Advanced Turning

- 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.

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.

- 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.

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.

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.

- 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 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.

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.

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.

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.

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.

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.