Metric threads are primarily classified based on their diameter and pitch, with further differentiation into coarse, fine, and extra fine thread types.
Here's a breakdown:
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Diameter: The major diameter of the thread, measured in millimeters (mm). This is the most basic dimension.
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Pitch: The distance between adjacent thread crests, also measured in millimeters (mm). It signifies the thread's "fineness."
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Thread Series (Coarse, Fine, Extra Fine): The pitch defines the thread series.
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Coarse Threads: These are the most common type of metric thread. They offer good strength and are suitable for general-purpose applications. Coarse threads are designated simply with the diameter and pitch (e.g., M8 x 1.25). The implied pitch for coarse threads is the standard pitch for a given diameter.
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Fine Threads: These have a smaller pitch than coarse threads for the same diameter. They offer greater strength due to the larger minor diameter, reduced tendency to loosen under vibration, and finer adjustment. They're often used in applications requiring high precision or resistance to vibration. Fine threads are specified with the diameter and the pitch (e.g., M8 x 1).
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Extra Fine Threads: Have even smaller pitches than fine threads. These are used in specific applications where extremely fine adjustments are needed, or where wall thickness is limited.
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Example:
An M8 bolt can have different thread classifications:
- M8: Implies a coarse thread (standard pitch for M8 is 1.25mm). This is equivalent to M8 x 1.25.
- M8 x 1: Fine thread with a 1 mm pitch.
- M8 x 0.75: Extra Fine thread with a 0.75mm pitch.
In summary, metric threads are classified according to their diameter and pitch, categorized into coarse, fine, or extra fine series. The combination of diameter and pitch determines the specific thread type and its suitability for different applications.
