How do you read a metric thread chart?


Quick Answer

Metric thread charts contain information regarding the internal and external diameter of the bolt or nut, the thread's spacing and the thread's minimum and maximum pitch. It also contains the ISO metric thread profile.

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Full Answer

As opposed to Unified Thread Size charts, metric thread charts refer to the actual dimensions of the thread and bolt rather than designated nominal sizes. Generally, the chart provides the ISO thread profile first. This includes the diameter in millimeters of the cross-section of the threaded portion of the screw or bolt, including the height of the threads. This number is called the major diameter. The thread designation follows, which includes the major diameter, typically preceded by the letter M, followed by the distance between the threads, which is the pitch.

The chart then reports the external, or bolt, dimensions, which are the major diameter, the minimum and maximum pitch, and the minimum and maximum minor diameter, which is the width of the screw or bolt not including the height of the threads. Internal dimensions, or the dimensions of the corresponding nut, are reported in the same manner, except that the minor diameter is often reported before the major diameter.

Thorough thread charts also include the class for both internal and external dimensions. The thread class refers to the tolerance of the threads, reported as a pairing of a number between one and six and the lowercase letter "e," "g" or "h" for bolts or a capital "G" or "H" for nuts. The number refers to the relative coarseness or fineness of the fitting, while the letters represent the fundamental deviation for the tolerance class.

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