GRINDING NON-METALLIC HARD MATERIALS

Abstract: In one method of cross-grinding in accordance with the present invention a non-planar surface (12) on a work piece (10), of a non-metallic material having a Vickers hardness value up to 5000, comprises, in each of two grinding steps, traversing the rotational axis of a grinding wheel (30) along a predetermined axis (22), relative to the workpiece surface. In the first step the radially extending plane of the grinding wheel includes the predetermined axis (22), and the required workpiece surface is produced with inevitable ridges. For the second grinding step the deformable working surface of the same, or different, grinding wheel (30) is shaped by a tool (34) capable of shaping in a normal manner the working surface suitable for the first grinding step. However, the working surface of the grinding wheel is altered by the radially extending plane of the wheel when presented to the tool being inclined in one sense at a selected angle (S), in the range 1° to 20°, to the direction of this plane if presented to the tool to obtain the shape suitable for the first grinding step. In the second grinding step the ridges on the workpiece are reduced by the radially extending plane of the wheel with said altered working surface being inclined in said one sense at the selected angle to the orientation of the radially extending plane of the grinding wheel in the first grinding step.

Description: This invention relates to a method of grinding a non-planar surface on a workpiece of non-metallic hard material having a Vickers hardness value up to 5000, by employing a rotating grinding wheel, according to the preamble of claim 1. The term non-metallic is employed in this specification and the accompanying claims to refer to any composition not comprising a metallic element, or an alloy of metallic elements, but, possibly, having at least some of the properties associated with a metallic element, or an alloy of metallic elements. Further, a surface on a workpiece of any such non-metallic composition is ground by the disintegration of the surface thereof, and the removal of small particles therefrom. It is, especially, to a method of grinding a required non-planar surface, on a workpiece of a non-metallic hard material, in which method the axis of rotation of the grinding wheel is caused to traverse along a predetermined axis, and relative to the surface of the work piece blank to be worked, so that the grinding wheel passes through the workpiece, that the present invention relates. It is known for the rotational axis either to reciprocate in the radially extending plane, or to move only in one linear direction, in this plane, relative to the workpiece surface, the predetermined axis being in this plane. Usually the wheel traverses relatively to the workpiece until the wheel has cut the full depth of its form into the workpiece, simultaneously there being incremental, relative perpendicular, movements between the wheel and the workpiece. Hence, any features, comprising protrusions, and/or depressions, of the required non-planar workpiece surface extend linearly parallel to the predetermined axis, and to the radially extending plane of the grinding wheel. As also is known that the required shape of the working surface of the grinding wheel is maintained by a tool with a complementary shape. The tool is required to pass through the wheel by traversing relatively to the wheel. The normal presentation is for the wheel to reciprocate relatively to the tool until the tool has cut the full depth of its form into the wheel, simultaneously there being incremental, relative perpendicular movements between the wheel and the tool. In particular, the present invention relates to a grinding method including such a method of shaping the working surface of the grinding wheel with a tool. Usually the rotating grinding wheel has the appropriately shaped working surface, comprising the radially outer periphery of the wheel, formed from gemstone, or synthetic diamond, particles bonded to a suitable substrate. A grinding wheel having a working surface of gemstone, or synthetic diamond particles, inevitably, provides a surface on the workpiece with grooves, having ridges therebetween, the ridges extending parallel to the direction of traverse of the axis of rotation of the wheel relative to the workpiece surface. Because the diamond particles have different sizes, and because of insufficient control over the way in which the particles are embedded in the working surface of the grinding wheel, inevitably, the working surface is irregular, with particles protruding therefrom, by different amounts from what can be considered to be the general level of the working surface. The finish of such a working surface, conveniently, can be defined by the maximum amount of protrusion of the diamond particles from the general level of the working surface, such maximum particle protrusion being greater for a relatively coarsely finished working surface than for a relatively finely finished working surface. It is known that if the required workpiece surface is to be flat the height of the ridges at least may be reduced by cross-grinding in a direction at right angles to the direction of the initial grinding action. However, in grinding a non-planar workpiece surface such cross-grinding cannot be employed. It is an object of the present invention to provide a novel and advantageous modifications of the known grinding method, including the known method of shaping the working surface of the grinding wheel with a tool, referred to above, to produce on a workpiece, of a non-metallic material having a Vickers hardness value up to 5000, a surface of a required non-planar form, either by employing a grinding wheel having a working surface with a desired finish, defined by the maximum particle protrusion as referred to above; or by employing two grinding wheels, possibly only one wheel having the desired finish, relatively finer than the finish of other wheel; and which modifications cause the height of ridges, otherwise inevitably formed on the workpiece surface, to be at least reduced. According to the present invention a method of grinding a required non-planar surface on a workpiece, of a non-metallic material having a Vickers hardness value up to 5000, includes, in each of two grinding steps, traversing the rotational axis of a grinding wheel along a predetermined axis, relative to the surface of the workpiece, in the first, known, grinding step the radially extending plane of the grinding wheel includes the predetermined axis, and the working surface of the grinding wheel, of gemstone, or synthetic diamond, particles, has the appropriate form, such that there is provided a non-planar workpiece surface with ridges thereon, subsequently, with a grinding wheel having a working surface of gemstone, or synthetic diamond, particles, and having either the same finish as, or a finer finish than, the working surface of the grinding wheel employed in the first grinding step, possibly initially the working surface having said appropriate form, but in any event, shaping the working surface with a tool; the tool either having a shaping surface capable of maintaining said appropriate form of said working surface when the grinding wheel is presented in a normal manner to the tool, and with the radially extending plane of the grinding wheel including the axis of traverse of the wheel relative to the tool, but the working surface being shaped in the desired way by the radially extending plane of the grinding wheel being inclined in one sense at a selected angle in the range 1° to 20° to the axis of traverse of the wheel relative to the tool; or said working surface is shaped to the desired form by being presented in the normal manner to a tool shaping surface of the complementary form, and with the radially extending plane of the wheel including the axis of traverse of the wheel relative to the tool; and in the second grinding step, arranging that the radially extending plane of the wheel is inclined in said one sense at the selected angle to the orientation of the radially extending plane of the grinding wheel in the first grinding step, at least to reduce the height of the ridges on the workpiece surface, but otherwise not affecting significantly the provided, required, non-planar shape of the workpiece surface. Previously, it has not been known either to incline the radially extending plane of the grinding wheel to the axis of traverse of the wheel relative to the workpiece, whilst maintaining the rotational axis of the wheel in the same plane as, say, in the first grinding step of the method in accordance with the present invention. Also it has not been known previously to incline the radially extending plane of the grinding wheel to the axis of traverse of the wheel relative to a tool to shape the working surface of the tool, again whilst maintaining the rotational axis of the wheel in the same plane as, say, in the normal manner of presentation of the wheel to the shaping tool. The desired shape of a grinding wheel when employed in the first grinding step may be maintained by employing an appropriately shaped tool in the manner referred to above for the normal presentation of the wheel to the tool. The desired shape of a grinding wheel when employed in the second grinding step may be maintained either by arranging that, with an appropriately shaped tool, the radially extending plane of the wheel is inclined in said one sense at the selected angle to the axis of traverse of the wheel relative to the tool, and with the axis of traverse of the wheel relative to the tool being parallel to the axis of traverse of the wheel relative to the workpiece; or by employing a differently shaped tool in the manner referred to above for the normal presentation of the wheel to the tool, but with the axis of traverse of the wheel relative to the tool being inclined at the selected angle to the axis of traverse of the wheel relative to the workpiece. The same grinding wheel as is employed in the first grinding step, may be employed also in the second grinding step, if the working surface of the wheel is readily capable of deformation, and has the desired finish for the working surface of the grinding wheel to be employed in the second grinding step. Otherwise different grinding wheels are employed in the two grinding steps, the working surface of the wheel to be employed in the second grinding step possibly having a relatively finer finish than the working surface of the wheel to be employed in the first grinding step, and especially, the grinding wheel employed in the first grinding step may not be readily capable of deformation. Because of the rotation of the working surface of the grinding wheel, in the second grinding step, the working surface is caused to enter, and, by grinding, to reduce the height of, the ridges. The arrangement is required to be such that it is unimportant if, in the second grinding step, the grinding wheel enters the side walls of protrusions, and/or depressions, of the non-planar workpiece surface, and previously formed in the first grinding step. The present invention will now be described by way of example with reference to the accompanying drawings, in which Figure 1 is a perspective view of a non-planar surface required to be produced on a workpiece of a non-metallic hard material, by a grinding method, employing a rotating grinding wheel, or wheels, in accordance with the present invention, one such method being shown in Figures 2 to 6, Figures 2 and 3 illustrate a first grinding step of the method, Figure 2 being a plan view, and Figure 3 being a perspective view, both these Figures indicating the manner in which the rotational axis of a grinding wheel traverses, relative to a work piece surface, along a predetermined axis, the predetermined axis being included in the radially extending plane of the wheel, only part of the wheel being shown in Figure 3 and this part is shown sectioned in a plane at right angles to the predetermined axis and including the rotational axis of the wheel, in particular, this Figure showing how ridges inevitably are formed on the workpiece surface, Figure 4 shows, in plan, the grinding wheel employed in the second grinding step, the working surface of the grinding wheel having the appropriate form required for the first grinding step, and there is shown, in particular, the working surface being shaped by a tool having the complementary shaping surface, the grinding wheel being presented to the shaping surface in the normal manner, Figure 5 corresponds to Figure 4, but indicates the further shaping of the working surface of the grinding wheel employed in a second grinding step of the method, the working surface having a shape determined by the tool of Figure 4, but the working surface being altered from that shown in Figure 4, by the radially extending plane of the grinding wheel being inclined in one sense at a selected angle to the direction of this plane shown in Figure 4, Figure 5 not giving any detail of the alteration of the working surface thereof, and Figure 6, corresponds to Figure 2, but illustrates the second grinding step, by indicating that the rotational axis of the wheel of Figures 4, and 5, traverses, relative to the workpiece surface, along the predetermined axis, but the radially extending plane of the wheel is inclined in said one sense at the selected angle to the orientation of the radially extending plane in the first grinding step of Figure 2, so that the ridges are reduced, but otherwise the provided, non-planar shape of the work piece surface is not significantly affected. The workpiece 10 shown in Figure 1 is of sintered silicon nitride, having a Vickers hardness value of approximately 2000. Shown in Figure 1 is a non-planar surface, indicated generally at 12, and required to be formed on the workpiece blank. The illustrated workpiece surface 12 is provided by a linearly extending, truncated 'V'-shaped depression to be ground in the workpiece blank, and the depression is bounded on either side of its longitudinally extending axis by two plane, ground, portions, of the non-planar surface, each such plane portion having a uniform width. The longitudinally extending axis of the depression, conveniently, can be considered to be a predetermined axis of the workpiece surface 12, and is indicated in Figure 1 by the dotted line 14. A known method of grinding the surface 12 of the workpiece 10, comprising the constituent first grinding step of a method of grinding the surface 12 in accordance with the present invention, is indicated in Figures 2 and 3. In these Figures a direction parallel to the predetermined axis 14 of Figure 1 is indicated by an arrow 22, and for the sake of clarity the predetermined axis 14 is not indicated in these Figures. A grinding wheel 20, only partially shown in Figure 3, has a radially extending plane including the predetermined axis 14. Figure 2 comprises a plan view of the grinding wheel 20 operating on the workpiece surface 12, and Figure 3 is a corresponding perspective view. The wheel 20 is shown at the forefront of the perspective view of the workpiece of Figure 3. The illustrated portion of the grinding wheel 20 is shown sectioned in a plane, at right angles to the predetermined axis, and including the rotational axis of the wheel, both not shown. Also for the sake of clarity the sectioned plane of the grinding wheel 20 is not hatched, and the portion of the workpiece 10 behind the grinding wheel 20, and otherwise in the background of the perspective view, is indicated in dotted line form. Also as shown in Figure 3, the working surface 24 of the grinding wheel 20 is provided by gemstone, or synthetic diamond, powder embedded in a suitable substrate. Because the diamond particles 25 have different sizes, and because of insufficient control over the way in which the particles 25 are embedded in the working surface 24, inevitably the working surface is irregular, with particles 25 protruding therefrom. The particles 25 protrude by different amounts from the general level of the working surface 24. The maximum amount of such particle protrusion, in greatly exaggerated form, is indicated by the dotted line 26, this maximum amount defining the finish of the working surface 24. As shown in both Figures 2 and 3 the rotational axis of the grinding wheel 20 is caused to traverse, relative to the workpiece surface 12, parallel to the arrow 22, and along the predetermined axis 14 shown in Figure 1, the direction of traverse being included in the radially extending plane of the wheel. The working surface 24 of the wheel 20 is appropriately formed, so that by the traversing of the rotational axis, the required non-planar surface 12 of the workpiece 10 is provided with ridges thereon, some of which ridges are indicated at 28. The ridges 28 inevitably are formed on the ground surface 12. The ridges 28 are formed because of the protrusion of some of the diamond particles 25 from the general level of the working surface 24 of the grinding wheel 20. Because of the traversing, relative to the workpiece surface 12, of the rotational axis of the wheel 20 along the predetermined axis 14 of the workpiece surface 12, the ridges 28 extend parallel to the predetermined axis. The limit of the height of the ridges 28, also in greatly exaggerated form, is indicated by the dotted line 29. Because the required workpiece surface 12 is non-planar it is not possible to reduce the height of the ridges 28 formed thereon by cross-grinding the surface 12 at right angles to the predetermined axis 14, and the arrow 22. Thus, in the method in accordance with the present invention there is performed a second grinding step in the manner described below. There is required for the second grinding step a grinding wheel with a working surface readily capable of deformation. Such a grinding wheel may comprise the grinding wheel 20 employed in the first grinding step, and shown in Figures 2 and 3. More conveniently, however, a different grinding wheel 30, shown in Figures 4 to 6, is provided for the second grinding step, this wheel having the readily deformable working surface 32, and the working surface 24 of the grinding wheel 20 employed in the first grinding step is not so readily deformable. The working surface 32 also is provided by gemstone, or synthetic diamond, particles 25, having different sizes. The working surface 32 is shaped by employing a tool 34, shown in Figures 4 and 5, and comprising a diamond faced former roller, the tool having a shaping surface 36 of the required form. The tool 34 traverses relative to the wheel 30, as indicated by the arrow 38, and is required to pass through the wheel. Usually the tool 34 is reciprocated, and reciprocation occurs until the tool has cut the full depth of its form into the wheel 30, simultaneously there being incremental, relative perpendicular, movements between the wheel and the tool. Initially, as shown in Figure 4, the normal manner of presentation of the grinding wheel 30 to the tool 34 is employed, and the radially extending plane of the wheel is parallel to the direction of traverse as indicated by the arrow 38. In this manner, there is provided on the wheel 30 a working surface 32 of the same shape as that of the working surface 24 required for the wheel 20 employed in the first grinding step. However, in the method in accordance with the present invention, the working surface 32 for the grinding wheel 30 to be employed in the second grinding step is required to be of a different shape from that shown in Figures 2 and 3. The desired altered shape for the working surface 32 of the grinding wheel 30 is obtained in the manner shown in Figure 5, also by employing the tool 34. The detail of the alteration of the working surface is not shown in Figure 5. Instead of obtaining the shape of the working surface 24 of the grinding wheel 20 employed in the first grinding step, and obtained by presenting the grinding wheel 30 in the normal manner to the tool 34, as shown in Figure 4, the tool 34 traverses relatively to the wheel 30 as indicated by the arrow 38 (also shown in Figure 5), but the radially extending plane of the grinding wheel 30 extends in a direction, indicated by the dotted arrow 40 in Figure 5 to the arrow 38. This direction 40 is inclined in one sense at a selected angle S, in the range 1° to 20°, to the arrow 38. The rotational axis 42 of the wheel 30 is indicated at 42 in Figure 4, and at 44 in Figure 5. The axis 44 is inclined at the selected angle S to the direction of the axis 42 for the normal presentation of the wheel to the tool, as shown in Figure 4, and is maintained in the same plane as, say, that in which the rotational axis is maintained in the normal manner of presentation. The grinding wheel 30 is then employed in the second grinding step, in the manner indicated in Figure 6, which Figure corresponds to Figure 2 indicating the first grinding step. In the second grinding step the rotational axis of the grinding wheel 30 is caused to traverse relative to the required non-planar workpiece surface 12 provided in the first grinding step, and along the predetermined axis 14 (shown in Figure 1), and parallel to the arrow 22. However, instead of the radially extending plane of the wheel 30 including the predetermined axis, this plane is inclined in said one sense at the selected angle S to the orientation of the radially extending plane in the first grinding step. In Figure 6 a direction parallel to the orientation of the radially extending plane in the second grinding step is indicated by the dotted arrow 46. The rotational axis of the wheel 30 is indicated at 50. The altered working surface (not shown) of the grinding wheel 30 also has diamond particles 48 protruding therefrom, by different amounts from the general level of the working surface. The pattern of diamond particle protrusion differs from that of the wheel 20. Further, the maximum particle protrusion for the wheel 30 is at most the same as the maximum particle protrusion for the wheel 20. Because, the radially extending plane of the grinding wheel 30 in the second grinding step is inclined at the selected angle S to the orientation of the radially extending plane of the grinding wheel 20 in the first grinding step, it is also inclined at the selected angle to the direction of the ridges 28 inevitably formed on the required workpiece surface 12 provided in the first grinding step. Thus, the protruding diamond particles enter the ridges 28 because of the rotation of the wheel 30, to grind the ridges and, at least, to reduce their height. The arrangement is required to be such that it is unimportant if, in the second grinding step, the grinding wheel enters the side walls of the non-planar surface 12 of the workpiece 10, and previously provided in the first grinding step. The desired shape 24 of the grinding wheel 20 employed in the first grinding step may be maintained by employing the tool 34 during the first grinding step, in the manner described above with reference to Figure 4 for the normal presentation of the wheel to the tool. The axis of traverse of the wheel relative to the tool is parallel to the axis of traverse of the wheel relative to the workpiece, with the radially extending plane of the wheel during the first grinding step including both such axes of traverse. The desired shape 32 of the grinding wheel 30 employed in the second grinding step may be maintained by employing the tool 34 during the second grinding step, in the manner described above with reference to Figure 5, the radially extending plane of the wheel being inclined in said one sense at the selected angle to the axis of traverse of the wheel relative to the tool, this axis of traverse being parallel to the axis of traverse of the wheel relative to the workpiece 10. It is possible that the shaping tool 34 does not have a shaping surface 36 with the form shown in Figures 4 and 5. Instead the shaping tool has a different shape for the shaping surface to that of the illustrated tool 34, and is of a form such that the same required working surface of the grinding wheel is obtained by presenting the grinding wheel to the tool in the normal manner as shown in Figure 4, and with the radially extending plane of the grinding wheel including the axis of traverse of the wheel relative to the tool. Then, in the second grinding step, the radially extending plane of the wheel is inclined at the selected angle S, in said one sense, to the axis of traverse of the wheel relative to the work piece, as shown in Figure 2. The required shape for the tool can be generated conveniently by employing conventional computer-aided-design techniques. With such an arrangement the desired shape of the grinding wheel employed in the second grinding step may be maintained by employing the altered tool, and presenting this altered tool to the wheel in the normal manner, as described above with reference to Figure 4. The axis of traverse of the wheel relative to the tool inclined at the selected angle S to the axis of traverse of the wheel relative to the workpiece. In one particular method in accordance with the present invention, the grinding wheel 20 has the maximum protrusion of the diamond particles in the range 39 to 180 microns, and the grinding wheel 30 has the maximum protrusion of the diamond particles in the range 6 to 39 microns. The shaping tool 34 has the maximum protrusion of the diamond particles being 4 microns. The material of the workpiece may be of any non-metallic material having a Vickers hardness value of up to 5000; and capable of being ground by the disintegration of the surface thereof, and the removal of small particles therefrom. The gemstone, or synthetic diamond, particles may be bound in a working surface not readily deformable by employing a suitable metal such as chromium, or by a suitable metal alloy. Such a working surface may be formed by employing spark erosion techniques. The gemstone, or synthetic diamond, particles may be bound in a working surface which is readily deformable by employing a vitreous binder, or by, for example, a binder comprising a mixture of copper and a suitable resin. The general shape of a surface which can be provided on a hard, non-metallic, workpiece by a grinding method in accordance with the present invention has protrusions, and/or depressions, extending linearly parallel to the predetermined axis along which the grinding wheel, or wheels, traverse relative to the workpiece surface.

Claims: A method of grinding a required non-planar surface (12) on a workpiece (10), of a non-metallic material having a Vickers hardness value up to 5000, includes, in each of two grinding steps, traversing the rotational axis of a grinding wheel along a predetermined axis, relative to the surface of the workpiece, in the first grinding step the radially extending plane of the grinding wheel (20) includes the predetermined axis (14), and the working surface (24) of the grinding wheel, of gemstone, or synthetic diamond, particles, has the appropriate form, such that there is provided a non-planar workpiece surface with ridges thereon, characterised by, subsequently, with a grinding wheel (30) having a working surface (32) of gemstone, or synthetic diamond, particles, shaping the deformable working surface (32) with a tool (34); either the tool (34) having a shaping surface (36) capable of maintaining said appropriate form of said working surface when the grinding wheel (30) is presented in a normal manner to the tool (34), and with the radially extending plane of the grinding wheel including the axis of traverse of the wheel relative to the tool, the working surface (32) being shaped in the desired way by the radially extending plane of the grinding wheel being inclined in one sense at a selected angle in the range 1° to 20° to the axis of traverse of the wheel relative to the tool; or said working surface (32) is shaped to the same desired form by being presented in the normal manner to a tool shaping surface of the complementary form, and with the radially extending plane of the wheel including the axis of traverse of the wheel relative to the tool; and in the second grinding step, arranging that the radially extending plane of the wheel (30) is inclined in said one sense at the selected angle to the orientation of the radially extending plane of the grinding wheel in the first grinding step. A method according to claim 1, characterised in that the same grinding wheel (20) is employed in both grinding steps. A method according to claim 1, characterised in that different grinding wheels (20,30) are employed in the two grinding steps. A method according to claim 3, characterised in that the working surface of the grinding wheel (20) employed in the first grinding step is provided by gemstone, or synthetic diamond, particles bonded by a suitable metal, or metal alloy. A method according to any one of the preceding claims, characterised in that a deformable working surface (24,32) of a grinding wheel employed is provided by gemstone, or synthetic diamond, particles bonded by a vitreous binder, or a binder comprising a mixture of copper and a suitable resin.

Inventors: DAWSON, DEREK JOHN
Applicants: T&N TECHNOLOGY LIMITED
Application year: 1990
Main classification: B24B1
IPC codes: B24B53, B24B1