Mean flank temperature measurement in high speed dry cutting of magnesium alloy

doi:10.1016/j.jmatprotec.2004.10.002

Journal of Materials Processing Technology

Volume 167, Issue 1, 25 August 2005, Pages 119-123

https://doi.org/10.1016/j.jmatprotec.2004.10.002 Get rights and content

Abstract

Magnesium in the molten state is flammable when exposed to oxygen. The risk of fire is the main concern during cutting operation. High speed dry cutting is preferable in cutting magnesium alloys due to there being no extra part cleaning work required, the environmental benefits and the ecological concerns. However, fire ignition could happen when the cutting temperature is close to the melting point of magnesium during high speed cutting. An experimental study of the mean temperature on the flank face is presented in this paper. In high speed cutting, the mean flank temperature is not likely to be less than that on the rake face, that is, the cutting temperature, as the undeformed chip thickness is very thin and of the same order of cutting edge radius. To measure the flank mean temperature, it is critically important to mount the artificial thermocouple properly in the workpiece. A new thermocouple mounting arrangement is proposed in this paper. The mean flank temperatures in various cutting conditions were measured and the collected chips were examined under SEM to find the burn marks. It was found that below the mean flank temperature of 302 °C, there were no burn marks on the chips. This indicates that the mean flank temperature can reasonably be used to predict the occurrence of fire in high speed cutting of magnesium alloys.

Introduction

Magnesium alloy, with a nominal alloy density of only 1.8 g/cm³, is one of the lightest structural metals. Magnesium alloys have excellent mechanical properties, including high strength-to-weight and high stiffness-to-weight ratios. Magnesium alloys are currently used in a wide range of applications in the electronics, automotive, and aerospace industries [1].

The cutting forces for magnesium alloy are extremely small. The relative power required to machine some common metals, with magnesium taken as unity, and the typical speeds of machining are indicated in Table 1 [1], [2], [3].

While it is possible to achieve a high cutting speed for magnesium alloy, there are concerns that with an increase in cutting speed, there may be serious flank build-up due to adhesion between the cutting tool and the workpiece. This may cause machining problems related to vibration and tolerances. Another major concern is the danger of fire ignition when dry machining magnesium alloys. Fires may be prevalent when the melting point (400–600 °C) is exceeded [4], [5], [6]. As this constitutes a serious problem in an industrial situation, it is necessary to be able to ascertain the temperature during cutting. This paper reports a study on the measurement of the mean temperature on the flank face in high speed dry cutting of magnesium alloys.

Section snippets

Experimental setup

The experiments were conducted on a Roeders 760 high speed machining centre that is capable of a spindle speed of up to 42,000 rpm. The ball-nose end-mills used were micro-grain tungsten carbide tools with diameter of 10 mm. The work material was magnesium alloy AZ91 (8.5% Al, 0.3–1.0% Zn, 0.17% Mn, <0.05% Si, <0.004% Fe, <0.015% Cu, <0.001% Ni) with a hardness of 65–85 in Brinell.

Table 2 shows the details of the cutting conditions. To avoid damage to the machine tool due to fire, an enclosure

Mean flank temperature measurement

The fire ignition, if it occurs, starts from the chips during cutting. This depends very much on the temperature on the tool rake face (cutting temperature). To determine the cutting temperature, a natural thermocouple is usually used to measure the potential between the chip and the rake face. This presents a problem in the milling process since the tool rotates at high speed. Hence, an artificial thermocouple is often preferred to be mounted in the workpiece. In this case, the temperature

Thermocouple mounting arrangment

A K-type of thermocouple is used in this study. As the voltage magnifying device varies in different environments, the voltage measured may not be perfectly proportional to the temperature. Calibration was conducted by using a CMU310 calibrator. For the calibration procedure, the temperature was preset accurately, and the output voltage was then measured as shown in Fig. 3. The right vertical axis shows the non-linearity, which is used to compensate for the measurement error. Here, the

Experimental results of the mean flank temperature

It is shown from the experiments that the temperature goes up with an increase in the cutting speed (see Fig. 7(a)). This is in agreement with that when cutting other metals such as the high speed cutting of hardened steels. Fig. 7(b) shows that the measured temperature decreases with an increase in the undeformed chip thickness. As mentioned in the previous section, the measured temperature consists of the temperature rise due to the deformation in the shear plane and due to the flank face

Conclusions

The main concern for machining magnesium alloys is fire ignition. Therefore, the temperature analysis is critical in the investigation of high speed dry cutting of magnesium alloy. An experimental study of the temperature in high speed dry cutting of magnesium alloy was conducted. The temperature was detected by using a thermocouple. From the study, it is concluded that

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the mounting arrangement of the thermocouple in the workpiece is critically important as improper mounting of the thermocouple

Acknowledgement

The authors would like to thank Mr. S.T. Ng for his support in conducting the experiments.

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International Magnesium Association Technical Committee, Machining of Magnesium and Magnesium Alloys, Metals Handbook,...

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