For machining centers, the cutting tool is a consumable tool, which will cause breakage, wear and chipping during the machining process. These phenomena are inevitable, but there are also controllable reasons such as unscientific and unstandardized operation and improper maintenance. Only by finding the root cause can we better solve the problem.
01
Symptoms of tool breakage
(1) The cutting edge is slightly chipped
When the workpiece material structure, hardness, and margin 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 sharpening quality is poor, the cutting edge is prone to chipping. That is, there are tiny chips, chips or peeling in the blade area. When this happens, the tool will lose part of its cutting ability, but it can still continue to work. As cutting continues, the damaged part of the edge area may expand rapidly, leading to greater damage.
(2) The cutting edge or tip is broken
This type of damage often occurs under cutting conditions that are more severe than those that cause micro chipping of the cutting edge, or is the further development of micro chipping. The size and range of chipping are larger than micro chipping, causing the tool to completely lose its cutting ability and have to terminate the work. The chipping of the knife tip is often called tip loss.
(3) The blade or tool is broken
When the cutting conditions are extremely harsh, the cutting amount is too large, there is impact load, there are micro-cracks in the blade or tool material, there are residual stresses in the blade due to welding and sharpening, and factors such as careless operation, the blade or tool may be damaged. Produces breakage. After this form of damage occurs, the tool cannot continue to be used and will be scrapped.
(4) The surface of the blade peels off
For materials with high brittleness, 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, during the cutting process It is easy to cause surface peeling when the tool surface is not stable enough or when it is subjected to alternating contact stress. The peeling may occur on the rake surface, and the knife may occur on the flank surface. The peeling material is flaky and the peeling area is large. Coated tools are more likely to peel off. After the blade is slightly peeled off, it can still continue to work, but after serious peeling, it will lose its cutting ability.
(5) Plastic deformation of cutting parts
Due to their low strength and low hardness, tool steel and high-speed steel may undergo plastic deformation in their cutting parts. When cemented carbide operates under high temperature and three-dimensional compressive stress, surface plastic flow will also occur, which may even cause plastic deformation of the cutting edge or tip to cause collapse. Collapse generally occurs when cutting volume 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 the blade
When the tool is subjected to alternating mechanical and thermal loads, the surface of the cutting part will inevitably generate alternating thermal stress due to repeated thermal expansion and contraction, which will cause fatigue and cracking of the blade. For example, when a carbide milling cutter performs high-speed milling, the cutter teeth are constantly subjected to periodic impacts and alternating thermal stresses, resulting in comb-shaped cracks on the rake face. Although some tools do not have obvious alternating loads and stresses, thermal stress will also occur due to inconsistent temperatures between the surface and inner layers. In addition, there are inevitable defects within the tool material, so the blade may also develop cracks. Sometimes the tool can continue to work for a period of time after the crack forms, and sometimes the crack expands rapidly causing the blade to break or the blade surface to peel off severely.
02
Causes of tool wear
(1) Abrasive wear
There are often tiny particles with extremely high hardness in the material being processed, which can draw grooves on the surface of the tool. This is abrasive wear. Abrasive wear exists on all surfaces, and is most obvious on the rake surface. Moreover, abrasive 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 damage will be.
(2) Cold welding wear
During cutting, there is a lot of pressure and strong friction between the workpiece, cutting and front and rear blade surfaces, so cold welding will occur. Due to the relative movement between the friction pairs, cold welding will cause cracks and be taken away by one party, resulting in cold welding wear. Cold welding wear is generally more serious at medium cutting speeds. According to experiments, brittle metals are more resistant to cold welding than plastic metals; multiphase metals are less resistant to cold welding than unidirectional metals; metal compounds are less prone to cold welding than elemental elements; group B elements and iron in the periodic table of chemical elements are less prone to cold welding. Cold welding is more serious during low-speed cutting of high-speed steel and cemented carbide.
(3) Diffusion wear
During the process of cutting at high temperatures and the contact between the workpiece and the tool, the chemical elements on both sides diffuse into each other in the solid state, changing the composition and 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 gradient to objects with low depth gradient.
For example, when the temperature of cemented carbide is 800℃, the cobalt in it will quickly diffuse into the chips and workpieces, and WC will decompose into tungsten and carbon and diffuse into the steel; when PCD tools are cutting steel and iron materials, when the cutting temperature is higher than 800℃ At this time, the carbon atoms in PCD will be transferred to the workpiece surface with great diffusion intensity to form a new alloy, and the tool surface will be graphitized. Cobalt and tungsten diffuse more seriously, while titanium, tantalum and niobium have strong anti-diffusion capabilities. Therefore, YT carbide has better wear resistance. When cutting ceramics and PCBN, diffusion wear is not significant when the temperature is as high as 1000℃~1300℃. Since the workpiece, chips and tool are made of the same material, a thermoelectric potential will be generated in the contact area during cutting. This thermoelectric potential promotes diffusion and accelerates tool wear. This kind of diffusion wear under the action of thermoelectric potential is called "thermoelectric wear".
(4) Oxidative wear
When the temperature rises, the tool surface is oxidized to produce softer oxides that are rubbed by chips and cause wear, which is called oxidative wear. For example: at 700℃~800℃, the oxygen in the air reacts with cobalt, carbide, titanium carbide, etc. in the cemented carbide to form a soft oxide; at 1000℃, PCBN reacts chemically with water vapor.
03
Blade wear patterns
(1) Rake face damage
When cutting plastic materials at a high speed, the parts on the rake surface close to the cutting force will wear into a crescent shape under the action of chips, so it is also called crater wear. In the early stage of wear, the rake angle of the tool increases, which improves the cutting conditions and is conducive to the curling and breaking of chips. However, when the craters further increase, the strength of the cutting edge is greatly weakened, which may eventually cause the cutting edge to be broken and damaged. Case. When cutting brittle materials, or cutting plastic materials at lower cutting speeds and thinner cutting thicknesses, crater wear generally does not occur.
(2) Tool tip wear
Tool tip wear is the wear on the arc flank surface of the tool tip and the adjacent secondary flank surface. It is a continuation of the wear on the flank surface of the tool. Due to poor heat dissipation conditions and concentrated stress, the wear rate is faster than that of the flank surface. Sometimes a series of small grooves with a spacing equal to the feed amount are formed on the secondary flank surface, which is called groove wear. They are mainly caused by the hardened layer and cutting lines on the machined surface. Groove wear is most likely to occur when cutting difficult-to-cut materials with a high tendency to work harden. Tool tip wear has the greatest impact on the surface roughness and machining accuracy of the workpiece.
(3) Flank surface wear
When cutting plastic materials at large cutting thicknesses, the flank face of the tool may not be in contact with the workpiece due to the presence of built-up edge. In addition, the flank surface usually comes into contact with the workpiece, forming a wear zone on the flank surface. Generally, in the middle of the working length of the cutting edge, the flank wear is relatively uniform, so the degree of flank wear can be measured by the flank wear band width VB of this section of the cutting edge.
Since various types of tools almost always undergo flank wear under different cutting conditions, especially when cutting brittle materials or cutting plastic materials with a small cutting thickness, the tool wear is mainly flank wear, and the wear band The measurement of width VB is relatively simple, so VB is usually used to indicate the degree of tool wear. The larger the VB, not only will the cutting force increase and cause cutting vibration, but it will also affect the wear at the tool tip arc, thus affecting the machining accuracy and machined surface quality.
04
How to prevent tool breakage
(1) According to the characteristics of the materials and parts to be processed, rationally select the materials and grades of various types of cutting tools. On the premise of having a certain hardness and wear resistance, the tool material must have the necessary toughness.
(2) Reasonably select tool geometric parameters. By adjusting the front and rear angles, main and auxiliary deflection angles, edge inclination angles and other angles, the cutting edge and tool tip are ensured to have good strength. Grinding a negative chamfer on the cutting edge is an effective measure to prevent tool collapse.
(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 surface quality and checked for cracks.
(4) Choose the cutting amount reasonably to avoid excessive cutting force and high cutting temperature to prevent tool damage.
(5) Try to ensure that the process system has good rigidity and reduce vibration.
(6) Adopt correct operating methods and try to prevent the tool from bearing sudden loads or less.
05
Causes and countermeasures for tool chipping
(1) Improper selection of blade grade and specifications, such as the thickness of the blade is too thin or a grade that is too hard and too brittle is selected during rough machining.
Countermeasures: Increase the blade thickness or install the blade vertically, and choose a grade with higher bending strength and toughness.
(2) Improper selection of tool geometric parameters (such as too large front and rear angles, etc.).
Countermeasures: You can redesign the tool from the following aspects.
1) Appropriately reduce the front and rear angles;
2) Use a larger negative edge angle;
3) Reduce the main deflection angle;
4) Use a larger negative chamfer or edge arc;
5) Grind the transition cutting edge and strengthen the tool tip.
(3) The welding process of the blade is incorrect, causing excessive welding stress or welding cracks.
Countermeasures:
1) Avoid using a blade slot structure that is closed on three sides;
2) Correctly select solder;
3) Avoid using oxy-acetylene flame for heating and welding, and keep it warm after welding to eliminate internal stress;
4) Use mechanical clamping structures as much as possible.
(4) Improper sharpening method will cause grinding stress and grinding cracks; after sharpening the PCBN milling cutter, the vibration of the teeth will be too large, making individual teeth overloaded, which will also cause knife breakage.
Countermeasures:
1) Use interrupted grinding or diamond grinding wheel grinding;
2) Choose a softer grinding wheel and trim it frequently to keep the grinding wheel sharp;
3) Pay attention to the sharpening quality and strictly control the vibration amount of the milling cutter teeth.
(5) The selection of cutting amount is unreasonable. If the amount is too large, the machine tool will be boring; during intermittent cutting, the cutting speed is too high, the feed amount is too large, and the blank margin is uneven, the cutting depth is too small; cutting high manganese When using materials with a high tendency to work harden, such as steel, the feed amount is too small, etc.
Countermeasure: Re-select the cutting amount.
(6) Structural reasons such as the uneven bottom surface of the tool groove of the mechanically clamped tool or the blade extending too long.
Countermeasures:
1) Trim the bottom surface of the tool groove;
2) Reasonably arrange the position of the cutting fluid nozzle;
3) The hardened tool holder adds a carbide gasket 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 may cause sudden heat and cracking of the blade.
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) Reduce the impact on the blade.
(9) The tool is installed incorrectly, such as: the cutting tool is installed too high or too low; the end milling cutter uses asymmetrical climb milling, etc.
Countermeasure: 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 and improve the rigidity of the workpiece clamping;
2) Reduce the overhang length of the tool;
3) Appropriately reduce the clearance angle of the tool;
4) Use other vibration-absorbing measures.
(11) Careless operation, such as: when the tool cuts into the middle of the workpiece, the tool moves too sharply; the tool stops before retracting the tool.
Countermeasures: Pay attention to the operation method.
06
Causes, characteristics and control measures of built-up edge
(1) Causes of formation
In the part close to the cutting edge, in the contact area between the tool and the chip, due to the large downward pressure, the underlying metal of the chip is embedded in the microscopically uneven peaks and valleys on the rake surface, forming a true metal-to-metal contact without gaps and causing bonding. , this part of the contact area between the knife and the chip is called the bonding area. In the bonding zone, a thin layer of metal material will be accumulated on the bottom layer of the chip on the rake face. The metal material of this part of the chip has undergone severe deformation and is strengthened under appropriate cutting temperatures. As the chips continue to flow out, under the subsequent cutting action, this layer of stagnant material will slip relative to the upper layer of the chips and separate, becoming the basis of built-up edge. Subsequently, a second layer of accumulated cutting material will form on top of it, and this layer of continuous accumulation will form a built-up edge.
(2) Characteristics and impact on cutting processing
1) The hardness is 1.5~2.0 times higher than the workpiece material. It can replace the rake face for cutting. It has the function of protecting the cutting edge and reducing the wear of the rake face. However, when the built-up edge falls off, the debris flowing through the contact area between the tool and the workpiece will Causes tool flank wear;
2) After the formation of built-up edge, the working rake angle of the tool increases significantly, which plays a positive role in reducing chip deformation and cutting force;
3) Because the built-up edge protrudes beyond the cutting edge, the actual cutting depth increases and affects the dimensional accuracy of the workpiece;
4) Built-up edge will cause "furrows" on the surface of the workpiece and affect the surface roughness of the workpiece;
5) The fragments of built-up edge will bond or embed into the surface of the workpiece to form hard spots, affecting the quality of the machined surface of the workpiece.
It can be seen from the above analysis that built-up edge is detrimental to cutting processing, especially finishing.
(3) Control measures
The generation of built-up edge can be avoided by not bonding or deforming the underlying material of the chip to the rake surface. Therefore, the following measures can be taken.
1) Reduce the roughness of the rake surface;
2) Increase the rake angle of the tool;
3) Reduce cutting thickness;
4) Use low-speed cutting or high-speed cutting to avoid cutting speeds that easily form built-up edge;
5) Proper heat treatment of the workpiece material to increase its hardness and reduce plasticity;
6) Use cutting fluids with good anti-adhesion properties (such as extreme pressure cutting fluids containing sulfur and chlorine).
