Development and application of additive manufactured fine grinding tools for the processing of fused silica

. The development or the improvement of production processes are necessary aspects, in order to enhance the quality and efficiency in optical manufacturing. This paper presents an approach to manufacture fine grinding tools in a very flexible and efficient way. A new filament composed of polyamide, ZrO 2 particles and diamond grains is developed and used in an additive manufacturing process for tool fabrication. The resulting tools are successfully applied in an ultra-fine grinding process on fused silica samples.


Motivation and research approach
The manufacturing of workpieces in the field of optical technology conventionally uses a process chain consisting of different grinding, lapping and polishing steps to gradually increase the surface quality of the part.[1] The special mechanical properties of inorganic-non-metallic materials, like fused silica, usually require complex manufacturing processes with often high demands on surface quality, contour accuracy and roughness of these brittle-hard materials.The component geometries to be produced are also increasingly characterised by a high degree of complexity, since the use of free-form surfaces and monolithic components is a current trend in modern optics manufacturing.
This leads to the motivation to improve the current manufacturing chain by developing new, or by improving existing production processes.One promising possibility is the (partial) replacement of loose grain processes like lapping and polishing with enhanced fine grinding processes, using bound grain.Apart from improvements of the machine technology and grinding parameter optimizations, this goal requires grinding tools adapted and optimized to the specific process.
The fabrication of tools by modern additive technologies shows a high potential in order to reflect the needed high flexibility in tool manufacturing.Additive manufacturing (AM) enables almost limitless possibilities for complex geometries, high production flexibilities and short production times.Individual part and small series production with individual design freedom is thus possible.Relevant tool features, like cooling channels or defined surface structures for better chip removal, can be included directly into the tool during additive manufacturing.This is why additive production of grinding tools is a promising market, which is currently still very much in its early stages.So far, there are mainly a few patent publications describing different approaches for additive grinding tool production.For example, methods for the production of an abrasive body with a vitreous bond and an abrasive body with a metal bond by means of selective laser sintering are presented.[2] Also, filament compositions suitable for polishing to produce tools by a fused layer modelling (FLM) process are described.[3] In this paper a new invention for extrusion based additive manufacturing of fine grinding tools is described and applied for the first time.This invention includes a special new filament consisting of a polyamide (PA) combined with small zirconium oxide particles and diamond grains (patent pending).The diamonds with small grain sizes act as the abrasive element of the tools.The polymer and the ZrO 2 form a hybrid resin-ceramic bond for the grinding tools, with the ZrO 2 acting as a support structure for stabilising the diamond grains in the soft, respectively elastic polymer matrix.
The approach for the presented investigations consists of the development of the filament material and the AM process on the one hand, and on the development of the grinding application on brittle-hard materials, in this case fused silica, on the other hand (Fig. 1).

Fig. 1. Schematic overview of the research approach 2 Additive manufacturing of grinding tools
The creation of the new grinding tools starts by creating the feedstock for the filament containing the three described components.A highly uniform distribution of the small ceramic and diamond particles within the polymer matrix is crucial.The feedstock is used for manufacturing a conveyable filament with regard to a requested concentration and size of diamond particles.
A desired tool geometry can be designed using a CAD program.The 3D volume model of the tool is then transferred into the STL format and prepared for additive manufacturing by slicing.
Additive manufacturing is done using a standard FLM printer.Printing parameters like temperature of the printing nozzle and the heating bed, flow rate and infill have to be optimized according to the desired tool geometry and size.Once the additive manufacturing process is completed, the produced part needs to be attached to a metal tool holder compatible to the used grinding machine.process steps from tool design to a ready-to-use tool are visualized in Fig. 2. The figure shows an example of a tool with ø 25 mm and D15 grain size for planar grinding, including a cooling structure.

Investigations of tool application on fused silica samples
The tools are tested in a planar surface grinding process on fused silica samples.The investigations are carried out on a 5 axis CNC machine "Ultrasonic 20 linear" (DMG MORI).First, the samples are grinded with a conventional metal bond tool (ø 24 mm, D35) from GÜNTER EFFGEN GMBH for creating a repeatable initial surface state.Prior to the application of the new fine grinding tools they need to be dressed using a fine corundum block.This creates a uniform tool surface with the cutting edges of the diamond grains exposed to the surface.
Fine grinding experiments, with the goal to achieve low surface roughness values, are performed with a 3D printed tool with grain size D15, grain concentration C180 and compared to a fine grinding process with commercially available resin bond tools from company EFFGEN.The comparison tools are also using D15 diamond grain with a standard concentration of D30 and a higher concentration of D120.The grinding parameters are kept constant with a cutting velocity of 17 m/s and a feed rate of 60 mm/min.The fine grinding process using the additive manufactured tool creates a semi-transparent surface (Fig. 3).The achieved surface quality was measured regarding RMS roughness Rq using a stylus instrument.The initial surface was characterized by a mean value roughness of 400 nm (Fig. 4).Ultra-fine grinding resulted in a strong roughness reduction of factor 8-14.It can be seen, that the AM tool even produced the lowest Rq of less than 30 nm compared with the commercially available ultra-fine grinding tools.
The investigations have shown the possibility to manufacture fine grinding tools in an additive way with successful application in fused silica grinding.The results demonstrate the huge potential of the presented approach.

Fig. 3 .
Fig. 3. Planar fused silica sample ultra-fine grinded on the left side using an additive manufactured tool, with comparison to the fine grinded initial surface on the right side

Fig. 4 .
Fig. 4. Comparison on achievable roughness values Rq: initial surface fine ground, additive tool (D15, C180) and two commercially available tools for ultra-fine grinding with same grain size but different concentration of diamond