sealPurdue News

August 20, 1999

New math method adds to likelihood
of super-reliable metal parts

WEST LAFAYETTE, Ind. -- It may soon be possible to manufacture ultrahard metal parts such as bearings, gears and jet engine components that are so reliable and long-lasting they never have to be replaced.

A technology called "superfinish hard machining" promises to make such a feat possible, while saving time and money and reducing pollution. Now, researchers at Purdue University have developed a mathematical method that may speed the emergence of hard machining. Details about the work will be released Tuesday (8/24) at a scientific conference in Switzerland.

Presently, parts that carry critical loads in everything from cars and appliances to jet engines are produced in many steps, including time-consuming and costly grinding and polishing operations. The parts are first machined out of metal that is relatively soft. Then, they are hardened by being subjected to high heat and quickly cooled in water, or "quenched." After those steps, they still require precision finishing processes to make their surfaces ultrasmooth to reduce friction and wear.

In superfinish hard machining, the metal is hardened first and then machined in a single-step process that yields smoother surfaces, reduces waste and eliminates the need for polluting oils now essential for cutting and grinding, says C. Richard Liu, a Purdue professor of industrial engineering who has been a pioneer in hard machining research.

Purdue is helping industry pursue ways to perfect hard machining and reap its potentially dramatic benefits. For example, it might be used one day to increase the service life of hardened metal parts by 20 to 50 times. Essentially, parts such as bearings and jet engine components that might ordinarily require replacement several times during the lifetime of a piece of equipment would never have to be replaced, Liu says.

One obstacle to the widespread use of hard machining is that, as the cutting tools that are used to machine hardened steel begin to wear, they cause thermal damage that weakens the metal being machined. The tools, which come in a variety of shapes, are small, sharpened bits like those used on a lathe to machine metal. Before superfinish hard-machining can be perfected, engineers need better methods to analyze precisely how heat is conducted between the cutting tool and the metal surface. They also need to take into account how much heat is released as it is carried away by metal shavings, or chips, removed from the metal during machining.

"It's a very complex heat-transfer system," Liu says.

To attack that problem, he has developed a new mathematical method to predict the precise temperature distribution at the interface of the cutting tool and the metal surface. A major benefit of the new model is that it can be used to predict which specific cutting tools will cause the least heat damage. Liu will present a paper detailing the work on Aug. 24, during the annual meeting in Switzerland of the International Institution for Production Engineering Research.

Purdue researchers have used the method to enhance a patented process for machining hardened components, which up until now have been extremely difficult to machine without causing thermal damage. During the process, the metal's surface is "prestressed," which means it is formed to counteract the stresses it will encounter in everyday use.

The research is funded by the National Science Foundation.

Source: C. Richard Liu, (765) 494-5413;

Writer: Emil Venere, (765) 494-4709;

Purdue News Service: (765) 494-2096;

NOTE TO JOURNALISTS: A copy of the research paper that will be presented Aug. 24 is available from Emil Venere at Purdue News Service, (765) 494-4709;

The Effect of Tool Flank Wear on the Heat Transfer, Thermal Damage and Cutting Mechanisms in Finish Hard Turning

Jia-Yeh Wang (former doctoral student) and C. Richard Liu, School of Industrial Engineering, Purdue University

Tool flank wear is a major cause of thermal damage in the machined surface layer in finish hard machining. A new methodology is developed to provide the knowledge needed for understanding the heat transfer regarding the formation of the thermal damage and the cutting mechanics. The methodology consists of a thermal model based on Green's function and a microstructure-based method using orthogonal hard turning. The coupling of the interface boundary conditions due to chip formation and flank wear is resolved using the proposed microstructure-based method, which is a departure from the conventionally incorrect approaches based on the assumption of constant chip formation. By incorporating the microstructure-based method with the thermal model, heat generated, heat partition, and the shear forces at the tool-chip and tool-work interfaces can be determined. Interface temperatures that are extremely difficult to be measured are obtained. The results quantitatively explain how the heat transfer and chip formation are altered as the tool flank is progressively worn.

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