Figure 6-81.--Screw thread nomenclature.
NOMINAL SIZE: The size that is used for
SINGLE THREAD: A single (single start) thread
identification. For example, the nominal size of a
whose lead equals the pitch.
1/2-20 thread is 1/2 inch, but its actual size is slightly
MULTIPLE THREAD: A multiple (multiple
smaller to provide clearance.
start) thread whose lead equals the pitch multiplied by
ACTUAL SIZE: The measured size.
the number of starts.
BASIC SIZE: The theoretical size. The basic
CLASS OF THREADS: A group of threads
size is changed to provide the desired clearance or fit.
designed for a certain type of fit. Classes of threads
are distinguished from each other by the amount of
MAJOR DIAMETER: The diameter of an
tolerance and allowance specified.
imaginary cylinder that passes through the crests of an
external thread or the roots of an internal thread.
THREAD FORM: The view of a thread along
MINOR DIAMETER: The diameter of an
the thread axis for a length of one pitch.
imaginary cylinder that passes through the roots of an
FLANK: The side of the thread.
external thread or the crests of an internal thread.
CREST: The top of the thread (bounded by the
HEIGHT OF THREAD: The distance from the
major diameter on external threads; by the minor
crest to the root of a thread measured along a
diameter on internal threads).
perpendicular to the axis of the threaded piece (also
called straight depth of thread).
ROOT: The bottom of the thread (bounded by
SLANT DEPTH: The distance from the crest to
the minor diameter on external threads; by the major
the root of a thread measured along the angle forming
diameter on internal threads).
the side of the thread.
THREAD ANGLE: The angle formed by
ALLOWANCE: An intentional difference
adjacent flanks of a thread.
between the maximum material limits of mating parts.
It is the minimum clearance (positive allowance) or
PITCH DIAMETER: The diameter of an
imaginary cylinder that is concentric with the thread
such parts.
axis and whose periphery passes through the thread
profile at the point where the widths of the thread and
TOLERANCE: The total permissible variation
the thread groove are equal. The pitch diameter is the
of a size. The tolerance is the difference between the
diameter that is measured when the thread is
limits of size.
machined to size. A change in pitch diameter changes
THREAD FORM SERIES: Threads are made
the fit between the thread being machined and the
in many different shapes, sizes, and accuracies.
mating thread.
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