Threads and Threading - Sportpilot.info

Machinery's Handbook 27th Edition
1784 METRIC SCREW THREADS M PROFILE
Fit: The relationship existing between two corresponding external and internal threads
with respect to the amount of clearance or interference which is present when they are
assembled.
Fundamental Deviation: For Standard threads, the deviation (upper or lower) closer to
the basic size. It is the upper deviation, es, for an external thread and the lower deviation,
EI, for an internal thread. (See Fig. 5.)
Limiting Profiles: The limiting M profile for internal threads is shown in Fig. 6. The limiting
M profile for external threads is shown in Fig. 7.
Lower Deviation: The algebraic difference between the minimum limit of size and the
corresponding basic size.
Nut Thread (Internal Thread): A term used in ISO metric thread standards to describe
all internal threads. All symbols associated with internal threads are designated with upper
case letters. This Standard uses the term internal thread in accordance with United States
practice.
Tolerance: The total amount of variation permitted for the size of a dimension. It is the
difference between the maximum limit of size and the minimum limit of size (i.e., the algebraic
difference between the upper deviation and the lower deviation). The tolerance is an
absolute value without sign. Tolerance for threads is applied to the design size in the direction
of the minimum material. On external threads the tolerance is applied negatively. On
internal threads the tolerance is applied positively.
Tolerance Class: The combination of a tolerance position with a tolerance grade. It
specifies the allowance (fundamental deviation) and tolerance for the pitch and major
diameters of external threads and pitch and minor diameters of internal threads.
Tolerance Grade: A numerical symbol that designates the tolerances of crest diameters
and pitch diameters applied to the design profiles.
Tolerance Position: A letter symbol that designates the position of the tolerance zone in
relation to the basic size. This position provides the allowance (fundamental deviation).
Upper Deviation: The algebraic difference between the maximum limit of size and the
corresponding basic size.
Basic M Profile.—The basic M thread profile also known as ISO 68 basic profile for metric
screw threads is shown in Fig. 1 with associated dimensions listed in Table 3.
Design M Profile for Internal Thread.—The design M profile for the internal thread at
maximum material condition is the basic ISO 68 profile. It is shown in Fig. 2 with associated
thread data listed in Table 3.
Design M Profile for External Thread.—The design M profile for the external thread at
the no allowance maximum material condition is the basic ISO 68 profile except where a
rounded root is required. For the standard 0.125P minimum radius, the ISO 68 profile is
modified at the root with a 0.17783H truncation blending into two arcs with radii of 0.125P
tangent to the thread flanks as shown in Fig. 3 with associated thread data in Table 3.
M Crest and Root Form.—The form of crest at the major diameter of the external thread
is flat, permitting corner rounding. The external thread is truncated 0.125H from a sharp
crest. The form of the crest at the minor diameter of the internal thread is flat. It is truncated
0.25H from a sharp crest.
The crest and root tolerance zones at the major and minor diameters will permit rounded
crest and root forms in both external and internal threads.
The root profile of the external thread must lie within the “section lined” tolerance zone
shown in Fig. 4. For the rounded root thread, the root profile must lie within the “section
lined” rounded root tolerance zone shown in Fig. 4. The profile must be a continuous,
smoothly blended non-reversing curve, no part of which has a radius of less than 0.125P,
and which is tangential to the thread flank. The profile may comprise tangent flank arcs
that are joined by a tangential flat at the root.
Copyright 2004, Industrial Press, Inc., New York, NY