Blog

lathe VS rolling thread

Size and material certainly determine product performance, but without the support of production technology, even the best product design is difficult to achieve its expected 

performance. Proper production technology can even achieve unexpected results.

If the process is different, even products made with the same size and material that look exactly the same will have different performance.

A classic case where the appearance and material are the same but the process is different, resulting in differences in product performance: lathe threads and rolling threads

Compare the two from the following four aspects, striving to comprehensively interpret the differences between the two processes.

process comparison

In short lathe thread refers to a machining method that uses cutting technology to "dig" threads on a round rod to produce bolts, as shown in Figure 1.

Figure 1: fixture for lathing thread

Material comparison

Imagine a scenario: you put a freshly made dough on a lathe and cut it. The consequence is that not only will you not be able to cut the shape you want, 

but the dough will also  be too soft and make the lathe covered in dough.The same applies to threading. If the material is soft (with good plasticity), the 

threading process may result in a certain degree of "stickiness" between the rod and the tool due to the good plastic deformation ability of the rod. 

This not only prevents the tool from fully utilizing its advantages, but also may result in irregular thread s

hapes during threading.

Generally speaking, only materials with lower ductility and higher hardness are more suitable for using threading technology to process threads, 

such as ductile iron.

On the contrary, if metal molds are used to process threads on materials with low ductility and high hardness, the risk is that the strong extrusion 

force during the rolling process may fracture the material, causing small cracks inside the product and seriously weakening the mechanical properties 

of the bolt.

However, materials with better plasticity are more suitable for wire rolling processes due to their excellent formability (plasticity). According to 

industry experience, the elongation at break of materials should reach at least 12% to be more suitable for wire rolling, such as most alloy steels.

In addition, any type of lead containing material is also more suitable for automotive wiring. The reason is that this type of material is prone to 

producing thin sheets during the extrusion process, resulting in material delamination and a poor appearance.

Cost comparison

To compare costs, it is necessary to focus on both materials and efficiency.

Firstly, the materials. As mentioned above, threading is done by "digging" out the thread, while rolling is done by "squeezing" out the thread. Therefore, compared to lathe thread 

wire, rolling wire has almost no material waste, it just changes the distribution of materials. lathe thread wire not only generates steel waste, but also requires other devices to 

prevent iron filings from splashing, and recycling devices to collect the waste.

In terms of time, rolling silk (especially rubbing silk) is much faster than threading. Although I don't have specific data comparisons, I can provide logical examples to illustrate.

For example, you need to process one bolt with L1=50mm and one bolt with L2=100mm.

If the threading method is used, the production time of the latter will inevitably be twice that of the former, mainly because the characteristics of the threading process itself 

determine that the processing time is positively correlated with the number of threads.

If the same two bolts are changed to twisted threads, the situation will be different. In fact, as long as the height of the screw plate is sufficient, regardless of the length of 

the bolts or the number of threads, all bolts are formed in one go. The production time for L1=50mm bolts and L2=100mm bolts is exactly the same.

Therefore, the longer the bolt, the more efficient the rolling process can be reflected. Some factories have reported that the rolling efficiency is 10 times higher than the 

threading efficiency.

This is assuming that both threading and rolling are processed in one go. In fact, rolling can be formed in one go, while threading often needs to be repeated many 

times (in order to meet the thread size requirements).

Comparison of Bearing Capacity

If the cost mentioned earlier can still be compromised, safety and quality cannot be compromised.

Generally speaking, the bearing capacity of rolling bolts is 15-30% higher than that of threaded bolts, and their fatigue performance is 50-75% higher than that of 

threaded bolts. These comparative data are not from first-hand information in the hands of the editor, but rather hearsay and are only for reference.

Similarly, the editor only analyzes the mechanical performance differences between the two from a logical perspective.

The different thread forming processes of "digging" and "squeezing" determine the different internal grain structures of the product after forming. Fastener 

manufacturers are familiar with the analysis of metal flow lines, which is one of the contents of bolt testing. It mainly tests whether the metal flow lines at the 

junction of the bolt rod and the plate twisting surface are smooth and regular. Generally speaking, the most basic requirement of fasteners for streamline is 

that the streamline must be continuous.

Why is the low performance of lathe thread bolts

The typical metal streamline diagram of a lathe thread wire bolt is shown in Figure 4- the streamline of its small diameter part is continuous (because it 

has not been damaged), but the streamline of the thread part is interrupted due to cutting, which looks like an excavator has cut through an underground cable.

Figure 4 Thread Flow Line of lathe thread

The streamline direction of the screw thread part is the same as that of the rod part, which is along the length direction of the bolt (horizontal direction). 

After the bolt and nut are adapted, the force transmission method is that the squeezing force between the nut thread and the bolt thread is transmitted to 

the screw along the axis direction of the bolt rod in a way that the thread is sheared, and finally presents as tension of the screw, as shown in Figure 5. 

Figure 5 Force transmission path of fasteners

For those interested in the transmission path, please move on to the thoughts triggered by the bolt tensile test in my article.

The reason why these types of threads have low stress performance can be summarized by the editor into three reasons:

1. The direction of the shear force acting on the thread is the same as the direction of the streamline, resulting in the entire thread being easily cut laterally.

2. In this case, the streamline of the thread does not actually play a role, mainly due to the interlayer shear strength between the streamlines.

2. The valley of the spiral tooth belongs to the stress concentration area (Figure 6), although it is reinforced by chamfers, it only alleviates the degree of stress 

concentration, rather than eliminating it. When force is transmitted through the screw thread to this point, it is easy to form a weak point here, and the first 

failure occurs - manifested as rod fracture or transverse failure of the screw thread. Figure 6 Stress concentration area of vehicle thread

3. During the process of forming threads, cutting tools are prone to produce machining defects on the surface of the thread, such as cracks or pits. 

These defects are most likely to become the source of fracture when the bolt is subjected to stress, especially fatigue loads. Under repeated loading, 

these microscopic defects will gradually expand until the bolt is destroyed.

Why do rolling bolts have strong performance

The typical metal streamline of a rolling bolt is shown in Figure 7- both the streamline of the small diameter part and the thread part are continuous 

without any interruption. The biggest feature is that the streamlines at the trough position are particularly dense.

Figure 7 Thread Flow Line

The reason why these types of threads have high stress performance can be summarized by the editor into four reasons:

1. The streamline of the spiral teeth is distributed along the shape of the spiral teeth like a dragon ridge terrace. The advantage of this distribution is

2. that the streamline and the spiral teeth are subjected to a certain angle of shear force, which means that the force of tension on the streamline can

3. completely resist the shear force of the spiral teeth, and this strength is much greater than the interlayer shear strength between the streamline.

2. The valley where stress is concentrated, although the stress is high, has a high streamline density and strong resistance. Not only does chamfering 

alleviate the degree of stress concentration, but it also improves its own strength through "own efforts".

Figure 8: The stress distribution of the rolling thread is more uniform

3. Rolling wire belongs to cold processing, and plastic deformation occurs during the extrusion process of the thread. As we have discussed before, after irreversible plastic deformation of the material, its strength will increase - that is, the strength at the valley position will be improved.

Although rolling is a one-time molding process, this "one-time" refers to the entire bolt being processed only once, and specifically for each thread, it is squeezed far more than once.

Taking threading as an example, although it is only processed once, each thread has already "rotated" several times. With each rotation, the thread is forced to form once. Multiple "rolling" processes make the surface of the thread very smooth and even, with almost no micro defects such as cracks and pits, resulting in better fatigue resistance.

Based on the excellent stress performance of thread rolling, many high-end manufacturing industries require the use of thread rolling bolts, such as the aerospace industry, nuclear power industry, and automotive industry.

The irreplaceable side of lathe threads

So much has been said above, it seems like there's a bit of pulling up the rolling wire and stepping down the lathe thread wire. So, is the lathe thread wire really useless? Not entirely so.

In addition to the high brittleness and hardness of raw materials mentioned at the beginning, which are more suitable for wire cutting, another advantage of it is its wide range of applications.

Especially for threads with larger angles and higher heights, it is difficult to achieve through wire rolling. Thread turning is relatively more flexible and can process almost any type of thread.

Also, is it possible that the die used for rolling (Figure 9) is also made by threading?! I don't know about this editor, welcome to the decompression module friends for science popularization. Figure 9: Rolling Thread Compression Mold