CNC milling tool wear is one of the most basic propositions in cutting processing. Defining and understanding tool wear can help tool manufacturers and users extend tool life. Today’s tool coating technologies offer an effective means to further extend tool life while significantly increasing productivity.
CNC Machining Tool Wear Mechanism
Heat and friction are forms of energy generated in metal cutting. The heat and friction generated by high surface loads and the high speed of the chip sliding along the rake face of the tool place the tool in a very challenging machining environment.
The magnitude of the cutting forces tends to fluctuate, depending on different machining conditions (such as the presence of hard components in the workpiece material or interrupted cutting). Therefore, in order to maintain its strength under high cutting temperatures, the tool must have some basic characteristics, including excellent toughness, wear resistance and high hardness.
Although the cutting temperature at the tool/workpiece interface is the key factor determining the wear rate of almost all tool materials, it is very difficult to determine the parameter value required to calculate the cutting temperature. However, the results of cutting test measurements can lay the foundation for some empirical methods.
It can be generally assumed that the energy generated in cutting is converted into heat, and usually 80% of this heat is carried away by the chips (this proportion varies depending on several factors – especially the cutting speed). The remaining 20% or so is transmitted to the tool. Even when cutting moderately hard steels, the tool temperature may exceed 550°C, which is the maximum temperature that high-speed steel can withstand without reducing its hardness. When cutting hardened steel with polycrystalline cubic boron nitride (PCBN) tools, the temperature of the tool and chips will usually exceed 1000°C.
Cutting Tool Wear and Tool Life
Tool wear usually includes the following types: flank wear; scoring wear; crater wear; cutting edge blunting; cutting edge chipping; cutting edge cracks; catastrophic failure.
There is no universally accepted definition of tool life, which usually depends on different workpieces and tool materials, as well as different cutting processes. One way to quantitatively analyze the end point of tool life is to set an acceptable maximum flank wear limit (denoted by VB or VBmax). Tool life can be expressed by Taylor’s formula for expected tool life, that is,
VcTn=C
A more common form of this formula is
VcTnĂDxfy=C
Where Vc is the cutting speed; T is the tool life; D is the cutting depth; f is the feed rate; x and y are determined experimentally; n and C are constants determined from experiments or published technical data, which represent the characteristics of the tool material, workpiece and feed rate.
The continuous development of optimal tool substrates, coatings and cutting edge preparation technologies is essential to limit tool wear and resist cutting high temperatures. These factors, together with the chip breaker and corner arc radius used on the indexable insert, determine the suitability of each tool for different workpieces and cutting operations. The best combination of all these factors can extend tool life and make cutting operations more economical and reliable.
Changing the Tool Base
By changing the particle size of tungsten carbide in the range of 1-5µm, tool manufacturers can change the matrix properties of carbide tools. The particle size of the base material plays an important role in cutting performance and tool life. The smaller the particle size, the better the wear resistance of the tool. On the contrary, the larger the particle size, the stronger and tougher the tool. The fine-grained matrix is ââmainly used for blades processing aerospace grade materials (such as titanium alloy, Inconel alloy and other high-temperature alloys).â
In addition, by increasing the cobalt content of cemented carbide tool materials by 6%-12%, better toughness can be obtained. Therefore, the cobalt content can be adjusted to meet the requirements of a specific cutting process, whether that requirement is toughness or wear resistance.â
The performance of the tool matrix can also be enhanced by forming a cobalt-rich layer close to the outer surface, or by selectively adding other alloying elements (such as titanium, tantalum, vanadium, niobium, etc.) to the cemented carbide material. The cobalt-rich layer can significantly increase cutting edge strength, thereby improving the performance of roughing and interrupted cutting tools.â
In addition, when selecting a tool matrix that matches the workpiece material and processing method, five other matrix properties are also considered – fracture toughness, transverse fracture strength, compressive strength, hardness and thermal shock resistance. For example, if a carbide tool experiences chipping along the cutting edge, a base material with higher fracture toughness should be used. In the case of direct failure or damage of the cutting edge of the tool, the possible solution is to use a base material with higher transverse fracture strength or higher compressive strength. For machining situations with higher cutting temperatures (such as dry cutting), tool materials with higher hardness should usually be preferred. In machining situations where thermal cracks in the tool can be observed (most common in milling), it is recommended to use tool materials with better thermal shock resistance.
Optimizing and improving the tool base material can improve the cutting performance of the tool. For example, the base material of Iscar’s Sumo Tec blade grade for machining steel parts has better resistance to plastic deformation, which can reduce the possibility of micro-cracks in the hard and brittle blade coating. Through the secondary processing of Sumo Tec blades, the surface roughness and micro-cracks of its coating are reduced, thereby reducing the cutting heat on the blade surface and the resulting plastic deformation and micro-cracks. In addition, a new base for inserts for machining cast iron has better heat resistance, allowing for higher cutting speeds.
Choose the Correct Coating
Coatings also help improve the cutting performance of the tool. Current coating technologies include:
- Titanium Nitride (TiN) coating: This is a general-purpose PVD and CVD coating that can increase the hardness and oxidation temperature of the tool.â
- Titanium carbonitride (TiCN) coating: By adding carbon element to TiN, the hardness and surface finish of the coating are improved.â
- Titanium aluminum nitride (TiAlN) and titanium aluminum nitride (AlTiN) coatings: The composite application of aluminum oxide (Al2O3) layer and these coatings can improve the tool life of high-temperature cutting processes. Aluminum oxide coatings are particularly suitable for dry and near-dry cutting. AlTiN coatings have a higher aluminum content and have higher surface hardness than TiAlN coatings which have a higher titanium content. AlTiN coatings are commonly used for high-speed cutting.
- Chromium Nitride (CrN) Coating: This coating has better anti-adhesion properties and is the preferred solution for fighting built-up edge.â
- Diamond coating: Diamond coating can significantly improve the cutting performance of tools for processing non-ferrous materials, and is very suitable for processing graphite, metal matrix composites, high-silicon aluminum alloys and other highly abrasive materials. However, diamond coating is not suitable for processing steel parts because its chemical reaction with steel will destroy the adhesion between the coating and the substrate.â
In recent years, the market share of PVD-coated tools has expanded, and its price is comparable to that of CVD-coated tools. The thickness of CVD coating is usually 5-15”m, while the thickness of PVD coating is about 2-6”m. When applied to a tool substrate, CVD coatings create undesirable tensile stresses; PVD coatings contribute to beneficial compressive stresses on the substrate. Thicker CVD coatings often significantly reduce the strength of tool cutting edges. Therefore, CVD coatings cannot be used on tools that require very sharp cutting edges.â
The use of new alloy elements in the coating process can improve the adhesion and coating performance of the coating.
Cutting Edge Preparation
In many cases, the preparation of the cutting edge (or edge passivation) of the insert has become the watershed that determines the success or failure of the machining process. The passivation process parameters need to be determined according to the specific processing requirements. For example, the edge passivation requirements of the insert used for high-speed finishing of steel parts are different from those of the insert used for roughing. Edge passivation can be applied to inserts for machining almost any type of carbon steel or alloy steel, but its application is somewhat limited to inserts for machining stainless steel and special alloys. The amount of passivation can be as small as 0.007mm or as large as 0.05mm. In order to enhance the cutting edge in harsh machining conditions, edge passivation can also form a tiny T-ribbed band.
In general, inserts used for continuous turning operations and milling of most steels and cast irons require a large degree of edge passivation. The amount of passivation depends on the carbide grade and the type of coating (CVD or PCD coating). For inserts with heavy interrupted cutting operations, heavy edge passivation or processing of T-ribbed bands has become a prerequisite. Depending on the type of coating, the amount of passivation can be close to 0.05mm.
In contrast, since inserts for machining stainless steel and high-temperature alloys are prone to built-up edge, the cutting edge is required to remain sharp and can only be slightly passivated (as little as 0.01mm), or even a smaller amount of passivation can be customized. Similarly, inserts for machining aluminum alloys also require sharp cutting edges. Spiral cutting edges can withstand greater cutting loads, achieve higher metal removal rates, and reduce stress. Another advantage of spiral cutting edges is that they can extend tool life due to reduced cutting pressure and cutting heat acting on the tool.
