Properties of Tool Material
The type of tool material and its properties will greatly influence the machining process. The type of machining process and parameters of machining like speed, feed, depth of cut etc., will be chosen for a particular combination of work and tool materials, thus the properties of tool and work materials play an important role in the machining process. The following are the important properties of tool materials.
1. Hot hardness
2. Strength, resilience and stiffness
3. Wear resistance; abrasion resistance
4. Coefficient of friction
5. Thermal Conductivity and Specific Heat
6. Toughness
7. Ease of fabrication
Tool material must be harder than workpiece material. Hardness at room temperature is not enough. Hardness must be maintained at elevated temperature prevalent at the cutting zone (1000oC) during machining. The critical temperature at which the hardness of a tool material starts to fall rapidly signifies the limit of its capability.
- Strength, Resilience and Stiffness
Tool material must have high tensile, compressive and torsional strength to resist deformation under cutting forces. Strength is required to resist disintegration of tool material under load. Strength of the tool must be greater than the workpiece material. Tool material must have sufficient impact resistance to withstand shock load experienced during the entry and exit of the tool. Greater impact resistance is required for interrupted cuts. Tool material must have resilience to absorb shock without permanent deformation and needs to have high stiffness, i.e. high ratio of load to deflection to suppress chatter and vibration.
Tool material must have high wear resistance. Wear resistance is dependent upon many properties of the material like hot hardness, strength (e.g. crushing), chemical inertness and propensity for diffusion.
- Low Coefficient of Friction
The tool material must have low coefficient of friction to reduce sliding friction between chip and tool. The friction between chip and tool is more due to welding between chip and tool and subsequent shearing of the welds. Hence low wettability by metals is a desirable property of tool material.
- Thermal Conductivity and Specific Heat
The tool must absorb heat and transmit it away quickly so that the temperature of the tool is not increased. In high speed machining, as possible with carbide and ceramics, the chips carry away most of the heat. Thus this is not an important property of a tool material. With our understanding of the desirable properties of tool materials, let us see to what extent these properties are present in the tool materials of today.