MEASUREMENT Figure 1. Example of machining after raw-part alignment. A typical alignment method consists of two steps: 1. Part characterisation, and 2. Part setting. • 1.- Part characterisation. Raw part characterisation could be done on a Coordinate Measuring Machine (CMM), but it is more fre- quent for it to be done by just putting the part on a flat surface and using height measurement devices. This alternative to the CMM-s is especially frequent when dealing with very large parts.The goal of this phase it to guarantee that the raw part - coming from foun- dry, forging or mechanization processes - contains in its entire volume the finished product to be obtained; that is, ensuring that there is enough material on all the surfaces of the raw part. For this, it is necessary to measure the raw pat and compare this volume with the theoretical measurements of the finished product. These comparisons should serve to calculate the correct fit of the part in machine to guarantee the mentioned overmaterial in each and every of the surfaces to be machined; being the result the definition of certain key references of the raw part in the machine coordinate-system. • 2.- Secondly, and with the part positioned on the machine, the part alignment operations are carried out according to the pre- vious calculations. Therefore, part characterisation is done by comparing several ‘heights’ with the sizes of the final part (Figure 2, left). As a result, some references are marked at the part (Fig. 2, right), which will be used in the next step. Figure 2. Example of manual raw part fitting process: part setting calculation (left) and reference marking (right). It should also be emphasized that in many occasions machining companies do not have the adequate resources to measure large size parts, due to weight, size, etc. In these cases, the parts are directly measured inside the machine. This implies that the machine is stopped – it is being used for measuring issues instead of for machining purposes. These downtimes can be long, and in case the part is not correct (i.e. not enough material), the part needs to be taken out of the machine, fixed or even scrapped. This supposes an added waste of time. Description In view of the difficulty and cost of raw part alignment process, the development of a methodology for part alignment was envisaged. The goal of the development was a system to characterise very large parts by ‘portable’ measuring devices (no CMMs), and for the alignment of them in the machine tool. Another goal was that the process could be performed by non-especially skilled operators. Regarding the accuracy required for the alignment process, it is necessary to start considering the average excess material of the part. For very large welded parts with quite near to shape tole- rances, a design excess material of 5 mm can be considered. In this case, an overall accuracy of 1 mm should be adopted. For the applications considered in the present work, with large parts from casting up to 5 m long, the design excess material can easily reach 20 mm. In that case, acceptable accuracy for the complete align- ment system is around 3 to 5 mm. There are already commercially available sophisticated methods for raw part characterisation. Among the different possible solu- tions stereometrics scanning and laser radar can be mentioned. Without entering too much details, both methods can provide a reasonably accurate 3-D representation of the complete part. Some of the limitations of these solutions can be the size of the part that can be processed, and the huge amount of information provi- ded. The latter is an important issue, as only the surfaces that have to be machined are really important for part setting. Processing the information of the complete part and manually establishing which surfaces have to be compared with the shape of the desig- ned part would represent a huge effort, well beyond the reach of unskilled labour. Therefore, both possibilities where abandoned. For measuring some specific points of the part, the most rele- vant methods are the laser tracker and photogrammetry. Photogrammetry was chosen, due to its lower cost and higher simplicity. Accuracy of photogrammetric measurement is in the order of 1/5.000 to 1/1.0000 of the size of the part, sufficient for the required specifications. Aware of this issue which concerns machining workshops, foun- dries and the metal fabrication of voluminous workpieces, Soraluce has developed with its technology centre Ideko an innovative technology which will enable the execution of these processes in a much faster and safer manner. A technology that is already paten- ted and called VSET, which will set new quality standards in the set-up phases. VSET is a new measuring system, based on 3D vision technology, which reduces the best fit calculation and alignment time of large raw parts. consists of various hardware and software systems which has been developed by IDEKO. The photogrammetric vision system makes it possible to achieve an alignment precision of up to 0.1 mm/m in a fast, reliable and simple manner. In short, tolerances that are more than enough for these processes. The information generated during the measuring stage is automa- tically compared to the 3D model of the part to calculate the best fit. Once the raw part is validated it can be aligned in the machine using the same reference points. The modular configuration of the product corresponds res- pectively with the previously described phases of the raw part measurement and best fitting. 21