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8
CONCLUSIONS AND RECOMMENDATIONS
8.1
Conclusions
At the end of this thesis-work some general conclusions are given about the performance of the micromilling setup according to the analysis that have been done.
About the static stiffness part, the goal has been to evaluate the global static stiffness behaviour of all the components of the setup. In order to do that, it has been calculated the stiffness of each element separately, modelling it as a spring, and assessing it via experimental testing, properly designed for each component. For each test done, the main sources of error have been considered and some corrective factors have been calculated to obtain the final value. Some components have been studied together, because it was impossible to disassembly them. In every analysis it has been found a range of stiffness values and for each component has been considered the lowest one, which is more cautionary. At the end a global value has been found and an analysis about the compliance budget of the stiffness chain has been done.
From the latter, it has been seen that the tool is the most flexible component of the compliance chain evaluated in the analysis; nevertheless the other components may represent at most 22% of the overall compliance. Furthermore the value obtained is bigger than others found in literature, so it is possible to conclude that the machine is quite stiff, especially for the presence of the granite bridge and table.
About the measurement of the spindle and the tool-holder some doubts have come out. In fact the hypothesis about the spindle-machine tool connection might be simplified too much and it has been noticed that the thermal expansion of the spindle might have a big influence to the performance of the setup.
As regards the dynamic and the geometric accuracy part, both of the analyses have been done because of some results found from a circular interpolation test. During this test, which was inside a Heidenhain workshop, some data regarding the dynamic accuracy and the straightness of the axes have been collected. From those, some inconsistencies have come out. In order to analyze this inconsistencies a dynamic and geometric accuracy analyses have been done.
As concern the dynamic accuracy has been done a test which the following error has been involved in. This allowed to analyse the behaviour of the machine as a function of the acceleration magnitude and study both the influence of the geometric accuracy and the control loop accuracy on the dynamic error. The results demonstrate that, in order to have a low following error, to improve the quality of the work-piece, it should be used a low acceleration magnitude. It is then possible to conclude that, in order to have a good quality of the products, it is advisable to not use high motion parameters magnitude.
Finally, for the geometric accuracy, a double check of the Heidenhain data has been done. It has been necessary to validate the results found during the workshop, which showed a non expected geometric error of the axes. In order to do that, an experimental setup has been designed and a comparison between the collected data and the precedents has been done.
Elaborating the data, at the end a correspondence has been found between the two tests. Therefore it has been possible to conclude that the accuracy of the machine does not obey the required specifications.
Given that one of the goal of this thesis work was to achieve a pretty deep knowledge of the machine setup (chapter 3.2), it can be concluded that with the data collected it is possible to draft an error budget of the micromilling setup, in order to rely on it during future work.
8.2
Error Budget of the micromilling setup
The performance analysis have permitted a better understanding of the micro-milling setup. Drafting a specific error budget of the machine under exam, given the data results, it can be said that:
118 1. the warming up and the consequent thermal expansion of the machine components originated by the control and servo-loop systems, influenced the final accuracy of the machine and therefore of the product. This aspect has been noticed during the experiments;
2. geometric errors are important aspects to evaluate although the specifications of the micro-milling show a high positional accuracy;
3. parameters of the control system do not compensate the errors due to the accuracy, straightness, orthogonality and temperature. It has been seen that the compensation’s action does not improve the behaviour of the machine;
4. dynamic errors related to the motion parameter of the machine influence the final accuracy of the product;
5. errors from the environment, such as vibrations, temperature and humidity, influence the behaviour of the machine.
This list of the sources of error can be taken into account for improving the performances of the machine tool and giving improvements for future work.
8.3
Recommendations for future works
Some recommendations for future works are given for the purpose to improve the performances of the micro-milling setup:
• design and built of a proper cooling and thermal compensation system, in order to prevent the errors due to the possible temperature fields that can be created during the machining. In particular it is recommended to take care of the spindle warming up, that is has been noticed as a heat source;
• it is necessary to place the machine in a controlled temperature and humidity and vibration-free room, in order to avoid all the possible errors caused by this factors;
• check the technical drawings of the stages in order to identify the relative positions of the machine’s components, for instance encoder and bearings, for understanding their influence on the geometric error of the machine;
• evaluate the positional accuracy again by using the laser interferometer, in order to check the calibration files of the axes.
