ANALYSIS: ADDITIVE MANUFACTURING


Figure 2: a schematic of the coaxial laser triangulation functioning principle (left) and the integration on an Additube LMD system at the Politecnico di Milano (right)


The coaxial laser triangulation is based on a simple principle, which becomes effective through correct implementation of hardware and software solutions. The strength of the method derives from the fact it provides a direct measurement of a fundamental process output through a non-invasive optical principle, rather than an indirect measurement which requires calibration for different configurations and materials. It is highly suitable for retrofitting on existing systems with monitoring portals available on the process head. The system can be used for online


The position of the probe beam is viewed through a coaxial commercial digital camera, and consequently the lateral displacement viewed by the camera is converted to the height information. The current system runs with a height resolution in the order of 0.1mm and an acquisition frequency higher than 100Hz, suitable for most industrial LMD applications.


Advantages of coaxial triangulation The implemented coaxial triangulation system has been tested with several different geometries. Figure 3 shows a case study where a cylindrical form is produced employing AISI 316 stainless steel powder. The robotic arm is programmed for a


constant height increment superimposed to a circular motion, resulting in a single helical trajectory. With a 35mm diameter and wall thickness of 1.5mm, such form is representative of geometries prone to heat accumulation and thus height mismatch. Extracting the position data from the


robot controller, the height measurement is used to generate the height error map along the deposition trajectory. The results showed that the final dimensional error can reach up to 5mm. The process settles to a constant error after the deposition of approximately 50 layers. It can be seen that the height information is highly correlated to the deposition width, since the real texture formation along the build direction can be viewed at the measured height colour map. Another critical condition is related to the deposition of thin-walled structures. Such geometries are prone to height variations, also due to the change in trajectory length, as well as the dynamics of the mechanical axes. Figure 4 shows a case study where a stepped thin-wall profile was produced


using AISI 316 stainless steel powder. It can be seen that the height error map follows the shape of the real part with high fidelity, including the acceleration zones at the end of the trajectory. The coaxial triangulation measurements were compared to measurements taken with a touch probe after the deposition. The results indicated in Figure 4 show the efficacy of coaxial triangulation as a contactless and inline measurement tool.


measurement of the workpiece height as a means for process monitoring, as well as for building control actions with the analysed signal. The output can be used as an aid for 3D reconstruction of the workpiece, hence part qualification. The measurement principle can allow for a real autonomous operation of the LMD systems with prolonged process duration, opening up new application fields. The development of such solutions, as well as industrialisation of the product, are ongoing. l


The authors acknowledge collaboration with BLM Group. This work was supported by the EU, Repubblica Italiana, Regione Lombardia and FESR.


References: a full list of references will be provided in the online version of this article


Figure 3: a cylindrical thin-walled structure built by LMD and the corresponding measured height error as a function of time. The colour map shows height error profile of a cylindrical sample. Positive height error refers to a decrease in standoff distance


Figure 4: the actual photograph and the height error profile of a thin-walled sample. Positive height error refers to a decrease in standoff distance


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