Taps and dies are
cutting tools used when
creating screw threads. A tap is used to cut the female portion of the mating pair (e.g. a nut). A die is used to cut the male portion of the mating pair (e.g. a bolt). The process of cutting the threads in a hole is called "tapping" the hole. The process of cutting with a die is called "threading" (or sometimes "chasing", although "chasing" is most commonly used when threads are already present but need to be "cleaned up").
Tap
Bottoming, plug and taper taps, from top to bottom, respectively.
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 ("tap chamfer") 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 a plug 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 (e.g., alloy steel) 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. Periodic reversing is usually not practical when power tapping is involved, and thus has led to the development of taps suitable for continuous rotation in the cutting direction.
The most common type of power driven tap is the "spiral point" plug tap (also referred to as a "gun 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 into the flutes, preventing crowding. 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 (usually by drilling) with a diameter slightly 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
TD is the tap drill size,
MD is the major diameter of the tap (e.g., ⅜ inch for a ⅜"-16 tap), and
N is the number of threads per inch (16 in the case of a ⅜"-16 tap). 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 result produces a tap drill size that results in an approximate 75 percent thread (recommended for most applications).
With soft or average hardness materials, such as
plastic,
aluminum or
carbon steel, the common practice is to use a plug tap to cut the threads. If the threads are to extend to the bottom of a blind hole, the plug 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 (plug) tap and then a bottoming tap to finish the operation.
In metal working, the use of a tap
lubricant is essential to achieve cleanly formed threads and to minimize friction. Failure to use the correct lubricant may result in ragged threads, as well as a substantial increase in the amount of torque required to turn the tap, possibly resulting in breakage.
Machine tapping
Tapping is essentially the internal threading of a hole. This may either be achieved by hand tapping by using a set of taps first tap, second tap & final (finish) 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, tappin
g operations have traditionally been very tricky to execute due to frequent tap breakage and inconsistent quality of tapping.
Research has shown that the important reasons causing tap breakage are as follows:
- Tap-related problems:
- Wearing of tap cannot be easily quantified (use of worn-out taps)
- Use of tap with improper tap geometry for a particular application.
- Use of non-standard or inferior quality taps.
- Clogging with chips
- Tapping does not follow the pre-tap hole (misalignment)
- Mismatch of machine feed and tap feed may cause the tap to break in tension or compression.
- Use of improper cutting fluid or not enough fluid.
- No safety mechanism to limit torque below torque breakage value of tap.
- Improper or zero float for use with screw machines (recommended feed .1 slower to establish float for 40 tpi or higher and .15 slower for 40 tpi or finer )
- 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. Addressed in detail in section below.
Precautions to be taken while tapping
- The tap should be made of a material that is suitable for machine tapping. The high-carbon steel used in hand tools is most likely too brittle; high speed steel (HSS) is preferable for virtually all workpiece materials.
- Proper cutting compound or coolant should be used during tapping.
- Spindle or tap is turning in the correct direction.
- Tap is lined up to the hole.
Tool holders for tapping operations
Various tool holders may be used for tapping depending on the requirements of the user:
Aids for hand-tapping (simple jigs and fixtures)
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 (a) produce good threads and (b) 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 (i.e., accurate coaxiality with the hole) without having to use freehand skill to approximate it:
- Hand-tapper: A simple fixture analogous to an arbor press in its basic shape (photo here). 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 (available in a range of tap sizes) or quick-change
- Quick-change drilling & tapping chucks (variations available for both CNC & manual-control tools)
- Rigid tapping attachments (for CNC)
Generally the following features are required of tapping holders:
- Twin chucking: tap is held both, on diameter as well as on the square thus giving it positive drive.
- Safety clutch: The built in safety mechanism, operates as soon as the set torque limit is crossed & 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
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 (already-drilled) 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.
Die
The
die cuts a thread on a preformed cylindrical rod, which cr
eates 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 (barely visible on the full image)
- right: two dies without adjusting screws
A cylindrical blank, which is usually slightly less than the required diameter, is machined with a
taper (chamfer) 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. The two rightmost dies shown in the image have no adjusting screws. However the die holder can exert pressure and decrea
se the size if required.
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.
Tap drill bit size table
Pipe
Threaded pipe is often used in plumbing and pneumatic applications. Because pipe joints must form a seal, the threaded portion is slightly conical rather than cylindrical. As a result, threaded pipe requires specialized taps and dies. Conventional pipe threads must be assembled with a jointing compound or use PTFE tape in order to achieve a leak proof seal.
A modified form of the basic pipe thread shape is the Dry-Seal thread. The Dry-Seal thread is formed so that during assembly, the tips of the male threads are slightly crushed into the roots of the female threads, effecting, in theory, a liquid-tight fit. In practice, a small amount of
pipe dope is usually necessary to assure a pressure-tight seal, and to prevent
galling of the mating parts.
BSP (British Standard Pipe) parallel threads are available in sizes 1/8, 1/4, 3/8, 1/2, 5/8, 3/4, 1 inch and over. Above 6 inch welding is usually done. They are also available in Tapered thread-form and called BSPT (British Standard Pipe Tapered) for British pipe sizes. Currently UK BSP male threads are tapered and the female is parallel.
North American equivalents to BSPT are called NPT (National Pipe Tapered), and range from 1/16 inch through large
integral sizes. Although BSPT and NPT are functionally identical, they are not mechanically interchangeable.
Thread classes and callouts
The Unified and American National Threads callout for a thread is usually shown as follows: 4-40UNF-2A or 1/4"-20UNC-2B
Where (A) - (B) (C) - (D) (E) mean the following:
A: major diameter of the thread in inches (or No. size)
B: threads per inch
C: Unified Nation Coarse(UNC) or Unified Nation Fine(UNF)
D: class of fit. 1 for loose tolerance. 2 for general purpose. 3 for tight tolerance
E: external or internal threads - A for external - B for internal
Metric threads are usually called out as follows: M5X0.8-6g
Where M (A) X (B) - (C) (D) mean the following:
M: ISO metric thread
A: nominal major diameter in millimeters
B: thread pitch in millimeters (distance from crest to crest on thread)
C: tolerance from 3 to 9. 3 being fine. 9 being coarse.
D: thread class as E,G or H. E being large allowance. G being tight allowance. H being no allowance. External threads are shown as lower-case e,g,or h. Internal threads are shown as upper-case E,G,or H.
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 (approximately 10%) 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 (15-25%) of petroleum-based cutting oil. WD-40 and 3-In-One Oil are acceptable substitutes in some cases.
- Brass
- Kerosene or mineral spirits.
- Bronze
- Kerosene or mineral spirits mixed with a small amount (10-15%) of petroleum-based cutting oil.
In power tapping and reaming operations, the tool and workpiece should be continuously flooded with lubricant.
