Tool Failure Mechanisms In High-Speed Intermittent Cutting Of Hardened Steel

In order to acquire good surface finish and high machining efficiency, high-speed machining (HSM) technology has been widely applied in manufacturing process. In the present study, the tool wear mechanisms in high-speed intermittent cutting of hardened steel were investigated theoretically and experimentally. Tool damage model and tool life prediction model were proposed in order to identify the tool wear mechanisms and predict the tool life and the tool wear evolution in high-speed intermittent turning of hardened steel. An analytical model was built to calculate and analyze the transient average tool temperatures in high-speed face milling of hardened steel. High-speed face milling experiments were designed and conducted with the calculated results of tool temperature considered. The wear mechanisms of tools tested with fixed metal removal rate and different cutting speeds were investigated. High-speed face milling of hardened steel was performed in order to explore the critical cutting speeds for cutting force, surface roughness and tool life. The tool wear mechanisms obtained in different cutting speed ranges were distinguished. The effects of cutting parameters on cutting force, surface roughness and tool life were investigated in different cutting speed regions divided by the critical cutting speed.Ceramic tool damage model was constructed based on continuum damage mechanics for high-speed intermittent turning of hardened steel. Theoretical analysis of tool damage mechanisms were conducted based on the established model. The tool damage accumulation can be attributed to the growth of the microcrack at the grain boundary. A mathematical method was applied to calculate the initial and critical damage value of the tool material. The concept of maximum damage equivalent stress (MDES) was proposed to analyze the tool damage mechanisms. The analysis results showed that fracture happened on the tool rake face was more serious than that happened on flank face. Lower exit angle was beneficial for reducing the degree of tool fracture. Smaller rake angle enhanced the possibility of fracture happened on the rake face at the initial cutting stage and reduced that of fracture happened on the flank face in the whole cutting process. When the clearance angle increased, larger fractured area was more likely to arise on the flank face.Taking the theoretical analysis results of tool damage mechanisms into consideration, high-speed intermittent turning of hardened steel was conducted in order to investigate the ceramic tool wear mechanisms. The contribution orders of cutting parameters for tool life was cutting speed, depth of cut and feed rate. The mechanical load influenced more on tool wear at the early stage of tool wear evolution process than it did in the whole tool life, while the thermal load influenced in the opposite way. At relatively low cutting speed, the mechanical load caused microscale damage of the tool material, leading to the fatigue crack growth. When the cutting speed was relatively high, the mechanical properties of the tool material were weakened due to the thermal damage caused by high tool temperature. Regional fracture happened on the tool because of the combined effects of mechanical and thermal loads. The established tool life prediction model can be used to predict tool life and tool wear evolution with acceptable level of accuracy.A transient average tool temperature model was established for high-speed face milling of hardened steel. This model can be applied to evaluate the transient average tool temperatures at any depth of the cutting tool. Based on the established model, the transient average tool temperatures for nine different cutting conditions with fixed cutting speed and metal removal rate were calculated and analyzed in order to identify the differences of the maximum tool temperatures. The proposed theoretical method increased the understanding of tool temperatures in face milling process without costly experimental time. Furthermore, it provided theoretical basis for the design of cutting tool.Based on the theoretical analysis of tool temperature, high-speed face milling of hardened steel was designed and conducted to investigate the CBN tool wear mechanisms experimentally. When the cutting speed increased, the normal wear stage became shorter and the tool wear rate growed larger. At relatively low cutting speed, the wear mechanisms of tool rake and flank faces were abrasive wear and adhesive wear. Due to the severer mechanical and thermal impact, flaking was likely to happen when the cutting speed was relatively high. Since the mechanical properties were weakened by the high cutting temperature, fracture was more inclined to arise on the tool flank face when relatively higher mechanical load is applied. For the purpose of acquiring high tool life of the CBN tool, the tool material should have sufficient capability of resisting adhesion when tested at relatively low cutting speed. At relatively high cutting speed, retention of mechanical properties to high cutting temperature and resistance to mechanical impact were crucial for enhancing the tool life.High-speed face milling of hardened steel with coated cemented carbide tool used was performed in order to investigate the critical cutting speed. At the critical cutting speed of1400m/min, relatively low cutting force, relatively low surface roughness Ra and relatively long tool life can be obtained at the same time. When the cutting speed surpassed1400m/min, cutting speed and feed per tooth became much more influential to cutting force and surface roughness Ra. As for tool life, the effect of cutting speed increased. When the tools were tested within cutting speed range200m/min～1000m/min, the main tool wear mechanisms were abrasive wear, adhesive wear and oxidation wear which mostly influenced the tool flank face. When the cutting speed was between1000m/min and1400m/min, owing to the severer mechanical and thermal impact resulting from the higher cutting speed, coating delamination and microchipping happened on the tool rake face and cutting edge, respectively. As the cutting speed increased from1400m/min to2400m/min, abrupt flaking occurred on the tool rake face due to the increase of tool temperature, mechanical impact and thermal impact.