METHOD AND APPARATUS FOR MACHINING HARD, BRITTLE AND DIFFICULTY-MACHINEABLE WORKPIECES

Abstract: Machining of a hard, brittle and difficultly-machinable workpiece (2) by a grinding wheel (12) made of superabrasive grains. The grinding wheel (12) is rotated at a peripheral speed of 1000 to 5500 m/min, and the grinding wheel (12) and the workpieces (2) are moved relative to each other in the machining direction at a feed speed of at least 30 mm/s.

Description: The invention relates to a method of machining a workpiece by means of a grinding wheel, wherein the workpiece and the grinding wheel are moved relatively to each other in the machining direction at a predetermined feed speed and the grinding wheel is rotating with a predetermined peripheral speed for machining the workpiece. Such a method is known from the publication "Hochgeschwindigkeitsschleifen - Möglichkeit zur Produktivitätssteigerung bei der Metallbearbeitung", published by K. Dittmann et al. in the periodical "Werkstatt und Betrieb", April 1987, pages 303 to 307. This publication is generally concerned with high speed grinding of metallic workpieces offering various possibilities for increasing the productivity in metal processing. Therefore, metallic workpieces are mentioned throughout the publication without considering any specific material. In particular, solid metallic workpieces are processed, for example various industrial types of steel as mentioned in connection with the corresponding examples given there, without dealing with the processing and machining of brittle, hard and difficultly-machinable workpieces like ferrite. In this publication, the metallic workpieces in question have a high stability so that there is no considerable risk of damaging the workpiece. Rather, care must be taken concerning the speed and feed motion or engagement not to damage the tool, as explained in connection with the diagrams shown in Bild 3 of this publication where the influence of the cutting speed in grinding on several properties of the tool are explained. From the explanations in this publication the impression is created that the machining operation is improved whenever the peripheral speed of the grinding wheel is increased, namely cutting forces decrease, surface coarseness decreases, grinding wheel wear decreases, lifetime of the tool increases, grinding time decreases, and cutting volume per time unit increases. Also, it is explained in this publication that the method of high-speed grinding should be used in the metal processing industry in those cases, where a large volume of material of the workpiece has to be removed economically during preprocessing. Also, it is explained that the productivity in the high-speed grinding technology has only an upper limit in practice depending on the driving power of the grinding wheel. In this publication, feed speeds in the range between 1000 to 10000 mm/min and peripheral speeds in the range between 60 to 250 m/s are mentioned, but it does not deal with the interrelation between the feed speed and the peripheral speed. In particular, the application of such a machining method in connection with hard and brittle material is not taken into account. As is well known, in the machining of a hard, brittle and difficultly-machinable workpiece, for example in a fluting operation in the manufacture of reading magnetic heads by providing a number of grooves in nearly rectangular ferrite blocks, a grinding wheel made of superabrasive grains and the workpiece are moved relatively to each other in a predetermined machining direction. However, in conventional machining operations of such materials, the relative feed speed of the workpiece and the grinding wheel should be maintained extremely low in order to perform good machining as desired without causing "chipping" to the workpiece and breakage to the grinding wheel. In fact, as far as the present inventor knows, the above relative feed speed in conventional machining operations is 17 mm/s at the most. Hence, the conventional machining operations take a considerably long time. Accordingly, the object underlying the present invention is to provide a method of machining a workpiece by means of a grinding wheel as specified above for machining specific hard and brittle workpieces within shorter time while ensuring good machining and avoiding chipping in the workpiece and breakage of the grinding wheel. This object is solved according to the invention by a method of the type specified above which is characterized in that a ferrite workpiece is used, in that a grinding wheel made of diamond abrasive grains is used, in that the feed speed of the workpiece relative to the grinding wheel is at least 50 mm/s and at maximum 72 mm/s depending on the peripheral speed of the grinding wheel, and in that the peripheral speed of the grinding wheel is 2000 to 5000 m/min. The present inventor has extensively conducted research and experimental work on the machining of hard, brittle and difficultly-machinable workpieces by a grinding wheel made of superabrasive grains as specified above. This work has unexpectedly led to the finding that in order to increase the relative feed speed greatly, it is critical to limit the peripheral speed of the grinding wheel to a predetermined range. In other words, the relative feed speed cannot greatly be increased when the peripheral speed of the grinding wheel is too low or too high. According to the invention, the object is solved in an advantageous manner. As explained in detail in connection with specific examples, the feed speed could be increased considerably so that the operation time could be decreased remarkably. The invention will be explained in more detail below with reference to experimental examples and the accompanying drawings. Brief Description of the Drawings Figure 1 is a simplified side view showing one mode of the method of this invention; and Figure 2 is a diagram showing the relation between the peripheral speed of the grinding wheel and the highest permissible relative moving speed in Experimental Example given hereinbelow. Detailed Description of the Preferred Embodiment The invention will now be described in detail with reference to the accompanying drawings. With reference to Figure 1, a hard, brittle and difficultly-machinable workpiece 2 (which may be of ferrite or various ceramics) to be machined is fixedly secured and held by a suitable method such as magnetic attraction or vacuum chucking onto a chuck table 4 mounted movable in the left-right direction. A driving source 6 which may be an electric motor is drivingly connected to the table 4. The driving source 6 constitutes moving means. Upon energization of the driving source 6, the table 4 and the workpiece 2 fixedly secured thereto are moved in the direction shown by an arrow 8 (or in an opposite direction). Above the table 4, a shaft 10 extending in a direction perpendicular to the sheet surface in Figure 1 is rotatably mounted on a suitable supporting structure (not shown). A circular grinding wheel 12 made of superabrasive grains is fixed to one end portion of the shaft 10. The grinding wheel 12 may be of any known type produced by bonding superabrasive grains such as natural or synthetic diamond abrasive grains or cubic boron nitride abrasive grains by a suitable method such as a metal bond method or a resin bond method. A driving source 14 which may be an electric motor is drivingly connected to the shaft 10. This driving source 14 constitutes rotating means for rotating the grinding wheel. Upon energization of the driving source 14, the grinding wheel 12 is rotated in the direction shown by an arrow 16. To provide a groove having a depth of d and extending in the left-right direction by machining the upper part of the workpiece 2, the relative height of the shaft 10 and the table 4 is adjusted so that the grinding wheel 12 interferes with the workpiece 2 from its upper surface to a predetermined cut depth d . The driving source 14 is energized to rotate the grinding wheel 12 in the direction of arrow 16, and the driving source 6 is energized to move the table 4 and the workpiece 2 secured to its upper surface in the direction of arrow 8. As a result, a groove is formed in the workpiece 2 by the machining action of the grinding wheel 12 as the workpiece 2 is fed (Figure 1 shows an intermediate stage of this fluting operation). If desired, instead of, or in addition to, feeding the workpiece 2, the shaft 10 and the grinding wheel 12 fixed to it may be moved in a direction opposite to the direction of arrow 8. In the above machining, let the force required to machine the workpiece 2 by the grinding wheel 12, i.e. the rotating resistance exerted on the grinding wheel 12 from the workpiece 2, be F and the peripheral speed of the grinding wheel 12 be V . If any effect by the feeding movement is ignored, the power P₁ consumed in the machining of the workpiece 2 by the grinding wheel can be expressed as P₁=F x V. Hence, the driving source 14 should impart power P₂ exceeding the power P₁ to the grinding wheel 12. The present inventor thought that the power P₃ obtained by subtracting the consumed power P₁ determined by ignoring any effect of the feeding movement from the power P₂ actually applied initially to the grinding wheel (P₂ - P₁) is consumed owing to the feeding movement during machining, and therefore the machining efficiency can be increased by simply increasing the power (horsepower) of the driving source 14 and thus increasing the relative feed speed. It was found, however, as can be understood from experiments described hereinafter, that the relative feed speed cannot be increased sufficiently by simply increasing the power of the driving source, and that to increase the relative feed speed sufficiently, it is critical to limit the peripheral speed of the grinding wheel to a predetermined range. More specifically, the present inventor has found that in order to obtain a feed speed of 30mm/s which is about 1,8 times the highest feed speed (17 mm/s ) in conventional machining operations, it is critical to limit the peripheral speed of the grinding wheel 12 to 1000 to 5500 m/s ; and that to obtain a feed speed of 50mm/s which is about 2,9 times the highest feed speed (17 mm/s ) in conventional machining operations, it is critical to limit the peripheral speed of the grinding wheel to 2000 to 5000 m/min. It has also been confirmed that this phenomenon remains basically the same irrespective of changes in cut depth and thickness, the material of which the workpiece 2 is made and the material of which the grinding wheel is made. It seems that when the peripheral speed of the grinding wheel 12 decreases, the consumed power p₁ (=F x V) determined by ignoring any effect of the feeding movement decreases and the feed speed can be increased. Actually, however, when the feed speed is increased by decreasing the peripheral speed of the grinding wheel 12 below the above-mentioned required values, chipping occurs in the workpiece 2 or breakage occurs in the grinding wheel 12. Although no clear reason can be assigned to it, the inventor presumes it to be due to the unique machining behavior of the grinding wheel by which the workpiece 2 is machined while causing ultrafine breakage of the grinding wheel 12 itself. On the other hand, even when the feed speed is increased by increasing the peripheral speed of the grinding wheel above the above-mentioned required values, chipping occurs in the workpiece 2 or the grinding wheel 12 breaks. No clear reason can be assigned to it, either. The inventor presumes that in addition to the above unique machining behavior of the grinding wheel 12, when the peripheral speed of the grinding wheel 12 becomes too high, cooling water to be impinged against the peripheral edge portion of the grinding wheel for cooling fails to collide sufficiently with the grinding wheel 12 owing to the centrifugal force and an air current formed near the grinding wheel by the centrifugal force, and that consequently, the cooling effect of the water is reduced. Experimental Example According to the mode shown in Figure 1, a groove with a depth of 9,0 mm was formed by machining a polycrystalline ferrite workpiece having a longitudinal (machining direction) size of 40 mm, a lateral size of 20 mm and a thickness of 10 mm. The grinding wheel used is a grinding wheel sold under the tradename "AIAIR03" by Disco Abrasive Systems, Ltd. which was produced by bonding synthetic diamond abrasive grains by a metal bond method. The grinding wheel had a diameter of 101,6 mm (4 inches) and a thickness of 0,5 mm. During machining, tap water was impinged against the machining part of the grinding wheel at a rate of 1,0 liters/min. Figure 2 is a diagram showing the relation between the peripheral speeds shown in Table 1 on the ordinate and the highest permissible feed speeds in Table 1 on the abscissa. It is understood from Figure 2 taken in conjunction with Table 1 that when the peripheral speed of the grinding wheel is adjusted to 1000 to 5500 m/min the feed speed can be increased to 30 mm/s which is about 1,8 times the highest feed speed (17 mm/s ) in conventional machining operations, or higher, and when the peripheral speed of the grinding wheel is adjusted to 2000 to 5000 m/min., the feed speed can be increased to 50 mm/s , which is about 2,9 times the highest conventional feed speed (17 mm/s ), or higher. The peripheral speed of the grinding wheel was changed from 250 m/min to 5750 m/min at intervals of 250 m/min At each of these peripheral speeds, the feed speed of the workpiece was increased incrementally by 2/mm/s , and the highest permissible feed speed was determined. The highest permissible feed speed is the feed speed at which spark occurred during machining. When the feed speed was increased beyond the speed at which spark occurred, chipping tended to occur in the groove formed in polycrystalline ferrite workpiece and the grinding wheel tended to break. The highest permissible feed speeds at the various peripheral speed of the grinding wheel are shown in Table 1.

Claims: A method of machining a workpiece (2) by means of a grinding wheel (12), wherein the workpiece (2) and the grinding wheel (12) are moved relatively to each other in the machining direction at a predetermined feed speed and the grinding wheel (12) is rotating with a predetermined peripheral speed for machining the workpiece (2), characterized in that a ferrite workpiece (2) is used, in that a grinding wheel (12) made of diamond abrasive grains is used, in that the feed speed of the workpiece (2) relative to the grinding wheel (12) is at least 50 mm/s and at maximum 72 mm/s depending on the peripheral speed of the grinding wheel (12), and in that the peripheral speed of the grinding wheel (12) is 2000 to 5000 m/min.

Inventors: SEKIYA, SHINJI
Applicants: DISCO ABRASIVE SYSTEMS, LTD.
Application year: 1987
Main classification: B24B1
IPC codes: B24B1