Metric thread specifications are provided in a format that differs significantly from that used by the American National Standards Institute (ANSI) for standard thread specifications. M3 metric machine screws are commonly used in computers and other applications. M3 machine screws used in computer cases commonly receive the designation "fine thread," which is a misnomer -- truly "fine thread" 3 mm fasteners are quite rare.
Thread Specification Basics
M3 fine thread machine screws come with multiple specifications: nominal size, tap drill size, thread pitch and thread height. The tap drill size is that of the bit used to drill a hole for the fastener prior to cutting the thread in that hole. Thread pitch is the angle between one surface of a single thread and the other surface, measured in degrees from the very ridge of the thread. Thread height is the distance between the very bottom (root) of the thread and the very top (crest).
Nominal diameter is the diameter for which a fastener is named. The metric system for naming fasteners is very straightforward -- an M3 fastener has a nominal diameter is 3 mm. The tap drill size for an M3 fastener is 2.5 mm for a coarse thread and 2.6 mm for a fine thread.
ANSI threads are specified in threads per inch, but metric threads are specified by the distance in millimeters between each thread. The type of 3 mm metric thread commonly designated "fine" and often used in computer cases and similar applications is actually a metric "coarse" thread. The metric coarse thread for M3 fasteners is 0.5 mm (1/2 mm between each thread). The metric fine thread for M3 fasteners is 0.35 mm. Thus, a M3 fine thread would receive the designation "M3x0.35."
Thread Diameter and Other Specifications
In an "M3x0.35" screw, the pitch diameter is 2.773; the minor diameter of the internal thread is 2.621 mm; the minor diameter of the external thread is 2.571 mm; the thread height is 0.215 mm; the thread overlap is 0.189 mm; the angle of the thread is 60 degrees; and the root radius is 0.051 mm.
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