Positioning error prediction in boring machines

This paper focuses the attention on the possibility to describe and predict the positioning error of the end-effector (or tool-holder) of a Cartesian boring machine. The aim of the work is i) to build a deterministic expression of the positioning error, starting from independent measurements of linear and angular errors for each independent axis, and ii) to investigate the precision enhancement, which may eventually be achieved by implementing such deterministic error-prediction into the numerical control of the boring machine. The six components of the end-effector positioning error are expressed on the basis of a symbolic analysis of the kinematic linkages, which represents the machine layout. Each component results as a function of the command position, some environmental parameters (mainly the ambient temperature), and alignment errors for each axis. In particular, alignment errors may be expressed as six functions of position for each axis: three position errors and three angular errors [1,2]. An accurate measurement of these errors (which may take place either on-line or at regular time schedule as a result of an auto-diagnosis procedure) allows expressing the end-effector positioning error as a deterministic vector field in the whole machine working space. Beside these considerations, the paper investigates the applicability of different techniques allowing the measurement of axes alignment errors.