Everything You Need to Know About Screw Thread

Threaded fittings are everywhere, and there are many kinds and standards.

For inexperienced people, the appearance and performance of threads are almost the same, which cannot be accurately identified.

Master the necessary thread structure and terminology, so as to effectively communicate with manufacturing partners to obtain the required.

What are the types of threads?

How to choose thread correctly?

What do the M, G, Tr and B of thread marks mean?

What are the differences between national standard, British standard Wyeth and American standard threads?

Let’s dive into it.

1. Brief history of thread

As early as the 4th century BC, the naturalists of ancient Greece learned the basic spiral form and applied it to pressing wine and oil.

Until today, we still use Archimedes’ spiral to get water.

Archimedes spiral water intake device still in use today

Fig. 1 Archimedes spiral water intake device still in use today

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 common among different industries or even different companies in the same industry.

handmade threads 200 years ago vs modern machine threads

Fig. 2 handmade threads 200 years ago vs modern machine threads

With the continuous progress of industrial technology, countries have formulated their own unified standards and international general standards in order to improve efficiency.

At present, there are mainly two kinds of thread measurement standards, British system and metric system.

China adopts the latter, and uses metric mm as the measurement and identification unit of thread.

2. Thread definition

Thread refers to the continuous convex spiral ridge made by winding on the cylinder or cone in the form of spiral.

The former is called straight thread, and the latter is called conical thread.

This spiral structure converts rotary motion into linear motion, so as to realize linear moving objects.

straight and tapered threads

Fig. 3 straight and tapered threads

The thread is fastened or driven 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;

Inside the workpiece is the internal thread, which is generally on the parts to be assembled.

3. Five elements of thread

A complete description of the thread should include at least five elements, namely:

  • tooth shape
  • nominal diameter
  • number of lines
  • pitch (or lead)
  • direction of rotation

The tooth profile is the key factor to determine the end use of thread.

Tooth shape

Tooth shape refers to the geometry of the thread.

Different tooth shapes determine whether the thread is used for connection or transmission.

At present, the common tooth types mainly include triangle, trapezoid, sawtooth and rectangle.

Triangle thread is mainly used for connection, and the other three tooth patterns are used for power transmission.

Comparison of four common thread types

Fig. 4 Comparison of four common thread types

Triangle thread is mainly composed of ordinary thread code m and pipe thread code G.

M thread is the most commonly used connecting thread, which is divided into coarse teeth and fine teeth.

Fine teeth are mainly used on small or thin-walled parts.

Pipe thread is used for the connection of water pipe, gas pipe and other pipelines.

The trapezoidal thread code is Tr, which is used for the power transmission of various machine tool lead screws.

The serrated thread of code B can only transmit power in one direction.

Fig. 5 shows the classification of threads by use and tooth type.

Fig. 5 Classification of threads


Indicates that the nominal diameter is used for thread size.

Except that the pipe thread takes the inner diameter (inch) of the pipe as the nominal diameter, other threads take the major diameter as the nominal diameter (metric unit).

The external thread and internal thread are represented by the upper and lower case letters “d” and “D” respectively.

For example, the large, medium and small diameters of external threads are represented by D, D1 and D2;

Internal thread is represented by D, D1 and D2.

Major diameter refers to the imaginary cylinder diameter tangent to the crown of external thread or the bottom of internal thread;

The small diameter is the imaginary cylindrical diameter tangent to the bottom of the external thread or the top of the internal thread;

Pitch diameter is the most important parameter for the thread to achieve fit and strength.

It is an imaginary cylinder diameter whose tooth width on the pitch line is consistent with the adjacent tooth width.

large, medium and small diameter of external thread and internal thread

Fig. 6 large, medium and small diameter of external thread and internal thread

Number of lines

The number of thread refers to the number of spiral lines when forming threads, which can be divided into single line and multi line.

Single thread refers to the thread formed along one spiral line, and multi thread refers to the thread formed along two or more spiral lines.

single and double line thread

Fig. 7 single and double line thread

Pitch (lead)

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

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

pitch and lead

Fig. 8 pitch and lead

thread direction

Fig. 9 thread direction

4. General standards of threads in various countries

The prevailing standards of threads in various countries are mainly the measurement and identification standards of class 2 threads in metric units and inch units.

Metric or international standard thread

Metric screw thread was first adopted in central France from 1898 to 1908, and then widely promoted among countries.

China adopts GB metric screw thread standard.

Metric thread code M is applicable to the connection of workpieces with a diameter of 0.25mm to 300 mm.

The thread angle is 60 °, the top of the tooth is flat, which is easy to turn, and the bottom of the tooth is circular arc, which can increase the thread strength.

Metric thread can be divided into coarse thread and fine thread.

metric thread standard

Fig. 10 metric thread standard

British standard Wyeth thread

Originated in Britain, this thread was invented by British citizen Joseph Whitworth in 1841, hence its name.

The thread angle of this thread is 55 °, and the top and bottom of the thread are circular arc, also known as b.s.w thread.

British standard Wyeth thread standard

Fig. 11 British standard Wyeth thread standard

British standard fine thread

In shape, this thread is similar to B.S.W thread, and the thread angle is also 55 °, but the number of threads per inch is more, that is, the thread thickness is thinner, which makes the grip stronger.

It is used for parts that need high strength roots or need to withstand greater vibration.

British standard fine thread standard

Fig. 12 British standard fine thread standard

American Standard 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 as a few teeth per inch, which is divided into three levels: coarse teeth (NC), fine teeth (NF) and super fine teeth (NEf).

American Standard thread

Fig. 13 American Standard thread

Unified standard thread

Developed by the United States, Britain and Canada, it is the British thread commonly used at present.

Compared with American Standard thread, the thread angle is 60 °, and the specification is also expressed by the number of teeth per inch, which is also divided into three levels: coarse tooth (UNC), fine tooth (UNF) and ultra-fine tooth (UNEF).

unified thread standards of the United States, Britain and Canada

Fig. 14 unified thread standards of the United States, Britain and Canada

German DIN round thread standard

It is the standard thread determined by din in Germany.

The thread is round, which is suitable for the connection of bulbs and rubber tubes.

The thread code is Rd.

Wyeth thread standard

Fig. 15 German DIN round thread standard

Trapezoidal Tr thread

Trapezoidal thread, also known as acme thread, is a special transmission thread that can be adjusted by nut after wear.

The thread angle when the metric thread standard is used is 30 °, and the thread angle when the British thread standard is used is 29 °.

Trapezoidal Tr thread

Fig. 16 Trapezoidal Tr thread

5. China standard metric thread identification and examples

China’s GB standard stipulates that the unit of ordinary thread marking is mm, the first letter is the thread code, the second digit represents the nominal diameter (major diameter) of the thread, and the subsequent symbols represent the tolerance code, the screw length code and the screw direction code in turn.

Among them, the fine pitch must be marked, and the coarse pitch can be omitted.

Fig. 17 marking contents of 5 parts of ordinary thread

Code notes of each part:

The feature code represents the thread type, for example, M represents ordinary thread; The size code indicates the nominal diameter × Pitch, for example, 8X1 means that the nominal diameter is 8 and the pitch is 1;

The tolerance zone code is composed of tolerance grade (number) and basic deviation (external thread is represented by lowercase letters and internal thread is represented by uppercase letters);

The code of Screwing Length uses code L, N and s to represent long, medium and short respectively.

When the thread is of medium screwing length, code N is not marked;

The rotation direction code is marked with LH for left-hand thread and not marked for right-hand thread.

Example interpretation

Example 1: what thread does M20x1.5LH-5g6g-S represent?

Example 2: what kind of thread does B36x14 (P7) -7H-L refer to?

Example 3: how to represent left-hand single thread fine thread ordinary thread with nominal diameter of 20 and pitch of 1.5?

6. Thread customization processing method

The thread can be customized by three methods: numerical control turning, three-axis linkage milling and tapping and threading.

Method 1 NC turning

NC turning with turning tool is the most commonly used customized processing method for single piece and small batch of threaded workpieces because of its simple structure.

Fig. 18 customized machining of turning thread

Method 2 NC Milling

Generally, disc milling cutters or comb milling cutters are used for milling.

Disc milling cutters are mainly used for milling trapezoidal external threads on workpieces such as screw rods and worms, while comb milling cutters are used for milling internal and external ordinary threads or conical threads.

Fig. 19 customized processing of NC milling thread

Method 3 tapping and threading

Tapping is to use a certain torque to screw the tap into the pre drilled bottom hole on the workpiece, so as to process the required internal thread.

Fig. 20 tapping

Threading is a forming method of cutting external thread on bar workpiece with die.

Fig. 21 threading

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 done by hand, or by lathe, drilling machine, tapping machine or threading machine.

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