'Cutting'-edge technology to better shape submarine propellers

Super smooth propellers of maximum structural integrity allow submarines to run silently in deep water. These nickel-aluminum-bronze alloy propulsion units take as long as 12 months to manufacture—a production time that the US Navy feels is too long. So, National Institute of Standards and Technology (NIST) high-speed machining experts are working with the US Navy and the University of North Carolina at Charlotte (UNCC) on a new machine tool and special metal-cutting strategies to decrease that time to four months.

A submarine propeller begins life as a greater than six-meter (20-foot) diameter, 50,000-kilogram (55-ton) metal casting. It must be machined down to a mass near 37,000 kilograms (41 tons) in its final form. Current machining methods leave the propeller with a rough surface, which if left unchanged, would betray a submarine’s movements in the ocean. So, months of hand finishing are required.

"Such a time-consuming process may soon be a thing of the past," said Tony Schmitz, a NIST engineer working on the project. "NIST’s tool wear and surface finishing experiments have led to a better understanding of the required parameters for high-speed machining of the propeller alloy. These discoveries have enabled us to increase the material removal rate during machining by a factor of 10. Additionally, refinements in the paths that the tool follows during metal cutting promise to substantially reduce roughness in the final milled propeller surface."

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