Machinery's Handbook, 31st Edition
Metric Screw Threads M Profile 2033 Metric Screw Thread Designations.— Metric screw threads are identified by the letter (M) for the thread form profile, followed by the nominal diameter size and the pitch expressed in millimeters, separated by the sign ( × ) and followed by the tolerance class separated by a dash ( − ) from the pitch. The simplified international practice for designating coarse pitch M profile metric screw threads is to leave off the pitch. Thus a M14 × 2 thread is designated just M14. However, to prevent misunderstanding, it is mandatory to use the value for pitch in all designations. Thread acceptability gaging system requirements of ANSI/ASME B1.3 may be added to the thread size designation as noted in the examples (numbers in parentheses) or as specified in pertinent documentation, such as the drawing or procurement document. Unless otherwise specified in the designation, the screw thread is right hand. Examples: External thread of M profile, right hand: M6 × 1 − 4g6g (22) Internal thread of M profile, right hand: M6 × 1 − 5H6H (21) Designation of Left Hand Thread: When a left hand thread is specified, the tolerance class designation is followed by a dash and LH. M6 × 1 − 5H6H − LH (23) Designation for Identical Tolerance Classes: If the two tolerance class designations for a thread are identical, it is not necessary to repeat the symbols. M6 × 1 − 6H (21) Designation Using All Capital Letters: When computer and teletype thread designations use all capital letters, the external or internal thread may need further identification. Thus the tolerance class is followed by the abbreviations EXT or INT in capital letters. Example: M6 × 1 − 4G6G EXT; M6 × 1 − 6H INT Designation for Thread Fit: A fit between mating threads is indicated by the internal thread tolerance class followed by the external thread tolerance class and separated by a slash. Examples: M6 × 1 − 6H/6g; M6 × 1 − 6H/4g6g Designation for Rounded Root External Thread: The M profile with a minimum root radius of 0.125P on the external thread is desirable for all threads but is mandatory for threaded mechanical fasteners of ISO 898/I property class 8.8 (minimum tensile strength 800 MPa) and stronger. No special designation is required for these threads. Other parts requiring a 0.125P root radius must have that radius specified. When a special rounded root is required, its external thread designation is suffixed by the minimum root radius value in millimeters and the letter R. Example: M42 × 4.5 − 6g − 0.63R Designation of Threads Having Modified Crests: Where the limits of size of the major diameter of an external thread or the minor diameter of an internal thread are modified, the thread designation is suffixed by the letters MOD followed by the modified diameter limits. Examples: External thread M profile, major diameter reduced 0.075 mm. M6 × 1 − 4h6h MOD Major dia = 5.745 − 5.925 MOD Internal thread M profile, minor diameter increased 0.075 mm. M6 × 1 − 4H5H MOD Minor dia = 5.101 − 5.291 MOD Designation of Special Threads: Special diameter-pitch threads developed in accor dance with this Standard ANSI/ASME B1.13M are identified by the letters SPL following the tolerance class. The limits of size for the major diameter, pitch diameter, and minor diameter are specified below this designation.
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