Machinery's Handbook, 31st Edition
Microcutting Tools
1161
Example 4, Required Included Angle: Find the minimum included angle for an ultra-fine grain carbide tool of 12 GPa Vicker hardness to machine the F799 Co-Cr alloy of 1500 MPa (0.2 3 10 6 psi) tensile strength. Solution: Referring to Fig. 5, locate workpiece strength at point A on the vertical axis (1500 MPa). Locate point B for tool hardness on the horizontal axis (12 GPa). The intersection at C of the horizontal line from A and the vertical line from B indicates that the minimum included tool angle should be 75°. Coating of microtools is still a technical challenge due to conflicting constraints for tool performance. Chemical or physical vapor deposition (CVD or PVD) techniques have been developed to coat cutting tools with mono/multiple layers of intermetallic or ceramic compounds (Table 1). Criteria for acceptable tool coating are numerous: uniformity, high hardness, high toughness, low friction, high wear resistance, surface smoothness, high chemical/diffusion resistance, and high temperature stability at a reasonable cost. Although a coating thickness of 2–4 m m (79–157 m in) is acceptable for a macrotool, the coating thickness on a microtool should be thinner to minimize fracture and peeling of the coating. Both CVD and PVD processes not only add the coating thickness to the edge radius, but the extra coating also increases the radius at sharp corners (Fig. 4b). This is unfortunate since the thicker coating reduces the tool sharpness by enlarging the tool edge radius and causes an unfavorable plowing effect with negative effective rake angle. An uncoated microtool might perform satisfactorily, but the same machining parameters can be devastating to an over-coated microtool (Fig. 6). A thin coating of less than 1.5 m m following by an edge sharpening process would improve the tool performance, however, at the expense of higher tool cost. Published data indicate that micrograin carbide tools with 1.5 m m TiN coating is the best for micromilling of H13 tool steel hardened to 45 RC. Table 1. Commercial Coatings for Microtools
Coating Thickness
Maximum Temperature
Hardness
Coefficient of Friction
Coating
Structure
GPa 10 6 psi
m m
m inch ºC ºF
TiN TiCN TiAlN AlTiN ZrN CrN
monolayer gradient multilayer gradient monolayer monolayer
24 37 28 28 38 20 18 20
3.5 5.4 4.1 4.1 5.5 2.9 2.6 2.9 6.5 6.1
0.55 0.20 0.30 0.60 0.70 0.40 0.30 0.15 0.45 0.35
1-5 39-197 600 1110 1-4 39-157 400 750 1-4 39-157 500 930 1-4 39-157 700 1290 1-3 39-118 900 1650 1-4 39-157 550 1020 1-4 39-157 700 1290 0.5-1.5 20-59 400 750 1-4 39-157 1200 2190
TiAlCN gradient
Diamond like gradient
AlTiN/Si 3 N 4 nanocomposite 45 AlCrN/Si 3 N 4 nanocomposite 42
1-5 39-197 1100 2010 Tool Offset and Positioning.— Tool offset and tool positioning are crucial in micromill ing and microdrilling because a tool is small and extremely fragile especially if it has a high aspect ratio (length to diameter ratio). Common shop practices to find tool offset and position often damage a tool or workpiece. Non-contact techniques using light, mag- netism, capacitance, ultrasound, etc. are the preferred choice for precisely locating the relative position between tool and workpiece. Selection of a suitable sensor depends at least on following criteria: • Better resolution compared to that of the machine tool axis
• Small working zone to cover a microtool • Fast sampling rate for intended tool speed
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