The CNC machining process of magnesium alloy must be operated continuously, and the parts covered with cutting fluid cannot be placed on the workbench for a long time.
(1) Process analysis
The object to be processed is the case of a notebook computer, and the material is magnesium alloy ME20. The part has a complex structure and high dimensional accuracy requirements, so it is formed by milling the magnesium alloy sheet as a whole. Magnesium alloy machining is very different from traditional aluminum alloy machining in terms of tool selection, cutting parameter selection, cutting plan selection, cutting fluid selection and anti-corrosion measures, and chip treatment.
(2) Tool selection
Magnesium alloy has good thermal conductivity, soft material, and low cutting force, so the heat dissipation rate during machining is very fast, and the amount of sticking is small, so the tool life can be very long. However, cutting tools used for magnesium alloy machining are required to keep the cutting edge sharp, because tools with larger cutting edges will increase the friction during the cutting process, resulting in a substantial increase in cutting temperature, causing flashing or even burning of magnesium chips, resulting in Unsafe factors in the cutting process increase. Therefore, magnesium alloy machining generally requires the selection of new carbide tools, and old tools that have been processed with other materials cannot be mixed.
The general tool design principles for machining steel and aluminum are also suitable for machining magnesium alloy tools. Due to the low cutting resistance of magnesium alloys and the relatively low heat capacity, milling cutters used for magnesium alloy machining have more teeth than other metals. Reducing the number of cutter teeth can increase the chip space and the amount of feed, which can reduce friction heating and increase chip clearance, reduce the distortion of chips, and reduce power consumption and heat generation. When the author's company processes magnesium alloys, three-edged carbide end mills are generally preferred.
In special circumstances, such as insufficient blade length of the three-blade tool, unsuitable diameter specifications, etc., four-blade carbide end mills can also be used.
(3) Cutting fluid selection
Magnesium alloy material is soft and easy to cut. It can achieve a very smooth and smooth surface regardless of whether it is used at high speed or low speed, with or without cutting fluid. Dry machining without cutting fluid can reduce machining costs, and waste chips can be easily collected, stored and transported. Therefore, in many references, dry machining is recommended.
However, there is a risk of fire when dry machining uses high speed and forms fine chips. This requires the CNC operator to observe the machining conditions at any time, in case of fire, it can be extinguished immediately, but this method still has immeasurable risks. This limits the operator's inability to achieve the one-man-multi-machine working mode, which is not cost-effective in terms of overall machining cost and efficiency.
In addition, magnesium alloys tend to expand when heated. According to the data, the linear expansion coefficient of magnesium alloy in the temperature range of 20～200℃ is 26.6～27.4μm/(m·℃) (related to the alloy composition). Taking the length dimension of 200mm as an example, if the temperature rises by 10°C during the machining, the resulting machining error will be between 0.053 2 and 0.054 8mm. It can be seen that if dry cutting is used without cooling fluid cooling, magnesium alloy parts will expand due to the rapid increase in temperature, which will affect the machining accuracy. The notebook casing has high requirements for dimensional accuracy, and such temperature effects cannot be ignored.
(4) Selection of cutting parameters
The cutting parameters of CNC milling include spindle speed, feed rate, tool depth of cut and tool width of cut.
We chose a domestic machine tool for magnesium alloy machining. The theoretical high speed of the machine tool can reach 8000r/min, the maximum feed rate is 15m/min, and the machining accuracy is 0.01mm. Using this machine tool to maintain the highest speed for a long time is detrimental to the machine tool. Too fast feed speed, for single-piece small batch production, does not save too much time, but greatly increases the quality risk and the probability of equipment failure. Therefore, we use large cutting depth and small feed to determine our cutting parameters.
According to our company's many years of CNC machining experience, when the carbide end mills are machining different materials, the speed and feed in the cutting parameters change, but the depth and width of cut generally do not change much: for rough machining, the recommended width of cut is 50%～100%D (D is the tool diameter), the recommended cutting depth is 0.3～0.5D. For finishing, the recommended cutting width is 0.1～0.5mm and the cutting depth is 0.5～1D.
In the preparation of machining strategies, the machining of magnesium alloy materials is not much different from common aluminum alloy materials, but when the finishing allowance is selected, a allowance of not less than 0.2mm is selected to avoid the generation of too small chips, thereby avoiding machining flash. Burn.
Magnesium alloy is soft and easy to cut, but too high speed and feed will easily cause greater cutting heat, which will cause flashover. Therefore, in consideration of efficiency and safety factors, the company conservatively adopts 1.5 to 2 times the speed and feed of aluminum alloy cutting. Take the notebook computer shell processed this time as an example, some machining parameters are shown in the attached table.
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