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By Wes Emerson
Taper pipe threads
are not like standard machine threads in the way they are used or
in the way they are made.
One of the more challenging
operations in metalcutting is taper pipe thread tapping. Those familiar
with the process consider it to be more difficult than machine thread
tapping. Taper pipe thread tapping requires greater accuracy, and
it subjects the tools to higher stresses. Pipe threads must be formed
more accurately, because 100% of the thread height must be cut by
the tap or other pipe-threading tool to maintain the standard thread
profile.
Machinists must use
special techniques to produce taper pipe threads, because the threads
must serve an entirely different purpose than machine threads or
nuts and bolts. Pipe threads are designed and cut so that they can
mechanically seal a threaded joint for pressure and prevent leakage.
The tapered thread permits pipes to be disassembled and reassembled
frequently. And it lets users assemble pipes with a new thread to
an old thread by just wrenching up tighter and relying on the taper
to increase fit tightness.
Pipe Thread Varieties
The taper pipe thread has been serving its purpose well for many
years. When the Briggs Standard taper pipe thread was developed
160 years ago, it made a substantial contribution to the improvement
of the steam engine. The basic dimensions for the thread remain
unchanged in today's standards. The rate of taper was and still
is 3/4" per foot (or 0.0625" per inch).
There are other pipe
threads designed for gas and hydraulic applications, hose couplings,
mechanical joints, and other purposes. These threads were developed
at later dates and their features differ from standard taper pipe
threads. Table 1 lists the various pipe threads, their designation
symbols, and uses. Some are straight threads and some are tapered.
Generally, the following recommendations for producing NPT-type
taper pipe threads apply to the other types of tapered threads as
well.
The regular NPT system,
in which both threaded parts have a taper thread, is generally used
in normal pressure fittings. Usually, some kind of chemical sealer
or tape is used to ensure a tight fit.
The NPTF, or dryseal,
system is similar to NPT, but the machinist must adhere to tighter
tolerances when cutting NPTF threads. An accurately cut thread profile
is necessary to ensure that when the connection between the threaded
parts is tightened the thread crest and root contact before the
thread flanks do so that contact is established along the entire
thread profile when the parts are assembled.
Full contact allows
the threads to serve as a mechanical seal and eliminates the need
for sealing material between the threads. Metal-to-metal contact
is desirable whenever contact between a sealing compound and the
fluid being carried in the pipe might cause problems. The performance
of substances such as some petroleum-base products and hydraulic
fluids can be compromised when contaminated by a sealing compound.
Conversely, the performance of the sealing compound may be compromised
by contact with the fluid carried in the pipe.
There are other threading
systems in addition to NPT and NPTF. ANPT, for instance, is basically
an improved version of NPT for aerospace applications that require
a higher level of gaging. The NPSI system, commonly used in automotive
brake systems and heater-hose connections, features a straight dryseal
thread as the internal thread mated to a tapered dryseal external
thread.
Taper Tapping Techniques
Machinists who are familiar with standard tapping will require some
re-education to produce taper pipe threads. Even the designations
for taper pipe thread taps are different. A machinist looking at
a 1/2-14 pipe tap is likely to note that the tap diameter is larger
than 1/2". In fact, the tap's diameter at the large
end is 0.865". The tap's designation refers to the inner diameter
(ID) of the standard iron pipe it is designed to thread. Therefore,
a 1/2-14 taper pipe thread tap is designed to tap a 1/2"
ID pipe. Keep in mind, the external thread on a pipe with a 1/2"
ID is at least the size of the ID plus the amount of two wall thicknesses.
An internally threaded coupling, or elbow, must be of comparable
size to assemble the threaded pipe.
Another difference between
taper pipe thread tapping and machine thread tapping is in the way
the machinist controls the thread diameter. Using a taper pipe thread
tap, the machinist can reduce or increase the thread diameter by
adjusting the depth that the tap cuts into the hole. To achieve
the basic thread depth, a machinist typically drives the tap into
the workpiece 12 turns. More turns will drive the tap deeper into
the workpiece and produce a larger diameter thread. Fewer turns
will produce a smaller diameter thread. There are no H-limits for
the taper pipe tap sizes, and there is no particular thread-product
class of fit, such as 2A/2B. The application, with proper gaging,
will determine the appropriate depth for the proper mating of the
parts.
Gaging Taps and Threads
Evaluating, or gaging, taper
pipe tap threads and actual pipe thread size is accomplished by the
use of taper thread gages (Figure 1). The L-1 ring gage is used to
check the external pipe thread size and is also used to measure the
pipe tap size.
Machinists gage taps by
measuring their "projection." The term refers to the amount
the tap projects through the L-1 ring gage, or the distance from the
extreme thread end of the tap to the front side of the gage (Figure
2). If a machinist knows this distance, he will also know the tap's
diameter, including its major, pitch, and minor diameters. Each taper
pipe tap size has a corresponding "standard projection"
value, as set by the industry.
When gaging a taper
pipe tap, the tap's diameter is measured at the gage line. This
point is where the backside of the L-1 ring gage touches the tap.
Therefore, the critical measurement that will determine the tap's
diameter is actually from the backside of the gage to the extreme
thread end. This measurement is called the run, and it is rather
difficult to measure. Machinists simplify this measurement by subtracting
the thickness of the ring gage from the specified run and using
this value to determine the proper projection from the front side
of the gage.
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| Figure 2: A pipe tap's projection is the
amount it extends beyond the front of a ring gage. For special
applications this projection may be shortened while the rest
of the tap's geometry remains the same. (Click
for larger image) |
In some applications the machinist may encounter a hole with a
restriction at the bottom that does not allow a standard taper pipe
tap to be driven to the depth needed to achieve the specified thread
diameter. In these cases the machinist will have to use a tap with
a nonstandard projection that will produce the correct size pipe
thread in fewer than 12 turns. This is commonly referred to as a
"short projection" pipe tap. It is basically a standard
pipe tap manufactured with a projection shorter than the industry
standard value (Figure 2). These taps can be ordered as specials.
Machinists use one or more tapered thread plug gages to evaluate
tapped taper pipe holes. An L-1 plug gage would be used to evaluate
an NPT threaded hole, for example. When a plug gage is inserted
into a properly threaded hole, a step, or notch, on the gage will
line up with the surface of the workpiece, plus or minus one thread.
If the thread size of the first hole is wrong, the machinist can
correct the problem on subsequent holes by increase the tapping
depth to enlarge the thread size or decreasing the depth to reduce
the size.
Machinists have adopted a quick way to monitor the depth of their
tapping. They wind a piece of wire or tape around the tap at the
gage line to serve as a depth indicator. This common practice is
a neat trick, but it severely restricts chip ejection and the flow
of coolant into the hole. Because this practice can lead to torn
threads and premature tap failure, it should not be done. A safer
alternative is to use a thread gage frequently to ensure the correct
tapping depth and thread size.
Machinists who are used to tapping threads with a 60% to 75% thread
height will find that cutting threads with a 100% thread height
presents a unique challenge. Actually, the taper pipe tap has to
cut more than 100% of the thread to ensure a fully profiled thread
when done. The taper makes the tapping operation even more difficult,
because it causes the tap to be extremely tight in the hole when
the tap has been driven to the required depth. The tap becomes wedged
at this point, and considerable force is required to break it loose
to reverse it.
Once the tap is broken loose, it becomes entirely loose in about
1/8 of a turn. This play presents a hazard when the tap is
reversed out of the hole. With only a little overfeeding or underfeeding,
the tap can wipe out the threads as it is backed out. For this reason,
lead control is highly critical in taper pipe thread tapping.
Reaming Risks
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| Table 2: The correct diameter at the wider
end of a taper reamed hole for various size taps. (Click
for larger image) |
One practice that is
frequently recommended for taper pipe tapping may not be necessary.
Many in the industry believe that holes should be reamed with a
taper pipe reamer before tapping. However, this reaming operation
can cause the tap to cut unacceptable threads, and it shortens the
tap life if the reaming depth is not closely controlled. Table 2
shows what the diameter should be at the large end of the taper
reamed hole prior to tapping. A hole reamed to this size is equal
to the minor diameter of the threads when the hole is tapped. Reaming
an oversize hole will result in insufficient thread height. If the
reamed hole is too small, it will subject the tap to an extreme
metal-cutting load.
Even a correctly reamed
hole will subject the tap to some extreme forces at the cutting
points, or thread crests, because all the teeth start cutting at
once. Starting at these points, the metal-cutting action is inward
toward the tap's cross-sectional center. When a tap cuts threads
in a straight-walled, nonreamed hole, the force progresses through
the chamfered threads and then through one thread at a time as the
tap rotates forward.
Unless there are compelling reasons for taper pipe reaming, it is
best to adjust the drilled holes so that they can be tapped without
reaming. In some cases-such as in a high-pressure assembly where
four or more fully profiled, or "perfect," threads are
required- reaming will be called for. But in most cases, tapping
a nonreamed hole will produce enough "perfect" threads
to satisfy thread sealing and holding requirements. In other words,
taper pipe reaming should not be considered standard operating procedure.
Styles and Hooks
When ordering pipe taps, it is particularly important to consider
and specify the type of material being tapped. The tap's style will
have a significant impact on thread quality and tap performance.
Styles such as full-thread, interrupted-thread, and spiral-flute
are available for specific applications.
Interrupted-thread pipe
taps are recommended for tapping hard or tough materials. If premature
chipping and breakage is a problem, an interrupted thread pipe tap
may perform better, because surface contact is reduced to help relieve
tapping torque and improve lubrication. The interrupted-thread tap
may not last as long because it has fewer cutting points, but where
taps are failing due to chipping rather than wear, wear resistance
is not of primary importance. Also, the reduced surface contact
of an interrupted-thread style might help prevent torn threads.
Threads can tear when the tap becomes stuck in the hole once it
stops rotating at the end of its forward motion. When the tap is
reversed, the material stuck to it is ripped away. By resisting
the material's tendency to stick to the tap threads, an interrupted
thread can be reversed without damaging the threads. Interrupted
threads are available in standard NPT and NPTF thread forms. The
style can be ordered in a variety of other pipe tap styles as well.
A tap's hook, surface
treatment, and coating also will influence its performance. Taps
are available with low (0°-3°), medium (8°-12°),
or high (15°-18°) hooks. There are a wide variety of
surface treatments and coatings available today to help extend tap
life, improve thread finish, and reduce tapping torque.
As an alternative to
tapping pipe threads, a shop might produce the thread by roll forming.
However, a shop should consider both the advantages and disadvantages
of roll forming before making such a choice. Not all manufacturers
supply this type of threadmaking tool. In use, thread-rolling tools
must be kept under tighter control than pipe thread taps. In thread
rolling, the hole size is very critical to the production of a 100%
thread height. The holder used for thread rolling should have a
torque-limit feature. Caution should be used when thread rolling
die-cast aluminum applications, as the material becomes brittle
when cold formed and may fracture. If a shop is considering thread
rolling, shop engineers should consult a tap application specialist
first.
It's true that taper
pipe thread tapping is a difficult operation full of risks and difficulties
not found in other types of tapping. But that doesn't mean it's
impossible to produce good quality threads. In general, all a machinist
needs to tap tapered threads successfully is a basic understanding
of the product requirements, the proper tap, and the correct gaging
process.
About the
Author
Wes Emerson is a technical specialist with Vermont Tap &
Die, Lyndonville, VT.
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