This study presents wetting characteristic of several lubricants for minimum quantity lubrication (MQL) by comparing their respective contact angles on different tool and workpiece materials. The size of an airborne droplet is estimated by measuring the profile of average droplets depositing on a polished and flat glass surface. The droplet velocity field in front of a nozzle is simulated numerically, and compared against measured data using anemometry and laser particle image velocimetry techniques.
A favorable contact angle of ∼5° is obtained for oil-based lubricants on titanium, stainless steel, and tungsten carbide samples. Such low contact angle is preferred over a higher contact angle of ∼30° typically found when using water-based cutting fluids. The micromist used in the study forms a conical flow of ∼ 20° in front of a coaxial nozzle. High air pressure can atomize lubricant into microdroplets with characteristics droplets of 4–11 μm in size, which are comparable with published data for microdroplet diameters. An optimum size of 3–5 μm microdroplets is preferred since airborne microdroplets below 2 μm may cause a health concern to some machine operators. When the input pressure is above 300 kPa, the air speed in front of a nozzle is at least 100 m/s (6,000 m/min) along a working distance of 20–100 mm from the nozzle tip. Since this droplet speed could be 5 times faster than the cutting speed of a diamond tool in ultrahigh speed machining, such microdroplet could penetrate the boundary layer of a fast rotating tool, adhere and wet the tool and workpiece surfaces for effective lubrication and heat removal. The axial microdroplet speed, however, is drastically reduced in the direction perpendicular to the flow due to vortices forming beyond ∼50 mm downstream from the nozzle tip. A single nozzle would be sufficient for a single point cutting tool — as in turning operation — if the working distance is short and the airstream is near the laminar regime, but multiple MQL nozzles should be utilized for a larger milling tool.