METHODOLOGY OF OFF-LINE SOFTWARE COMPENSATION FOR ERRORS IN THE MACHINING PROCESS ON THE CNC MACHINE-TOOL

Off-line software machining accuracy enhancement has been suggested ba sed on the prediction of the machining error <(Delta)over right arrow> = [Delta(x), Delta(y), Delta(z)]. It has been assumed that errors in the machine tool workspace can be analysed as a superposition of the c omponents <(Delta)over right arrow>(g), <(Delta)over right arrow>(m), <(Delta)over right arrow>(n) where <(Delta)over right arrow>(g) result s from the geometric surface description, <(Delta)over right arrow>(m) results from the limited machine tool stiffness and <(Delta)over righ t arrow>(n) results from the cutting tool deflection during machining. Geometric description error <(Delta)over right arrow>(g) = [Delta(xg) , Delta(yg), Delta(zg)] arises from the surface geometric modelling ph ase in the CAD system. The NURBS (Non-Uniform Rational B-Spline) techn ique has been selected as the most effective for the complex shape sur face description, supported by the least square principle, Then the di stances between the measuring points and the calculated points on the described surface patch have been determined and a function representi ng the inaccuracy of the surface description has been defined. The <(D elta)over right arrow>(m) = [Delta(xm), Delta(ym), Delta(zm)] error ha s been calculated based on periodic measurements of the machine tool p ositions while applying static forces to the spindle endpoint. The poi nts in the factor have been selected according to principles of mathem atical experiment planning. The polynomial regression-based relationsh ips have been derived for <(Delta)over right arrow>(m) error descripti on. The results have been validated using statistical correlation anal ysis, A similar approach has been adopted for the estimation of the cu tting tool deflection error <(Delta)over right arrow>(n) = [Delta(xn), Delta(yn), Delta(zn)]. The resulting Delta(x), Delta(y), and Delta(z) components have been introduced as compensation values adding up to t he regular tool movements along the programmed tool path. A special po stprocessing program has been developed for doing this job, herein cal led active task in order to differentiate it from standard postprocess ing functions named as passive tasks. (C) 1998 Elsevier Science S.A. A ll rights reserved.