Machinery's Handbook, 31st Edition
1166 Microcutting Tools mutually across their interfaces, therefore degrading their properties and causing diffusion wear. Diamond with a carbon-rich matrix cannot be used with low-carbon ferrous alloys like steels or stainless steels because diamond carbonizes at temperatures exceeding 1112ºF (600°C) and carbon diffuses to the steel due to its lower carbon content and high affinity to carbon. The useful life of a tool can be extended by proper application of coolant to reduce thermal damage, or by use of a protective coating that blocks undesirable thermal diffusion from/to a tool surface. Chemical damage of a tool is due to a chemical reaction between a tool material and its environment like air, cutting fluid, or workpiece material. Tool oxidation is common when cutting in air at high speed. An oxidation reaction is accelerated with temperature, but can be eliminated when inert gas is used to shield the cutting tool from surrounding oxygen. A chain reaction can also occur and further degrade a tool. For example, iron in steel is first oxidized at high cutting temperature to form iron oxide; the iron oxide then weakens the aluminum oxide coating of a tool and leads to peeling and chipping of the coating. Adhesion tool damage happens when a built-up-edge (BUE) welds strongly to a tool sur face and then breaks away with a minute amount of tool material. When machining soft materials, a chip tends to adhere to the tool and grow in size. When the BUE is large and becomes unstable, it is removed with the chip while also shearing off part of the cutting tool due to the higher adhesion strength between BUE and tool than the intergrain binding strength of the tool. Stainless steel, nickel and titanium alloys are known for causing adhe sion wear on carbide microtools. Adhesion damage can be reduced by using proper lubri cant to reduce friction between chip and tool, by coating the tool with a smooth and low friction layer, by reducing tool edge radius, or by increasing cutting speed to raise the tool surface temperature and soften the BUE while reducing its weldability to the tool surface. Microtool failures occur due to a combination of the above mechanisms. For example, peeling of tool coating might be due to coating defects, or to mechanical mechanisms when a large gradient of stress exists across a thick coating layer; the loosened coating particles then rub and cause mechanical abrasive wear on a tool. Thermal mechanisms may cause workpiece atoms to diffuse, weaken, and dislodge several tool grains as microchipping. Table 2. Categories of Tool Damage Microtool damage Damage size Mechanism m m μ inch Abrasion < 1 < 39 Mechanical, thermal Attrition 1–3 39–118 Mechanical, thermal Peeling 1–3 39–118 Mechanical, chemical Microchipping 3–10 118–394 Mechanical, adhesion Chipping 10–30 394–1180 Mechanical Fracture > 100 > 3940 Mechanical Tool Life.— Tool life criteria in macromachining are documented in ANSI/ASME B94.55M-1985 (R2019), Tool Life Testing with Single-Point Turning Tools . This standard suggests an end of tool life when a tool exhibits: • An average flank wear of 300 m m (0.0118 in), or • Any maximum flank wear land of 600 m m (0.0236 in), or • Any tool wear notch of 1000 m m (0.0394 in), or • A crater wear of 100 m m (0.0039 in). It is obvious that such criteria for a macrotool cannot be applied to a microtool because (i) it would be cost prohibitive to continue testing until 300 m m flank wear, and (ii) the wear criteria are even larger than most tool dimensions. In the absence of a microtool standard, researchers have set their own criteria based on direct observation and/or indirect monitoring of microtool tool wear effects. Published data varies on microtool wear thresholds: 5 m m flank/nose wear on diamond tools, or
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