Threads and Threading - Sportpilot.info

TRAPEZOIDAL METRIC THREADS 1807
Tolerances: The thread tolerance system is based on ISO 965/1, Metric Screw thread
System of Tolerance Positions and Grades. Tolerances are positive for internal threads and
negative for external threads, that is, in the direction of minimum material.
For aerospace applications, except for fluid fittings, tolerance classes 4H5H or 4G6G
and 4g6g should be used. These classes approximate classes 3B/3A in the inch system.
Aerospace fluid fittings use classes 4H5H or 4H6H and 4g6g.
Tolerance classes 4G5G or 4G6G and 4g6g are provided for use when thread allowances
are required. These classes provide a slightly tighter fit than the inch classes 2B/2A at minimum
material condition.
Additional tolerance classes 6H/6g are included in this Standard to provide appropriate
product selection based on general applications. These classes and the selection of standard
diameter/pitch combinations are the same as those provided for the M profile metric
screw threads in ANSI/ASME B1.13M. Classes 6H/6g result in a slightly looser fit than
inch classes 2B/2A at minimum material condition.
Symbols: Standard symbols appearing in Fig. 1 are:
D=Basic major diameter of internal thread
D 2 =Basic pitch diameter of internal thread
D 1 =Basic minor diameter of internal thread
d=Basic major diameter of external thread
d 2 =Basic pitch diameter of external thread
d 1 =Basic minor diameter of internal thread
d 3 =Diameter to bottom of external thread root radius
H=Height of fundamental triangle
P=Pitch
Basic Designations: The aerospace metric screw thread is designated by the letters “MJ”
to identify the metric J thread form, followed by the nominal size and pitch in millimeters
(separated by the sign “×”) and followed by the tolerance class (separated by a dash from
the pitch). Unless otherwise specified in the designation, the thread helix is right hand.
Example:MJ6 × 1 − 4h6h
Machinery's Handbook 27th Edition
For further details concerning limiting dimensions, allowances for coating and plating,
modified and special threads, etc., reference should be made to the Standard.
Trapezoidal Metric Thread
Comparison of ISO and DIN Standards.—ISO metric trapezoidal screw threads standard,
ISO 2904-1977, describes the system of general purpose metric threads for use in
mechanisms and structures. The standard is in basic agreement with trapezoidal metric
thread DIN 103. The DIN 103 standard applies a particular pitch for a particular diameter
of thread, but the ISO standard applies a variety of pitchs for a particular diameter. In ISO
2904-1977, the same clearance is applied to both the major diameter and minor diameter,
but in DIN 103 the clearance in the minor diameter is two or three times greater than clearance
in the major diameter. A comparison of ISO 2904 and DIN 103 is given in Table 1.
Copyright 2004, Industrial Press, Inc., New York, NY