Understanding Metric Thread (Screw Threads) (2024)

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Precise threading is crucial in manufacturing. To achieve accuracy and optimal function, engineers and designers need to understand screw threads design requirements and standards. While different types of threads may look similar, they may not be compatible, affecting your project. For example, NPT external threads are not compatible with metric internal threads. A metric screw size chart can help in identifying and matching screw components during procurement and assembly.

This article focuses on one of the most widely adopted screw threads standards worldwide – the iso metric screw thread standard. We’ll explore the fundamentals of metric fasteners as well as the metric thread chart.

Metric thread sizes are usually indicated by nominal diameter, pitch, and length. These dimensions are all in millimeters (mm).

Metric threads are either categorized as fine or coarse depending on their pitch. Coarse threads are more common than their fine counterparts and are assumed if the pitch is not given.

Let’s consider a metric thread of size M10-2.0 x 15. Here is a breakdown of these thread dimensions.

• M – Metric thread designation

• 10 – Nominal outer diameter in millimeters

• 2.0 – Thread pitch in millimeters

• 15 – Thread length in millimeters

A larger diameter points to a larger bolt or screw. As the thread pitch increases, you get a coarser thread (fewer threads per millimeter).

The minor diameter is the smallest diameter of a threaded cylinder or the distance between opposite roots of a thread profile. This critical parameter is specified in millimeters and serves as a measure of the strength and quality of a thread. There are several methods of measuring the minor diameter of a thread. They include;

• Optical measurement using profile projectors

• Specialized tooling such as micrometer depth gauge

Additionally, the nominal diameter is a key dimension in thread specifications, used alongside the thread pitch and crucial in standardized thread specifications according to ISO standards.

Pitch refers to the distance between two consecutive crests of a screw thread, measured in millimeters (mm). It’s typically obtained by measuring the distance between multiple crests and dividing it by the number of spaces between them. Trying to directly measure the distance between two succeeding peaks using a ruler can be challenging since threads are often tiny. Alternatively, pitch can be measured using specialized equipment such as pitch gauges and vision systems.

Depending on the pitch size, screw threads can be grouped into two categories: coarse threads and fine threads. Fine threads feature a smaller pitch and shallow ridges while coarse threads inherently have a larger pitch and deep ridges. Thread pitch is a crucial dimension that impacts the mechanical properties of a thread as well as its compatibility with mating components. The nominal diameter is used alongside the thread pitch in thread specifications and is a crucial component in standardized thread specifications according to ISO standards.

The root of a screw thread is the bottom surface that connects the two flanks of the thread, whether internal or external. The root radius is simply the radius of this rounded surface.

Additionally, the nominal diameter is a key dimension in thread specifications, used alongside the thread pitch and crucial in standardized thread specifications according to ISO standards.

Thread angle refers to the angle between the flanks of the thread. This crucial component is a defining factor for the engagement and tightness of the threads.

Additionally, the nominal diameter is a key dimension in thread specifications, used alongside the thread pitch and essential for standardized thread specifications according to ISO standards.

This is the diameter of a hole that is created through an object and enables a screw or bolt to pass through without the threads engaging with the material. In other words, a clearance hole is slightly larger than the fastener’s diameter, facilitating easy assembly and disassembly.

Clearance hole diameter can be measured using either digital calipers or pin gauges. The nominal diameter is a key dimension in thread specifications, used alongside the thread pitch and crucial in standardized thread specifications according to ISO standards.

The metric thread size chart specifies thread dimensions such as pitch, major and minor diameters, tolerances, and tap drill. Here are some of the most commonly used coarse threads up to M68. A metric screw size chart can help in visualizing the dimensions of coarse pitch threads.

Thread size Major diameter Pitch Pitch diameter Minor diameter Minor diameter Tap drill (male thread) (female thread)