Machinery's Handbook, 31st Edition
1178 CUTTING FLUIDS IN MICROMACHINING droplet dimension. Knowing the lubricant drop size allows proper calibration of a micro mist system to maintain the system effectiveness. Table 6 summarizes different techniques to measure the liquid drop size. The techniques are basically intrusive and nonintrusive methods to either collect the droplets for subsequent analysis, or for in situ imaging of the in-flight droplets. Nonintrusive techniques use dedicated laboratory research instruments to provide accurate dimensions and comprehensive statistical information of microdroplets. The effect of variables like air pressure on drop size and speed can be automatically calculated and analyzed. Intrusive techniques use less sophisticated instruments to collect and analyze droplets directly or indirectly. These simpler techniques, however, depend on operator skills for collecting reliable data. Table 6. Liquid Droplet Measurement Techniques Intrusive Nonintrusive Slide: collect droplets on a slide for microscopic assessment. Solidification: transform droplets to solid for sieving or weighting. Momentum: analyze droplet impact. Heat Transfer: analyze cooling effect of droplets with a hot wire anemometer. Light shadowing: analyze shadows of in flight droplets. Laser Doppler Anemometry: analyze visibility, intensity and phase shift of scattered laser from a small sample of droplets. Laser Diffractometry: scan and analyze a large group of droplets. The slide technique is a simple way to study drop size and its wetting characteristic. The setup is shown in Fig. 20a, in which a mask and glass plate are quickly exposed to a steady stream of micromist droplets. Only a few droplets are able to pass through the mask opening and deposit on a clean glass plate behind it.
P
Micromist nozzle
Droplet
θ
h
R
Mask Glass plate
Fig. 20a. Setup for microdroplet collection. Fig. 20b. Analysis of droplet geometry. It is assumed that (i) droplet volume remains the same before and after touching the glass plate, (ii) gravity effect on a microscale droplet is negligible, and (iii) the droplet forms part of a sphere on the plate to minimize its total surface energy. Using Equations (13) and (14) that follow, the average volume of a single droplet can be calculated by measuring the average projected droplet diameter P and its height h on a toolmaker’s microscope:
2
h P D 6 2 π
3
π
(13) (14)
V h h =
+ = a k
2 3 4
1 ⁄ 3
1 ⁄ 3
2
6
P
h h 3 3 4 2 + a :
D V a
k
k
D
= π =
h
where V = volume of microdroplet (mm 3 , in 3 ) P = projected droplet diameter (mm, in) h = height of a microdroplet (mm, in)
D = air-borne diameter of microdroplet (mm, in)
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