From assembling a bicycle with dozens of components to building a car with hundreds of thousands of parts, how are all these components securely fastened together? Threads play a crucial role—they are an indispensable method of connection in mechanical design. Whether it’s plastic parts, sheet metal components, or precision CNC machined pieces, threads can easily combine multiple components into a cohesive whole. Properly designed threads make equipment durable and reliable, while poorly designed ones can lead to loosening or damage.

In the following sections, we will guide you through the fundamentals of threads, common thread types, how to choose the right thread size, and thread fabrication techniques, helping you work more effectively in mechanical design and manufacturing.

What is a Thread?

A thread is a geometric structure formed along the surface of a cylinder following a specific helical pattern, consisting of continuous ridges (crests) and grooves (roots). Its primary function is to securely connect two or more components while allowing for easy assembly and disassembly. Threads can withstand tensile, compressive, and torsional forces, and by adjusting the fastening torque, they help ensure the stability of mechanical structures.

In mechanical design, understanding the definition and function of threads is essential. Through the spiral slope of the thread, rotational motion is converted into linear clamping force, allowing screws, nuts, or other fasteners to hold components firmly in place. This design ensures reliable connections and effective force transmission.

Thread schematic diagram illustrating thread type, pitch, diameter, and assembly details for mechanical components.

How to Distinguish Threads?

You can easily distinguish between internal threads and external threads: for example, the threads on a screw are typical external threads, while the threads inside a nut are internal threads. Simply put, threads processed on the outer surface of a part are called external threads, while those processed on the inner surface are called internal threads.

Besides internal and external threads, there are many other types of threads, such as trapezoidal threads, pipe threads, coarse threads, and fine threads. Next, we will give you a detailed overview of thread classifications and characteristics.

Common methods of distinguishing threads mainly include the following categories:

1. Classification by Thread Standard:

  • ISO Metric Thread (M), e.g., M10×1.5
  • Unified Thread (UNC/UNF), e.g., 1/4-20 UNC or 1/4-28 UNF
  • British Standard Pipe Thread (BSPP/BSPT), e.g., G1/2 BSPP or R1/2 BSPT
  • American National Pipe Thread (NPT/NPTF), e.g., 1/2 NPT or 1/2 NPTF
  • British Whitworth Thread (BSW), e.g., 1/4 BSW

2. Classification by Thread Profile (Cross-Section Shape):

  • Triangular Thread: Thread cross-section is an equilateral or nearly equilateral triangle, with a thread angle of 60° or 55°
  • Trapezoidal Thread: Thread cross-section is an isosceles trapezoid, with a thread angle of 30°
  • Sawtooth Thread: Thread cross-section has one inclined side and one nearly vertical side (like a sawtooth), with flank angles typically 3° and 30°, forming an asymmetrical profile
  • Square Thread: Thread cross-section is square, with a thread angle of 90°

3. Classification by Pitch and Thread Spacing:

  • Coarse Thread: Compared to fine threads, coarse threads have a larger pitch and fewer threads per unit length, e.g., 1/4-20 UNC (UNC indicates coarse thread)
  • Fine Thread: Compared to coarse threads, fine threads have a smaller pitch and more threads per unit length, e.g., 1/4-28 UNF (UNF indicates fine thread)

4. Classification by Thread Direction:

  • Right-Hand Thread: Viewed along the axis of the screw, rotating clockwise moves the screw forward
  • Left-Hand Thread: Viewed along the axis of the screw, rotating counterclockwise moves the screw forward

In addition, threads can also be classified as single-start, double-start, and multiple-start threads. The more thread starts there are, the greater the axial distance the screw advances per turn (the lead), and the fewer turns are required, resulting in faster assembly. Conversely, the fewer the thread starts, the smaller the axial movement per turn, requiring more turns and resulting in slower assembly.

What geometric parameters are involved in threads?

Thread information diagram showing thread type, pitch, diameter, and tolerance for mechanical fasteners.
Threads come in many types, but their geometric parameters remain essentially unchanged.
  • Root: The lowest point of the thread groove.
  • Crest: The highest point of the thread tooth.
  • Pitch: The axial distance between corresponding points of adjacent thread forms.
  • Thread Angle: The angle between the flanks of the thread.
  • Minor Diameter: The diameter measured at the bottoms of the thread grooves.
  • Pitch Diameter: The diameter at which the width of the thread ridge and the width of the thread groove are equal. It is the most important controlling dimension for thread fit.
  • Major Diameter: The diameter measured at the crests of the thread teeth.
For thread inspection, matching thread ring gauges or plug gauges are commonly used for quick verification, while the three-wire method is often employed to accurately measure the pitch diameter when higher precision is required.

How are threads manufactured?

Threads are generally produced using two main process methods:
  1. Cutting – removing material to form the desired thread profile.
  2. Plastic forming – such as extrusion or rolling, where the threads are directly formed by plastic deformation of the material.
External thread machining is usually performed with a turning tool. The tool must be selected according to the thread profile angle. If the wrong profile angle is chosen, the part may fail to assemble properly. Thread rolling can also be used to manufacture external threads.Compared with external threads, internal thread machining offers more tool options. For example, a tap can be used for direct threading, or a special thread milling cutter can be used for milling.
When machining external threads and blind hole threads, some process details must be noted:
1.External threads – Due to tool geometry limitations, external threads cannot usually be machined all the way to the thread root. To prevent bolts or nuts from failing to fully engage, a relief groove is often left at the thread end to ensure smooth assembly.
2.Blind hole internal threads – The front end of a tap is designed for guidance and has no cutting teeth, which makes it difficult to cut threads to the very bottom. There are two ways to address this issue:
  • Limit the effective thread depth according to design requirements;
  • Leave a relief groove at the thread end (especially for larger-diameter threaded holes).
When receiving 2D drawings from customers, if the effective thread depth is not specified, the machined part may fail to meet the requirements. Therefore, timely communication with the customer is essential.
3.Through-hole internal threads – Although there is no relief groove issue, attention must still be paid to the effective cutting depth of the tap. If threading is performed from both ends, misalignment of threads may occur, preventing the bolt from fully engaging.
In conclusion, whether machining external or internal threads, the thread depth and end treatment must be arranged reasonably according to tool characteristics and design requirements to ensure machining quality and assembly reliability.

Design Tips

Designing threaded parts is not difficult, but for manufacturability and cost control, clarity and adherence to standards are essential. In some cases, threads can be formed directly during manufacturing (e.g., metal 3D printing); in most cases, however, threads are created after the part is formed through drilling and tapping.
Key guidelines to follow include:

1.Follow standards
Unless there is a strong reason not to, always prefer commonly used thread series (such as UN or M) and standard sizes. If necessary, adjust the design slightly to match a standard thread size—this is usually more beneficial than insisting on non-standard dimensions.
2.Specify thread depth clearly
In blind-hole designs, the effective thread depth is always less than the total hole depth. Therefore, the exact thread depth should be clearly indicated in CAD models and technical drawings.
3.Consider manufacturing limits
Thread depth is constrained by diameter, material, and tooling capability. Excessively deep threads may reduce part strength or be impractical to manufacture.

4.Leverage CAD/CAM tools
Modern CAD/CAM software often provides thread design and simulation features, which help optimize designs and minimize manufacturing risks.

As a prototyping expert, Horizon is experienced in a wide range of manufacturing processes. We can help you avoid potential design issues and improve manufacturability from the very beginning. Contact us today for a free quote.

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