With an optimal selection of parameters, burnished micro spherical stylus tips in variety diameters can be produced instantaneously. This process is particularly for production of micro ball-end stylus tips with diameters smaller than 0.1mm which are not available by conventional machining processes. With further research into probing systems, μ-CMM in the near future will be capable of measuring such micrometer-sized products as micro molds, micro dies and micro holes more precisely.

1. Background
During the previous decade, the trend towards the miniaturization of complex commercial micro products has led to an increased need for high-precision machining, assembling and measuring devices. As the accuracy of micro products increases, so does the need for highly accurate 3D measurements. The most reliable way to measure the size, form and space position of 3D components is through use of a ball-ended, contact touch-probe measurement system such as a coordinate measurement machine (CMM).
A new range of high-accuracy micro CMMs has recently been developed that are capable of measuring micro-scale features on millimeter-sized objects [1]. Commercial literature commonly states that these instruments can be used to measure the structure of micro-electro-mechanical systems (MEMS). However, the majority of commercially available micro CMMs incorporate stylus tips with diameters in the order of 0.12 mm, as determined by the availability of high-quality ruby spheres used for the ball tip, and this makes them incapable of measuring many MEMS structures such as deep micro trenches with high aspect ratio lateral side-walls, and micro nozzles for diesel or spark ignition engines. Therefore, one of the key factors required to achieve micro metrology by µ-CMM will be the availability of high-quality, micro ball-ended stylus tips.

2. Shaping micro tools and OPED process
 Prior to production of micro spherical stylus tips, straight micro tools need to be produced by WEDG [2] as shown in Figure 1. In general, micro rod tools of various diameters, even straight ones with diameter less than 0.08mm, are available on the commercial market. However, these micro tools without ball-tip are not useful for μ-CMM and difficult to set precisely on any machine. Therefore, WEDG technology is easily employed and combined with OPED for micro spherical stylus tip production on one machine [3]. WEDG technology is similar to turning on a lathe except that material is removed by erosion utilizing electrical discharge. A resistor-capacitor (RC) circuit generates pulses that produce electrical discharges between the electrode workpiece and brass (or tungsten) sacrificial wire. Electro-discharges between the two polarities cross a small gap (of a few micrometers) filled with dielectric oil. The cylindrical material is held in a mandrel that rotates at 2500 RPM, and its position is fed in the z-direction utilizing an electro-discharge current detector. The brass wire is supported on a guide, and its position is controlled with an optical linear scale by personal computer interface. To maintain the same gap between both polarities, the sacrificial wire travels on the guide at a very slow speed. With precise position controlling, the system can produce a micro tool smaller than 5 µm in diameter [4].

Figure 1. Schematic view of Shaping micro tools and OPED process

After shaping the micro electrode tool by WEDG, one pulse-electro-discharge was applied to both of the small polarities. The micro electrode tool melted due to the high thermal energy and the melted part solidified into a micro sphere instantaneously. As shown in Figure 2 μ-EDM’s in-process measurement function is used to evaluate the forming quality and measure the largest outer profile of micro ball-ended stylus tips. 

 
Figure 2. In-process measurement of micro stylus tips by micro EDM in electro sensing mode

3. Results of micro spherical stylus tips fabrication
First, a thin rod of tungsten, with a stylus shank typically 50μm in diameter, is produced using WEDG. The tip of the rod is then subjected to a single, high-energy electro-discharge pulse (OPED) that melts the material. The melted material rapidly solidifies into a spherical shape due to surface tension. The diameter of the resulting stylus tip can be controlled by varying the duration and peak electro discharge current of the applied pulse. Previous investigations have shown that styluses with a tip diameters of 0.06 mm can be replicated using this process. The surface roughness of the micro spherical tips produced using this technique is significantly better than those produced using WEDG or micro EDM alone. Figure 3 is an SEM image of a stylus produced using WEDG/OPED showing the region between the shank and the spherical tip. The difference in surface roughness between the two regions is clearly evident. The surface of the ball-ended tip measured using a confocal microscope was found to have an Ra value of approximately 0.02 μm. 
The use of tungsten for the manufacturing material ensures that the stylus has the appropriate mechanical properties necessary for contact probing applications.

Figure 3. Surface roughness of ball-ended stylus tips

During OPED high voltage spark electro discharge occurs between both polarities of the electrode tool and running wire to shape the stylus tip. However, eliminating the capacitance and reducing the voltage can make the WEDG unit act as an electro sensor trigger unit without any electro-spark. In the configuration shown in Figure 2., after solidification of the sphere, the ball-ended stylus tip moves downward to the contacting trigger position. The tip electrically contacts the stationary wire repeatedly without causing any vibration of the wire. The largest outer profile of the ball-ended stylus tip can be measured in-process automatically.
Figure 4 shows a set of tip roundness profiles taken using the EDM in electro sensing mode. The tips were created under the same OPED electro discharge conditions, but using different dimensions of precursor shank. The roundness of a standard 0.2 mm diameter shank was also measured to confirm the roundness of the EDM spindle. The plot shows that the variation in tip roundness achieved under these discharge conditions was approximately 1μm.

In order for manufactured styluses to be used in contact probing applications, it is necessary that the diameter of the shank be smaller than the diameter of the tip. Experimental results indicate that the tip diameter should be approximately 20 μm smaller than the target tip diameter as determined by the choice of electro discharge parameters shown in Figure 5. This enables formation of a high-quality spherical tip while also ensuring that no undesirable contact of the shank will occur when the stylus is used for contact probing. Figure 6 illustrates another example of ball-ended tip production. It is evident that micro ball-ended tips with excellent sphericity can be produced by the OPED process. With selection an optimal parameters, the OPED process is able to fabricate ultra micro ball-ended stylus tips as per the one shown in Figure 7, which is the smallest in the current commercial market.
 

Figure 4 The largest outer profile measurement by EDM in electro sensing mode.

Figure 5 View of micro spherical stylus tips 

 
Figure 6. SEM images of micro ball-ended stylus tip for micro CMM (F-25)

 
Figure 7. Ultra micro ball-ended stylus tip by OPED process
  
4.  Conclusion
An OPED system combined with WEDG technology was applied to produce micro spherical stylus tips for which conventional machining methods are not available. A burnished micro spherical stylus with tip of about ¢50µm in diameter could be formed instantaneously with the selection of an optimal machining parameter. The dimensions of the spherical stylus tips are almost the same at the polarities where electro-discharge of energy occurs. In addition, material elements with large surface tension will affect the qualities of the spherical stylus tips. Tungsten is an excellent material for micro spherical stylus tips because of its hardness and spherical precision. With advanced research into the probing system for micro CMMs, it will be possible for micro CMMs to measure micrometer-sized products more precisely.

References
[1] Leach, R. K., 2009. Fundamental principles of engineering nanometrology (Elsevier).
[2] Masuzawa, T., Fujino, M. and Kobayashi, K.: Wire Electro-Discharge Grinding for Micro machining. Annals of CIRP, Vol. 34,1 (1985) PP. 431-434.
[3] Sheu, D.-Y., 2005. Micro-Spherical Probes Machining by EDM. Journal of Micromechanics and Microengineering Vol.15(2005) pp.185-189.
[4] Egashira Kai, Mizutani Katsumi, 2002 Micro-drilling of monocrystalline silicon using a cutting tool. Journal of International Societies for Precision Engineering and Nanotechnology, Vol.26 (2002) pp. 263-268.

Dong-Yea Sheu
Department of Mechanical Engineering, Micro Machining Laboratory
National Taipei University of Technology, Taiwan
1, Sec. 3 Chung Hsiao E. Rd. Taipei 106, Taiwan
Email: dongyea@ntut.edu.tw

Biography

 
Dong-Yea Sheu received his initial degree in Mechanical Engineering from Chung Yuan Christian University in Taiwan in 1994. After compulsory two year military service, he received a scholarship from TDK Corporation Taiwan and matriculated in Japan for five years. He received his doctorate from the Department of Precision Engineering of The University of Tokyo in 2000. Since 2002 he has been associate professor at national Taipei University of Technology (NTUT) in Taiwan. His research is currently focused on micro machining technologies such as micro-CMM's stylus tip fabrication, micro EDM, micro cutting and micro drilling on brittle materials.  Email:dongyea@ntut.edu.tw