Micromilling of hardened tool steels

Miniaturized parts are increasingly demanded in different fields like medical, transportation, environmental, and communication industries. In order to manufacture these parts in an economical way, mass replication methods, such as micro injection molding, have to be applied. Currently, Electro Discharge Machining (EDM) process is mainly used to manufacture the needed moulds for micro injection molding in industries. In order to achieve the final shape, several electrodes have to be made by milling with different levels of geometrical accuracy. Besides, the material removal rate of EDM is relatively low. This results in a long throughput-time and high manufacturing cost. Therefore, industries are looking for alternative technologies to overcome these drawbacks; micromilling is one of the promising technologies. The advantages of the micromilling technology include the applicability of a broad range of materials including hardened tool steels, the capability of manufacturing three dimensional geometries, accurate machining of complex features, and it is economical for small and medium lot sizes, etc. However, although micromilling in principle is a good alternative for the EDM process, it is found in research that some challenges have to be overcome before this technology is ready to be adopted in industrial applications. The literature survey shows that the fundamental micro cutting mechanism has been well investigated and understood through the study of micro orthogonal cutting and ultraprecision machining. Issues related to the application of micromilling have however not yet been well studied. Besides, inconsistent observations are commonly seen in literature. This is because observed results in micromilling are highly dependent on the experimental conditions. Based on the literature survey and initial micromilling tests, the general goal of this research has been defined as to develop and describe a reliable micromilling process for precision machining of hardened tool steels. It was decided to first improve lives of micro endmills to achieve a reliable cutting, and then to improve the performance of the process through process planning. In this research, experiments were mainly done with Ø 0.5 mm square endmills on hardened tool steels (AISI H11, H13, etc.). Experimental investigations were done to identify the main problems in micromilling. It was observed that the used commercial micro tools suffered severe wear, the tool life was too short to conduct a successful task, and the workpiece quality was not achieving the requirements. Investigations were conducted to understand tool wear types and mechanism. The factors which influence the tool performance were analyzed. It was found that the geometry of commercial tools is mainly derived from macro endmills, with which the cutting edge corners have the highest stress level. The machining parameters and tool paths are two factors that have significant effect on the tool performance; however, there was no good method available for the planning of the micromilling process. The geometry of micro endmills was studied theoretically by means of analytical modeling and FEA method. Having understood the relationship between geometrical features of the cutting tool and their influence on the tool performance (stiffness and strength of the cutting edge corners), the geometry of the micro endmill can be designed specifically for a given application to achieve the desired performance. This method was demonstrated by designing the micro square endmill especially for hard milling applications. The newly designed tools were manufactured and validated through experiments in comparison with the commercial tools. The experimental results have shown that the new designs have improved the tool performance as expected. The planning of the micromilling process has been divided into two parts. In the first part, design of experiments has been used to understand the relationship between input variables (machining parameters and tool paths) and process response (tool wear and surface finish). With this method, the significant variables can be identified by means of ANOVA analysis, and the cutting conditions can be planned accordingly to optimize the process output. For example, to have a long tool life is important for the roughing operation, and to achieve a good surface finish is of interest for the finishing operation. In the second part of the process planning, a knowledge-based method is used to plan cutting conditions for the machining of micro features with high aspect ratios. The selection of machining parameters was done by means of a force model, which describes the relation between machining parameters and average forces. The tool paths were tested by a FEM model. An improved tool path was proposed to overcome the disadvantage of the conventional tool path. Experiments were done with conditions chosen by the theoretical analysis, and the results proved the validity of the developed method. Micro ribs with aspect ratios higher than 50 could be machined successfully.Precision and Microsystems EngineeringMechanical, Maritime and Materials Engineerin