Types of cutting edge damage and solutions
Normal damage
Wear of the flank and rake faces caused by abrasion with workpiece material.
Abnormal damage
Wear that occurs suddenly. It is necessary to choose tools and set cutting conditions that prevent abnormal damage.
Flank wear
Causes
The flank face is always in contact with the workpiece, therefore the rubbing between the two causes flank wear. As the workpiece hardness increases so too does the amount of flank wear.
The cutting heat generated can also accelerate flank wear. When machining materials with high strength and adhesiveness the rate of flank wear development accelerates.
In these cases, if the cutting edge succumbs to plastic deformation caused by the heat, it can lead to a blunt cutting action. This will result in higher cutting resistance leading to increased flank wear. This particular type of damage is known as a secondary cause.
Solutions
- Choose a higher wear-resistant tool grade and lower the cutting speed to a correct level.
- Low feed is likely to cause flank wear, increase the feed rate.
Crater wear
Causes
Crater wear is a tool failure that arises due to the chips’ removing small particles of the cutting edge.
When the cutting edge is exposed to high temperatures the chemical stability of the cutting edge reduces. Under this condition the amount of particles removed by the chips increases. This can often be seen when machining workpiece materials that are abrasive, adhesive or generate a lot of heat.
Additionally, when machining workpiece materials with low thermal conductivity, heat accumulates around the cutting edge leading to crater wear.
Crater wear is also caused when the cutting edge and workpiece react easily to each other (have a high affinity).
Solutions
- When the insert is an uncoated carbide grade, change to a coated grade.
- Decrease the cutting speed and feed, thereby delaying tool wear development.
- Increase the rake angle.
Built-up wear
Causes
A built-up edge is a piece of work hardened material that adheres to the top of the cutting edge. The built up edge itself protrudes from the cutting edge and actually machines the workpiece, generally causing a greater depth of cut and therefore creating component inaccuracy. The following are the main possible causes of a built up edge.
- Low cutting temperatures under the recrystallization temperature of a workpiece material.
- High affinity between a tool grade and the workpiece material.
Solutions
- Increase cutting speed to increase cutting temperature.
- To avoid built-up edge, use a larger rake angle, use a small honed edge, use of a high-lubricity coolant.
- Change the tool grade to one with low affinity to the workpiece material such as cermet and coated grades.
Notch wear
Causes
Notching of the main cutting edge is greatly influenced by the workpiece material’s work hardened layer, a cast or forged surface. The major part of notch wear is usually seen at the depth of cut line where the tool contacts the work-hardened layer. Similarly, hard surfaces of castings or forgings can also cause notch wear to develop.
When machining materials that are easily work hardened and other problematical materials such as castings, hot forged and heat treated components, care should be taken to avoid notching.
Oxidization damage at the flank of the end cutting edge is caused by high temperatures and makes the surface structure chemically unstable. Therefore, oxidization damage is often seen when machining high strength, adhesive materials and when machining materials with low thermal conductivity.
Solutions
The following solutions can decrease chip thickness and increase chip width, therefore cut the hardened layer more effectively.
- Increase the lead angle of the insert.
- Increase the insert corner radius and set the depth of cut under the value of the corner radius.
- When a large depth of cut is necessary to machine difficult-to-cut materials, choose a round insert.
Fracture
Causes
Fracture of the cutting edge is generally caused by impacts encountered during machining. Machining constantly exposes the cutting edge to thermal shocks and continuous impacts with the workpiece. Therefore, the cutting edge can fracture during machining of any type of workpiece material.
This is especially so when machining harder workpiece materials. When machining hard workpiece materials the cutting edge is exposed to severe impacts and care should be taken to avoid fracture, especially in the initial stages of machining.
Solutions
- Soften the tool grade.
- Enlarge the insert honing.
- If possible, increase the corner radius.
- Decrease the rake angle but maintain cutting edge strength.
- Change the chip breaker and enlarge the land.
- For heavy interrupted cutting, decrease the feed and increase the shank size to maintain tool rigidity.
Chipping
Causes
Chipping is seen as small fractures of the cutting edge and is caused by impacts and vibration. It can also be caused when welded sections peels off and removes a part of the cutting edge. Therefore, chipping can occur both when cutting hard and soft materials.
Chipping of the cutting edge causes higher cutting resistance, leading to the development of simultaneous flank and chipping wear.
Solutions
- Soften the tool grade.
- Enlarge the insert honing.
- If possible, increase the corner radius.
- Decrease the rake angle but maintain cutting edge strength.
- Change the chip breaker and enlarge the land.
- For heavy interrupted cutting, decrease the feed and increase the shank size to maintain tool rigidity.
Breakage
Causes
If a cutting edge is broken, it is rarely caused by the mechanical properties of the workpiece material and therefore it is necessary to review the cutting conditions, tool geometry and tool grade material. Breakage is a tool failure pattern that can often occur if the insert in not correctly set in its location pocket.
In all cases, it is important to review tool installation, tool geometry and the combination of the insert and a shim.
Solutions
Ensure good clamping conditions.
- Remove dirt from the insert and location pocket and clamp appropriately.
- Fasten with the appropriate torque.Lower the cutting conditions.
- Check the workpiece clamping and use cutting conditions where no vibrations occur.
Flaking (Spalling)
Causes
Flaking is a type of conchoidal fracture and has the following possible causes.
- Compression stress is generated on the cutting edge by elastic deformation of the workpiece material.
- Spalling occurs when welded material peels away.
A typical case in which flaking leads to shortened tool life is machining of hardened materials. When machining hardened materials the back force tends to be very high, causing compression stress on the cutting edge. Stress from the flank face direction by back force causes flaking on the rake face.
Solutions
It is very difficult to avoid flaking during the machining of hardened materials. Decreasing cutting resistance could improve results. However, decreasing the honing width or increasing the rake angle will decrease cutting edge strength so both are not ideal answers.
- Decrease the feed and cutting speed to prevent flank wear.
- Decrease the corner radius.
This is because when machining hardened materials, a small depth of cut within the corner radius value, which is often used for finishing with a small machining allowance, causes an increase in back force.
Welding
Causes
Welding of the workpiece material on the cutting edge can easily occur when machining materials that are easily affected by heat.
Also, as in crater wear, when the affinity between a workpiece material and a tool is high, welding can occur.With chip welding, there is sometimes an advantage that it becomes a built-up edge, therefore delaying tool wear. However, a built-up edge can lead to problems such as poor surface finish and chipping of the cutting edge. Therefore it is recommended to avoid chip welding conditions as much as possible.
Solutions
- Increase cutting speed.
- Aluminium alloys also cause welding on the flank faces due to elastic deformation. Make the flank angle larger than is usual for machining steel and cast iron.
- To improve chip flow, a mirror finish rake face is recommended.
Plastic deformation
Causes
Plastic deformation of the cutting edge is a result of both high cutting resistance and cutting edge temperatures. When the tool is exposed to cutting resistance the loads on the cutting edge are extreme. These high load and heat condition can result in softening of the cutting edge. When this occurs the cutting edge is prone to plastic deformation.
Plastic deformation of the cutting edge easily arises when machining materials with low thermal conductivity.
Solutions
- Change to a harder tool grade.
- To reduce cutting heat, decrease the cutting speed and feed.
- Increase the corner radius.
- Use coolant.
Thermal cracks
Causes
Thermal cracks are caused by thermal shocks. Thermal shock comes from rapid and repeated heating and cooling of the cutting edge. Under these conditions the material will repeatedly expand and contract. This continuous expansion and contraction, known as thermal shock causes separation of the particle boundary*. Thermal cracks can lead to sudden damage such as fracturing and often occurs when the cutting temperatures are high. Additionally, cutting conditions such as interrupted cutting and wet cutting become factors that can easily lead to the development of thermal cracks.
Solutions
The measures below are effective.
- Use a tool grade with a sharper geometry to prevent heat generation as much as possible.
- Choose a highly thermal-shock-resistant tool grade.
- Use air blow
- Use copious amounts of coolant when necessary for suitable applications.
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