Automated Polishing for the European Tooling Industry
The poliMATIC research project has received funding from the European Community´s 7th Framework Programme (FP7/2007-2013) in the call
Robot Polishing The overall objective of the scheduled activities for the Robot Polishing solution approach is the development of automated robot-assisted fine machining, grinding, lapping and polishing processes (figure 1) by the means of a force controlled finishing spindle and CAD/CAM-based process strategies. These activities will support the manual worker, providing a brush, glossy or mirror finish on freeform steel moulds and dies.

Figure 1: Considered force controlled processes

Force controlled finishing spindle:
Efforts on the development of a modular force controlled concept to rotational and translational finishing tools were elaborated through the design of prototypes. Figure 2 depicts this concept, where the prototypes of the modular force controlled tools are mounted in a conventional electric spindle. The adoption of such an approach is based fundamentally on the possibility to develop robust modules capable to maintain the necessary forces and cutting velocities of the finishing process, with affordable additional costs for the implementation in existing robot cells or even in NC-milling machines.

Figure 2: Modular force controlled concept for rotational and translational finishing tools

Both module prototypes work essentially with the same basic system, a pneumatic valve, to control the process force during finishing, a pneumatic valve. With the integration of an electric valve unit, it is possible to set a desired finishing pressure by a signal from the control unit of the robot and according to the CAM-data generated previously, the pressure can be pre-programmed for distinctive positions of the tool on the mould surface. This possible active control permits the addition of the following functionalities during the process:

• Finish distinct regions of a surface with more or less aggressive finishing parameters, due to identified defects.
• Finish distinct regions of a surface with more or less aggressive finishing parameters, to achieve gradient of glossiness or generating desired structures.

CAD/CAM-based process strategies:
The appliance and development of strategies and different movement paths are important during the automated finishing process trials with the robot to improve the machined surface. According the surface roughness, different rotational finishing tools were adopted and appropriate process parameter were identified. The investigated tools differed in the type of machining technology (grinding, lapping and polishing tools) and in the specific characteristics as abrasive grain size, tool stiffness, contact geometry, etc.

Figure 3: Abrasion simulation - surface quality optimization

Furthermore each investigated tool and corresponding processing step presented a tendency to an optimal process parameter combination, regarding basically to the abrasive cutting velocity (spindle rotation), the process infeed (movement velocity of the robot), the process pressure (prototype force appliance) and the tool path strategy (path strategy). At this stage of the project development, three basic tool path strategies provided by the CAx framework prototype were applied in the multi-stage finishing process according with the objectives of each finishing steps (figure 3). The paths are named as:

- Parallel paths,
- Spiral paths,
- Stochastic paths.

Parallel paths are derived directly from existing milling paths and are used primarily in the initial finishing step to remove a high material volume of the surface through grinding and lapping. Both new spiral and stochastic paths are reserved to final finishing stages, when a more homogeneous material removal is requested and it is adopted from lapping until polishing finishing steps. The programmed simulation tool was built in MATLAB and presented as a useful qualitative tool to forecast the abrasion effect of the selected parameters, especially regarding the CAM-paths.

Successful demonstrations:
RMP-I Rotational module prototype I
The realisation of the rotational prototype was fundamental to assure the capabilities of the pneumatic force-controlled system as an appropriate solution and to provide an adequate equipment to initiate the development of other tasks for the Robot Polishing. Initial automated finishing process trials were conducted on planar milled surfaces of steel probes - 1.2343 ESU, a material usually adopted by the end-users in the consortium - and progressively enhanced with different finishing tools and higher surface quality requirements up to the finishing grade N1 according to the standard DIN/ISO 1302. Due to this progress, the involved parameters had to be adapted and new strategies and finishing paths were developed.

Figure 4 shows one cavity of a plastic injection airbag mould which was finished with a robot at Fraunhofer IPT. The mould material (hardened steel 1.2343 ESU) was previously fine milled until the desired geometry, however still presented several waviness and pitch imperfections on the surface. Due to remaining 15 µm deep holes during the milling process, the subsequent automated finishing process had to be able to remove uniformly at least the correspondent 15 µm high of material to clear the optical defects on the surface and to keep a uniform concave surface geometry.

Figure 4: Robot based automated finished cavity of a plastic injection airbag mould

TMP-I Translational module prototype I
The translational module TMP-I was tested and evaluated in order to design a subsequent mature prototype. Initial tests were made on flat surfaces and included the whole selected abrasive tools for mould finishing from milled until glossy surfaces (figure 5).
The initial milled surface presented a roughness of approximately:
Ra = 0,35 µm
Rt = 4 µm
After the multistage finishing process depict on figure 4, the reached surface quality was of:
Ra = 0,01 µm
Rt = 0,8 µm

Figure 5: Experiments of the TMP-I on a flat surface with different finishing tools

However, these values are an average of the roughness measured on different and random spots over the surfaces. As the surface quality is improved, the roughness differs in singular spots, due to defects generated on early and further stages of the finishing process. The defects (predominantly deep scratches) are caused by different reasons and will become a main focus on the following process development, seeking a higher homogeneity of the finished surfaces.
last update 2013-03-12 © poliMATIC 2010