Machinery's Handbook, 31st Edition
1160 Microcutting Tools Tool Materials.— Using the right microtool is essential for micromachining. A mi- crotool that successfully drills through holes on a plastic printed circuit board is not nec- essarily able to drill deep blind holes on titanium alloys. Understanding the requirements and selecting the right microtool for each condition saves time, money, and frustration. It has been theoretically derived and experimentally proven that the smaller is the chip, then the higher is the stress to generate it. Microcutting tools, therefore, have to be designed for higher stress with extreme geometrical constraints. When depth of cut is smaller than the average grain size of a workpiece, each grain generates different stress on the cutting edge and eventually fatigues the tool. Microtools as small as 25 m m (0.001 inch) are commercially available. Common tool materials are high-speed steel (HSS), cermet, carbide, cubic boron nitride (CBN), poly crystalline diamond (PCD), and single crystalline diamond (SCD). HSS is commonly not used in micromachining of metal since it does not have required hardness and strength to resist plastic deformation. A SCD tool is available for microturning, but not for micro drilling or micromilling. Carbide and cermet, having properties between HSS and dia mond, are most suitable for microcutting tools. They are sintered from random abrasive grains in either cobalt or nickel binder with a small addition of molybdenum or chromium. A higher binder content increases the tool toughness and crack resistance, but reduces the tool bulk hardness. Using ultra fine grain (submicron size) abrasives in a lesser amount of binder is the optimal solution because a tool with submicron carbide grains can maintain a high hardness while improving its crack resistance against chattering, interrupted cuts, or cyclic deflection due to spindle runout. Microtool failure modes include shearing, chipping, and wear. To minimize shearing and catastrophic tool failure, a tool should be made from a high hardness substrate and with a geometry suitable for micromachining, i.e., large included angle and sharp cutting edge (Fig. 4). A tool with smaller than minimum included angle will be deformed and fractured in service.
Rake angle
Included angle
Tool
Uniform coating Nonuniform coating
r (uncoated)
Tool edge radius
Relief angle
(a)
(b)
Fig. 4. (a) Tool Geometry, and (b) Change of Tool Edge Radius Due to Coating.
2500
2000
A
C
1500
1000
500
B
5
10
15
20
Tool Hardness (HV, GPa)
Fig. 5. Microtool Minimum Included Angles.
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