Mist generation at a machining center

Control of occupational exposure to metalworking fluid mist generally involves enclosing the machining center and exhausting to an air cleaner that returns cleaned air to the workplace. To select an appropriate air cleaner, particle size and generation rate of the mists need to be known. Mist particle size and concentration were measured as a function of tool speed, fluid flow rate, and cutting rate at an enclosed machining center. A vertical machining center was totally enclosed and the air from this enclosure was exhausted into a duct where mist concentration and size distribution were measured using a time-of-flight aerosol spectrometer and a cascade impactor. Mist generation during the face milling of a 30 x 31-cm piece of aluminum with a 10-cm diameter face mill was studied. Machining parameters were varied as a 2 x 2 x 3 factorial experiment with these variables: coolant flow rate (18 and 44 m/sec), tool rpm (1900 and 3800 rpm), and metal removal (no removal, two teeth on face mill, and six teeth on face mill). Mist concentration increased with increasing tool speed and fluid application velocity. Whether the tool was actually removing metal did not affect the mist generation. Thus, mist generation is a function of fluid and tool motion. During a second experiment, effect of tool speed and diameter on mist generation was studied. Mist concentrations measured with the aerosol spectrometer were proportional to the 2 and 3.5 powers of the tool speed for the face mill and end mill, respectively. In both experiments the shape of the size distribution was largely unaffected by the experimental variables.