proved the ability of increasing the cutting speed without affecting tool wear intens- ity by using cooling agents [1, 2]. Since then, improvement in cooling and ...
Witold F. Habrat, Piotr Laskowski and Angelos P. Markopoulos
3 Machining with High-Pressure Cooling Abstract: The importance of cooling in machining operations is incontestable. Cooling fluids prolong the tool life and aid in the control of the workpiece geometrical properties. Both these features play a significant role on the final part quality, surface integrity, and life cycle. For this reason, most machine tools possess rather simple coolant supply systems. Although these systems offer adequate cooling and lubrication conditions in many machining practices, the effect of cooling and lubrication can be further improved, especially in the case of hard-to-machine materials like titanium alloys. Such improvements include the promising method of high-pressure cooling, among others. The ongoing studies indicate that the application of a coolant with high pressure in the cutting zone can be beneficial in many ways. This chapter reveals the characteristics and benefits of such a cooling approach in machining and describes the supply and tooling systems required for its implementation.
3.1 Introduction The beneficial effect of application of cooling agents during machining has been known for a long time. Already at the beginning of the twentieth century, F. Taylor proved the ability of increasing the cutting speed without affecting tool wear intensity by using cooling agents [1, 2]. Since then, improvement in cooling and lubricating techniques used for machining is being observed. Cooling agents commonly used in machining are not only a coolant but also a lubricant. Therefore, cooling agents can perform a number of tasks: – reduce friction between tool and workpiece and between tool and chip, – reduce the amount of heat and improve its transfer from the cutting zone, – limit the diffusion of workpiece material elements and cutting edge elements, – increase the cutting edge durability, – reduce the workpiece material strength (Rebinder effect), – ease the chip breaking and evacuation from the cutting zone, – clean and preserve machined surfaces and tooling against corrosion.
The influence of cooling agents on the geometrical structure of machined surfaces and outer layer condition is also known [3–8]. The selection of the cooling agent and the cooling and lubricating technique has a significant influence on the process performance and is decisive in terms of its quality. Currently, several different methods are known based on the selection performed by the manufacturing engineer, taking into consideration the type of workpiece material, the requirements defined for the manufactured component, and technological
3.1 Introduction
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Methods of cooling and lubrication in machining processes
"Wet" cutting
Minimum quantity lubrication (MQL)
Dry cutting
Cryogenic cooling
Flood cooling low pressure (LP)
With use of the compressed gas medium
High-pressure cooling (HPC)
Without use of the compressed gas medium
Ultra-highpressure cooling (UHPC) Fig. 3.1: Methods of cooling and lubrication in machining processes.
capabilities. In Fig. 3.1, an account of the cooling and lubricating methods employed in machining can be seen. However, the most commonly used cooling and lubricating method is flood cooling (FC). The cooling agent is supplied at 0.6–1 MPa pressure through external supplying ducts. Temperature reduction during conventional cooling compared with dry cutting equals about 15%. For this type of cooling technique, the effective supply of cooling agent into the cutting zone is hindered due to the creation of vapor near the cutting insert and chip surfaces. This type of cooling is characterized, however, by simple design of the coolant supply system. Ecological and health protection aspects led to the development of minimum quantity lubrication technique. This method is characterized by the application of minimum volume of water and soluble oil supplied in a high-pressure air jet directed into the cutting zone of the tool [9, 10]. Dry cutting also limits the dangers for operator’s health and reduces pollution. At the same time, it results in thermal shock effects reduction and reduces cooling liquids purchase and maintenance costs and the costs of coolant supply system. For such approach, however, several limitations are present due to significant adhesion between the chip and the tool, temperature increase in the cutting zone, intense tool wear and workpiece deformation exceeding dimensional and geometrical tolerances. Thus, the ability to completely eliminate cooling agents is limited, especially in terms of machining hard-to-machine materials including flammable titanium or magnesium alloys [11]. Cryogenic cooling consists in directed cooling by using very low temperature cooling agent. The goal is to avoid high process temperatures during machining. It is achieved by providing the coolant, usually in liquid state, to the cutting tool and
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finishing, respectively. The model correlated cutting conditions, coolant pressure, tool type, forces, roughness, and circularity to tool life and wear rate [34]. Three-layered feed-forward, back-propagation models were proposed that were able to produce reliable results for the analysis and optimization of the cutting process. Furthermore, the optimization of machining performance in HPC turning of Ti– 6Al–4V alloy was presented with the use of multiregression analysis. Then, genetic algorithms were employed in order to determine the optimum machining parameters, i.e., cutting conditions and coolant pressure; surface roughness, material removal rate, and cutting power were considered as optimization criteria in this analysis [35]. It is worth noting that maximum tool life was achieved for the highest level of pressure of the analysis. Finally, a Taguchi design of experiment investigation was reported to determine the effects of HPC on surface integrity in machining of Inconel 718 [36]. More specifically, the effect of cutting speed, feed rate, and coolant pressure on residual stresses was investigated and was concluded that turning parameters have a different effect on radial and circumferential residual stresses.
3.6 Conclusion This chapter presents the use of HPC as a means to dissipate extensive heat of the cutting tool–workpiece–chip system more effectively than the traditional cooling methods employed so far in machining. With this approach, coolant penetrates the cutting zone deeper and more focused, allowing for better coolant flow and higher temperature reduction through convective heat transfer. Furthermore, the high pressure of the coolant invokes the bending and consequently the breaking of chip, differentiating the chip formation processes. This action leads to a reduced contact length between the cutting tool and the chip and thus to minimization of friction and more space for the coolant to penetrate the cutting zone. It is analyzed theoretically, experimentally, and with the use of modeling and simulation techniques that HPC is connected to lower cutting forces and temperatures, resulting in turn to lower vibration of the tool system, lower cutting tool wear, better surface quality, and tolerances of the finished parts. The aforementioned features are especially beneficial for the machining of materials like titanium alloys used extensively in the aerospace industry.
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