Strategies and Processes to Measure the Dimensional Geometry of Sheet Metal Parts for Exact Laser Cutting

Abstract

This paper describes strategies and processes to measure the dimensional geometry and deformations of sheet metal parts, in order to improve accuracy in laser cutting. A robust circular triangulation laser probe was integrated into a CNC machining centre. For the development of the measuring system a noncontact laser sensor and a personal computer (PC) were integrated information-technically with the NC-controller of the machining centre, as introduced in this study. To meet the demands of the measuring tasks in laser machining, the measuring uncertainty of the selected laser probe with respect to different measuring procedures, such as the fixed height procedure, surface tracing procedure, and profile-orientated procedure for steel and plastic materials, was investigated in this study. When measuring uncertainty is considered, the profileorientated procedure best meets the accuracy requirements. According to the experimental results, the uncertainty could be minimised to within 20 m in the middle measuring range. To shorten the measuring time, a twofold measuring strategy was developed. The first step checks for existing deviations quickly. If necessary, in the case of deviations of the actual machining path from the theoretical machining path, these can be quantified in the second step. A nine-point measuring strategy was proposed in this study in order to calculate the surface normal of the deformed sheet metal. An algorithm for calculating the path correction using these deviations and the calculated surface normal with respect to the coordinates and the surface normal of theoretical path position, was also presented. The algorithms were verified using the developed automatic measuring system on a 4-axis machining centre with satisfactory results. One specific characteristic of the strategy presented is that the machining path can be verified for the case of laser cutting for workpieces without edges or marks.