Machinery's Handbook, 31st Edition
1942 Screw Thread Systems ISO Metric Thread System.— ISO refers to the International Organization for Standard ization, a worldwide federation of national standards bodies (for example, the American National Standards Institute is the ISO national body representing the United States) that develops standards on a very wide variety of subjects. The basic profile of ISO metric threads is specified in ISO 68 and shown in Fig. 2. The basic profile of this thread is very similar to that of the Unified thread, and as previously discussed, H is the height of a sharp V-thread, P is the pitch, D and d are the basic major diameters, D 2 and d 2 are the basic pitch diameters, and D 1 and d 1 are the basic minor diameters. Here also, capital letters designate the internal thread dimensions ( D , D 2 , D 1 ), and lowercase letters designate the external thread dimensions ( d , d 2 , d 1 ). This metric thread is discussed in detail in the section METRIC SCREW THREADS starting on page 2012.
H 8
Internal threads
P
P 8
3 8
60 °
H
P 2
30 °
D , d
5 8
H
H
P 4
P 2
H 4
D 2 , d 2
D 1 , d 1
90 °
Axis of screw thread
External threads
P = 0.866025404 P 0.125 H = 0.108253175 P 0.250 H = 0.216506351 P 0.375 H = 0.324759526 P 0.625 H = 0.541265877 P 3 2 H × =
Fig. 2. ISO 68 Basic Profile
Definitions of Screw Threads The following definitions are based on American National Standard ANSI/ASME B1.7- 2006 (R2016) “Screw Threads: Nomenclature, Definitions, and Letter Symbols,” and refer to both straight and taper threads. Actual Size: An actual size is a measured size. Allowance: An allowance is the prescribed difference between the design (maximum material) size and the basic size. It is numerically equal to the absolute value of the ISO term fundamental deviation. Axis of Thread: Thread axis is coincident with the axis of its pitch cylinder or cone. Basic Profile of Thread: The basic profile of a thread is the cyclical outline, in an axial plane, of the permanently established boundary between the provinces of the external and internal threads. All deviations are with respect to this boundary. Basic Size: The basic size is that size from which the limits of size are derived by the ap - plication of allowances and tolerances. Bilateral Tolerance: This is a tolerance in which variation is permitted in both directions from the specified dimension. Black Crest Thread: This is a thread whose crest displays an unfinished cast, rolled, or forged surface. Blunt Start Thread: “Blunt start” designates the removal of the incomplete thread at the starting end of the thread. This is a feature of threaded parts that are repeatedly assembled
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