An example of a pipe thread is shown here.
NPT = tapered pipe thread
1/4 = inside diameter of the pipe in inches
18 = threads per inch
Straight pipe threads are similar in form to
Figure 6-87.--Typical taper pipe thread.
tapered pipe threads except that they are not tapered.
The same nominal outside diameter and thread
dimensions apply. Straight pipe threads are used for
important. These threads require consistency that is
joining components mechanically and are not
available only through high quality production
s a t i s f a c t o r y for high-pressure applications.
methods combined with a very efficient system of
Sometimes a straight pipe thread is used with a
gauging and inspection.
tapered pipe thread to form a low-pressure seal in a
threads in which the external threaded member is
CLASSES OF THREADS
larger than the internally threaded member when both
members are in the free state and which, when
Classes of fit for threads are determined by the
assembled, become the same size and develop a
amount of tolerance and allowance allowed for each
holding torque through elastic compression, plastic
particular class. The tolerance (amount that a thread
movement of the material, or both. There are a
may vary from the basic dimension) decreases as the
number of different thread designations within class
class number increases. For example, a class 1 thread
5. They distinguish between external and internal
has more tolerance than a class 3 thread. The pitch
threads and the types of material the external thread
diameter of the thread is the most important thread
will be driven into. This information may be found in
element in controlling the class of fit. The major
diameter for an external thread and the minor
diameter or bore size for an internal thread are also
important, however, since they control the crest and
MEASURING SCREW THREADS
root clearances more than the actual fit of the thread.
Thread measurement is needed to ensure that the
A brief description of the different classes of fit
thread and its mating part will fit properly. It is
important that you know the various measuring
Classes 1A and 1B: Class 1A (external) and
methods and the calculations that are used to
class 1B (internal) threads are used where quick and
determine the dimensions of threads.
easy assembly is necessary and where a liberal
The use of a mating part to estimate and check the
allowance is required to permit ready assembly, even
needed thread is common practice when average
with slightly bruised or dirty threads.
accuracy is required. The thread is simply machined
Classes 2A and 2B: Class 2A (external) and
until the thread and the mating part will assemble. A
class 2B (internal) threads are the most commonly
snug fit is usually desired with very little, if any, play
used threads for general applications, including
between the parts.
production of bolts, screws, nuts, and similar threaded
You will sometimes be required to machine
threads that need a specific class of fit, or you may not
have the mating part to use as a gauge. In these cases,
Classes 3A and 3B: Class 3A (external) and
you must measure the thread to make sure you get the
class 3B (internal) threads are used where closeness
of fit and accuracy of lead and angle of thread are