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A screw thread is an inclined plane wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread. More screw threads are produced each year than any other machine element. [1] Threads are generally produced according to one of the many standards of thread systems.
Screws in the Thury thread system are given nominal sizes, with the base size "0" being six millimeters in diameter and having a thread pitch of one millimeter. Sizes are proportional, so a size "1" is ten percent smaller in diameter that a size "0", while a size −1 is ten percent larger than a size "0".
The standard designation for a UTS thread is a number indicating the nominal (major) diameter of the thread, followed by the pitch measured in threads per inch.For diameters smaller than 1 / 4 inch, the diameter is indicated by an integer number defined in the standard; for all other diameters, the inch figure is given.
The term fine adjustment screw typically refers to screws with threads from 40–100 TPI (Threads Per Inch) (0.5 mm to 0.2 mm pitch) and ultra fine adjustment screw has been used to refer to 100–254 TPI (0.2 mm to 0.1 mm pitch). These screws are most frequently used in applications where the screw is used to control fine motion of an object.
The Thury thread form had the crests rounded at 1 / 6 p and the roots rounded at 1 / 5 p so the thread angle was close to 47.5° but not exactly. This was simplified in the BA thread definition by defining the thread angle to be 47.5° exactly and the thread form to be symmetrical with a depth of 3 / 5 p.
Many thread sizes have several possible tap drills, because they yield threads of varying thread depth between 50% and 100%. Usually thread depths of 60% to 75% are desired. People frequently use a chart such as this to determine the proper tap drill for a certain thread size or the proper tap for an existing hole.
The thread form follows the British Standard Whitworth standard: Symmetrical V-thread in which the angle between the flanks is 55° (measured in an axial plane) One-sixth of this sharp V is truncated at the top and the bottom; The threads are rounded equally at crests and roots by circular arcs ending tangentially with the flanks where r ≈ 0 ...
British Standard Fine (BSF) is a screw thread form, as a fine-pitch alternative to British Standard Whitworth (BSW) thread. It was used for steel bolts and nuts on and in much of Britain's machinery, including cars, prior to adoption of Unified, and later Metric, standards.
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