Why Coil Angle Matters - and What We Did About It

Let’s talk about coils. Those unassuming spirals of wire are hiding in plain sight everywhere. From the guidewires in minimally invasive medical procedures to the cable housings for your bicycle brakes and the heating elements in your toaster, coils are essential to modern life. At Pilot Line, we’re often asked about the basics of coil winding-especially the difference between open pitch and closed pitch coils. With the launch of our upgraded coil winder, we thought it was the perfect time to offer a practical overview of how coil winding works, why precision matters, and where these spirals end up.

Making Open Pitch Coils

Open pitch coils are characterized by the visible gaps between each turn of the wire. These spaces are intentional and give the coil flexibility, making them ideal for applications where bending and movement are important but where the coil doesn’t need to resist crushing forces.

On a coil winder, open pitch coils are made by precisely coordinating the rotation of the mandrel (the rod or tube the wire is wound around) with the lateral movement of the wire feeder. The pitch—the distance from the center of one wire to the next—is set by adjusting the speed at which the feeder moves relative to the mandrel’s rotation. Achieving a uniform pitch is crucial, especially for medical devices, as even slight inconsistencies can affect the performance and safety of the final product.

The process requires careful tension control. If the tension is too loose, the coil can end up sloppy and inconsistent; too tight, and it becomes difficult to remove the mandrel from a catheter with the coil wound in place.. Pilot Line’s coil winders are designed to maintain this balance, allowing users to program precise parameters for each project.

Closed Pitch Coils and the Importance of Angle

Closed pitch coils, on the other hand, are tightly wound so that each turn of the wire is in direct contact with the next. This design gives the coil strength and resistance to crushing, which is essential in applications like endoscope shafts, control cables, and any structure that needs to maintain its shape under compression.

Winding a closed pitch coil is a more demanding process. The key variable here isn’t just how fast the feeder moves, but the angle at which the wire is laid onto the mandrel. Even minute variations in wire diameter can accumulate over hundreds of turns, leading to a coil that gradually opens up or overlaps itself, neither of which is acceptable in high-precision manufacturing.

To solve this, our upgraded coil winder uses a machine vision system called the Angle Pilot. This system employs a video microscope to continuously measure the lay angle of the wire as it’s wound. If the angle drifts, the machine automatically adjusts the feeder speed to bring it back in line. This real-time feedback loop ensures that even with ultra-fine wires, the coil remains consistent from end to end.

Maintaining the correct angle is also critical for controlling the coil’s preload, or the internal tension between the turns. A coil wound with too steep an angle may have too much preload and be difficult to bend. Too shallow, and the coil will lack the necessary strength. The Angle Pilot makes it possible to dial in exactly the right amount of preload for each application, which can be tested with methods like the “droop test” (measuring how far a coil can extend horizontally before bending).

Coils in Medical Devices: Small, Precise, and Essential

In medical manufacturing, the stakes for coil quality and precision are exceptionally high. Coils are used to reinforce guidewires, catheters, and steerable instruments, where they must be flexible enough to navigate the human body but strong enough to transmit force and resist collapse. The dimensions involved are astonishingly small; Wires can be as thin as a human hair, and tolerances are often measured in ten-thousandths of an inch.

Achieving this level of precision means every aspect of the coil winding process must be tightly controlled, from the pitch and angle to the tension and speed. Pilot Line’s machines are designed with these requirements in mind, offering programmable recipes and advanced monitoring to ensure every coil meets the exacting standards of medical device engineering.

Supporting Every Spiral

Coils may not spend a lot of time in the spotlight, but they’re critical to the function and reliability of countless products. Whether you’re developing a next-generation catheter or building industrial heating elements, understanding the basics of coil winding—and the difference between open and closed pitch designs—can help you choose the right approach for your needs.

Pilot Line is proud to support engineers and manufacturers across industries with tools that make coil production more precise, efficient, and customizable. We’re always excited to see the creative ways our machines are put to use, and we’re here to help you wind the perfect coil, no matter your application!