Research on Electrochemical-Mechanical Composite Polishing Electrode Design and Process Testing of 316 Stainless Steel Deep Long Holes

Deep inside the machinery of nuclear power plants, aerospace engines, and advanced energy systems, there are narrow, tunnel-like passages that must be flawless. These are deep holes drilled into 316 stainless steel, a material chosen for its strength and its ability to resist corrosion in the harshest environments. However, the inner walls of these tunnels are notoriously difficult to finish. When manufacturers try to smooth them out using traditional mechanical tools, the results are often uneven, leaving behind microscopic scratches and roughness that can compromise the safety and efficiency of the entire machine. The challenge lies in reaching deep into a tiny tube, only 3.5 millimeters wide and 80 millimeters long, to make the surface as smooth as glass without damaging the delicate structure.

To solve this, researchers at Xi'an Technological University turned to a method that combines two distinct forces: electricity and physical grinding. This approach, known as electrochemical-mechanical composite polishing, works by using a specialized tool that acts as both an electrical conductor and a mechanical grinder. The process relies on a simple but powerful principle: when electricity passes through a conductive liquid between two metal surfaces, it dissolves the high points of the metal surface, while the physical motion of the tool scrapes away the stubborn, insoluble bits. By balancing these two actions, the team aimed to create a surface so smooth that it would be invisible to the naked eye, meeting the extreme standards required for high-end industrial components.

The researchers began by designing a new type of polishing electrode, the tool that does the actual work inside the hole. They realized that the key to success was not just the speed or the pressure, but the specific ratio of the tool's surface that conducts electricity versus the part that grinds mechanically. They created a mathematical model to understand how this ratio, which they called the proportion of electrochemically active interfaces, would affect the final surface. In their model, the tool was divided into sections: some parts were designed to let electricity flow and dissolve the metal, while other parts were covered with ceramic fibers to physically grind the surface. They simulated how different mixes of these two zones would behave inside the deep, narrow hole.

Through these computer simulations, the team discovered that the balance between electricity and grinding was critical. When the tool had too much grinding surface and not enough electrical contact, the smoothing effect was weak. However, when they increased the electrical contact area, the current distribution became more even across the entire inner wall of the hole. This even distribution allowed the electricity to dissolve the microscopic peaks on the metal surface more uniformly. The simulations showed that a specific configuration worked best: a tool where 80 percent of the surface was designed to conduct electricity and 20 percent was dedicated to mechanical grinding. At this ratio, the tool could effectively level out the rough surface, turning a chaotic landscape of tiny hills and valleys into a flat, uniform plane.

With the design optimized in the computer, the researchers moved to the laboratory to test their creation on real 316 stainless steel tubes. They set up a precise experimental station where the new electrode could rotate inside the deep holes while a controlled voltage and pressure were applied. They ran the process for exactly 144 seconds, using a voltage of 3.25 volts and a polishing pressure of 0.08 megapascals, with the tool spinning at 564 revolutions per minute. The results were striking. Before the treatment, the inner surface of the holes had a roughness level of Ra 3.2, a texture that would feel gritty and look dull under magnification. After just two and a half minutes of this composite polishing, the surface transformed completely.

When the team examined the finished holes under powerful microscopes, they found a surface that was pristine. There were no visible scratches, pits, or defects. The roughness measurement dropped dramatically to Ra 0.1, a level of smoothness that rivals high-precision optical components. The study confirmed that by carefully tuning the ratio of electrical to mechanical action on the tool, it is possible to achieve a level of finish in deep, narrow holes that was previously difficult to reach. This work provides a clear path for improving the quality of critical components in industries where the integrity of every square millimeter of a surface matters, proving that a balanced combination of electricity and mechanics can solve some of the most stubborn manufacturing problems.