How to Choose Screw Thread Rolling Machine?

Important rolling processes and tools

Specialist topic: An exciting overview of thread rolling... The basic, processes, tools, and applications of rolled high-tech threads

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Various rolling technologies

In industry, several rolling technologies have become established. Even in the early days of thread rolling, a distinction was made between processes with round tools and those using flat tools (flat dies). Sometimes the rolling processes are subdivided according to the number of tools used.

In the following description of the different rolling processes for high-tech threads, we will only deal with general issues.

Rolling process with two round tools, symmetrical or asymmetrical

When thread rolling with two externally profiled tools, the two rollers rotate in the same direction. A distinction is made between a symmetrical and an asymmetrical variant. In the symmetrical variant, both tools move towards the blank, while in the asymmetrical variant, one of the (rotating) tools remains stationary while the other moves towards the blank. The major advantage of the symmetrical variant is the significantly longer tool life.

The blank is always positioned between the rolling tools. If the moving tool touches the (still) stationary blank, the latter is set in rotation by the frictional connection that builds up very quickly. The tool profile is then formed as a thread in the blank.

Grooving process

Every mechanic is familiar with grooving during turning: The turning tool pierces the blank and forms a predetermined geometry. In the grooving process, the two movable tools move towards the blank and cut into it with the profile. The tools have grooves with which the pitch angle of the thread is rolled.

Fig. 1: Thread rolling using the grooving process: The two tools pierce the workpiece symmetrically with the wedge-shaped pitch groove.

Very precise, but finite threads

The principle of the grooving process allows the production of very precise threads. The disadvantage is obvious: Because the blank is not fed in, the maximum thread length is limited to the tool length.

The accuracy of the thread largely depends on the tool, which is manufactured in several steps. The tool profile is most often produced by grinding. Grinding is only carried out after hardening in order to eliminate the distortion caused by heat treatment. The surface quality of the roller profile is decisive for the quality of the threads produced.

Their manufacturers – specialist companies – use high-quality tool steel for the tools. Most manufacturers prefer special alloys, but some also offer tools made of other materials and with different heat treatments on request.

Through-feed process

The two tools for the through-feed process have several grooves without a pitch on their circumference, which run in a circle around the tool. The two rollers are positioned horizontally in the machine and are inclined in opposite directions around their horizontal transverse axis, in each case by the pitch angle of the thread to be rolled. If the front end of the right-hand roller is pivoted downwards and the left-hand roller upwards, a right-hand and a left-hand thread are created respectively.

Fig. 2: Thread rolling in the through-feed process: The inclined position of the two tools without pitch can be clearly seen

As the tools swivel and force is applied, the blank also moves forward in the horizontal axis (parallelogram of forces!). This allows the rotating blank to pass through the machine; the throughput speed can be influenced by the rotation speed of the rollers.

“Infinite threads”

The advantage of the through-feed process is the ability to produce a thread of (theoretically) infinite length. The length ultimately only depends on the length of the blank. However, it is a fact that the dimensional accuracy compared to the grooving process is slightly lower.

The machine must have swiveling roller spindles for the through-feed process with two tools without pitch.

One pair of tools for different core diameters

A further advantage is that the thread manufacturer can roll the same pitch on workpieces of different diameters with a single pair of tools, apart from a few restrictions. However, if very tight thread tolerances are required, it may still be necessary to produce a separate tool for each batch of material.

The choice of material for continuous tools is the same as for grooving tools. The grooves are created by grinding and, more recently, increasingly by hard turning with special plates. Because of the axial feed-in, the tools have three zones with different outside diameters, analogous to the three phases of the rolling process: a run-in zone, a calibration zone, and a run-out zone.

Fig. 3: Representation of a tool for the through-feed method (a diameter of the calibration zone)

The transition from the run-in zone to the calibration zone determines whether the tools achieve the required service life. Through-feed tools from different manufacturers differ relatively greatly in this respect. If “difficult” base materials are to be rolled, it is recommended to seek the advice of the tool manufacturer.

Partially corrected continuous process

The partially corrected through-feed method is a combination of the grooving and through-feed methods. The grooving process controls the pitch on the tools, while the swivel ability of the rolling spindle is transferred from the through-feed process. The profile pitch of the tools is not identical to the pitch of the thread to be rolled. The angular difference is compensated by tilting the roller spindles, as described for the through-feed method. Thread rolling tools for the partially corrected through-feed process also have a run-in, calibration, and run-out zone.

Large pitches

With the partially corrected through-feed method, considerably greater gradients can be achieved than with the pure through-feed method. This can be illustrated with an example: The thread must have a pitch angle of 15°. However, a roller spindle for the through-feed process can be swiveled by 5° at the most. To achieve the required 15°, the tools are given a profile pitch of 10° and the missing 5° is set on the machine.

Fig. 4: Thread rolling in the partially corrected through-feed process: The tools have a pitch groove and are also inclined.

The blank is moved in an axial direction during the forming process, as in the through-feed process. The service life of the rollers depends to a large extent on the tool design and the material of the raw part.

Through-feed process with corrected tools

The through-feed process with corrected tools makes it possible to work with a machine without swiveling roller spindles. However, this is only recommended if a machine with swivel roller spindles is not available.

The through-feed process with corrected tools is similar to the partially corrected through-feed process. The pitch angle of the tool profiles is either larger or smaller than that of the thread. Unlike the partially corrected through-feed method, however, it is not possible to correct the pitch by swiveling the roller spindles. The disadvantage is therefore that you have to accept pitch errors.

Rolling process with three or more tools

Thread rolling with three or more tools is always a process based on the methods with two rollers. Three rollers are generally used and arranged symmetrically around the blank at angles of 120° to each other. The blank is gripped centrally by the tools and set in rotation, just as with the grooving and through-feed processes. The multi-roller principle makes support blades and holding devices superfluous.

Fig. 5: Thread rolling machine with three tools

Suitable for pipes and hollow parts

It is mainly used for pipes and hollow parts. The disadvantage is the high investment for the machine and tools.

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Supplementary rolling procedures

The most important rolling processes for high-tech threads – especially for screw spindles – are the grooving process, the through-feed process and the partially corrected through-feed process. For those readers who are interested, we are also providing as comprehensive an overview as possible of additional thread rolling processes and their relevant tools that used in industry, but without any claim to completeness.

Cross rollers with two flat tools

Cross rolling is characterized by two opposing flat tools (flat jaws) that engage with the rotating blank. They generally move past the blank in opposite directions during thread rolling. There is also a variant in which one flat jaw is fixed in the machine while the second flat jaw – mounted on a slide – moves past the workpiece. This process has found its place in mass production – e.g., of standard screws – and where accuracy requirements are low. It is a standard procedure, but its possibilities have been exhausted.

Fig. 6: Flat tools

Thread rollers with the segmented roller

In thread rolling with the segmented roller, three fixed thread segments in the machine body with a run-in and run-out zone press the rotating and passing blank against a rotating thread ring. The advantage of this thread rolling method is the uniform rotary movement of the threaded ring seated on the shaft.

Fig. 7: Segmented roller with three segments and a threaded ring

Thread rolling with rolling heads

Thread rolling with rolling heads is a process that is used quite frequently when threads are to be produced on a lathe. The tools used are not actuated. The rotary movement of the blank is generated by the machine tool. This production method is very economical. Turned parts – including the threads – can be completely produced on automatic machines, i.e., without reworking (bar turning).

Thread rolling with thread rolling dies

Thanks to its compact design, the thread rolling die enables threaded bolts to be produced without clamping, even in the tightest of spaces, e.g., in the machining area of a machine tool. The thread length that can be produced is limited by the machine tool. After the rolling process, the thread rolling dies must be turned back in the resulting thread.

A distinction is made between adjustable and non-adjustable tools. Adjustable thread rolling dies are set with the aid of a holding device. The advantage of thread rolling with adjustable rolling dies is not insignificant in machine building. Without special rolling machines, the use of adjustable rolling dies is extremely economical and therefore recommended for small and medium-sized series. In the case of non-adjustable thread rolling dies, it is particularly important to specify the thread tolerances and the material in advance. Non-adjustable rolling dies are suitable for larger series.

Fig. 8: Thread rolling dies

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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