Regrinding and recoating of cutting tools
regrinding and recoating of cemented carbide and high speed steel cutting tools are common processes at present. Although the price of tool regrinding or recoating is only a small part of the manufacturing cost of new tools, it can prolong the tool life. Regrinding process is a typical treatment method for special cutting tools or expensive cutting tools. Tools that can be reground or recoated include drill bits, milling cutters, hobs and forming tools
in the regrinding process of drill or milling cutter, the cutting edge needs to be ground to remove the original coating, so the grinding wheel used must have sufficient hardness. Regrinding is very critical to the pretreatment of cutting edges. It is not only necessary to ensure that the geometric shape of the original cutting edge can be completely and accurately retained after regrinding, but also requires that regrinding must be "safe" for PVD coated tools. Therefore, it is necessary to avoid unreasonable grinding processes (such as rough grinding or dry grinding in which the surface of the tool is damaged due to high temperature)
Figure 1 tin coated high speed steel gear cutter
removal of coating
before the tool is recoated, all the original coatings can be removed by chemical methods. Chemical removal method is often used for complex tools (such as hobs, Broaches), or tool cupping testing machine with multiple recoating. The technical principle of testing machine and tools with problems due to coating thickness. The chemical coating removal method is usually limited to high-speed steel tools, because this method will damage the cemented carbide matrix: the chemical coating removal method will filter cobalt from the cemented carbide matrix, resulting in the matrix surface porosity, pores and even difficult to recoat
Figure 2 Comparison before and after coating removal
"chemical removal method is preferred for corrosion removal of high-speed steel hard coating" Mr. Dennis Quinto, technical director of balcha step 1 to reduce pump station noise, said. "Since the cemented carbide matrix contains chemical components similar to those in the coating, the chemical removal solvent is more likely to damage the cemented carbide matrix rather than the high-speed steel matrix"
"the time that the tool stays in the coating removal solution is very important," said Bill langendor fer, vice president of Jinxing coating company. "The longer the tool is left in the solution, the more serious the corrosion of the tool is. Although the corrosion rate is much lower for high-speed steel, the tool should be taken out and cleaned immediately after the original coating on the tool is removed"
in addition, there are some patented chemical methods suitable for removing PVD coatings. Among these chemical methods, the coating removal solution has only a small chemical reaction with the cemented carbide matrix, but these methods have not been widely used at present. In addition, there are other methods to clean the coating, such as laser processing, grinding and sandblasting. Chemical removal is the most commonly used method because it can provide good surface coating removal consistency
at present, the typical recoating process is to remove the original coating of the tool by regrinding process
Fig. 3 tin coated shaving cutter
economy of recoating
the most common tool coatings are tin, tic and TiAlN. Other superhard nitrogen/carbide coatings are also used, but not very common. PVD diamond coated tools can also be reground and recoated. During the recoating process, the tool should be "protected" to avoid damage to the critical surface
this is often the case: after using and staying in this state for a long time, users buy uncoated tools, and then coat them when the tools need to be reground, or apply different coatings on new tools or reground tools
mr. bill langendorfer said: "In many cases, we remove the tin coating on the tool and re coat it with TiAlN coating. Because users want to improve the productivity of the tool, and TiAlN coated tools have higher cutting speed and higher temperature resistance than tin coated tools. Users often hope to obtain new coated tools with better performance from tool manufacturers, so 'tool manufacturers may have to re develop a new tool with TiAlN coating'. However Compared with the redevelopment of this new tool, it takes much less time to remove the tin coating from the old tool and apply the TiAlN coating. "
limitation of recoating
like a tool can be reground many times, the cutting edge of the tool can also be coated many times. "The key to improve tool performance is to obtain a coating with good adhesion on the tool surface that has been reground." Rob bokram, director of domestic sales of ionbond LLC, pointed out
except for the cutting edge, the rest of the tool surface may not need to be coated or recoated during each grinding of the tool, which depends on the type of tool and the cutting parameters used in processing. Hobs and broaches are tools that need to remove all the original coatings when recoating, otherwise the tool performance will be reduced. Before the adhesion problem caused by stress becomes prominent, the tool can be recoated a few times without removing the old coating. Although PVD coating has residual compressive stress conducive to metal cutting, this pressure will increase with the increase of coating thickness, and the coating will begin to appear delamination after exceeding a fixed limit. When recoating without removing the old coating, a thickness is added to the outer diameter of the tool. For the bit, it means that the hole diameter is getting larger. Therefore, the influence of the additional thickness of the coating on the outer diameter of the tool must be considered, and the influence of the two on the dimensional tolerance of the machined aperture must also be considered
a drill bit can be coated 5 ~ 10 times without removing the old coating, but after that, it will face serious error problems. Dennis Klein, vice president of spec tools, believes that the coating thickness will not be a problem within the error range of ± 1 m; However, when the error is within the range of 0.5 ~ 0.1 M, the influence of coating thickness must be considered. As long as the thickness of the coating is not a problem, the tool with recoating and regrinding may have better performance than the original tool. (end)