1. Manifestations of tool damage
1) Micro-collapse of cutting edge
When the workpiece material structure, hardness, and allowance are uneven, the rake angle is too large, resulting in low cutting edge strength, the process system is not rigid enough to produce vibration, or intermittent cutting is performed, and the grinding quality is poor, the cutting edge is prone to micro-collapse, that is, small collapse, notch or peeling occurs in the cutting area. When this happens, the tool will lose some of its cutting ability, but it can continue to work. During continued cutting, the damaged part of the cutting area may expand rapidly, resulting in greater damage.
2) Cutting edge or tool tip collapse
This type of damage often occurs under more severe cutting conditions than those that cause micro-collapse of the cutting edge, or it is a further development of micro-collapse. The size and range of the collapse are larger than micro-collapse, causing the tool to completely lose its cutting ability and have to stop working. The collapse of the tool tip is often called tip drop.
3) Blade or tool breakage
When the cutting conditions are extremely bad, the cutting amount is too large, there is an impact load, there are microcracks in the blade or tool material, there are residual stresses in the blade due to welding and grinding, and factors such as careless operation may cause the blade or tool to break. After this form of damage occurs, the tool cannot be used anymore and is scrapped.
4) Blade surface peeling
For very brittle materials, such as cemented carbide, ceramics, PCBN, etc. with high TiC content, due to defects or potential cracks in the surface structure, or residual stress in the surface due to welding and grinding, it is very easy to produce surface peeling when the cutting process is not stable enough or the tool surface is subjected to alternating contact stress. Peeling may occur on the front blade face, and on the back blade face. The peeling material is flaky and the peeling area is large. Coated tools are more likely to peel. After slight peeling, the blade can continue to work, but it will lose its cutting ability after severe peeling.
5) Plastic deformation of cutting parts
Due to their low strength and hardness, tool steel and high-speed steel may undergo plastic deformation in their cutting parts. When cemented carbide works under high temperature and triaxial compressive stress, it will also produce surface plastic flow, and even cause the cutting edge or tip to undergo plastic deformation and collapse. Collapse generally occurs when the cutting amount is large and hard materials are processed. The elastic modulus of TiC-based cemented carbide is smaller than that of WC-based cemented carbide, so the former's ability to resist plastic deformation is accelerated, or it fails quickly. PCD and PCBN basically do not undergo plastic deformation.
6) Thermal cracking of blades
When the tool is subjected to alternating mechanical loads and thermal loads, the surface of the cutting part will inevitably produce alternating thermal stress due to repeated thermal expansion and contraction, which will inevitably cause fatigue and cracking of the blade. For example, when a cemented carbide milling cutter is milling at high speed, the teeth are constantly subjected to periodic impact and alternating thermal stress, and comb-like cracks are generated on the front face. Although some tools do not have obvious alternating loads and alternating stresses, thermal stress will be generated due to inconsistent temperatures between the surface and inner layers. In addition, there are inevitable defects inside the tool material, so the blade may also crack. After the crack is formed, the tool can sometimes continue to work for a period of time, and sometimes the crack expands rapidly, causing the blade to break or the blade surface to peel off severely.
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2. Tool wear
1. According to the cause of wear, it can be divided into:
1) Abrasive wear
There are often some tiny particles with extremely high hardness in the processed material, which can scratch grooves on the surface of the tool, which is abrasive wear. Abrasive wear exists on all surfaces, and the front cutting surface is the most obvious. Moreover, hemp wear can occur at various cutting speeds, but for low-speed cutting, due to the low cutting temperature, the wear caused by other reasons is not obvious, so abrasive wear is the main reason. In addition, the lower the hardness of the tool, the more serious the abrasive hemp wear.
2) Cold welding wear
During cutting, there is a lot of pressure and strong friction between the workpiece, the cutting and the front and rear cutting surfaces, so cold welding will occur. Due to the relative motion between the friction pairs, cold welding will cause cracks and be carried away by one side, thus causing cold welding wear. Cold welding wear is generally more serious at medium cutting speeds. According to experiments, brittle metals have stronger cold welding resistance than plastic metals; multiphase metals are less resistant to cold welding than unidirectional metals; metal compounds have less tendency to cold welding than single substances; and the cold welding tendency of group B elements and iron in the chemical periodic table is less. Cold welding is more serious when high-speed steel and cemented carbide are cut at low speeds.
3) Diffusion wear
During high-temperature cutting and contact between the workpiece and the tool, the chemical elements of both parties diffuse with each other in the solid state, changing the composition structure of the tool, making the surface of the tool fragile, and aggravating the wear of the tool. The diffusion phenomenon always maintains the continuous diffusion of objects with high depth gradients to objects with low depth gradients. For example, when the temperature of cemented carbide is 800℃, the cobalt in it will diffuse rapidly into the chips and workpieces, and WC will decompose into tungsten and carbon and diffuse into the steel; when the PCD tool is cutting steel and iron materials, when the cutting temperature is higher than 800℃, the carbon atoms in the PCD will be transferred to the surface of the workpiece with a large diffusion intensity to form a new alloy, and the surface of the tool will be graphitized. Cobalt and tungsten diffuse more seriously, and titanium, tantalum, and niobium have strong anti-diffusion ability. Therefore, YT cemented carbide has better wear resistance. When cutting ceramics and PCBN, when the temperature is as high as 1000℃-1300℃, the diffusion wear is not significant. Due to the same material, the workpiece, chips and tools will generate thermoelectric potential in the contact area during cutting. This thermoelectric potential has the effect of promoting diffusion and accelerating the wear of the tool. This diffusion wear under the action of thermoelectric potential is called "thermoelectric wear".
4) Oxidation wear
When the temperature rises, the surface of the tool is oxidized to produce softer oxides, which are rubbed by the chips and formed by oxidation wear. For example, at 700℃~800℃, oxygen in the air reacts with cobalt, carbide, titanium carbide, etc. in cemented carbide to form softer oxides; at 1000℃, PCBN reacts chemically with water vapor.
2. According to the wear form, it can be divided into:
1) Rake face wear
When cutting plastic materials at a high speed, the part of the rake face close to the cutting force will wear into a crescent shape under the action of the chips, so it is also called crescent wear. In the early stage of wear, the tool rake angle increases, which improves the cutting conditions and is conducive to the curling and breaking of the chips. However, when the crescent further increases, the strength of the cutting edge is greatly weakened, which may eventually cause the cutting edge to collapse and damage. When cutting brittle materials, or cutting plastic materials at a lower cutting speed and a thinner cutting thickness, crescent wear generally does not occur.
2) Tool tip wear
Tool tip wear is the wear on the back face of the tool tip arc and the adjacent secondary back face. It is a continuation of the wear on the back face of the tool. Due to the poor heat dissipation conditions and stress concentration here, the wear rate is faster than the back face. Sometimes a series of small grooves with a spacing equal to the feed amount will be formed on the secondary back face, which is called groove wear. They are mainly caused by the hardened layer and cutting lines of the machined surface. Groove wear is most likely to occur when cutting difficult-to-cut materials with a strong tendency to harden. Tool tip wear has the greatest impact on the surface roughness and machining accuracy of the workpiece.
3) Back face wear
When cutting plastic materials with a large cutting thickness, the back face of the tool may not contact the workpiece due to the presence of built-up edge. In addition, the back face usually contacts the workpiece, and a wear band with a back angle of 0 is formed on the back face. Generally, in the middle of the working length of the cutting edge, the wear of the back face is relatively uniform, so the wear degree of the back face can be measured by the width VB of the back face wear band of the cutting edge in this section. Since all types of tools will almost always experience flank wear under different cutting conditions, especially when cutting brittle materials or cutting plastic materials with a smaller cutting thickness, the wear of the tool is mainly flank wear, and the measurement of the wear band width VB is relatively simple, so VB is usually used to indicate the degree of tool wear. The larger the VB, the greater the cutting force and causes cutting vibration, and the wear at the arc of the tool tip, thereby affecting the processing accuracy and surface quality.
2. Methods to prevent tool breakage
1) According to the characteristics of the processed materials and parts, reasonably select the types and grades of tool materials. Under the premise of having a certain hardness and wear resistance, it is necessary to ensure that the tool material has the necessary toughness;
2) Reasonably select tool geometric parameters. By adjusting the front and rear angles, the main and secondary deflection angles, the blade inclination angles and other angles;
Ensure that the cutting edge and the tip have good strength. Grinding a negative chamfer on the cutting edge is an effective measure to prevent chipping;
3) Ensure the quality of welding and sharpening, and avoid various defects caused by poor welding and sharpening. The tools used in key processes should be ground to improve the surface quality and check for cracks;
4) Rationally select cutting parameters to avoid excessive cutting force and high cutting temperature to prevent tool damage;
5) Ensure that the process system has good rigidity and reduce vibration as much as possible;
6) Take correct operating methods to minimize or minimize sudden loads on the tool.
III. Causes and countermeasures of tool chipping
1) Improper selection of blade brand and specification, such as too thin blade thickness or too hard and brittle brand used for rough processing.
Countermeasures:
Increase the blade thickness or install the blade vertically, and select a brand with higher bending strength and toughness.
2) Improper selection of tool geometry parameters (such as too large front and rear angles, etc.).
Countermeasures:
The tool can be redesigned from the following aspects.
① Appropriately reduce the front and rear angles.
② Use a larger negative edge inclination angle.
③ Reduce the main deflection angle.
④ Use a larger negative chamfer or edge arc.
⑤ Grind the transition cutting edge to strengthen the blade tip.
3) The welding process of the blade is incorrect, resulting in excessive welding stress or welding cracks.
Countermeasures:
① Avoid using a blade slot structure that is closed on three sides.
② Select the correct solder.
③ Avoid using oxyacetylene flame to heat welding, and keep it warm after welding to eliminate internal stress.
④ Use a mechanical clamping structure as much as possible
4) Improper grinding method causes grinding stress and grinding cracks; the oscillation of the teeth of the PCBN milling cutter after grinding is too large, which makes individual teeth overloaded and also causes knife beating.
Countermeasures:
① Use intermittent grinding or diamond grinding wheel grinding.
② Use a softer grinding wheel and frequently dress it to keep the grinding wheel sharp.
③ Pay attention to the quality of grinding and strictly control the oscillation of the milling cutter teeth.
5) Improper selection of cutting parameters, such as excessive amount, which will cause the machine tool to stall; when cutting intermittently, the cutting speed is too high, the feed rate is too large, the blank allowance is uneven, and the cutting depth is too small; when cutting materials with a high tendency to work hardening such as high manganese steel, the feed rate is too small, etc.
Countermeasures:
Reselect cutting parameters.
6) Structural reasons such as uneven bottom surface of the tool groove of the mechanical clamping tool or excessive extension of the blade.
Countermeasures:
① Repair the bottom surface of the tool groove.
② Reasonably arrange the position of the cutting fluid nozzle.
③ Hardened tool bar adds carbide gaskets under the blade.
7) Excessive tool wear.
Countermeasures:
Change the tool or replace the cutting edge in time.
8) Insufficient cutting fluid flow or incorrect filling method causes the blade to heat up suddenly and crack.
Countermeasures:
① Increase the flow of cutting fluid.
② Reasonably arrange the position of the cutting fluid nozzle.
③ Use effective cooling methods such as spray cooling to improve the cooling effect.
④ Use * cutting to reduce the impact on the blade.
9) Improper tool installation, such as: the cut-off turning tool is installed too high or too low; the end mill uses asymmetric down milling, etc.
Countermeasures:
Reinstall the tool.
10) The rigidity of the process system is too poor, causing excessive cutting vibration.
Countermeasures:
① Increase the auxiliary support of the workpiece to improve the rigidity of the workpiece clamping.
② Reduce the overhang length of the tool.
③ Appropriately reduce the back angle of the tool.
④ Use other vibration elimination measures.
11) Careless operation, such as: when the tool cuts in from the middle of the workpiece, the action is too violent; the tool is stopped before retracting.
Countermeasures:
Pay attention to the operation method.
IV. Built-up edge
1) Causes of formation
In the part near the cutting edge, in the tool-chip contact area, due to the large downward pressure, the metal of the bottom layer of the chip is embedded in the microscopic uneven peaks and valleys on the front cutting edge, forming a gapless real metal-to-metal contact and producing a bonding phenomenon. This part of the tool-chip contact area is called the bonding area. In the bonding area, a thin layer of metal material will be accumulated on the front cutting edge of the chip bottom layer. The metal material of this part of the chip has undergone severe deformation and is strengthened at an appropriate cutting temperature. With the continuous outflow of the chips, under the push of the subsequent cutting flow, this layer of accumulated material will slide relative to the upper layer of the chips and leave, becoming the basis of the built-up edge. Subsequently, a second layer of accumulated cutting material will form on it, and this continuous accumulation will form a built-up edge.
2) Characteristics and influence on cutting
① The hardness is 1.5~2.0 times higher than that of the workpiece material. It can replace the front cutting face for cutting, which has the effect of protecting the cutting edge and reducing the wear of the front cutting face. However, when the built-up edge falls off, the fragments flowing through the tool-workpiece contact area will cause wear on the tool back cutting face.
② After the built-up edge is formed, the working front angle of the tool increases significantly, which plays a positive role in reducing chip deformation and reducing cutting force.
③ Since the built-up edge protrudes beyond the cutting edge, the actual cutting depth increases, affecting the dimensional accuracy of the workpiece.
④ The built-up edge will cause "plowing" phenomenon on the workpiece surface, affecting the surface roughness of the workpiece. ⑤ The fragments of the built-up edge will adhere or embed on the workpiece surface to form hard points, affecting the quality of the machined surface of the workpiece.
From the above analysis, it can be seen that the built-up edge is not conducive to cutting, especially for finishing.
3) Control measures
The formation of built-up edge can be avoided by preventing the chip bottom material from adhering to or deforming the front cutting face. The following measures can be taken today.
① Reduce the roughness of the front cutting edge.
② Increase the front angle of the tool.
③ Reduce the cutting thickness.
④ Use low-speed cutting or high-speed cutting to avoid cutting speeds that are prone to forming built-up edge.
⑤ Perform appropriate heat treatment on the workpiece material to increase its hardness and reduce plasticity.
⑥ Use cutting fluid with good anti-adhesion performance (such as extreme pressure cutting fluid containing sulfur and chlorine).
