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What should I do if the CNC tool is damaged, worn or chipped?

Jun 06, 2019

ALLLES CNC today wants to tell you about the maintenance of CNC machine tools.


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What should I do if the CNC tool is damaged, worn or chipped

First, the tool is damaged

 

1. Tool damage performance

1) The cutting edge is slightly collapsed

 

When the workpiece material structure, hardness, and margin are not uniform, the rake angle is too large, the cutting edge strength is low, the rigidity of the process system is insufficient to generate vibration, or the interrupted cutting is performed. When the sharpening quality is poor, the cutting edge is prone to micro-collapse. That is, there is a slight avalanche, gap or flaking in the blade area. When this happens, the tool will lose some of its cutting capacity, but it will continue to work. During continued cutting, the damaged portion of the blade area may expand rapidly, resulting in greater damage.

 

2) The cutting edge or the tip of the blade is broken

 

This type of damage is often produced under cutting conditions that are more severe than the micro-collapse of the cutting edge, or a further development of micro-cracking. The size and extent of the chipping are larger than the micro-collapse, causing the tool to completely lose its cutting ability and having to terminate the work. The fact that the tip of the knife is broken is often referred to as the tip.

 

3) Blade or cutter break

 

When the cutting conditions are extremely bad, the cutting amount is too large, there is an impact load, and the blade or the tool material has micro-cracks. Due to the residual stress in the blade due to welding and sharpening, and the inadvertent operation, etc., the blade or the tool may be caused. Breaking occurs. After this type of damage occurs, the tool cannot be used any more, resulting in scrapping.

 

4) Blade surface peeling

 

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 layer due to welding and sharpening, during the cutting process When the surface is not stable enough or the tool surface is subjected to alternating contact stress, surface peeling is likely to occur. Peeling may occur on the rake face, and the knife may occur on the flank, the exfoliation is in the form of a sheet, and the peeling area is large. The coating tool is more likely to peel off. After the blade is slightly peeled off, it can continue to work, and the cutting ability will be lost after severe peeling.

5) Plastic deformation of the cutting part

 

Steel and high-speed steel may have plastic deformation at the cutting part due to their low strength and low hardness. When the cemented carbide is working in the high temperature and three-direction compressive stress state, it will also produce surface plastic flow, and even the plastic deformation surface of the cutting edge or the tool tip will collapse. Collapse generally occurs when the amount of cutting is large and the hard material is machined. The elastic modulus of TiC-based cemented carbide is smaller than that of WC-based cemented carbide, so the former has accelerated resistance to plastic deformation or rapidly fails. 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 portion is inflated and contracted due to repeated thermal expansion, which inevitably produces alternating thermal stress, which causes the blade to fatigue and crack. For example, in the case of high-speed milling of carbide milling cutters, the teeth are constantly subjected to periodic impact and alternating thermal stress, and comb cracks are generated on the rake face. Although some tools do not have obvious alternating load and alternating stress, due to the inconsistent temperature of the surface layer and the inner layer, thermal stress will also occur, and inevitably there are defects inside the tool material, so the blade may also generate cracks. After the crack is formed, the cutter can sometimes continue to work for a while, and sometimes the crack spreads rapidly, causing the blade to break or the blade to be severely peeled off.

 

Second, tool wear

1. According to the cause of wear, it can be divided into:

 

1) Abrasive wear

 

The material to be processed often has some extremely fine particles, which can be grooved on the surface of the tool. This is the abrasive sanding loss. Abrasive wear is present on all sides and the rake face is most pronounced. Moreover, hemp wear can occur at various cutting speeds, but for low speed cutting, the wear is not obvious due to the low cutting temperature, so abrasive wear is the main reason. The lower the hardness of the tool, the more severe the abrasive loss of the abrasive.

 

2) Cold welding wear

 

During cutting, there is a large pressure and strong friction between the workpiece, the cutting and the front and back flank, and cold welding occurs. Due to the relative movement between the friction pairs, the cold welding will cause the rupture to be carried away by one side, resulting in cold welding wear. Cold weld wear is generally more severe at moderate cutting speeds. According to experiments, the brittle metal has stronger cold welding resistance than the plastic metal; the multiphase metal is smaller than the unidirectional metal; the metal compound tends to be colder than the elemental cold welding; the B element of the chemical periodic table has a tendency to be cold welded with iron. Cold welding of high speed steel and hard alloy at low speed is more serious.

3) diffusion wear

 

During the cutting at high temperature and the contact between the workpiece and the tool, the chemical elements of the two sides diffuse in the solid state, changing the composition of the tool, making the surface of the tool weak, which aggravates the wear of the tool. The diffusion phenomenon always maintains a high depth gradient. The object continues to diffuse toward a low depth gradient object. For example, at 800 °C, the cobalt in the cemented carbide rapidly diffuses into the chips and the workpiece, and the WC is decomposed into tungsten and carbon to diffuse into the steel. The cutting temperature of the PCD cutter is higher than 800 °C when cutting steel and iron materials. At the time, the carbon atoms in the PCD will transfer to the surface of the workpiece with a large diffusion intensity to form a new alloy, and the surface of the tool is graphitized. The diffusion of cobalt and tungsten is more serious, and the anti-diffusion ability of titanium, niobium and tantalum is stronger. Therefore, the YT type hard alloy has good wear resistance. When ceramic and PCBN are cut, the diffusion wear is not significant when the temperature is as high as 1000 °C - 1300 °C. Due to the same material, the workpiece, the chip and the tool will generate a thermoelectric potential in the contact zone during cutting. This thermoelectric potential promotes the diffusion and accelerates 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 oxidizes to produce a softer oxide. The wear caused by the friction of the chips is called oxidative wear. For example, at 700 ° C ~ 800 ° C, the oxygen in the air and the cobalt and carbide in the cemented carbide, titanium carbide, etc. oxidation reaction, forming a soft oxide; at 1000 ° C PCBN and water vapor chemical reaction.

 

2. According to the form of wear, it can be divided into:

 

1) rake face damage

 

When cutting plastic material at a large speed, the portion of the rake face close to the cutting force will wear into a concave shape under the action of the chip, so it is also called crater wear. In the early stage of wear, the rake angle of the tool is increased, which improves the cutting conditions and facilitates the curling and breaking of the chips. However, when the crater is further enlarged, the strength of the cutting edge is greatly weakened, which may eventually cause the cutting edge to collapse. Case. In the case of cutting brittle materials, or cutting plastic materials at lower cutting speeds and thinner cutting thicknesses, crater wear is generally not produced.

 

2) Tool tip wear

 

The wear of the tool tip is the wear of the flank face of the tool edge arc and the adjacent minor flank face, which is a continuation of the wear of the flank face on the tool. Because the heat dissipation conditions here are poor, the stress is concentrated, so the wear speed is faster than the flank face. Sometimes a series of small grooves with a spacing equal to the feed amount are formed on the secondary flank face, which is called groove wear. They are mainly caused by the hardened layer of the machined surface and the cutting lines. When it is difficult to cut a material with a high degree of work hardening, it is most likely to cause groove wear. Tool tip wear has the greatest impact on workpiece surface roughness and machining accuracy.

3) flank wear

 

When cutting plastic material at a large cutting thickness, the flank of the tool may not come into contact with the workpiece due to the presence of built-up edge. In addition, usually the flank face comes into contact with the workpiece, and a wear band with a back angle of 0 is formed on the flank face. Generally, in the middle of the working length of the cutting edge, the flank wear is relatively uniform, so the degree of wear of the flank can be measured by the flank wear band width VB of the cutting edge.

 

Since all types of tools will 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 the flank wear, and the wear band The measurement of the width VB is relatively simple, so VB is usually used to indicate the degree of wear of the tool. The larger the VB, the more the cutting force will increase, causing the cutting vibration, and it will affect the wear of the arc of the tool tip, thus affecting the machining accuracy and the quality of the machined surface.

 

2. How to prevent damage to the tool

 

1) Reasonable selection of various types and grades of tool materials for the characteristics of the materials and parts to be processed. Under the premise of certain hardness and wear resistance, it must be ensured that the tool material has the necessary toughness;

 

2) Reasonably select the tool geometry parameters. By adjusting the front and rear angles, the main and auxiliary declination, the blade inclination angle, etc.;

 

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 the chipping;

 

3) Guarantee the quality of welding and sharpening, and avoid all kinds of defects caused by welding and sharp grinding. The tool 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 excessive cutting temperature to prevent the tool from being damaged;

 

5) As far as possible to 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 load.

Third, the causes and countermeasures of tool chipping

 

1) The blade number and specifications are not properly selected. For example, if the thickness of the blade is too thin or rough, the grade is too hard and too brittle.

 

Countermeasure: Increase the thickness of the blade or stand up the blade, and choose the grade with high flexural strength and toughness.

 

2) Improper selection of tool geometry parameters (such as excessive front and rear angles, etc.).

 

Countermeasure: You can start redesigning the tool from the following aspects.

 

1 Reduce the front and back angles appropriately.

 

2 Use a large negative edge angle.

 

3 Decrease the lead angle.

 

4 Use a large negative chamfer or edge arc.

 

5 Grind the transition 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 the use of a three-sided closed blade slot structure.

 

2 Use solder properly.

 

3 Avoid the use of oxy-acetylene flame heating welding, and should be insulated after welding to eliminate internal stress.

 

4 use mechanically clamped structure as much as possible

 

4) Improper sharpening method, causing grinding stress and grinding crack; the oscillating of the cutter teeth after the sharpening of the PCBN milling cutter is too large, so that the individual cutter teeth are overloaded and the knife is also caused.

 

Countermeasures:

 

1 Use intermittent grinding or diamond grinding wheel grinding.

 

2 Use a softer grinding wheel and often trim to keep the grinding wheel sharp.

 

3 Pay attention to the quality of the sharpening and strictly control the amount of the milling cutter teeth.

 

5) The selection of cutting amount is unreasonable. If the dosage is too large, the machine will be boring. When the cutting is interrupted, the cutting speed is too high, the feed rate is too large, and the blank margin is too small, the cutting depth is too small; cutting high manganese steel When the material having a large work hardening tendency is used, the feed amount is too small.

 

Countermeasure: Reselect the amount of cutting.

 

6) The reason why the bottom surface of the turret of the mechanical clamping tool is not flat or the blade protrudes too long.

 

Countermeasure: 1 Trim the bottom surface of the sipe. 2 Reasonably arrange the position of the cutting fluid nozzle. 3 Hardened shank adds carbide insert under the blade.

 

7) The tool is excessively worn.

 

Countermeasure: Change the knife or replace the cutting edge in time.

 

8) The cutting fluid flow is insufficient or the filling method is incorrect, causing the blade to heat up and break.

 

Countermeasures:

 

1 Increase the flow rate of the cutting fluid.

 

2 Arrange the position of the cutting fluid nozzle reasonably.

 

3 Improve cooling by using effective cooling methods such as spray cooling.

 

4 Use *cutting to reduce the impact on the blade.

 

9) The tool is not installed correctly, such as: the cutting tool is installed too high or too low; the end milling cutter adopts asymmetric milling.

 

Action: Reinstall the tool.

 

10) The process system is too rigid and the cutting vibration is too large.

 

Countermeasures:

 

1 Increase the auxiliary support of the workpiece to improve the rigidity of the workpiece.

 

2 Reduce the overhang of the tool.

 

3 Reduce the back angle of the tool appropriately.

 

4 Use other vibration suppression measures.

 

11) Inadvertent operation, such as: when the tool is cut in from the middle of the workpiece, the action is too strong;

 

Countermeasures: Pay attention to the operation method.

Fourth, built-up edge

 

1) Reason for formation

 

In the part close to the cutting edge, in the blade-chip contact area, due to the large down-pressure, the underlying metal of the chip is embedded in the microscopic uneven peaks and valleys on the rake face, forming a true intermetallic contact without gap and causing a bonding phenomenon. This part of the knife-chip contact area is called the bonding area.

 

In the bonding area, the underside of the chip will have a thin layer of metal material deposited on the rake face. This part of the chip metal material undergoes severe deformation and is strengthened at an appropriate cutting temperature. As the continuous flow of the chips is pushed, the layer of stagnant material slips away from the upper layer of the chip and becomes the basis of the built-up edge. Subsequently, a second layer of stagnant cutting material is formed on top of it, so that it is continuously laminated to form a built-up edge.

 

2) Features and impact on machining

 

1 The hardness is 1.5~2.0 times higher than 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, but the debris when the built-up edge falls off flows through the tool-workpiece contact area. The flank of the tool is worn.

 

2 After the formation of the built-up edge, the working 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 is increased, which affects the dimensional accuracy of the workpiece.

 

4 The built-up edge will cause a “furrow” phenomenon on the surface of the workpiece, which will affect the surface roughness of the workpiece. 5 The fragments of the built-up edge will bond or embed the surface of the workpiece to create a hard spot, which affects the quality of the machined surface.

 

It can be seen from the above analysis that the built-up edge is disadvantageous for the cutting process, especially for finishing.

 

3) Control measures

 

The following measures can be taken for the day when the chip bottom material is not bonded or deformed by the rake face.

 

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 Appropriate heat treatment of the workpiece material to increase its hardness and reduce plasticity.

 

6 Use a cutting fluid with good adhesion resistance (such as extreme pressure cutting fluid containing sulfur and chlorine).



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