Machining and Rolling Threads - Test Result Comparison

Hello everyone! This is Yamazato from Hanshin Neji. It's amazing how quickly the year has flown by, isn't it?
This year, I've experienced a lot of changes in my mind and body. I feel like I've grown as a human. Even when I make progress, it feels like it doesn't lead to the next step.
I have a stronger desire to make good thing and last it for next generation (though I've felt this way for a while).
I have not only gained some weight, but I also moved to a new country to take on new challenges! (lol)

Machining and Rolling Threads – Test Result Comparison

    There are two main ways to manufacture screw threads: Machining and Rolling, as their names representing, we can guess the manufacturing method by their names.

For more information on these methods, please refer to our previous blog as Fasteners Thread Anatomy : Cutting and rolling thread, what are their differences ?

    Generally, rolled threads are stronger because they retain all the metal fibers, making them denser with smoother surfaces and fewer burrs. Because of their advantages, we recommend this method for those who have experienced breakage issues with machined threads.

Then, what are the causes? Here, I gather the verified test results for your reference. I hope this information will be useful.

Contents

1. Is there a difference in strength?
2. How do their surfaces differ?
3. Summary
   

1. Strength Comparison – Machining vs. Rolling Threads

    We conducted tensile and shear strength tests to see the difference in term of strengths between Machined threads (where metal fibers have been cut) and Rolled threads (where all threads and fibers are firmly rolled), showing in numerical results.The experiment involved pulling a “Rolled thread 10.9 strength class hexagon bolt size M16x100 (standard product)” and a “Bolt of the same conditions but with Machined threads”. We pulled them until they broke to check the tensile strength.

Tensile Strength Test Results Comparison

• Rolled Threads: 171.85 KN (1,094.5N/mm²)

• Machined Threads: 177.40 KN (1,129.9N/mm²)

Both threads met their tensile strength specification requirements. The only difference was in the fracture surface, with the machined screw breaking without stretching. This could be characteristic of machined screws.

Tensile Test:

• No wedge applied to the test.

• Specification: Minimum 163KN (1040N/mm² × 157 mm²)

Shear Test Results Comparison

• Sample 1: 10.9 hexagon bolt (Rolled thread)

• Sample 2: 10.9 hexagon bolt (Machined thread)

Shear test (Provided by JQA) 

2. Surface Inspection with Microscope

    Our experienced partner factory used a microscope to see the differences between machined and rolled screws. Even though machined screws look nice to the naked eye, the high-end Keyence VHX Series microscope revealed otherwise.

The surface of a machined screw shows some burrs remaining on the thread surface.

Meanwhile, the surface of a rolled screw shows no burrs or dents, probably because the dies are pressed against the threads.

    This difference in surface quality means that the most important element of a screw, its fastening strength is maintained. Rolled screws, which have a smoother surface, do not lose torque to imperfections like burrs, and generate the expected axial force with high precision.

With this microscope, you can even see dents like this.

Conclusion

    In this issue, we conducted experiments to compare machined and rolled threads in terms of strength and surface quality. In strength tests, both types showed no significant differences in tensile and shear strength. However, in real applications where threads' functions are actually used with repeating force, additional testing is necessary to determine durability and resistance to loosening.
    Machined threads are easier to make because you only need a lathe machine with fewer tool adjustments. It's hard to say one method is definitively better than the other; it depends on your application requirements.

Personally, I love the shine and smoothness of rolled threads—they make me want to rub them on my cheek! (lol)

Thank you for reading my blog. If you have any opinions or guidance, I’d love to hear from you. As i am still on my journey of discovering more about fasteners.