Application of a contour boring tool in an ultra precision machine.

Bliedtner, Jens ; Buerger, Wolfgang ; Dick, Lars 等

1. INTRODUCTION

The development in ultra precision manufacturing with shape cutting manufacturing technologies allows for manufacturing micro lens arrays with a high surface quality and low surface deviations in many different polymers and metals. Surface roughness of Ra < 10 nm and form deviations (p-v-values) of under 1 [micro]m are possible achieving micro topography and macro topographies that meet the performance requirements for the optical industry in many applications.

Lens arrays can be manufactured using different shape cutting ultra precision machining technologies, where varying process times are required. Production is realized using servo-tool options (slow tool-fast tool-servo), micro milling technology and contour boring technology (Badrawy, 2008), (Bliedtner & Graefe, 2008). The large variation in manufacturing times, costs and equipment utilization drive the costs of this process.

The notion contour boring (form boring) is a shape cutting ultra precision manufacturing technology utilizing a half arc diamond form tool (Fig. 1) The diamond tool is positioned on the rotating machine spindle, opposite to the ultra precision diamond turning process. The accuracy of the bored lens arrays is dependent on the accuracy of the diamond tool geometry and the exact position of the diamond tool in relation to the rotation center of the spindle. Due to the cooperation between the research partners JENOPTIK Polymer systems GmbH (located in Triptis) and the university of applied sciences Jena, extensive F & E work have been done realize a contour boring module.

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2. DEVELOPMENT, EXPERIMENTAL PROCEDURE AND RESULTS

2.1 Development and design engineering of a contour boring module

Figure 2 shows the integrated module into a 2 axis ultra precision diamond turning machine. The module consists of a work piece and the precision tool alignment.

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During the development of the module it was important to understand the minimal technical complexity for the integration of the tool position alignment to the ultra precision diamond turning machine. The position accuracy of the tool to the rotating center of the spindle has to be [+ or -] 1 [micro]m. The assembly of the module is characterized in Bliedtner et al. 2009.

2.2 Integration of the module into the ultra precision manufacturing machine

In preparation for the experimental research the contour boring module was implemented into an Precitech optimum 2400 ultra precision diamond turning machine. The positioning of the diamond tool was very critical. The adjustment is achieved through the regulation of the workpieces at the Y-axis. With a 2 CNC controlled axis (X, Y) on the ultra precision diamond turning machine only spherical micro lens arrays can be manufactured.

2.3 Testing and optimization of the process parameters

The lenses were bored with different technological parameters such as revolution speed, feed rate and cutting depth in numerous tests during optimization. Two materials (PMMA and alloy RSA 905) common for the optical industry were used for the experiments. The micro topography and macro topography of the manufactured lenses was measured using tactile profile measurement equipment.

At the determination of technological parameters were all the parameters retention time, feed rate and cutting speed considered. Retention time is the time, which the diamond tool remains at the end of the cutting process at his position, while the machine spindle is still in rotation (no feed rate).

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Figure 3 and 4 show the experimental results for the surface roughness Ra and form deviation p-v-values of the manufactured optical surfaces in connection with retention time.

Analysis of the metrology data from the speciemens manufactured in PMMA show a minimized surface roughness by processing with a known retention time (reduced from Ra = 13 nm to Ra = 7 nm). The form deviation however declines in quality from p-v = 0,45 [micro]m to p-v = 0,55 [micro]m for this same retention time. The results of material alloy RSA 905 revealed the same results. Therefore, for the contour boring process controlling the retention time at the end of contour boring process determines the roughness of the material surface.

Based on the results of this research manufacturing processes have been established to control retention time , achieveing a minimal surface roughness for the manufacturing of optical tool inserts and prototypes to an [R.sub.a] < 10 nm). Analogue experiments have been completed to explore the influence of the feed rate on surface roughness and form deviations. In conclusion of the experiments optimal technological parameters for the contour boring process were deducted (Bliedtner et al., 2009).

2.4 Manufacturing of selected micro lens arrays

Based on the optomized from our experimental results selected micro lens arrays were manufactured and analysed. REM analysis was also used to help evaluate the manufacturing results.

All in all 4 different micro lens arrays were manufactured from materials PMMA and nickel-phosphorous surface coating steel inserts using 2 different half arc diamond tools. The following figures show exemplary results from the REM analysis of this micro lens arrays. In figure 5 microscopic magnifications oft he manufactured surfaces (shifted placement) of both test materials with a cutting depth of [a.sub.p] = 50 [micro]m are shown. On the lens array manufactured in PMMA, some pollution is effident. The reason for this is the static effect of polymers. The direct comparisons indicate that the machined lens array in nickel-phosphorous appears much cleaner than that of the PMMA.

With the same lens array composition and a larger cutting depth of [a.sub.p] = 100 [micro]m, the result is a hexagonal (comp shaped) micro lens array.

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Figure 6 shows a zoomed-in detail of both arrays with the typical overlap of each lens geometries. This picture demonstrates the result from the high stress in PMMA material from the shape cutting manufacturing (cracking).

3. CONCLUSIONS

The research presented here shows that the contour boring technology offers an interesting alternative for manufacturing micro lens arrays and that the high demands from the optical industry for precise form accuracy and good surface quality can be produced with this technology. Therefore the operator has the possibility to use an additional technological process.

4. REFERENCES

Badrawy, S.J. (2008). Moore Precision Tools: Milling--A complimentary Solution for Diamond Turning Lens Arrays

Brinksmeier, E.; Glabe, R. & Autschbach, L. (2008). Novel ultra precise Tool alignment setup for Contour Boring and Ball end Milling, Universitat Bremen, Labor fur Mikrozerspanung

Roth, M. (2009). Integration eines "Contour-Boring--Moduls" in eine Ultraprazisionsdrehmaschine sowie messtechnische Charakterisierung erzeugter Funktionsflachen (engl. integration of a "Contour-Boring- Module" in an ultra precision turning machine as well as the metrological characterization of generated functional surfaces) , Diploma thesis, Fachhochschule Jena

Bliedtner, J.; Burger, W.; Dick, L. & Roth, M. (2009). Modul zum Formbohren optischer Mikrostrukturen (Module for conture boring of optical micro structures), Mikroproduktion, Vol. 3, 14-18, Carl Hanser Verlag, Munchen, ISSSN 1614-4538, Germany

Bliedtner, J. & Grafe, G. (2008). Optiktechnologie, Fachbuchverlag Leipzig im Carl Hanser Verlag, ISBN: 978-3-446-40896-8, Germany