Figure 4 Milling cutter diameter and required speed diagram: The smaller the tool diameter, the higher the speed required for optimal cutting (Source: Precise)
The use of synchronous motors has become a growing trend in the drive of high-speed spindle components. An important advantage of such a motor is that it has a large torque or power density and the resulting temperature rise is also low, which is advantageous for avoiding the thermal elongation of the spindle.
The combined application of air bearing and synchronous motor on the electric spindle is undoubtedly the best spindle system for micro-machining due to its smooth operation, long service life and good thermal characteristics.
Tool holder and holder interface
In order to adapt to high-speed machining, the interface between the toolholder and the spindle is mainly composed of the structurally symmetrical HSK-E32 (maximum speed of 60,000r/min) and HSK-E25 (maximum speed of 80,000r/min). If the speed exceeds 80,000r/min, or if higher precision is required (reducing the number of interfaces), the method of directly clamping the tool holder in the spindle of the machine tool should be used, but the disadvantage of this method is that all tools are required. Must have the same shank diameter.
The circular runout of the tool holder has an important influence on the surface quality and surface contour accuracy of the micro-milling. Therefore, for the micro-milling, the hot-shrinking tool holder with a small circular runout error (≤0.003mm) and a symmetrical structure is used. . Here, a finishing stamping die with 90MnCrV8 material is used to illustrate: the cutting parameters used in the finishing of the double-edged milling cutter are the speed n=30000r/min, the side cutting knife amount ae=0.05mm, the backing knife amount ap=0.05mm And feed rate Vf = 200mm / min. This gives a feed per tooth of fz = 0.003 mm. If the tool holder has a round trip error of 3 祄, the double-edged cutter quickly becomes a tool with only one effective cutting edge. Therefore, based on the accuracy and surface quality considerations, it is recommended to use a hot-packed cold-shrinking high-precision tool holder.
Three-axis and five-axis micromachining machine tools
In order to achieve micro-machining, not only small-sized tools and corresponding tool holders, but also milling machines or machining centers for micro-machining designed for micro-machining are required. At present, Primacon's Mikro-BAZ-PFM24 NGd five-axis (or three-axis) machining center has a spindle speed of up to 140,000 r/min. The control system uses the HEIDENHAIN iTNC50. Kugler offers the Micromaster∕M five-axis micromachining machine, which takes many structural measures to ensure high machining accuracy: its body is made of fine-grained monolithic granite, thus ensuring long-term stability of the machine. The XYZ-axis linear guides are hydrostatically supported, and the XZ-axis gantry frame is also made of precision-machined granite to ensure thermal stability and high rigidity. The linear positioning is used to improve the positioning accuracy of the shaft, and the air-supported electric spindle is used to increase the rotational speed. The maximum speed is up to 160,000 r/min to ensure the optimal process of micro-milling. Wissner's GAMMA202 EconoMic milling machine is a three-axis milling machine designed for micromachining.
Figure 5 Micromachining machine: Kugler's misaligned micromachining machine
The main advantage of the five-axis machining is that a shorter tapered shank and a small diameter milling cutter can be used to machine deeper cavities and ribs, and a good surface quality is obtained, thereby eliminating manual polishing. The machining accuracy of the three-axis machining is higher than that of the five-axis machining. In the machining time, the three-axis machining process can be shortened by 15% to 50% compared with the five-axis machining process.
At present, the control system of the micro-machining machine can generally be connected with the CAD/CAM system of different manufacturers. Through this connection, the complex contour of the mold can be processed well, and the adjustment time and programming time can be shortened.
Conclusion
In recent years, with the development of miniaturization of technical product components, micro-machining technology has increasingly become a key technology for micro-component processing. Micro-milling plays an important role in the manufacture of micro-components and molds. The further development of tools, tool holders and high-speed electric spindles for micro-milling, as well as micro-machining machines, is the key to the development of micro-machining technology. In micro-mold manufacturing, micro-milling can not only improve economic efficiency, but also further exploit the process potential of milling.
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