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 diameter of 20mm, a spindle power of 12kW, a maximum feed rate of 10m/min, and a spindle speed range of 1000 to 15000rpm. It uses the FIDIA system, which ensures smooth deceleration 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 level. For smooth cast iron parts, the rotational speed is set between 7000-8000rpm, with a feed rate of approximately 6000mm/min and a feed per tooth of 0.3–0.4mm. We use Walter SΦ20 ball end mills (model P3202-D20), with a tool material of WXK15. According to Walter's cutting parameters, the tool is already reaching its upper limit of cutting speed at 8000rpm.
2. Tool Cutting Analysis
The cutting speed of a tool depends on the tool material, the workpiece material, and geometric parameters such as 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 varies depending on the position of the tool tip. At the center of the tool, the speed is zero, while the outer edge moves faster. Since mold surfaces are often complex, the cutting point of the tool changes constantly, causing significant variations in cutting speed. This greatly limits the effectiveness of high-speed machining for automotive molds.
Currently, some companies use high-speed machining parameters with step speeds up to 30,000rpm. However, in the case of automotive molds, high-speed machining is mainly suitable for flat external panels because higher rotational speeds lead to larger variations in linear speed. The allowable cutting speed range of the tool is much smaller than this variation. To address this issue, slicing is used to isolate flat areas and process them separately. By using an appropriate tilt angle, the problem of zero tool centerline speed can be avoided. For example, the outer panel of a hood drawing die has a flat surface that can be machined at high speed, while the rest of the part is not suitable for high-speed processing. Inner panels typically have very few smooth areas, except for the pressing region. The flat surface must be large enough; otherwise, too many slices will result in short tool paths and frequent turns, which are not ideal for high-speed machining. Even if some outer plates cannot be separated, like the front wall with limited flat areas, the portion that can be processed at high speed may be less than 20%. Improving efficiency by 30% in this area could increase overall efficiency by more than 5%, and in parts like the hood door, it could even reach over 20%. However, if a part must be divided into small sections for high-speed machining, it might not necessarily improve overall efficiency.
3. Considerations for High-Speed Machining Programming
3.1 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, which in turn influences both efficiency and accuracy. Based on our past experience, we always perform a detailed surface quality analysis before machining the mold surface.
(1) Main content of the analysis:
Checking the smoothness of the product surface, identifying cracks or defects, ensuring there are no negative angles in the drawing direction, and verifying whether the process supplementary parts are rounded, to ensure the quality of the final product.
(2) Analysis methods:
1) Using section curvature comb and Gaussian radius analysis to detect surface quality issues in the digital model.
2) Checking for distortions, overlaps, and cracks through view operations and deviation analysis.
3) Analyzing the precision of the surface model and predicting how production tolerances might affect surface processing quality.
4) Entering the UG processing environment to verify the digital model and handle any abnormal surfaces (e.g., dotted areas) in advance to prevent tool collisions. Careful analysis is especially important for small transitional and filleted surfaces, as errors during high-speed machining could lead to serious issues like overcutting.
3.2 Slicing and Selection Principles
(1) When machining mold surfaces, block programming is used to separate flat and steep areas, improving processing efficiency. As shown in Figure 1, program 1 in the flat area is suitable for high-speed machining.
(2) Cutting method
For the outer cover drawing die, we pay special attention to the cutting direction, following the body of the car as shown in Figure 2.
(3) Feed Retraction Mode
In the feeding and retraction process, circular cutting is adopted to improve surface finish and accuracy without leaving marks. In the program design, a circular arc segment is added to the cut-in and cut-out positions, so that the arc corresponds to the initial cutting element. This allows the tool to enter the workpiece in a circular motion, ensuring a smooth surface finish, as shown in Figure 3.
(4) Selection of Up and Down Movements
During profile machining, fixed-direction cutting is used, and climb milling is applied.
4. Five-Axis Linkage Function
Our high-speed milling machine is equipped with 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 it would be difficult to machine certain components like impellers without it. However, automotive mold surfaces are highly complex, and it’s impossible to always follow the part’s normal direction. If the machine swings too much, it may interfere with the mold surface. Also, the tool can’t always cut from the same area, which reduces the benefits of five-axis linkage. Another critical issue is that with three-axis linkage, the A and C axes are locked via the hydraulic system. With five-axis linkage, these axes are controlled by the machine’s drive system, which significantly reduces rigidity and negatively affects five-axis accuracy. Additionally, the interpolation speed of five-axis machining is slower than that of three-axis.
Recently, our factory started processing inspection tools, which provides a good application of five-axis linkage. These inspection tools are mostly made of soft resin materials, avoiding the issue of poor rigidity associated with five-axis linkage. In inspection tools with variable angle contours and multiple inclined holes, five-axis linkage greatly improves machining efficiency. Without five-axis linkage, the changing tool shaft profile would need to be machined using a ball end mill in profiling mode. However, with five-axis linkage, side cutters can be used, resulting in a significant improvement in machining efficiency.
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