Polishing process

Abstract

A workpiece (1) is rotationally symmetrical about an axis (L0), wherein a cross-section of a surface (5) to be polished, cut by a virtual plane encompassing the axis (L0), has an arc (5a) of more than 120° from a point intersecting the axis (L0) to one side of the axis (L0). When the workpiece (1) is polished with a rotary brush (10), the workpiece (1) is rotated about the axis (L0), and the rotary brush (10) is rotated and pressed against the surface (5) to be polished and moved along the arc (5a), while a rotation axis (L1) of the rotary brush (10) is kept perpendicular to a tangential surface of the surface (5) to be polished.When the surface to be polished (5) and the rotary brush (10) are viewed from a rotation axis direction (L1) along the rotation axis (L1), a rotation direction (R2) of the rotary brush (10) on a front side of a movement direction (M1) of the rotary brush (10) is opposite to a rotation direction (R1) of the workpiece (1).

Description

Field of InterestThe present invention relates to a polishing method for polishing a spherical part of a workpiece using a rotary brush.BackgroundA polishing method for polishing a surface of a rotationally symmetric workpiece with a polishing tool is described in Patent Literature 1. In this literature, the workpiece is a lens, and a surface to be polished of the workpiece is a curved surface protruding toward one side in the axial direction along the central axis. The polishing tool has an annular working portion with abrasive grains embedded in an elastic material. The elastic material is a rubber, a resin or a sponge. When the surface to be polished is polished, the workpiece is rotated about the central axis. While the polishing tool is rotated about the central axis of the working portion, the working portion is pressed against the surface to be polished and moved along the surface to be polished.A rotary brush with a bundle of grinding elements is known as a polishing tool. A rotary brush described in Patent Literature 2 includes: a brush body including a bundle of grinding elements and a holder of the bundle of grinding elements that holds a proximal end part of the bundle of grinding elements; and a brush housing that radially surrounds the brush body from the outside. The bundle of abrasive members is a bundle of linear abrasive members made by impregnating and curing aggregate yarn of inorganic filaments such as alumina filaments with a resin. The brush housing holds the brush body with a distal end portion of the bundle of grinding elements exposed to the outside.List of ReferencesPatent LiteraturePatent Literature 1: Japanese Patent Application Publication No. 2011-36974Patent Literature 2: Japanese Patent Application Publication No. 2009-50967SummaryTechnical ProblemA surface of a rotationally symmetric workpiece can be polished with the rotary brush described in Patent Literature 2. For example, a workpiece to be polished having an approximately spherical surface may be polished with a rotary brush. However, no such polishing method has been developed.In view of the above problem, a polishing method for polishing a spherical part of a workpiece with a rotary brush has not been developed.Solution to ProblemsIn order to solve the above problem, the present invention provides a polishing method for polishing a workpiece. The workpiece is rotationally symmetric about a predetermined axis and has a surface to be polished, wherein a cross section of the surface to be polished cut by a virtual plane including the axis has an arc of more than 120° from a reference point cutting the axis toward a side of the axis (L 0). A rotary brush is used as the polishing tool. The workpiece is rotated about the axis. The rotary brush is rotated and pressed against the surface to be polished and moved along the arc of the cross section while a rotation axis of the rotary brush is kept perpendicular to a tangential surface of the surface to be polished. When the surface to be polished and the rotary brush are viewed from a rotation axis direction along the rotation axis, a rotation direction of the rotary brush is opposite to a rotation direction of the workpiece on a front side of a movement direction of the rotary brush.In the present invention, the workpiece and the rotary brush are rotated when the surface of the workpiece to be polished is polished. The rotary brush is pressed against the surface to be polished and moved along the arc of the cross section while the rotational axis of the rotary brush is kept perpendicular to a tangential surface of the surface to be polished. Here, the rotational direction of the workpiece and the rotational direction of the rotary brush are opposed to each other on the front side of the moving direction of the rotary brush when the surface to be polished and the rotary brush are viewed from the rotational axis direction along the rotational axis. With this configuration, the brush bite into the surface to be polished, compared to when the rotational direction of the workpiece and the rotational direction of the rotary brush on the front side of the moving direction of the rotary brush are the same direction. As a result, the spherical surface to be polished can be polished more easily.In the present invention, the rotary brush may be rotated in a first rotational direction and pressed against the surface to be polished, and may be moved toward one side of the axis along the arc of the cross section from a position offset on another side of the axis relative to the reference point, via the reference point, to an apex 90° away from the reference point. The rotary brush can then be moved away from the surface to be polished. The rotary brush may thereafter be rotated in a second rotational direction opposite to the first rotational direction and pressed against the surface to be polished, and may be moved from the terminal end opposite to the reference point to the vertex in the arc of the cross section.Here, when the surface to be polished of the workpiece is formed by a cutting operation of cutting the workpiece by bringing a cutting tool into contact with an outer circumferential surface of the workpiece rotating about the axis, the spheroidicity of the part to be polished may decrease beyond a preset tolerance range due to the accuracy of the cutting process. For example, the cross section of the surface to be polished may have an oval shape that is longer in the direction orthogonal to the axis than in the axial direction. In this case, when the rotary brush is moved from the reference point toward the terminal end of the arc of the cross section of the surface to be polished, the depth of cut on the front side of the moving direction of the rotary brush is decreased as compared with the depth of cut on the back side of the moving direction of the rotary brush at a time when the rotary brush crosses the apex. As a result, the polishing performance on the surface to be polished by the rotary brush can be reduced when the rotary brush polishes the terminal end side with respect to the apex in the arc of the cross section. In order to solve such a problem, in the present invention, when the rotary brush polishes the terminal end side with respect to the apex in the arc of the cross section, the rotation direction of the rotary brush is set as the second rotation direction opposite to the first rotation direction. In addition, the moving direction of the rotary brush is set as the direction from the terminal end to the vertex, and the rotating direction of the workpiece and the rotating direction of the rotary brush are set as directions opposing each other on the front side of the moving direction of the rotary brush. This setting can avoid the reduction of the cutting depth on the front side of the moving direction of the rotary brush compared to the cutting depth on the back side of the moving direction of the rotary brush, thereby avoiding the reduction of the polishing performance on the surface to be polished by the rotary brush. As a result, the polishing accuracy can be maintained in the same manner as when the cross section of the surface to be polished is circular, even when the cross section of the surface to be polished has an oval shape that is longer in a direction perpendicular to the axis.In the present invention, the rotary brush may be rotated in a first rotational direction and pressed against the surface to be polished, and may be moved along the arc of the cross section from an apex 90° away from the reference point toward one side of the axis via the reference point to a position offset relative to the reference point on another side of the axis. The rotary brush can then be moved away from the surface to be polished. The rotary brush may then be rotated in a second rotational direction opposite to the first rotational direction and pressed against the surface to be polished, and may be moved from the apex to a terminating end opposite the reference point in the arc of the cross section.Here, when the surface to be polished of the workpiece is formed by a cutting operation in which the workpiece is cut by bringing a cutting tool into contact with an outer circumferential surface of the workpiece rotating about the axis, the cross section of the surface to be polished may have an oval shape longer in the axial direction than in the direction orthogonal to the axis. In this case, when the rotary brush is moved from the reference point toward the terminal end of the arc, the depth of cut on the front side of the moving direction of the rotary brush is decreased from the reference point toward the vertex as compared with the depth of cut on the back side of the moving direction of the rotary brush. As a result, when the rotary brush polishs the reference point side with respect to the vertex in the arc of the cross section of the surface to be polished, the polishing performance on the surface to be polished by the rotary brush can be reduced. In order to solve such a problem, in the present invention, the rotation direction of the rotary brush is set in the first rotation direction when the rotary brush polishes the reference point side with respect to the apex in the arc of the cross section, and the moving direction of the rotary brush is set in a direction from the apex to the reference point. When the rotary brush polishes the terminal end side with respect to the apex in the arc of the cross section, the rotation direction of the rotary brush is set in the second rotation direction, and the movement direction of the rotary brush is set in a direction from the apex to the terminal end. This setting can avoid the reduction of the cutting depth on the front side of the moving direction of the rotary brush compared to the cutting depth on the back side of the moving direction of the rotary brush, thereby avoiding the reduction of the polishing performance on the surface to be polished by the rotary brush. As a result, the polishing accuracy can be maintained in the same manner as when the cross section of the surface to be polished is circular, even when the cross section of the surface to be polished has an oval shape that is longer in the axial direction.In the present invention, the rotary brush may be rotated in a first rotational direction and pressed against the surface to be polished, and may be moved along the arc of the cross section from a position offset on another side of the axis relative to the reference point, across the reference point, to a terminal end opposite to the reference point in the arc of the cross section. In other words, when the spheroidity of the part to be polished is high and the cross section of the surface to be polished has a circular shape, the depth of cut on the front side of the moving direction of the rotary brush does not change even if the rotary brush goes beyond the apex of the arc in the cross section of the surface to be polished. Thereby, the polishing performance can be maintained on the surface to be polished by the rotary brush while the rotation direction of the rotary brush is set in the first rotation direction and the movement direction of the rotary brush is maintained in a direction from the reference point to the termination end.In the present invention, it is preferable that the workpiece rotation speed at which the workpiece is rotated about the axis is lower than the brush rotation speed at which the rotary brush is rotated about the rotation axis, and that the brush rotation speed is not divisible by the workpiece rotation speed. This configuration can prevent occurrence of a striped polishing pattern on the surface to be polished. In other words, when the brush rotation speed is divisible by the workpiece rotation speed, stripe-shaped polishing tracks occur at equal angular intervals around the axis on the surface to be polished. In contrast, the occurrence of such stripe-shaped polishing tracks can be avoided when the brush rotation speed is not divisible by the workpiece rotation speed.In the present invention, the rotary brush may include a bundle of grinding elements of a plurality of linear grinding elements, and a holder of the bundle of grinding elements holding a proximal end portion of the bundle of grinding elements. The bundle of grinding elements may be pressed at a central part of a distal end surface of the bundle of grinding elements. A distal end portion of the bundle of grinding elements may be brought into contact with the surface to be polished. This configuration makes it easier to bring the entire distal end surface of the bundle of grinding members into contact with the surface to be polished, thereby ensuring the polishing performance on the surface to be polished by the rotary brush.In the present invention, the rotary brush may comprise a plurality of annularly arranged bundles of grinding elements, and a holder of the bundle of grinding elements holding respective proximal end portions of the bundles of grinding elements. Each of the bundles of abrasive elements may comprise a plurality of linear abrasive elements. Respective distal end portions of the bundles of grinding elements may be brought into contact with the surface to be polished. In such a rotary brush, there is no bundle of abrasive members at the center of rotation where the peripheral speed during polishing is zero. Here, when the linear grinding elements of the bundle of grinding elements come into contact with the surface to be polished of the workpiece at a circumferential speed of zero and the brush is pressed against the surface to be polished, there is a risk that the linear grinding elements break. However, the occurrence of such a situation can be avoided if the rotary brush comprises a plurality of annularly arranged bundles of grinding elements.In the present invention, the rotary brush may include an annular bundle of grinding elements of a plurality of linear grinding elements annularly arranged, and a holder of the bundle of grinding elements that holds a proximal end portion of the bundle of grinding elements. A distal end portion of the bundle of grinding elements may be brought into contact with the surface to be polished. In such a rotary brush, there is no linear grinding element at the center of rotation of the brush where the peripheral speed during polishing is zero. Here, when the linear grinding elements come into contact with the surface to be polished of the workpiece at a peripheral speed of zero and the brush is pressed against the surface to be polished, there is a risk that the linear grinding elements break. However, the occurrence of such a situation can be avoided if the rotary brush comprises a bundle of grinding elements of a plurality of linear grinding elements which are arranged annularly.In the present invention, the rotary brush may include: a brush body including a bundle of grinding elements of a plurality of linear grinding elements and a holder of the bundle of grinding elements that holds a proximal end portion of the bundle of grinding elements; and a sleeve surrounding the bundle of grinding elements and the holder of the bundle of grinding elements, a distal end portion of the bundle of grinding elements being exposed to the outside, the sleeve facing the bundle of grinding elements with a predetermined gap in the radial direction. A distal end portion of the bundle of grinding elements may be brought into contact with the surface to be polished. With this configuration, the sleeve can prevent the bundle of grinding members from spreading to the outer circumferential side, thereby ensuring the polishing performance on the surface to be polished by the rotary brush. In addition, since the linear grinding elements of the bundle of grinding elements can be prevented from being out of control, the formation of scratches on the surface of the workpiece to be polished can be prevented or suppressed.In the present invention, the workpiece may be an artificial joint. The rotary brush may comprise a bundle of abrasive elements of a plurality of linear abrasive elements. The linear abrasive members may each comprise an aggregate yarn of inorganic filaments and a resin impregnating the aggregate yarn. The rotary brush having such a bundle of linear grinding elements has a high grinding performance. Thus, such a rotary brush can be used for polishing an artificial joint of cobalt, chromium, titanium alloy or the like.Brief Description of the DrawingsFIG. 1 is a side view of a workpiece to be subjected to a polishing method of the present invention. FIG. 2 is a perspective view of a rotary brush used for polishing a workpiece. FIG. 3 is a cross-sectional view of the rotary brush. FIG. 4 is a diagram illustrating a first polishing method. FIG. 5 is a diagram illustrating a rotation direction of the workpiece, a rotation direction of the rotary brush, and a movement direction of the rotary brush in the first polishing method. FIG. 6 is a diagram illustrating a second polishing method. FIG. 7 is a diagram illustrating a rotation direction of the workpiece, a rotation direction of the rotary brush, and a movement direction of the rotary brush in the second polishing method. FIG. 8 is a diagram for explaining a problem occurring when the first polishing method is applied. FIG. 9 is a diagram illustrating a third polishing method. FIG. 10 is a diagram illustrating a rotation direction of the workpiece, a rotation direction of the rotary brush, and a movement direction of the rotary brush in the third polishing method. FIG. 11 is a diagram for explaining a problem occurring when the first polishing method is applied. FIG. 12 is a diagram illustrating another example of the rotary brush. FIG. 13 is a diagram illustrating still another example of the rotary brush.DESCRIPTION OF EMBODIMENTSNext, a polishing method according to embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a side view of a workpiece to be subjected to a polishing method of the present invention. FIG. 2 is a perspective view of a rotary brush used for polishing a workpiece. FIG. 3 is a cross-sectional view of the rotary brush.(Workpiece)A workpiece 1 to be polished by the polishing method in this example is an artificial joint. More specifically, the workpiece 1 in this example is an artificial hip joint. The workpiece 1 is made of metal, for example cobalt, chromium or a titanium alloy. As illustrated in FIG. 1, the workpiece 1 has a rotationally symmetrical shape about a predetermined axis L 0. The workpiece 1 includes, in this order from one side to the other side in the axial direction X along the axis L 0, a rod-shaped part 2, a disk-shaped part 3, and a spherical part 4. a surface to be polished 5 of the workpiece 1 is an outer circumferential surface of the spherical part 4. the cross section of the surface to be polished 5 cut by a virtual plane including the axis L 0 has an arc 5 aof more than 120° from a reference point P 1 at which the surface to be polished 5 cuts the axis L 0 toward one side of the axis L 0. Here, the cross section of the workpiece 1 cut by the virtual plane including the axis L 0 has the same shape as the side view illustrated in FIG. 1.In this example, the diameter of the spherical part of the workpiece 1 is 20 mm to 40 mm. The arc 5 aof the surface to be polished 5 cut by the virtual plane extends to a position at an angle of 135° from the reference point P 1 toward the one side of the axis L 0. When the surface 5 to be polished is polished, the workpiece on the rod-shaped part 2 is clamped onto the spindle of a machine tool.(Rotary brush)A polishing tool that polishs the workpiece 1 in the polishing method in this example is a rotary brush 10. The rotary brush 10 also includes a fixing screw 13 for fixing the brush body 11 to the brush housing 12.The brush body 11 includes a bundle of grinding elements 15 and a holder 16 of the bundle of grinding elements holding a proximal end portion of the bundle of grinding elements 15. The bundle of grinding members 15 is a bundle of a plurality of linear grinding members 17. the linear grinding members 17 are produced by impregnating and curing inorganic filament aggregate yarn with a binder resin. In other words, the linear grinding members 17 each include an aggregate yarn of inorganic filaments such as alumina filaments and a resin impregnating the aggregate yarn. The inorganic filaments are, for example, alumina filaments, silicon carbide fibers, boron fibers or glass fibers.As illustrated in FIG. 3, the holder 16 of the bundle of grinding members includes a cylindrical portion 18 surrounding an end part on the proximal end side of the bundle of grinding members 15 from the outer circumferential side, and a bottom portion 19 closing one of the openings of the cylindrical portion 18. The bottom portion 19 is disk-shaped. The inner circumferential side of the cylindrical portion 18 and the end surface of the bottom portion 19 on the side closer to the cylindrical portion 18 form a grinding member bundle holding hole 31 holding the proximal end part of the grinding member bundle 15. An end of the bundle of grinding elements 15 inserted into the cylindrical section 18 abuts against the bottom section 19. The bundle of grinding elements 15 is fixed to the holder 16 of the bundle of grinding elements by adhesive injected into the bundle of grinding elements holding hole 31. The bundle of grinding elements 15 is held in the bundle of grinding elements holding hole 31 to form a circular bundle. The bottom portion 19 has a screw hole 20 radially passing through the bottom portion 19. The screw hole 20 is used for fixing the brush body 11 to the brush housing 12.In this example, the bundle of grinding elements 15 is depressed at a central part of its distal end face 15a. As shown in FIG. 3, when the bundle of grinding members 15 is cut along a rotation axis L 1 of the rotary brush 10, the distal end surface 15 ahas a cross section shaped like an arc 5 athat is depressed toward the proximal end side of the bundle of grinding members 15. The curvature of the arc 5 aincludes a curvature that corresponds to the curvature of the surface 5 to be polished of the workpiece 1. The diameter of the bundle of grinding elements 15 is preferably greater than 1 / 3 and equal to or less than 1 / 2 of the diameter of the spherical part 4 of the workpiece 1.As illustrated in FIG. 2, the brush housing 12 includes a sleeve 21, a circular closing portion 22 closing a rear end of the sleeve 21, and a shaft 23 extending from the center of the closing portion 22 toward the opposite side of the sleeve 21. The sleeve 21 has a slot 24 which extends in the axial direction X. The axis of the shaft 23 is the rotation axis L 1 of the rotary brush.When the brush body 11 is held in the brush housing 12, the brush body 11 is inserted into the sleeve 21 so that the sleeve 21 surrounds the bundle of grinding elements 15 and the holder 16 of the bundle of grinding elements. The distal end portion of the bundle of grinding elements 15 protrudes from the sleeve 21 by a predetermined dimension. In this state, the fixing screw 13 is screwed through the slit 24 of the sleeve 21 into the screw hole 20 of the holder 16 of the grinding element bundle of the brush body 11.As illustrated in FIG. 3, when the fixing screw 13 is screwed into the screw hole 20, the fixing screw 13 penetrates the holder 16 of the bundle of grinding elements in a direction orthogonal to the axis L 0, and a distal end of the fixing screw 13 abuts against an opposing surface part facing the slit 24 in the inner circumferential wall surface of the sleeve 21 When the fixing screw 13 is further screwed in in this state, the holder 16 of the bundle of grinding elements is pressed against a circumferential wall surface part (an inner circumferential wall surface part including an opening edge of the slit 24) facing the opposing surface in the inner circumferential wall surface of the sleeve 21. The brush body 11 is thus fixed to the brush housing 12. When the brush body 11 is fixed to the brush housing 12, the sleeve 21 faces the bundle of grinding members 15 with a predetermined gap in the radial direction.At this time, the rotary brush 10 is rotated about the rotation axis L 1 during polishing of the workpiece 1. When the bundle of grinding elements 15 of the rotary brush 10 is worn out by polishing the workpiece 1, the fixing screw 13 is loosened and the brush body 11 is moved relative to the brush housing 12 to the opposite side of the shaft 23 so that the distal end portion of the bundle of grinding elements 15 protrudes from the sleeve 21 by a predetermined dimension. Thereafter, the fixing screw 13 is screwed to fix the brush body 11 to the brush housing 12.(Polishing Method)A method of polishing the workpiece 1 with the rotary brush 10 will now be described. FIG. 4 is a diagram illustrating a first polishing method. FIG. 5 is a diagram illustrating the relationship among the rotation direction of the workpiece 1, the movement direction of the rotary brush 10, and the rotation direction of the rotary brush 10 in the first polishing method. FIG. 6 is a diagram illustrating a second polishing method. FIG. 7 is a diagram illustrating the relationship among the rotation direction of the workpiece 1, the movement direction of the rotary brush 10, and the rotation direction of the rotary brush 10 in the second polishing method. FIG. 8 is a diagram for explaining a problem occurring in the first polishing method when the workpiece 1 has an oval shape that is longer in a direction orthogonal to the axis L 0. FIG. 9 is a diagram illustrating a third polishing method. FIG. 10 is a diagram illustrating the relationship among the rotation direction of the workpiece 1, the movement direction of the rotary brush 10, and the rotation direction of the rotary brush 10 in the third polishing method. FIG. 11 is a diagram for explaining a problem occurring in the first polishing method when the workpiece 1 has an oval shape that is longer in the axial direction X.As illustrated in FIGS. 4 and 7, when polishing the workpiece 1, the rod-shaped part 2 of the workpiece 1 is clamped on the spindle of a machine tool and rotated about the axis L 0. The rotary brush 10 is rotated about the rotation axis L 1 with the shaft 23 being clamped to the head of the machine tool. The rotary brush 10 is pressed against the surface 5 of the workpiece 1 to be polished by the machine tool and moved along the arc 5 aof the cross section of the surface 5 to be polished while the rotation axis L 1 is held perpendicular to a tangential surface of the surface 5 to be polished.Here, "the rotary brush 10 is rotated and pressed against the surface to be polished 5" means that the bundle of grinding elements 15 of the rotary brush 10 is brought into contact with the surface to be polished 5 at a predetermined cutting depth. A rotation direction R 1 of the workpiece 1 and a rotation direction R 2 of the rotary brush 10 are opposed to each other on the front side of a movement direction M of the rotary brush 10 when the surface to be polished 5 and the rotary brush 10 are viewed from a rotation axis direction Y along the rotation axis L 1.A workpiece rotation speed at which the workpiece 1 is rotated about the axis L 0 is lower than a brush rotation speed at which the rotary brush 10 is rotated about the rotation axis L 1. The brush rotation speed and the workpiece rotation speed are in such a relationship that the brush rotation speed is not divisible by the workpiece rotation speed. Here, it is preferable that the speed difference between the workpiece rotation speed and the brush rotation speed is larger. A feed speed at which the rotary brush 10 is moved along the arc 5 ais set based on the workpiece rotation speed and the like. The feed rate is constant.In this example, the rotational speed of the workpiece is 300 revolutions / minute. The rotational speed of the rotary brush 10 is 10000 revolutions / min. Thus, the brush speed cannot be divided by the workpiece speed. The feed speed of the rotary brush 10 is 0.4 mm / revolution relative to the workpiece rotational speed.Here, the polishing method for polishing the workpiece 1 using the rotary brush 10 includes: a first polishing method in which the moving direction M 1 of the rotary brush 10 and the rotating direction R 2 of the rotary brush 10 are not changed during polishing of the surface to be polished 5; and first and second polishing methods in which the moving direction M 1 of the rotary brush 10 and the rotating direction R 2 of the rotary brush 10 are changed during polishing of the surface to be polished 5. In each of the first polishing direction, the second polishing method, and the third polishing method, the workpiece rotation speed, the rotational brush rotation speed, and the feed speed of the rotational brush 10 are all the same.The first, second and third polishing methods will be described in detail below. In the following description, a point intersecting the axis L 0 on the surface to be polished 5 of the workpiece 1 is defined as a reference point P 1. In the arc 5 aof the cross section of the surface to be polished 5, a position 90° away from the reference point P 1 toward one side of the axial direction X is defined as the vertex P 2 of the surface to be polished 5. Moreover, in the arc 5 aof the cross section of the surface to be polished 5, an end opposite to the reference point P 1 is defined as a terminating end P 3 of the surface to be polished 5.(First Polishing Method)The first polishing method is used when a plurality of workpieces 1 are polished one by one and the spheroidicity of the spherical part 4 is confirmed to be high in all the workpieces 1 in advance. In other words, the first polishing method is used when the cross section of the surface to be polished 5 of the workpiece 1 has a perfect circular shape within a predetermined tolerance range or less.As illustrated in FIG. 4, the workpiece 1 is rotated in a predetermined rotational direction R 1 in the first polishing method. The rotary brush 10 is rotated in a first rotational direction R 2( 1) and pressed against the surface to be polished 5 of the rotating workpiece 1 and moved toward the finish end P 3 along the arc 5 aof the cross section of the surface to be polished 5 from a position offset on the other side of the axis L 0 relative to the reference point P 1 beyond the reference point P 1 on the surface to be polished 5 of the workpiece 1 (polishing step ST 1). As illustrated in FIG. 5, when the surface to be polished 5 and the rotary brush 10 are viewed from the direction of the rotation axis L 1 along the rotation axis L 1 of the rotary brush 10, in the polishing step ST 1, the first rotation direction R 2( 1) of the rotary brush 10 is on the front side (the side on which the apex P 3 is located) of the moving direction M 1 of the rotary brush 10 opposite to the rotation direction R 1 of the workpiece 1.Thereafter, the polishing step ST 1 is repeated a predetermined number of times. The polishing of the workpiece 1 is thus completed.(Effects of Operation)In the present invention, the workpiece 1 and the rotary brush 10 are rotated when the surface 5 to be polished of the workpiece 1 is polished. The rotary brush 10 is pressed against the surface to be polished 5 and moved along the arc 5a of the cross section while the rotational axis L1 of the rotary brush 10 is kept perpendicular to the tangential surface of the surface to be polished 5. At this time, the rotation direction R 1 of the workpiece 1 and the rotation direction R 2 of the rotary brush 10 are opposed to each other on the front side of the movement direction M 1 of the rotary brush 10 when the surface to be polished 5 and the rotary brush 10 are viewed along the rotation axis L 1 from the rotation axis L 1. With this configuration, the brush bite into the surface to be polished 5 as compared with when the rotational direction R 1 of the workpiece 1 and the rotational direction R 2 of the rotary brush 10 face the same direction on the front side of the moving direction M 1 of the rotary brush 10. Thus, it is easier to polish the spherical surface 5 to be polished.In this example, polishing is started from a position that is offset by 5° from the reference point P 1 on the other side of the axis L 0. Thus, the reference point P 1 and its periphery where the peripheral speed on the surface to be polished 1 is zero can be sufficiently polished.A workpiece rotation speed at which the workpiece 1 is rotated about the axis L 0 is lower than a brush rotation speed at which the rotary brush 10 is rotated about the rotation axis L 1, and the brush rotation speed is not divisible by the workpiece rotation speed. This configuration can prevent occurrence of a striped polishing pattern on the surface to be polished 5. In other words, when the brush rotation speed is divisible by the workpiece rotation speed, stripe-shaped polishing tracks occur at equal angular intervals around the axis L 0 on the surface 5 to be polished. In contrast, the occurrence of such stripe-shaped polishing tracks can be avoided when the brush rotation speed is not divisible by the workpiece rotation speed.In this example, the rotary brush 10 comprises a bundle of grinding elements 15 of a plurality of linear grinding elements 17 and the holder 16 of the bundle of grinding elements holding the proximal end portion of the bundle of grinding elements 15. The bundle of grinding elements 15 is depressed at the central portion of its distal end face 15a. The rotary brush 10 brings the distal end portion of the bundle of grinding members 15 into contact with the surface to be polished 5. this configuration makes it easier to bring the entire distal end surface 15 aof the bundle of grinding members 15 into contact with the surface to be polished 5, thereby ensuring the polishing performance on the surface to be polished 5 by the rotary brush 10.The rotary brush 10 includes: the brush body 11 including the bundle of grinding elements 15 of a plurality of linear grinding elements 17 and the holder 16 of the bundle of grinding elements holding the proximal end portion of the bundle of grinding elements 15; and the sleeve 21 surrounding the bundle of grinding elements 15 and the holder 16 of the bundle of grinding elements, the distal end portion of the bundle of grinding elements 15 being exposed to the outside and facing the bundle of grinding elements 15 with a predetermined gap in the radial direction. The rotary brush 10 brings the distal end part of the bundle of grinding members 15 into contact with the surface to be polished 5. according to this example, the sleeve 21 can prevent the bundle of grinding members 15 from spreading to the outer circumferential side, thereby ensuring the polishing performance on the surface to be polished by the rotary brush 10. In addition, since the linear grinding elements 17 of the bundle of grinding elements 15 can be prevented from being out of control, the generation of scratches on the surface 5 to be polished of the workpiece 1 can be prevented or suppressed.The workpiece 1 is an artificial joint. The rotary brush 10 comprises the bundle of grinding members 15 of a plurality of linear grinding members 17. the linear grinding members 17 each comprise an aggregate yarn of inorganic filaments and a resin impregnating the aggregate yarn. The rotary brush 10 having such a bundle of linear grinding elements 17 has a high grinding performance. Thus, an artificial joint made of cobalt, chromium, titanium alloys or the like can be polished.(Second Polishing Method)When the surface 5 to be polished of the workpiece 1 is formed by a cutting operation in which the workpiece is cut by bringing a cutting tool into contact with an outer circumferential surface of the workpiece 1 rotating about the axis L 0, the spheroidicity of the spherical part 4 may decrease beyond a preset tolerance range due to the accuracy of the cutting operation. When the spheroidity decreases due to the accuracy of the cutting operation, the cross section of the surface to be polished 5 of the workpiece 1 may tend to assume an oval shape longer in the Y direction orthogonal to the axis L 0 than in the axial direction X. The second polishing method is used when a plurality of workpieces 1 are to be polished, and may include a workpiece 1 having such a shape.As illustrated in FIG. 6, in the second polishing method, first, the workpiece 1 chucked on the spindle of a machine tool is rotated in the rotational direction R 1, and the rotary brush 10 chucked on the head of the machine tool is rotated in the first rotational direction R 2( 1). The rotary brush 10 is pressed against the surface to be polished 5 of the rotating workpiece 1 and moved along the arc 5 aof the cross section of the surface to be polished 5 from a position offset on the other side of the axis L 0 relative to the reference point P 1 to the vertex P 2 via the reference point P 1 (first polishing step ST 11). In the second polishing direction, the rotational direction R 1 of the workpiece 1 is the same as in the first polishing method.Here, as the rotary brush 10 moves, the rotation axis L 1 of the rotary brush 10 is held perpendicular to the tangential surface of the surface 5 to be polished. As illustrated in FIG. 7( a), when the surface to be polished 5 and the rotary brush 10 are viewed from the direction of the rotation axis L 1 along the rotation axis L 1 of the rotary brush 10, the second rotation direction R 2( 2) of the rotary brush 10 is on the front side (the side on which the apex P 2 is located) of the movement direction M 1 of the rotary brush 10 opposite to the rotation direction R 1 of the workpiece 1.Next, the rotary brush 10 is moved away from the surface to be polished 5 (setting changing step ST 12).Thereafter, the rotary brush 10 is rotated in the second rotation direction R 2( 2) opposite to the first rotation direction R 2( 1) and pressed against the surface to be polished 5 and moved from the termination end P 3 to the vertex P 2 of the sheet 5 a(second polishing step ST 13). Thus, the moving direction M 2 of the rotary brush 10 is opposite to the moving direction M 1 in the first polishing step. The rotational direction R 1 of the workpiece 1 remains unchanged.Here, as the rotary brush 10 moves, the rotation axis L 1 of the rotary brush 10 is held perpendicular to the tangential surface of the surface 5 to be polished. As illustrated in FIG. 7( b), when the surface to be polished 5 and the rotary brush 10 are viewed from the direction of the rotation axis L 1 along the rotation axis L 1 of the rotary brush 10, the second rotation direction R 2( 2) of the rotary brush 10 is opposite to the rotation direction R 1 of the workpiece 1 on the front side (the side on which the apex P 2 is located) of the movement direction M 1 of the rotary brush 10.Thereafter, the first polishing step ST 11, the setting changing step ST 12, and the second polishing step ST 13 are repeated a predetermined number of times. The polishing of the workpiece 1 is thus completed.(Effects of Operation of Second Polishing Method)In the second polishing method, the polishing accuracy can be maintained in the same manner as when the cross section of the surface to be polished 5 is circular, even when the cross section of the surface to be polished 5 has an oval shape that is longer than in the axial direction X due to the machining accuracy of the workpiece 1 in the direction Y orthogonal to the axis L 0. In FIG. 8, the cross section of the surface to be polished 5 of the workpiece 1 has an oval shape that is longer in the direction Y orthogonal to the axis L 0 than in the axial direction X.For example, assuming that the cross section of the surface to be polished 5 has an oval shape that is longer in the direction Y orthogonal to the axis L 0, the first polishing method is employed to perform polishing by moving the rotary brush 10 from the reference point P 1 to the terminal end P 3 of the sheet 5 a. In this case, as illustrated in an exaggerated manner in FIG. 8, when the rotary brush 10 moving from the reference point P 1 to the terminal end P 3 of the sheet 5 aexceeds the vertex P 2, at the time when the rear part in the bundle of grinding elements 15 in the moving direction M 1 comes into contact with the surface to be polished 5, the front part in the bundle of grinding elements 15 in the moving direction M 1 does not come into contact with the surface to be polished 5 at the same depth as the rear part. Thus, the depth of cut on the front side of the moving direction in the rotary brush 10 is reduced as compared with the depth of cut on the back side of the moving direction in the rotary brush 10. As a result, when the rotary brush 10 is polishing the terminal end P 3 side with respect to the apex P 2 in the arc 5 aof the cross section, the polishing performance on the surface 5 to be polished by the rotary brush 10 can be reduced.In contrast, in this example, when the rotary brush 10 polish the terminal end P 3 side with respect to the vertex P 2 in the arc 5 aof the cross section, the rotation direction R 2 of the rotary brush 10 is set in the second rotation direction R 2( 2) opposite to the first rotation direction R 2( 1). In addition, the moving direction M 1 of the rotary brush 10 is set in the direction from the terminal end P 3 to the vertex P 2, and the rotating direction R 1 of the workpiece 1 and the rotating direction R 2 of the rotary brush 10 are set in directions opposing each other on the front side of the moving direction M 1 of the rotary brush 10. This setting can avoid the reduction in the depth of cut on the front side of the moving direction of the rotary brush 10 compared to the depth of cut on the back side of the moving direction of the rotary brush 10, thereby avoiding the reduction in the polishing performance on the surface 5 to be polished by the rotary brush 10. As a result, the polishing accuracy can be maintained even when the cross section of the surface to be polished 5 has an oval shape.The second polishing method can achieve similar effects of operation to the first polishing method.(Third Polishing Method)When the surface 5 to be polished of the workpiece 1 is formed by a cutting operation in which the workpiece is cut by bringing a cutting tool into contact with an outer circumferential surface of the workpiece 1 rotating about the axis L 0, the spheroidicity of the spherical part 4 may decrease beyond a preset tolerance range due to the accuracy of the cutting operation. When the spheroidity decreases due to the accuracy of the cutting operation, the cross section of the surface 5 to be polished of the workpiece 1 may tend to assume an oval shape longer in the axial direction X than in the direction Y orthogonal to the axis L 0. The third polishing method is used when a plurality of workpieces 1 are to be polished, and may include a workpiece 1 having such a shape.As illustrated in FIG. 9, in the second polishing method, first, the workpiece 1 chucked on the spindle of a machine tool is rotated in the rotational direction 10, and the rotary brush R 2 chucked on the head of the machine tool is rotated in the first rotational direction 1(R 1). The rotary brush 10 is pressed against the surface to be polished 5 of the rotating workpiece 1 and moved along the arc 5 aof the cross section of the surface to be polished 5 from the apex P 2 via the reference point P 1 to a position offset on the other side of the axis L 0 relative to the reference point P 1 (first polishing step ST 21). In the third polishing direction, the rotational direction R 1 of the workpiece 1 is the same as in the first and second polishing methods. The first rotational direction R 2( 1) in the third polishing direction is the opposite rotational direction to the first rotational direction R 2( 1) in the second polishing method. The moving direction M 1 of the rotary brush 10 in the third polishing direction is the direction opposite to the moving direction M 1 of the rotary brush 10 in the third polishing direction.Here, as the rotary brush 10 moves, the rotation axis L 1 of the rotary brush 10 is held perpendicular to the tangential surface of the surface 5 to be polished. When the surface to be polished 5 and the rotary brush 10 are viewed from the direction of the rotation axis L 1 along the rotation axis L 1 of the rotary brush 10, as illustrated in FIG. 10( a), the first rotation direction R 2( 1) of the rotary brush 101 is opposite to the rotation direction R 1 of the workpiece 1 on the front side (the side on which the reference point P 1 is located) of the movement direction M 1 of the rotary brush 10.Next, the rotary brush 10 is moved away from the surface to be polished 5 (setting changing step ST 22).Thereafter, the rotary brush 10 is rotated in the second rotation direction R 2( 2) opposite to the first rotation direction R 2( 1) and pressed against the surface to be polished 5 and moved from the apex P 2 to the termination end P 3 (second polishing step ST 23). Here, the second rotational direction R 2( 2) in the third polishing direction is the opposite rotational direction to the second rotational direction 2( 2) in the second polishing method. The moving direction M 2 of the rotary brush 10 in the third polishing direction is the direction opposite to the moving direction M 1 of the rotary brush 10 in the third polishing direction, and is the same direction as the moving direction M 1 in the first polishing method. The rotational direction R 1 of the workpiece 1 remains unchanged.Here, as the rotary brush 10 moves, the rotation axis L 1 of the rotary brush 10 is held perpendicular to the tangential surface of the surface 5 to be polished. When the surface to be polished 5 and the rotary brush 10 are viewed from the direction of the rotation axis L 1 along the rotation axis L 1 of the rotary brush 10, as illustrated in FIG. 10( b), the second rotation direction R 2( 2) of the rotary brush 10 is opposite to the rotation direction R 1 of the workpiece 1 on the front side (the side on which the apex P 3 is located) of the movement direction M 1 of the rotary brush 10.Thereafter, the first polishing step ST 21, the setting changing step ST 12, and the second polishing step ST 13 are repeated a predetermined number of times. The polishing of the workpiece 1 is thus completed.(Effects of Operation of Third Polishing Method)In the third polishing method, the polishing accuracy can be maintained in the same manner as when the cross section of the surface to be polished 5 is circular, even when the cross section of the surface to be polished 5 has an oval shape that is longer in the axial direction X due to the machining accuracy of the workpiece 1. This effect will be explained with reference to Fig. 11. In FIG. 11, the cross section of the surface to be polished 5 of the workpiece 1 has an oval shape that is longer in the axial direction X than in the direction Y orthogonal to the axis L 0.For example, assuming that the cross section of the surface to be polished 5 has an oval shape that is longer in the direction Y orthogonal to the axis L 0, for example, the first polishing method is employed to perform polishing by moving the rotary brush 10 from the reference point P 1 toward the terminal end P 3 of the sheet 5 a. In this case, as illustrated in an exaggerated manner in FIG. 11, from the reference point P 1 to the vertex P 2 of the arc 5 a, at the time when the rear part in the moving direction M 1 in the bundle of grinding elements 15 comes into contact with the surface to be polished 5, the front part in the moving direction M 1 of the bundle of grinding elements 15 does not come into contact with the surface to be polished 5 at the same depth as the rear part. Thus, the depth of cut on the front side of the moving direction in the rotary brush 10 is reduced as compared with the depth of cut on the back side of the moving direction in the rotary brush 10. As a result, when the rotary brush 10 is polishing the reference point P 1 side with respect to the vertex P 2, the polishing performance on the surface 5 to be polished by the rotary brush 10 can be reduced.In contrast, in this example, when the rotary brush 10 polish the reference point P 1 side with respect to the vertex P 2, the moving direction M 1 of the rotary brush 10 is set in the direction from the vertex P 2 to the reference point P 1. The rotational direction R 1 of the workpiece 1 and the rotational direction R 2 of the rotary brush 10 are set in directions opposing each other on the front side of the moving direction M 1 of the rotary brush 10. This setting can avoid the reduction in the depth of cut on the front side of the moving direction of the rotary brush 10 compared to the depth of cut on the back side of the moving direction of the rotary brush 10, thereby avoiding the reduction in the polishing performance on the surface 5 to be polished by the rotary brush 10. As a result, the polishing accuracy can be maintained even when the cross section of the surface to be polished 5 has an oval shape.The third polishing method can achieve similar effects of operation to the first polishing method.(Modifications)The rotary brush 10 does not necessarily include the sleeve 21, for example, the brush body 11 may be used as a rotary brush.When polishing the workpiece 1, the rotary brush 10 may be fixed to the head of a machine tool via a floating holder so that the rotary brush 10 is pressed against the surface 5 to be polished with a predetermined biasing force.(Another example of a rotary brush)FIG. 12 is a diagram showing another rotary brush for use in the polishing method in this example. Fig. 13 is a diagram showing still another rotary brush for use in the polishing method in this example. A rotary brush 10A shown in Fig. 12 comprises a plurality of annularly arranged bundles of grinding elements 15 and a holder 16 of the bundles of grinding elements which holds the respective proximal end portions of the bundles of grinding elements 15. Each of the bundles of grinding elements 15 includes a plurality of linear grinding elements 17. the holder 16 of the bundles of grinding elements includes a disc portion 32 having a plurality of annularly arranged on its front side bundle holding holes 31 and a shaft portion 33 extending rearward from the center of the disc portion 32. Each of the bundles of grinding elements 15 has a proximal end portion which is inserted into the corresponding holding hole 31 for the bundle of grinding elements and fixed to the holder 16 of the bundles of grinding elements by adhesive. When polishing the workpiece 1, the respective distal end portions of the bundles of grinding members 15 are brought into contact with the surface to be polished 5.In the rotary brush 10A in this example, there is no bundle of grinding members 15 at the center of rotation where the peripheral speed during polishing is zero. Here, when the linear grinding elements 17 of the bundle of grinding elements 15 come into contact with the surface to be polished 5 of the workpiece 1 at a peripheral speed of zero while the brush is pressed against the surface to be polished 5, there is a risk that the linear grinding elements may be broken. However, the occurrence of such a situation can be avoided if the rotary brush 10A includes a plurality of annularly arranged bundles of grinding members 15. The rotary brush 10A may also include a sleeve 21 surrounding a plurality of bundles of grinding members 15 and the holder 16 of the bundles of grinding members, with a distal end portion of each of the bundles of grinding members 15 exposed to the outside.A rotary brush 10B shown in FIG. 13 includes an annular bundle of grinding members 15 of a plurality of linear grinding members 17 arranged annularly, and a holder of the bundle of grinding members 16 that holds a proximal end portion of the bundle of grinding members 15. The abrasive-bundle holder 16 includes a disc portion 32 having a plurality of abrasive-bundle holding holes 33 annularly disposed on its front side, and a shaft portion 32 extending rearward from the center of the disc portion 31. The bundle of grinding elements 15 has a proximal end portion inserted into the bundle of grinding elements holding hole 31 and fixed to the holder 16 of the bundle of grinding elements by adhesive. In a polishing process, the respective distal end portions of the bundles of grinding elements 15 are brought into contact with the surface 5 to be polished.In the rotary brush 10B, there is no bundle of grinding members 17 at the center of rotation where the peripheral speed during polishing is zero. Thus, it is possible to prevent the breakage of the linear grinding member 17 coming into contact with the surface to be polished 5 of the workpiece 1 at a circumferential speed of zero. When the rotary brush 10B has the annular bundle of grinding members 15, the bundle of grinding members 15 continuously comes into contact with the surface to be polished 5. Thus, as compared with the rotary brush 10A in which a plurality of linear grinding elements are annularly arranged and the bundles of grinding elements 15 intermittently come into contact with the surface to be polished 5 during polishing, it is easier to improve the surface roughness of the surface to be polished 5 after polishing. the rotary brush 10B may also include a sleeve 21 surrounding a plurality of bundles of grinding elements 15 and the holder 16 for the bundle of grinding elements, with a distal end part of each of the bundles of grinding elements 15 exposed to the outside.References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedJP 2011-36974

[0003] JP 2009-50967

[0003]

Claims

A polishing method for polishing a workpiece, the workpiece being rotationally symmetric about a predetermined axis and having a surface to be polished, wherein a cross section of the surface to be polished cut by a virtual plane including the axis has an arc of more than 120° from a reference point intersecting the axis toward a side of the axis (L0), wherein as the polishing tool, a rotary brush is used, the workpiece is rotated about the axis, the rotary brush is rotated and pressed against the surface to be polished and moved along the arc of the cross section while a rotation axis of the rotary brush is held perpendicular to a tangential surface of the surface to be polished, and when the surface to be polished and the rotary brush are viewed from a rotation axis direction along the rotation axis, a rotation direction of the rotary brush is opposite to a rotation direction of the workpiece on a front side of a movement direction of the rotary brush.The polishing method according to claim 1, wherein the rotary brush is rotated in a first rotational direction and pressed against the surface to be polished and moved along the arc of the cross section from a position offset on another side of the axis relative to the reference point toward one side of the axis via the reference point to an apex 90° away from the reference point, the rotary brush is then moved away from the surface to be polished, and the rotary brush is thereafter rotated in a second rotational direction opposite to the first rotational direction and pressed against the surface to be polished and moved from the terminating end opposite to the reference point to the apex in the arc of the cross section.The polishing method according to claim 1, wherein the rotary brush is rotated in a first rotational direction and pressed against the surface to be polished and moved along the arc of the cross section from an apex 90° away from the reference point toward one side of the axis via the reference point to a position offset relative to the reference point on another side of the axis, the rotary brush is then moved away from the surface to be polished, and the rotary brush is thereafter rotated in a second rotational direction opposite to the first rotational direction and pressed against the surface to be polished and moved from the apex to a termination end opposite to the reference point in the arc of the cross section.The polishing method according to claim 1, wherein the rotary brush rotates in a first rotational direction and is pressed against the surface to be polished, and moves along the arc of the cross section from a position offset on another side of the axis relative to the reference point, across the reference point, to a termination end opposite to the reference point in the arc of the cross section.The polishing method according to claim 1, wherein a workpiece rotation speed at which the workpiece is rotated about the axis L0 is lower than a brush rotation speed at which the rotating brush is rotated about the rotation axis L1, and the brush rotation speed is not divisible by the workpiece rotation speed.The polishing method according to claim 1, wherein the rotary brush comprises a bundle of grinding elements of a plurality of linear grinding elements, and a holder of the bundle of grinding elements that holds a proximal end part of the bundle of grinding elements, the bundle of grinding elements is pressed at a center part of a distal end surface of the bundle of grinding elements, and a distal end part of the bundle of grinding elements is brought into contact with the surface to be polished.The polishing method according to claim 1, wherein the rotary brush comprises a plurality of annularly arranged bundles of grinding elements, and a holder of the bundle of grinding elements that holds respective proximal end portions of the bundles of grinding elements, each of the bundles of grinding elements comprises a plurality of linear grinding elements, and respective distal end portions of the bundles of grinding elements are brought into contact with the surface to be polished.The polishing method according to claim 1, wherein the rotary brush comprises an annular bundle of grinding elements of a plurality of linear grinding elements annularly arranged, and a holder of the bundle of grinding elements that holds a proximal end part of the bundle of grinding elements, and a distal end part of the bundle of grinding elements is brought into contact with the surface to be polished.The polishing method according to claim 1, wherein the rotary brush comprises: a brush body having a bundle of grinding elements of a plurality of linear grinding elements and a holder of the bundle of grinding elements that holds a proximal end part of the bundle of grinding elements; and a sleeve surrounding the bundle of grinding elements and the holder of the bundle of grinding elements, a distal end part of the bundle of grinding elements being exposed to the outside, the sleeve facing the bundle of grinding elements with a predetermined gap in the radial direction, and the distal end part of the bundle of grinding elements is brought into contact with the surface to be polished.The polishing method according to claim 1, wherein the workpiece is an artificial joint, the rotary brush comprises a bundle of grinding members of a plurality of linear grinding members, and the linear grinding members each comprise an aggregate yarn of inorganic filaments and a resin impregnating the aggregate yarn.

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