Development of flexible polishing tools for synchro-speed polishing processes using additive manufacturing

. A new concept for synchro-speed polishing of flat and spherical surfaces is introduced: 3D printed gradient index (GRIN) polishing tools. By using additive manufacturing technologies in combination with photopolymer plastics, GRIN tools can be fabricated that are individually adapted to the workpiece geometry. By using two different plastics, the hardness and therefore the removal rate of certain tool areas can be defined. Surface structures, benefiting material removal rate and tool wear rate, are possible as well as lightweight structures with high mechanically stability. Tools can be fabricated as thin foils as well as solid pads, ranging from small (few mm) to large diameters. Additionally, the pads can be fabricated with an individual radius. This can enable the replacement of radius-dependent tool holders, because the pads can be mounted on flat tool interfaces, since the radius is not dependent from the tool body anymore. First results from the experimental setup are showing, that by using GRIN foils similar surface quality results can be achieved in comparison to conventional polyurethane foils, while the GRIN foils are offering a lot more possibilities regarding process optimization.


Synchro-speed polishing
The applications of micro-optical systems in the fields of refractive and diffractive optics have increased tremendously.This paper reports on an experimental study using a new type of 3D printed polishing pads for synchro-speed polishing.
The Synchro-speed polishing process was first published by Erhard Brück in 1981 [1] and is known to be a common mass production method for optical components.The polishing process is initiated due to area contact between the lens and a tool by using a polishing slurry, with a tool diameter twice the size of the lens diameter.Both, tool and lens, rotate in the same direction by a certain speed ratio, to achieve a constant cutting speed for the whole lens surface.The tool axis needs to be tilted in a defined angle in relation to the workpiece axis to achieve a certain lens radius.This polishing process can be used for spherical as well as for flat surfaces.[2]

GRIN polishing tools
The developed 3D printed foil and pad geometries consist of two different photopolymer plastic types of material: a black and white photopolymer, which differ in hardness.Previous investigations have proved, that the wear rate of the white pad is larger than the black pad, which resulted in a four times higher removal rate of the black pad when polishing N-BK7.Both materials enabled the generation of P3 optical surface roughness levels of less than 2 nm RMS (1.4 nm RMS using the white pad and 1.8 nm using the black pad).[3] By using additive manufacturing technologies, gradient index pads or GRIN pads can be produced, consisting of both materials.The polyjet process is used for the polishing tool generation in this paper.Due to the flexibility of multi-material 3D printing with a high lateral resolution of less than 10 µm, various attempts to improve conventional synchro-speed polishing can be made.Individual structured foils with a thickness of less than 1 mm can be produced to substitute conventional polishing foils (fig. 1 a and b).Those one side planar pads can be mounted on flat tool interfaces for the CNC tool axis, reducing the amount of traditional metal synchro-speed tools needed, since the radius is no longer dependent from the tool body.Therefore, the disadvantages of conventional polishing foils, such as expensive materials (foils and metal tool bodies for specific radii), processes (foil cutting and gluing) and problematic applications for foils with a diameter less than 30 mm, can be compensated.For tool dressing, the same methods as for conventional polishing tools can be applied.
Another example is the modification of certain areas in terms of hardness by using different photopolymers to adjust the polishing process to a specific geometry (fig.3).Those modifications are applicable for huge diameters as well as small diameters of a few mm, enabling also polishing of miniature lenses (fig.3).Fig. 3. GRIN pads with 9 mm diameter used for polishing lenses with a diameter of 4.5 mm

First results of basic experiments and conclusion
A sample geometry, a biconvex lens with a diameter of 32 mm with radii of 90 mm and 180 mm, was successfully fabricated using GRIN foils (fig.4).First results of comparing GRIN foils with polyurethane foils could be established as well, using white light interferometry.In terms of roughness, the GRIN foils (Sq=0.9nm) achieve similar or partly better results compared to the conventional foil (Sq=1.7 nm) after the same polishing time using a cerium oxide polishing slurry.In conclusion, GRIN polishing pads, a novel 3D printed kind of synchro-speed polishing pads, have been developed and successfully tested on N-BK7 lenses during first experiments.They offer a promising alternative to conventional polishing foils and spherical metallic polishing foil carriers can possibly be replaced by cost-efficient 3D printed spherical pads.The feasible applications for polishing micro-lenses in terms of diameter, smallest radii processable, long-term stability, shape durability as well as industrial applicability are currently under investigation in the labs of the universities of applied sciences EAH (Jena, Germany) and OCB (Buchs, Switzerland) as well as industrial partners.

Fig. 2 .
Fig. 2. Polishing, pads diameter of 64 mm: a) pad with radius of 90 mm mounted on flat tool interface b) lightweight structure

Fig. 4 .
Fig. 4. Biconvex lens polished with GRIN foils, diameter 32 mm, with radii 90 mm and 180 mm Fig. 5 shows the difference topography of a N-BK7 lens after 10 minutes polishing time with a GRIN foil in the central area of 400x400 µm² with a 20x magnification measured by white light interferometry.Only small deviations in the range ∆z < 30 nm are noticeable.

Fig. 5 .
Fig. 5. Difference topography of GRIN polished N-BK7 lens in the center of the lens, measured by white light interferometry