History

While modern taps and dies 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 evolutionary 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.

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: The tap illustrated in the top of the image has a continuous cutting edge with no taper. This feature enables a bottoming tap to cut threads to the bottom of a blind hole. A bottoming tap is never used to cut threads in an unthreaded hole, as the cutting edges lack the taper required to successfully start into such a hole

Plug Tap: Also known as an intermediate tap (US only - In Australia and Britain, the Bottoming tap is the Plug), 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. Plug taps are the most commonly used type of tap.

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. 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.

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 to the tapping. Machines like Lathe, Radial Drilling machine,Bench Type Drill M/c,Pillar Type Drill M/c, Vertical Milling Machines, HMCs VMCs. Machine tapping is faster, generally more accurate as 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 & 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/inferior quality taps.
• Chip clogging
• Tapping does not follow the pre-tap hole (misalignment)
• Mismatch of machine feed & tap feed may cause the tap to break in tension/compression.
• Use of improper cutting fluid.
• No safety mechanism to limit torque below torque breakage value of tap.

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 & CNC tool holders. Addressed in detail in section below.

Precautions to be taken while tapping

1. Taps have self feed (Taps once engaged get pulled into the job) due to helical angle & hence length compensation arrangement is required in tool holder to avoid breakage of tap.
2. High Carbon taps should not be used as they cannot sustain high speed. These were used for Hand Tapping Earlier.
3. Only HS Taps should be used.
4. In order to retract tap from the hole, the machine should be reversible.
5. Proper Lubricant should be used during tapping.

tool Holders for Tapping Operations

Various tool holders may be used for tapping depending on the actual requirement of the user.

• Tapping Attachments : These may be normal ( available is a range of tap sizes ) or Quick Change
• Quick change drilling & tapping chucks ( Variations available for both CNC & Conventional 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

toolsOf Carbide"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 (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 decrease 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.

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 pipe dope 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 (further details shown on website source: http://www.tapdie.com). They are also available in Tapered thread-form and called BSPT (British Standard Pipe Tapered) for British pipe sizes. 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.