Machinery's Handbook, 31st Edition
1162
Microcutting Tools
b 20.0 µ m
a
Fig. 6. Peeling (a) and cracking (b) of 4 m m-thick TiN coating layer on a WC micromill. Back scattered electron technique shows high contrast of a dark TiN coating layer against the bright WC/Co substrate in the background.
Example 5, Sensor Requirement: Select a sensor for microdrilling using Microdrill: 100 m m diameter, 1 mm flute length (Ø0.004 inch, 0.040 inch flute length). Machine tool: 1 m m (40 m inch) repeatability and 500,000 rpm capability. Solution: A laser displacement sensor is selected to satisfy the following specifications: Resolution: 0.1 m m (4 m inch) Spot size: 25–75 m m (0.001–0.003 m inch). Although most drill shanks are Ø3.175 mm (Ø0.125 inch), the working zone should be as small as possible to detect the shank center. In order to make 6 measurements when a tool is rotating at 500,000 rpm, the time between measurements is: , min min t 6 1 500 000 1 1 60 2 10 20 measurements rev rev s s s 5 µ = = = − a a a k k k # A laser with minimum 20 m s sampling rate (50 kHz) would be sufficient. A mechanical edge finder is adequate for most macromachining setups, but it is not suitable for micromachining especially with small and pliable parts. Fig. 7a shows a non- contact technique to detect part edge or find lateral tool offset. A rotating precision plug gage, mounted on a machine spindle, is positioned between a stationary laser sensor and the workpiece. The small laser beam is aimed at the plug gage center and on the part edge when the plug gage is withdrawn away from the beam path. These two laser sensor readings allow computing the tool center offset. A precision plug gage should be used instead of a cutting tool shank for better repeatability. Example 6, Lateral Tool Offset Calculation: Use a laser displacement sensor and a Ø3.175 mm (Ø1/8 inch) plug gage to detect the edge of a ground block. Solution: i) Mount the plug gage on the machine spindle and rotate it at 5000 rpm. ii) Scan a laser beam across the plug gage and stop when the distance from the laser source to the target is minimum, i.e., the beam is at the gage center. Read L 1 = 35 mm. iii) Jog the plug gage away from the beam path, read distance to the part edge L 2 = 55 mm. iv) The lateral offset from the spindle center to the workpiece edge is then: Lateral offset = L 2 − L 1 − D ⁄2 = 55 − 35 − (3.175⁄2) = 18.412 mm
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