Atti del IV Congresso “Metrologia & Qualità” (Febbraio 2005)

The dimensional and geometric compensation for Cartesian robots and machine tools is becoming a manufacturing engineering tool and the volumetric diagonal measurement is confirmed a modern instrument for the preventive maintenance. The laser Doppler make this change-up possible.

Gianmarco Liotto          OPTODYNE   

Giuseppe   Capasso      ALENIA  Aeronautica

Cosimino   Corona        ALENIA  Aeronautica

Abstract

In the actual word competition it is always requested the design and the manufacturing of complex machines for the automatic manufacturing of object, also of complex type, as the aircraft body. Are always requested more sophisticated equipments , more accurate, higher speed of actuation , but always less expensive. The massive mechanics and accurate have always been the prerequisite for the satisfaction of the accuracy to the detriment of the cost and the dynamic performances. With the progress of the power of calculation for the CNC is now possible to generate the errors model and compensate it with opposite movements. The theoretical limit of this technique is the mechanics repeatability, but the practical limit is in the difficulty to measure all the errors in an economical reasonable time. By the Optodyne laser Doppler it is possible to measure the error parameters much more rapidly and make the electronic error correction possible and profitable, including the angular deformation of the cross beams . The results of correction of a large dimensional robot (working volume 780 cubic meter ), measured by the Optodyne laser Doppler and corrected, in all the working area with matricial and vectorial techniques by means of 52 compensation tables, are reported in this article.

The residual error, measured with the volumetric diagonal technique meeting the new ISO 230-6 or the well established ASME B5-54 standards, is less than the dimensional variation due to thermal expansion. The dimension of the diagonals (fast to be measured) will be used as stability reference for the determination of eventual maintenance actions.

Introduction

The automated manufacturing machines, in this case the CNC machine tools and the Cartesian robots, but also all the CMM measuring machines have to be both Accurate and Fast but also and especially Cost effective. The cost for the manufacture of complex production tooling is transformed in increased repeatability better productivity with consequent better yield in constant higher quality level. The quality is a saving tool, because are reduced the cost at the downstream of the production process including maintenance and safety. The feed is related to the inertia reduction, and thereafter of the weight reduction. The new composite materials, ceramics, aluminum alloys and titanium con solve the weight problem but often are too expensive and not enough robust for practical application in the manufacturing industry. For the manufacture of large machines the steel alloys in welded structures is normally used in order to increase the inertia momentum and reduce the weight .

The construction of accurate machines with orthogonal axes is based on the capability of manufacturing and installation of structures and guide-way, that in spite of the gravity force, have to be perfectly straight and orthogonal between themes. Alternatively to the absolute accuracy of the mechanical structures are becoming popular alternative methods based on the electronics and servo systems. Is the way to cancel electronically the error produced by the mechanics providing new positioning data, that pushes the machine in the correct position. The electronic compensation by error tables is commonly used by the CMM manufactures that in this way have overcame the compromise between accuracy and massive construction. On the CMM, with the support of powerful electronic processor are compensated up to 21 errors obtaining accuracy of few micrometers on machines of large dimensions, too. This technique is experienced by several year and now, with the capability of new numerical controllers that contains the possibility of the utilization of multi axis electronic compensation, synchronized and conditioned. Synchronized is a compensation that is applied in a direction different by the axis movement of the axis that is moving, used for instance for the compensation of the vertical or horizontal straightness compensation. Conditioned is a compensation where the comp value depends both