DUCTILE REGIME MICROMACHINING Machinery's Handbook, 31st Edition
1174
where d c = critical depth of cut (m, inch) A = constant E = Young’s modulus (Pa, psi) K c = surface fracture toughness (Pa·m
0.5 , psi·in 0.5 )
H = surface microhardness (Pa, psi) A shallow depth of cut, therefore, would energetically promote plastic flow rather than brittle fracture in the substrate and the chips. Table 5 tabulates properties of some brittle materials and their experimental critical depths of cut. Table 5. Selected Properties of Some Brittle Materials Materials Young modulus (GPa) Fracture toughness (MPa·m 0.5 ) Knoop hardness (GPa) Critical depth of cut ( m m) α -Al 2 O 3 275–393 3.85–5.90 19.6–20.1 1.0 SiC 382–475 2.50–3.50 24.5–25.0 0.2 Si 168 0.6 10 0.5 The constant A in Equation (12) varies in the range 0.1–0.6 due to measuring uncertainty of surface toughness K c , elastic modulus E , and microhardness H in a testing environment. These properties depend on crystalline orientation of the materials, surface conditions, and tool geometry. • The critical resolved shear stress, on a crystalline plane due to the cutting action, is directly proportional to the Schmid factor cos λ cos φ , where φ and λ are the orientations of the slip plane and slip direction. An ideal ductile mode machining would happen when the cutting shear stress is parallel to both the slip plane and the slip direction, otherwise a pseudo ductile mode with micro cleavages occurs. True ductile-regime machining happens only along certain crystalline orientations, but brittle machining occurs at other crystalline orientations. This explains why micromachining a crystal line specimen at the same speed, depth of cut, and coolant produces ductile machined surfaces in one direction but brittle machined surfaces on others. • Cutting fluid changes the surface properties of materials ( K c , E , and H ) and affects conditions for ductile regime micromachining. When micromachining the (100) germanium using a single crystalline diamond tool, the critical depth of cut changes from 0.13 m m (5 m in.) with distilled water as cutting fluid to 0.29 m m (11 m in.) in dry machining. • Tool geometry also affects the results. Plowing and fracture of material occurs when depth of cut is less than approximately half of the tool cutting edge radius (see Microcutting Tools on page 1157). Tools with negative top rake angle are usually utilized because a negative rake causes a compressive zone in the workpiece ahead of and below the tool and suppresses microcrack formation. Example 10, Mirror-finish Micromachining: Diamond tools with sharp cutting edge radii are very effective for machining brittle or ductile material with the exception of ferrous alloys such as tool steels or stainless steels. The cutting speed has minimum effect on surface finish, but a reduction of the feedrate leads to improvement of surface finish. An optical quality surface of 1.4–1.9 nm R max was obtained when turning single crystalline quartz with a diamond tool ( − 20° rake, 0.8 mm nose radius) at < 0.3 m m depth of cut, 3 m/s speed, and 8.1 m m/rev feedrate. Case Study.— A study used polished (001) p-type silicon wafers of Ø100 mm (Ø4 inch). Small grooves were faced at different constant depth of cut or gradually changing depth of cut to study the ductile behavior (Fig. 14). Single crystalline diamond tools with (001) rake surface, 10–40 nm edge sharpness, +5° rake angle, and 0.51 mm or 2.00 mm nose radii were used for a facing operation. The complete tool nomenclature follows the American Standards Association (back rake angle, side rake angle, end relief angle, end clearance
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