A Complete Guide to Thread: History, Definition, Elements, Standards & Manufacturing


Threaded fittings are almost everywhere, there are many thread types and standards. How to identify different threads? What are the types of threads and how to choose the correct one? Understanding necessary structure and terminology is important. Let’s get into a complete guide to the thread.

Brief History of Thread

As early as the 4th century BC, the ancient Greek naturalists learned the basic spiral form and applied it to wine and oil extraction. By the 14th century, handmade threads and screws appeared. During the industrial revolution, it was more necessary than ever to connect mechanical objects with bolts. These early nuts, bolts, and screws were made by hand or simple lathes, and they could not be interchanged and used among different industries or even different companies in the same industry. With the continuous progress of industrial technology, countries have developed their own unified standards and international general standards in order to improve efficiency. At present, there are mainly two kinds of thread measurement standards: imperial and metric.

Definition of Thread - What is a Thread?

Thread refers to the continuous convex spiral ridge made by winding the cylinder or cone in the form of a spiral. The former is called a straight thread, and the latter is called a tapered thread. This spiral structure converts the rotational motion into linear motion to realize the linear movement of the object. The thread plays the purpose of fastening or transmission through the connection of internal and external threads. The thread on the outer surface of the workpiece is the external thread, such as the common bolt or screw. The internal thread in the workpiece is generally on the parts to be assembled.

Elements of Thread

A complete description of the thread should contain the thread type + nominal diameter + the number of threads + pitch (or lead) + direction of rotation. The thread profile is the key factor to determine the end use of the thread.



The profile refers to the geometry of the thread. Different profiles determine whether the thread is used for connection or transmission. There are mainly four kinds of common thread types: triangle, trapezoid, serration, and rectangle. The triangular thread is mainly used for connection, and the other three tooth types are used for power transmission. The triangular thread is mainly represented by M and the pipe thread represented by G. M thread is the most commonly used connecting thread, which is divided into coarse thread and fine thread. Fine thread is mainly used on small or thin-walled parts. Pipe thread is used for the connection of water pipes, gas pipes, and other pipelines. The trapezoidal thread code is tr, which is used for power transmission of various machine tool lead screws. The serrated thread of code B can only transmit power in one direction.



Indicates that the nominal diameter is used for thread size. Except that the inner diameter (inch) of the pipe thread is used as the nominal diameter, the major diameter is used as the nominal diameter (metric unit) for other threads. The external thread and internal thread are represented by the upper and lower case letters “d” and “D” respectively. The major diameter refers to the imaginary cylinder diameter tangent to the crest of the external thread or the bottom of the internal thread; The small diameter is the imaginary cylinder diameter tangent to the external thread bottom or the internal thread top; Pitch diameter is the most important parameter for the thread to achieve fit and strength, and is the imaginary cylinder diameter whose tooth width on the pitch line is consistent with that of the adjacent tooth slot.


Number of lines

The number of thread lines refers to the number of spiral lines when forming threads, which can be divided into a single line and a multi-line. A single thread refers to the thread formed along one spiral line, and a multi-thread refers to the thread formed along two or more spiral lines.



The axial distance between the corresponding two points on the pitch diameter line of two adjacent teeth on the thread is called the pitch; The axial distance between the corresponding two points on the pitch diameter line of two adjacent teeth on the same thread is called the lead.

Thread Standards and Forms

Metric or International Standard Thread

The metric thread was first adopted in central France from 1898 to 1908, and then widely promoted among countries. Metric thread code m is applicable to the connection of workpieces with a diameter of 0.25 mm to 300 mm. The thread angle is 60 °, the tooth top is flat, and easy to turn, and the tooth bottom is a circular arc, which can increase the thread strength. The metric thread has a coarse thread and fine thread.


British Standard Whitworth (BSW) Thread 

Originated in Britain, the thread was invented by British citizen Joseph Whitworth in 1841, hence the name. The thread angle of this thread is 55 °. The crest and the bottom are arc-shaped, also known as BSW thread.


British Standard Fine (BSF) Thread 

In shape, this thread is similar to the BSW  thread, and the thread angle is also 55 °, but the number of threads per inch is more, that is, the thinner the thread thickness makes the grip stronger. It is used for parts that need high-strength roots or need to bear large vibrations.


United States Standard (USS) Thread 

The top and bottom of American Standard thread are flat, with good strength and stress resistance. The thread angle is 60 °, and the specification is expressed in several teeth per inch, which is divided into three grades: coarse tooth (NC), fine tooth (NF), and super fine tooth (NEF).


Unified Thread Standard (UTS) Thread 

It is jointly developed by the United States, Britain, and Canada and is the commonly used British thread. Compared with the American Standard thread, the thread angle is 60 °, and the specification is also expressed by the number of teeth per inch. It is also divided into three grades: coarse tooth (UNC), fine tooth (UNF), and ultra-fine tooth (UNEF).


DIN Standard Thread 

It is the standard thread specified by din in Germany. The thread is round and suitable for the connection of light bulbs and rubber pipes.

Thread Manufacturing Methods

The thread can be manufactured by different methods.

CNC turning: it is the most commonly used customized processing method for single pieces and small batches of threaded workpieces with a simple tool structure.

– CNC Milling: generally, a disc milling cutter or comb milling cutter is used for milling. The disc milling cutter is mainly used for milling trapezoidal external threads on workpieces such as screw rods and worms, while the comb milling cutter is used for milling internal and external common threads or tapered threads.

– Tapping and threading: tapping is to use a certain torque to screw the tap into the bottom hole drilled on the workpiece in advance, so as to process the required internal thread. Threading is a molding method of cutting external thread on the bar workpiece with a die. On the one hand, the accuracy of tapping and threading depends on the accuracy of tap or die, on the other hand, it depends on the processing experience value. Many non-standard small-diameter internal threads can only be processed by tap. Tapping and threading can be manually operated, or a lathe, drilling machine, tapping machine, or threading machine can be used.