written 5.5 years ago by |
Properties Required for Tool Materials:
In order for a machining operation to proceed at a fast rate with minimum tool and machining cost the cutting tool material must satisfy certain basic requirements. The more important of these requirements are given below:
1] Hot hardness:
This represents the capacity of the tool to retain its cutting ability and hardness at the high temperatures developed at the chip-tool interface. To be effective the tool
material must remain harder than the work material at all temperatures.
Hot hardness of the tool material becomes more significant as the cutting speed is increased or the hardness of the metal to be machined becomes higher.
2] Wear resistance:
The life of a tool is determined by the wear developed on its cutting face due to motion of the chip and on its flanks due to contact with the machined surface. In order for the tool to continue to perform its duties satisfactorily it is important that the wear characteristics of its material relative to that of the workpiece are such that excessive tool wear does not occur during the machining process.
3] Toughness:
Toughness is necessary to enable the tool to withstand cutting forces, to absorb shock and to prevent the chipping of the cutting edge. The tool must not become so hard that it becomes brittle. Toughness is particularly important for tools like milling cutters which are subjected to impact loading due to interrupted cutting.
4] Low friction:
The coefficient of friction between the tool material and the chip should be low. This is important for reducing tool forces, keeping chip-tool interface temperature low, increasing tool life and improving surface finish.
5] Thermal conductivity:
A material with a high conductivity can conduct heat away from the chip tool interface faster. This results in a lower chip tool interface temperature, less interface welding and longer tool life.
6] Low cost:
This includes the cost of material, cost of grinding and the cost of replacement when the tool is worn out. A cheap material that requires frequent stopping of the machine for tool changing may prove much costlier in the long run compared to the one which has a higher initial cost but can be operated for longer time at a higher speed.
The properties of the tool machine as outlined above are often contradictory and inter-dependent. For example, a material that has a good wear resistance will not generally have high toughness. There is no single tool material that satisfies all the requirements specified above.