Understanding Metric Thread (Screw Threads)

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Introduction

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.

What is a Metric Thread?

Metric Thread

A metric thread is the most common general-purpose thread type. It features a cylindrical shape and is measured in millimeters (mm). The nominal diameter is used alongside the thread pitch and is essential for defining screw and bolt threads. Controlled by the International Organization for Standardization (ISO), the metric thread standard is used to define screw and bolt threads worldwide. This comprehensive system facilitates precise thread design and compatibility.

Metric Thread Size Explained

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).

Metric Thread Dimension Properties

Stainless Steel Screw

The size of a screw thread is generally measured and specified using the geometric dimensions of its tooth profile. These dimensional parameters include pitch, major diameter, minor diameter, and thread angle. A metric screw size chart can help in visualizing these geometric dimensions of screw threads.

Major Diameter (mm) Explained

The major diameter refers to the largest diameter of a threaded element and is equal to the distance between opposite crests (or tips) in a thread profile. It is measured in millimeters and this crucial dimension dictates all metric bolt sizes. The major diameter can be accurately measured using vernier calipers. The nominal diameter is often used interchangeably with the major diameter in some contexts and is a key component in standardized thread specifications according to ISO standards.

Minor Diameter (mm) Explained

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.

Thread Pitch (mm) Explained

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.

Root Radius Explained

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 Explained

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.

Clearance Hole Diameter (mm) Explained

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.

Metric Thread Chart: Coarse Pitch (M1-M48)

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)

Document
Thread Major Diameter (d=D) Pitch (p) Root Radius(r) Pitch Diameter (d2=D2) Minor Diameter Male Thread (d3) Minor Diameter Female Thread (D1) Thread Height Male Thread (h3) Thread Height Female Thread (H1) Tap Drill Diameter
M1 1 0.25 0.036 0.838 0.693 0.729 0.153 0.135 0.75
M1.1 1.1 0.25 0.036 0.938 0.793 0.829 0.153 0.135 0.85
M1.2 1.2 0.25 0.036 1.038 0.893 0.929 0.153 0.135 0.95
M1.4 1.4 0.3 0.043 1.205 1.032 1.075 0.184 0.162 0.162
M1.6 1.6 0.35 0.051 1.373 1.171 1.221 0.215 0.189 1.25
M1.8 1.8 0.35 0.051 1.573 1.371 1.421 0.215 0.189 1.45
M2 2 0.4 0.058 1.74 1.509 1.567 0.245 0.217 1.6
M2.2 2.2 0.45 0.065 1.908 1.648 1.713 0.276 0.244 1.75
M2.5 2.5 0.45 0.065 2.208 1.948 2.013 0.276 0.276 2.05
M3 3 0.5 0.072 2.675 2.387 2.459 0.307 0.271 2.5
M3.5 3.5 0.6 0.087 3.11 2.764 2.85 0.368 0.325 2.9
M4 4 0.7 0.101 3.545 3.141 3.141 0.429 0.379 3.3
M4.5 4.5 0.75 0.108 4.013 3.58 3.688 0.46 0.406 3.8
M5 5 0.8 0.115 4.48 4.019 4.134 0.491 0.433 4.2
M6 6 1 0.144 5.35 4.773 4.917 0.613 0.541 5
M8 8 1.25 0.18 7.188 6.466 6.647 0.767 0.677 6.8
M10 10 1.5 0.217 9.026 8.16 8.376 0.92 0.812 8.5
M12 12 1.75 0.253 10.863 9.853 10.106 1.074 0.947 10.2
M14 14 2 0.289 12.701 11.564 11.835 1.227 1.083 12
M16 16 2 0.289 14.701 13.546 13.835 1.227 1.083 14
M18 18 2.5 0.361 16.376 14.933 15.394 1.534 1.353 15.5
M20 20 2.5 0.361 18.376 16.933 17.294 1.534 1.353 17.5
M22 22 2.5 0.361 20.376 18.933 19.294 1.534 1.353 19.5
M24 24 3 0.433 22.051 20.319 20.319 1.84 1.624 21
M20 20 2.5 0.361 18.376 16.933 17.294 1.534 1.353 17.5
M27 27 3 0.433 25.051 23.319 23.752 1.84 1.624 24
M30 30 3.5 0.505 27.727 25.706 26.211 2.147 1.894 26.5
M33 33 3.5 0.5051 30.727 28.706 29.211 2.147 1.894 2.147
M36 36 4 0.577 33.402 31.093 31.67 2.454 2.165 32
M39 39 4 0.577 36.402 34.093 34.67 2.454 2.165 35
M42 42 4.5 0.65 39.077 36.479 37.129 2.76 2.436 37.5
M45 45 4.5 0.65 42.077 39.479 40.129 2.76 2.436 40.5
M48 48 5 0.722 44.752 41.866 42.857 3.067 2.706 43

Advantages of Coarse Threads

Fast installation

One of the main benefits of coarse threads is the quick and simplified installation process. Thanks to the larger thread design, you can insert fasteners into materials without much hassle. Coarse threads are often preferred for applications that require speed and convenience.

Enhanced resistance to vibrations

Due to their larger pitch, coarse threads can absorb vibrations more efficiently. They provide stability and can endure vibrations without loosening.

Resilience

Coarser threads are tougher than fine pitch threads. They can withstand scratches and nicks and still function well.

Disadvantages of Coarse Threads

Limited strength

Compared to fine threads, coarse threads provide reduced tensile strength. Therefore, they’re not as suited to high-strength applications. When picking the ideal thread for your project, you need to take into account the specific load requirements.

Reduced holding power

Another disadvantage of coarse threads is their limited holding power. As the thread pitch increases, the level of grip reduces. This means that coarse threads may not be a good choice in applications where secure connections are crucial.

Metric Thread Chart: Fine Pitch (M1-M24)

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

Document
Thread Size Major Diameter d=D Pitch (p) Root Radius(r) Pitch Diameter (d2=D2) Minor Diameter Male Thread (d3) Minor Diameter Female Thread (D1) Thread Height Male Thread (h3) Thread Height Female Thread (H1) Tap Drill Diameter
M1.0 x 0.2 1 0.2 0.029 0.87 0.755 0.783 0.123 0.108 0.8
M1.1 x 0.2 1.1 0.2 0.029 0.97 0.855 0.883 0.123 0.108 0.9
M1.2 x 0.2 1.2 0.2 0.029 1.07 0.955 0.983 0.123 0.108 1
M1.4 x 0.2 1.4 0.2 0.029 1.27 1.155 1.183 0.123 0.108 1.2
M1.6 1.6 0.2 0.029 1.47 1.355 1.383 0.123 0.108 1.4
M1.8 1.8 0.2 0.029 1.67 1.555 1.583 0.123 0.108 1.6
M2 x 0.25 2 0.25 0.036 1.838 1.693 1.729 0.153 0.135 1.75
M2.2 2.2 0.25 0.036 2.038 1.893 1.929 0.153 0.135 1.95
M2.5 2.5 0.35 0.051 2.273 2.071 2.121 0.215 0.189 2.1
M3 3 0.35 0.051 2.773 2.571 2.621 0.215 0.189 2.6
M3.5 3.5 0.35 0.051 3.273 3.071 3.121 0.215 0.189 3.1
M4 4 0.5 0.072 3.675 3.387 3.459 0.307 0.271 3.5
M4.5 4.5 0.5 0.072 4.175 3.887 3.959 0.307 0.271 4
M5 x 0.5 5 0.5 0.072 4.675 4.387 4.459 0.307 0.271 4.5
M5.5 x 0.5 5.5 0.5 0.072 5.175 4.887 4.959 0.307 0.271 5
M6 x 0.75 6 0.75 0.108 5.513 5.08 5.188 0.46 0.406 5.2
M8 x 0.75 8 0.75 0.108 7.513 7.08 7.188 0.46 0.406 7.2
M8 x 1.0 8 1 0.144 7.35 6.773 6.917 0.613 0.541 7
M10 x 0.75 10 0.75 0.108 9.513 9.08 9.188 0.46 0.406 9.2
M10 x 1 10 1 0.144 9.35 8.773 8.917 0.613 0.541 9
M10 x 1.25 10 1.25 0.18 9.188 8.466 8.647 0.767 0.677 8.8
M12 x 1 12 1 0.144 11.35 10.773 10.917 0.613 0.541 11
M12 x 1.25 12 1.25 0.18 11.188 10.466 10.647 0.767 0.677 10.8
M12 x 1.5 12 1.5 0.217 11.026 10.16 10.376 0.92 0.812 10.5
M14 x 1 14 1 0.144 13.35 12.773 12.917 0.613 0.541 13
M14 x 1.25 14 1.25 0.18 13.188 12.466 12.647 12.647 0.677 12.8
M14 x 1.5 14 1.5 0.217 13.026 12.16 12.376 0.92 0.92 12.5
M15 x 1 15 1 0.144 14.35 13.773 13.917 0.613 0.541 0.541
M15 x 1.5 15 1.5 0.217 14.026 13.16 13.376 0.92 0.812 13.5
M16 x 1 16 1 0.144 15.35 14.773 14.917 0.613 0.541 15
M16 x 1.5 16 1.5 0.217 15.026 14.16 14.376 0.92 0.812 14.5
M18 x 1 18 1 0.144 17.35 16.773 16.917 0.613 0.541 17
M18 x 1.5 18 1.5 0.217 17.026 16.16 16.376 0.92 0.812 16.5
M18 x 2 18 2 0.289 16.701 15.546 15.835 1.227 1.083 16
M20 x 1.0 20 1 0.144 19.35 18.773 18.917 0.613 0.541 19
M20 x 1.5 20 1.5 0.217 19.026 18.16 18.376 0.92 0.812 18.5
M20 x 2.0 20 2 0.289 18.701 17.546 17.835 1.227 1.083 18
M22 x 1.0 22 1 0.144 21.35 20.773 20.917 0.613 0.541 21
M22 x 1.5 22 1.5 0.217 21.026 20.16 20.376 0.92 0.812 20.5
M22 x 2.0 22 2 0.289 20.701 19.546 19.835 1.227 1.083 20
M24 x 1.0 24 1 0.144 23.35 22.773 22.917 0.613 0.541 23
M24 x 1.5 24 1.5 0.217 23.026 22.16 22.376 0.92 0.812 22.5

Advantages of Fine Threads

Enhanced strength

One of the key benefits of fine threads is that they are stronger than coarse pitch threads in terms of both tension and shear. This is because a fine pitch thread has more threads per inch creating a larger surface area for load distribution. The improved stress distribution makes this thread type ideal for high-strength applications.

Increased holding power

Thanks to their smaller thread incline, fine threads have a tighter grip when fastened. Therefore, they create more secure connections that are less prone to loosening under load.

Precise Adjustments

Fine threads are best known for their precision. They allow for finer adjustments and are ideal for sensitive assemblies.

Disadvantages of Fine Threads

Challenging to install

Given their smaller size and tighter tolerances, finer threads are generally trickier to align and install. This is especially true in sections where access is limited. It’s recommended that you employ the right tools and techniques during installation to ensure proper alignment and prevent damage to the screws or bolts.

Poor resistance to vibrations

With fine pitch threads, there is a very small distance between threads. This means they have a reduced capacity to dampen vibrations compared to their coarse counterparts. As such, finer threads are not well-suited to applications where vibrations are an issue.

Long thread engagement

Due to their delicate nature, fine-threaded fasteners require longer thread engagements to prevent damage and fouling.

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