Failure of Metal Cutting Tools: 3 Factors | Industries | Metallurgy

The following points highlight the three main factors responsible for failure of tools in the industries. The factors are: 1. Temperature Failure 2. Rupture of Tool Point 3. Gradual Wear at the Tool Point.

Factor # 1. Temperature Failure:

During machining at high speeds, very high temperature exists at tool chip interface. When temperature exceeds the critical limit, the tool point gets softened. Due to this high temperature, localised phase transformation occurs. This gives rise to high residual stresses due to which cracks appear in the tool point and in such a state, it is more prone to failure.

In some cases tool point might even melt. This type of failure occurs quite rapidly, and is frequently accompanied by sparking and is easily recognised. Large nose radii result in smaller stresses at the tool point and less failure.

Thermal cracking occurs when there is a steep temperature gradient due to intermittent cutting. Thermal shock combined with mechanical impact lead to failure. This type of failure can be reduced by proper selection of the cutting parameters and using tougher tool materials.

Factor # 2. Rupture of Tool Point:

At slow speeds, built up edge is formed on the tool. When it grows too much, it is unstable and breaks away with the underside of the chip, taking away a small portion of tool with it. This is so with brittle tool materials, like carbides. This problem can be overcome by increasing the cutting speed.

Tools are generally hard. A large degree of brittleness is associated with hardness especially in case of carbide and diamond tipped tools. Whenever the cutting forces exceed the critical limit, small portions of the cutting edge begin to chip off, or the entire tip may break away in some cases.

The high forces, which produce this type of failure are not generally associated with steady state cutting but rather with variations in the cutting process or when cutting with excessive vibration (chatter).

These forces, however, can be avoided by giving more rigidity to work holding and tool holding devices, and the failure can be avoided by redesigning so as to redirect the forces (providing negative rake angle).

Factor # 3. Gradual Wear at the Tool Point:

Wear means any process by which material is removed from the tool surface in the form of very small particles. Depending on the environmental conditions, wear could occur due to abrasion, adhesion with material transfer at asperities, corrosion with removal of product by chemical action.

As a result of direct contact of tool with the work material, there are three regions of wear:

(a) Face

(b) Flank

(c) Nose radius.

(a) Face Wear (Crater Wear):

On the face of the tool there is a direct contact of tool with the chip. Wear takes the form of cavity or crater, which has its origin above the cutting edge. With time, cavity goes on widening. This is prominent in ductile materials. The crater occurs on the rake face of the tool at the point of impingement of the chip with tool and does not actually reach the cutting edge but ends near the nose and on the periphery which serves as the focal points of development of crack and extends to the cutting edge causing a rapid rupture.

It leads to weakening of tool, increase in cutting temperature, friction and cutting forces. The tool life due to crater wear can be determined by fixing the ratio of width of crater to its depth. Various theories have been advanced to explain the crater formation. Trent stated that the work material alloys with tool material due to extremely high temperature developed at the cutting edge and then the alloyed layer is carried away with the chip due to high pressure at the tool face.

This sort of alloying phenomenon where the metals have not reached the melting point and yet alloyed, is known as “diffusion of metal”. The reason for the alloying is the increased activity of the molecules on the nascant surfaces of the chip and tool at high temperature. This type of wear is a major cause of failure when using tungsten carbide tools to cut steel, but is less prevalent when using H.S.S. or tungsten titanium carbide.

(b) Flank Wear (Edge Wear):

This wear is also called “wearland”. Work and tool are in contact at cutting edge only. Usually wear first appears on the clearance face of the toll in the form of a wear land, and is mainly the result of friction and abrasion.

Adhesion is also a factor because welding of the tool to the work material causes a built-up edge which is torn away, taking particles of the tool material with it. Thermal cracking, due to thermal shock, is also a cause of breakdown of small particles, leading to flank or edge wear.

Flank wear starts at cutting edge and then starts widening along the clearance face. It is independent of cutting conditions and tool/work materials. Crater wear is prominent in ductile metals, but the flank wear becomes predominant in materials having brittle and flaky chip and discontinuous chip. It is important even in ductile materials if surface finish is the main criteria.

While all other modes of tool failure can be effectively reduced by changing speed, feed or depth of cut, the flank wear is a progressive form of deterioration which will ultimately result in failure inspite of best precautions.

The flank wear of cutting tool progress is shown in Fig. 24.13. It may be noted that the primary stage is one of rapid wear due to the very high stresses at the tool point. The wear rate is more or less linear in the secondary stage, but in the tertiary stage wear rate increases rapidly resulting in catastrophic failure. With increase in cutting speed, the secondary stage keeps on reducing and tertiary stage is attained quickly.

 

(c) Nose Wear:

It is similar to flank wear in certain operations like finish turning. This is more prominent than flank wear.

The fracture may not occur due to wear only. Wear might only assist the cause. Wear will give rise to high energy input due to which more heat dissipation will be present, leading to failure.

It is evident that the greatest tool life can be obtained by regrinding the tool, which produces the smoothest surfaces, free from flaws on both the tool face and the clearance face, with the least change in tool microstructure at these surfaces. However, it may be noted that severe grinding without proper coolant may lead to annealing re-hardening and cracking of the surface.