Papers by Author: Roberto González

Abstract: The general trend in the field of machine tools is to increase the speed of feed axes with the aim of reducing manufacturing times, high speed machining is a significant example of such increase. Ball screws have proven their ability to fulfill requirements of workpiece positioning accuracy, nevertheless, the heat generated in the screw-nut interaction increases proportionally with the speed of the axes. As a result, an increase in temperature takes place, which causes thermal expansion of the ball screw, producing positioning errors and decreasing the service life of the ball screws, therefore it is important to have effective methods for predict the temperature.The main objective of the present work is to predict the heat generated in the screw-nut contact based on analytical models in order to estimate the temperature distribution in the nut. For this purpose a heat transfer FE model of a preloaded high speed ball screw nut has been developed. Additionally, experimental heating tests have been performed to validate the FE numerical model. In this way, the temperature predictions obtained at different working conditions have less than 6% of deviation comparing with the experimental results.

Abstract: The development of Fe-based metal matrix composites (MMCs) with high content of hard phase has been approached by combining the use of advanced powder metallurgy techniques like high-energy milling (HEM), cold isostatic pressing (CIP) and vacuum sintering. The most innovative is the use of HEM for the obtaining of a composite powder avoiding the formation of clusters in the microstructure of the sintered material, and the study of hardenability of sintered materials. A 30 % vol. of NbC particles was mixed with Fe powder by HEM in a planetary mill under Ar atmosphere to prevent oxidation. The optimal milling time was determined by sampling every two hours until 10 hours of milling, characterising the powder by the observation of morphology and microstructure by scanning electron microscopy (SEM), and controlling the carbon content by a LECO analyser. Composite powders were compacted by CIP and then sintered in vacuum at temperatures between 1300 °C and 1375 °C, during 30 min and 60 min. The variation of density, hardness and carbon content with sintering temperature and time, besides the microstructural changes observed, permits to find the optimal conditions of processing. Afterwards, a heat treatment study was performed to study the hardenability of the cermet.