What Are The ISO Metric Threads Of Fastener?

ISO metric threads are a worldwide standard for fasteners and are part of the International Organization for Standardization (ISO) system. These threads are described by their pitch (the distance between thread peaks) and nominal diameter (the basic major diameter of the thread). Metric threads are widely used because of their simplicity and worldwide acceptance.

The main ISO standards for metric threads include:

ISO 68-1: Specifies the basic profile for ISO general purpose metric screw threads (M series).

ISO 261: Defines the general plan for metric screw threads, specifying the basic dimensions. It does not specify tolerances or other specific design features, but it does outline the thread form and its basic dimensions, similar to what ISO 68-1 covers but with a broader scope.

ISO 262: Specifies the coarse thread pitches and the preferred combination of diameter and pitch for ISO metric threads.

Metric threads are identified by their diameter (d) followed by the pitch (P). For example, M8x1.25 describes a metric thread where “M” indicates it is a metric thread, “8” is the nominal diameter in millimeters, and “1.25” is the pitch, also in millimeters. This means that each thread is 1.25mm apart from the next.

There are two main types of ISO metric threads:

Coarse threads (ISO 261/262): These have a larger pitch, are more common, and are used where fastening and unfastening might be frequent. They provide a good balance between strength and ease of use.

Fine threads: These have a smaller pitch, which allows for finer adjustments in applications that require greater precision. They also have a slightly higher strength in tapped holes, but are more prone to damage and are less suitable for high-speed assembly or disassembly.

ISO metric thread sizes can range from very small (like those used in watches) to very large (used in heavy machinery). The selection between coarse and fine threads, as well as the specific size, typically depends on the application’s specific requirements, including the needed strength, precision, and the materials involved.

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