In recent years, high-speed cutting technology has been more and more widely used in developed countries such as Europe and the United States. Taking mold processing as an example, a large number of high-speed cutting machine tools are gradually replacing electrical processing equipment to perform efficient precision processing on mold cavities. At present, domestic mold manufacturing and processing are mainly based on ordinary machining and EDM. The process is cumbersome, inefficient, and has a long cycle. It seems increasingly incapable under the trend of increasingly accelerated product updates in the current market. High-speed cutting technology has a bright application prospect in shortening mold manufacturing cycles and reducing costs due to its high speed, high quality, and ability to directly process hardened steel.
High-speed cutting technology can be traced back to the high-speed cutting theory proposed by Dr. Carl Salomon of Germany in the 1930s. Compared with traditional cutting, high-speed cutting has higher cutting speed and processing efficiency; and the surface quality after processing is high, and hardened materials with a hardness of 50-60HRC can be directly processed to achieve “cutting instead of grinding”.
Compared with EDM in traditional mold processing, high-speed cutting saves the process of electrode design and processing, and the processing accuracy is significantly improved. The polishing and grinding work of fitters is greatly reduced or even eliminated, and the processing efficiency is greatly improved. Statistics show that for molds of average complexity, high-speed cutting can reduce the processing cycle by at least 40% or even more. Even for some mold cavity surfaces with particularly complex shapes (such as deep grooves and narrow slits), EDM is still required, and high-speed milling can also help obtain higher quality EDM graphite electrodes.
Study on the Surface Roughness of Dies in High-Speed Cutting
Surface roughness is a very important indicator of mold surface quality. The effect of high-speed cutting on surface roughness can be completed through experiments. Experimental conditions: cutting material is mold steel 3Cr2Mo, tool material is SG4 ceramic, tool diameter is 100mm, main deflection angle is 75°, axial rake angle and radial rake angle are both 0°, single blade. The experiment observes the effect on surface roughness by changing cutting speed, feed speed, axial and radial cutting depth.
It can be seen from the experimental results that: with the increase of cutting speed, the roughness tends to decrease. When the speed reaches 1000mm/min, the surface roughness reaches the minimum value, and the grinding effect is fully achieved. In the process of high-speed cutting, the contact and extrusion time between the tool and the workpiece is shortened due to the increase in cutting speed, and the plastic deformation of the workpiece is reduced.
High cutting speed is also not conducive to the formation of built-up edge, so better surface quality can be obtained. On the other hand, the high speed of the spindle also makes the excitation frequency of the machine tool during cutting very high, which is much larger than the natural frequency of the process system, reducing the possibility of resonance, which is conducive to improving processing accuracy and surface quality. In the experiment, when the cutting speed exceeds 1000mm/min, Ra shows an upward trend again, which is mainly caused by tool grinding.
Relative to the cutting speed, the increase of feed speed, axial cutting depth and radial cutting depth in high-speed cutting will make the surface roughness tend to increase. Therefore, it can be concluded from the experiment that when selecting cutting parameters for actual high-speed cutting, a higher cutting speed should be selected, and a smaller feed speed and cutting depth are more conducive to improving surface roughness.
High-speed Cutting Technology for Molds
When determining the mold processing technology, the cutting method should consider the requirements of high-speed cutting. Try to use down milling. In down milling, the chip thickness generated when the tool just cuts into the workpiece is the largest, and then gradually decreases. In reverse milling, it is just the opposite, so the friction between the tool and the workpiece is greater in reverse milling, and the heat generated on the blade is more than in down milling. The radial force is also greatly increased, which reduces the life of the tool.
When processing the mold, avoid the direct vertical downward feed method. The use of oblique feed or spiral feed is more suitable for the needs of high-speed processing of the mold cavity. The oblique feed method is to gradually increase the axial cutting depth to the set axial cutting depth value. The milling force is gradually increased, and the impact on the tool and the spindle is small, which can significantly reduce the phenomenon of blade chipping. The spiral feed starts from the top of the workpiece and spirals down into the workpiece. Due to the continuous processing method adopted, it is relatively easy to ensure the processing accuracy, and there is no sudden change in speed, so it can be processed at a higher speed.
The tool path setting in high-speed cutting puts forward higher requirements for the tool path setting. In high-speed cutting, since the cutting speed and feed speed are very fast, if the tool path is unreasonable, it is very easy to cause a sudden change in the cutting load during the cutting process, which will bring impact to the processing, damage the processing quality, damage the tool and even the equipment. This damage is much more serious than in ordinary cutting. Therefore, in high-speed cutting, the corresponding tool path should be selected according to different processing objects and shapes, and high speed and high efficiency should not be pursued blindly.
In the processing of mold cavities, most of the tool movement trajectories are not simple straight lines but curved movements. At this time, the inertia effect brought by high-speed movement should be paid special attention. When the cutting direction changes, the change is gradual rather than sudden. For example, when cutting the corners of the mold cavity, try to use arc transition to make the steering smooth. At the same time, if you can appropriately reduce the feed speed, the effect will be better. Such a setting can reduce the impact on the system and avoid damage to the tool or workpiece caused by overcutting.
In the traditional processing method of cavity corners, straight line cutting is generally used. When approaching the corner, the movement speed slows down and the feed reversal is completed at the same time. During this period, the tool movement is discontinuous, and a lot of friction and heat will be generated in the intermittent process; after setting the corner to arc transition, the arc interpolation movement of the CNC machine tool is a continuous process, and there will be no intermittent movement of the tool, thereby reducing the contact length and time between the tool and the workpiece, and avoiding the surface quality of the mold due to overheating.
In high-speed cutting, the tool trajectory must be kept stable and sudden speed changes must be avoided. Because sudden acceleration or deceleration will cause instantaneous changes in cutting thickness, which will lead to changes in cutting force, making the processing unstable, thereby reducing the quality of workpiece processing. Many modern CAM software provide the function of optimizing cutting speed, so it is necessary to select appropriate cutting speed and acceleration and deceleration strategies according to needs to reduce the impact of speed changes on processing.
The use of high-speed cutting in mold cavity processing can greatly improve the processing efficiency of mold manufacturing, and has a good promotion prospect for the domestic mold industry. In the actual application of high-speed cutting, unlike traditional processing, it is necessary to select reasonable cutting parameters according to the specific requirements of the mold and the characteristics of high-speed cutting, and combine with appropriate processing technology to give full play to the advantages of high-speed cutting.
