Not to be confused with Tool and die. Taps and dies are cutting tools used to create
screw threads, which is called threading. A tap is used to cut the female portion of
the mating pair. A die is used to cut the male portion of the mating pair. The process
of cutting threads using a tap is called tapping, whereas the process using a die is called
threading. Both tools can be used to clean up a thread, which is called chasing.
History While modern nuts and bolts are routinely
made of metal, this was not the case in earlier ages, when woodworking tools were employed
to fashion very large wooden bolts and nuts for use in winches, windmills, watermills,
and flour mills of the Middle Ages; the ease of cutting and replacing wooden parts was
balanced by the need to resist large amounts of torque, and bear up against ever heavier
loads of weight. As the loads grew ever heavier, bigger and stronger bolts were needed to resist
breakage. Some nuts and bolts were measured by the foot or yard. This development eventually
led to a complete replacement of wood parts with metal parts of an identical measure.
When a wooden part broke, it usually snapped, ripped, or tore. The splinters having been
sanded off, the remaining parts were encased in a makeshift mold of clay, and molten metal
poured into the mold, so that an identical replacement could be made on the spot.
Metalworking taps and dies were often made by their users during the 18th and 19th centuries,
using such tools as lathes and files for the shaping, and the smithy for hardening and
tempering. Thus builders of, for example, locomotives, firearms, or textile machinery
were likely to make their own taps and dies. During the 19th century the machining industries
evolved greatly, and the practice of buying taps and dies from suppliers specializing
in them gradually supplanted most such in-house work. Joseph Clement was one such early vendor
of taps and dies, starting in 1828. With the introduction of more advanced milling practice
in the 1860s and 1870s, tasks such as cutting a tap's flutes with a hand file became a thing
of the past. In the early 20th century, thread-grinding practice went through significant evolution,
further advancing the state of the art of cutting screw threads, including those of
taps and dies. During the 19th and 20th centuries, thread
standardization was evolving simultaneously with the techniques of thread generation,
including taps and dies. The largest tap and die company to exist in
the United States was Greenfield Tap & Die of Greenfield, Massachusetts. GTD was so irreplaceably
vital to the Allied war effort from 1940–1945 that anti-aircraft guns were placed around
its campus in anticipation of possible Axis air attack. The GTD brand is now a part of
Widia Products Group. Tap
A tap cuts a thread on the inside surface of a hole, creating a female surface which
functions like a nut. The three taps in the image illustrate the basic types commonly
used by most machinists: Bottoming tap or plug tap
The tap illustrated in the top of the image has a continuous cutting edge with almost
no taper — between 1 and 1.5 threads of taper is typical. This feature enables a bottoming
tap to cut threads to the bottom of a blind hole. A bottoming tap is usually used to cut
threads in a hole that has already been partially threaded using one of the more tapered types
of tap; the tapered end of a bottoming tap is too short to successfully start into an
unthreaded hole. In the US, they are commonly known as bottoming taps, but in Australia
and Britain they are also known as plug taps. Intermediate tap, second tap, or plug tap
The tap illustrated in the middle of the image has tapered cutting edges, which assist in
aligning and starting the tap into an untapped hole. The number of tapered threads typically
ranges from 3 to 5. Plug taps are the most commonly used type of tap. In the US, they
are commonly known as plug taps, whereas in Australia and Britain they are commonly known
as second taps. Taper tap
The small tap illustrated at the bottom of the image is similar to an intermediate tap
but has a more pronounced taper to the cutting edges. This feature gives the taper tap a
very gradual cutting action that is less aggressive than that of the plug tap. The number of tapered
threads typically ranges from 8 to 10. A taper tap is most often used when the material to
be tapped is difficult to work or the tap is of a very small diameter and thus prone
to breakage. The above illustrated taps are generally referred
to as hand taps, since they are, by design, intended to be manually operated. During operation,
it is necessary with a hand tap to periodically reverse rotation to break the chip formed
during the cutting process, thus preventing an effect called "crowding" that may cause
breakage. The most common type of power driven tap is
the "spiral point" plug tap, whose cutting edges are angularly displaced relative to
the tap centerline. This feature causes the tap to continuously break the chip and eject
it forward into the hole, preventing crowding. Spiral point taps are usually used in holes
that go all the way through the material, so that the chips can escape. Another version
of the spiral point plug tap is the spiral flute tap, whose flutes resemble those of
a twist drill. Spiral flute taps are widely used in high speed, automatic tapping operations
due to their ability to work well in blind holes.
Whether manual or automatic, the processing of tapping begins with forming and slightly
countersinking a hole to a diameter somewhat smaller than the tap's major diameter. The
correct hole diameter may be determined by consulting a drill and tap size chart, a standard
reference item found in many machine shops. If the hole is to be drilled, the proper diameter
is called the tap drill size. In lieu of a tap drill chart, it is possible
with inch-sized taps to compute the correct tap drill diameter as follows:
where is the tap drill size, is the major diameter of the tap, and is the thread pitch.
For a ⅜"-16 tap, the above formula would produce 5⁄16 as a result, which is the correct
tap drill diameter for a ⅜"-16 tap. The above formula ultimately results in an approximate
75 percent thread. The correct tap drill diameter for metric-sized
taps is computed as:
where is the tap drill size, is the major diameter of the tap, and pitch is the pitch
of the thread and so the correct drill size is 8.5 mm. This works for both fine and coarse
pitches, and also produces an approximate 75 percent thread.
With soft or average hardness materials, such as plastic, aluminum or mild steel, the common
practice is to use an intermediate tap to cut the threads. If the threads are to extend
to the bottom of a blind hole, the intermediate tap will be used to cut threads until the
point of the tap reaches bottom, after which a bottoming tap will be used to finish the
hole. Frequent ejection of the chips must be made in such an operation to avoid jamming
and possibly breaking the tap. With hard materials, the machinist may start with a taper tap,
whose less severe diameter transition reduces the amount of torque required to cut the threads.
If threads are to be cut to the bottom of a blind hole, the taper tap will be followed
by an intermediate tap and then a bottoming tap to finish the operation.
Machine tapping Tapping may either be achieved by hand tapping
by using a set of taps first tap, second tap & final tap or using a machine to do the tapping,
such as a lathe, radial drilling machine, bench type drill machine, pillar type drill
machine, vertical milling machines, HMCs, VMCs. Machine tapping is faster, and generally
more accurate because human error is eliminated. Final tapping is achieved with single tap.
Although in general machine tapping is more accurate, tapping operations have traditionally
been very tricky to execute due to frequent tap breakage and inconsistent quality of tapping.
Common reasons for tap breakage are: Tap-related problems:
Wearing of tap cannot be easily quantified Use of tap with improper tap geometry for
a particular application. Use of non-standard or inferior quality taps.
Clogging with chips. Misalignment between tap and hole.
Over- or under-feeding the tap, causing breakage in tension or compression.
Use of improper and/or insufficient cutting lubricant.
Absence of a torque limiting feature. Improper or zero float for use with screw
machines Improper spindle speed.
In order to overcome these problems, special tool holders are required to minimize the
chances of tap breakage during tapping. These are usually classified as conventional tool
holders and CNC tool holders. Tool holders for tapping operations
Various tool holders may be used for tapping depending on the requirements of the user:
Aids for hand-tapping The biggest problem with simple hand-tapping
is accurately aligning the tap with the hole so that they are coaxial—in other words,
going in straight instead of on an angle. The operator must get this alignment rather
close to ideal in order to produce good threads and avoid tap breakage. The deeper the depth
of thread, the more pronounced the effect of the angular error becomes. With a depth
of 1 or 2 diameters, it matters little. With depths beyond 2 diameters, the error becomes
too pronounced to ignore. Another fact about this alignment task is that the first thread
or two that is cut establishes the direction that the rest of the threads will follow.
In other words, you can't make corrections to the angle once you have cut the first thread
or two. To help with this alignment task, several
kinds of jigs and fixtures can be used to provide the correct geometry without having
to use freehand skill to approximate it: Hand-tapper: A simple fixture analogous to
an arbor press in its basic shape. Its spindle is thus held accurately perpendicular to the
work. Standard taps are held in the spindle, and the operator turns the spindle manually
via a handlebar. This fixture obviates the need for the operator to carefully and skillfully
approximate perpendicularity, which even for a skilled operator can easily result in a
2–5° error. Tapping guide, or "tap and reamer aligner/holder",
a simple conical guide slipped over a tap when using a regular tap handle. As with a
hand-tapper, the basic principle is simply that of a jig or fixture to provide the correct
alignment. Heads for machine tool spindles
Tapping attachments: these may be normal or quick-change
Quick-change drilling and tapping chucks Rigid tapping attachments
Generally the following features are required of tapping holders:
Twin chucking: tap is held at points of both its circular and square cross-section. Gripping
the circular section assures concentricity to the machine spindle, and gripping the square
produces positive rotational drive. Safety clutch: The built in safety mechanism
operates as soon as the set torque limit is attained to save the tap from breakage.
Float radial parallel: small misalignments are taken care of by this float.
Length compensation: built in length compensation takes care of small push or pull to the spindle
or feed difference. Tapping case studies with typical examples
of tapping operations in various environments are shown on source machinetoolaid.com [1]
Tapping stations Tapping stations are worktables with a tapping
head attached to the end of a pantograph-style arm similar to that of a balanced-arm lamp.
The operator guides the tapping head to each hole and quickly taps it.
Drilling and tapping centers, whose name sounds similar to that of tapping stations, are actually
light-duty, affordable machining centers of 2, 2.5, or 3 axes that are designed for a
life of mainly drilling and tapping with limited milling use.
Tap drill sizes
Die
The die cuts a thread on a preformed cylindrical rod, which creates a male threaded piece which
functions like a bolt. The dies shown are top left: an older split die, with top adjusting
screw bottom left: a one piece die with top adjusting
screw center: a one piece die with side adjusting
screw right: two dies with side adjusting screws
A cylindrical blank, which is usually slightly less than the required diameter, is machined
with a taper at the threaded end. This chamfer allows the die to ease onto the blank before
it cuts a sufficient thread to pull itself along.
The adjusting screws allow the die to be compressed or expanded to accommodate slight variations
in size, due to material, manufacture, or die sharpness.
Each tool is used independently, but are usually sold in paired sets of both types, one die
and three taps. Some sets may provide a lesser number of taps. The common sets shown are
designed for hand operation, but different types such as helical or spiral may be used
in production tools such as CNC machining tools, which employ die heads to make large
volumes of threaded parts. Die nuts, also known as rethreading dies,
are dies made for cleaning up damaged threads, have no split for resizing and are made from
a hexagonal bar so that a wrench or shifter spanner can be used to turn them. Die nuts
cannot be used to cut new threads. Lubricants
The use of a suitable lubricant is essential with most tapping and reaming operations.
Recommended lubricants for some common materials are as follows:
Carbon steel Petroleum-based or synthetic cutting oil.
Alloy steel Petroleum-based cutting oil mixed with a small
amount of kerosene or mineral spirits. This mixture is also suitable for use with stainless
steel. Cast iron
No lubricant. An air blast should be used to clear chips.
Aluminum Kerosene or mineral spirits mixed with a small
amount of petroleum-based cutting oil. In some cases, products such as WD-40, CRC 5-56
and 3-In-One Oil are acceptable substitutes. Brass
Kerosene or mineral spirits. Bronze
Kerosene or mineral spirits mixed with a small amount of petroleum-based cutting oil.
References
Bibliography Degarmo, E. Paul; Black, J T.; Kohser, Ronald
A.. Materials and Processes in Manufacturing. Wiley. ISBN 0-471-65653-4.
Roe, Joseph Wickham, English and American Tool Builders, New Haven, Connecticut: Yale
University Press, LCCN 16011753 . Reprinted by McGraw-Hill, New York and London, 1926;
and by Lindsay Publications, Inc., Bradley, Illinois,.
External links Photo of a hand-tapper