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Thread

Updated: Oct 25, 2023


Figure 1: Thread tap

Threaded connection is one of the most common types of mechanical connection of parts. It is used in a variety of applications and industries, both in everyday life and in industrial environments. The most important information that you need to know as a cutting machine operator, engineer or craftsman is collected here!


Threaded connection offers several advantages that have contributed to its widespread use. It is easy to assemble and usually does not require any special tools or equipment. After dismantling, a connection can be easily reassembled, making maintenance, repairs or replacement of parts easier. In the form of screws, nuts, bolts, pipe connections, valves, machine tools, vehicles, aircraft and much more, threads are used in many technically essential products and structures.

There are a variety of thread types that have different properties depending on the intended use and connection requirements.

Some of the most common thread types are:

- Metric thread (ISO thread):

The metric thread is the most widely used thread system and is standardized according to the ISO standard 68-1. It is used in most countries for a variety of applications. Metric threads have a uniform profile and are identified by their nominal diameters and pitches (e.g. M8x1.25).

- Pipe thread (Unified Thread Standard - UTS):

Pipe thread is primarily used in the United States and some other countries. It is specified by the inch size (e.g. 1/4-20) and the thread pitch. NPT (National Pipe Thread): Common in plumbing and piping applications in the United States. BSP (British Standard Pipe): Widely used in the United Kingdom and many other countries.

- Whitworth thread:

The Whitworth thread was the first standardized thread system and was developed in Great Britain. Although it is no longer widely used today, it can still be found in some older machines and systems.

- Trapezoidal thread:

Trapezoidal threads have a trapezoidal thread profile and are often used in applications that require high load capacity and wear resistance, such as threaded spindles in machine tools.

- Knuckle thread:

Knuckle threads are special threads with a rounded profile used in some applications to prevent damage or provide a better seal.

- Saw thread:

Buttress threads have a triangular thread profile and are often used for wood screws because they easily cut into the wood.

- Special thread:

There are many special thread types for specific applications, such as gas threads (e.g. G-thread), pipe threads (e.g. R-thread) and fine threads (e.g. M-Fine).

A thread is defined by various parameters that determine its size, shape and properties, as shown in Figure 2.



The most important parameters include:

  1. Thread diameter (D for internal thread and d for external thread): The thread diameter is the distance from one side of the thread profile to the opposite side, measured either over the thread height for metric threads or over the flanks for imperial threads.

  2. Pitch (P): The pitch is the distance between consecutive thread crests or flanks. It is often measured in millimeters per revolution (mm/rev) for metric threads or in inches per revolution (TPI) for imperial threads.

  3. Thread tolerance: Thread tolerance defines the permissible deviation in relation to the theoretical ideal thread diameter and pitch. It is usually expressed by letter combinations such as "6g" for metric threads or "2A" for imperial threads.

  4. Thread depth (h3 for external thread and H1 for internal thread): Thread depth is the distance between the thread tip and the thread root. It can be expressed as a percentage of the thread diameter, e.g. B. 75% or 60%.

  5. Flank angle (α): The flank angle is the angle between the flanks of the thread. This angle varies depending on the thread type. For example, the flank angle on metric threads is usually 60 degrees.

  6. Number of Threads: Number of threads refers to the number of threads per unit length, often expressed as revolutions per meter or inch (U/M or U/IN).

  7. Thread Type (Internal or External): A thread can be either internal or external depending on whether it is a hollow hole or a bolt.

There are several thread forming methods that can be used depending on the requirements and material of the workpiece. Thread cutting is the most commonly used method for creating threads. A tap or die is used to cut the thread into a hole or external thread on a rod. There are different taps for metric and imperial threads as well as for different thread pitches. The core hole must also have a suitable diameter in order to be able to continue cutting the threads. The following table lists the most common nominal dimensions for the threads.

There are several thread forming methods that can be used depending on the requirements and material of the workpiece. Thread cutting is the most commonly used method for creating threads. A tap or die is used to cut the thread into a hole or external thread on a rod. There are different taps for metric and imperial threads as well as for different thread pitches. The core hole must also have a suitable diameter in order to be able to continue cutting the threads. The following table lists the most common nominal dimensions for the threads.


Metric ISO thread according to DIN 13

Nominal diameter

Pitch

Core hole diameter

d

P

d3

M3

0,5

2,4

M3,5

0,6

2,8

M4

0,7

3,2

M4,5

0,75

3,6

M5

0,8

4,1

M6

1,0

4,8

M8

1,25

6,7

M10

1,5

8,2

M12

1,75

9,9

M14

2,0

11,6

M16

2,0

13,6

M18

2,5

15,0

M20

2,5

17,0

M22

2,5

19,0

M24

3,0

20,4

M27

3,0

23,4

M30

3,5

25,8

M33

3,5

28,8

M36

4,0

31,1

M39

4,0

34,1

M42

4,5

36,5

M45

4,5

39,5

M48

5,0

41,9

M52

5,0

45,9

M56

5,5

49,3

M60

5,5

53,3

M64

6,0

56,7

M68

6,0

60,7

During thread rolling, a smooth workpiece is pressed between two rollers or dies with the desired thread shape. This process is often used for mass production of threads and produces threads with high strength and accuracy.

Thread drawing involves pulling the thread through an opening, forming it onto the workpiece. This process is often used for pipes and thin-walled parts.

Thread milling is done using a special thread milling cutter that mills the thread into the workpiece. This process is suitable for high-precision threads and is often used for larger diameters or special thread profiles.

Thread forming involves pressing a specially shaped tool into the workpiece to form the thread. This process is often used for screws with high strength and accuracy.

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