It is well known that the geometry of the cutting edge of a tool is a key factor in determining its processing efficiency. In general, increasing the blunt radius of the cutting edge helps to cut metal material more efficiently and extends tool life. Scientific research institutions and tool manufacturers have conducted detailed research on the mechanisms involved.
Simply put, sharp cutting edges with grinding defects and burrs are prone to chipping during the initial stages of the cut, causing localized edge breakage. A detailed analysis of the frictional shear stress around the cutting edge shows that the chip thickness decreases as the radius of the blunt circle of the cutting edge increases. Increasing the blunt radius of the cutting edge greatly reduces the contact length of the tool and the chip and heat accumulation, thereby reducing tool wear. Figure 1. Wet blasting machine 1 Modern tools rely on super-hard surface coatings (such as TiN or CrN coating) to improve their cutting performance. These coatings applied by physical vapor deposition (PVD) or chemical vapor deposition (CVD) processes can significantly increase the hardness and durability of the cutting edge of the tool. Since the hardness of these coatings (up to HV3400) is usually twice as high as the hardness of the matrix material (hard alloy is about HV1700), the adhesion between the coating and the substrate tends to be one of the limiting factors for tool life. .
Therefore, the blunt radius of the cutting edge of the tool base and its surface finish are critical to the final performance of the tool. The coating deposition process tends to deposit more material at the sharper edges. Therefore, a smaller blunt radius of the cutting edge will attract thicker deposits. Since the area is subjected to extreme stress when the workpiece is cut, cracks are more likely to occur and become the starting point for subsequent accelerated damage.
In order to maximize the adhesion between the coating and the substrate, the surface of the substrate must be very clean. In a study called "Improving the cutting performance of highly adherent PVD films on cemented carbide inserts by micro-blasting," the researchers found that grinding alone did not provide the desired coating adhesion. The ideal surface finish. Therefore, the blade usually needs to be ground + micro-blasted or polished + micro-blasted. The study found that the polishing + micro-blasting treatment is better (in both treatment methods, the micro-blasting uses dry blasting). The blasting head in the work of Figure 2 In this research project, the adhesion of the blade surface was measured by repeating the oblique impact test until the surface was broken. A comparative test (simple C Rockwell hardness test) can also be used to observe the damage around the indentation. The adhesion of the coating was divided into 6 grades from HF1 to HF6 (HF1 indicates the best adhesion). One of the main benefits of wet micro-blasting is that excess cobalt binder can be removed from the surface of the blade, which can adversely affect the adhesion of the surface coating. However, obtaining a blunt radius of the cutting edge by means of a brush or dry blasting accumulates static electricity on the substrate, so electrostatically adsorbed debris or dust must be removed from the blade surface.
In order to obtain a high quality pre-coating pre-treated surface, wet blasting can be used instead of two-stage polishing and dry blasting. With wet blasting technology, the coating adhesion of HF1 can be achieved, and the surface of the tool does not generate static electricity or absorb dust.
One of the main advantages of pre-coating pre-coating (especially flushing and drying immediately after grit blasting) of a cemented carbide tool by wet blasting is that the treated tool surface is extremely active. In fact, its surface activity is so high that existing surface activity measuring devices (such as angle measuring systems or Dyne ink testing methods) cannot measure them (because the instrument's measurement threshold is exceeded). This active surface further enhances the adhesion of the coating. A key to successful wet blasting prior to coating is process control to ensure consistent surface cleanliness, including rinsing, rinsing in deionized (DI) water, and immediate drying. Figure 3: The blasted blade must be cleaned and dried. Just as the tool base benefits from the micro-blasting process, the coated tool itself benefits. The researchers of the above projects tested the cemented carbide milling inserts treated with the grinding + micro-blasting process and the polishing + micro-blasting process. Studies on the micro-blasted PVD film after coating showed that there is a superficial layer with a large deformation and high nanohardness (the depth is about 0.6 μm). Two kinds of blasting pressures (2 bar and 5 bar) were used in the process test. The results show that the blasting pressure of 5 bar can increase the hardness of the superficial layer.
Subsequently, the coated blade was subjected to a milling test. In the test, the flank wear of the blade was measured, and the flank wear of 0.2 mm was set as the tool scrapping standard. The tool life of the ground and micro-blasted blade samples is extended by up to 15%, while the blade life of the polished and micro-blasted (can be replaced by wet micro-blasting) is extended by up to 70% ( The blasting pressure is 5 bar).
Two distinct tool failure modes were identified by examining the wear marks and performing a finite element analysis of the associated stresses. For grinding and micro-blasting tool samples, the main cause of failure is insufficient adhesion of PVD coatings. For polishing + micro-blasting tool samples, the failure mode is fatigue in PVD coatings. To. Therefore, in order to maximize the processing efficiency by increasing the nano-hardness of the PVD coating by post-coating micro-blasting, a reasonable pre-coating pretreatment must be performed.
Automated wet blasting lines already on the market can perform all of these functions, including edge rounding, pre- and post-coating of PVD/CVD coatings, rinsing, deionized water rinsing and subsequent drying. For aesthetic reasons, the blade should be spray rinsed, cleaned and blown dry with hot air to the standard for packaging.
The time for edge rounding of a typical (200 piece) blade by wet blasting is 4-20 minutes, depending on how many process steps the wet blasting machine needs to complete. The time required to apply the coating to the pre- and post-coating treatment is about 1/2 of the rounding time of the edge.
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