1. Status of High-Speed Milling Machines in the Mold Factory
Our factory is equipped with a Rambudi five-axis high-speed milling machine, which has a maximum tool clamping diameter of 20mm, a spindle power of 12kW, a maximum feed rate of 10m/min, and a spindle speed range from 1000 to 15000r/min. It uses the FIDIA control system, which allows the machine to automatically decelerate when there are sharp changes in curvature, preventing damage caused by inertia at high speeds. This makes it a typical high-speed machining center.
Although the machine can reach up to 15,000rpm, it is currently operating at a quasi-high speed. For smooth cast iron parts, the spindle speed ranges between 7000 to 8000r/min, with a feed rate of about 6000mm/min and a feed per tooth of 0.3–0.4mm/tooth. We use Walter SΦ20 ball end mills (model P3202-D20) with a material of WXK15. According to the cutting parameters provided by Walter, the tool has already reached its upper limit of cutting speed at 8000r/min.
2. Tool Cutting Analysis
The cutting speed of the tool depends on the tool material, the workpiece material, and factors like the rake and clearance angles. Once the tool is selected, the cutting speed is determined. However, when machining mold surfaces with ball end mills, the linear speed of the tool varies due to changes in the tool's position. At the tip of the tool, the speed is zero, while the speed increases toward the tool's radius. Since mold surfaces are often complex, the cutting point constantly changes, leading to significant variations in cutting speed. This greatly limits the effectiveness of high-speed machining for automotive molds.
Some companies have started using high-speed machining parameters with step speeds of up to 30,000rpm. However, for automotive molds, high-speed machining is mostly suitable for flat external panels. The higher the rotational speed, the greater the variation in tool linear speed, but the allowable cutting speed range for the tool is much smaller. To address this, slicing is used to isolate flat areas for separate processing. By using an appropriate tilt angle, the issue of zero tool centerline speed can be avoided. For example, the outer panel of a hood drawing die can be processed at high speed, while other areas, such as the inner panel or small features, are not suitable. If the flat area is too small, excessive slicing leads to short tool paths and frequent turns, which are not ideal for high-speed machining. Even if some parts cannot be separated, like the front wall with limited flat surfaces, only less than 20% of the surface can be processed at high speed. Improving this by 30% could increase overall efficiency by more than 5%, and in some cases, such as door panels, even over 20%. However, if a part must be divided into small slices, it may not improve efficiency overall.
3. Considerations in High-Speed Machining Programming
3.1 Digital Surface Quality Inspection
The quality of the digital model plays a crucial role in high-speed machining, directly affecting the smoothness of the tool path and thus the efficiency and quality of the process. Based on our experience, it's essential to analyze the surface quality of the die before machining.
(1) Main Content of the Analysis:
Evaluate the smoothness of the product surface, check for cracks or defects, identify any negative angles in the drawing direction, and ensure that process additions are rounded, all to guarantee the quality of physical processing.
(2) Analysis Methods:
1) Use section curvature comb and Gaussian radius analysis to detect surface quality issues in the digital mold.
2) Check for distortions, overlaps, or cracks in the digital model through view operations and deviation analysis.
3) Analyze the precision of the surface model, assess tolerances during production, and predict their impact on surface processing quality in advance.
4) Enter the UG machining environment, verify the digital model, and handle abnormal surfaces (such as dotted areas on the machined surface) to prevent issues like tie-breakers. Careful analysis is especially important for small transitional and filleted surfaces, as errors like overcutting can lead to serious accidents during high-speed machining.
3.2 Principles of Slicing and Selection
(1) When machining the mold surface, block programming is used to separate flat and steep areas, improving processing technology and efficiency. As shown in Figure 1, program 1 in the flat area can be considered for high-speed machining.
(2) Cutting Method
For the outer cover drawing die, we pay special attention to the direction aligned with the car body, as shown in Figure 2.
(3) Feed Retraction Mode
We use circular and plunge cutting methods during feeding and retracting. To enhance surface finish and accuracy, no tool marks are left. In the program design, a circular arc segment is added to the cut-in and cut-out positions, ensuring that the arc corresponds to the initial cutting element. This allows the tool to enter the workpiece in a circular motion, ensuring the surface finish, as shown in Figure 3.
(4) Selection of Up and Down Movements
When using fixed cutting mode in profile machining, climb milling is applied.
4. Five-Axis Linkage Function
Our high-speed milling machine has a five-axis linkage function, but I believe it is not very suitable for automotive molds. For large and regular parts, five-axis linkage performs well, and without it, parts like impellers would be difficult to machine. However, automotive mold surfaces are more complex, making it hard to always follow the normal direction of the part. If the machine tool swings too much, it may interfere with the mold surface. Additionally, the tool cannot always cut from the same area, reducing the benefit of five-axis linkage. Another major issue is that when three-axis linkage is used, the A and C axes are locked by the hydraulic system. With five-axis linkage, the A and C axes are controlled by the machine’s drive system, which significantly reduces rigidity and negatively affects five-axis accuracy. Moreover, five-axis interpolation speed is slower than three-axis.
Recently, our factory has begun processing inspection tools, which allows us to fully utilize the five-axis linkage. The materials used for the inspection tools are mainly resin, which is soft and avoids the problem of poor rigidity in five-axis linkage. For inspection tools with variable angle contours and many inclined holes, the use of five-axis linkage programs greatly improves machining efficiency. Without five-axis linkage, the changing tool shaft profile would need to be machined using ball end mills in profiling mode. With five-axis linkage, side cutters can be used, resulting in a significant difference in machining efficiency.
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