No description available.

No description available.

No description available.

No description available.

No description available.

Selective Laser Sintering (SLS) or Laser Sintering (LS) allows functional parts to be produced in a wide range of powdered materials using a dedicated machine, and is thus gaining popularity within the field of rapid prototyping. Two current manufacturers of LS equipment and materials are EOS GmbH and 3D Systems. The PA2200 semi-crystalline polyamide powder studied here was developed by EOS and was processed using the 3D Systems Sinterstation 2500 HiQ LS machine. One of the advantages of employing LS is that the loose powder of the building chamber can be recycled. However, the properties of some recycled powders such as polyamide 12 (PA 12) deteriorate by comparison to those of fresh powder. Fabricating parts using only new powder, although providing the best quality, is considerably more expensive than using recycled powder. On the other hand, using recycled powder creates the problem of the coarse, rough, and uneven surface texture. This thesis examines LS fabricated parts which are affected by the "orange peel" phenomenon due to the usage of recycled PA 12 powder. This problem must be addressed before the technology can be widely accepted. This thesis presents the problematic areas and proposes solutions to manage and utilise the recycled powder. Further, the thesis discusses experimental work on the deterioration or ageing of PA 12 powder properties in the LS process, the microstructure of "orange peel" texture and the improvement of part surface finish to avoid the "orange peel" problem.

Due to the high precision and good surface quality that it can give, Electrical Discharge Machining (EDM) is potentially an important process for the fabrication of micro tools, micro components and parts with micro features. However, a number of issues remain to be solved before micro EDM can become a reliable process with repeatable results and its full capabilities as a micro manufacturing technology can be realised. This work presents some developments in advancing the state-of-the-art in the micro EDM process. EDM drilling and EDM milling are regarded as separate processes as they require different approaches in investigating and implementing the results of the study. At the beginning, special attention is paid to factors and procedures influencing the accuracy achievable, including positioning approaches during EDM and electrode grinding. In particular, the main parameters affecting the size and position of a machined feature are discussed and new techniques for minimising errors are proposed. The technological capabilities of different methods of setting up and dressing the electrode on the machine are analysed. Factors contributing to electrode wear, the main systematic cause for inaccuracy of the dimensions achieved, during the micro EDM process are studied. A method for calculating the volumetric wear ratio based only on geometrical information obtained from the process is proposed. This study investigates the suitability of micro EDM electrode wear compensation methods. Electrode shape deformation and random variations in the volumetric wear are also investigated as the two main factors affecting the applicability of the wear compensation methods as well as indicating the accuracy achievable with micro EDM. When producing features and parts on the micro scale, the phenomena that take place between the electrodes in EDM is not fully understood. A barrier to a complete exploitation of the potential natural tolerance of this process and to the further development of the process towards the production of components on the nano-scale is therefore in place. An analytical micro EDM model of electrode wear based on electrode shape deformation and wear ratio is suggested, verification of which requires experimental work with pure metals. Electrode-tool wear is studied during the micro EDM process of pure metals and the effect of electrode wear on the process accuracy and process variability. Objectives in this case are to advance the experimental knowledge of the electrical discharges during micro EDM operations which often conflicts with existent theoretical models of the EDM process. In particular, the remit of this investigation is to identify the effects that electrode materials have on selected electrical characteristics of the discharge process. An exploratory data analysis (EDA) approach is adopted in order to draw conclusions from the performed experimental activity. The material removal mechanism in the micro EDM process was confirmed to be mainly attributed to the melting and vaporisation phenomenon. Metal removal takes place as a result of the extremely high temperature generated by the discharge sparks. It was also found in this study that the volumetric wear ratio depends not only on the sparking conditions but also on the electrode materials. In addition, the research also proved that the electrode material severely influences the energy distribution between the electrodes during the sparks.

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No description available.

This investigation was motivated by the need for accurate prediction of electrochemical machined surfaces relative to corresponding tool geometries for given sets of machining parameters. A mathematical model was formulated which simulates the electrochemical erosion achieved by primary current distribution under steady tool feed rate, but with correction for variable efficiency. The equations comprising the mathematical model were programmed for solution by a digital computer, using discrete steps and a quasi-steady approach. The model was not completely analytical; it utilised an empirical values for specific metal removal rates. The efficiency of machining with NaNO₃ electrolyte was estimated from experimental results of other investigators. To assess the validity of the model, drilling test runs were performed with tubular electrodes having two geometries at the leading edge of the tool. Work specimens were made out of EN58J stainless steel, both NaC1 and NaNO₃ electrolytes were used. The correlation between experimentally obtained drilled surfaces and the computer predicted surfaces were satisfactory, justifying the assumptions made during the development of the model and the numerical methods of the solution used. This investigation has provided a method which could be successfully employed to predict the electrochemically machined profiles relative to tool geometries. This undoubtedly helps the production engineer in achieving the desired tolerances of the finished component eliminating the high cost of trial and error techniques.