Selection of cutting tools and geometric parameters for coating cutting

       In the process of popularization and application of thermal spraying technology, in order to solve the problem of coating processing, a series of new cutting tools have been developed one after another, and good processing results have been achieved. At present, cutting tool materials are roughly divided into three categories, namely cemented carbide tools, ceramic tools and cubic boron nitride tools. Carbide tools are still commonly used.



Cemented carbide tools are divided into four categories according to their chemical composition and performance, namely tungsten-cobalt (YG), tungsten-titanium-cobalt (YT), tantalum carbide (niobium)-added (YW, YA) and titanium carbide-based ( YN). For the processing of spray coating with high hardness, fine-grained alloy tools with added tantalum carbide (niobium) should be selected.



Ceramic knives are divided into ordinary ceramic knives and combined ceramic knives. Ordinary ceramic tools are sintered at high temperature with alumina as the main component. The composite ceramic tool is made by adding a certain amount of carbide (usually TiC) to ordinary alumina ceramics and sintering at high temperature and high pressure. The density, hardness, bending strength and impact toughness of the combined ceramic tool are higher than those of ordinary ceramic tools, and can be used for coating processing.



The cubic boron nitride tool is transformed from hexagonal boron nitride by adding a catalyst under high temperature and high pressure. It is characterized by extremely high hardness and excellent wear resistance, as well as low friction coefficient, good thermal stability and thermal conductivity. Due to the advantages of this tool, it can be used to process spray-welded layers with high hardness.



Due to the different requirements for rough turning and finishing of the coating, the geometric parameters of the tool should be selected differently, and the tool material and cutting amount should also be considered. The following optional parameters are for reference during processing.



1) Main declination angle. The size of the entering angle directly affects the strength of the tool tip and the heat dissipation conditions of the tool during cutting. Using a smaller entering angle can increase the nose angle, improve the strength of the tool nose and increase the heat dissipation area, but too small entering angle will cause the increase of radial force during cutting. Generally, it is recommended to be 45-75°. When roughing the coating with higher hardness, the main declination angle can be appropriately reduced to 20° to ensure the strength of the tool. When processing slender workpieces, it is advisable to use about 45°. When finishing turning, the main declination angle can be increased to reduce the axial cutting force and make the workpiece less prone to vibration. However, if the main declination angle is too large, the tool strength will be reduced and the edge will be chipped.



2) Front corner. Negative rake angle (-3~-8°) is generally used in rough turning, and zero value can be used in fine turning.



3) Rear corners. Generally, 8-12° is used. For example, when the coating has high hardness or brittleness, a smaller relief angle (4-6°) can be used.



4) Blade inclination. 6-15° can be used for rough turning, which can enhance the strength of the tool and change the impact position, which is beneficial to cutting. The inclination angle of the edge can be set to zero value during fine turning, so as to control the flow direction of the chips and avoid affecting the finish.



5) The corner of the knife tip. When the spray welding layer is rough turned, the tip angle of the tool can be arc-shaped, generally R0.8-1.0 is appropriate, which can ensure the strength and wear resistance of the tool. When machining the spray-welded layer with high hardness, the arc of the tool nose for rough turning can be correspondingly increased (R1.5-2), and the arc of the tool nose should be reduced to about R0.75 during fine turning. For the processing of the spray coating, a suitably sharp tool should be ground at the contact point. This leveling has the effect of troweling the metal, and the finish of the processing is better. However, this leveling should not be excessive, otherwise the temperature of the contact surface will be high, and overheating may cause damage to the coating.