Analytical and finite element modeling of a machining system to minimize inaccuracy in milling and using rapid prototyping for die manufacturing

摘要

The end milling process is used extensively in a gamut of manufacturing areas. It accounts for up to 40% of the cost of fabrication of non-electrical parts for a high performance aircraft. This economically justifies the effort to find ways to reduce inaccuracy caused in milling by workpiece deformation, fixture deflection and cutter deflection to improve the quality of parts. The process is also used extensively for roughing and finishing of dies. However, the conventional die manufacturing process, which uses the milling process, is too time consuming because of the extensive CNC programming involved. Furthermore skilled labor required for CNC programming accounts for the high cost of die manufacturing. Therefore new processes need to be developed that will eliminate CNC programming and possibly reduce the usage of the milling process thereby reducing the cost and time required to produce parts;An analytical non-linear optimization model has been developed which can determine the maximum inaccuracy due to workpiece deformation and the optimal clamping forces that are required to minimize work piece deformation while ensuring that the workpiece will not slip during machining. However, this model assumes rigid fixturing elements and is only suitable for simple workpiece shapes;A finite element model and a simple novel algorithm has been developed which has the same objective as the analytical non-linear optimization model. This model can be used for any complex shaped workpiece or fixture. The model also takes into account the flexibility of fixtures;Inaccuracy in machining is also caused by deflection of the tool. A study of the deflection of a milling cutter due to the action of the cutting forces was performed. An analytical equation was developed to determine the deflection of an end mill under a cutting force. The equation was verified by modeling the complete geometry of a four flute milling cutter using the finite element analysis module of I-DEAS software. The deflections obtained by the finite element model were exactly the same as those obtained by using the analytical equation. Previous researchers modeled the milling cutter as a simple cylinder which resulted in some error in the result;Three die manufacturing processes are proposed, namely, the casting prototype process, the EDM milling process and the copy milling process. All three processes use rapid prototyping to eliminate costly and time consuming CNC programming. All the three processes are economical compared to conventional processes provided there are large number of surfaces on the part. If the part has very few surfaces the conventional process will require less time for CNC programming making it more efficient. The Casting prototype process does not use milling whereas the EDM milling uses milling for rough machining. These two process would minimize inaccuracy in parts by eliminating milling or using milling to remove the rough stock only. The copy milling process uses milling but the models developed here can be used to minimize error in this process. All three processes have the additional advantage that they are more time efficient and economical than the conventional process of making dies.