Features

Fasteners and threaded parts are among the many small, precision-machined components found in medical devices. Because threaded parts can play a critical role in the operation and assembly of the device, they must be designed for high strength and dependability.

Fig. 1 – Pictured is a single thread rolling-die. They are typically made of hardened steel and are very resistant to wear and efficient during long production runs. (Credit: Kenneth Rinier)
During the product design process, in-depth consideration is given to material, biocompatibility (when appropriate), finish, contamination- and corrosionresistance, development cost, and much more. Reliable operation in correct proportion with production cost is key.

Many product designers are unaware of the fact that they can significantly elevate quality while also containing costs and maintaining development timelines by making one simple change: specifying a rolled machining method for their threaded parts. Why is the designer unaware of this potential for improvement? Traditionally, the machining process is left to the machining partner. If material capability, timeline, cost, certifications and other requirements are met, how a part will be machined is not often specified and, thus, not a key consideration.

But by understanding the benefit of specifying that threaded parts be rolled and not cut, product designers can experience the following benefits:

  • Stronger components,
  • More reliable components,
  • Higher degree of tolerances and higher precision,
  • Superior surface finishes, and
  • Chip-less, flake-less, and chatter-free operation.

Understanding threading methods, such a rolling or cutting, can help designers choose the right method for their particular need. It’s best practice to work with the machining partner to identify exactly the right approach, but the following are insights to point the designer in the right direction.

Traditional Methods for Machining Threads

Most threads are either cut or rolled. Thread cutting is a subtractive threading method. Material is removed from a metal work piece or hole using sharpened cutting tools. This process forms both internal and external threads, removing sections from the work piece each time the cutting tool passes.

Thread cutting is typically considered for applications that favor full or very deep threads, when the blank is uneven or unpolished, or when low quantities are required. Other threading methods do not always provide a great enough angle to form the deep, non-precise threads that cutting can.

But, the emergence of smaller and more complex medical devices makes it difficult to achieve precise thread geometry and tight tolerances with traditional machining methods, like cutting. In these instances, the product designer should consider rolled threads in order to meet the component’s functional requirements.

Fundamentals of Thread Rolling

Thread rolling is a cold-working, or work-hardening, process in which hardened steel dies force the material of the blank work piece outward into the shape of the threads. Pictured in Figure 1 is a single thread roller. Unlike subtractive methods, rolling dies do not physically remove the material but instead displace it. This displacement provides physical qualities, such as tensile strength, fatigue resistance, and smooth surface finish that are highly desirable to device designers.

Fig. 2 – Pictured is an illustration of rolled threads. These threads are unbroken and the contours are smooth and continuous to provide increased shear strength.
Components with cut threads are acceptable for some applications but cut threads can yield negative effects such as loosening, seizing, or tearing that will be noticed almost immediately after assembly or during operation. Choosing to roll the threads for medical device components can provide inherent advantages, such as strength, smooth finishes, improved machinability of materials, and lower production costs.

Benefits of Rolled Threads

Increased Strength: Rolling does not remove material, so threads are harder and have higher shear and tensile strengths than threads produced by subtractive methods. The cold-working process of thread rolling strains the metal beyond the level of pressure needed to permanently form threads. These strained areas become much harder and stronger. Roots of rolled threads can be as much as 20 to 30 percent harder than cut. American Standard Test Methods (ASTM) tensile tests have shown up to 10 percent increases in tensile strengths.

Fatigue strength can also be improved with the tight tolerances of rolled threads. Medical devices often require tolerances of +/- .001" to .0005" and must perform reliably under extreme conditions. Rolled threads can maintain tight tolerances and fatigue strength under temperatures up to 500°F for several hours. In some instances, the fatigue strengths on cut threads can decrease by up to 25 percent under similar temperatures.

Another benefit is increased shear strength. During rolling, the microscopic material fibers are not disconnected like cut threads and form natural and continuous grains. As shown in Figure 2, the material grains of rolled threads are left unbroken and the contours are smooth and continuous. This provides increased shear strength because shear failures and thread stripping typically occur across the grains, not along them.

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