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Is CNC tool broken, worn or chipped? And its solution (cnc machining)

Feb 23,21

Is CNC tool broken, worn or chipped? And its solution (cnc machining

1、 Tool breakage
1. Performance of tool breakage
1) Micro chipping of cutting edge
When the material structure, hardness and allowance of the workpiece are not uniform, and the rake angle is too large, the cutting edge strength is low, the rigidity of the process system is insufficient, vibration occurs, or intermittent cutting is carried out, and the grinding quality is poor, the cutting edge is prone to micro collapse, that is, micro collapse, notch or peeling occurs in the edge area. When this happens, the tool will lose part of its cutting ability, but it can continue to work. In the process of continuous cutting, the damaged part of the edge area may expand rapidly, leading to greater damage.
2) Chipping of cutting edge or tip
This kind of damage often occurs under more severe cutting conditions than micro chipping, or is the further development of micro chipping. The size and range of chipping are larger than that of micro chipping, which makes the tool lose its cutting ability completely and have to stop working. Chipping of the blade tip is often called tip dropping.
3) Broken blade or cutter
When the cutting conditions are very bad, the cutting parameters are too large, there are impact loads, there are micro cracks in the blade or tool material, and there are residual stresses in the blade due to welding and grinding, combined with careless operation and other factors, the blade or tool may break. After the occurrence of this kind of damage, the cutting tool can not be used any more, resulting in scrapping.
4) Surface peeling of blade
For brittle materials, such as cemented carbide, ceramics and PCBN with high tic content, there are defects or potential cracks in the surface layer structure, or residual stress in the surface layer due to welding and grinding, so it is easy to produce surface spalling when the cutting process is not stable or the tool surface bears alternating contact stress. Spalling may occur on the rake face and on the back face. The spalling material is flaky and the spalling area is large. The possibility of coating tool spalling is high. The blade can continue to work after slight peeling off, and will lose cutting ability after severe peeling off.
5) Plastic deformation of cutting part
Because of low strength and low hardness, plastic deformation may occur in the cutting parts of tool steel and high speed steel. When cemented carbide is working under high temperature and three-dimensional compressive stress, plastic flow will occur on the surface of cemented carbide, and even plastic deformation will occur on the cutting edge or tool tip, resulting in surface collapse. Collapse usually occurs in the case of large cutting amount and hard material processing. The elastic modulus of TiC based cemented carbide is smaller than that of WC based cemented carbide, so the former has faster plastic deformation resistance or rapid failure. There is no plastic deformation in PCD and PCBN.
6) Hot cracking of blades
When the cutting tool is subjected to alternating mechanical load and thermal load, the surface of the cutting part will inevitably produce alternating thermal stress due to repeated thermal expansion and contraction, which will cause the blade to crack due to fatigue. For example, when the carbide milling cutter is used for high-speed milling, the cutter teeth are constantly subjected to periodic impact and alternating thermal stress, resulting in comb like cracks on the rake face. Although some cutting tools do not have obvious alternating load and alternating stress, because the temperature of the surface layer and the inner layer is not consistent, thermal stress will also occur. In addition, there are inevitable defects in the material of the cutting tools, so the blade may also have cracks. Sometimes the tool can continue to work for a period of time after the formation of the crack, and sometimes the rapid propagation of the crack leads to the fracture of the blade or serious peeling off of the tool face.
2、 Tool wear
1. According to the causes of wear, it can be divided into: 1
1) Abrasive wear
There are often some very high hardness micro particles in the processed materials, which can draw grooves on the tool surface, which is abrasive wear. Abrasive wear exists on all surfaces, especially on the rake face. Moreover, hemp wear can occur at all kinds of 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 tool hardness is, the more serious the abrasive pitting is.
2) Cold welding wear
When cutting, there is great pressure and strong friction between the workpiece, cutting and the front and rear tool faces, so cold welding will occur. Because of the relative movement between the friction pairs, the cold welding will break and be taken away by one side, resulting in cold welding wear. Cold welding wear is generally serious at medium cutting speed. According to the experimental results, brittle metals have stronger resistance to cold welding than plastic metals; multiphase metals have less resistance to cold welding than unidirectional metals; metal compounds have less resistance to cold welding than simple materials; and group B elements in the periodic table of chemical elements have less resistance to cold welding than iron. The cold welding of high speed steel and cemented carbide is serious in low speed cutting.
3) Diffusion wear
In the process of cutting at high temperature and the contact between workpiece and tool, the chemical elements of both sides diffuse each other in the solid state, changing the composition and structure of the tool, making the surface layer of the tool vulnerable and aggravating the wear of the tool. Diffusion always keeps the object with high depth gradient diffusing continuously to the object with low depth gradient. For example, cobalt in cemented carbide rapidly diffuses into chips and workpieces at 800 ℃, WC decomposes into tungsten and carbon and diffuses into steel; when PCD tool is cutting steel and iron materials, when the cutting temperature is higher than 800 ℃, carbon atoms in PCD will transfer to the workpiece surface with great diffusion strength to form new alloy, and the tool surface will be graphitized. The diffusion of cobalt and tungsten is serious, and the anti diffusion ability of titanium, tantalum and niobium is strong. So YT cemented carbide has better wear resistance. When cutting ceramics and PCBN, when the temperature is up to 1000 ℃ - 1300 ℃, the diffusion wear is not significant. Due to the same material, the workpiece, chip and tool will produce thermoelectric potential in the contact area during cutting, which can promote the diffusion and accelerate the tool wear. This kind of diffusion wear under the action of thermoelectric force is called "thermoelectric wear".
4) Oxidation wear
When the temperature increases, the surface of the tool is oxidized to produce soft oxide, and the wear caused by chip friction is called oxidation wear. For example, oxygen in the gas reacts with cobalt, carbide and titanium carbide in cemented carbide at 700 ℃ ~ 800 ℃ to form soft oxides; PCBN reacts with water vapor at 1000 ℃.
2. According to the wear form, it can be divided into:
1) Rake face loss
When cutting plastic materials at a high speed, the part near the cutting force on the rake face will be worn into crescent concave under the action of chips, so it is also called crescent concave wear. At the initial stage of wear, the larger the rake angle of the tool, the better the cutting condition and the better the chip curling and breaking. However, when the crater is further increased, the strength of the cutting edge will be greatly weakened, which may eventually lead to the breakage of the cutting edge. When cutting brittle materials or plastic materials with low cutting speed and thin cutting thickness, crater wear will not occur.
2) Tool tip wear
The tool tip wear is the wear on the flank of the tool tip arc and the adjacent secondary flank, which is the continuation of the tool flank wear. Due to the poor heat dissipation condition and stress concentration, the wear rate is faster than that of the flank. Sometimes a series of small grooves with the distance equal to the feed rate are formed on the flank, which is called groove wear. They are mainly caused by the hardened layer and cutting lines on the machined surface. It is easy to cause groove wear when cutting hard cutting materials with high work hardening tendency. The tool tip wear has the greatest influence on the surface roughness and machining accuracy of the workpiece.
3) Flank wear
When cutting plastic materials with large cutting thickness, the flank of the tool may not contact with the workpiece due to the existence of chip build-up. In addition, usually the flank will contact with the workpiece, and a wear band with 0 rake angle is formed on the flank. Generally, in the middle of the working length of the cutting edge, the wear of the flank is relatively uniform, so the wear degree of the flank can be measured by the width VB of the flank wear band of the cutting edge.
Because all kinds of cutting tools will have flank wear under different cutting conditions, especially when cutting brittle materials or plastic materials with small cutting thickness, the tool wear is mainly flank wear, and the measurement of wear band width VB is relatively simple, so VB is usually used to express the degree of tool wear. The larger the VB is, the larger the cutting force will be, which will not only cause cutting vibration, but also affect the wear of the tool tip arc, thus affecting the machining accuracy and surface quality.
2. Methods to prevent tool breakage
1) According to the characteristics of the materials and parts to be processed, the types and grades of tool materials should be reasonably selected. On the premise of certain hardness and wear resistance, the tool material must have the necessary toughness;
2) Reasonable selection of tool geometry parameters. By adjusting the front and rear angles, the main and auxiliary deflection angles, the blade inclination angle and other angles;
Ensure that the cutting edge and tip have good strength. Grinding negative chamfering on the cutting edge is an effective measure to prevent tool collapse;
3) Ensure the quality of welding and grinding, avoid all kinds of defects caused by poor welding and grinding. The cutting tools used in key processes should be ground to improve the surface quality and check for cracks;
4) Reasonable selection of cutting parameters can avoid excessive cutting force and high cutting temperature to prevent tool breakage;
5) Ensure the rigidity of the process system and reduce the vibration as much as possible;
6) Take the correct operation method, try to make the tool not bear or less bear the sudden load.
3、 Causes and Countermeasures of tool edge collapse
1) Improper selection of blade grade and specification, such as too thin blade thickness or too hard and brittle blade grade in rough machining.
Countermeasures: increase the blade thickness or install the blade vertically, and select the brand with higher bending strength and toughness.
2) Improper selection of tool geometric parameters (such as excessive rake angle, etc.).
Countermeasures: redesign the cutting tool from the following aspects. Reduce the front and back angles appropriately. Large negative blade angle is adopted. Reduce the main deflection angle. Large negative chamfering or cutting edge arc is adopted. Sharpen the transition cutting edge to enhance the tool tip.
3) The welding process of the blade is not correct, resulting in excessive welding stress or welding cracks.
Countermeasures: avoid the blade groove structure with three sides closed. Correct selection of solder. Avoid using oxyacetylene flame to heat welding, and keep heat after welding to eliminate internal stress. Try to use mechanical clamping structure as far as possible
4) Improper grinding method will result in grinding stress and grinding crack; excessive shimmy of PCBN milling cutter after grinding will result in overload of individual cutter teeth, which will also cause tool striking.
Countermeasures: intermittent grinding or diamond wheel grinding. Choose the softer grinding wheel, and often trim to keep the wheel sharp. Pay attention to the grinding quality and strictly control the shimmy of milling cutter teeth.
5) The selection of cutting parameters is unreasonable, such as too much, the machine tool will be stuffy; when intermittent cutting, the cutting speed is too high, the feed rate is too large, the cutting depth is too small when the blank allowance is not uniform; when cutting high manganese steel and other materials with high work hardening tendency, the feed rate is too small, etc.
Countermeasures: re select cutting parameters.
6) There are some structural reasons for the mechanical clamping cutter, such as the bottom surface of the groove is not flat or the blade extends too long.
Countermeasures: trim the bottom of the groove. The position of cutting fluid nozzle should be arranged reasonably. The hardened tool bar is provided with a hard alloy gasket under the blade.
7) Excessive tool wear.
Countermeasures: change tool or cutting edge in time.
8) The cutting fluid flow is insufficient or the filling method is incorrect, which causes the blade to crack due to sudden heating.
Countermeasures: increase the flow of cutting fluid. The position of cutting fluid nozzle should be arranged reasonably. Effective cooling methods such as spray cooling are used to improve the cooling effect. Use * cutting to reduce the impact on the blade.
9) The tool installation is not correct, such as: cutting tool installation is too high or too low; the end milling cutter adopts asymmetric milling.
Countermeasure: re install the tool.
10) The rigidity of the process system is too poor, resulting in excessive cutting vibration.
Countermeasures: increase the auxiliary support of the workpiece and improve the clamping rigidity of the workpiece. Reduce the tool overhanging length. Reduce the rake angle of the tool appropriately. Other vibration elimination measures are adopted.
11) Careless operation, such as: cutting tool from the middle of the workpiece, the action is too fierce; not yet back the tool, that line stop.
Countermeasures: pay attention to the operation method.
4、 Chip lump
1) Cause of formation
In a part of the tool chip contact area near the cutting edge, due to the great downward pressure, the underlying metal of the chip is embedded in the micro uneven peaks and valleys on the rake face, forming a real metal to metal contact without gap, resulting in bonding phenomenon. This part of the tool chip contact area is called bonding area.
In the bonding area, there will be a thin layer of metal material in the bottom layer of the chip, which will remain on the rake face. The metal material of this part of the chip will undergo severe deformation and strengthen under the appropriate cutting temperature. With the continuous flow of the chip, pushed by the flow of the subsequent cutting, this layer of stagnant material will slip away from the upper layer of the chip and become the basis of the chip build-up. Then, a second layer of stagnant cutting material will be formed on the top of it, which will continuously accumulate and form a chip lump.
2) Characteristics and influence on cutting
The hardness is 1.5 ~ 2.0 times higher than that of the workpiece material, which 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, the debris flowing through the tool workpiece contact area will cause the wear of the tool flank.
The rake angle of the tool increases obviously after the chip build-up, which plays a positive role in reducing the chip deformation and cutting force.
Because the chip bump protrudes out of the cutting edge, the actual cutting depth increases and the dimensional accuracy of the workpiece is affected.
Chip build-up will cause "furrow" phenomenon on the surface of the workpiece and affect the surface roughness of the workpiece. The debris of the built-up chip will stick or embed into the surface of the workpiece, causing hard points, which will affect the quality of the machined surface of the workpiece.
From the above analysis, it can be seen that chip build-up is unfavorable for cutting, especially for finishing.
3) Control measures
In order to avoid the chip buildup, the following measures can be taken.
Reduce the roughness of the rake face.
Increase the rake angle of the tool.
Reduce the cutting thickness.
Low speed cutting or high speed cutting is adopted to avoid the cutting speed which is easy to form chip buildup.
Proper heat treatment is carried out to improve the hardness and reduce the plasticity of the workpiece.
The cutting fluid with good anti sticking property (such as extreme pressure cutting fluid containing sulfur and chlorine) shall be used.