For the machining center, the tool is a consumable tool, which will be damaged, worn, chipped and so on during the machining process. These phenomena are inevitable, but there are also controllable reasons such as unscientific and irregular operation and improper maintenance. Only by finding the root cause can we better solve the problem.
01
Symptoms of tool breakage
1) Chipping of the cutting edge
When the workpiece material structure, hardness, and margin are uneven, the rake angle is too large, resulting in low cutting edge strength, insufficient rigidity of the process system to generate vibration, or intermittent cutting, poor grinding quality, the cutting edge is prone to chipping, That is, small chipping, nicks or peeling appear in the edge area. When this happens, the tool will lose some of its cutting ability, but will continue to work. As the cutting continues, the damaged part of the edge area may expand rapidly, resulting in greater damage.
2) Chipping of the cutting edge or tip
This type of damage often occurs under harsher cutting conditions than the chipping of the cutting edge, or is the further development of chipping. The size and scope of the chipping are larger than the chipping, so that the tool completely loses its cutting ability and has to stop working. Chipping of the tip is often referred to as point drop.
3) The blade or knife is broken
When the cutting conditions are extremely harsh, the cutting amount is too large, there is an impact load, there are microcracks in the blade or tool material, there is residual stress in the blade due to welding and sharpening, and factors such as careless operation may cause damage to the blade or tool. break off. After this form of damage occurs, the tool cannot continue to be used, so that it is scrapped.
4) The surface layer of the blade peels off
For materials with high brittleness, such as hard alloys with high TiC content, ceramics, PCBN, etc., due to defects or potential cracks in the surface structure, or residual stress on the surface due to welding and sharpening, during the cutting process It is easy to peel off the surface layer when it is not stable enough or the tool surface is subjected to alternating contact stress. The peeling may occur on the rake face, and the knife may occur on the flank face. The peeling is in the form of flakes and the peeling area is relatively large. Coated tools are more likely to flake off. After the blade is slightly peeled off, it can continue to work, but after severe peeling off, it will lose its cutting ability.
5) Plastic deformation of cutting parts
Due to the low strength and low hardness of tool steel and high-speed steel, plastic deformation may occur in the cutting part. When the cemented carbide works directly at high temperature and in a state of three-dimensional compressive stress, it will also produce plastic flow on the surface, and even cause plastic deformation of the cutting edge or tip to cause collapse. Collapse generally occurs when the amount of cutting is large and when processing hard materials. 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 rapidly. 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 the blade to fatigue and crack. For example, when the cemented carbide milling cutter is used for high-speed milling, the cutter teeth are constantly subjected to periodic impact and alternating thermal stress, and comb-shaped cracks are generated on the rake face. Although some tools do not have obvious alternating load and alternating stress, thermal stress will also be generated due to the inconsistent temperature of the surface layer and the inner layer. In addition, there are inevitably defects inside the tool material, so the blade may also crack. The tool can sometimes continue to work for a period of time after the crack is formed, and sometimes the crack expands rapidly and causes the blade to break or the blade surface to peel off severely.
02
Causes of Tool Wear
1) Abrasive wear
There are often some tiny particles with extremely high hardness in the processed material, which can draw grooves on the surface of the tool, which is abrasive abrasive wear. Abrasive wear exists on all surfaces, most obviously on the rake face. 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 damage.
2) Cold welding wear
When cutting, there is a lot of pressure and strong friction between the workpiece, the cutting and the front and rear cutter faces, so cold welding will occur. Due to the relative movement between the friction pairs, the cold welding will produce cracks and be taken away by one side, resulting in cold welding wear. Cold welding wear is generally severe at moderate cutting speeds. According to experiments, brittle metals have stronger resistance to cold welding than plastic metals; multi-phase metals are smaller than unidirectional metals; metal compounds have a lower tendency to cold welding than simple substances; B group elements and iron in the periodic table of chemical elements have a smaller tendency to cold welding. Cold welding is more serious when high-speed steel and cemented carbide are cut at low speed.
3) Diffusion wear
During cutting at high temperature and contact between the workpiece and the tool, the chemical elements on both sides diffuse 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. Diffusion phenomenon always maintains the continuous diffusion of objects with high depth gradient to objects with low depth gradient.
For example, when the cemented carbide is at 800°C, 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 the cutting temperature of PCD tools is higher than 800°C when cutting steel and iron materials At this time, the carbon atoms in 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. The diffusion of cobalt and tungsten is relatively serious, and the anti-diffusion ability of titanium, tantalum and niobium is relatively strong. Therefore, YT cemented carbide has better wear resistance. When cutting ceramics and PCBN, when the temperature is as high as 1000°C-1300°C, the diffusion wear is not significant. Due to the different materials of the workpiece, chip and tool, a thermoelectric potential will be generated in the contact area during cutting. This thermoelectric potential can promote diffusion and accelerate the wear of the tool. This kind of 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 that are rubbed by chips, which is called oxidative wear. For example: at 700°C~800°C, oxygen in the air reacts with cobalt, carbide, titanium carbide, etc. in cemented carbide to form soft oxides; at 1000°C, PCBN reacts chemically with water vapor.
03
Blade wear pattern
1) Rake face damage
When cutting plastic materials at a high speed, the part of the rake face close to the cutting force will wear into a crescent concave 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 crescent crater further increases, the strength of the cutting edge is greatly weakened, which may eventually cause the cutting edge to break. 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 nose wear is the wear of the flank of the tool nose arc and the adjacent secondary flank, which is the continuation of the wear of the upper flank of the tool. Due to the poor heat dissipation conditions and concentrated stress here, the wear speed is faster than that of the flank, and sometimes a series of small grooves with a distance equal to the feed amount are formed on the auxiliary flank, which is called groove wear. They are mainly due to the hardened layer and cutting lines on the machined surface. When cutting difficult-to-cut materials with a high tendency to work hardening, groove wear is most likely to be caused. Tool tip wear has the greatest impact on workpiece surface roughness and machining accuracy.
3) flank wear
When cutting plastic materials at large cutting thicknesses, the flank of the tool may not be in contact with the workpiece due to the presence of built-up edge. In addition, usually the flank will come into contact with the workpiece, and a wear zone with a relief angle of 0 is formed on the flank. 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 zone width VB of the cutting edge.
Because various types of tools almost always have flank wear under different cutting conditions, especially when cutting brittle materials or cutting plastic materials with a small cutting thickness, the wear of the tool is mainly flank wear, and the wear zone The measurement of the width VB is relatively simple, so VB is usually used to indicate the degree of tool wear. The larger the VB, not only will increase the cutting force and cause cutting vibration, but also affect the wear at the arc of the tool tip, thereby affecting the machining accuracy and surface quality.
04
How to prevent breakage of knives
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 the geometric parameters of the tool. By adjusting the front and back angles, the main and auxiliary deflection angles, and the blade inclination angles, etc., it is possible to ensure that the cutting edge and tool tip have better 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 knives used in the key process should be ground to improve the surface quality and check for cracks.
4) Reasonably select the cutting amount to avoid excessive cutting force and high cutting temperature to prevent tool damage.
5) As far as possible, ensure that the process system has better rigidity and reduce vibration.
6) Take the correct operation method, and try to make the tool not bear or bear the sudden change load as much as possible.
05
Causes and countermeasures of tool chipping
1. Improper selection of the grade and specification of the blade, such as the thickness of the blade is too thin or the grade that is too hard and too brittle is selected for rough machining.
Countermeasures: increase the thickness of the blade or install the blade vertically, and choose a grade with higher bending strength and toughness.
2. Improper choice of tool geometry parameters (such as too large front and rear angles, etc.).
Countermeasures:
You can start to redesign the tool from the following aspects.
1) Appropriately reduce the front and rear angles.
2) Use a larger negative edge inclination.
3) Reduce the entering angle.
4) Use a larger negative chamfer or edge arc.
5) Grinding the transitional cutting edge to enhance the tip.
3) The welding process of the blade is incorrect, resulting in excessive welding stress or welding cracks.
Countermeasures:
1) Avoid adopting a three-sided closed blade groove structure.
2) Correct selection of solder.
3) Avoid using oxyacetylene flame heating welding, and keep warm after welding to eliminate internal stress.
4) Use mechanical clamping structure 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 cutting teeth is too large, which makes the load of individual cutting teeth too heavy, and will also cause cutting.
Countermeasures:
1) Grinding with intermittent grinding or diamond grinding wheel.
2) Choose a softer grinding wheel, and often dress to keep the grinding wheel sharp.
3) Pay attention to the sharpening quality and strictly control the vibration of the milling cutter teeth.
5. The choice of cutting amount is unreasonable. If the amount is too large, the machine tool will be boring; when cutting intermittently, the cutting speed is too high, the feed rate is too large, and when the blank allowance is uneven, the cutting depth is too small; cutting high manganese steel For materials with a large tendency to work hardening, the feed rate is too small.
Countermeasure: Reselect the cutting amount.
6. Structural reasons such as the bottom surface of the groove of the mechanical clamping tool is uneven or the blade protrudes too long.
Countermeasures:
1) Trim the bottom surface of the sipe.
2) Reasonably arrange the position of the cutting fluid nozzle.
3) The hardened shank 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 rate or incorrect filling method will cause sudden heat and crack damage of the blade.
Countermeasures:
1) Increase the flow rate 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) Adopt high-speed cutting to 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 adopts asymmetrical down milling, etc.
Countermeasure: Reinstall the tool.
10. The rigidity of the process system is too poor, resulting in excessive cutting vibration.
Countermeasures:
1) Increase the auxiliary support of the workpiece to improve the clamping rigidity of the workpiece.
2) Reduce the overhang length of the tool.
3) Properly reduce the back angle of the tool.
4) Adopt other damping measures.
11. Inadvertent operation, such as: when the tool cuts in from the middle of the workpiece, the action is too violent; before the tool is retracted, stop immediately.
Countermeasures: Pay attention to the operation method.
06
Causes, characteristics and control measures of built-up edge
1. Causes
In the part close to the cutting edge, in the tool-chip contact area, due to the large down force, the underlying metal of the chip is embedded in the microscopic uneven peaks and valleys on the rake face, forming a real metal-to-metal contact without gaps and causing bonding. , this part of the knife-chip contact area is called the bonding area. In the bonding zone, there will be a thin layer of metal material deposited on the rake face at the bottom of the chip. The metal material of this part of the chip has undergone severe deformation and will be strengthened at an appropriate cutting temperature. With the continuous flow of chips, under the push of the flow of subsequent cutting, this layer of stagnation material will slip relative to the upper layer of chips and leave, becoming the basis of built-up edge. Subsequently, a second layer of stagnant cutting material will be formed on it, and this continuous layering will form a built-up edge.
2. Characteristics and influence on cutting process
1) The hardness is 1.5~2.0 times higher than that of the workpiece material. It can replace the rake face for cutting, and has the effect of protecting the cutting edge and reducing the wear of the rake face. However, when the built-up edge falls off, the debris flows through the tool-workpiece contact area. Cause tool flank wear.
2) After the built-up edge is formed, the working rake angle of the tool increases significantly, which plays a positive role in reducing chip deformation and cutting force.
3) Since the built-up edge protrudes beyond the cutting edge, the actual cutting depth increases, which affects the dimensional accuracy of the workpiece.
4) Built-up edge will cause a "furrow" phenomenon on the surface of the workpiece, which will affect the surface roughness of the workpiece.
5) Fragments of built-up edge will bond or embed into the surface of the workpiece to cause hard spots, which will affect the quality of the processed surface of the workpiece.
From the above analysis, it can be seen that built-up edge is not good for cutting, especially for finishing.
3. Control measures
The generation of built-up edge can be avoided by not bonding or deforming and strengthening the bottom material of the chip and the rake face. For this day, the following measures can be taken.
1) Reduce the roughness of the rake face.
2) Increase the rake angle of the tool.
3) Reduce the cutting thickness.
4) Use low-speed cutting or high-speed cutting to avoid the cutting speed that is easy to form built-up edge.
5) Carry out proper heat treatment on the workpiece material to increase its hardness and reduce plasticity.
6) Use cutting fluid with good anti-bonding properties (such as extreme pressure cutting fluid containing sulfur and chlorine).
