Threads and Threading - Sportpilot.info

Machinery's Handbook 27th Edition
METRIC SCREW THREADS M PROFILE 1783
METRIC SCREW THREADS
American National Standard Metric Screw Threads M Profile
American National Standard ANSI/ASME B1.13M-1983 (R1995) describes a system of
metric threads for general fastening purposes in mechanisms and structures. The standard
is in basic agreement with ISO screw standards and resolutions, as of the date of publication,
and features detailed information for diameter-pitch combinations selected as to preferred
standard sizes. This Standard contains general metric standards for a 60-degree
symmetrical screw thread with a basic ISO 68 designated profile.
Application Comparison with Inch Threads.—The metric M profile threads of tolerance
class 6H/6g (see page 1790) are intended for metric applications where the inch class
2A/2B have been used. At the minimum material limits, the 6H/6g results in a looser fit
than the 2A/2B. Tabular data are also provided for a tighter tolerance fit external thread of
class 4g6g which is approximately equivalent to the inch class 3A but with an allowance
applied. It may be noted that a 4H5H/4h6h fit is approximately equivalent to class 3A/3B
fit in the inch system.
Interchangeability with Other System Threads.—Threads produced to this Standard
ANSI/ASME B1.13M are fully interchangeable with threads conforming to other
National Standards that are based on ISO 68 basic profile and ISO 965/1 tolerance practices.
Threads produced to this Standard should be mechanically interchangeable with those
produced to ANSI B1.18M-1982 (R1987) “Metric Screw Threads for Commercial
Mechanical Fasteners—Boundary Profile Defined,” of the same size and tolerance class.
However, there is a possibility that some parts may be accepted by conventional gages
used for threads made to ANSI/ASME B1.13M and rejected by the Double-NOT-GO
gages required for threads made to ANSI B1.18M.
Threads produced in accordance with M profile and MJ profile ANSI/ASME B1.21M
design data will assemble with each other. However, external MJ threads will encounter
interference on the root radii with internal M thread crests when both threads are at maximum
material condition.
Definitions.—The following definitions apply to metric screw threads — M profile.
Allowance: The minimum nominal clearance between a prescribed dimension and its
basic dimension. Allowance is not an ISO metric screw thread term but it is numerically
equal to the absolute value of the ISO term fundamental deviation.
Basic Thread Profile: 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. (See Figs. 1 and 5.)
Bolt Thread (External Thread): The term used in ISO metric thread standards to
describe all external threads. All symbols associated with external threads are designated
with lower case letters. This Standard uses the term external threads in accordance with
United States practice.
Clearance: The difference between the size of the internal thread and the size of the
external thread when the latter is smaller.
Crest Diameter: The major diameter of an external thread and the minor diameter of an
internal thread.
Design Profiles: The maximum material profiles permitted for external and internal
threads for a specified tolerance class. (See Figs. 2 and 3.)
Deviation: An ISO term for the algebraic difference between a given size (actual, measured,
maximum, minimum, etc.) and the corresponding basic size. The term deviation
does not necessarily indicate an error.
Copyright 2004, Industrial Press, Inc., New York, NY