Re-grinding and recoating of CNC tools for CNC tooling technology lectures

Re-grinding and recoating of cemented carbide and high speed steel tools is a common process today. Although tool regrind or recoating is only a fraction of the cost of new tooling, it extends tool life. The regrind 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.

1. Re-grinding of the tool

During the regrind of the drill bit or milling cutter, the cutting edge needs to be ground to remove the original coating, so the grinding wheel used must have sufficient hardness. Re-grinding of the cutting edge is critical, not only to ensure that the geometry of the original cutting edge is completely accurately retained after re-grinding of the tool, but also that re-grinding must be “safe” for PVD coated tools. Therefore, unreasonable grinding processes must be avoided (for example, rough or dry grinding where the surface of the tool is damaged by high temperatures).

Chemically remove all of the original coating before coating. Chemical removal is often used for complex tools (such as hobs, broaches), or for multiple overcoating tools and tools that have problems with coating thickness. The method of chemically removing the coating is usually limited to high-speed steel tools because it damages the cemented carbide substrate: the chemical removal coating method removes cobalt from the cemented carbide substrate, causing the surface of the substrate to be loose, causing pores to be difficult to carry out. Recoating.

“Chemical removal is preferred for corrosion removal of high speed steel hardcoats,” said Dennis Quinto, technical director of Balzers Coatings. “Because the cemented carbide matrix contains similar chemical components as the coating, chemical removal of the solvent is more likely to damage the cemented carbide matrix than the high speed steel matrix”.

“The time it takes for the tool to stay in the coating removal solution is critical,” said Bill Langendor fer, 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 high speed steel, the tool should be removed immediately after the original coating on the tool is removed. Remove and clean."

In addition, there are some patented chemical methods 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 typical recoating process is to remove the original coating from the tool by a regrind process.

2. The economics of recoating

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

This is often the case: after the user purchases an uncoated tool, the coating is applied when the tool needs to be reground, or a different coating is applied to the new tool or the reground tool.

Mr. Bill Langendorfer said: “In many cases, we remove the TiN coating on the tool and reapply the TiAlN coating. Because the user wants to increase the tool production efficiency, the TiAlN coating tool has a higher cutting speed than the TiN coating tool. It is also more resistant to high temperatures. Users often want to get new coated tools with better performance from tool manufacturers, so 'tool manufacturers may have to redevelop a new tool with TiAlN coating'. But with Re-developing this new tool takes a much shorter time to remove the TiN coating from the old tool and apply the TiAlN coating."

The recoating limit is like a tool that can be reground multiple times, and the cutting edge of the tool can be coated multiple times. “Getting a good adhesion to the surface of a tool that has been reground is the key to improving tool performance,” said Rob Bokram, Director of Domestic Sales, IonBond LLC.

Except for the cutting edge, the rest of the tool surface may not need to be removed or recoated during each grinding of the tool, depending on the type of tool and the cutting parameters used in the machining. Hobs and broaches are tools that need to remove all of the original coating when recoating, otherwise the tool performance will be reduced. The tool can be recoated a small number of times without removing the old coating before the stress-induced adhesion problem becomes prominent. Although PVD coatings have residual compressive stresses that favor metal cutting, such pressures increase with increasing coating thickness and the coating will begin to delaminate beyond a certain fixed limit. When recoating without removing the old coating, a thickness is added to the outer diameter of the tool. For a drill bit, it means that the drilled hole is getting larger. Therefore, the effect of the additional thickness of the coating on the outer diameter of the tool must be considered, as well as the effect of both on the tolerance of the machined hole size.

A drill bit can be recoated 5 to 10 times without removing the old coating, but will face serious error problems after that. Dennis Klein, vice president of Spec Tools, believes that coating thickness is not an issue within ±1 μm of error; 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 is not an issue, the recoated, reground tool may be better than the original.

About Yaken turning tools

The composite compression form of the Yaken translatable tool holder is divided into three types according to the shape of the blade:

The blade for the W type is: TN##, WN##
The blades for the T-type are: DN##, CN##, SN##
The M-shaped blade is: VN##
With the above-mentioned blade pressing form, the blades of different shapes are stably and reliably installed. And has the following characteristics:
W type: blade installation stability is the highest;
T type: blade replacement is the easiest and quickest;
T and M models: Higher blade repeatability.