DUCTILE REGIME MICROMACHINING Machinery's Handbook, 31st Edition
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Example 9, Grain Size Consideration: The speed and feed table on page 1118 recommends a chip load (feed) of 0.1 mm/tooth (0.004 in/tooth) for macro-scale end milling 316L stainless steel using an uncoated carbide tool. Assume the average material grain size is 15 m m. Macromilling: Using a Ø1⁄2 inch (Ø12.5 mm) end mill, the number of grains being cut by each tooth would be . . 15 01 100 667 grain size chip load mm m grains µ = µ m ⁄ grain = 15 µ m ⁄ grain = Micromilling: Selecting a Ø0.1 mm (Ø0.004 in) end mill, the recommended chip load would be 13 mm for stainless steel (see Table 13b, page 1195). The number of grains being cut by each tooth is . 13 087 = The cutting force on the macrotool and resulting surface finish are uniform due to the averag ing effect from seven grains. Because a microtool shears less than one single grain at a time, the micromachined surface is irregular due to different spring-back amounts of each individual grain, and the cutting force on the microtool fluctuates depending on each grain orientation. Ductile Regime Micromachining grain size chip load m grains µ = 15 µ m ⁄ grain Crystallographic Directions and Planes.— When machining in micro or nano scale, workpiece atom orientation affects machining performance because material properties change with crystalline orientation. Fig. 12a and Fig. 12b show blocks of the same mate- rial but with different surfaces. For example, the surface of the silicon block shown in Fig. 12a is harder, stiffer (higher elastic modulus), and is more difficult to machine than the same silicon block in Fig. 12b . Miller indices are commonly used to specify particular crystallographic orientations of atoms. (110) (100) Fig. 12b. Block of Same Material with (100) Surface. Consider a simple cubic system where atoms are located at corners (cubic as with manganese), at corners and inside (body centered cubic as with iron and chrominum), or at corners and on the surfaces (face centered cubic as with aluminum and copper) systems. For convenience, we will set a coordinate system Oxyz, as shown in Fig. 13a, and the size of the cube is set at one atomic spacing unit (OA = OC = OD = 1). Fig. 12a. Block of Material with (110) Surface.
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Fig. 13b.
Fig. 13a. Fig. 13c. During a micro/nano machining process, a cutting tool pushes and shears a grain. The cutting action forces some atoms to slide in certain directions and planes to form chips
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