Quoting from the Machinery's Handbook, 29th Edition, p. Uppercase positioning letters indicate internal threads while lowercase positioning letters indicate external threads. Positions denote the distance of the tolerance from the pitch diameter. Grades provide a measure of the tolerance itself: the smaller the number, the tighter the tolerance. Metric threads have a slightly more complex tolerancing method that uses tolerancing grades, designated by a number 3 through 9 and tolerancing positions, which use letters e through h. Unified thread classes are alphanumeric identifiers starting with a number from 1 through 3, where 1 is the loosest tolerance and 3 is the tightest, and either A for external threading or B for internal threading. The tolerances and allowances on a thread series are given by a thread class. The underlined grades are used with normal lengths of thread engagement. The tolerance becomes looser as the grade increases.The term M4 x 0.7 is often shortened to just M4. In metric nomenclature, an M4 x 0.7 cap screw has a 4 mm barrel and the pitch is 1 thread per 0.7 mm. For example, in the Unified nomenclature, a 1/4"-20 cap screw has a 1/4" diameter barrel and the pitch is 20 threads per inch (TPI). Unified threads specify density in threads per inch, while Metric threads specify the thread pitch. Thread series are denoted by the major diameter and density of threads. Most screws are identified by their thread series. Those forms which are not standardized by the Unified screw thread system are designated UNS. For example, an extremely fine thread with a flat root contour is designated UNEF. These can be further described by appending more letters. There are many thread forms in the Unified screw thread standard designated by either UN, which defines a flat root contour, or UNR, which defines a round root contour. Common thread forms include the Unified Screw Thread Form, used in the United States of America and measured in imperial units, and the ISO Metric Screw Thread Form, used in many parts of the world and measured with the International System of Units. The pitch diameter is approximately halfway between the major and minor diameters.Ī thread form is a set of rules that define the features' scale relative to one another. For most screws, crests and roots do not terminate at a sharp point, so crest and root truncation values are included in the definitions of major and minor diameter. The major diameter is taken from the crests of a thread while the minor diameter is taken from the roots. Each feature is shown in the diagram to the right. Unless otherwise stated, threads have a flank angle of 30°, resulting in a total angle between flanks of 60°. The flank angle is defined as the angle the flank makes with a perpendicular ray drawn from the screw axis. The distance between corresponding points on adjacent threads is known as the pitch of the thread. Except in special cases, threads have symmetrical sides inclined at equal angles when a vertical line is drawn through the center of a crest or root. For more details regarding specifications and dimensions, please consult the Machinery's Handbook, available for purchase at many bookstores.Ī thread consists of three repeating features: a crest, flank, and root (see drawing to the right). The following is a general overview of screw threading. You can also find an article on how to drill and tap a hole properly here.Click to Enlarge Threading Specifications You can find a metric drill and tap chart here. Below is an SAE drill and tap chart on both coarse thread and fine thread. When drilling and tapping a hole you need to drill your workpiece to the right size and in order to tap your workpiece to the correct size.
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