Avoiding The Pitfalls Of Incorrect Thread Size Screws

Choosing the right screw is essential for a stable, secure connection. The screw thread size, which refers to the dimensions of the threads on a screw, is a critical factor in this selection process. Incorrectly sized screws can lead to incompatibility and instability in bolted joints, which are prevalent across industries. Understanding the basics of screw threads, including their measurements and styles, is crucial for ensuring the successful intertwining of parts. This knowledge will enable individuals to select the most suitable screw for their specific applications, preventing issues and ensuring the integrity of their projects.

How to measure screw thread size

Choosing the correct screw thread size is essential for a stable, secure connection. There are several different systems of measurement used for screw thread sizes, including the Unified Thread Standard (UTS), also known as Imperial, and the metric system.

Step 1: Determine the Type of Thread

External threads, or male threads, are visible on the outside of a bolt, screw, or threaded post. Internal threads, or female threads, are found inside a part such as a tapped hole or a nut. If the threads are not all the same size and are tapered, they are likely pipe threads, commonly used for containing liquids or gases.

Step 2: Measure the Thread Diameter

The thread diameter refers to the width of the threaded part of the screw. Measure the outer diameter of the bolt, known as the nominal or major diameter, by holding the fastener against a ruler and measuring from one side of the thread crest to the other. Ensure the head of the fastener is parallel with the ruler for accuracy.

Step 3: Identify the Thread Spacing

Thread spacing can be identified by counting the number of threads per inch (TPI) or measuring the pitch, depending on the measurement system. For inch-based measurements, count the number of thread peaks along a one-inch length. For metric measurements, measure the distance between adjacent threads at their peaks in millimetres.

Step 4: Compare to a Standard Measurement

Once you have the diameter and TPI or pitch measurements, compare this information to a thread chart. If using imperial fasteners, refer to an imperial thread chart. If the dimensions don't match well, try a metric thread chart or consider if the fastener is worn, as older fasteners may have a slightly smaller diameter over time.

Tools for Measuring Screw Thread Size:

• Ruler
• Thread pitch gauge
• Caliper
• Nut/bolt gauge

By following these steps and using the appropriate tools, you can accurately measure screw thread size to ensure a secure and compatible fit.

Differences between imperial and metric threads

The choice between using imperial or metric fasteners depends on several factors, including the target market and the specific application. Here are the key differences between imperial and metric threads:

Measurement System

Metric fasteners are measured in millimetres and use the metric system, whereas imperial fasteners are measured in inches and use the imperial system. This difference in measurement systems leads to variations in thread sizes and pitches, making metric and imperial fasteners incompatible.

Expression of Thread Characteristics

Metric fasteners express thread characteristics using thread pitch, which is the distance between adjacent threads in millimetres. In contrast, imperial fasteners use thread count or threads per inch (TPI), indicating the number of threads within one inch.

Markings and Prefixes

Metric fasteners are typically prefixed with an "M" followed by the outer diameter in millimetres, such as “M8” for a metric thread with an 8-millimetre outer diameter. Imperial fasteners, on the other hand, are commonly marked with "W*N", where "N" represents the number of threads per inch.

Thread Patterns

Thread patterns in metric fasteners are classified as coarse (longer pitch) or fine (shorter pitch). In imperial fasteners, thread patterns are categorised as Unified National Coarse (UNC) with lower TPI values, or Unified National Fine (UNF) with higher TPI values.

Target Market

The choice between metric and imperial fasteners often depends on the target market. For example, manufacturers based in the United States, where imperial fasteners are standard, might opt for metric fasteners if they sell a higher volume in Europe, where metric fasteners are prevalent.

Range of Sizes

Metric fasteners offer a wider range of sizes within a small range, making them a more flexible option when working with applications that require various sizes. The metric system also provides easier subdivision of units of measurement.

How to read screw sizes

To avoid choosing the wrong screw size, you need to understand screw thread sizes. Here is a step-by-step guide on how to read screw sizes:

Step 1: Understand Screw Thread Size

The screw thread size refers to the dimensions of the threads on a screw, which determines how it fits into a threaded hole.

Step 2: Learn the Basic Screw Terminology

• External thread: Also known as male threads, external threads are found on the outside of a screw or bolt.
• Internal thread: Also known as female threads, internal threads are on the inner surface of a hollow cylinder, such as a cap or nut.
• Thread angle: The angle between the thread flanks.
• Major diameter: The largest diameter of the screw.
• Pitch: The distance from a point on the screw thread to the point on the next thread.

Step 3: Understand the Different Measurement Systems

There are two different systems of measurement used for screw thread sizes: the Unified Thread Standard (UTS) and the metric system.

Unified Thread Standard (UTS)

The UTS system uses inch-based measurements and includes different thread series, such as the coarse thread series (UNC) and the fine thread series (UNF). Threads are measured in threads per inch (TPI) by counting the number of thread peaks along a one-inch length.

Metric System

The metric system uses millimetres (mm) to measure screw thread sizes. To get the screw size, you need to measure the screw's major diameter and the thread pitch, which is the distance between two adjacent threads at their peaks.

Step 4: Measure the Screw Diameter and Thread Spacing

To measure the screw size, you need to measure the screw diameter and the thread spacing.

• Screw diameter: Measure the width of the threaded part of the screw. This corresponds to the first number of the thread size. For inch measurements, any diameter smaller than 1/4" is indicated by a number (e.g. #8 corresponds to 0.164"). For metric screws, this is the number of millimetres preceded by the letter "M" (e.g. M6 corresponds to 6 mm).
• Thread spacing: This corresponds to the second number of the thread size. For inch measurements, count the number of threads in one inch. For metric screws, measure the length in millimetres between threads.

Step 5: Understand the Different Thread Types

In addition to the different measurement systems, there are also different thread types, such as coarse and fine threads.

• Coarse threads: Coarse threads have a larger pitch and are used for general applications.
• Fine threads: Fine threads have a smaller pitch, making them stronger and more resistant to damage, such as from vibration.

How to measure threads

To measure screw thread size, you need to determine the type of thread, measure the thread diameter, identify the number of threads per inch (TPI) or pitch, and then compare this information against a standard thread chart. Here is a step-by-step guide:

Determine the Type of Thread:

External threads, known as male threads, are visible on the outside of a bolt, screw, or threaded post. Internal threads, known as female threads, are found inside a part such as a tapped hole or a nut. If the threads are not all the same size and are tapered, they are likely pipe threads used for containing liquids or gases, and you should refer to a pipe thread chart.

Measure the Thread Diameter:

The thread diameter corresponds to the first number of the thread size. For the imperial system, measure the width of the threaded part of the screw in inches, using a ruler or measuring tape. Any diameter smaller than 1/4" will be indicated by a number (e.g., #10 or No.10). For metric screws, measure the width in millimetres, and this will be preceded by the letter "M" (e.g., M6 corresponds to 6 mm).

Identify the Number of Threads per Inch (TPI) or Pitch:

Threads per inch (TPI) and pitch are inverse measurements of thread spacing. TPI is used for imperial fasteners, while pitch is used for metric fasteners. Pitch must be measured in millimetres. To find TPI, count the number of threads in one inch. To find the pitch, measure the distance between adjacent peaks or valleys of the thread in millimetres.

Compare to a Standard Measurement:

Once you know the diameter and TPI or pitch, compare this information to a thread chart. In the US, imperial fasteners are most common, but if the dimensions don't match well, try comparing to a metric thread chart, pipe thread chart, or consider if the fastener is worn and has a smaller diameter due to age.

There are also tools that can help with measuring threads, such as a thread pitch gauge, calipers, or a caliper and pitch gauge combination. These tools provide faster and more accurate measurements than a standard ruler or measuring tape.

Four other accurate ways to measure threads

There are several accurate ways to measure screw threads, ensuring you select the correct screw size for your project. Here are four reliable methods:

• Using a Thread Pitch Gauge: A thread pitch gauge is a simple and quick tool to determine the thread size. Place the leaves of the gauge against the fastener to find a perfect match. The teeth of the pitch gauge should align with the fastener's threads with no visible gap. These gauges typically range from $10 for introductory models to $50 or more for professional-grade sets.
• Measuring with a Caliper: While a tape measure can be used, a caliper provides faster and more accurate measurements. For external threads, place the caliper jaws along the length of the fastener, at a slight angle to ensure the jaws rest on the thread peaks. For internal threads, carefully centre the caliper in the hole to achieve a straight and accurate measurement.
• Combining a Caliper and a Pitch Gauge: This method is a quick and simple option. First, use the caliper to measure the diameter of the fastener. Then, use the pitch gauge to determine the pitch or TPI (threads per inch). Finally, compare these measurements to a standard reference for fastener sizes to identify the correct size.
• Utilising a Nut/Bolt Gauge: This specialty tool is designed specifically for measuring nuts and bolts. There are three types of nut/bolt gauges: a flat card style that provides a rough measurement, a wall hanging style with threaded posts, and a series of labelled headless fasteners. While this method is fast and effective, it is important to remember that nut/bolt gauges have no other uses beyond measuring these specific fasteners.

Frequently asked questions