What Is Tapping? Machining Conditions, Types, and Precautions

What Is Tapping?

Tapping is a technique for machining the inner surface of a female thread. First, let's review the basics of tapping and how it differs from drilling and reaming.

What Is Tapping

Tapping is a type of thread machining technique to create thread grooves on the inner surface of a female thread. A cutting tool called a tapping tool is used to machine the inside of a pre-drilled pilot hole. Tapping is also called thread cutting, and is generally performed on a drill press or machining center.

The cutting method, size of hole that can be machined, and pitch (distance between crests or roots of adjacent threads) vary depending on the tapping tool used. When tapping, it is important to know the characteristics of each tool and select the best one for the material and type of thread to be machined.

Differences From Drilling and Reaming

Tapping is a machining technique for forming roots and crests on the inner surface of a female thread. Drilling focuses on creating a hole itself, and is performed with the objective of creating a pilot hole for the female thread in thread machining. Furthermore, reaming is machining to enlarge the inner hole diameter or to smooth out the inner surface of a pilot hole created by drilling.

• Tapping: Machining method for cutting thread grooves inside holes that had already been created
• Drilling: Machining method dedicated to creating holes
• Reaming: Machining method for finishing the inner diameter of holes for accuracy and for improving surface roughness.

Tapping Methods

There are two types of machining method for tapping: cutting and rolled. Let's take a look at how they differ.

Characteristics of Cutting Tapping

Cutting tapping is a method of forming the crests and roots of a threaded hole by cutting away the inner surface of a female thread with a tapping tool. To improve accuracy, it is important to accurately calculate not only the tapping process but also the creation of the pilot hole. Moreover, because it produces chips, care must be taken to ensure that the chips are properly disposed of.

Characteristics of Rolled Tapping

Rolled tapping is a method of forming threads by pushing outward to widen the pilot hole from the inside. Also known as plastic machining, it is characterized by the fact that it does not produce chips, unlike cutting. In some cases, rolled tapping cannot be performed if the workpiece is too hard or too soft.

Advantages of Tapping

There are various types of tap, such as spiral, point, and hand. Hence, the advantage of tapping is that it can be used in a wide array of machining to match the material and machining required.

When forming grooves inside a female thread using a lathe, a tool called a female thread cutting tool is used. The cutting tool is pressed against the rotating workpiece to cut through. The tip of the female thread cutting tool is L-shaped, so machining cannot be performed if the inner diameter is too small. In contrast, a tap can carve grooves even in threaded holes with small inner diameters.

In addition, machining centers and multi-tasking machines are equipped with an automatic tool change function, so the process from pilot hole machining to tapping can be automated. This significantly reduces man-hours, which ultimately leads to manufacturing cost reduction and productivity improvement.

Types of Tap

In this section, we will introduce five major types of tap. Let's review the uses, features, and precautions when using each.

Spiral Tap

Spiral taps are taps with teeth forming a spiral. The spiral flute directs chips upward, making them less likely to clog the hole. However, fragments from the workpiece may get caught in the tool and lead to lower machining accuracy.

Point Tap

Point taps are tools used for thread cutting through holes. While spiral taps evacuate chips upward, point taps evacuate chips in the same direction as the moving tool. Since chips can clog the bottom of blind holes, which do not fully penetrate the workpiece, point taps are mainly used for machining through holes.

Hand Tap

Hand taps are tools for manually carving grooves inside female threads. Hand taps normally come in a set consisting of three taps: tap No. 1 (taper tap), No. 2 (center tap), and No. 3 (finishing tap). Each tap is used to gradually widen the threaded hole. They are used for purposes such as creating prototypes, because they allow threaded holes to be made manually. However, a certain level of skill is required to be able to properly adjust the force and angle needed.

Roll Tap

Roll taps are tools used for rolled tapping. Instead of cutting, they use pressure to extrude the pilot hole and form threads. They are less likely to lose accuracy due to chips, and are suited for workpieces with relatively low hardness, such as aluminum and copper.

Pipe Tap

Pipe taps are tools used to create thread grooves in pipe joints, and are broadly classified into two types: tapered thread and parallel thread. Tapered thread taps are excellent for airtightness and are used for machining water pipes, vacuum pipes, and others. On the other hand, parallel thread pipe taps are often used for machining machine parts that require strength. Note that pipe taps are sometimes classified by thread machining rather than tapping tools.

Thread Mill

Thread mills are different from tapping tools, but are special milling tools used to machine threads. Grooves are cut into the inner circumference of a pilot hole using a helical interpolating motion. A helical interpolating motion is the movement of the tool as it continuously spins and revolves around its axis. The tool rotates at high speed (spins) and moves in a circular arc (revolves) along the inner circumference of the pilot hole to cut the thread.

How to Perform Tapping and Tapping Steps

Tapping is broadly divided into three steps.

1. Create pilot hole
2. Inject cutting oil
3. Form thread grooves with tap

We will explain the process of each step in detail.

Create Pilot Hole

Tapping starts with creating the pilot hole first. The size of the pilot hole is determined by the diameter of the thread. There are also two types of pitch: fine thread and coarse thread. Fine threads have pitches with small distances between threads. Unless there is a special reason, coarse threads are generally used.

The following is a chart showing thread diameter and pilot hole diameter. The chart shows that, for example, the diameter of the pilot hole that fits an M8 thread (8 mm thread diameter) is 6.647 – 6.912 mm.

The depth of the pilot hole is typically machined "slightly deeper than the length of the male thread." In general, the depth of the pilot hole is calculated using the following formula:

Pilot hole depth = Length of male thread + Length of drill tip (height of triangular pyramid) + Length of incomplete thread + Allowance (1 pitch)

Inject Cutting Oil

After creating the pilot hole, the next step is to inject cutting oil. This process reduces friction between the workpiece and the tool, as well as lubricates and enables smooth cutting.

Not using cutting oil will shorten the lifespan of the tapping tool. Note that there are various types of cutting oil, such as mineral oil, synthetic cutting oil, and straight oil. Choose the appropriate oil based on the workpiece material, hardness and other factors.

Insert the Rotating Tap

Finally, use the tapping tool attached to the machine tool to cut thread grooves inside the pilot hole. It is important to set the workpiece and tool so that the insertion angle is perpendicular to the pilot hole. If the cutting conditions are set incorrectly, the tap may break or the groove may be deformed. Make sure to carry out the work appropriately according to the required machining details.

What Are the Precautions to Take When Tapping?

In this section, we will explain the precautions to take when tapping and points to consider while working. Be sure to check these carefully. They are all important points to remember in order to maintain the required accuracy.

Select Suitable Tap for the Work

When tapping, tap selection is first conducted based on the diameter and depth of the thread. Next, the appropriate tool is selected according to the work required, such as whether the hole is a through hole or a blind hole, or whether the workpiece is made of a hard or soft material. For example, taps with long chamfers are often selected for through holes, while taps with short chamfers are selected for blind holes because the depth of the hole is limited. Note that the chamfer is the portion of the tap at the tip with teeth tapering at an angle.

Securely Fix the Workpiece

It is important to firmly fix the workpiece in place not only during tapping but in any other cutting process. If the workpiece is not properly fixed, there is a risk of lower machining accuracy or damage to the tap. Consider the shape and properties of the workpiece and use a jig, such as a clamp or machine vise, to securely fix it in place if necessary.

Check Pilot Hole Diameter

The pilot hole diameter can be determined based on the pilot hole quick reference chart for thread diameters, or it can also be calculated by subtracting the pitch from the thread diameter.

[Formula]
Pilot hole diameter = thread diameter – pitch

For example, for thread diameter: M12 and pitch: 1.75 mm, the pilot hole diameter can be calculated as 10.25 mm (= 12 – 1.75).

Use Cutting Oil

Cutting oil has three functions: lubrication, coolant, and anti-adhesion. Anti-adhesion is the function of preventing the workpiece and the tool from sticking together, in order to limit variations in roughness and accuracy of the machined surface.

Cutting oils are also broadly classified into the following types: insoluble cutting fluid and water-soluble cutting fluid. The former has excellent lubrication properties, while the latter has excellent cooling performance and carries no risk of catching fire.

Cutting oils containing chlorine were widely used in the past, although chlorine generates dioxins. Because dioxins have a negative environmental impact, environmentally friendly chlorine-free cutting oils are increasingly being used.

Set Cutting Speed

Increasing the cutting speed shortens the work time, but it also increases cutting heat and wears out the tool more quickly. Therefore, setting the cutting speed properly when tapping is crucial to improving accuracy and extending tool life. Cutting speed is generally determined by the type of tap and the workpiece material. Based on this, set the rotational speed of the tool.

Note that the cutting speed of the tap can be calculated using the following formula.

[Formula]
Cutting speed: V (m/min) = D x n x π / 1,000
D: tap cutter diameter (mm)
n: rotations per minute (min-1)
π: pi

Tapping Terminology That You Should Know

Lastly, we will discuss some terms that are often used in tapping.

Thread Accuracy

In thread machining, thread accuracy is a measure indicating the accuracy of the shape and dimensions of the threads. Generally expressed as a symbol or number, it determines the precision and tolerance range of the thread for manufacturing.

For female threads, there are two types of tolerance positions: G (positive allowable tolerance from the basic size) and H (0 allowable tolerance from the basic size). Moreover, the larger the number associated with it, the wider the tolerance range. S, N, and L refer to the length of the fit: S for short, L for long, and N for intermediate length.

Thread Engagement Rate

Thread engagement rate is a measure indicating the amount of meshing between the male and female threads. It is expressed as the ratio of the "height of thread where male and female threads mesh" to the "standard thread crest height." The closer the thread engagement rate is to 100%, the smaller the pilot hole diameter becomes, and the larger the engagement area between the crests and roots of the male and female threads.

[Formula]
Thread engagement rate (%) = (outer diameter of male thread – inner diameter of female thread) ÷ (2 x standard thread height of male thread) x 100

Gouging, Thread Skipping

Gouging and thread skipping are both cutting defects that occur during tapping. Gouging occurs when the tool tears the workpiece when machining, causing the cutting surface to burr and reducing machining accuracy.

Collapsing crests of female threads, called thread skipping (pitch skipping), are caused by chips getting caught between the tool and workpiece during cutting.

Making Tapping More Efficient With a Machining Center

Tapping is the machining process to create thread grooves for female threads using a wide range of available tools. It is essential to select the appropriate tool according to the work required, such as the workpiece material and the required accuracy. Also, be sure to remember the basics, such as using cutting oil and inserting the tap perpendicularly. In particular, when working manually, screwing the tap diagonally into the hole may lead to a decline in quality.

One way to maintain tapping accuracy and improve productivity is to use a machining center. Because tools can be controlled by programming, even complex thread shapes can be finished with consistently high quality. If necessary, consider investing capital in equipment.