For a machining center, cutting tools are considered consumable tools, and during the machining process, they can suffer damage, wear and chipping.
These occurrences are inevitable, but can also be caused by controllable factors such as unscientific and non-standard operation, inadequate maintenance, etc.
Finding the root cause is essential to solving the problem more effectively.
1. Expression of cutting tool damage
1. Microchipping of the cutting edge
When the material structure, hardness and stock of the workpiece are uneven, the face angle is too large, causing the cutting edge strength to be low, the rigidity of the process system is insufficient, causing vibration or carrying out intermittent cutting, and the grinding quality is poor, the cutting edge is subject to microchipping, that is, small fractures, gaps or flaking appear in the edge area.
Once this occurs, the tool will lose some of its cutting ability, but will still be able to continue functioning.
During continuous cutting, the damaged area of the edge can expand rapidly, causing further damage.
2） Breakage of cutting edge or tip
This type of damage occurs under more severe cutting conditions than micro-chipping or as a result of the development of micro-chipping.
The size and scope of the break are greater than those of microchipping, causing the tool to completely lose its cutting capacity and stop working. Tip breakage is commonly called tip breakage.
3) Blade or tool breakage
Under extremely severe cutting conditions, excessive cutting volume, impact loads, microcracks in the blade or tool material, residual stress from welding or grinding and other factors such as careless operation, the blade or tool may break. After this type of damage occurs, the tool can no longer be used and is discarded.
4) Peeling of the surface layer of the blade
For materials with high brittleness, such as hard alloys with high TiC content, ceramics, PCBN, etc., the surface layer may peel off due to defects or possible cracks in the surface layer or residual stress from welding or grinding.
When the surface is not sufficiently stable during cutting or the tool surface is subjected to alternating contact stresses, peeling is likely to occur.
Peeling can occur on the front or back face of the tool, with peeled material in the form of sheets and a large peeling area. The likelihood of peeling is greater for coated tools.
After light peeling, the tool can still continue to work, but after severe peeling, it will lose its cutting ability.
5) Plastic deformation of the cutting portion
Due to low strength and low hardness, plastic deformation may occur in the cutting portion of carbon steel and high-speed steel.
When hard alloys work under conditions of high temperature and triaxial stress, surface plastic flow may also occur, causing plastic deformation of the cutting edge or tip, leading to collapse.
Collapse generally occurs under conditions of high volume cutting and processing of hard materials. The antiplastic deformation capacity of TiC-based hard alloys is faster or fails faster than that of WC-based hard alloys because the former has a lower modulus of elasticity. 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 repeated thermal expansion and contraction of the surface of the cutting portion will inevitably generate alternating thermal stress, causing fatigue and cracking of the blade.
For example, in high-speed milling with a hard alloy cutter, the teeth are constantly subjected to periodic impacts and alternating thermal stresses, resulting in comb-shaped cracks on the front face.
Although some tools do not have obvious alternating loads and stresses, temperature differences between the surface and inner layers will also generate thermal stress.
Furthermore, defects are inevitably present in the tool material, so the blade can also crack. After cracks form, the tool can sometimes continue to function for a period of time, but sometimes the cracks expand quickly, causing the blade to break or the surface to peel severely.
2. Causes of tool wear
1） Abrasive Wear
The workpiece usually contains some small particles of extremely high hardness that can scratch grooves on the surface of the tool, this is abrasive wear.
Abrasive wear exists on all surfaces and is most evident on the front cutting edge.
Abrasive wear can occur at all cutting speeds, but when cutting at low speed, other types of wear are not obvious due to the low cutting temperature, so abrasive wear is the main cause. The lower the hardness of the tool, the more severe the abrasive wear will be.
2）Cold welding wear
During cutting, great pressure and intense friction occur between the part and the cutting edge, resulting in cold welding.
Due to the relative movement between the friction pair, cold welding causes the fracture to be removed by one of the parts, resulting in cold welding wear. Cold welding wear is generally more severe at moderate cutting speeds.
According to experiments, brittle metals have better resistance to cold welding than plastic metals; multiphase metals are smaller than single-phase metals; metallic compounds are less prone to cold welding than pure metals; Group B elements in the periodic table and iron are less prone to cold welding.
High speed steel and hard alloys are more susceptible to cold welding at low cutting speeds.
3）Diffusion Wear
During high-temperature cutting and contact between the workpiece and the tool, the chemical elements on both sides diffuse into the solid state, changing the composition and structure of the tool, making the tool surface brittle and accelerating tool wear.
Diffusion always maintains a deep gradient from the high-gradient object to the low-gradient object.
For example, when cobalt in a hard alloy is cut at 800°C, it quickly diffuses into the chips and workpiece; WC decomposes into tungsten and carbon and diffuses into steel; When cutting steel and iron with a PCD tool, if the cutting temperature is higher than 800°C, the carbon atoms in PCD will be transferred to the surface of the workpiece with high diffusion resistance to form a new alloy and the surface of the tool will be graffitied.
Cobalt and tungsten have more severe diffusion, while titanium, tantalum and niobium have stronger anti-diffusion ability, therefore YT-type hard alloys have better wear resistance.
During cutting of ceramics and PCBN, diffusion wear is not significant when the temperature is as high as 1000°C-1300°C.
Due to the material of the workpiece, chips and tool, a thermoelectric potential will be generated during cutting contact, which promotes diffusion and accelerates tool wear.
This type of diffusion wear under the action of thermoelectric potential is called “thermoelectric wear”.
4）Oxidation wear
When the temperature increases, the surface of the cutting tool can oxidize, creating a soft oxide that is abraded by chips and causing wear. This type of wear is called oxidation wear.
For example, in the temperature range of 700-800°C, oxygen in the air reacts with cobalt and carbides in cemented carbides and with titanium carbide, forming a softer oxide. At 1000°C, PCBN reacts with water vapor.
3. Forms of blade wear
1）Front cutting edge wear:
During cutting at a higher speed of plastic materials, the front cutting edge close to the cutting force will wear in an increasing shape due to the effect of chips, so it is also called increasing groove wear.
In the initial wear phase, the tool's face angle increases, improving cutting conditions and helping to bend and break chips.
However, as the growing groove becomes larger, the strength of the cutting edge weakens and ultimately the cutting edge may break.
Increasing groove wear does not occur when cutting brittle materials or when cutting plastic materials at lower cutting speed and thinner cutting thickness.
2）Cutting tip wear:
Cutting point wear is the wear on the back cutting edge and the back cutting edge adjacent side of the cutting point arc.
It is a continuation of wear on the back cutting edge of the tool. Due to the poor heat dissipation conditions here, the stress is concentrated and therefore the wear rate is faster than the later cutting edge.
Sometimes a series of grooves spaced equal to the feed rate will be formed on the side of the back cutting edge, called groove wear.
They are mainly caused by the hardening layer and cutting pattern on the machined surface.
Groove wear is most likely to occur when cutting difficult-to-cut materials with a high tendency to harden.
Cutting tip wear has a greater impact on surface roughness and machining precision of the part.
3）Rear cutting edge wear:
When cutting plastic materials with large cutting thickness, the rear cutting edge of the tool may not be in contact with the workpiece due to the existence of built-up edge.
In addition, the back cutting edge often comes into contact with the workpiece, forming a wear band with a back angle of 0.
Normally, in the middle of the working length of the cutting edge, the wear of the back cutting edge is relatively uniform, so the wear degree of the back cutting edge can be measured by the width of the wear band VB in this section of the edge cutting.
Since almost all types of tools will experience back cutting edge wear under different cutting conditions, especially when cutting brittle materials or when cutting plastic materials with smaller cutting thickness, the main tool wear is back cutting edge wear.
And the measurement of wear band width VB is simple, so VB is often used to represent the degree of tool wear.
The higher the VB is, the more the cutting force will increase, causing cutting vibration and also affecting the wear on the arc of the cutting tip, thus affecting the machining accuracy and surface quality.
4. Methods to prevent blade breakage:
Based on the characteristics of the material and parts being machined, select the type and quality of blade material reasonably. Make sure the blade material has the required toughness under the premise of having a certain hardness and wear resistance.
Select the blade geometry parameters reasonably. Adjust the front and back angles, major and minor relief angles, and rake angles to ensure that the cutting edge and tip have good strength. Grinding with a negative slope on the cutting edge is an effective measure to prevent blade breakage.
Ensure the quality of welding and grinding and avoid various defects caused by improper welding and grinding. Blades used in the process must be ground to improve surface quality and checked for cracks.
Choose the cutting amount reasonably to avoid excessive cutting force and high cutting temperature and avoid blade breakage.
Make the process system as rigid as possible to reduce vibration.
Use the correct method of operation to minimize blade exposure to sudden loads.
5. Causes and countermeasures for tool breakage:
1. Improper selection of tool blade specification and class, such as too thin a blade or using a blade with too high hardness and brittleness during rough machining.
Countermeasures: Increase the blade thickness or install the blade vertically and select a grade with greater flexural strength and toughness.
2. Inadequate selection of tool geometry parameters (such as excessive front and back angles).
Countermeasures:
Redesign the tool from the following aspects:
1）Reduce the front and back angles appropriately.
2）Adopt a larger negative blade inclination angle.
3) Reduce the main tilt angle.
4）Adopt a larger negative slope or circular arc on the edge of the blade.
5) Sharpen the transition cutting edge to strengthen the tool tip.
3. Incorrect tool blade welding process, resulting in excessive welding stress or welding cracks.
Countermeasures:
1）Avoid using a slot blade structure with a three-sided cabinet.
2）Select the correct welding material.
3) Avoid using oxygen-acetylene flame heating and keep the blade warm after welding to eliminate internal stress.
4）Replace the structure with mechanical fixation as much as possible.
4. Improper grinding method, resulting in grinding stress and cracking. For PCBN milling tools, excessive tooth oscillation after grinding may cause individual teeth to bear excessive loads and cause breakage.
Countermeasures:
1) Use intermittent grinding or diamond abrasive grinding.
2) Select a softer abrasive disc and sharpen it frequently.
3) Pay attention to the grinding quality and strictly control the oscillation of the milling tool teeth.
5. Improper selection of cutting quantity, such as excessive cutting force and temperature, causing tool breakage.
Countermeasures: Select the cutting amount again.
6. Structural reasons for mechanical clamping type tools, such as uneven tool groove bottom or blade protruding too much.
Countermeasures:
1）Repair the bottom of the tool groove.
2) Arrange the position of the cutting fluid nozzle reasonably.
3) Increase the hardness of the alloy spacer under the blade by quenching the rod.
7. Excessive tool wear.
Countermeasures: Change the blade or change the cutting edge in time.
8. Insufficient cutting fluid flow or incorrect addition method, causing blade overheating and breakage.
Countermeasures:
1) Increase cutting fluid flow.
2) Arrange the position of the cutting fluid nozzle reasonably.
3) Use effective cooling methods such as spray cooling to improve cooling efficiency.
4) Use interrupted cutting to reduce impact on the blade.
9. Incorrect tool installation, such as cutting tool installed too high or low, end mill using asymmetrical milling, etc.
Countermeasures: Reinstall the tool.
10. Poor rigidity of the process system, causing excessive cutting vibration.
Countermeasures:
1) Increase the auxiliary support for the workpiece and improve the clamping rigidity of the workpiece.
2) Reduce the tool swing length.
3）Reduce the back angle of the tool appropriately.
4）Adopt other vibration dampening measures.
11. Unsafe operation, such as too abrupt when cutting the workpiece from the center or stopping the machine before retracting the tool.
Countermeasures: Pay attention to the operation method.
6. Formation, characteristics and control measures of the built edge
1. Training
In the area close to the cutting edge where tool-chip contact occurs, due to the high downward pressure, the metal at the bottom of the chips becomes embedded in the microscopic peaks and valleys in front of the cutting tool, forming a tight metal-to-chip contact. metal that results in bonding.
This part of the contact area between the tool and the chip is known as the bonding area. In the gluing area, a thin layer of metallic chip material accumulates on the front of the cutting tool, which undergoes severe deformation and strengthening at appropriate cutting temperatures.
As the chips continue to flow outward, this accumulated material is pulled away from the subsequent cutting flow and becomes the basis for the built-up edge.
Then, a second layer of accumulated cutting material will form on top of it, and this process will continue to accumulate, forming a built-up edge.
2. Characteristics and Impact on the Court Process
1) The hardness of the built-up edge is 1.5 to 2 times that of the workpiece material, and can replace the front of the cutting tool, protecting the cutting edge and reducing wear on the front of the cutting tool, but edge fragments Falling inserts can cause wear on the back of the cutting tool when they flow through the tool-workpiece contact area.
2) After the built-up edge is formed, the front working angle of the tool increases, which actively reduces chip deformation and reduces cutting force.
3) Due to the built-up edge protruding from the cutting edge, the actual cutting depth increases, affecting the dimensional accuracy of the workpiece.
4）The built-up edge causes a “plowing” effect on the surface of the part, affecting its surface roughness.
5）Built edge fragments may adhere or embed into the surface of the workpiece, causing hard spots and affecting the quality of the machined surface of the workpiece.
From the above analysis, it can be seen that the built-up edge is harmful to cutting processing, especially precision machining.
3. Control measures
To prevent the formation of built-up edges, the following measures can be taken to prevent the metal at the bottom of the chips from bonding or strengthening with the front of the cutting tool:
1）Reduce the roughness of the front of the cutting tool.
2) Increase the front angle of the tool.
3) Reduce the cutting thickness.
4） Use low or high speed cutting to avoid cutting speeds that are prone to forming built-up edges.
5）Heat treat the workpiece material appropriately to increase its hardness and reduce its plasticity.
6) Use cutting fluids with good anti-stick properties (such as extreme pressure cutting fluids containing sulfur and chlorine).
