For the process of plastic materials working，there is a minimum cutting thickness. When the realeffective cutting thickness is less than the minimum cutting thickness of workpiece materials in the actual cuttingprocess，it will cause abnormal cutting of the tool and the workpiece，mainly in the sliding friction. So the machining accuracy is hard to guarantee. It is significant to determine the minimum cutting thickness of the materials topmprove the machining quality and precision. A test method was provided to determine the minimum cutting thickness of workpiece material，focusing on the material of hardened steel Cr12MoV and the minimum cutting thicknessfor different cutting speed was determined by this test method. The test results show that:the thermal softeningeffect has greater effect on the chip formation mechanism of hardened steel Cr12MoV(58 HRC)than strain rateeffect in the selected speed range. The determination of the minimum cutting thickness has provided a theoreticalbasis for selecting the allowance of finish machining of automobile covering mold.