Parts of a Screw: The In-Depth Anatomy

In construction, home improvement, and DIY projects, screws play a critical role. But have you ever wondered about the intricacies that go into designing a screw? Though small and seemingly simple, screws have a diverse set of designs and components, each tailored for specific materials and applications.

This guide will take you through the different parts of a screw, demystifying this everyday tool.

Different Parts of a Screw

The following image illustrates the anatomy of a screw.

1. Head

The screw head is the top portion of the screw and, in most cases, is wider than the shaft. It plays a pivotal role in securing materials together.

Although all screw heads may seem similar at first glance, there are three types of screw heads that perform different functions.

• Countersunk
• Domed
• Raised

The basic function of the different head designs is based on their final application.

The raised screw head stays above the surface when fully driven into the material. The domed screw head also stays above the surface but is flatter. While the countersink is designed to be driven into the material until it sits flush with the material, providing a seamless finish.

2. Drive Profile

This is the part of the screw head where the screwdriver connects. You may have noticed that the drive may have different shapes, which must match the type of screwdriver that is used.

There are several different types of screw drives, but the three basic ones are as follows.

• Philips
• Pozidriv
• Slotted
• Square drive
• Hex and Torx

Each of these drive types has its unique advantages in terms of grip and torque.

The Philips is the cross-shaped drive that fits the Philips screwdriver. The Pozidriv is a variation of the Philips that features the same cross shape but has smaller additional slots for a better grip. And the slotted screw drive is arguably the simplest and most recognizable.

3. Shank

The shank is the body of the screw where you’ll find the thread. It’s an integral part of a screw that holds everything together. The thread on the shank is what keeps the screw in place when it’s driven into a material.

For partial thread screws, the shank is measured by the non-threaded surface just below the screw head. And the rest of the shank holds the thread, tip, and pitch of the screw.

4. Thread

The thread is the spiral-shaped ridge that runs down the length of the shaft. This is one of the most recognizable parts of the screw. Starting from just below the screw head down to the tip, the thread is what keeps the screw in place when driven into the material.

The spiral-shaped ridge is also known as a helical groove, referencing its shape. This helical structure enables the screw to stay firmly in place when embedded into the material, providing strong and durable fastening.

5. Tip or Point:

The tip or point is the end of the screw opposite the screw head. This is the first part that contacts the material into which the screw will be driven.

Although one of the most common types of tips is pointed, there are other shapes and styles as well. The sharp, pointed tip is most often used for jobs in which the screw may have to be removed at some point.

Generally, drywall screws, wood screws, self-tapping sheet metal screws, lag screws, etc., have the tip. Whereas metal screws, such as cap screws, usually have a flat end.

6. Crest and Root

The crest is the topmost point of the thread. This is where you measure the major diameter, which is often denoted by the screw size.

The root is the bottom-most point of the thread. This is the minor diameter.

The crest and root determine the major and minor diameter of the screw and hence play a key role in defining the screw size. Understanding these aspects helps in selecting the appropriate screw for your task.

Screw Specifications

Screw specifications, while not physical parts of a screw, are integral to understanding its functionality and ideal use. These measurements and angles, including pitch, thread angle, thread length, and screw size (diameter), help determine how the screw interacts with the material it’s penetrating and how securely it fastens objects together.

Let’s delve deeper into these essential screw specifications.

7. Thread Angle

The angle of a thread of a screw is the angle between the flanks of the thread measured in an axial plane. It’s a crucial factor in screw design as it affects the screw’s strength, the ease of turning, and the amount of material displaced by the thread.

Typical thread angles are 60° for Metric, Unified, and American screws and 55° for British Standard Whitworth (BSW) and pipe thread.
Screws with different thread angles are not interchangeable as the threads will not match correctly, leading to reduced holding power and might damage the threads.

8. Thread Length

Thread length refers to the portion of the screw that is threaded.

In a fully threaded screw, the thread runs the entire length of the screw’s shank. In a partial thread screw, the thread covers only a portion of the shank, leaving an unthreaded area.

9. Pitch

The pitch is the distance between the two consecutive crests on the screw threads.

This is an important dimension because similar-sized screws can have different pitches, and you cannot use them interchangeably.

The pitch may seem like a small detail, but the pitch plays a significant role in how a screw functions and is used. Here’s why:

• Material Compatibility: Different materials require screws with different pitches. For example, a fine pitch is better suited for softer materials like wood and plastic, where it provides a better grip and reduces the risk of material splitting. On the other hand, a coarse pitch is typically used for harder materials like metal, which helps in reducing the effort required to drive the screw.
• Strength and Durability: The pitch also impacts the strength of the screw. Fine threads, which have a smaller pitch, offer a larger stress area and hence provide stronger load-bearing capacity. However, they are also more prone to wear and tear compared to coarse threads.
• Ease of Use: The pitch of the screw can affect how easily it can be inserted or removed. Screws with a coarse pitch are typically easier to start driving into a material and can be inserted more quickly. They also tend to be more resistant to cross-threading, where the threads of the screw and the threaded hole it is being driven into don’t align properly.
• Vibration Resistance: If your project will be subjected to significant vibrations, the pitch of the screw can make a difference. Screws with a coarse pitch have a better ability to resist vibration, which can prevent them from loosening over time.

So, while it might seem like a minor detail, the pitch of a screw is critical to ensure the success of your project. Always consider the material you’re working with, the load the screw needs to bear, and the conditions it will be subjected to when choosing a screw with the right pitch.

In Closing:

Understanding the different parts of a screw is crucial when you’re working on any project that involves these fundamental components.

From the head, drive profile, shank, thread, tip, to the crest, root, and pitch, every part plays a specific role in the function of the screw. This knowledge is not only essential for understanding how screws work but also helps you make informed decisions when choosing the right screw for your project.

Choosing the right screw involves considering the material you’re working with, the size of the screw, its drive type, and more. This can seem complex, but don’t worry – armed with the information from this guide, you’re well-prepared to take the next step. For a more detailed guide on how to choose the perfect screw for your project, check out our screw selection guide.

This in-depth understanding of screws will make your DIY and construction tasks smoother and more effective. Happy building!