Choosing The Right Screw Size For Your Panel

Screws come in a wide range of sizes and types, and are used for a variety of applications. Panel screws are used in access panels for electronic enclosures and in equipment consoles across industries, such as food and beverage production, oil and gas, and power generation facilities. They are also used in computers, where the most common types are #6-32 UNC and M3, which differ in thickness and thread type. In addition, there are self-drilling sandwich panel screws, which are used for fast assembly and fastening sandwich panels to steel framing.

Screw size standards

Screw sizes can be incredibly confusing, especially when trying to select the correct one for your project. There are a variety of standards used around the world, with different countries using different systems.

Metric System

The Metric system is the most common standard used globally. Metric screws are designated by the letter "M" followed by a number, which indicates the nominal outer diameter of the screw thread in millimetres. For example, an M6 screw has a nominal outer diameter of 6 millimetres. The length of the screw is then indicated by an "x" and the length expressed in millimetres. For example, M8×30.

Imperial System

The Imperial system is used in the United States. Here, the Unified Thread Standard (UTS) is common. The UTS uses an alphanumeric designation, such as #6-32 UNC, where the # number refers to the major thread diameter, and the second number refers to the threads per inch (tpi).

Conversion

It is possible to convert between the two systems. For example, #6-32 UNC is equivalent to a major thread diameter of 3.51 millimetres and a thread pitch of 0.7938 millimetres.

Other Standards

In addition to the Metric and Imperial systems, there are other standards in use. For example, the British Standard Whitworth (BSW) is a British thread standard with a 55-degree profile. Similarly, the United States Standard thread is another standard that uses an inch-based set of diameter and pitch combinations.

Screw head types

Cap Head Screws:

Cap head screws are characterised by their cylindrical, flat top, which allows for higher torque application. They are known for their durability and strength, making them a preferred choice in precision engineering and assembly. Cap head screws are commonly used in aerospace, automotive, and heavy machinery applications.

Button Head Screws:

Button head screws feature a round, low-profile top that blends smoothly with surfaces. They offer a balance between strength and aesthetics, making them versatile and widely used. This type of screw is often found in consumer electronics, furniture assembly, and automotive interiors.

Pan Head Screws:

Pan head screws have a rounded top and a flat bearing surface underneath, allowing them to sit above the surface of the material. They are versatile and suitable for use in a variety of materials, including metal, wood, and plastic. Pan head screws are commonly used in electronics to secure components within devices.

Countersunk Head Screws:

Countersunk head screws are designed for applications where a smooth surface finish is essential. They have a conical top that sits flush with or below the surface of the material. This type of screw is commonly used in metal fabrication and aerospace industries, such as aeroplane fuselage assembly, to maintain a flush surface and reduce aerodynamic drag.

Hexagon Head Bolts:

Hexagon head bolts are recognised by their six-sided head, providing a robust grasp for tools like wrenches and spanners. They are versatile and suitable for a wide range of applications, from heavy machinery to household furniture. Hexagon head bolts are commonly used in structural engineering, automotive, and manufacturing industries.

Cheese Head Screws:

Cheese head screws resemble a wheel of cheese, with a disc-shaped head and a tapered outer edge. They offer a practical combination of torque application and simplicity. Cheese head screws are commonly used in electronics and precision engineering, such as in the assembly of laptops and cameras.

These are just a few examples of the many screw head types available. Each type has been engineered to meet specific requirements, offering unique benefits that cater to the diverse needs of various industries.

Screw length

• For most screws with heads that are flat on the bottom (such as pan head, round head, or truss head), the length is measured from the bottom of the head to the tip.
• Flat head, countersunk screws, which are flat on the top of the head, are measured from the top of the head to the tip.
• Hex and hex washer head screws, despite being flat on both the top and bottom, are typically measured from the bottom of the head.
• Oval head screws are an exception, with their length measured from the widest point of the head to the tip.
• Self-drilling screws are measured from the bottom of the head to the end of the drill point tip.

When determining the appropriate screw length, it is essential to consider the thickness of the material you are working with. As a general rule, the screw should penetrate at least half of the thickness of the attached material. For thicker materials, longer fasteners may be required to ensure a secure hold. Conversely, shorter screws may be adequate for thinner materials.

In certain situations, such as when using non-countersinking screws, it is recommended to aim for approximately two-thirds of the screw to be threaded into the "secondary" piece. However, as the thickness of the material increases, this ratio can be adjusted, and having half of the screw's shank threaded into the secondary piece may be sufficient.

The intended application of the screw also plays a role in determining the appropriate length. For example, #4 screws, which are designed for small crafts, jewellery boxes, and attaching hinges, typically range from 3/8 inch to 3/4 inch in length. On the other hand, #10 screws, suitable for general construction, heavy-duty furniture, and outdoor projects, are available in lengths ranging from 3/4 inch to 4 inches.

Screw thread

A screw thread is a helical structure that enables the conversion of rotational and linear movement or force. It is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter a tapered thread. The screw thread is an essential feature of a screw as a simple machine and as a threaded fastener.

The mechanical advantage of a screw thread depends on its lead, which is the linear distance travelled by the screw in one revolution. The lead of a screw thread is typically chosen to ensure sufficient friction, preventing linear motion from being converted to rotary motion. This characteristic is essential for most applications.

The helix of a thread can be right-handed or left-handed, with right-handedness being the default. Right-handed threads are oriented so that when the threaded item is turned clockwise, it moves away from the viewer. Left-handed threads are oriented in the opposite direction.

The cross-sectional shape of a thread is called its form or threadform and can be square, triangular, trapezoidal, or other shapes. The most common triangular threadforms are based on an isosceles triangle and are known as V-threads or vee-threads.

The pitch of a screw thread refers to the distance from the crest of one thread to the next, measured parallel to the axis. Coarse threads have a larger pitch, while fine threads have a smaller pitch. Coarse threads are more resistant to stripping and cross-threading, while fine threads are stronger and less likely to vibrate loose.

Tapered threads are commonly used on fasteners like wood screws and pipes. The seal created by a tapered thread is achieved by tightening an externally threaded end into an internally threaded end.

Standardisation of screw threads has evolved since the early 19th century to ensure compatibility between manufacturers and users. The most common threads in use today are the ISO metric screw threads (M) and BSP threads (R, G) for pipes.

Screw applications

Screws are used to assemble or position two or more components in relation to each other. They are most commonly seen in wood, drywall, and machinery.

Machine Screws

Machine screws are used to join two metal components together. They are threaded along the entire length of the shank and usually have a countersink. They are used in electronic devices, appliances, and manufacturing equipment.

Sheet Metal Screws

Sheet metal screws are used to attach wood, plastic, or metal to a piece of sheet metal. They have sharp tips to penetrate tough metal surfaces, and fine threads for superior grip.

Wood Screws

Wood screws are used to join multiple pieces of wood together. They have a sharp, piercing tip that can easily penetrate wooden surfaces. They are available in a wide variety of sizes and drives.

Drywall Screws

Drywall screws have coarse threads suitable for securing drywall to wooden studs or beams. They usually have flat heads that are designed to countersink just below the surface of the drywall.

Set Screws

Set screws, also known as grub screws, are headless threaded rod segments. They are used when the screw head should not protrude outside the product, or when there is limited space available for the screw head.

Frequently asked questions