Magnetic Field Characteristics of the Flat Type Electro-Magnetic Barrel Finishing Machine O. Sugiura

Transcription

1 Key Engineering Materials Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Magnetic Field Characteristics of the Flat Type Electro-Magnetic Barrel Finishing Machine O. Sugiura Faculty of Education & Human Sciences, Yamanashi University Takeda Kofu-Shi, Yamanashi-Ken, Japan Keywords: Magnetic Abrasive Grain, Barrel Finishing Machine, Magnetic Field Rotation, Magnetic Media Abstract. The effect of increasing the magnetic flux density in the container is examined in an attempt to improve the electro-magnetic barrel finishing machine. A rolled steel core was placed on the cover of the container as the ferro magnetic material to increase the magnetic flux density. The magnetic flux density in the container could then be varied by the height of the container and the applied voltage. The effect of the magnetic flux density on the finishing characteristics was investigated under various conditions. It was found that the magnetic flux density in the container continuously increased when the upper iron core end of the finishing machine was installed near the cover of the container. Optimum values of the magnetic flux density with respect to the finishing characteristics were obtained, and revealed to vary with respect to the material of the workpieces. Introduction Barrel finishing normally involves the loading of workpieces to be finished into a container together with some abrasive media, water and compound. Action is applied to the container to cause the media to rub against the surfaces, edges, and corners of the workpieces or for workpieces to rub against each other, or both. This action may deburr, generate edge and corner radii, clean the workpieces by removing rust and scale, and modify the surface stress [1, 2]. Recently, various kinds of magnetic abrasive machines have been studied and developed, including the magnetic barrel finishing machine developed by the present author as described in [3]. The magnetic barrel finishing machine processes workpieces using a rotating magnetic field generated by either a permanent magnet or an electro-magnet, and both models are currently in practical use. In the permanent magnet machine, the permanent magnets are fixed on a rotating disk to produce a rotating magnetic field in the container. The electro-magnet machine generates the rotating magnetic field using a three-phase induction motor with the rotor removed. The electro-magnet machine generally offers better performance than the permanent magnet machine, particularly with respect to the much simpler structure, which allows for a much smaller machine and hence lower production cost. Furthermore, in the electro-magnetic configuration, the machine produces less noise and vibration resulting in higher safety, and allows for the magnetic flux to be varied by changing the current of the stator winding, thereby providing adjustment to achieve the optimal finishing condition with respect to the material, size and form of the workpieces. In this paper, I describe a new flat type electro-magnetic barrel finishing machine which was manufactured on trial, and experiments were performed in order to prove operating characteristics. Principle of Magnetic Barrel Finishing Figure 1(a) shows the stator of the flat-type three-phase induction motor. The inside and outside diameters of the stator are 130 mm and 214 mm, respectively. Figure 1(b) shows the barrel container of the barrel finishing machine and the stator of the induction motor that is placed under the container. Figure 2 shows a photograph of the flat type barrel finishing machine manufactured by the author. The All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-19/02/16,17:04:50)

2 516 Advances in Abrasive Technology VI (a) Stator of flat type induction motor (b) Stator and barrel container Fig.1 Schematic of flat type electro-magnetic barrel finishing machine Fig.2 Apparatus of finishing machine Fig. 3 Electric circuit and cross section of finishing machine barrel finishing machine is primarily composed of the stator of the axial gap induction motor. The rotor of the motor is removed, and in its place, the container of the finishing machine is placed on the stator core. The rolled iron core was placed on the cover of the container. The electrical circuit and the cross section of the barrel finishing machine are shown in Fig. 3. The magnetic media, the workpieces, the compound and the water are placed into the container. This stator winding generates the rotating magnetic field by applying three-phase voltage, as shown in Fig. 3. A spacer is placed between the barrel and the stator core so that the stator winding can be cooled by a fan. As magnetic media, stainless steel pins of mm in diameter and 4-8 mm in length are normally used. Both ends of the pins are approximately half radiused in order to avoid possible scratches on the workpiece surface. Then, the media rotates in the same direction as the rotating magnetic field. The rotation of the media causes collisions with the workpieces. This sliding action removes burrs and refines the surfaces of the workpieces. This machine was developed for finishing the casting surface of minute gaps, inner sides and fine lines, which are difficult to finish by hand tools. Various metals such as gold, platinum, silver, or even copper alloys can be finished by the barrel finishing machine. Electro Magnetic Characteristics Before the finishing experiments were performed on the magnetic barrel finishing machine, the electrical and magnetic aspects of the machine were investigated. The length Hg was defined as the length between the upper surface of the stator core and the lower surface of the rolled iron core, as shown in Fig. 3. Thus, if the stator current is constant, the magnetic flux density in the container increases when the length Hg decreases. Figure 4 shows the results of the experiments on the

3 Key Engineering Materials Vols voltage characteristics at Hg = 30 mm. Specifically, this figure shows the curves of the stator current I, the input power W and the magnetic flux density B as functions of the impressed voltage V. The stator current I and the magnetic flux density B are nearly proportional to the impressed voltage V in both cases, and the input power W is nearly proportional to the square of the impressed voltage V. The magnetic flux density B was measured on the stator core of r m = 86 mm, as shown in Fig. 5. However, the proportional relationship is the same at other positions. In addition, these relationships apply at arbitrary values of Hg. B-h characteristics for the space over the stator core at various Hg values are shown in Fig. 5, in which h = 0 mm is the position at the upper surface of the stator core. As Hg decreases, the magnetic flux density B increases for h = constant. As h decreases, the magnetic flux density B increases for Hg = constant. However, the magnetic effect of the rolled iron core is slight in the range Hg > 100 mm. Figure 6 shows the variations in magnetic flux density of the upper surface on the stator core Bb and of the under surface on the rolled iron core Bu and the stator winding current I with respect to the length Hg at h = 6 mm. The values of Bb and Bu are large if the length Hg is short. In addition, current I does not vary greatly with respect to length Hg. The magnetic flux density of the space between the stator core and the rolled iron core is fairly high at short Hg. The relationship between the magnetic flux density and the radius of the stator is shown in Fig. 7 at h = 6 mm. The magnetic flux exist almost between the stator core and the rolled iron core, and the magnetic flux density rapidly decreases outside the stator core. As the length Hg decreases, the magnetic flux density increases. Fig. 4 Voltage characteristics at Hg=30mm Fig. 5 B - h curves (V=200V, r m =86mm) Fig.6 Hg characteristics (V=200V, r m =86mm) Fig. 7 B - Hg curves (V=200V, h=6mm)

4 518 Advances in Abrasive Technology VI Finishing Condition The desirable finishing conditions were first investigated for various Hg at V = 200 V. The workpieces which used a cast silver bar of diameter d = 3 mm and length l = 20 mm were manufactured by the lost wax casting process. The average value of five workpieces is used in each experiment. The finishing time is 20 minutes for the surface roughness Ra of the workpiece, as described later. Three experiments using Hg =, Hg = 60 mm and Hg = 30 mm were performed, as shown in Fig. 8 ~ Fig. 10. In the experiments, 0.7 kg of magnetic media, 5 g of compound and 0.4 L of water were used as the desirable finishing conditions. The relationship between the surface roughness Ra and water volume is shown in Fig. 8. One of the conditions was varied to confirm the desirable finishing conditions. The water volume was varied from 0.4 L to 1.2 L. And, it became clear that the effect of water volume on the surface roughness Ra is not recognized. The relationship between Ra and the weight of the compound are shown in Fig. 9. As the weight of the compound increases, Ra degreases rapidly. As the weight of the compound reaches 3 g ~ 5 g, Ra becomes nearly constant with respect to Hg. Thus, 5 g of the compound necessary to finished the workpieces. The relationship between Ra and the weight of the media is shown in Fig. 10. Ra has a minimum value at a specific weight of media. When the weight of the media increases, Ra decreases. After reaching the minimum value, Ra remains nearly constant. The desirable finishing conditions described above are induced from the results obtained by Fig. 8 ~ Fig. 10. Next, the relationship between Ra and finishing time was investigated in order to determine the finishing time for the desirable finishing condition. Silver, brass and stainless steel were used for the Fig. 8 Desirable finishing condition(water) Fig. 9 Desirable finishing condition(compound) Fig. 10 Desirable finishing condition(media) Fig. 11 Finishing process(ra-t curve)

5 Key Engineering Materials Vols workpieces. The finishing results are shown in Fig. 11. The finishing time for good finishing was approximately 10 minutes at Hg = 30 mm. Since the good finishing time for good finishing is approximately 20 minutes at Hg = as described in [4], the time for all finishing experiments was set as 20 minutes. Effect of Magnetic Flux Density on the Finishing Machine Two types of finishing experiments are performed using the various desirable finishing conditions described above. In one experiment, we varied the impressed voltage and observed the change in the magnetic field density. In the other experiment, we varied the length Hg and observed the change in the magnetic field density. Finishing Characteristics in the Case of Variable Voltage. The relationship between the surface roughness Ra and the impressed voltage V is shown in Fig. 12. The magnetic flux density B is proportional to V, as shown in Fig. 4. The workpieces cannot perform the finishing in the range V<60 V, because the magnetic media do not move for weak B. The finishing of the workpieces is performed in the range V>60 V for the magnetic media that are moved by strong B. In the range Hg>60 mm, Ra always decreases as the impressed voltage V increases. In the range Hg<60 mm, Ra shows a minimum value at a specific value of V. When the length Hg decreases, the minimum value of Ra shifts toward lower voltage V. The relationship between the Vicker s hardness Hv and the impressed voltage V is shown in Fig. 13. The value of Hv before finishing is in the range of V<60 V, as shown in Fig. 13. However, the magnetic media move actively in case of V>60 V and Hv increases gradually. As Hg decreases, Hv increases for the same voltage. The photographs in Fig. 13 show cross section in the neighborhood of the cast silver surface both before and after finishing. The photographs of the cross section show clearly that the finishing has been completed. Fig. 12 Ra-V curves in case of various Hg Fig. 13 Hv-V curves in case of various Hg Finishing Characteristics in the Case of Variable Hg. The relationship between the surface roughness Ra and the length Hg at the rated stator current is shown in Fig. 14. A minimum value of Ra exists for each material. With the exception of stainless steel, the minimum Hg value is the optimum value for the barrel finishing condition for all materials. No minimum Hg was found for the stainless steel. As the hardness of the material increases, Hg becomes shorter in order to allow the higher magnetic flux density B. The relationship between Hv and Hg is shown in Fig. 15. As the magnetic flux density B increases, Hv becomes harder. However, Hv is approximately constant with respect to the magnetic flux density B (namely Hg). The finishing characteristics under the desirable finishing condition before and after finishing to the optimum Hg are shown in Table 1 for each material.

6 520 Advances in Abrasive Technology VI Fig. 14 Ra-Hg curves in case of various metals Fig. 15 Hv-Hg curves in case of various metals Conclusion Improvement of the flat-type electro-magnetic barrel finishing machine was achieved, as described below: 1) As the length Hg between the stator core of the finishing machine and the rolled iron core is short, the magnetic flux density increases gradually. 2) The desirable finishing condition is consistent regardless of magnetic flux density. 3) A magnetic flux density which provides the best finishing characteristics exists for each material. 4) As the magnetic flux density increases, the Vickers hardness Hv increases gradually. 5) A rolled iron core was placed on the cover, improving the finishing characteristics of the flat-type electro-magnetic finishing machine. Acknowledgement I should like to express my grateful thanks to the members of Yamanashi University who extended me their kind assistance. References [1] D.B. Dallas: Tool and Manufacturing Engineers Handbook (McGRAW-HILL Book Company, 1976). [2] T. Takazawa: Technical Collection of Surface Abrasive and Finishing (Niikei Gijyutsu Tosho Ltd., 1984). [3] O. Sugiura et al: PCIM 95 Conference, California, USA, Vol. 28 (1995), p.130. [4] O. Sugiura et al: JSGE, Vol. 40 (1996), p. 51.

7 Advances in Abrasive Technology VI / Magnetic Field Characteristics of the Flat Type Electro-Magnetic Barrel Finishing Machine /