Selection of cutting strategy and parameters in multi-axis machining operations for improved productivity
Tunç, Lütfi Taner (2010) Selection of cutting strategy and parameters in multi-axis machining operations for improved productivity. [Thesis]
Official URL: http://192.168.1.20/record=b1302886 (Table of Contents)
Importance and application of multi-axis machining operations has been continuing to increase in several industries such as aerospace, automotive, die and mold, where parts with complex surfaces. In such industries achieving tight tolerances is required with minimum number of setups, as well. Two and half, three and five axis milling can be considered in such a class together with parallel machining operations. The cutting tool has simultaneous translation in two axes, and the third axis is used to change the axial level. In three axis milling the cutting tool is able to have simultaneous translation in –x, –y and –z axis of Cartesian coordinates. However, in five axis milling, tool orientation changes with respect to the machined surface due to the rotary axes in addition to the linear motions in x, y, and z directions. Parallel machining, which can also be considered as a multi-axis process, consists of simultaneous machining operations on a given part. For instance, while the outer diameter of a part is machined, a drilling operation can be conducted in parallel. Increasing the productivity and part quality in such operations is important due to the high cost of machine tools, equipment and raw material involved. For this purpose, the right machining strategy and appropriate set of process parameters should be selected for a given part. Moreover, proper scheduling of parallel machining operations is also of great importance for decreased machining time on a part. This objective can be achieved using of process simulations based on process modeling. It is practically known that as the cutting speed is decreased, process stability increases with the effect of process damping. Considering this fact, high productivity conditions can be achieved by selecting low cutting speeds and high cutting depths, besides the high speed cutting conditions. Although there are several models for estimation of process stability at high cutting speeds, there has not been a practical method for estimation of stability limit considering the process damping effect for low cutting speed conditions. Therefore, current models fail to accurately estimate the stability limits, especially at low cutting speeds. In this thesis, a new and practical method is proposed for modeling of process stability at low cutting speeds. The model predictions are also verified through experiments. Thus, it is one of the major contributions of this thesis to the machining research. In this thesis, selection of cutting strategies and parameters is studied for multi axis milling and turning operations through process modeling. Process stability, spindle torque and power, quality requirements on the workpiece, form errors and tool life are considered as the constraints. The proposed model for estimation of process damping and stability limits at low cutting speed is used together with the previously developed process models. By doing so, different cutting strategies are compared for industrial parts. Considering that there is not much study dealing with such a problem, it can be said that this thesis contributes to literature in this respect. Parallel machining operations, where more than one machining units are allowed to work on the part provide several advantages, while bringing additional challenges. By the help of parallel machining physical space can be saved, tolerance integrity can be achieved easier, total machining time can be decreased and flexibility in process scheduling can be achieved. The literature on scheduling of parallel machining operations is relatively scarce. Moreover, current studies do not consider the dynamic and mechanic interaction between the processes conducted in parallel. In this thesis, such interactions are also considered in scheduling of parallel machining operations. The applicability of the method and models proposed in the thesis is shown on industrial workpiece geometries and the improvements are also presented together with the results.
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