J. Eur. Opt. Society-Rapid Publ. 21, 12( 2025) 125
Figure 1. Schematic representation of a new process chain for freeform fabrication with a combination of grinding processes, including ultra-fine grinding, and plasma jet polishing.
2 Experimental
2.1 Grinding
The process chain that was investigated consists of three grinding steps and the final PJP process. Grinding was carried out on a 5-axis CNC machine( Ultrasonic 20 linear, DMG MORI). The three main grinding steps include pregrinding for fast material removal, fine grinding for shape generation and finally ultra-fine grinding for a first surface smoothing. For the pre-grinding process coarser( D126 or D64) cylindrical diamond grinding tools are applied in combination with a 3-axis tool path movement, following the freeform contour. The material removal is achieved level by level, whereas layers of a certain cutting depth( in z-direction) are removed from the workpiece with the tool moving in the x-y-plane. This is done until the rough surface geometry is achieved, which at this point is characterized by a step-like appearance.
Afterwards the fine grinding process with ball shaped diamond tools is applied to remove these steps and further approximate the component surface to the required geometry. This manufacturing step is characterized by certain( kinematic) characteristics shown in Figure 2, such as the necessary 5-axis simultaneous path movements and pointlike tool contact with the workpiece.
For this, D30 diamond grain, which can be described as a medium to fine diamond grain size, is used in a metal bond. The fine grinding step serves for shape correction. Also, mid-spatial frequency errors should be removed during this process.
For ultra-fine grinding however, special diamond tools with a resin bond and fine grain size( D16) from company Günter Effgen GmbH are applied to reduce surface roughness. The bond material has a high porosity and slight elasticity, that can reduce the penetration depth of the diamond grains into the workpiece material and thus enable a kind of ductile grinding regime. This results in already partially reflective surfaces. The main purpose of this process is to achieve a further reduction in roughness and depth damage to the freeform surface while the material removal rate has a minor significance. This should lead to a smooth surface sufficient for the polishing step afterwards.
To investigate and improve the freeform grinding processes several parameters were specifically varied and their influence on periodic surface errors, form deviation and surface roughness were examined. These parameters include machine and program settings like path tolerance in CAM, sample positioning in the machine, as well as kinematic settings and grinding parameters.
2.2
Plasma jet polishing
The principle of atmospheric PJP in general has been explained in detail in several publications during the last decades [ 30 – 32 ]. The PJ source is based on a coaxial conductor system equipped with a gas supply. The PJ is excited at the gas outlet nozzle by microwave power( 2.45 GHz). The power provided by a solid-state microwave generator( CP250, Trionplas Technologies GmbH) is coupled into the source via a coaxial cable. The microwave propagates as a continuous wave( CW) along the inner electrode, through which gases are also passed. A high-frequency electric field generated by the microwave power at the tip of the nozzle ignites the gas flow and forms a PJ discharge. As a non-mechanical tool, the PJ is wear-free as long as MW-power and gases are supplied. Since only noble gases( He and Ar) are used, there is no chemical attack on the substrate and material is locally redistributed rather than removed.
A stable polishing process requires a uniform surface temperature. Hence, a closed-loop power control was established using a PID controller in a data acquisition system( RedLab 2408-2AO, Meilhaus Electronic GmbH) to maintain the maximum surface temperature at the contact point of PJ and the surface at a predefined setpoint by adjusting the microwave power accordingly.
The measurements of the local surface temperature were performed with an infrared( IR) camera( Pyroview 380L,