Machinery's Handbook, 31st Edition
1176 CUTTING FLUIDS IN MICROMACHINING not effective in microdrilling because coolant cannot flow into a partially drilled micro - hole. This section recommends how to select and apply a cutting fluid for effective micro milling/drilling. Micro milling/drilling requires high rotation speed exceeding 25,000 rpm of a small tool to achieve an acceptable surface cutting speed for material removal. When drilling steel, 50 percent of the heat generated conducts into the drill, but 80 percent of heat will go to the tool when drilling titanium. A microdrill with sharp cutting edges subjected to high temperature and high stress will fail easily if cutting fluid is not adequate. When rotating a microtool at a very high rotating speed, flood coolant is not effective since it does not have enough momentum to penetrate the boundary layer (fast moving air layer) around a fast rotating tool, or wet the bottom of a deep microhole. In addition, any unfiltered chip from recycled coolant can damage a microtool or fragile workpiece. Micromist (minimum quantity lubrication, MQL) has been studied by many researchers and is proven to provide proper cooling and lubricating in micromachining. In ideal conditions, a stream of micron-size lubricant particles in micromist: • Does not contain any chip or solid contaminant • Has enough momentum to penetrate the boundary layer of a fast rotating tool • Adheres to the fast rotating tool despite high centrifugal force, and • Wets the tool and workpiece to provide effective cooling and lubricating. The following section discusses safety, selection of cutting fluid, application method, and recommends optimal setup for micromachining. Safety.— The aspect of health and safety when using micromist is a concern. A mist does not only cause potential health issues for workers in the environment, but also contami nates other instruments and machines nearby. Biodegradable fluids must be used; polyol esters are superior to common vegetable oils because the former have higher biodegrad ability, are less “sticky” due to oxidizing, and increase in molecular weight with time and temperature. Due to the aerosol formation during mist flow at high pressure, an air purification unit or proper ventilating fan should be installed to minimize breathing of the aerosol particles by operators, and prevent damage to adjacent equipment. Benefits.— Most conventional machining processes like turning, milling, drilling, and grinding can benefit from micromist lubrication when applied properly. Although applica tion of micromist is limited when the mist flow is obstructed — as in gun drilling — successful microdrilling has been reported for microholes with 10:1 aspect ratio (depth/diameter). At optimal conditions, micromist significantly minimizes built-up-edges, reduces burr size and cutting force, and therefore improves tool life for both coated and uncoated tools. Depending on which cutting fluid is used and how it is applied, researchers have found the effect of micromist ranges from “the same as flood cooling” to “extending tool life 3–10 times over flood cooling.” There is yet any published paper on inferior results of micromist over dry and flood cooling.
Air Inlet
Reservoir Manifold
Hypodermic Needle Tip
Pulse Generator
Hose
Metering Pump
Fig. 17. Schematic of a Micromist System for Micromachining. Courtesy of Unist, Inc. Systems that can generate micromist for minimum quantity lubrication machining are commercially available. A typical design (Fig. 17) includes a resevoir for biocompatible oil, feeding tubes, and an atomizing unit that mixes a compressed air flow with a controlled volume of oil. A needle is necessary to direct the mist to a predetermined location.
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