For machining centers, tools are a kind of consumable tools.During the processing process, it will produce breakage, wear, chipping and other phenomena.These phenomena are inevitable, but there are also such phenomenaThe operation is not scientific and standardized,Improper maintenance and other controllable conditionscause, find the root cause to better solve the problem.

 

01Performance of tool breakage

1) The cutting edge is slightly chipped

When the material structure, hardness and allowance of the workpiece are uneven, the rake angle is large, the cutting edge strength is low, the rigidity of the process system is insufficient to produce vibration, or the cutting edge is not good quality, the cutting edge is prone to micro-chipping, that is, the cutting edge has a small chip, chip or peeling in the cutting area. After this happens, the tool loses some of its cutting power but continues to work. As you continue to cut, the damaged part of the cutting area may expand rapidly, leading to even greater breakage.

2) The cutting edge or tip is crushed

This type of breakage often occurs under harsher cutting conditions than causing micro-chipping of the cutting edge, or is a further development of micro-chipping. The size and range of the chipping are larger than that of microchipping, causing the tool to completely lose its cutting ability and have to stop working. The situation where the tip of the knife is broken is often called tip drop.

3) Blade or knife break

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, due to the residual stress in the blade due to welding and edge sharpening, coupled with careless operation and other factors, the blade or tool may be broken. After this form of breakage, the knife cannot be used anymore and is scrapped.

4) The surface of the blade peels off

For materials with great 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 on the surface layer due to welding and edge grinding, it is very easy to cause surface peeling when it is not stable enough during the cutting process or the tool surface is subjected to alternating contact stress. Peeling may occur on the front blade surface, and the knife may occur on the back blade surface, and the peeling material is in the form of sheets and the peeling area is large. The coated tool is more likely to peel off. After the blade peels off slightly, it can continue to work, and after severe peeling, it loses its cutting ability.

5) Plastic deformation of the cutting part

Due to the low strength and low hardness of the steel and high-speed steel, plastic deformation may occur at their cutting parts. When cemented carbide works directly at high temperature and three-way compressive stress, it will also produce plastic flow on the surface, and even cause the cutting edge or tip to have plastic deformation surfaces and cause collapse. Collapse generally occurs when the amount of cutting 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 plastic deformation resistance of the former is accelerated or 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 cutting part of the surface will inevitably produce alternating thermal stress due to repeated thermal expansion and contraction, resulting in fatigue and cracking of the blade. For example, when carbide milling cutters are milled at high speeds, the teeth are constantly subjected to periodic impact and alternating thermal stress, resulting in comb-like cracks on the front cutting surface. Although some tools do not have obvious alternating load and alternating stress, due to the inconsistency of the temperature of the surface and inner layers, thermal stress will also occur, and there will inevitably be defects inside the tool material, so the blade may also produce cracks. Sometimes the tool can continue to work for a while after crack formation, and sometimes the crack spreads rapidly, causing the blade to break or the blade to peel off severely.

 

02Causes 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 damage. Abrasive wear is present on all sides, with the front face being the most noticeable. 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 numb damage.

2) Cold welding wear

During cutting, there is a lot of pressure and strong friction between the workpiece, cutting and the front and rear tool 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 side, resulting in cold welding wear. Cold welding wear is generally more serious at moderate cutting speeds. According to experiments, brittle metals have stronger cold welding resistance than plastic metals. Multiphase metals are smaller than unidirectional metals; metal compounds have a smaller tendency to cold weld than elemental materials; In the periodic table of chemical elements, group B elements and iron have a small tendency to cold weld. Cold welding is more serious when cutting high-speed steel and cemented carbide at low speed.

3) Diffusion wear

During cutting at high temperatures and the contact between the workpiece and the tool, the chemical elements of both sides diffuse each other in the solid state, changing the compositional structure of the tool, making the surface of the tool fragile and aggravating the wear of the tool. The diffusion phenomenon always maintains that objects with high depth gradients continue to diffuse from objects with low depth gradients.

For example, the cobalt in cemented carbide at 800°C will quickly diffuse into chips and workpieces, and WC will decompose into tungsten and carbon and diffuse into steel. When the cutting temperature is higher than 800°C when the PCD tool cuts steel and iron materials, the carbon atoms in the PCD will be transferred to the surface of the workpiece with great diffusion strength to form a new alloy, and the surface of the tool will be graphitized. Cobalt and tungsten diffusion are serious, and titanium, tantalum and niobium have strong diffusion resistance. Therefore, YT cemented carbide has good wear resistance. When cutting ceramics and PCBNs, the diffusion wear is not significant at temperatures as high as 1000°C-1300°C. Due to the same material of the workpiece, chips and tools, a thermal potential will be generated in the contact area during cutting, which has the effect of promoting diffusion and accelerating the wear of the tool. This diffusion wear under the action of thermal potential is called "thermoelectric wear".

4) Oxidation wear

When the temperature rises, the surface of the tool oxidizes to produce softer oxides, which are rubbed by chips, and the wear formed by the chips 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 softer oxides; At 1000°C, PCBN reacts chemically with water vapor.

 

03Wear form of the blade

1) Front knife surface damage

When cutting plastic materials at a large speed, the part of the front cutting surface that is close to the cutting force will wear out into a crescent depression under the action of chips, so it is also called crescent depression 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, but when the crescent depression is further enlarged, the strength of the cutting edge is greatly weakened, which may eventually cause the chipping and damage of the cutting edge. Crescent wear generally does not occur when cutting brittle materials, or when cutting plastic materials at lower cutting speeds and thinner cutting thicknesses.

2) The tip of the knife is worn

The wear of the tip is the wear on the back of the tip arc and the adjacent sub-rear face of the cutter point, which is the continuation of the wear of the rear face on the tool. Due to the poor heat dissipation conditions and stress concentration here, the wear speed is faster than that of the rear cutting surface, and sometimes a series of small grooves with spacing equal to the feed will be formed on the rear cutting surface of the sub-level, which is called groove wear. They are mainly caused by the hardened layer and cutting pattern of the machined surface. When cutting difficult-to-cut materials with a large tendency to work hardening, it is most likely to cause groove wear. Tip wear has the greatest impact on the surface roughness and machining accuracy of the workpiece.

3) The rear blade surface is worn

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

Because all types of tools almost always have rear tool surface wear in different cutting situations, especially when cutting brittle materials or cutting plastic materials with a smaller cutting thickness, the wear of the tool is mainly the rear tool surface wear, and the width of the wear band VB is relatively easy to measure, so VB is usually used to indicate the wear degree of the tool. The larger the VB, the more cutting force will not only increase, cause cutting vibration, but also affect the wear at the tip arc, thereby affecting the machining accuracy and surface quality.

 

 

 

04Methods to prevent knife breakage

1) According to the characteristics of the materials and parts to be processed, reasonably select the types and grades of tool materials. Under the premise of a certain hardness and wear resistance, the tool material must have the necessary toughness.

2) Reasonable selection of tool geometric parameters. By adjusting the front and rear angles, the main and secondary declination angles, the blade inclination angle and other angles;Ensure good strength of the cutting edge and tip. Grinding negative chamfers on the cutting edge is an effective measure to prevent chipping.

3) Ensure the quality of welding and edge grinding, and avoid various defects caused by poor welding and edge sharpening. The knives used in key processes 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 tool damage.

5) Ensure that the process system has good rigidity and reduce vibration as much as possible.

6) Adopt the correct operation method to make the tool bear the load of sudden degeneration as little as possible.

 

05Causes and countermeasures of knife chipping

1. Improper selection of blade grades and specifications, such as the thickness of the blade is too thin or the grade that is too hard and brittle for rough processing.

Countermeasure:Increase the thickness of the bladeOr install the blade vertically and choose a grade with high bending strength and toughness.

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

Countermeasure:

The tool can be redesigned in the following ways.

1) Appropriately reduce the front and rear angles.

2) Adopt a large negative blade inclination angle.

3) Reduce the main declination angle.

4) Larger negative chamfers or cutting edge arcs are used.

5) Regrind 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.

Countermeasure:

1) Avoid using a three-sided closed blade groove structure.

2) Choose the solder correctly.

3) Avoid using oxyalkyne flame heating welding, and keep warm after welding to eliminate internal stress.

4) Switch to mechanically clamped structures as much as possible

4. Improper sharpening method causes grinding stress and grinding cracks; The vibration of the blades after grinding the PCBN milling cutter edge is too large, so that the load of individual teeth is too heavy, which will also cause punching.

Countermeasure:

1) Intermittent 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 quality of edge grinding and strictly control the vibration of the milling cutter teeth.

5. The selection of cutting dosage is unreasonable, such as the amount is too large, and the machine tool is stuffy; When cutting intermittently, the cutting speed is too high, the feed is too large, and the blank allowance is uneven, the cutting depth is too small; When cutting materials with a high manganese steel and a large tendency to work hardening, the feed rate is too small.

Countermeasure:Re-select the cutting amount.

6. Structural reasons such as uneven bottom surface of the tool groove or blade protruding too long of mechanical clamping tools.

Countermeasure:

1) Trim the bottom of the knife groove.

2) Reasonably arrange the position of the cutting fluid nozzle.

3) Hardened cutter bar to add carbide gasket under the blade.

7. Excessive tool wear.

Countermeasure:Replace the tool or the cutting edge in time.

8. Insufficient cutting fluid flow or incorrect filling method, causing the blade to heat up suddenly and crack.

Countermeasure:

1) Increase the flow rate of cutting fluid.

2) Reasonably arrange the position of the cutting fluid nozzle.

3) Adopt effective cooling methods such as spray cooling to improve the cooling effect.

4) Adopt * 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 face milling cutter adopts asymmetric forward milling, etc.

Countermeasure:Reinstall the tool.

10. The rigidity of the process system is too poor, resulting in excessive cutting vibration.

Countermeasure:

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) Other vibration suppression measures are adopted.

11. Careless operation, such as: when the tool cuts from the middle of the workpiece, the action is too violent; Before the knife was withdrawn, he stopped.

Countermeasure:Pay attention to how to do it.

 

06Causes, characteristics and control measures of the formation of edges

1. Formation causes

In the part of the cutting edge close to the cutting edge, in the knife-chip contact area, due to the large downforce, the chip bottom metal is embedded in the microscopic uneven peak and valley on the front cutter surface, forming a real metal-to-metal contact without gaps and producing a bonding phenomenon, which is called the bonding area. In the bonding area, a thin layer of metal material is trapped on the front cutting surface at the bottom of the chip, and this part of the metal material is violently deformed and strengthened at the appropriate cutting temperature. With the continuous outflow of chips, under the push of the flow of subsequent cuttings, this layer of stagnant material will slide relative to the upper layer of chips and leave, becoming the basis of accumulated edges. Subsequently, a second layer of stagnant cutting material will be formed on top of it, so that the continuous accumulation of edges will form.

 

2. Characteristics and impact on cutting processing

1) The hardness is 1.5~2.0 times higher than that of the workpiece material, which can replace the front cutting surface for cutting, which has the effect of protecting the cutting edge and reducing the wear of the front cutting surface, but the debris flowing through the tool-workpiece contact area when the accumulated edge falls off will cause the rear cutting surface wear of the tool.

2) After the formation of edges, the working angle of the tool is significantly increased, which plays a positive role in reducing chip deformation and reducing cutting force.

3) Because the accumulated edge protrudes outside the cutting edge, the actual cutting depth increases, affecting the dimensional accuracy of the workpiece.

4) Accumulation of edge will cause a "furrow" phenomenon on the surface of the workpiece, affecting the surface roughness of the workpiece.

5) The fragments of the accumulated edge will bond or embed on the surface of the workpiece, causing hard spots, affecting the quality of the processed surface of the workpiece.

From the above analysis, it can be seen that the edges are not good for cutting, especially finishing.

 

3. Control measures

The following measures can be taken to avoid the occurrence of accumulated edge by not bonding or deforming the bottom material of the chip and the front cutting surface.

1) Reduce the roughness of the front blade surface.

2) Increase the rake angle of the tool.

3) Reduce the cutting thickness.

4) Use low-speed cutting or high-speed cutting to avoid cutting speed that is easy to form edges.

5) Appropriate heat treatment of the workpiece material to improve its hardness and reduce plasticity.

6) Use cutting fluids with good anti-adhesion performance (such as extremely high-pressure cutting fluids containing sulfur and chlorine).