Micro-grinding of titanium

Licentiatavhandling, 2018

Titanium and its alloys are difficult-to-cut materials, commonly used in several application fields, such as: medicine, aerospace, automotive and turbine manufacturing due to their biocompatibility, corrosion resistance, excellent mechanical and thermal properties, and light weight. However, its machining is associated with several difficulties, such as high tool wear, low surface quality, high cutting forces and high costs. To overcome these problems, using a proper and efficient manufacturing process seems essential. Micro-grinding provides a competitive edge in the fabrication of small-sized features and parts with superior surface quality compared with other processes. The quality aspects such as surface integrity of the parts produced by micro-grinding is influenced by various factors related to the induced mechanical and thermal loads during the process. Therefore, the machining parameters must be carefully chosen and controlled. Hence, developing an advanced, highly effective and efficient method, which can produce high quality micro-parts without inducing sub-surface damage, seems essential.

In this study, experimental and analytical investigations on 2D micro-grinding of titanium are presented. The run-out of micro-tools can be affected by the relatively high forces induces by mechanical dressing, meaning that the dressing and tool-conditioning possibilities are limited. Therefore, a proper set of dressing parameters is obtained for dressing of micro-grinding tools. An analytical model, which considers grits interaction, heat transfer and actual micro-grinding tool topography is developed which is able to predict the surface roughness and cutting forces for a given set of dressing and grinding parameters. It is shown that the topography of the tool varies with changing the dressing parameter which affects the grinding forces and surface roughness. In the analytical model the actual topography of the tool is considered in the simulation for the first time.  Additionally, the model is able to determine grinding parameters that generate minimum surface roughness with minimizing the grinding forces. To determine the correct chip thickness with the maximum material removal rate, an appropriate grinding tool and optimum process parameters to generate highly accurate contours in a micron scale will be further analyzed.

Using the analytical model, the effects of process parameters and tool surface topography are mapped to the process outputs, i.e. surface roughness and grinding forces. The results show that the analytical model enables the prediction of micro-grinding forces with a total error of 13.5% and surface roughness with the total error of 16%. The simulation results match with the experimental results to a greater degree in the low cutting speed range, rather than at higher cutting speeds. The results also indicate that the dressing parameters, such as the dressing overlap ratio and the speed ratio are influential factors, affecting surface roughness and grinding forces. Using higher values of dressing overlap ratio (Ud up to 1830) reduced the surface roughness, however, leads to approximately 70% higher cutting forces. The observed 40% reduction in the grinding forces is achieved by increasing the cutting speed from 6 to 14 m/s, but this increases the surface roughness. Higher values of the dressing overlap ratio reduce the chip cold-welding on the abrasive grains and causes less loading of the tool in form of chip nests. Welded clogging of the grinding pin at lower Ud values deteriorates the surface quality resulting in increased surface roughness. Using the up-dressing method leads to lower chip loading over the surface of the grinding tool, which improves the ground surface. Moreover, the down-dressing of micro-grinding pins results in higher value of surface roughness and lower grinding forces compared with up-dressing.