M - Metric

A metric thread refers to a specific system of thread measurement that is commonly used for screws, bolts, and nuts in most countries around the world, except for the United States and a few other regions. The metric thread system is based on the metric system of measurement, which uses units like millimeters and millimeters per pitch for thread specifications.

Key characteristics of metric threads include:

1. Thread Profile: Metric threads use a standardized thread profile known as the ISO metric screw thread profile. This profile is characterized by a 60-degree included angle between the thread flanks and a rounded root and crest.

1. Thread Size: Metric threads are specified using two main dimensions:

• Major Diameter (D): This is the maximum diameter of the thread, measured across the outer edges of the thread crests.
• Pitch (P): This is the distance from one thread crest to the next along the axis of the screw or bolt. Pitch is measured in millimeters.

1. Thread Pitch: The pitch of a metric thread is one of the most critical dimensions. It is expressed as "P" and represents the distance between threads. For example, an M8 x 1.25 metric thread has a major diameter of 8 millimetres and a pitch of 1.25 millimetres.

1. Thread Designation: Metric threads are typically designated with the letter "M" followed by the major diameter and the pitch in millimetres. For example, M10 x 1.5 represents a metric thread with a major diameter of 10 millimetres and a pitch of 1.5 millimetres.

1. Tolerance Class: Metric threads often have specified tolerance classes that indicate the acceptable range of variation for the thread dimensions. Common tolerance classes for metric threads are 6g for external threads (bolts and screws) and 6H for internal threads (nuts and tapped holes).

Metric threads are widely used in various industries, including manufacturing, automotive, construction, and machinery. They are known for their precision, and the use of the metric system simplifies thread design and specification. The metric thread system has largely replaced the older imperial or Unified Thread Standard (UTS) system in most parts of the world due to its ease of use and international standardization.