Influence of the Cooling Environment on Cutting Temperature and Specific Energy when Turning AISI 4340 Steel
Abstract
Temperature monitoring and controlling is a critical aspect influencing the technical and economic success of metal cutting operations owing to the fact that it affects both the quality of the machined component and the performance of the cutting tool. The aim of this work is to investigate the influence of the cutting parameters on tool temperature when turning AISI 4340 steel under three cooling environments: flooding, minimal quantity lubrication and dry cutting. In addition to that, the specific cutting energy required under each cooling environment was measured using a modified impact testing machine, thus allowing the estimate of the temperature associated with the chip formation mechanism. Furthermore, it is aimed at verifying whether this test can replace force measurement using a dynamometer. The findings indicate that the lowest tool temperature was achieved under flood cooling, followed by minimum quantity lubrication and dry cutting. Inversely, lowest specific cutting energy was obtained when dry cutting, followed by minimum quantity lubrication and flooding, thus suggesting that the higher temperatures achieved under dry cutting aid the softening of the work material and the reduction of its shear strength.
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Introduction
The usage of cutting fluids aiming at reducing temperature in cutting zone was first reported by Taylor [1], who manage to increase cutting speed by 40% in turning with the provision of ordinary water as cutting fluid. Ever since, considerable progress has been achieved with regard to cutting fluids formulation and means of application, thus leading to increasing cutting tool life, superior surface finish [2], tighter dimensional accuracy, easiness of chip breakage and transportation [3], machined surface protection against corrosion and reduced cutting forces and energy consumption [4].
Despite the above mentioned benefits, approximately 85% of the cutting fluids used in industry is of mineral basis and therefore offers considerable risk to environment and occupational health and safety during their life cycle [5]. Prolonged contact of cutting fluids may cause dermatitis [6], skin and breathing problems [7], intoxication and cancer [8]. Furthermore, cutting fluid purchase, maintenance and disposal increase production costs. Boubekri, Shaikh & Foster [8] estimate that between 7.5% and 20% of the production costs is related to cutting fluids, while the costs associated with cutting tools are approximately 4%.
Hadad&Sadeghi [2] highlight minimal quantity lubrication (MQL) as an important alternative to flood cooling due to the fact that it combines cooling function with extremely low fluid consumption, which ranges from 10 to 500 ml/h. The influence of the cooling environment (flooding, MQL or dry cutting) on the machinability of TiAl4V when turning with polycrystalline diamond tools was investigated by Revankar et al. [9], who noticed that while MQL provided better surface finish (owing to the reduced friction between tool and workpiece), dry turning promoted higher surface hardness of the machined specimen due to the increase in workpiece temperature combined with higher plastic flow rate when cutting under this condition.
Cutting fluid application at the tool-chip or tool-workpiece interface directly affects cutting temperature. Varadarajan, Philip &Ramamoorthy [10]performed turning trials using AISI 4340 steel as work material and reported lowest temperature using MQL in comparison with dry turning and flooding, under both low and high cutting speeds. The results are explained by the fact that cutting fluid droplets are capable of reaching the interface, making heat transfer more effective.
Conclusion
After conducting continuous turning tests on AISI 4340 steel using coated tungsten carbide tools under flooding, minimal quantity lubrication (MQL) and dry cutting in order to measure the cutting tool temperature (using the implanted thermocouple method) and performing pendulum scratch tests on a modified impact testing machine in order to assess the specific cutting energy associated with each cooling environment, the following conclusions can be drawn:
As far as the influence of the cutting parameters is concerned, temperature in the cutting tool increased with the elevation of cutting speed, feed rate and depth of cut, however, the effect of cutting speed was considered not statistically significant with a confidence level of 95%. This behaviour can be explained by the fact that the tool-chip contact area is reduced as cutting speed is elevated, therefore the heat source is moved farther from the thermocouple location.
Cooling environment statistically affected cutting tool temperature and lowest tool temperatures were recorded when flood turning, followed by minimal quantity lubrication and dry turning.
Inversely, lowest specific cutting energy was obtained during the dry pendulum tests, thus suggesting that the higher temperature achieved under this condition aid the softening of the work material and the reduction of its shear strength. Highest specific cutting energy was recorded for the tests under flooding and intermediate results were achieve using minimal quantity lubrication, which indicates that the oil droplets produced by MQL are capable of reducing the friction coefficient and, consequently, the fraction of the specific energy associated with sliding.
The modified pendulum scratch test allowed the calculation of the temperature associated with the chip formation mechanism.