Background information
Cutting tools are some of the most crucial gears used during turning, drilling, milling, and all other machining operations.
It is imperative to note that machining operations are associated with extremely high forces and temperatures that impact highly on the cutting tools. As such, subjecting cutting tools to extremely high forces drastically increases the chances of the tools being broken. Additionally, extremely high temperatures make a cutting tool too soft to be effective. A cutting tool may withstand too high temperature and forces but that does not mean that it can withstand wear if used over a considerable period.
In the cutting tool expertise, two key aspects are outstanding. One of the two most outstanding aspects is the material used in making the cutting tools. Experts strive to develop materials that can work under high temperatures and forces while minimizing the proneness to wear of a cutting tool during machining operations. The other aspect is the tool geometry where experts strive to optimize the geometry of a cutting tool to make it as efficient and effective as possible for a certain machining operation.
Probable ways of a cutting tool failure in mechanical operations
Cutting tool failure can be grouped into three major possibilities, including fracture failure, temperature failure, and gradual wear.
Fracture failure
A cutting tool working under extremely high cutting force is prone to brittle breakages, which lead to its failure.
Failure resulting from high temperature
A tool working in an environment with extremely high temperatures is prone to softening, especially if the material cannot withstand the high temperatures. Plastics are the most affected by temperature failures since they can easy get deformed and loss the required edges.
Failure resulting from gradual wears
When a tool is exposed to frictional forces for a considerable time, the cutting edges gradually deform, and loss the required shape. Hence, the tool cutting efficiency is gradually reduced culminating to total failure comparable to failure resulting from high temperature/forces.
Mechanisms that lead to wear of cutting tools
- Abrasion: in most cases, work materials have hard particles, which gradually gouge and remove small particles from the cutting tool. Scratching and gouging of the tool particles result in the wearing of the tool through abrasion. Abrasive wearing is common in both flank wear and crater wear and it is linked to considerable cases of flank wear.
- Adhesion: during the machining processes, the chip and the rake face of the cutting tool come into contact under the high temperature and pressure and are likely to have adhesive forces that bring them together (welding). The continuous movements of the chip across the tool result in abrasion of the tool.
- Diffusion: when the cutting tool surface is in contact with the chip surface, there is an exchange of atoms between the two materials. The tool surface loses significant amounts of atoms, which makes it hard. Losing of vital atoms responsible for hardness increases the proneness of the tool to abrasion and adhesion. It is believed that diffusion mechanisms play a major role in many cases of crater wear.
- Chemical reactions: the conditions at the tool-chip boundary are conducive to chemical reactions. The favorable high temperature and the clean surfaces of the tool-chip boundary enhance oxidation. The oxidized layer covering the tool surface is softer relative to the tool material and, therefore, it is easily moved away exposing the tool to new oxidation.
- Plastic deformation: tool wear can also result from plastic deformation, especially if the cutting edge is affected. Plastic deformation happens due to the tool exposure to cutting forces associated with extremely high temperatures. As a result, the cutting edge becomes more prone to abrasion. Notably, most flank wears of cutting tools are linked to plastic deformation.
From the earlier mentioned ways that could lead to failure in cutting tools, it is easy to identify the vital characteristics that a material should exhibit for it to be used in making an effective cutting tool.
Three crucial characteristics necessary for cutting tool materials
- Toughness: A cutting tool material must have the capacity to withstand excessive energy. The toughness of a material is a factor of its strength and level of ductility. A material that poses high toughness is less likely to fail due to fracture failure relative to a material with low toughness.
- Hot hardness: An effective cutting material should possess the ability to retain its hardness even at high temperatures. As such, the material must not be easily softened, especially in the high-temperature environment associated with machining operations.
- Wear resistance: One of the most vital characteristic of a material that enhances wear resistance is material hardness. As such, cutting tool materials must possess the optimal degree of hardness. Nonetheless, material hardness alone is not a sufficient characteristic required to augment the wear resistance property of a material. More than one mechanisms result in the wearing of a tool and, therefore, other factors such as appropriate surface finish of the tool, the harmony of the tool and work material, and the probable use of a cutting fluid influences the wear resistance of a material.
Ceramic materials for cutting tools
The use of cutting tools made of ceramics has a somewhat long history. Ceramic cutting tools have been in gradual development and use for more than a century in Europe. The US, on the other hand, has commercially used ceramic cutting tools since the mid-1950s.
Ceramic tools have gradually developed to the current forms that are predominantly made of the fine-grained aluminum oxide (Al2O3). The aluminum oxide (Al2O3) is subjected to relatively high but regulated temperature and pressures to bond the partly fused particles.
In most cases, manufacturers use extremely pure oxides of aluminum but some add substantively small quantities of other oxides. For instance, zirconium oxide is a popular oxide used by manufacturers who do not use pure aluminum oxide (Al2O3).
Some of the vital aspects and techniques to be followed in the manufacturing of ceramic tools include the use of very fine grain size in the alumina powder, optimizing the density of the mixture by subjecting it to high pressure compressing the particles to augment the material’s low stiffness.
The success of Aluminum oxide cutting tools in the high-speed turning of cast iron and steel is more apparent relative to other oxides. Additionally, Aluminum oxide tools are used for turning of hardened alloys of iron (steels) using high cutting speeds, low feeds and depths and rigid working environments.
However, there are cases of premature fracture failures of ceramic tools. A good number of the cases of premature fracture failures could be linked to a number of mechanical issues, including non-rigid machine tool setups that make the tool be prone to mechanical shock.
Proper application and use of ceramic tools can result in an exceptionally good surface finish. Nonetheless, Ceramic cutting tools are not endorsed for hefty interrupted cut procedures since they are not as tough as required by the said operations.
Apart from being used as inserts in mainstream machining operations, Al2O3 is commonly adopted as an abrasive in many rough and scratchy procedures such as grinding.
Although Al2O3 is commonly used, there are other commercially obtainable cutting tool materials made of ceramic, including silicon nitride, sialon (SiN–Al2O3), (Al2O3–TiC), and aluminum oxide mixed with silicone carbide (for reinforcement)
One of the prerequisite conditions for an effective cutting tool is the suitability of its shape to the specific machining operation.
A key technique of classifying cutting tools is according to the specific machining process a tool is manufactured to accomplish. As such, there are turning tools, cutoff tools, grinding cutters, drill bits, reamers, and taps among others. It is important to note that each tool has a unique geometry depending on its intended purpose.
Further, cutting tools can be divided into two broad categories to incorporate those tools with a single cutting edge and those with more than one cutting edges.
Single-point cutting tools are majorly used in turning, boring, and planning processes. On the other hand, tools with more than one cutting edges are used in boring, reaming, tapping, grinding, drilling of holes, and severing.
Evidently, most of the principles adopted in the single-point tools geometry are similar to those used in multiple- edge tools. The similarity in the principles adopted in both single and multiple edge tools is due to the similarity in the mechanism of chip formation for all machining operations.