The development of high-speed machining centers lays the foundation for the improvement of the milling efficiency and production quality of graphite electrodes, especially the milling of small electrodes with complex shapes and thin-walled properties.
Graphite electrodes used in EDM tooling require high-speed machining centers (HSMs) to mill them, so the demand for HSM machine tools is also increasing. However, for some time due to technological developments in hard steel processing, it seems likely that graphite electrodes will become an outdated technology. However, there are many factors that determine the need to continue to produce graphite electrodes, so high-speed machining has become a key production process in the mold manufacturing industry.
Compared to the time required for graphite electrode milling, the processing of high-rigidity hard steels still takes longer. When milling pockets, cavities, and complex 3D profiles on hard steel, runout errors and tool misalignment often result in tool breakage. When the tool breaks, it will not only stop the continuous processing of steel, but also affect the quality of the product, resulting in scrap parts and material waste.
For the foreseeable future, graphite electrodes will occupy a very high position. Mold makers who use the most efficient equipment to process graphite electrodes will continue to expand their efforts with those who are determined to use hard steel processing. The competition gap.
Miniature tool foundation
For graphite electrodes, one of the most magical words is "high speed." The spindle speed of the high-speed machining center can reach 30,000 to 60000 r/min, and the feed speed can be increased to shorten the processing cycle and improve the quality of the surface and edges. The motors required to drive such spindles are small and light, which helps reduce cutting forces and reduce tool breakage. This is critical because many electrodes are complex and their production involves small, easily broken miniature tools (see Figure 1).
Fig. 1 The thin wall on the graphite electrode, such as the circular part of the above sample, requires the use of micro-tools and efficient high-speed machining
1. Spindle
The smaller the tool, the higher the required spindle speed. Only in this way can the machining quality of the part be improved and the tool can be prevented from breaking. For high-frequency spindles up to 60,000 r/min, micro-tools are ideal for milling, drilling, and engraving graphite electrodes. High-speed machining technology uses extremely high speeds, small spans, and greatly increased feed rates.
When you pass your hand over the flame of a burning candle, if you move slowly, the flame will have enough time to burn your hand. But if the hand quickly passes the flame, the flame will not have enough time to burn the skin. The principle of high-speed machining using microtools is the same. Moving too fast, the heat does not have enough time to feed back into the part, causing some problems.
The use of small micro-tools to machine graphite or other materials is actually not an easy task, especially on the 40-taper spindle of an ordinary CNC machine. It is difficult to find a chuck that can hold such a small tool. Because this kind of main shaft is specially designed for holding the 3in (1in = 25.4mm) flying knife, uses in the deep cutting of the high density matrix material. Therefore, the torque is very large, it is easy to break small tools, and it is inefficient and costly.
In this case, the only way the operator can choose is to reduce the speed and feedrate, and make the machine work in a slow state. This will inevitably lead to very low efficiency and the length of the processing cycle is unacceptable. If you think of it in a more visual way, it's like the relationship between a semi-powered truck and a car. In reality, the two are incomparable. Because trucks are designed to use power and power to drag and transport large quantities of cargo, racing cars are designed for speed and mobility. In essence, any machine tool manufacturer who says that its ordinary CNC machine can use micro-tools is like a car manufacturer. It installs spoilers on SUV models and sprays racing stripe, and claims that it also has the same quality as Porsche. . In fact, after installing baffles and spraying racing stripe on SUV models, it cannot be used as a racing car, nor can a refurbished high-speed spindle be installed on ordinary machines, hoping that it can be used. Miniature tool for high speed machining.
2. Geometric shapes
Like an ordinary CNC machine tool, it is impossible to renovate a machine that can effectively use micro-tools. The geometry of ordinary tools (large-size tools) cannot be reduced to miniature tools. The geometry of the tool used to mill the electrode plays a key role in the quality and efficiency of the process electrode. The small format tool made by reducing the geometry of large tools in proportion is not satisfactory for its feed rate and surface quality.
As the diameter of the tool is reduced and the speed of the spindle is reduced, the requirements for the tool change. Ordinary cutters using inserts are not suitable as miniature tools. This is mainly because of its high speed, not the diameter of the tool.
3. Tool wear
Increasing the rotational speed requires a proper balance of the tools to greatly increase the cutting space and ensure that the chips are well eliminated. When milling graphite, the geometry of the tool helps to increase productivity and product quality. However, due to the inherent abrasive properties of this material, tool wear is still a concern. The special coating is a powerful weapon to protect the tool from excessive wear. The coatings that can be considered for use include a silicon carbide coating, a titanium aluminum alloy coating (TiAlN) used in steel processing, and an AlCrN coating. Generally used for the processing of high hardness aluminum materials (such as cast aluminum or sand cast aluminum), in general, this kind of coating shows very good advantages for processing other abrasive matrix materials such as glass fiber.
System performance considerations
1. Dust prevention measures
Since graphite is a kind of abrasive material, it is extremely important to protect the workpieces of CNC machine tools from graphite dust (see Figure 2). Some machine tool manufacturers develop a solution that meets the needs of these special applications.
In this high-speed milling machine, around the graphite workpiece and the main shaft is a cylinder from which a spiral stream (or vortex) can be blown out so that dust can remain in motion without the chance of settling. In order to achieve the purpose of vacuuming
The search for graphite machine tools should meet the following requirements:
(1) Linear guides and ball screws should be protected with positive pressure air flow to prevent abrasive dust from entering the high-speed machining center.
(2) All motion control electronics and motors must be sealed to prevent contamination of the machine's control system.
(3) The full seal with safety lock shall be equipped with an air suction system. This processing system is suitable for processing graphite, glass fiber composites, ceramics or any abrasive material (see Figure 3).
Figure 3 This high-speed graphite milling machine is ideally suited for use with miniature tools. The linear guides and ball screws are equipped with a sealed protective sleeve and positive pressure airflow protection. All motion control electronics and motors are also equipped with a sealed protective sleeve and air suction. System to prevent the entry of abrasive graphite dust
2. Tool length measurement
The advanced machine tool should also have another feature. The machine tool should be conducive to the milling of graphite. The length measurement of the tool should be able to compensate for the wear of the tool. This tool sensor/compensation replacement system consists of three independent but interrelated components:
(1) Tool changer. The tool changer is a tool holder or a cutter head with spare tool space, and a free tool position for placing broken tools. The operator of the machine can store the spare tool on the tool holder so that it can be automatically replaced if the tool breaks during a no-light operation or unattended operation in the night shift.
(2) Tool inspection device. The tool inspection device is actually a mechanical sensor that measures the length of the tool. This is an instrument that can actually test tool wear and tear.
(3) Software. The software is a macro program that can be set to run a tool check program after executing a series of code lines. For example, a tool check macro program can perform a check after every 500 code lines. This kind of inspection is called the "if/then" statement. In other words, it needs to have a process: "Measure this tool. If the length is lower than the listed parameters, then you need to replace the tool."
Therefore, when the machining center gradually advances toward the ultimate goal of high-performance milling of hard steel in a single-step manner, the technical development of the high-speed machining center stays in front of the curve in terms of providing the efficiency and quality required for the milling of the graphite electrode. In the mold manufacturing process, this process is very helpful, and will become the mainstream process of the industry for a long period of time.
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