Machinery's Handbook, 31st Edition
CUTTING FLUIDS IN MICROMACHINING 1179 Drop size varies with air pressure and volume of oil for atomization. In general, higher air pressure and velocity give more uniform and smaller drop size. Table 7 lists results of average drop sizes measured using this technique. Among different cutting fluids, the average diameter of in-flight droplets is approximately 1 m m for the CL2210EP lubricant, but it can be as large as 9 m m for other fluids. Table 7. Properties of Selected Lubricants Lubricant 2210EP 2210 2200 2300HD Surface tension (mN/m) 26 29 34 34 Droplet diameter ( m m) 0.97 2.3 6.7 8.4 Viscosity (Pa-s) @270s –1 shear rate 0.016 0.014 0.023 0.061 Contact angle on 316L (°) 7 14 10 18 Contact angle on WC (°) 7 7 10 12 Example 11, Drop Size Calculation: Set the Unist system at 32 strokes/min, 3.6 mm stroke length, 3.78 bar pressure. Collect droplets of CL2100EP and measure with a toolmaker’s microscope. The average projected drop size is 2 m m and average drop height is 0.3 m m. The drop volume is calculated using Equation (13) to be . . . . V h h h P 2 3 4 2 03 3 03 4 03 2 049 m m m m m 2 2 2 2 3 # π π µ µ µ µ µ = + = + = a ^ c ^ k h h m Using Equation (14), the average air-borne diameter of a droplet is . . D V 6 6 049 097 m m 3 # π π µ µ = = = a a k k 1 ⁄ 3 1 ⁄ 3 The following practical guides will assist in obtaining reliable results using the slide technique. • The solid surface (glass plate, workpiece, or cutting tool) should be as smooth as pos sible, flat and polished, clean and positioned horizontally. Any surface defect such as machining marks or burrs will distort the droplet profile. • The solid must be cleaned thoroughly before testing to avoid contamination of the tested liquid and distorted data. Ultrasonic cleaning in alcohol or degreaser following by dry air blowing should be adequate. • For meaningful information, about 10–20 droplets should be measured. Ignore very large drops that are coalesced from smaller droplets, and very small satellite drops that are splashed off upon impact of a droplet on the glass plate. • A measurement should be as quick as possible, using minimum light since a tiny liquid droplet might evaporate or spread when heated in bright light. Contact Angle Measurement.— Droplet volume is calculated in the previous section by the sessile drop technique, and the same volume can be used for calculating the droplet contact angle. Alternatively, a predetermined droplet volume can be set and dispensed on a solid surface using a micropipette. The contact angle of a sessile droplet on a flat surface can be computed from the following equation: (15) where P = projected droplet diameter (mm, in) V = droplet volume (mm 3 , in 3 ) θ = contact angle (°) K = 0 for θ between 90° and 180°, K = 1 for θ between 0° and 90° ( ) cos cos cos V P K 2 3 24 1 3 2 π i i i = − + − < F 3 ⁄ 2 1 ⁄ 3 1 ⁄ 3
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