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Research on Graphite Machining Process

Graphite is a common non-metallic material, black, with high and low temperature resistance, good electrical and thermal conductivity, good lubricity and stable chemical characteristics; good electrical conductivity, can be used as an electrode in EDM. Compared with traditional copper electrodes, graphite has many advantages such as high temperature resistance, low discharge consumption, and small thermal deformation. It shows better adaptability in the processing of precision and complex parts and large-size electrodes. It has gradually replaced copper electrodes as electric sparks. The mainstream of machining electrodes [1]. In addition, graphite wear-resistant materials can be used under high-speed, high-temperature, and high-pressure conditions without lubricating oil. Many equipment widely use graphite material piston cups, seals and bearings [2].

At present, graphite materials are widely used in the fields of machinery, metallurgy, chemical industry, national defense and other fields. There are many types of graphite parts, complicated parts structure, high dimensional accuracy and surface quality requirements. Domestic research on graphite machining is not deep enough. Domestic graphite processing machine tools are also relatively few. Foreign graphite processing mainly uses graphite processing centers for high-speed processing, which has now become the main development direction of graphite machining.

This article mainly analyzes graphite machining technology and processing machine tools from the following aspects.

①Analysis of graphite machining performance;

② Commonly used graphite processing technology measures;

③ Commonly used tools and cutting parameters in processing graphite;

Graphite cutting performance analysis

Graphite is a brittle material with a heterogeneous structure. Graphite cutting is achieved by generating discontinuous chip particles or powder through the brittle fracture of the graphite material. Regarding the cutting mechanism of graphite materials, scholars at home and abroad have done a lot of research. Foreign scholars believe that the graphite chip formation process is roughly when the cutting edge of the tool is in contact with the workpiece, and the tip of the tool is crushed, forming small chips and small pits, and A crack is produced, which will extend to the front and bottom of the tool tip, forming a fracture pit, and a part of the workpiece will be broken due to the tool advancement, forming chips. Domestic scholars believe that the graphite particles are extremely fine, and the cutting edge of the tool has a large tip arc, so the role of the cutting edge is similar to extrusion. The graphite material in the contact area of ​​the tool - the workpiece is squeezed by the rake face and the tip of the tool. Under pressure, brittle fracture is produced, thereby forming chipping chips [3].

In the process of graphite cutting, due to changes in the cutting direction of the rounded corners or corners of the workpiece, changes in the acceleration of the machine tool, changes in the direction and angle of cutting in and out of the tool, cutting vibration, etc., a certain impact is caused to the graphite workpiece, resulting in the edge of the graphite part. Corner brittleness and chipping, severe tool wear and other problems. Especially when processing corners and thin and narrow-ribbed graphite parts, it is more likely to cause corners and chipping of the workpiece, which has also become a difficulty in graphite machining.

Graphite cutting process

The traditional machining methods of graphite materials include turning, milling, grinding, sawing, etc., but they can only realize the processing of graphite parts with simple shapes and low precision. With the rapid development and application of graphite high-speed machining centers, cutting tools, and related supporting technologies, these traditional machining methods have gradually been replaced by high-speed machining technologies. Practice has shown that: due to the hard and brittle characteristics of graphite, tool wear is more serious during processing, therefore, it is recommended to use carbide or diamond coated tools.

Cutting process measures
Due to the particularity of graphite, in order to achieve high-quality processing of graphite parts, corresponding process measures must be taken to ensure. When roughing graphite material, the tool can directly feed on the workpiece, using relatively large cutting parameters; in order to avoid chipping during finishing, tools with good wear resistance are often used to reduce the cutting amount of the tool, and Ensure that the pitch of the cutting tool is less than 1/2 of the diameter of the tool, and perform process measures such as deceleration processing when processing both ends [4].

It is also necessary to reasonably arrange the cutting path during cutting. When processing the inner contour, the surrounding contour should be used as much as possible to cut the force part of the cut part to always be thicker and stronger, and to prevent the workpiece from breaking [5]. When processing planes or grooves, choose diagonal or spiral feed as much as possible; avoid islands on the working surface of the part, and avoid cutting off the workpiece on the working surface.

In addition, the cutting method is also an important factor that affects graphite cutting. The cutting vibration during down milling is less than that of up milling. The cutting thickness of the tool during down milling is reduced from the maximum to zero, and there will be no bouncing phenomenon after the tool cuts into the workpiece. Therefore, down milling is generally selected for graphite processing.

When processing graphite workpieces with complex structures, in addition to optimizing the processing technology based on the above considerations, some special measures must be taken according to the specific conditions to achieve the best cutting results.

Cutting tool
In graphite high-speed machining, due to the hardness of the graphite material, the interruption of chip formation and the influence of high-speed cutting characteristics, alternating cutting stress is formed during the cutting process and a certain impact vibration is generated, and the tool is prone to rake face and flank face Abrasion seriously affects the service life of the tool, so the tool used for graphite high-speed machining requires high wear resistance and impact resistance [6].

Diamond coated tools have the advantages of high hardness, high wear resistance, and low friction coefficient. Currently, diamond coated tools are the best choice for graphite processing.

Graphite machining tools also need to choose a suitable geometric angle, which helps to reduce tool vibration, improve machining quality, and reduce tool wear. German scholars' research on graphite cutting mechanism shows that graphite removal during graphite cutting is closely related to the rake angle of the tool [3]. Negative rake angle cutting increases the compressive stress, which is beneficial to promote the crushing of the material, improve the processing efficiency, and avoid the generation of large-size graphite fragments.

Common tool structure types for graphite high-speed cutting include end mills, ball-end cutters and fillet milling cutters. End mills are generally used for surface processing with relatively simple planes and shapes. Ball-end milling cutters are ideal tools for processing curved surfaces. Fillet milling cutters have the characteristics of both ball-end cutters and end mills, and can be used for both curved and flat surfaces. For processing.

Cutting parameters
The selection of reasonable cutting parameters during graphite high-speed cutting is of great significance to the improvement of workpiece processing quality and efficiency. Since the cutting process of graphite high-speed machining is very complicated, when choosing cutting parameters and processing strategies, you need to consider the workpiece structure, machine tool characteristics, tools, etc. There are many factors, which mainly rely on a large number of cutting experiments.

For graphite materials, it is necessary to select cutting parameters with high speed, fast feed, and large amount of tool in the rough machining process, which can effectively improve the machining efficiency; but because graphite is prone to chipping during the machining process, especially at the edges, etc. The position is easy to form a jagged shape, and the feed speed should be appropriately reduced at these positions, and it is not suitable to eat a large amount of knife.

For thin-walled graphite parts, the reasons for the chipping of edges and corners are mainly caused by cutting impact, letting the knife and elastic knife, and cutting force fluctuations. Reducing the cutting force can reduce the knife and bullet knife, improve the surface processing quality of thin-walled graphite parts, and reduce corner chipping and breaking [6].

The spindle speed of graphite high-speed machining center is generally larger. If the spindle power of the machine tool allows, selecting a higher cutting speed can effectively reduce the cutting force, and the processing efficiency can be significantly improved; in the case of selecting the spindle speed , The feed amount per tooth should be adapted to the spindle speed to prevent too fast feed and large amount of tool to cause chipping. Graphite cutting is usually carried out on a special graphite machine tool, the machine speed is generally 3000 ~ 5000r/min, and the feed speed is generally 0. 5~1m/min, select a relatively low speed for rough machining and a high speed for finishing. For graphite high-speed machining centers, the speed of the machine tool is relatively high, generally between 10000 and 20000r/min, and the feed rate is generally between 1 and 10m/min.

Graphite High Speed ​​Machining Center

A large amount of dust is generated during graphite cutting, which pollutes the environment, affects the health of workers, and affects machine tools. Therefore, graphite processing machine tools must be equipped with good dust-proof and dust-removing devices. Since graphite is a conductive body, in order to prevent the graphite dust generated during processing from entering the electrical components of the machine tool and causing safety accidents such as short circuits, the electrical components of the machine tool should be protected as necessary.

Graphite high-speed machining center adopts high-speed electric spindle in order to achieve high speed, and to reduce the vibration of the machine tool, it is necessary to design a low center of gravity structure. The feed mechanism mostly adopts high-speed and high-precision ball screw transmission, and designs anti-dust devices [7]. The spindle speed of graphite high-speed machining center is usually between 10000 and 60000r/min, the feed speed can be as high as 60m/min, and the processing wall thickness can be less than 0. 2 mm, the surface processing quality and processing accuracy of the parts are high, which is the main method to achieve high-efficiency and high-precision processing of graphite at present.

With the wide application of graphite materials and the development of high-speed graphite processing technology, high-performance graphite processing equipment at home and abroad has gradually increased. Figure 1 shows the graphite high-speed machining centers produced by some domestic and foreign manufacturers.

OKK's GR400 adopts a low center of gravity and bridge structure design to minimize the mechanical vibration of the machine tool; adopts C3 precision screw and roller guide to ensure the high acceleration of the machine tool, shorten the processing time, and adopt the addition of splash guards The fully enclosed sheet metal design of the machine top cover prevents graphite dust. The dust-proof measures adopted by the Haicheng VMC-7G1 is not a commonly used method of vacuuming, but a water curtain sealing form, and a special dust separation device is installed. The moving parts such as guide rails and screw rods are also equipped with sheaths and Powerful scraping device to ensure long-term stable operation of the machine tool.

It can be seen from the specification parameters of the graphite high-speed machining center in Table 1, that the spindle speed and feed speed of the machine tool are very large, which is the characteristic of graphite high-speed machining. Compared with foreign countries, domestic graphite machining centers have little difference in machine tool specifications. Due to machine tool assembly, technology and design, the machining accuracy of machine tools is relatively low. With the widespread use of graphite in the manufacturing industry, graphite high-speed machining centers have attracted more and more attention. High-performance and high-efficiency graphite machining centers are designed and manufactured. The optimized processing technology is adopted to give full play to its characteristics and performance to improve graphite. The processing efficiency and quality of the parts are of great significance to improving my country's graphite cutting processing technology.

to sum up

This article mainly discusses the graphite machining process from the aspects of graphite characteristics, cutting process and the structure of graphite high-speed machining center. With the continuous development of machine tool technology and tool technology, graphite high-speed machining technology needs in-depth research through cutting tests and practical applications to improve the technical level of graphite machining in theory and practice.


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