you are screwing into, and whether the screw will be used indoors or outdoors.
Steel
Steel is the most common material used for manufacturing screws due to its low cost. It is available in different grades, with Grade 2 being the most commonly used as it is highly workable. Steel screws are sturdy and strong, making them suitable for various tasks such as construction and woodworking. However, they are less durable and may require a coating for protection.
Stainless Steel
Stainless steel screws are made from an iron alloy with at least 10.5% chromium, which provides excellent corrosion resistance. They are perfect for outdoor, marine, and coastal applications and are commonly used in external construction and marine projects. Stainless steel screws are also used indoors, especially in kitchens and bathrooms, where corrosion resistance is beneficial.
Copper
Copper screws are known for their corrosion-resistant properties, making them suitable for long-term durability if exposed to the elements. They are also good conductors, making them ideal for electrical equipment, wiring, and plumbing systems. Copper screws may be preferred for visible or decorative applications due to their attractive appearance.
Aluminum
Aluminum screws are lightweight and corrosion-resistant. They are often used in situations where weight reduction is essential, such as in the electronics, automotive, and aerospace industries. However, aluminum is less durable and has relatively low strength compared to other materials.
Titanium
Titanium screws offer a blend of strength and lightness, making them ideal when both robustness and lightweight properties are required. They are commonly used in aerospace, medical implants, and high-performance racing cars. Titanium screws come at a premium cost but offer excellent corrosion resistance even in challenging conditions.
Brass
Brass is an alloy of copper and zinc, which provides good corrosion resistance and electrical conductivity. Brass screws are suitable for both indoor and outdoor use and are often selected for decorative and ornamental projects due to their golden appearance. However, brass is a relatively soft metal with low tensile strength.
Plastic
Plastic screws, made from materials such as nylon, polypropylene, or polycarbonate, offer good chemical and corrosion resistance. They are non-conductive, making them suitable for electrical applications. Plastic screws are lightweight, reasonably priced, and are commonly used in electronics, food processing machinery, and automobile interiors.
When selecting screw material, it is important to consider the specific requirements of your project, including the environment, load-bearing needs, and aesthetic preferences. Additionally, coatings can be applied to screws to enhance their performance and provide additional protection or aesthetic qualities.
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Screw use
Screws are used to fasten materials together. The type of screw used depends on the application and the materials being joined.
Basic Types of Screws
- Sheet metal screws are threaded all the way to the head without interruption.
- Drywall screws have a curved junction connecting the head to the shaft to prevent tearing.
- Wood screws are the standard type that forms its own internal mating thread in wood or other resilient materials.
Screw Size
The size of a screw is extremely important. The screw thickness needs to work for the width of the material you’re placing the screw into, as well as the weight it will have to support. The pre-load should be at least twice the weight of any external loads to reduce risk.
Screw Length
The length of a screw is also important. The nail should be three times as long as the thickness of the material it will be driven into. The screw should enter at least half the thickness of the bottom material. For instance, a screw should enter at least 3/4" into a 2 x 4.
Indoor vs. Outdoor Use
For indoor use, you can use less expensive zinc screws or select the material/coating for visual appeal. For outdoor use, the screws need protection against corrosion from moisture and temperature change. The best outdoor solutions are silicon-coated bronze or stainless steel.
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Frequently asked questions
The three most common types of screws are sheet metal screws, drywall screws, and wood screws. Sheet metal screws are threaded all the way to the head, while drywall screws have a curved junction connecting the head to the shaft to prevent tearing. Wood screws are standard screws that form their own internal mating thread in wood or other resilient materials.
You can measure screw size using either the imperial or metric system. For the imperial system, measure the length in inches from the tip of the screw to where the screw head rests when it is fully embedded. Then, measure the width of a thread in fractions of an inch to get the diameter. For the metric system, measure the length in millimetres from the tip of the screw to the flat underside of the screw head. Measure the width of a thread in millimetres to get the diameter.
The size of a screw is important and depends on the width of the material you're screwing into and the weight it will have to support. The screw thickness needs to be suitable for the material width and the weight it will support. The screw length should be three times the thickness of the material it will be driven into.
Wood screws must be long enough to hold the two boards together securely. If they are too short, they may not hold the boards together, and if they are too long, they will pierce through to the other side. Wood screws must also be thick enough to grab onto the two boards without being so thick that they split the wood.
For indoor use, you can use less expensive zinc screws or choose a material/coating for visual appeal. For outdoor use, screws need protection against corrosion from moisture and temperature changes. The best outdoor solutions are silicon-coated bronze or stainless steel.