Hardware Knowledge Base: Regrind and Recoat of CNC Tool

The re-grinding and re-coating of cemented carbide and high-speed steel tools is a common process at present. Although the cost of regrinding or recoating a tool is only a fraction of the cost of a new tool, it can increase tool life. The regrinding process is a typical treatment for special tools or expensive tools. Tools that can be reground or recoated include drills, milling cutters, hobs, and forming tools.

Regrind

During re-grinding of a drill or cutter, the cutting edge needs to be ground to remove the original coating, so the grinding wheel used must have sufficient hardness. Re-grinding is very critical for the pretreatment of the cutting edge, not only to ensure that the original cutting edge geometry after tool re-grinding can be completely accurately retained, but also that re-grinding must be "safe" for PVD-coated tools. Therefore, it is necessary to avoid unreasonable grinding processes (for example, rough grinding or dry grinding in which the surface of the tool is damaged due to high temperatures).

Prior to coating, the entire coating can be chemically removed. Chemical removal methods are often used for complex tools (eg, hobs, broaches), or for multiple overcoating tools and tools that cause problems due to coating thickness. Chemical removal of coatings is usually limited to high-speed steel tools because they damage the cemented carbide substrate: the use of a chemical removal coating removes cobalt from the cemented carbide substrate, causing the surface of the substrate to loosen, create porosity, and make it difficult Recoat.

"Chemical removal is the first choice for the corrosion removal of high-speed steel hard coatings," said Dennis Quinto, technical director at Balzers Coatings. "Since the cemented carbide substrate contains chemical components similar to those in the coating, chemical removal of the solvent can easily damage the cemented carbide substrate rather than the high-speed steel substrate."

“The time it takes for the tool to stay in the coating removal solution is critical,” notes Bill Langendorfer, vice president of Venus Coatings. "The longer the tool is left in the solution, the more severe the corrosion of the tool. Although the corrosion rate is much lower for HSS, the tool should be removed immediately after the original coating on the tool is removed. Remove and clean."

In addition, there are patented chemical methods that are suitable for removing PVD coatings. In these chemical methods, the coating removal solution has only a slight chemical reaction with the cemented carbide substrate, but these methods are not currently widely used. In addition, there are other methods of cleaning the coating, such as laser processing, abrasive blasting, and the like. Chemical removal is the most common method because it provides good surface coating removal consistency.

The current typical recoating process is to remove the original coating of the tool through a regrind process.

Recoat economics

The most common tool coatings are TiN, TiC, and TiAlN. Other superhard nitrogen/carbide coatings are also used but are less common. PVD diamond coated tools can also be reground and recoated. During the recoating process, the tool should be "protected" to avoid critical surface damage.

This is often the case: After the user purchases an uncoated tool, he or she will apply the coating again when the tool needs to be reground, or perform a different coating on the new or reground tool.

Mr. Bill Langendorfer said: “In many cases, we removed the TiN coating on the tool and re-applied the TiAlN coating. Because users want to increase tool productivity, the TiAlN coating tool has higher cutting speed than the TiN coating tool. It is also more resistant to heat. Users often want to get better coated tooling from toolmakers, so 'tool makers may have to redevelop a new tool with TiAlN coating'. But with Compared to the development of this new tool, it takes much less time to remove the TiN coating from the old tool and apply the TiAlN coating."

The re-coating limit is like a tool that can be re-grinded several times, and the cutting edge of the tool can also be coated multiple times. “The key to improving tool performance is to obtain a coating with good adhesion on the surface of a tool that has been reground,” said Rob Bokram, director of domestic sales at IonBondLLC.

In addition to the cutting edge, the rest of the tool surface may not need to be removed or re-coated at each tool dressing, depending on the type of tool and the cutting parameters used in the process. Hobs and broaches are tools that require removal of all of the original coating when recoating, otherwise tool performance will be reduced. Before the stress-induced adhesion problem becomes prominent, the tool can be recoated a few times without removing the old coating. Although the PVD coating has residual compressive stress that favors metal cutting, this pressure increases as the thickness of the coating increases, and the coating begins to delaminate beyond a certain fixed limit. When recoating without removing the old coating, a thickness is added to the outside diameter of the tool. For the drill bit, it means that the drilled hole diameter becomes 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 both on the dimensional tolerance of the machined hole diameter must also be considered.

A drill bit can be recoated 5 to 10 times without removing the old coating, but after this will face serious errors. Dennis Klein, vice president of Spec Tools, believes that the coating thickness will not become a problem within a tolerance of ±1 μm; however, when the error is in the range of 0.5 to 0.1 μm, the effect of coating thickness must be considered. As long as the coating thickness does not become a problem, the re-coated, reground tool may well perform better than the original.