# Machinability of Stone--Plastic Materials During Diamond Planing
## 3 Results and Discussion
### Chip Deformation  
Machining is a material failure process. When the cutting edge bit into the workpiece, the unneeded SPC was separated and sheared along the cutting plane. Then, with further movement of the cutter, the rake face squeezed and accumulated the unneeded SPC. Finally, the unneeded material fell out and formed into chips with different shapes (Figure 8). Figure 8: Chip formation at different cutting depths: (**a**) 0.1 mm, (**b**) 0.6 mm, (**c**) 1.1 mm, (**d**) 1.6 mm, and (**e**) 2.1 mm. Figure 7: The maximum cutting temperature at varying cutting depths for the (**a**) cutting edge and (**b**) chips. Each point is the average of five measurements, and the error bars of each data point represent one standard deviation. For more details of the cutting process, the full-field related mechanics of SPC machining were analyzed, and the von Mises strain was examined at the interface of the SPC and the cutting tool based on DIC analysis. As displayed in Figure 9a, when the cutting edge cut into the SPC, the highest von Mises strain response was in the SPC along the cutting tool edge, primarily in the cutting direction at the area oriented along the edge and into the SPC. Due to the considerably lower stiffness of the SPC compared to the cutting tool, the measurable strain was determined in the SPC. The strain localization at the edge in front of the cutting tool differs between the DIC stages, as shown in Figure 9a,b. This is related to the SPC material elasticity limit; thus, it is an explanation for the systematic discontinuity of the chip formation shape for arc chips at this moderate cutting depth of 1.1 mm. The strain gradually builds up until it reaches the material's elasticity limit, at which point a new strain localization connection forms and creates an arc chip. It can be concluded based on the von Mises strain that the cutting tool is influencing the SPC even at a considerable distance from the cutting tool. It influences not only the cutting surface, but also material below the surface. Measurement of von Mises strain indicates that the machined material partially passed its elastic limit, as it did not fully recover after the tool left the area (Figure 9b). This indicates that the material below the machined surface of the SPC reached its plastic yield point, and eventually, the unneeded SPC was removed by the rake face and then deformed into chips. Regarding the uncertainty quantification, values larger than the von Mises strain noise-floor mean of 0.09 with an SD of 0.05, indicate the experimentally tested material's actual mechanical response (Figure 9). Based on the observation of their shapes, the SPC chips were divided into five categories: tubular, ribbon, arc, segmental, and helical chips. Figure 10 reveals the chip shapes produced when SPC was machined with different cutting depths. SPC consists of PVC and stone; while the chip shapes produced during its machining are more similar to the chip shapes of PVC materials than of the stone material [29]. Tubular chips had a cylindrical shape with a chip-breaking length of 188-248 mm. They were mainly produced at a cutting depth of 0.1 mm. The machined surface was smoother than the free surface of the tubular chip; some irregular pits could be found on the free surface of tubular chips. Ribbon chips were flake-like, with the chip-breaking length ranging from 114-124 mm. They were primarily generated with a cutting depth of 0.6 mm. Cracks were observed on both the free and machined surfaces of ribbon chips. Arc chips had a C-shape, with a chip-breaking length of 13-35 mm. They were mainly observed with a cutting depth of 1.1 mm. Some clear serrated borders could be observed on both the machined and free surfaces of arc chips. Figure 9: Full-field mechanics of SPC machining at moderate cutting depth of 1.1 mm: (**a**) The first captured stage; (**b**) the following capture stage. Segmental chips were a discontinuous chip, formed at a cutting depth of 1.6 mm. Segmental chips had low strength, with a breaking-chip length varying from 6-11 mm.