Graphite has excellent self-lubrication, corrosion resistance, wear resistance, low friction coefficient and expansion coefficient, etc. It is often used as a mechanical seal friction pair material. In the machining process of the graphite ring turning, there are frequent chipping, resulting in unsatisfactory product appearance inspection and sealing performance inspection, high product rejection rate and high production cost. This paper analyzes the cause of the edge chipping in the graphite ring turning, and optimizes the tool material, the tool angle, the amount of cutting tool, and the process flow, and solves the chipping problem of turning.
1 Graphite Machining Mechanism
Metal cutting is the shear stress and strain generated by the external force of the material. When the shear stress reaches the yield limit of the material, the internal metal grains are slipping elastically deformed, plastically deformed, partially microcracked, crack propagation, fracture separation process. The graphite is a brittle material with non-uniform structure of multi-layered crystals. The energy required for its brittle fracture is less than the elastic deformation energy. Under the action of the cutting force, the graphite first breaks brittle without elastic deformation.
In the actual machining process, the graphite material will produce an expanding crack when the tool comes into contact with the workpiece, and some of the material will be broken by the cutter advancement to form chipping chips. Therefore, the graphite processing process is particularly prone to chipping, and the production process is difficult to control.
2 Causes
In order to analyze the cause of the graphite chipping, statistics were made on the chipping of the various processes in the turning of the graphite ring. The chipping mainly occurred at the intersection of all parts of the part, and the chipping was most serious at the inner surface of the cut surface. Observing the same cutting process, when the tool comes in contact with the part and starts cutting, no chipping occurs, and when the cutter leaves the workpiece and finishes cutting, chipping easily occurs. Graphite machining is actually a material brittle fracture process. When the machining state changes, the impact of cutting changes greatly, and the chipping is prone to occur. When the tool ends cutting and leaves the workpiece, the residual material strength of the part is insufficient to withstand the impact of external force, and it is particularly easy to break and chipping. In the outer circle, inner hole and face turning process, the tool feed direction is different, the radial force cutting force difference is great, the radial force is less when the outer circle and the inner hole are turned, and the chipping is not easy to occur, and the radial force is great when the face is turned., prone to chipping. The part is cut by a cutting knife, which has a large amount of knife and a large radial cutting force. When the part is cut off, a large chipping will occur instantaneously.
3 Process Optimization
3.1 Tool Material
The tool material is the fundamental factor that determines the cutting performance of the tool. Graphite is a brittle material and must be made of wear-resistant, sharp tools. After high-speed steel and cemented carbide tools were used for multiple machining tests, YG8 carbide tools were selected. The main components were WC and Co. Grinding with diamond grinding wheels ensured that the cutting tools were sharp and durable. YG cemented carbide has good bending strength and toughness, and is suitable for brittle materials processing. The tool can withstand high cutting shocks. YG carbide has good machinability and can sharpen cutting edges and reduce cutting force.
3.2 Tool Angle
The main declination angle Kr and the declination angle Kr’ are the two main tool angles of the cutting blade. The main declination angle determines the cross-sectional shape of the cutting layer, the proportion of the cutting force, the strength of the cutting edge, and the heat dissipation conditions. The declination angle affects the maximum height of the remaining surface area of the machined surface and directly affects the surface roughness of the product. According to theoretical analysis and experimental comparison, the cutting knife was optimized. Use a double cutting edge to cut the knife and increase the tool rake and relief angle to 10 to 13°. When the conventional cutting blade is cut radially, the radial cutting force acts on the cutting direction of the component, and the chipping angle is easily generated. In the case of the double cutting blade cutting tool, the radial cutting force generates the component force in the oblique direction and the radial force of the component is decrease. With two main cutting edge cutting blades, when the two cutting edges of the tool do not leave the workpiece, the part has been cut off. The chipping angle appears outside the end face of the part and can be removed by subsequent processing without affecting the final appearance of the product. When using a double-cutting cutting tool, the tool declination angle should be 45° to avoid tool misalignment.
3.3 engagement
The depth of cut has a great influence on the chip formation process of the graphite. When the depth of cut is small, the chip is pseudo-band or floccular; when the depth of cut is large, the cutting is broken or broken. In order to verify the effect of cutting depth on the processing chipping angle, a machining contrast test was conducted with cutting depths of 0.01 mm, 0.05 mm, 0.10 mm, 0.15 mm, and 0.2 mm. When the cutting depth increases, the chipping becomes more pronounced, and the small depth of cut is 0.01 mm and 0.05mm, there is no processing chipping. Therefore, a smaller cutting depth should be selected for the graphite processing, and the appropriate cutting depth should be selected according to the part acceptance requirements and production efficiency.
3.4 Process flow
In order to completely solve the problem of part processing and chipping, the processing sequence of the outer ring, inner hole and end face of the graphite ring is optimized. The machining process of the outer circle and the inner hole cannot completely avoid the processing chipping, but the chipping can be effectively avoided by adjusting the processing sequence when the end face is processed. When the graphite ring face is turned, it can be machined by internal and external machining and center-knife processing. The tool is processed from the inner and outer sides to the center of the part, and the knife is connected at the center of the end face. Since the inner and outer sides are processed to the center of the part, the cutter never leaves the workpiece. There will be no phenomenon that the residual material strength of the part is insufficiently squeezed, which can effectively avoid the occurrence of processing chipping. In actual production, in order to meet the requirements of processing and measurement, the turning of the face can be divided into two processes. The first machining process reserves the machining allowance, and then the outer circle and the inner hole are processed. Finally, the inner and outer machining and the center-knife processing method are used to finish graphite ring‘s end surface.
4 production verification
All corners of a mechanical seal graphite ring must not have chipping, and the yield of the seal ring processed by conventional methods is less than 50%. A total of 286 pieces of products were produced using the above-mentioned optimized turning process. Only 32 pieces of turning were found to have a smaller chipping, of which 20 pieces were eliminated in the subsequent face grinding process, and the sealing rings were all tested for airtightness after assembly. ,The product processing pass rate increased to about 96%.
5 Conclusion
Through the research of the turning process of graphite rings, YG hard alloy cutting tools were selected, and cutting blades were used to reduce the amount of cutting tools. Internal and external machining and center-joint machining methods were used to solve the problem of chipping of graphite ring turning.
Author: YongFeng Ren, Yanming Zhang
