The main goal of semi finish machining is to make the workpiece contour flat and the surface finish machining allowance uniform, which is particularly important for tool steel mold, because it will affect the change of tool cutting layer area and tool load during finish machining, thus affecting the stability of the cutting process and the surface quality of finish machining. Rough machining is based on volume model, and finish machining is based on surface model.
However, the geometric description of the parts in the previous CAD / CAM system is discontinuous. Because there is no intermediate information of the machining model after rough machining and before finishing machining, the distribution of the residual machining allowance and the maximum residual machining allowance of the rough machined surface are unknown. Therefore, the semi finish machining strategy should be optimized to ensure that the workpiece surface has uniform residual machining allowance after semi finish machining.
The optimization process includes: the calculation of the contour after rough machining, the calculation of the maximum residual machining allowance, the determination of the maximum allowable machining allowance, the partition of the surface with the residual machining allowance greater than the maximum allowable machining allowance (such as the areas where the transition radius is less than the rough machining tool radius such as grooves and corners), and the calculation of the tool center path during semi finish machining. Most of the existing CAD / CAM software for high-speed die machining have the function of residual machining allowance analysis, and can adopt reasonable semi finishing strategy according to the size and distribution of residual machining allowance. For example, hyper mill and hyper form software of open mind company provide pencil milling and rest milling methods to remove corners with large residual machining allowance after rough machining, so as to ensure uniform machining allowance in subsequent processes. Local milling of Pro / Engineer Software Milling) has similar functions. For example, the value of residual machining allowance in local milling process is equal to that in rough machining. In this process, only a small diameter milling cutter is used to remove the corners not cut in rough machining, and then semi finish machining is carried out. If the residual machining allowance in local milling process is taken as the residual machining allowance in semi finish machining, the process can not only remove the corners not cut in rough machining Corner, also can complete semi finishing.
The high-speed finishing strategy of finishing die depends on the contact point between the tool and the workpiece, and the contact point between the tool and the workpiece changes with the change of the surface slope and the effective radius of the tool. For the machining of complex curved surface composed of multiple curved surfaces, continuous machining should be carried out in one process as far as possible, instead of machining each curved surface separately, so as to reduce the times of tool lifting and cutting. However, due to the change of surface slope in machining, if only the step over is defined, the actual step distance on the surface with different slope may be uneven, thus affecting the machining quality.
Pro / engineer can solve the above problems by defining the scallop machine at the same time of defining the side feed; hyper mill provides equidistance machining Machine) mode, which can ensure the uniform side feed between the tool paths, and is not limited by the surface slope and curvature, to ensure that the tool always bears uniform load in the cutting process. In general, the curvature radius of finishing surface should be greater than 1.5 times of the tool radius to avoid the sudden change of feed direction. In the high-speed finish machining of the die, when cutting in and out the workpiece each time, the change of the feed direction should adopt the arc or curve transfer as far as possible, and avoid using the straight line transfer, so as to maintain the stability of the cutting process.
Feed speed optimization at present, many CAM software have the function of feed speed optimization adjustment: in the semi finishing process, when the cutting layer area is large, the feed speed is reduced, while when the cutting layer area is small, the feed speed is increased. The application of feed rate optimization can make the cutting process stable and improve the surface quality. The area of cutting layer is automatically calculated by CAM software, and the feed rate can be adjusted by the user according to the processing requirements.
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