ABSTRACT

This paper deals with the mechanisms governing influence of magnetic field on wear resistance of cutting tools in drilling process. The results show that the rate of crater wear of both HSS and carbide drills submitted to a high d.c magnetic field intensity, is drastically reduced compared to the one obtained without magnetisation. Moreover, opposite variations of thrust force have been recorded in the presence of magnetic field when 0.38% carbon steel work material were drilled with HSS and carbide tools. These results suggest that even the magnetic effect seems to be manifold, it can be basically grouped into two categories (i) a change in the cutting mechanics (ii) a material properties variation ofworkpiece and tool.

KEYWORDS: magnetic field, wear, dry drilling, HSS and carbide tools, drilling forces.

1. INTRODUCTION

Drilling, for lack of reliable managing drill wear and avoiding catastrophic failures, is still under extensive research and development. Much of the works to control drill wear follows two approaches [10, 18]. One is to develop a reliable real-time drill wear measurements as an automation issue of traditional drilling operations [10]. The second is to look for new alternative methods to cut difficult holes with adequate precision. Methods applied to real-tool wear monitoring and in spite of many attempts have not yet proven to be attractive economically nor technically. New holemaking alternative is a task amenable to in-shop research and development. Recently, two alternative methods include the Thriller tool from Emuge Corp (Northborough, MA), which enables hole drilling, thread milling, chamfering and spot facing all with a single tool, and the Novator AB (Spanga, Sweden) orbital drilling tool currently undergoing testing for aerospace and automotive applications [18].

This paper will attempt to present study regarding the "wearlessness" effects by application of external electromotive force (EMF) sources (e.g. magnetic field) with special emphasis in its illustrations and discussions on drilling process. Indeed, monitoring wear resistance of cutting tools by applying an external electrical current have been first studied by Bobrovoskii [3], Kanji and Pal [11]. An increase of tool life time was observed. However no viable reasons for this improvement were given. Later, Bagchi and Ghosh [1, 2] reported that residually magnetized HSS tools while machining mild steel exhibit increased wear-resistance when compared to no-magnetized tools. To provide some reasons explaining why a magnetized cutting tool has a greater life, Chakravarty proposed a qualitative model [4]. In his attempt of modelling, Chakravarty assume a physical reorientation of elementals magnets. Although the results of the modeling analysis were encouraging, the model itself was subject to question.

Two years later, Pal and Gupta [17] studied the effect of an alternating magnetic field on the wear behaviour of HSS drills. The authors observed that the presence of magnetic field considerably reduces the wear rate and possible reasons for this improvement have been suggested. More significant explanation of the "wearlessness" by application of magnetic field are given by Muju and Ghosh [13,15] when conducting tool wear experiments with magnetized HSS turning tools on mild steel and brass. The authors state that the magnetic filed influences adhesive wear behaviour in such a way as to reduce the wear rate of the body with the lowest magnetic permeability. Moreover, they foresee that an increase of the temperature reduces the magnetic field effect. Following this reasoning, Muju and Radhakrishna [16] conclude that the application of a magnetic field to a contacting pair reduces the activation energy of wear and diffusion and is advantageous only when H₁ / H₂ > 2 where H₁ and H₂ are the harnesses of bodies with high and low magnetic permeability respectively. Even this criterion supports some of the experimental data reported in preceding works, it seem to be only suitable for describing the improvement of wear behaviour in the presence of a magnetic filed when an adhesive wear mechanism is a predominant. Thus the criterion which indicates the beneficial effect of the magnetic field may not be suitable for the selection of contacting pair.

Recently, we published some data on the magnetic field effects on HSS tools durability during machining [5, 6]. Cutting tests consisted on turning experiments. We observed high values of tool durability and significant reduction of wear rates at higher level of magneto-mechanical excitation, which is characterized by higher magnetic field and cutting speed.

In this study an attempt has been made to confirm this phenomenon in drilling process and to clarify the "wearlessness" mechanisms by application of external electromotive force (EMF) sources (e.g. magnetic field).

2. MAGNETIC-DRILLING SET-UP AND PROCEDURES

2.1. Materials

Workpiece and tool materials selection requires a specific approach to achieve better understanding of the magnetic field influence on the tool wear. In this connection, the effect of external magnetic field has been considered in respect to materials properties. Hence, all drilling tests were run on test specimens machined from block of ferromagnetic 0.38 percent carbon steel.

Therefore, two kinds of twist drills were used as reported in Table 1. One is non-magnetic carbide twist drill. The other one is ferromagnetic HSS twist drill. The selection of HSS and carbide tools was motivated respectively by their magnetic reactivity and inertness with respect to the application of external electromotive force sources. The basic objective is to compare their wear response in order to appreciate whether the magnetic field effect is much concerned by a possible variation in materials properties rather than a probable change in the mechanisms of cutting.

2.2. Magnetic-drilling set-up

The drilling tests were carried out on vertical machining centre MIKRON VCE 500. The machine's rated maximum spindle speed is 7500 rpm with a maximum of 1 IkW power. To establish some effects of d.c. magnetic field on the growth of drill wear under dry cutting conditions, a special drilling-fixture was built-up. An appropriate means of hole machining using an external magnetic field is through the work-piece holding device.

Drilling is hence accomplished by feeding the magnetized rotating drill to the stationary work-piece already linked by magnetic flux. The external view of the used holding device is shown in figure 1. The last is designed to hold work solidly on the machining centre, to apply the magnetic field and to permit the associated drill forces to be gathered during a single test. Therefore, the drill jig consists of stiffened plate with three steel blocks of cylindrical shape, which serve as a fixed support of coil. Enclosed the body of the drill jig, the work-piece is held by a 3-jaw spur chuck.

A Kistler drill Dynamometer is mounted between the stiffened plate and spur chuck to monitor forces during hole making in order to follow the drill wear growth. To check the spindle head perpendicularity to the worktable and to make sure that coil and the work-piece are situated coaxially an positioning indicator was shop made. It consists of pipe whose its inside and outside diameters are respectively equal to the outer diameter of the work-piece and inner diameter of the coil.

The magnetic field which crosses both the drill and the work, in line with the drill axis, was created by an applied d.c. electric current in a coil fixed around the workpiece as shown in figure 1. The coil had an inside diameter, 6 mm, in excess of the tool diameter and the length of the coil was such that a considerable portion of the tool was linked by magnetic flux. The magnetic field strength was varied in the range of 0-3x10⁴ A/m according to the electric current intensity.

2.3. Procedures

The experiments consist of drilling a blind holes of 8mm in diameter and 20 mm in depth using 10 deburring cycles as shown in figure 2. This manner to drill under magnetic field has been taken for efficient chip and heat extraction. It seems to be an adequate approach to estimate the contribution of magnetization factor to the total drill wear compared to conventional drilling that build up a lot of friction and heat. Indeed, when considering wear of drill subjected to external magnetic field, it should be taking into account that it results from both mechanical and magnetic actions, which are closely interrelated. Thus, all dry drilling experiments have been performed using this procedure under the machining conditions given in Table 1. The cutting conditions chosen are the results of several initial tests under the used drilling configuration (see figure 2) in magnetic-free process. To get reliable results, each experiment was repeated five times under identical conditions. For every single test, a new tool and a work-piece sample were used. Average thrust force and torque for each test are also estimated for further analysis.

Tool wear features were investigated ex-situ by Optical Microscopy and quantified using a white light interferometry profilometer NT-3300 (Veeco device). A first important advantage of the white light interferometer is that the height (or 'Z') resolution is independent of objective. This allows to study wide areas -as often required on cutting tools- with low magnification objectives, while maintaining the high resolution. Secondly, due to its large Z range (up to 2 mm), the system can be used to measure both nanometer details and millimeter-sized steps, permitting to study the earlier stages of wear process as well as the more severe ones. Moreover, this technique allows the study of some supplementary criteria like the volume of the crater, the width of the crater and the roughness of the crater.

Fig. 1 Experimental set-up of hole making under magnetic field

Fig. 2 Schematic diagram showing the deburring cycles for chip removal during a blind hole drilling.

REFERENCES

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