J. Eur. Opt. Society-Rapid Publ. 21, 37( 2025) 67
Fig. 1. Optical micrographs showing( a) initial WEDM cut surface of the tungsten carbide channel and( b) surface condition after fluid jet finishing.
chances of abrasive particle embedding into the workpiece, and non-uniform removal inside the grain and grain boundary regions [ 10 ]. FJP polishing was tried on tungsten carbide channels where similar conclusions could be drawn. The initial WEDM( Wire-electric discharge machining) cut surface showed poor surface morphology with crater and pits, as shown in Figure 1a. After FJP polishing, the grains were exposed clearly as the removal depth was higher at the grain boundaries, as shown in Figure 1b. The surface roughness reduced to 1.12 lm Ra from the initial roughness 1.55 lm Ra. The surface roughness could not be reduced further using FJP polishing.
Shape adaptive grinding( SAG) uses an elastic tool with abrasive pad attached on top of it, shown in Figure 2a. Elastic tools can achieve conformity with the shape of the workpiece while removing the material by virtue of contact pressure between tool and the workpiece. Shape adaptive grinding with optimum process parameters combinations and suitable abrasive pad can achieve Ra ~ 1nmandform error within 100 nm [ 11 ]. There have been quite a lot of developments in tooling and processing for shape adaptive grinding. Beaucamp et al. [ 12 ] proposed pressurized airfilled SAG tools to achieve high removal rate and subnanometric surface finish. CVD deposited silicon carbide could be polished to 0.4 nm Ra with removal rate as high as 100 mm 3 / min. It could be established that these tools gave stable performance over 10 h of finishing time. Ductile mode removal during SAG was associated with subnanometer chip thickness and very high specific energy, while fracture mode was associated with chip thickness above 10 nm and specific energy an order of magnitude lower than ductile mode. An aspheric silicon mirror could be polished down to form error 1.5 lm P-V and brittle mode material removal could be controlled to less than 10 % by determining ductilebrittle transition limit for SAG process [ 13 ]. Zhu et al. [ 14 ] performed shape adaptive grinding of low expansion
ceramics such as Nexcera TM, Zerodur TM and Cordierite using SAG. Shape adaptive grinding with fine abrasive grain size of 3 lm could realize better surface roughness finish when compared to that obtained with 9 lm abrasive size. Ghosh et al. [ 15 ] finished WC-Co( Tungsten carbidecobalt) coating using SAG tools and could attain surface roughness 7 nm Sa. It was concluded that sequential reduction in abrasive size over the finishing runs leads to better surface finish in SAG. In all the above discussed research on SAG, various shaped SAG tools, as shown in Figure 2b, have been developed and one chooses amongst them for the process in accordance with the workpiece size and shape. Different workpiece sizes and shapes such as flat, spherical, aspherical and freeform could be effectively finished using these available tool shapes. However, the cylindrical optics with narrow channel shape is difficult to process using existing SAG tooling as it is not feasible to fit and precess the ball-end, teardrop or cap type tools inside narrow channels. Therefore, a new tooling system is needed to effectively process such surfaces demanded in optical applications.
The aim of this research is to develop a new SAG tooling system which is capable of effectively finishing narrow channel shaped optical surfaces. In this research, a new thin wheel type finishing tool and a tool jig is designed which could be mounted on ultra-precision polishing machines for the form correction of narrow channels. Removal characteristics of the wheel tool are modeled and compared with that of the experiments. The finishing and form correction capability of the proposed tooling system was also established.
2 Experiments
The workpiece measurement, the tooling development, and finishing experiments are detailed in the following subsections.
2.1 Workpiece
This study targets ultra-precision finishing of narrow channel made of tungsten carbide( having cobalt as binder material). The channel was manufactured by wire-cut electric discharge machining( WEDM) technology. The channel and the surface micrographs are shown in Figure 3a. A large number of pits and craters induced by electric discharge machining are quite evident on the surface. The channel has concave conical shape with radius ~ 3 mm.