Why parallelism of workpieces becomes convergent during double-sided lapping?

Double-sided lapping (DSL) is a precision process widely used for machining flat workpieces, such as optical windows, wafers, and brake pads owing to its high efficiency and parallelism. However, the mechanism of parallelism error reduced by the DSL process was rarely investigated. Furthermore, the relationship between parallelism and the flatness was not clearly illustrated. To explain why the parallelism of workpieces becomes convergent by the DSL, a theoretical model has been developed in this paper by calculating the parallelism evolution with the consideration of variation contact situations between workpieces and lapping plates for the first time. Moreover, several workpieces, including a slanted one rendering the model close to the actual process, are taken to calculate the parallelism evolution, and the mechanism of the parallelism error reduced by the DSL process is clarified. The calculation result has indicated that the parallelism error was reduced from 100.0 μm to 25.6 μm based on the parallelism evolution model. The experimental results showed that the parallelism improved from 108.6 μm to 28.2 μm, which agreed with the theoretical results well.