Basics of thread rolling

The Library of Thread Rolling 3/9

Find out more about thread rolling in this blog article. You can expect exciting insights into the different types of rolled thread systems and their applications, as well as into how they are tested.

What is a thread

Physically speaking, a thread is a spiral-shaped inclined plane; a screw and a matching nut form a sliding combination. The following rule of thumb applies to the use of threads: Threads with a small pitch are used for fastening and threads with a larger pitch for movement.
Thread designations are made up of a code letter for the thread profile and the specification of the outside diameter in addition to any other details. Take for example the M10 thread designation, where M stands for a metric standard thread with an outside diameter (nominal size) of 10 mm and a pitch of 1.5 mm. If M10 is followed by the abbreviation LH (left hand), it is a counterclockwise thread. However, threads that rotate clockwise are more common. These threads are abbreviated to RH (right hand).

The thread systems and their thread profiles described below apply to all known manufacturing methods, such as milling, turning, swirling, grinding, or rolling. The manufacturer may partially modify the standard profile for rolled threads.

Types of rolled thread systems

The three best-known rolled thread systems are:

• The metric ISO thread
• The Whitworth thread
• American thread types - the ISO inch thread

All three thread systems have both standard and fine threads. Although no globally harmonized thread standard is currently anticipated, the thread is considered to be the most comprehensively standardized machine element. In addition to generally applicable standards, movement threads have particular “specifications”. These specifications, however, are usually dependent on the manufacturer.

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The metric ISO thread (M)

The metric thread is a globally standardized thread with metric dimensions and a 60 degree flank angle (see Fig. 2, Basic profile of the metric ISO thread according to DIN 13). The outer edges run together in a wedge shape, which is why it is also called a sharp or V thread. The “M” stands for metric standard thread, while the number following it indicates the outside diameter in millimeters. The exact definition can be found in DIN 13 and DIN 14. The designation “MF” stands for the metric ISO fine pitch thread, which has a smaller pitch and is used in the watch or measuring device industries, among others.

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Fig. 1: Basic profile of the metric ISO thread according to DIN 13

The MJ thread (ISO 5855) is a modification of the metric ISO thread used in the aerospace industry. This thread has an enlarged core diameter and radius at the base.
The following abbreviations of the metric thread profile apply to the external threads on the bolt.

• D = nominal diameter (outside diameter)
• P = pitch
• h₃ = 0.61343 P (thread depth)
• R = 0.14431 P (radius at the thread root)
• R = 0.15...0.18 P (for MJ thread)

Whitworth thread

Named after its inventor Sir Joseph Whitworth, the Whitworth thread is specified in inches. Unlike the metric ISO thread, the flank angle of the Whitworth thread is 55°.
The thread profile of the Whitworth thread, which is standardized according to British Standard 84 (Fig. 5), can be divided into: BSW (British Standard Whitworth; Whitworth standard thread), BSF (British Standard Fine; fine thread) and BSP/G (British Standard Pipe; pipe thread; according to DIN ISO 228 with the designation “G”).

The pitch P is calculated using the number of turns per inch specified in the dimension tables. The following rough definition applies to external threads on the bolt.

Fig. 2: Basic profile of the Whitworth thread according to BS 84

• D = nominal diameter (outside diameter)
• P = pitch (specified in practice in turns per inch)
• h₃ = 0.64033 P (thread depth)
• R = 0.13733 P (radius at the thread root)
  
  

American thread types—the ISO inch thread

The American ISO inch threads (UST threads according to ASME B 1.1 and B 1.2):
UNC Unified National Coarse (coarse thread), UNF Unified National Fine (fine thread), UNEF Unified National Extra Fine (extra fine thread), UNS Unified National Special (“free” special thread with additional specifications) and UNJ Unified National thread series with external thread controlled root radius (ISO inch thread with enlarged core diameter and radius at the thread root), comparable to the MJ thread.

Fig. 3: Basic profile of the ISO inch thread

The specifications for the outside diameter and the number of turns indicated in the dimension tables are based on inches as the unit of length (1 inch = 24.4 mm). As with the metric ISO thread, the flank angle is 60°. The profile of the ISO inch thread shown in Figure 6 therefore corresponds to that of the metric ISO thread.

• D = nominal diameter (outside diameter)
• P = pitch (specified in practice in turns per inch)
• h₃ = 0.61343 P (thread depth)
• R = 0.14434 P (radius at the thread root)
• R = 0.15...0.18 P (for UNJ thread)

Special rolled threads

This section describes other thread profiles that are important for thread rolling: the trapezoidal thread, the round thread, and the Gothic thread profile.

Trapezoidal thread

The nominal diameter and pitch for trapezoidal threads according to DIN 103 follow the ISO metric thread standard; the thread depth h3 is 0.5 P, the flank angle 30° (Fig. 7). For trapezoidal threads according to DIN 380 (lower load-bearing depth), the core diameter is increased by 0.4 P; otherwise, the profile is identical to that to DIN 103. A radius is permissible on the core diameter (P/2 0.15) for thread rolling.

Fig. 4: Basic profile of the trapezoidal thread according to DIN 103

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Round thread

Round threads according to DIN 405 are particularly suitable for process valve fittings, as they are especially resistant to dirt and damage. Fire service pipes and hoses have threads that are insensitive to impact and dirt (Fig. 5). The outside diameter is given in millimeters, the pitch in inches. It is worth mentioning that the thread of the nut has a different radius than that of the screw.

• D = nominal diameter
• P = pitch
• h₃ = 0.5 P (thread depth)
• R₁ = 0.238 P (same radius at the outside diameter and thread root of the bolt)

Fig. 5: Basic profile of the round thread according to DIN 405

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The Gothic ball screw thread

The profile of the Gothic ball screw (Fig. 6) is used for threaded spindles in ball screws.

Fig. 6: Gothic thread profile for ball screws
P pitch, x radius offset

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Other special threads

In addition to the standard rolled thread systems listed at the beginning of the article, there are also various special threads that have their own profile:

• The Löwenherz thread has a flank angle of 53° 8' and is known as a precision engineering thread. It has now been replaced by the metric basic profile according to DIN 13.
• The E thread (Edison thread according to DIN 49689) is the well-known lamp socket thread found in every household. Two specially interlocking radii ensure good electrical contact.
• The Thury thread named after Marc Thury from Geneva is a special ultra-fine watch thread.
• The BA thread, a fine pitch thread with a flank angle of 47° 30', is available up to 6 mm diameter. It was developed after 1900 for the watch industry and precision mechanics.
• The Swiss NHS thread is another special thread for the watch industry. It has even made its way to America, but its profile there is defined in inches.
• Special threads for fuel and oil lines often have profiles with better sealing properties.

Determining the accuracy of fit

The accuracy of fit of an external thread in a nut element with internal thread is determined by the following elements:

• Outside diameter (nominal diameter)
• Core diameter
• Gradient
• Flank angle
• Flank diameter

The pitch of standard threads is directly related to the outside diameter (diameter-pitch series). The flank diameter is important because it represents the tolerance base (zero line) (Fig. 7).

The ISO thread tolerances

Similar to the fitting tolerance system, the ISO thread tolerances are made up of a specification for the tolerance position and one for the size of the tolerance zone.
The tolerance position is determined by one of the code letters a to h, while the size of the tolerance zone is indicated by a number between 3 and 9 (Fig. 7). To ensure the fitting tolerances are clearly differentiated, the number for the thread—the size of the tolerance zone—is stated first, e.g., 6h and not h6. The smaller the number, the more accurate the tolerance.

Fig. 7: Tolerance position (x) and size of the tolerance zone (y) for a thread (Language: German)

The pitch diameter and the outside diameter of a thread can have different tolerances. For example, M10-4h-6g means that the tolerance of the flank diameter is 4h and that of the outer diameter is 6g. Tight tolerances of a thread allow larger tolerances of the corresponding nut element and vice versa.

Tolerance positions a to g are particularly suitable if there are plans to have the surface protected (by hot-dip galvanizing, electroplating or chromium plating) with the coating thicknesses commonly used nowadays.
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Measuring methods and measuring equipment

The three-wire test method

In the three-wire test method, two cylindrical test wires are placed in the threads on one side and one test wire on the opposite side (Fig. 8). The outer distance between the test wires, the test dimension M, is then measured with the flat and parallel test surfaces of a measuring device, e.g., a micrometer; this test dimension can then be used to determine the pitch diameter d2 of the external thread. Two conditions must be met for the test wire diameter: The diameter of the test wires must be large enough to protrude beyond the thread tips. The test wires must be as close as possible to the theoretical flank diameter so that existing flank angle deviations cannot significantly influence the measurement result. The theoretical measuring wire diameter is calculated.

Fig. 8: Three-wire test method for measuring a thread:
M dimension via measuring wire

Thread measuring and testing

In most cases, the thread is checked using a ring gauge. A micrometer with measuring inserts is rarely used. The thread is measured directly on the thread flank (Fig. 9). The measuring inserts are ground and have the thread flank angle. The measuring tool, a micrometer with holes for the inserts, is available as standard for metric and imperial threads. Ring gauges are used to check the thread tolerance (run-out): The go ring gauge must run on the thread, the no-go ring gauge should not.

Fig. 9: Outside micrometer

Another test method is the thread gauge (also known as a limit gauge), which is used to check the pitch diameter of a thread. The good side of the jaw gauge represents the maximum permissible size and must slide over the test point due to its own weight. The reject side is smaller by the tolerance and represents the smallest dimension and must withstand its own weight. The result is good or bad or rework is required.

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With Eichenberger, you can implement your individual requirements for a screw thread right from the start of development. Don't hesitate to contact us before you even know what you need. We will be happy to work with you to develop a customized solution for your thread system. Contact one of our experts directly, we are there for you right from the start.
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With state-of-the-art production methods, many years of expertise and a tool inventory of over 1,000 rolling tools, we produce rolled screw drives that meet even the most unusual requirements:

• Gradients up to 6x in diameter
• Slope accuracy class G5
• Spindle lengths up to 6 meters
• Spindle diameter from 2 to 160 millimeters
• All standard profiles (M, Tr, UNC, UNF, UNEF, Whitworth)
• Multi-start threads, also as right/left-hand threads
• Steep thread profiles
• Ball screw profiles
• Special profiles
• Worm thread profiles (special quality and price advantages!)
• Serrations and knurling
• Conical thread
• Threads on prefabricated and/or bulky parts, e.g., also on forged parts
• Freely designed thread geometry
• Responding to customer requirements, such as tailored nut geometry
  
  

Eichenberger leaves nothing to chance and places the highest value on quality. This is what has been impressing our customers since 1953. See for yourself!

> 100% Swiss Quality
> Thread specialist since 1953

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Literature and sources

Apel, H. (1952). Gewindewalzen: Kaltverformen von Präzisionsgewinden und Spindeln. Hanser.
DeWiki (2022, 3.August). Lexikon Gewinde. https://dewiki.de/Lexikon/Gewinde
Kübler, K. & Mages W.J. (1986). Handbuch der hochfesten Schrauben, (1. Aufl.). Girardet.
Peters, H. (2003). Mathematisch-Technisch-Algorithmisch-Linguistisches Sammelsurium. http://www.hp-gramatke.de
Trösch, B. & Husistein, K. (2007). Bibliothek der Technik -, Band 286, Gewinderollen. Moderne Industrie.
Verein Deutscher Eisenhüttenleute (1984) (Hrsg.). Werkstoffkunde Stahl, Bd. 1. Springer,
Wikipedia (2022, 3. August). Metrisches ISO-Gewinde. https://de.wikipedia.org/wiki/Metrisches_ISO-Gewinde

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Abbildungen: Nr. 1, 23-25 RWT Rollwalztechnik GmbH, Engen; Nr. 2 Foto Deutsches Museum, München; Nr. 3 Musée du tour automatique et d'histoire de Moutier, Moutier (Schweiz); Nr. 4 Fette GmbH, Schwarzenbek; Nr. 5 Meinrad Plaz, Staufen (Schweiz); Nr. 6 Habegger SA, Court (Schweiz); Nr. 34-36 FBT Fahrzeug- und Maschinenbau AG, Thörigen (Schweiz); Nr. 7, 8 Schleuniger AG, Thun (Schweiz); Nr. 9, 10 Max-Planck-Institut für Physik (Heisenberg-Institut), München; Nr. 11 Saurer AG, Arbon (Schweiz); Nr. 12 Line Tech AG, Glattbrugg (Schweiz); alle übrigen Eichenberger Gewinde AG, Burg (Schweiz). Satz: abavo GmbH, D-86807 Buchloe. Druck und Bindung: Sellier Druck GmbH, D-85354 Freising. Printed in Germany 889030.