Machinery's Handbook, 31st Edition
MICROFABRICATION PROCESSES AND PARAMETERS 1189 Microdrilling.— Microdrilling is a more complex operation compared to turning or milling. Chip removal and effective supply of cutting fluid is easy with the latter, but not with microdrilling due to extremely limited space around a microdrill. • Tool material. As with a micromill, a carbide microdrill should be sintered from fine grains, and ground to small cutting edge radius ( Tool Sharpness on page 1158). • Hole quality. Spindle runout, tool eccentricity, and wandering of a microdrill causes cyclic bending of the tool that leads to catastrophic failure. To control drill wandering, precision pre-drilling of a center hole can be tried, or the drilled surface must be ground to minimize deflection of a slender drill when starting on an irregular surface. • High aspect ratio. Pecking is essential for microhole drilling since chips have to be extracted and cutting fluid must penetrate into a small and deep microhole. The pecking depth can be deep in the beginning, but it must be reduced when drilling deeper. Start with an initial pecking depth of 2 3 drill diameter and gradually reduce it to 0.5 3 diameter at a depth of 10 3 diameter. Pecking depth and cycles can be calculated from: (19) . R 10 # = − + ^ h
. 9 1 15 195 05 .
D P D P
for for
R R
10
=
2
P = incremental pecking depth (mm, in) D = drill diameter (mm, in) R = drill aspect ratio = hole depth / drill diameter See Example 14 for more on the the use of Equation (19).
• Apply micromist with a fixed nozzle pointing to the drill tip; making an angle of 60– 70° with the tool axis is recommended (Fig. 34). In this way, the chip is blown away after a pecking cycle and the microdrill is re-lubricated before re-entering into the hole. Adequate ventilation and filtering are required to avoid inhaling of micromist.
Micromist nozzle
Micromill
60–70°
Workpiece
Fig. 34. Positioning of a Micromist Nozzle in Microdrilling. The Nozzle and Workpiece should be Stationary. Commercial microdrills are available for drill diameters of 100 m m (0.004 inch) and above (Table 10).
Drill diameter
Shank diameter
Flute length
Overall length
Fig. 35. Microdrill Nomenclature.
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