Machinery's Handbook, 31st Edition
1192 MICROFABRICATION PROCESSES AND PARAMETERS Micromist is required to lubricate and cool both the tool and the machined surface. The micromist nozzle should move with the tool while blowing the micro/nano chips away from the machined surface. Adequate ventilation and filtering of micromist are required to avoid environmental issues.
400 nm
200
µ m
1 μ m Fig. 36b. Deep Scratch on Machined Surface Caused by a Broken Beryllide. The Arrow Shows Tool Cutting Direction.
15
10
5
Fig. 36a. Beryllide Particles at Grain Boundaries of Diamond Turned Beryllium Copper.
Speeds and Feeds.— In macromachining of engineering materials, the material grains are very small compared to the cutting tool dimensions and edge radius. Only bulk mate- rial properties such as material strength, hardness, and thermal conductivity affect ma- chining performance. However, when the workpiece grain size is similar to the tool edge radius then the workpiece material will have more influence on micromachining perfor - mance (see Workpiece Materials on page 1170). Since plastically deformed materials are normally harder than the original material, machining feed must be (i) deeper than this hardened layer, and (ii) at least half of the tool edge radius (see Microcutting Tools on page 1157). Example 15, Work Hardening of Titanium Alloy: Micromachining Ti 6Al 4V alloy with a carbide tool generates a work-hardening depth of about 1–2 m m (40–80 m inch). The microhardness increases 8– 14% above the base material hardness to ~350 Vicker. Chip load for microdrilling or micromilling should be greater than this hardened zone depth. Microturning Parameters: Criteria for selecting turning speed and feed are tool life and surface finish, and less concern on tool fracture since a turning tool is robust and rigid. Cutting speeds for single crystalline diamond are very high and depend mostly on the rigidity of the setup and the machine tool capability. Turning at 100–500 m/min (330–1650 ft/min) is common for most metals and metal matrix composites. Selection of feed depends on the surface finish required because this is the main objec tive for using diamond for turning in the ductile regime mode. Models for a macroma chined surface finish have been proposed which assume: • The depth of cut is less than tool nose radius but more than cutting edge radius • There is no built-up-edge on the tool, therefore, effect of cutting speed is negligible • Workpiece of polycrystalline material with fine grains is used and grain orientation does not significantly affect surface finish of large areas • Chips are completely removed without side-burr, and • No error is introduced from imperfect machine kinematics (such as asynchronous spindle error motion). The theoretical surface finish values are functions of feed and tool nose radius, and can be computed from Equations (20) and (21) and plotted as in Fig. 37a and Fig. 37b.
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