tailieunhanh - Handbook of Advanced Ceramics Machining Episode 6

Tham khảo tài liệu 'handbook of advanced ceramics machining episode 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 136 Handbook of Advanced Ceramics Machining TABLE Grain Size of Used Diamond Grinding Wheels Mesh Size Grain Size mm Average Grain Size mm 325 40-90 600 20-30 1200 8-16 2000 5-10 4000 2-6 6000 8000 finish parameters Ra Rz and Rmax were measured after the grinding operations. A mirror finish surface was obtained when ELID grinding was performed with a 4000 mesh-size wheel or finer. Figure and Figure show a significant improvement in surface finish when grinding using a 4000 mesh wheel compared with 2000 mesh wheel. Better surface finish was obtained with the SRBSN material than Si3N4 especially when using rougher wheels. However with finer wheels 4000 almost the same surface finish was obtained with both materials. The results are shown in Figure 27 . To produce a mirror surface finish by ELID grinding a three-step operation was required. The silicon nitride specimens were first ground with a 325 mesh-size wheel. These specimens were further ground Ra Rmax Rz FIGURE The effect of wheel grit size on surface finish. Highly Efficient and Ultraprecision Fabrication of Structural Ceramic Parts 137 Ra Rmax Rz FIGURE The effect of wheel grit size on surface finish. with a 600 grit-size wheel and finally ground with a 4000 mesh-size wheel. The mechanism of material removal in ceramic grinding is a combination of microbrittle fracture and micro- or quasiplastic cutting mechanism 28 29 . The quasiplastic cutting mechanism typically referred Silicon nitride SRBSN SÌ3N4 FIGURE The effect of wheel grit size on surface finish. 138 Handbook of Advanced Ceramics Machining to as ductile-mode grinding results in grooves on the surface that are relatively smooth in appearance. By a careful choice of grinding parameters and control of the process ceramics can be ground predominantly in this mode 30 31 . On the other hand the microbrittle fracture mechanism results in surface fracture and fragmentation. .