Threads and Threading - Sportpilot.info

METRIC SCREW THREADS M PROFILE 1803
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 B1.3M 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.
Example: 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.
Example: 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.
Examples: 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
Machinery's Handbook 27th Edition
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 accordance
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.
Copyright 2004, Industrial Press, Inc., New York, NY