Machinery's Handbook, 31st Edition
Microcutting Tools
1163
z
Precision plug gage
x
Laser displacement sensor
Workpiece
L 1
D /2
L
L 2
Fig. 7a. Setup for Lateral Edge Detection Using Laser Sensor. Fig. 7b compares the accuracy and repeatability of the non-contact method shown in Fig. 7a against those of a mechanical edge finder.
1 0 2 3
Laser on 5/16 inch drill Mechanical edge drill Laser on 1/8 inch plug gage
–1 –2 –3
0
10
20
30
Trial Number
Fig. 7b. Superior Accuracy and Repeatability of Laser Edge Detection Technique Compared to Mechanical Technique.
It is rather simple to find the lateral offset as illustrated in Example 6, but it is more difficult to find the exact vertical offset for a slender microdrill or micromill without damaging it. One can attempt to use the common “paper technique” or take a risk with an available contact sensor for z-setting. In the paper technique, one would use a hand to slide a piece of paper on top of a workpiece while gradually lowering a tool. The tool stops when a resistance on the paper is felt. The paper technique is tedious, subjective, and tool dependent. Fig. 8b shows scattering of data up to ± 5 m m when finding z-offset for a center drill, but it is ± 15 m m for a milling cutter with 4 teeth. A commercial contact sensor requires a tool to move down and press against a solid surface. A pressure sensor then triggers an audible or visual signal to indicate a positive contact. The pressure level on such sensor is preset for macrotool setting and cannot be adjusted for a microtool. In both cases, the tool tip is one paper thickness or one contact sensor height above the workpiece—if the tool survives. A non-contact sensor is more practical and reliable. The same laser displacement sensor used for lateral tool offset can also be used for vertical tool offset. Fig. 8a shows a precision ring with secured circular plastic membrane that is used for indirect measurement. The membrane center is marked with a reference (e.g., crossing lines) at which the height can be measured with the laser displacement sensor. Upon placing the fixture on top of a work piece and then lowering a tool onto the reference mark, a slight contact of the tool and the flexible membrane is precisely detected with the laser beam pointing near the contact point. When this happens, the tool tip is at the same height as the membrane. The repeat ability of tool offset using this technique is well within the positioning repeatability 3 m m of the tested Haas OM2 machine tool (Fig. 8b).
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online