Machinery's Handbook, 31st Edition
CUTTING FLUIDS IN MICROMACHINING 1177 The resulting oil microdroplets—size and speed—should be adjustable to effectively penetrate and wet a tool/part interface. This can be done by adjusting the air pressure, type of oil, and volume of oil released into the air stream. Selection of Cutting Fluid.— A cutting fluid is selected for both cooling and lubricating purposes in micromachining. It should be environmentally friendly, should not interact chemically at high temperature with tool or workpiece, and can be cleaned and disin- fected from the machined parts. It must have low surface energy relative to the surface energy of the cutting tool and workpiece material, high thermal diffusivity, and lubricity. For micromist applications, a cutting fluid must be able to flow easily in a small tube (low viscosity) and to form microdroplets. Complete wetting is desirable for a cutting fluid because it covers large surface areas of tool and workpiece and effectively removes heat from the source. Its self-spreading capability due to the differential surface energy allows cutting fluid to penetrate deep into the chip/tool interface to effectively lubricate and cool this zone. Wetting condition can be assessed by two methods: Pendant drop technique: A drop of liquid is formed and suspended vertically at the end of a solid tube. The side view of a drop is analyzed to compute the liquid surface tension using a tensiometer (Fig. 18a and Fig. 18b).
28 26 24 22
10 18 16 14 12
0 26 56 84 112 140 Time into Run (s) 20 Fig. 18b. Calculated Surface Tension from Starting to Full Forming of a Droplet.
Fig. 18a. Side View of a Pendant Drop Below a Stainless Steel Tube.
Sessile drop technique: A drop of liquid is placed on a horizontal surface. The side view of the drop is analyzed to calculate the liquid contact angle or measure it with a goniometer (Fig. 19).
Contact angle θ
Liquid
Complete wetting
Non-wetting Partial wetting
Solid
Fig. 19. Sessile Drop Technique to Assess Wetting of Cutting Fluid. The following section presents a simpler approach to calculating the contact angle and drop size using a modified sessile drop technique. This technique uses a toolmaker’s microscope, available at most manufacturing shops, to measure the top view of a drop, instead of a goniometer to measure the side view. Drop Size Measurement.— A microdroplet must have sufficient momentum to penetrate the boundary air layer moving around a fast rotating microtool and to wet the tool after ward. Calculation of momentum and contact angle for wetting assessment requires the
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