An Investigation of Tool Entry in Orthogonal Metal Cutting Using an Optical Quickstop Device and Machine Dynamics Methods

Abstract

The entry and steady-state phases of plate orthogonal metal cutting were examined using an improved optical quickstop device (OQSD) in an experimental design that included material, hardness, depth of cut, cutting speed, rake angle, and tool stiffness. The results showed that stress/strain analysis using laser-marked circle deformation may be substituted for etched grain structure with great confidence. Payton’s observation of Merchant’s “direction of crystal elongation” as the direction of shear is proven to be easily measured with the laser-marked circles. The use of circle deformation led to new methods for measuring shear strain, shear strain rate, and shear flow stress, which are compared to previous models. A new empirically derived, statistically based expression for predicting the direction of shear that includes the effect of material properties such as stacking fault and hardness is proposed. A new method of identifying the transition from entry-state to steady-state cutting in orthogonal machining based on deflection and force measurements allows the analyst to analyze the two states individually. The use of modal stiffness as a factor level showed that for lower stiffness tools, the strain rate (𝛾̇_snell) increased by 28 percent. The entry-state and steady-state signals were analyzed through nonlinear dynamic methods, specifically employing approximate entropy (ApEn) and the Largest Lyapunov Exponent (LPE). ApEn is higher in steady-state signals than entry-state signals. LPE of the entry-state for thrust deflection showed that the signal changed from linear to nonlinear across the factor levels of material, depth of cut, and cutting speed, but overall was found to be inconclusive in low-speed machining.