UKO carbide circle thread rolling dies are made of the highest quality materials. Manufactured to exacting standards, we can offer standard and customized thread rolling dies for in-feed and through-feed applications. Thread forms are precision ground and dies are produced to fit most types of circular screw nail thread rolling machine.

Figure 1 is a schematic drawing of two heat-treating cycles which give approximately the same hardness but with one having a significantly shorter transformation time, and Figure 2 is a schematic drawing of two heat-treating cycles with the same transformation time but with one having significantly higher hardness.

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Feeding: There are three basic techniques for feeding the stock into the dies: radial in-feed, tangential feed, and through feed. In radial in-feed, the dies move radially towards the axis of the stock. For tangential feed, the pitch of the stock approaches the rollers from its side making square, tangential contact. Lastly, through feed involves a cylindrical die that mates against the stock causing it to move axially.

Conclusion：Through the above explanation, we summarize the three points of the thread rolling dies to make a clear comparison.

Material Requirements: A known disadvantage of thread rolling is its incompatibility with hard materials. The materials to be rolled must have a hardness not greater than HRC 40. Materials that can be rolled are low-carbon steels, mild steels, stainless steels, copper alloys, and often, aluminum. Moreover, the material must have the right degree of ductility. The recommended range is 12 to 20% elongation factor.

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As an illustration, it is showed a method of complete bainite hardening of steel for use in bearings and other load carrying components, wherein bainite transformation is performed at a temperature just above the martensite formation temperature, transforming 25%—99% of the austenite into bainite at said temperature, and then increasing the temperature to speed up the transformation of the remaining austenite into bainite.

OSGthread rolling dies

Bainite hardening or austempering has been a developing hardening technique over the past decades and is particularly well applied in the bearings industry.Some key advantages of bainite hardening include a reduced size of the crystal lattice, which in turn leads to a reduced distortion of the part. Furthermore, in most applications a bainitic material shows desirable compressive stresses at the surface.

Choose the most suitable thread rolling tool for your workpiece, or we can suggest you for the case as long as we have your workpiece for the study in ahead!

In the in-feed process, the profile grooves of the rolls have the pitch of the thread to be produced. The rolls, driven at the same speed, move in the same direction. During rolling, the workpiece moves due to friction with no axial shifting. As the thread produced by this means is an exact copy of the rolling dies, threads of this kind have a very high pitch accuracy. The maximum length of the thread is limited by the width of the dies (30-200 mm).

Thread rolling is an integral cold-forming process, used to produce a variety of threads, knurls and helical gears. The process involves deforming a metal blank by rolling it through a die. This process forms external threads along the surface of the metal blank. Internal threads can be formed using the same principle. In contrast to other widely applied threading operations such as thread cutting, thread rolling is not a subtractive process. That means it doesn’t remove metal from the stock. Rolled threaded fasteners offer advantages such as stronger threads, precise final dimensions, good surface finish, and a lower coefficient of friction.

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Thread Rolling Speeds: Thread rolling speeds depend on the mechanical and power limitations of the machine, the thread diameter, and the material and hardness of the metal stock. Rolling speeds can range from 30 to 100 m/min. Low rolling speeds are required for hard materials while high speeds are for soft and ductile materials.

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The advantages or disadvantages of the process might vary in dependence with a special application, however, some general rules can be listed: As mentioned before, bainite consist of ferrite with fine carbide precipitations. Therefore, the changes in volume are reduced, followed by a reduced size of the crystal lattice, which leads to a reduced distortion of the parts. Furthermore, in most applications the tempering process can be saved. The bainitic material shows desirable compressive stresses at the surface, which are significantly higher than with martensitic hardening. Further advantages are higher yield strength with same hardness as tempered steel, higher tensile stress and improved notch impact strength. Finally, the residual austenite is lower than with martensitic hardening. As a disadvantage it has to be mentioned, that the dwell time might be very long and therefore the process might get uneconomical. Furthermore, the process parameters have to be carefully adjusted to the parts geometry and the material. Finally, the austempered material usually does not achieve the same maximum hardness as martensitic hardened material.

Habeggerthread rolling dies

Bainite hardening or austempering has been a developing hardening technique over the past decades and is particularly well applied in the bearings industry.Some key advantages of bainite hardening include a reduced size of the crystal lattice, which in turn leads to a reduced distortion of the part. Furthermore, in most applications a bainitic material shows desirable compressive stresses at the surface.

1. T. Lund et al: Method of complete Bainite hardening, United States Patent, Patent Number 6 149 743, 21 Nov 21, 2000; Accessed May 2020;2. H. Altena, K. Buchner: Process technology and plant design for bainite hardening, La Metallurgia Italiana - Nº. 3, 2016, p.23-26; Accessed May 2020.

Circular thread rolling die gives more control to the rolling process, with power to all rolls and controlled penetration rates. This allows harder and more difficult materials to be threaded.

Thread rollinghead for lathe

Coolant and Lubricant: Coolants or cutting fluids are extensively used in thread cutting, but these are also necessary for thread rolling. Deforming the metal also generates heat which can compromise both the dies and stock. Moreover, coolants can act also as lubricants to reduce the friction between the dies and stock.

In Figure 1, the temperature versus log time is plotted for two cycles giving approximately the same hardness, i.e., 60 HRC. Ms denotes the martensite start point, which normally varies between about 180°C and 280°C, depending on the composition of the steel alloy. Bs denotes the start of bainite transformation and Bf denotes the finish of bainite transformation. In order to obtain maximum hardness, the temperature should be close to the martensite start point. This, however, results in a very long transformation time Which is not economical, which is illustrated with the dotted line i in Figure 1.

Bainite hardening is sometimes preferred in steels instead of martensite hardening. This is because a bainitic structure in general has better mechanical properties, e.g., higher toughness, higher crack propagation resistance, etc. Therefore, for critical components such as bearings or other load carrying components especially subjected to fatigue, a bainitic structure could be most suitable. Bainite hardening is widely used to produce components with high strength and hardness. For a given steel, the bainite structure often exhibits superior mechanical proper ties and structural stability compared to a martensite structure. The disadvantage with bainite hardening, however, is the long process time. To reduce the processing time for isothermal bainite transformation, it is necessary to increase the bainite transformation temperature. However, this will reduce the hardness which could affect the properties of the component.

In-feed rolling uses a timed machine cycle. This is used for the general rolling of parts up to the maximum width of the face of the dies, less die chamfers. In-feed rolling dies, also known as plunge rolling, are suitable for threading with a shoulder or a headed workpiece.

In the last years an increased interest in bainite hardening processes for different applications can be found. Besides the applications in the bearing industry, where salt bath quenching is state of the art, more and more applications can be found, where bainite hardening generates advantages in comparison with martensitic hardening and tempering. Bainite hardening allows higher yield strength with same hardness as tempered steel, higher tensile stresses and leads to desirable compressive stresses at the surface. However, it is necessary to carefully select the material grade according with the wall thickness or weight of the parts and the desired yield strength. Furthermore, the cooling strategies have to be optimized to minimize the dwell time in the salt bath.

According to the presently claimed invention, illustrated with the solid line 1 in Figure 1, the bainite transformation is performed slightly above the Ms point until more than 25% up to 99%, preferably 50%—90%, most preferably 60%—80%, transformation has occurred, and the remaining hardening is performed at an increased temperature at which the transformation proceeds with increased velocity to 100%. The temperature can be increased any amount so as to result in a complete bainite transformation. The amount of temperature increase and time to complete transformation can be determined by one of ordinary skill using known techniques. The fraction of bainite formed at the higher temperature has not shown any noticeable effects on the hardness.

Thru-feed rolling is used for rolling threads that exceed the maximum face width of the dies, as well as for continuous rolling of long threaded bars. Here, the rolling dies have grooves with no pitch; the groove cross-section is that of the standard flank profile. The thread pitch is produced by skewing the die axes by the pitch angle of the thread. This means the workpiece is given an axial thrust and moves one pitch length in the axial direction for every full revolution. As the axial thrust kicks in immediately when the workpiece enters the assembly, larger threads are not formed to full depth in one pass.

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Another Way to use the process according to the presently claimed invention, is to increase the hardness for a given transformation time, which is illustrated in Figure 2. In the process illustrated with a dashed line, the transformation takes place at the optimum temperature in view of the velocity of transformation, versus time consumption for 100% transformation to bainite. This optimum temperature can be determined by the skilled artisan using conventional techniques. The resulting material has a hardness of 58 HRC. According to the presently claimed invention, the bainite hardening is performed along the solid line at a lower temperature to more than 50%, preferably 60%—80%, bainite transformation, and then the temperature is increased to the optimum temperature. This results in a steel alloy with a hardness of 60 HRC, using the same transformation time. In practice, both features could be achieved compared to traditional bainite hardening, i.e., shorter transformation time and higher hardness. The results also show increased structural stability Which is very important in many applications.

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Chamfer Angle: Chamfer is the tapered conical surface at the start of a thread. Before rolling, the edge at one end of the stock must be machined to have a chamfer. A correct chamfer angle must be set to properly shape the thread at the end. The recommended chamfer angle is 30° for most cases.

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