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However, one of the challenges in this area is that the tool path during milling increases the material removal rate. In a typical high-speed roughing path with a depth of cut and a span of 10% of the tool diameter, the material removal rate will reach 10 times the assumed value when the tool begins to enter the channel, and will reach this level when the tool enters the inner corner. 5 times. This peak load is the number one cause of tool failure. The general treatment of the factory is to change the feed rate, depth of cut or span. Although reducing any of these cutting parameter values ​​may cause the peak load condition to return to a level below the threshold, this will also reduce the overall tool path metal removal rate, thereby reducing productivity. Therefore, a better way to solve this problem can be solved.
Tool path adjustment
The purpose of some tool path optimization methods is to achieve a consistent material removal rate by differentiating the tool path and frequently adjusting the feed rate. This strategy can produce a constant level of material removal at a macro level. However, it raises complex questions about machine tools. The machine controller's built-in high-speed machining processor completes the geometrically smooth tool path. In the case of a high feed rate, the controller needs to dynamically smooth the tool path. Adjusting the feedrate at a small length interval causes the controller to interpret some toolpath data for precise positioning that would otherwise qualify for smooth interpolation. If this happens, the machine will slow down and make the cycle time longer. When the spacing is very small, precise adjustments can also cause jumper machine actions that detract from surface roughness.
Another issue is related to spindle speed. Adjusting the feed rate without adjusting the corresponding spindle speed causes a change in the chip thickness, which has a decisive influence on the surface roughness of the long-term machining and the effectiveness of the tool.
An alternative approach to some toolpath processors can be described as a preventive approach. These processors can plan the geometry of the toolpath to avoid excessive loads. For example, whenever the tool finishes slotting or enters a smaller corner, the CAM software can apply a swing function that automatically initiates an additional swing tool path loop. As in the NX CAM of UGS, the user not only specifies parameters such as depth of cut and span, but also the allowable percentage of overload. Then, the metal removal rate is controlled within the threshold. The software controls the load by retracting and re-engaging the tool by pressing one of the tool paths shown on these two pages. Although this geometry introduces additional air cuts, it optimizes the tool load.
Programming small tools
Another CAM programming problem that causes intermittent tool loading is the irregularity of the margin left for finishing. Finishing operations typically use smaller diameter and longer overhanging tools. In order to ensure safe cutting and good surface roughness, these tools must consistently bite into the part material and cut off the uniform amount of material.
A typical Z-direction semi-precision operation leaves a non-uniform margin in the shallow zone that causes an irregular load on subsequent tools. The more complex Z-direction capability automatically adds toolpaths to these shallow areas, helping to ensure a more uniform margin.
Another feature is that the flat level is automatically identified during the roughing process, leaving a residual margin on these faces. This also avoids excessive load on subsequent tools.
Tool bite
For efficient hard milling, the tool and workpiece must be strictly controlled. The chip thickness determined by the spindle speed and feed rate is part of this factor. But horizontal and vertical occlusion angles (which are often overlooked) also play an important role. The horizontal occlusion angle represents the amount of material removed as each cutting edge snaps and leaves the workpiece. The vertical occlusion angle indicates the maximum instantaneous cutting edge and the amount of engagement of the workpiece.
These factors combine to determine instantaneous cutting forces and thermal spread. For high speed, efficient hard milling, they need to be as consistent as possible.
Conclusion: A constant material removal rate can be used as an integral part of the tool path generation. By providing high-speed machine tools with tool paths designed to maintain a constant material removal rate, mold shops can benefit from hard milling.
Constant material removal rate: the key to hard milling
Tool breakage is an important concern for mold makers who are reluctant to use EDM (Electrical Discharge Machining) and directly mill molds with hard materials. Unexpected tool breakage due to exceeding the tool's allowable load conditions not only wastes money, but also destroys the entire process. Therefore, the mold shop can get the most out of it by always keeping the tool at the optimum load level.