How are internal and external threaded screws and nuts manufactured?

The Engineering Paradox: When Threads Meet Thin Walls

In the world of precision fasteners, some designs seem to defy standard manufacturing logic. At Ever Power, we often handle custom requests for components that feature both internal and external threads. A classic example is a specialty screw with an M6 external thread and an M4 internal thread.

The primary engineering challenge here is structural deformation. Because the gap between an M6 outer diameter and an M4 inner diameter is minimal, the wall thickness of the fastener is extremely thin. Traditional high-speed methods like thread rolling (using a rolling machine) or thread rubbing (using thread plates) are unsuitable. The massive compressive force exerted by these machines would instantly crush the thin wall, deforming the internal bore and rendering the internal threads useless.

The “CNC Trap” and the Reality of Production Costs

When faced with complex thread geometries, many engineers reflexively suggest CNC (Computer Numerical Control) machining. While CNC is capable of extreme precision—even for aerospace-grade engines—it is often an “engineering sin” to overlook the cost-to-benefit ratio in mass production.

If you were to produce these dual-threaded nuts on a standard CNC lathe, even if the material cost was zero, the processing fee alone would rarely fall below 1 RMB (approx. $0.14 USD) per unit. For high-volume industrial applications, this cost is prohibitive. Professional manufacturing is not just about making a part; it is about making it efficiently and affordably.

The Industrial Solution: High-Efficiency Thread Milling

So, how do we achieve precision without the crushing force of rolling or the high cost of CNC? The answer lies in Thread Milling Attachments (铣牙座) for automatic cam lathes. This is the “hidden secret” of high-efficiency fastener manufacturing.

The Mechanics of a Thread Milling Attachment

A thread milling head is a specialized accessory installed on high-speed cam-driven automatic lathes. Its operation is a masterpiece of mechanical synchronization:

• Synchronized Transmission: The milling head is connected to the lathe’s main spindle via a synchronous toothed belt. This ensures a fixed rotational ratio between the workpiece and the cutting tool.
• Blade Customization: By selecting specific carbide blades, we can mill any thread profile—from standard ISO metric threads to specialized sharp-edged threads used in self-tapping brass inserts.
• Pitch Precision: The transmission ratio is adjusted according to the required thread pitch, allowing the tool to “trace” the thread onto the part with minimal radial pressure, preventing any deformation of the internal hole.

This method allows us to create exceptionally sharp and non-standard external threads that would be impossible to “roll” and too slow to “turn” on a standard lathe.

Internal Threading: The Final Touch

While the external thread requires specialized milling, the internal thread is processed using standard tapping techniques. Once the external profile and the internal bore are prepared, high-precision taps create the internal M4 threads. Since the milling process has already formed the exterior without stress, the tapping stage can proceed with high accuracy, ensuring the concentricity of the two thread systems.