Machinery Repairman

where
An example of a pipe thread is shown here.
NPT 1/4-18
NPT = tapered pipe thread
1/4 = inside diameter of the pipe in inches
18 = threads per inch
Figure 6-87 shows the typical taper pipe thread.
Straight pipe threads are similar in form to
tapered pipe threads except that they are not tapered.
The same nominal outside diameter and thread
dimensions apply. Straight pipe threads are used for
joining components mechanically and are not
satisfactory for high-pressure applications.
Sometimes a straight pipe thread is used with a
tapered pipe thread to form a low-pressure seal in a
vibration-free environment.
CLASSES OF THREADS
Classes of fit for threads are determined by the
amount of tolerance and allowance allowed for each
particular class. The tolerance (amount that a thread
may vary from the basic dimension) decreases as the
class number increases. For example, a class 1 thread
has more tolerance than a class 3 thread. The pitch
diameter of the thread is the most important thread
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
root clearances more than the actual fit of the thread.
A brief description of the different classes of fit
follows:
Classes 1A and 1B: Class 1A (external) and
class 1B (internal) threads are used where quick and
easy assembly is necessary and where a liberal
allowance is required to permit ready assembly, even
with slightly bruised or dirty threads.
Classes 2A and 2B: Class 2A (external) and
class 2B (internal) threads are the most commonly
used threads for general applications, including
production of bolts, screws, nuts, and similar threaded
fasteners.
Classes 3A and 3B: Class 3A (external) and
class 3B (internal) threads are used where closeness
of fit and accuracy of lead and angle of thread are
6-52
Figure 6-87.—Typical taper pipe thread.
important. These threads require consistency that is
available only through high quality production
methods combined with a very efficient system of
gauging and inspection.
Class 5: Class 5 threads are interference-fit
threads in which the external threaded member is
larger than the internally threaded member when both
members are in the free state and which, when
assembled, become the same size and develop a
holding torque through elastic compression, plastic
movement of the material, or both. There are a
number of different thread designations within class
5. They distinguish between external and internal
threads and the types of material the external thread
will be driven into. This information may be found in
the interference-fit threads section of Machinery’s
Handbook.
MEASURING SCREW THREADS
Thread measurement is needed to ensure that the
thread and its mating part will fit properly. It is
important that you know the various measuring
methods and the calculations that are used to
determine the dimensions of threads.
The use of a mating part to estimate and check the
needed thread is common practice when average
accuracy is required. The thread is simply machined
until the thread and the mating part will assemble. A
snug fit is usually desired with very little, if any, play
between the parts.
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,
you must measure the thread to make sure you get the
required fit.