The production of rolled threads

The Library of Thread Rolling - 6/9

In this blog, you will learn more about how rolled threads are produced, the raw materials that are used, the phases of thread rolling, the time and duration, the correct temperature, and monitoring.

Regardless of the individual processes or variants, thread rolling always involves a wedge-shaped tool geometry that cuts into the raw part material. The tool displaces the material, which flows into the free spaces of the tool contour during the process. The process is completed when either the intended flow height of the material is reached, or the tool contour is filled with material.

Raw material

For thread rolling, the raw material must be rotationally symmetrical, i.e., cylindrical with a constant diameter. However, thick-walled pipes and other hollow bodies may also be suitable for rolling. Filigree workpieces such as thin-walled pipes, on the other hand, are not suitable, as the material flows in all directions with large changes in material thickness. The required diameter of the raw part is determined based on empirical values or can be calculated. Two parameters are decisive: the unwinding ratio between the workpiece and the tool, and the flow volume of the material required for the intended final form.

Material requirement: elongation of at least 6%

In principle, any material is suitable for thread rolling, as long as it can be plastically deformed. In other words, the material must allow an elongation of more than 6% without fracturing during the thread rolling process.

Tensile strength of max. 1200 N/mm²

The information on the maximum permissible tensile strength – e.g., kf0 = 1200 N/mm2 – is less about the formability of the material, but more about a reasonable service life of the tool. The lower the tensile strength and the more correct the diameter, the longer the service life of the tool.

It is important to remember that the material to be rolled has a significant influence on the forming process, the rolling force applied by the machine, and the service life of the tools.

The plasticity of metals is determined by their crystal structure and their chemical composition. For example, the carbon content significantly changes the plasticity of steels: as the content increases, the possibility of forming decreases, and the tendency to cold work hardening increases.

When deciding which materials are suitable for thread rolling, special attention needs to be paid to the hardening of the material during deformation. The hardening is based on complex plastomechanical processes in the crystal structure and can be demonstrated on the ground cross-section of a rolled thread (Fig.1). It cannot be emphasized enough that, although materials that solidify result in highly resilient components, the rolling process requires special measures and a great deal of experience.

Phases of the thread rolling process

There are three phases during the thread rolling process (see Fig.2): the preparation phase, where the tool touches the surface, the first grooving phase, and then the second grooving phase, when the tool cuts into the material. These three phases are described in more detail below.

Fig.2: Three phases of thread rolling

Preparation phase

At the beginning of the forming process, the wedge-shaped tool touches the surface of the cylindrical workpiece. The contact zone is pointed or linear, depending on the geometry of the rolling tool. The contact surface of the workpiece with the tool is small at this point, taking into account the elastic properties of the materials. The force applied by the machine leads to a surface pressure that is usually much greater than the strength of the material. In this first rolling phase, i.e., the preparation phase, the compressive stresses applied to the blank are so high that the material begins to deform plastically.

First grooving phase

In the next phase of the rolling process, the first grooving phase, the wedge-shaped tool profile presses only slightly into the material. The base material changes its internal structure and solidifies in line with the described forming mechanisms. In this phase, the contact surface between the tool and the workpiece increases and becomes significantly larger than in the preparation phase. As the contact area increases, the surface pressure decreases, which means that the machine would have to apply a significantly greater rolling force in order to maintain the grooving speed. In most cases, however, the machine is already operating at maximum rolling force during the preparation phase. The grooving speed therefore usually decreases during the grooving process.

Second grooving phase

In the last rolling phase, i.e., the second grooving phase, the footprint increases and is thus significantly larger than in the first grooving phase. When the material starts to harden the grooving process is made even more difficult.

The progression of thread rolling

The maximum grooving distance specified by the gaps in the rolling tool is reached at different speeds depending on the material. The continuous decrease in surface pressure with the constant rolling force of the machine and the material hardening associated with the forming process have a significant influence on the progression of the grooving process.

All material-independent influences, especially the machine parameters and the tool geometry, must be known and matching in order to be able to make and evaluate statements about the rolling time of different materials. A more pronounced tread depth and form increase rolling time.

Temperature behavior and cooling during thread rolling

The increase in temperature is caused by the external and internal friction during the rolling process, and depends on the material and the machine parameters.

Using a cooling medium

In practice, forming is carried out using a cooling lubricant that flows over the workpiece and the tool. The chemical composition of the cooling medium and its application parameters are the manufacturer's well-kept secret, as the resulting differences in quality can be significant. Consistent temperature conditions on the workpiece and tool during rolling are especially important for through-feed machines (more on this topic in blog 8/9 Through-feed process) for producing threaded spindles with very small pitch tolerances. If this is not achieved, an undefined pitch distortion occurs.

Preparing coolant

To ensure that the temperature conditions remain constant during the rolling process, the machines have supply systems that provide the coolant in a consistent quantity, temperature, and purity. The temperature of the coolant is thus kept constant within a range of ±1.5 °C.

The change in temperature of the workpiece reveals how well the flow process has been achieved. The base material should not heat up or heat up only slightly. The expert knows from experience when to use which medium for cooling.

Monitoring

There are different ways to control and monitor the rolling process. A distinction must be made between production on conventional thread rolling machines, which are used for about 90% of rolled threaded parts, and CNC (Computerized Numerical Control) production, which is only used for manufacturing less than 10% of parts. The measuring and monitoring systems used nowadays record variables whose changes over time influence the forming process. The measured parameters can be force, torque, or power.

Workpiece measurement

In order to produce a perfect workpiece using conventional methods, the pre-machining diameter that determines the deformation must be precisely maintained. Hence, the workpieces are measured several times at suitable points during the production process.

If the workpiece is processed on a CNC machine, the groove depth and grooving speed of the tool into the workpiece are specified by the user interface and stored in a program. The controller constantly checks the values during processing and, if necessary, adjusts. Any differences in homogeneity in the material that cause the machine to be loaded differently can thus be compensated for.

Image processing systems and mechanical measuring equipment

In industry, image processing systems or mechanical measuring devices are used for this purpose. These machines can also be used to check the workpieces after processing.

Rolling force detection systems

Bar turning systems, which are used to produce millions of small and very small parts for the watch industry, are a special case. In these special systems, the material to be processed is fed fully automatically as blank bars. The forming process during the grooving procedure is tested and evaluated using rolling force detection systems. These systems record the force or torque curve over a specific distance or over a specified period of time. The two recorded parameters are then evaluated, resulting in envelope curves that may not be exceeded or fallen short of.
Another option for automated monitoring is the definition of windows through which the force progression must pass. The workpieces are rejected if they do not meet these criteria. These procedures provide indirect information as to whether the pre-machining diameter or the material met the specifications. They can eve n signal a tool breakage that is about to develop.

Find out in the next blog why the rolling process must be stopped before the intended shape is reached and how the expert can recognize this.
> The Library of Thread Rolling Blog 7/9

Fig.3: Thread rolling in the grooving process in blogs 8/9: Important rolling processes and tools

Eichenberger – your service provider 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
  
  

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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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