Baity Air and Engineering Laboratory at the
University of North Carolina at Chapel Hill

Past Projects

Mist Generation During Metal Machining

The objective of former doctoral student Jon Thornburg's research is to identify and characterize factors that determine the generation rate and size distribution of cutting fluid mists.

Cutting fluids cool and lubricate the tool used in machining operations such as turning, milling, and boring. The three types of cutting fluids are mineral, soluble, or synthetic oils. Each type of oil may contain biocides or rust inhibitors. During the machining process, a mist is formed by the interaction between the tool and the cutting fluid covered surface of the metal, impaction of the cutting fluid on the machine, the centrifugal force acting on the cutting fluid covering the rotating workpiece, and vaporization/condensation of the cutting fluid. The size distribution and the generation rate of the mist is dependent on the type of coolant used and the machining process.

Extensive epidemiological research has indicated a relationship between the cutting fluid mists and diseases in the workers exposed to them. Concentrations of mists in the workplace have decreased since the implementation of the ACGIH recommended TLV of 5 mg/m3 in 1966. However, studies indicate exposure to oil mists are associated with benign and malignant health effects in exposed workers.

Local exhaust ventilation and mist collection equipment are frequently used to remove cutting fluid mists from the air and protect the workers. However, a more basic control option is to minimize the formation of the mist in the first place by altering the characteristics of the cutting fluid or the machining process. Therefore, the objective of this research is to identify and characterize factors that determine the generation rate and size distribution of cutting fluid mists. A mist will be generated by flooding a small lathe with a cutting fluid while turning aluminum rods. The lathe will be surrounded by an acrylic container to collect the mist, and the mist size distribution and generation rate will be measured by an Aerosizer LD (Amherst Process Instruments, Amherst, MA).

The first step of the research will be to identify the characteristics of the machining process and the properties of the cutting fluid that influence the generation rate and size distribution of mists by conducting fractional factorial experiments at two levels. Once the important characteristics are identified, the functional relationship between the mist generation rate and the factors that determine this rate will be derived using dimensional analysis. The important dimensionless groups include the Weber Number, Reynolds Number, Capillary Number, and Power Number, and Ohnesorge Number.  A similar correlation will be derived to predict the mist size distribution. Finally, the influence of the four mechanisms of mist formation (impaction, centrifugal force, mechanical break-up, and evaporation/condensation) on the mist generation rate and size distribution will be determined.

This research will result in a complete predictive model relating the contributions of the four mist formation mechanisms on the cutting fluid mist generation rate and size distribution to the important machining characteristics and cutting fluid properties. Therefore, machining characteristics and cutting fluid properties can be modified to minimize mist generation, and minimize a worker's exposure to coolant mists.

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