Basics About Tool Wear And Tool Life In Metal Cutting

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The tool will wear gradually in the cutting process.

When the tool is worn to a certain extent, the cutting force increases rapidly, the cutting temperature increases sharply, and even vibration occurs.

At the same time, the dimensional accuracy of the workpiece decreases and the machined surface deteriorates obviously.

At this time, sharpen or change the knife.

Tool wear and tool life are related to the efficiency, quality and cost of cutting, so it is one of the most important problems in cutting.

（1） Form of tool wear

In the cutting process, the rake face and flank often contact with chips and workpieces, and there is strong friction in the contact area.

At the same time, there is high temperature and pressure in the contact area.

Therefore, the rake face and rake face will gradually wear with the progress of cutting, as shown in Fig. 1-35.

Fig. 1-35 tool wear

• a) Tool wear pattern
• b) Measuring position of tool wear

1. The rake face is worn

When cutting plastic materials, if the cutting speed and cutting thickness are large, crescent pits will be formed on the rake face of the tool.

It starts with the position of the highest cutting temperature as the center, and then gradually expands forward and backward, and the depth increases continuously.

When the crescent depression develops to the edge between its leading edge and the cutting edge becomes very narrow, the strength of the cutting edge decreases, which is easy to lead to the damage of the cutting edge.

The crater wear value of the rake face is expressed by its maximum depth.

2. Rear tool face wear

In fact, the contact area between the flank and the workpiece surface is very small, the contact pressure is very large, and there is elastic and plastic deformation.

Therefore, wear occurs on this contact surface.

This kind of wear mainly occurs when cutting cast iron and plastic materials with small cutting thickness.

The width of the flank wear band is often uneven and can be divided into three areas:

(1) The tool tip in Zone C is worn.

The strength is low, the heat dissipation condition is poor, and the wear is serious. The maximum value is VC.

(2) The boundary of zone N is worn.

When cutting steel, the main cutting edge is close to the back cutting surface (zone n) of the surface to be machined, which is ground into a deep groove, expressed in VN.

This is mainly caused by the work hardening layer of the workpiece at the boundary and the large stress gradient and temperature gradient of the tool at the boundary.

(3) Wear in the middle of zone B.

In the middle part of the flank wear belt, the wear is relatively uniform, and its average width is expressed in VB, while its maximum width is expressed in VBmax.

（2） Causes of tool wear

Tool wear is different from the wear of general mechanical parts.

Because the chip bottom surface in contact with the tool surface is a fresh surface with high activity, the contact pressure on the tool surface is very large (up to 2 ~ 3GPa) and the contact temperature is very high (such as cemented carbide processing steel, up to more than 800 ~ 1000 ℃), the tool wear has mechanical, thermal and chemical effects, including the wear caused by the hard engraving of the workpiece material, as well as the wear caused by bonding, diffusion and corrosion.

1. Abrasive wear

Although the hardness of chips and workpieces is lower than that of the tool, they often contain some tiny hard points with extremely high hardness, which can draw grooves on the tool surface, which is abrasive wear, as shown in Fig. 1-36.

Fig. 1-36 abrasive wear

2. Adhesive wear

In the cutting process, when the friction surface between the tool and the workpiece material has the conditions of high temperature, high pressure and fresh surface, and the contact surface reaches the distance between atoms, adsorption bonding will occur, also known as cold welding (see Fig. 1-37).

The degree of wear mainly depends on the affinity and hardness ratio between workpiece material and tool material, cutting temperature, pressure and lubrication conditions.

Adhesive wear is the main cause of cemented carbide tool wear when machining difficult materials with heavy load and low speed.

Fig. 1-37 adhesive wear

3. Diffusion wear

When the cutting temperature is very high, in the process of contact between chip, workpiece and tool, the chemical elements of both sides diffuse each other in the solid state, which changes the original material composition and structure, makes the tool material fragile, and intensifies the wear of tool.

Diffusion wear is one of the main causes of cutting wear of cemented carbide tools at high temperature (800 ~ 1000 ℃).

Generally, starting from 800 ℃, Co, C, W and other elements in the cemented carbide will diffuse into the chips and be taken away.

At the same time, Fe in the chips will also diffuse into the cemented carbide, reducing the hardness and strength of the tool surface, increasing brittleness and aggravating wear (see Fig. 1-38).

The diffusion rate of different elements is different.

For example, the diffusion rate of Ti is much lower than that of C, Co, W and other elements, so the diffusion resistance of p-type cemented carbide is stronger than that of K-type cemented carbide.

Fig. 1-38 schematic diagram of diffusion wear

4. Oxidation wear

When the cutting temperature is 700 ~ 800 ℃, the oxygen in the air oxidizes with cobalt, tungsten carbide and titanium carbide in cemented carbide to form loose and fragile oxides, which are then wiped off by chips or workpieces to form wear, accelerating tool wear (see Fig. 1-39).

Fig. 1-39 oxidation wear

（3） Tool blunt standard

1. Tool wear process

As shown in Fig. 1-40, the tool wear process can be divided into three stages:

Fig. 1-40 tool wear process

(1) Initial wear stage.

The slope of the wear curve is large.

Because the cutting edge of the new tool is very sharp, the actual contact area between the flank and the machining surface is very small and the pressure is very high, so it wears out very quickly.

(2) Normal wear stage.

After the initial wear, the rough surface of the tool has been ground flat, the defects are reduced, the contact area between the tool flank and the machined surface becomes larger, the pressure decreases, and enters a relatively slow normal wear stage.

The wear of the flank increases approximately in proportion to the cutting time.

During normal cutting, this stage takes a long time, which is the effective working period of the tool.

(3) Sharp wear stage.

When the wear zone of the tool reaches a certain degree, the friction between the tool surface and the workpiece is too large, resulting in the rapid increase of cutting force and cutting temperature, resulting in large cutting force.

If the cutting tool continues to work, not only the machining quality and accuracy cannot be guaranteed, but also the cutting efficiency will be reduced and the wear will increase sharply.

The cutting tool should be changed in time before the sharp wear occurs.

2. Blunt standard of cutting tools

When the tool is worn to a certain limit, it can not be used anymore.

This wear limit is called the blunt standard. Because the uniform wear of the flank may occur in most cutting cases.

In addition, VB value is easy to measure and control, so VB value is often used to study the wear process as the standard to measure the blunt of tools.

The ISO standard uniformly stipulates that the wear band width VB measured on the flank at 1 / 2 back cutting depth is used as the tool blunt standard.

For finishing tools in automatic production, the tool wear size along the radial direction of the workpiece is often used as the tool blunt standard, which is called radial wear NB.

The national standard GB / T16461-1996 stipulates the blunt standards of high-speed steel tools and cemented carbide tools, as shown in table 1-6.

Table 1-6 grinding standards of high speed steel cutting tools and cemented carbide cutting tools

Table 20-3 common blade wear and improvement measures

（4） Tool life

In practice, in order to judge the tool wear more conveniently, accurately and intuitively, the tool life is generally used to indirectly reflect the tool blunt.

The cutting time of the sharpened tool from the beginning of cutting until the wear reaches the blunt standard is called the tool life, and the unit is min.

Tool life reflects the speed of tool wear.

Long tool life indicates that the tool wear speed is slow;

On the contrary, it indicates that the tool wear speed is fast.

The factors affecting cutting temperature and tool wear also affect tool life.

The influence of cutting parameters on tool life is obvious.

Through cutting experiments, we can get the relationship between vc, f and ap on tool life T:

When cutting carbon steel with Rm = 0.637GPa (f > 0.7mm / r) with P40 cemented carbide turning tool, the relationship between cutting parameters and tool life is as follows:

It can be seen from the above formula that the cutting speed has the greatest impact on the tool life, followed by the feed rate and the back cutting depth.

According to the experience in actual production, the cutting speed increases by 20% and the blade wear increases by 50%;

The feed rate increases by 20%, and the blade wear increases by 20%;

The back feed increases by 50% and the blade wear increases by 20% (see fig. 1-41).

This is completely consistent with the influence order of the three on the cutting temperature, which reflects that the cutting temperature has a great influence on the tool life.

Fig. 1-41 relationship between cutting parameters and tool life

Tool life is an important parameter with many uses.

It can be used to determine the tool change time, measure the machinability of workpiece materials and the cutting performance of tool materials, and judge whether the selection of tool geometric parameters and cutting parameters is reasonable.

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