Experimental Characterization of Contact Stiffness for Fixture Design

Abstract

When cutting flexible metal workpieces, the mechanical characteristics of the fixture play a crucial role in suppressing deformation and vibration. Previously, the influence of fixture design elements on workpiece vibration has been studied with the aim of gaining a fundamental understanding of the phenomenon. Meanwhile, there has been limited research on the contact area between the fixture and workpiece, and there is a need for further research to understand the phenomenon and establish design guidelines for fixtures. In this paper, an experimental approach is proposed to characterize the relationship between stress on the contact surface and the resulting displacement. The main goal is to analyze the influence of contact surface shape and material on the deformation (or vibration) of the fixture and the workpiece. To this end, an on-machine measurement device was introduced. This device was used to obtain displacement-load curves without setup errors for materials such as PTFE, MC nylon, and aluminum alloy, as well as contact surface shapes such as flat or conical. From the obtained curves, the repeatability stability of the fixture was qualitatively evaluated. When using MC nylon, the plastic deformation was minimal even when loads were applied multiple times under flat conditions or conical conditions with a tip angle of 170° or more. This indicates that these contact conditions are stable as fixtures. On the other hand, when using PTFE, buckling occurred due to its low Young’s modulus, and the workpiece was damaged when using aluminum alloy. This suggests that these materials are not suitable for use as fixtures in this paper. For a more quantitative evaluation, an assessment based on contact stiffness was conducted. The displacement-load curves were modeled using a power-law function, following existing elastoplastic models. The contact stiffness was calculated from the slope of the tangent line to the displacement-load curve at maximum load. When using MC nylon as the material, the contact stiffness was found to be a maximum of 6×106 N/m. Furthermore, the rate of change of contact stiffness during loading and unloading was used to quantitatively evaluate the fixture stability.