Threading a workpiece is an important aspect of metalworking, especially for parts created through CNC machining. Thread milling and tapping are both very popular processes, with distinct differences, used for threading. Each has its advantages, disadvantages, and applications.

This article takes a look at these two threading processes, the differences between them, their pros and cons, and when each should be used.

What Is Thread Milling?
Thread milling is a machining process that involves using a specialized cutting tool called a thread mill to create internal or external threads in a workpiece. It involves cutting threads, whether they are internal threads within a hole or external threads encircling a workpiece, through the controlled circular movement of a rotating tool. The key feature of thread milling is the production of thread pitch through the lateral movement of one revolution. In general, thread milling cutters can produce threads with a minimum diameter of about ⅛” (3.175 mm) and depths of roughly three diameters.

For internal threads, thread milling diverges from tapping. Instead of a rotating tool, it employs a CNC (computer numerical control) machine to guide the tool in a spiral or “corkscrew” pattern. This precision, enabled by computer-controlled machinery, empowers thread milling cutters to generate a wide range of thread types and sizes tailored to specific requirements.

What Does Thread Mill Look Like?
A thread mill bears a resemblance to an end mill but features a thread profile on its side. A thread mill features a shank for secure mounting in tool holders, multiple teeth for cutting threads, a tailored thread profile, an end mill design for precise sizing, and optional coatings to enhance performance. Thread mills come in various sizes and lengths. Their appearance varies based on factors like size and design.

How Does Thread Milling Work?
Thread milling works by an operator inserting the thread milling cutter into the hole along the spindle axis until the desired full thread depth is achieved. Subsequently, the controller guides the thread cutter to the entry position, initiating the cutting of threads on the hole’s sidewalls. Afterward, it moves in a 360° circular path and returns to its initial position.

As the threading tool follows this circular trajectory, it must incline towards the upper edge of the hole or travel one increment along the machine’s Z-axis to form the thread effectively. Typically, the extent of the threading’s depth remains within a range of one and a half times the diameter of the hole to minimize deflection and ensure precise results.

How Is Thread Milling Used in Manufacturing?
Thread milling plays a crucial role in manufacturing as a versatile and precise machining process. It is extensively utilized across various industries for creating threads on a wide range of materials and components. One of its primary applications is in the production of precision components. It excels at generating high-precision threads, ensuring tight tolerances and accurate profiles. Thread milling offers versatility in thread types, capable of producing internal and external threads, right-hand and left-hand threads, and various thread profiles to meet diverse manufacturing needs. It is particularly suitable for machining large threaded holes, eliminating the need for costly rigid taps. Custom thread designs are achievable, making it advantageous for unique or proprietary thread requirements. Thread milling is versatile enough to work with materials spanning metals like: aluminum, stainless steel, titanium, and high-temperature alloys, as well as non-metallic materials like plastics and composites.

What Are the Advantages of Thread Milling?
The advantages of thread milling are as follows:

Offers precise control over the fit. By milling a threaded hole at high RPM and helixing it into a pre-milled hole, the operator can adjust thread size using a strategy akin to using an end mill, enabling tight tolerance control and allowances for finishing processes, such as painting.
Allows for the use of a single tool to create a broad spectrum of hole sizes.
Produces both interior and exterior threads, accommodating right-hand and left-hand threads.
Proficient at machining very large threaded holes, such as pipe threads, negating the requirement for large, expensive rigid taps.
Can design custom threads without the need to invest in costly and time-consuming custom taps.

What Are the Disadvantages of Thread Milling?
The disadvantages of thread milling are as follows:

The necessity for a high-speed spindle to execute the process effectively. Achieving the desired results relies on equipment equipped with high-speed spindles rotating at up to 60,000 RPM..
May have limited availability for extremely tiny threads, such as those found in wristwatches and certain medical devices.

What Is Tapping?
Tapping is a process used to create threads, typically internal threads, in a hole or opening in a workpiece. It involves the use of a tap, which is a dedicated cutting tool for threads. The tool essentially functions similarly to an ordinary drill. The cutting element of a tap is its thread, while the indented section is referred to as a groove. This groove serves the purpose of housing and facilitating the material that the cutting edge removes. This technique has been in use for a considerable period and can be carried out manually or with the assistance of power tools. In cases of high-speed material machining, like aluminum and steel, machinists commonly opt for tapping technology. Furthermore, CNC machines, encompassing lathes, drilling machines, and vertical milling machines equipped with power tools, employ taps that can generate up to 20 threads within a length equivalent to one hole diameter of the workpiece.

What Does Thread Tapping look like?
A tap resembles a screw or a bolt, characterized by grooves running along its sides. These grooves effectively guide the removal of chips from the workpiece during the machining process. The sharp edges at the end and outer periphery of the tap are responsible for cutting the threads. Taps function in a way similar to other rotary tools: they are securely held in a collet, chuck, or specialized “floating” tool holder and are then advanced into the workpiece at a specific feed rate.

How Does Tapping Work?
Tapping begins with the preparation stage, in which a hole of the appropriate size is drilled into the material. The size of the hole must be sufficient to accommodate the tap while leaving room for the creation of threads. The hole’s dimensions are critical to ensure a proper fit for the threads. Next, the tap is perfectly aligned with the prepared hole. Cutting fluid or lubricant is then applied to both the tap and the hole. Lubrication serves several purposes, including reducing friction and heat generation during the tapping operation. With the hole prepared, aligned, and lubricated, the tap is inserted into the hole. The tap is then rotated clockwise (for normal, right-hand threads) while being fed into the material. As it rotates, the tap’s cutting edges engage with the material, gradually cutting and forming the threads. It’s important to periodically reverse the tap’s rotation to break the chips created during the cutting process. This prevents the chips from clogging the hole and interfering with the threading process.

After the tapping process is complete, the workpiece undergoes a clean-up phase. This involves removing any remaining chips or debris from the hole, as well as cleaning away any residual cutting fluid or lubricant.

How Is Tapping Used in Manufacturing?
Thread tapping is used to create interior threads in holes. One of the most common applications of tapping is in the production of threaded holes and nuts that receive threaded fasteners, such as screws and bolts. These components are essential for assembling machinery, structures, and consumer products.

Furthermore, thread tapping plays a crucial role in industries such as: automotive, aerospace, electronics, medical, and construction. Thread tapping is utilized in the automotive sector to establish internal threads in transmission housings, engine blocks, suspension components, brake systems, and other vehicle parts. The aerospace industry also relies on thread tapping to manufacture threads in critical aircraft components, including landing gear, engine parts, and structural elements. Additionally, thread tapping is essential in the electronics industry for creating threads that accommodate fasteners and screws in a variety of devices, such as smartphones, computers, and consumer electronics. Finally, in the medical field, thread tapping is used to make threads in surgical instruments and medical equipment.

What Are the Advantages of Tapping?
Tapping has the following advantages:

High-speed tapping centers, equipped with rigid taps, can efficiently create threaded holes in significantly less time compared to thread milling the same holes.
Tapping can thread deeper holes in materials like steel, which is a bit harder than some other metals.
Excels in small threads. In cases in which these miniature threads are more than a few diameters deep, taps might become the preferred choice due to their wider availability and applicability for such intricate applications.

What Are the Disadvantages of Tapping?

Here are some drawbacks associated with thread tapping:

With tapping, different sizes of taps are necessary for different hole sizes. This increases cycle time.
Does not allow you to adjust thread fit. If you’ve tapped the hole, then the size and the position of the thread are final.
It’s used exclusively for the inner threading of holes.
How To Choose Between Thread Milling and Tapping?
Selecting between thread milling and tapping depends on various factors. The decision should be based on the specific needs of your machining application. Generally, it is best to use tapping when you need to make multiple holes with little variations in size. Some key considerations to help you weigh the two options are: speed, power, size, tool life, flexibility, thread fit, thread quality, blind holes, and chip control.

For example, tapping generally offers a slight speed advantage over thread milling. For typical workpiece materials and thread depths, tapping can be faster, taking around 4–5 seconds to create a 1/4″-20 thread, while thread milling may take roughly twice that time. However, for large production quantities, thread milling can become more efficient. Taps also require substantial torque, especially when dealing with larger threads. Machining centers, particularly those with geared heads, are needed for threads beyond approximately 3/4″ in diameter. In contrast, thread milling has no inherent size limitations and can handle various thread sizes effortlessly.

Thread milling’s versatility can be limited by extremely tiny threads, such as those found in wristwatches and certain medical devices. If these miniature threads extend beyond a few diameters, taps might be the preferred choice due to their wider availability. Thread milling also has the advantage of longer tool life, thanks to the use of carbide tools compared to most taps, which are made of high-speed steel (HSS). When taps wear and break inside a hole, workpiece damage or scrapping is more likely. Thread mills are more predictable, making it more likely to salvage the workpiece if breakage occurs during operation. Taps can also only create threads based on manufacturer specifications, requiring a different tap for each variation in thread size and pitch. Whereas, thread milling offers the flexibility to adjust thread size and pitch through CNC machine tool programming.

How do Thread Milling and Tapping Differ in the Cutting Process?
The primary distinction between tapping and thread milling lies in their approach to creating threads. Tapping employs a tap specifically designed to cut a predetermined thread, while thread milling relies on the controlled movement of the tool to define the thread. Consequently, tapping is exclusively used to cut threads on the inner surface, whereas a thread milling cutter can be employed for both inner and outer surface threading.

What Is the Accuracy of Thread Milling?
The accuracy of thread milling is typically quite high. Thread milling allows for precise control over thread dimensions and profiles, making it well-suited for applications that require tight tolerances and accurate threads. The CNC machines equipped with thread milling cutters can provide consistent and accurate results, ensuring that threads meet the desired specifications.

What Is the Size of the Thread Mill?
The size of a thread mill can vary widely depending on the specific application and requirements. Thread mills come in various sizes to accommodate different thread dimensions and hole sizes. They can range from small, compact thread mills for threading small holes to larger, more robust thread mills for machining larger threads. The size of the thread mill is typically determined by factors such as the thread pitch, diameter, and depth, as well as the material being machined and the machine’s capabilities.

How do Thread Milling and Tapping Differ in Applications?
Thread milling is particularly suited for machining thin-walled parts, asymmetric or non-rotating components, and materials that generate high cutting forces, where chip evacuation might be problematic. It also excels in reducing tool inventory, accommodating unstable component setups, and customizing threads. Furthermore, it efficiently handles large threaded holes. In contrast, tapping shines when speed is of the essence, standard thread sizes are required, and deep threads need to be produced, especially in tougher materials. Tapping is also economical for small threads like those in wristwatches and medical devices. The choice between thread milling and tapping depends on specific application requirements and considerations.

How Flexible Are Thread Milling and Tapping?
Thread milling shines in terms of flexibility. When dealing with tapped threads that fall out of tolerance, the usual solution is to acquire a different “H-size” tap with minor size adjustments available in increments of 0.0005″ larger or smaller. Thread milling simplifies this process by allowing for straightforward offset adjustment to bring threads back to specification, eliminating the need for multiple tap sizes. Moreover, traditional taps are designed to produce a single size and type of thread and often feature material-specific geometries. In contrast, thread mills excel in versatility. A full-profile 16-pitch thread mill can effectively cut any 16-pitch thread, and the same principle applies to 20-pitch thread mills and so on, as long as the thread fits within the hole dimensions. Single-plane thread mills can accommodate a wide range of thread sizes and pitches simply by modifying the machining program. This adaptability makes thread milling an excellent choice when precision, customization, and efficient thread adjustments are essential.

What Is the Typical Life Span of Thread Mills and Taps?
The life span of thread mills and taps in manufacturing can vary depending on several key factors. In general, thread mills tend to have a longer life span compared to taps. High-quality thread mills, often made of durable materials like carbide, can last for thousands of holes or more before requiring replacement. Proper maintenance, such as keeping the tool clean and well-maintained, can further extend their lifespan. On the other hand, taps have a comparatively shorter life span. The longevity of taps depends on factors like their material (e.g., high-speed steel or carbide), the hardness of the workpiece material, and the specific cutting conditions. Taps may last from several dozen to a few hundred holes before showing signs of wear or reduced cutting performance, particularly in demanding materials like stainless steel or hardened steel. Regular monitoring of tool condition and adherence to recommended cutting speeds, feeds, and lubrication practices are essential to maximizing the life span of both thread mills and taps in manufacturing operations.

Are Thread Milling and Tapping Common Methods Used in Milling Processes?
Yes, both thread milling and tapping are common methods used in milling processes. These methods provide precision and reliability in producing threads for fasteners, components, and various parts. Thread milling is known for its versatility and ability to create a wide range of threads, offering flexibility in manufacturing. Tapping, on the other hand, is favored for its speed and efficiency, making it a practical choice for high-volume production.

This article presented thread milling and tapping, explained each of them, and discussed their key differences. To learn more about thread milling and tapping, contact a Proto MFG representative.

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