The magnetic field kept the powder in the working gap and created normal forces in the contact of the powder with the inductor poles and the work surface [2]. The magnetic forces and friction forces between the poles and the powder were larger than the ones between the work surface and the powder. For this reason, the powder followed the poles of the rotating inductor. The vector and the value of the magnetic force F_m that acted on each grain of the powder was determined using the formula [2]:

      in which µ₀ is the magnetic conductivity of the vacuum; V_gr is the volume of the grain; and  and gradB are the magnetic induction and its gradient at a point of arrangement of the grain. The quantity of the powder placed in the working gap was determined using the recommendation in [2].

Vibrating Table. Previous researches [2,3, and 6] have shown that oscillation of a workpiece or inductor poles during MAF increases productivity and improves the roughness of a work surface. Additional oscillation increases the mobility of the grains of the powder in the working gap and thus raises the cutting ability of the powder. The interrupted contact induced by oscillation reduces the friction between the powder grain and the workpiece surface, which helps keep the powder from scattering. Besides, the additional oscillation perpendicular to the feed direction increases the relative motion and improves the roughness of the work surface.
      In this research, a vibrating table was made to find out the role of oscillation during deburring using the MAF method. The vibrating table shown in Fig. 2 allows the imparting of the oscillation movement to samples in a cross or longitudinal direction to feed motion. Samples were fixed on a top plate, which was made from ferromagnetic steel. The construction of the vibration table allow operate the frequency and the amplitude of the oscillation.

Characteristic Equation for the Magnetic Abrasive Finishing Method. Magnetic abrasive powders carry out the role of an abrasive tool. The efficiency of deburring depends on the powders’ characteristics. The powders are characterized by many parameters, the properties of which are shown under the magnetic field and depend on the operating condition. A method of evaluating the performance of the powders is proposed here, which can be applied in any shop with machines for MAF. The machining characteristics and tool life of the powder in surface finishing are very important for efficient MAF operation. The experimental relationship for the specific removal (q) in surface finishing is represented in Fig. 3. The following characteristic equation for the specific removal mass in the surface finishing operation by MAF is suggested based on experimental experience[2]:

in which q, mg/cm² is the removed mass from the 1cm² area of the work surface in time; q₁, mg/(cm²*min) is the removed mass from the 1 cm² area in the first minute (1 second or 1 hour can also be chosen as the time unit); and m is an exponent that describes the productivity of the MAF process, which is less than 1.

      The parameter q₁ is offered for use to compare the cutting properties of powders in the given conditions of MAF. The parameter m is convenient for an indirect estimation of the tool life of the powder. Certainly, the parameters q₁ and m in Eq. (2) depend not only on the properties of the powder but also on the magnetic inductor design and all other conditions of the MAF operation. Therefore, comparison of the powders is recommended for maintenance of constant conditions of the MAF. In this case, parameters q₁ and m will fully reflect the cutting properties of the powders. The cutting ability of the powder placed in a working gap is reduced due to the wear and destruction of the abrasive grains of the powder. And the reduction of the amount of the powder because of its dispersion.

The parameters q₁ and m were obtained by mathematical processing from the experimental relations q = f(t). The relationship of parameter "m" to the tool life of the powder is visible in the following equation. Derivatives q to t refer to the surface removal rate by the same portion of the powder at the moment tq_i:

It follows from Eq. (3) that the longer the operation time t_i, the lower the removal rate q^’_img/(cm²*min). By finding the minimum removal rate q^’_i= q^’_min, when the powder should be replaced,it becomes possible to calculate the allowable powder operation duration, T, of the powder using Eq.(4). The time T refers to the tool life of the powder and may be used to compare powder performance.

Experiments to Determine the Performance of Magnetic Abrasive Powders

Experiment to Compare Powder Performance in Surface Finishing. A series of tests was executed for composite magnetic abrasive powders that were made by the company SPC “Reactivelectron” NAN Ukraine specially for polishing using the MAF method. The powders are listed in Table 1. The grains of each powder contain abrasives such as TiC or NbC and a ferromagnetic component such as iron or polymer (the structure of the last one is unknown). The ferromagnetic component simultaneously provided the magnetic and mechanical properties of the grains and served as a binding material (bundle). The powders had different granularities and locations of abrasives in their composite grain i.e., on their surface or distributed throughout its volume.

      This series of tests was executed to obtain the experimental relationship q = q₁t^m in surface finishing for comparison of magnetic abrasive powders. Similar samples were provided without holes and placed in section “c,” as shown in Fig. 1b, for homogeneity of performance. The surfaces were grounded to maintain the same surface condition. The magnetic inductor described in Fig. 1a was used. The test conditions were as follows: the sample material is alloy steel SM45C; the volume of the powder portion V_p is 19 cm³; n = 130 min^-1; f = 187 mm/min; I = 1.0A; the height of gap 2 mm; and the duration of the test of each powder t= 6.4 min (15 strokes of the table). A coolant was injected periodically into the working gap during the test.
      For each powder from the list in Table 1, the characteristic Eq. (3) is obtained by experiment and approximation. The parameter q₁ and the tool life T were chosen as criteria for the comparison of the powders. The initial productivity q₁ characterizes the cutting ability of the powder in its initial condition. To calculate the tool life T using Eq. (4), it was conditionally accepted that the minimum removal rate, q^’_min, is equal to half of the initial removal rate at t=1 and q^’_min= 0.5*q^’₁. By substituting t=1 into Eq. (3), q^’1=m*q₁ and q^’_min=0.5*q^’₁. Therefore q^’_min=0.5*m*q₁ can be substituted into Eq. (4). Then the tool life of the powder is obtained simply from T = 21/(1-m). The tool lives are calculated and listed in Table 1. The general level of the tool life of the powders in Table 1 is small, which can be explained by a rather small magnetic induction, B=0.47*T, at the working gap, and the gradual scattering of the powder from the working gap for this reason. Nevertheless, the results allow the analysis of the characteristics of the compared powders.

      Values q₁ and T for the powders with identical chemical structures were averaged and are represented in Fig. 3. Powders Fe-TiC showed higher cutting properties than powders Polymer-TiC, although they had the same abrasive component as titanium carbide. It is possible to assume that the higher cutting ability q₁ of the powder Fe-TiC is explained by the higher magnetic properties of its ferromagnetic component, Fe. Powders Polymer-TiC had higher tool lives because of the higher level of durability of its ferromagnetic component, ‘polymer.’ Unfortunately, the manufacturer of the powders has not provided data on the powders’ magnetic and mechanical characteristics, due to which we cannot confirm our assumptions of the figures.

      The influence of the sizes of the grains of the powders is shown in Fig. 4, in the example of the Fe-TiC composition. The data for the powders with different abrasive particle sizes and different composite grain arrangements were averaged to obtain more authentic results. The reduction of the grain size V results in the decrease of the cutting ability of the powder Fe-TiC and the increase of its tool life. These results conform to Eq. 1. Smaller magnetic forces work on grains with smaller sizes (V). Accordingly, the cutting forces are also reduced.

Fig.4. Initial productivity and tool life of the powders with different graininess

      Powder no. 4 had the highest initial productivity q₁ and powders nos. 7, 10, 12 and 13 had high tool lives at a high enough initial productivity. Because of their high tool lives, the last powders are preferable due to the stability of the MAF process. Therefore, powders nos. 4, 5, 7 and 13 are used in subsequent researches on deburring.

Deburring using the MAF Method. It is possible to predict the cutting performance of powder according to the characteristic curve obtained from the surface finishing test. Deburring performancewas examined using the same powder as in surface finishing. Powders for experiment are selected according to the characteristic curve. Those with three different compositions were selected for the burr removal experiment on the basis of the powder test results–i.e., Fe-TiC (no. 4), Polymer-TiC (no. 7), and Fe-NbC (no. 13) which showed good finishing performance. They had the same composite grain sizes of 500/400 µm. Burrs were formed by drilling an SM45C plate using a carbide drill with a diameter of 3 mm. Regular burrs were produced and measured with a laser sensor. Samples were located in section “c,” as shown in Fig. 1, to produce a uniform deburring effect. The same cutting conditions as those in the surface finishing test are used for deburring. The averaged burr heights are represented in Fig. 5 according to the scheduled number of table strokes using the three powders selected before. The reliability of the measured data with a laser sensor has been discussed in a previous work [6].
      In powder no. 4, burrs are removed almost after three strokes, as shown in Fig. 5. They remain after three strokes, when powder numbers. 7 and 13 were used for deburring. After 7 strokes of all powders, however, they were removed completely. It took about 40–60 seconds to remove burrs with an initial height of 0.15 mm using powder no. 4. At the same time, the top surface is polished. Burrs are removed from each hole in section “c” of the fixture. The result of this comparison proves the reliability of the characteristic curve obtained from the finishing test. This result suggested the very important application of the characteristic curve to the selection of the proper powder for efficient deburring. Likewise, the method could be used to evaluate the performance of newly developed powders.

Fig.5. Decreasing of burr heights by different powder

      Regarding deburring efficiency, it was well proven that the magnetic abrasive deburring method is very effective. The edge shape after deburring was very sharp. The corner shape could be managed by controlling deburring conditions such as deburring duration, proper powder, and current, among others.

Summary

1. The electromagnetic inductor applied here is sufficient for removing the burr after drilling on steel components. The workpieces have to be located in sector “c” of the top surface of a fixture for uniform deburring.
2. The developed technique for the comparison of magnetic abrasive powders allows the selection of the proper powders for concrete technological operations. The initial productivity q₁ and the tool life Ň are suggested as criteria for the comparison and optimization.
3. The performance of the deburring operation of each powder can be evaluated from the results of the powder experiment. Among the tested powders, the powder Fe-TiC, with grain sizes 500~400 µm and abrasive sizes 40~28 µm and located on the surface of the composite grains, is best for removing burrs of steel workpieces.
4. Magnetic abrasive deburring could be more efficient in deburring many kinds of precision parts such as microelectric parts, since its cutting force can be changed by controlling the current.

References

1. S.L.Ko and J.K.Lee: Analysis on Burr formation in Drilling with New Concept Drill, J. of Materials Processing Technology, Vol,113/1-3(2001), pp392-398.
2. Y.M.Baron: Technology of abrasive finishing in the magnetic field (Mashinostroenie, - Leningrad 1975).
3. Y.M.Baron, M,Manchenko and A.V.Chubais: Magnetic-abrasive finishing of parts made from non-magnetic steel, Proc. «Technology of mechanical engineering for light and food industry and household device. – Moscow: TSIITAI light and food industry, 1976. Issue.10, p.9-19 (Rus).
4. S.L.Ko, J.W.Chae, J.I.Park, Y.M.Baron and V.S.Polishuk: Proc. On 7th Int. Conf. “Burr Formation, Deburring & Surface Finishing”, 2004, Berkeley CA, USA, pp355-363.
5. T.Shinmura, K.Takazawa and E.Hatano: Study on Magnetic-abrasive Finishing (1 st Report) –On Process Principal and a Few Finishing Characteristics, JSPE-52-05(1986).
6. .L.Ko: 3rd PSFDT, 2003, Melborn, Australia.