Ultra-precision machining of optical surfaces with slow-tool servo.

Bliedtner, Jens ; Buerger, Wolfgang ; Froehlich, Maik 等

1. INTRODUCTION

The continuous technical development in the field of ultra-precision machining enables a high-quality, ultra-precise manufacturing of moulds for the injection moulding of plastic optics in accordance with the users' requirements. The high requirements in terms of roughness demand the application of high-quality surface measuring technology for assuring the quality of the component ranges.

By means of examples the article shows results of experimental investigations of precision-machined toric surfaces, which were realised in co-operation with Jenoptik Polymer Systems GmbH.

2. EXPERIMENTAL PROCEDURE, RESULTS

The investigated component range (see fig. 1) consists of tree metallic moulds and two plastic prototypes with toric surfaces (metallic moulds: radiuses R 800 and R 600; plastic moulds: radiuses R 800 and R 100).

[FIGURE 1 OMITTED]

There were three specimens of each material available, so that fifteen specimens had to be measured in five positions each. The components with toric surfaces were manufactured on the ultra-precision turning lathe Nanoform 350 of the company Precitech (see fig. 1). It has three servo-controlled axes (XZC) and provides different machining options.

The component range under investigation was exclusively manufactured with the Slow-Tool-Servo (STS) option. This machining option is one of three Servo-Tool options which are currently used in ultra-precision manufacturing of optical surfaces (Osm2007).

The advantage of the Servo-Tool options is the economical manufacturing of non-rotation-symmetric geometries, as they can be manufactured at significantly lower costs in comparison with a conventional manufacturing. Furthermore, the ServoTool options enable an on-axis manufacturing of e.g. aspheres compared to previous off-axis manufacturing. During the STS machining the work piece is mounted on the air-bedded spindle (C-axis). The C-axis is mounted above the hydrostatically bedded linear guiding of the X-axis and rotates around the Z-axis (see fig. 2).

[FIGURE 2 OMITTED]

The STS procedure is used for manufacturing of non-rotation-symmetric connected surfaces such as toric surfaces. Simple arithmetic relations between excitation frequency [f.sub.an] [Hz] and rotation speed [[min.sup.-1]] are used for creating the optical structures.

If a surface is to be machined at a rotation speed of n = 400 [min.sup.-1], a tool excitation frequency of [f.sub.an] = 400/60 Hz = 6.66 Hz is necessary. For the creation of a toric surface where two radiuses are staggered by 90[degrees], the tool has to make two moves for one rotation. The optical surfaces of the component range are toric surfaces with radiuses of R = 600 mm and R = 800 mm (metallic moulds) or radiuses of R = 800 and R = 100 (plastic moulds) which are staggered by 90[degrees].

All turning work pieces were scrubbed at a constant rotation speed of n = 149 [min.sup.-1] with a cutting edge radius of [r.sub.[epsilon]] = 0.5 mm and finished at a rotation speed of n = 133 [min.sup.-1] with a cutting edge radius of [r.sub.[epsilon]] = 1.5 mm. The following materials were machined with MKD tools in the investigations:

1. CuproNickel: good machinability in cold state, good thermo formability

2. Aluminium RSA-905 AE: universally applicable, corrosion resistant, high stiffness

3. Aluminium AlMgSi1: versatilely applicable, good corrosion resistance

4. Zeonex E48R (Plastics/Cyclo Olefin Polymer (COP)): excellent heat resistance, low water absorption, high transparency

5. PMMA gs (Polymethylmethacrylat (PMMA)): good transparency, UV-resistance, low weight compared to glass.

The mechanical precision machining was carried out with selected technological parameters. For determining the micro topography (roughness) in the nanometer range an AFM (Atomic Force Microscope) was used in the investigations. The scanning range for determining the roughness was 50 x 50 [micro][m.sup.2]. Furthermore, the Power Spectral Density-Function (PSD) was determined as a useful instrument for analysing the surface roughness. This function determines the amplitude for a roughness value in the roughness profile of the technical surface as a function of the spatial frequency or the wavelength. Fig. 3 exemplarily shows the PSD-function of all surfaces manufactured by means of STS.

[FIGURE 3 OMITTED]

An elevation image, an image of the three-dimensional surface, the PSD-function for each specimen and the PSD-functions for all specimens of a material (compilation of all PSD-functions of the investigated material) were realised in the investigations. From the individual PSD-functions of a material an averaged PSD-function can be derived, which was also illustrated. From the results of the investigations important technical expertise can be gained, which can only briefly be introduced in this article (Dupa2002); (Weck2000).

Concerning the roughness of the technical surfaces the following values (average values) could be achieved with Slow-Tool-Servo (STS) and the same technological machining parameters:

Material roughness
 [R.sub.q]
 [nm]

CuproNickel 5.4
Aluminium RSA-905 5.2
Aluminium AlMgSi1 7.4
Plastics Zeonex E48R 14.1
PMMA gs 7.7

Except for the material Zeonex E48R, with all other materials the restriction [R.sub.q] [less than or equal to] 10 nm was kept. From the illustrations of the surfaces (e.g. see fig. 4) it can be realised very well that the specific micro topography of the machined technical surfaces mainly results from the different machinability of the used materials (material influence/machinability). Further investigations on the problem specific roughness differences in relation to the machining position on toric surfaces have to be carried out.

[FIGURE 4 OMITTED]

A comparison of the individual surfaces clearly shows the different micro topography creation with the different materials, although all components were manufactured with the same procedure (STS) and with the same technological parameters. The specific micro topography of the machined technical surfaces results mainly from the different machinability of the used materials (material influence). It is a consequence of the overlay of the roughness, which is due to the geometry of the diamond tool and the technological parameters with the micro roughness, due to the machinability of the material.

The comparison of the PSD-functions (see fig. 4) of the STS-machined surfaces shows that the PSD-function of all finished components has nearly the same gradient. The deviations result from the differing RMS-values, which again result from the specific machinability of the individual materials (micro roughness).

3. CONCLUSIONS

The evaluation of the determined power spectral density distributions (PSD-functions) of selected surfaces of the tooling inserts and their plastic mouldings aims at increasing the statistical certainty of the quality evaluation of these technical surfaces on the one hand and detecting the differences between the technical surfaces on the other hand. The evaluation of these specific technical surfaces can be qualitatively increased by the power spectral density distribution.

By means of Slow-Tool-Servo sophisticated technical surfaces can be manufactured from the different materials (moulds for injection moulding). Furthermore the extensive experimental investigations have proved valuable for:

--objectification of production issues by components with optical surfaces

--further optimization of the manufacturing process

--the quality assurance of such components.

4. REFERENCES

Osmer, J.; Weingartner, S.; Riemer, O.; Brinksmeier, E.; Bliedtner. J.; Burger, W.; Frohlich, M.; Muller, W.

Diamond Machining of Free-Form Surfaces: A Comparison of Raster Milling and Slow Tool Servo Machining. In: Proceedings of the 7th International Conference euspen Volume I--Volume II, May 20 th--May 24th 2007, Bremen, Germany, S. 189--192, Published by euspen, Bedford, 2007

Duparre', A.; Notni, G.: Charakterisierung nanorauher Oberflachen. DAKOM 2002: Charakterisierung von optischen und technischen Oberflachen. Darmstadt, 27. Februar 2002

Weckenmann, A.; Ernst, R.: Anforderungen und Randbedingungen fur den Einsatz von MeBsystemen in der Mikro--und Nanotechnik. VDI-Berichte Nr. 1530 "Sensoren und Messsysteme 2000", pp. 297-307, VDI Verlag GmbH, Dusseldorf, 2003