A process for machining an edge portion of a ceramic article preform without chipping

Abstract: A process for machining edge portions of a ceramic article preform without chipping in such consecutive machining steps and machining directions that a chipped portion occurring in a machined part of said edge portion in a certain machining step is removed by any succeeding step, and a lastly remaining part of the edge portions of ceramic article preform is machined in a final step without chipping, thereby enabling the edge portion of the ceramic article preform to be ground without chipping.

Description: Background of the Invention Field of the Invention The present invention relates to a process for machining edge portions of a ceramic article preform, particularly, of a ceramic article preform having a rotationary symmetrical shape. Related Art Statement When a workpiece such an a preform of a ceramic article is to be ground, a machining order and machining directions are subjected to arbitrary discretion of a worker, or such machining order and machining directions are selected from the standpoint of shortening the machining time. When the ceramic article preform is to be machined by a machining tool such as a grinding stone in a traverse machining, no chipping occurs at a side of a machining face of the ceramic article preform which the machining tool enters because mainly compression stress applies, whereas because tensile stress applies to a side of the machining face of the preform through which the grinding tool leaves, chipping inevitably occurs there. Since the ceramic article is conventionally finished after being ground, it is an actually common practice to machine the preform in such a machining order and machining directions as causing chipping without taking any countermeasure to preventing the chipping. However, if the ceramic article is chipped, it is considered that the ceramic article, particularly a structural article such as an engine valve, begins to be broken from a chipped portion. The above chipping phenomenon is explained with reference to the conventional process for grinding a preform of a ceramic valve by way of example. Fig. 1 is a schematic view for illustrating an apparatus which is to continuously machining ceramic valve preforms each having a rotation symmetrical shape. The ceramic valve preforms are machined according to the following four steps (1) to (4). (1) The ceramic valve perform is conveyed to a station 1 from a feed side by means of a chuck 1-1, and chucked by a chuck C1-2. Then, a head portion and a rear end portion of a shaft of the valve preform shown by circles in Fig. 1 are ground by a grinding tool not shown. (2) The ground ceramic valve preform is chucked by the chuck 1-1, and placed on a reversing station where the preform is reversed at 180. (3) Then, the reversed ceramic valve is arranged between chuck heads C2-2a and C2-2b at a station 2 by means of a chuck 2-1, and is finish ground by a finish grinding tool not shown. (4) The finish ground ceramic valve is conveyed to a discharge side. The chipping phenomenon discussed in the present application occurs in the grinding step in the station. Fig. 2 shows portions of the ceramic valve preform W chucked by the chuck 1-2 in the station 1, said portions being to be machined by a grinding stone T as a grinding tool in the state that the valve preform is being rotated around its axis. Figs. 3 and 4 show the order and directions for machining a head portion and a rear end portion of a shaft of the valve preform according to a conventional machining process, respectively. In this conventional machining process, in order to decrease a grinding load, the movement of the grinding stone is controlled under the rotation of the valve preform W. As to the valve head portion, the grinding stone is moved from right to left as shown by a reference numeral "1" in Fig. 3 to grind a peripheral face of the head portion. Then, the grinding stone is moved obliquely upwardly from left to right as shown by a reference numeral "2". Thereafter, an end face of the valve head portion is ground by moving the grinding stone not through the entire end face but up to a central portion of the end face as shown by a reference numeral "3", different from the machining in the above "1" and "2". Finally, a corner portion of the head portion formed by the machining in the above "1" and "3" is chamfered by moving the grinding tool obliquely downwardly from left to right as shown in a reference numeral "4". Similarly, as to the rear end portion of the shaft of the valve preform, in order to decrease a grinding load, the movement of the grinding stone is controlled under the rotation of the valve preform W in Fig. 3. As to the rear end portion of the valve shaft, the grinding stone is moved from left to right as shown by a reference numeral "1" to grind a peripheral face of the shaft. Thereafter, as to an end face of the shaft, this end face is ground by moving the grinding stone not through the entire end face but up to a central portion of the end face as shown by a reference numeral "2", different from the machining in the above "1". Finally, a corner portion of the head portion formed by the machining in the above "12" and "2" is chamfered by moving the grinding tool obliquely downwardly from right to left as shown by a reference numeral "3". According to the above machining process, when the grinding stone grinds the ceramic valve preform through the entire grinding face, chipping may occur (See A and B in Fig. 3 and C in Fig. 4). Each of the valve head portion and the shaft portion of the ground ceramic valve preform is trimmed into a desired shape in the station 2 by the finish grinding (See shapes designated by dotted lines in Figs. 3 and 4). However, there is possibility that a chipping race remains even after the finish grinding. In order to completely remove the chipping trace, a finish grinding amount needs to be increased. Figs. 5 and 6 are enlarged photographs for showing chipping occurred at an edge portion of the shaft and the head portion of the valve preform ground in the first station according to the conventional machining process, respectively (See B in Fig. 3 and C in Fig. 4). As is seen from these figures, the chipped portions remain in the head portion and the edge portion of the shaft of the valve. US-A-4 344 260 discloses a method for the precision shaping of wafer materials. A single crystal rod is first ground to have a cross-section larger than the desired wafer by a margin to grind, and sliced into margined wafers. The margined wafer is subject to contour grinding, tracing a prototype of the desired wafer. Small chipping at the edges of the wafer is acceptable if it is limited within the margin removed by the contour grinding. Summary of the Invention The present inventors decreased the machining speed and decreased the grain size of the grinding stone to prevent or reduce the occurrence of the above chipping phenomenon, but the occurrence of the chipping could not be prevented or reduced. On the other hand, it was clarified that although the chipping phenomenon may be slightly mitigated by decreasing the machining speed in some cases, this largely increases the machining time and makes the process unsuitable for the mass-production. Further, it was also clarified that in order to incorporate the production of ceramic articles into a mass-production line without suffering the chipping phenomenon, a succeeding chipping-removing step needed to be effected after an initial grinding step, or a post-grinding step needed to be effected in such an excess extent as presumably meeting a chipped level. In those cases, the number of the producing steps increases or the production cost rises due to a low yield. The term "edge portion" is concept which includes a border line at which two different faces intersect as well as a vicinity thereof (totally referred to a boundary line portion). For example, a boundary line portion between an end face and a cylindrical peripheral face, a border line portion between an end face and a truncated conical face, a border line portion where different truncated conical faces intersect, a border line portion where a cylindrical face and a curved face intersect, etc. may be recited. The end face may be included in the edge portion. Under the circumstances, it is an object of the present invention to solve the above-mentioned problems, and to provide a process for machining an edge portion of a ceramic article preform without chipping at the surface thereof. The process for machining edge portions of the ceramic article preform without chipping according to the present invention is defined in claim 1. According to the invention, consecutive machining steps and machining directions are selected such that a chipped portion occurring in a machined part of said edge portion in a certain machining step is removed by any succeeding step, and a lastly remaining part of the edge portions of ceramic article preform is machined in a final step without chipping, thereby enabling the edge portion of the ceramic article preform to be ground without chipping. As preferred embodiments of the process for machining the edge portion of ceramic article without chipping according to the present invention, the following are recited. (1) Said edge portion of the ceramic article is included in any one of an edge face, a peripheral face continuing to the edge face, an inclined face continuing to the peripheral face, and a concaved face continuing to the inclined face, and said consecutive machining steps and said machining directions are selected such that a side of a machined part of the ceramic article through which a grinding tool left in a certain machining step is set at a side of a part of the ceramic article through which the grinding tool enters in any succeeding step. (2) The process for machining the edge portion of ceramic article preform without chipping according to the present invention is particularly suitable for grinding an end portion of a head portion and a rear end portion of a shaft of a ceramic valve preform, end portions of ceramic tube, edge portions of a ceramic support pin jig, edge portions of an all-ceramic turborotor, etc., each having a rotation symmetrical shape. (3) Said machining of said lastly remaining part of the edge portions of the ceramic article preform in said final step without chipping is to machine an end face of the ceramic valve preform, said end face exteding in a direction substantially perpendicular to the axis in said rotation or being concave in an axially outward direction. These and other objects, features and advantages of the invention will be appreciated upon reading of the following description of the invention, with the understanding that some modifications, variations and changes of the same could be easily made by the skilled person in the art. Brief Description of the Drawings For a better understanding of the invention, reference is made to the attached drawings, wherein: Fig. 1 is a schematic view of the apparatus for continuously machining the ceramic valve preforms each having a rotationally symmetrical shape; Fig. 2 shows portions of the portions of the ceramic valve preform W chucked by the chuck 1-2 in the station 1, said portions being to be machined by a grinding stone T as a grinding tool in the state that the valve preform is being rotated around its axis; Fig. 3 shows the order and directions for machining a head portion of the valve preform according to a conventional machining process; Fig. 4 shows the order and directions for machining a rear end portion of a shaft of the valve preform according to a conventional machining process; Fig. 5 is an enlarged photograph for showing chipping occurred at an end portion of the shaft of the valve preform ground in the first station according to the conventional machining process; Fig. 6 is an enlarged photograph for showing chipping occurred at the head portion the valve preform ground in the first station according to the conventional machining process; Fig. 7 shows an example of process for grinding a valve head portion of an engine valve preform made of a ceramic material as a ceramic article without chipping according to the present invention; Fig. 8 shows the process for grinding a rear edge portion of a shaft portion of the engine valve preform made of a ceramic material as a ceramic article without chipping according to the present invention; Figs. 9 and 10 show another example of the process for grinding a valve head portion and a rear edge portion of a shaft portion of an engine valve preform made of a ceramic material as a ceramic article, without chipping, according to the present invention, respectively; Figs. 11 and 12 show still another example of the process for grinding a valve head portion and a rear edge portion of a shaft portion of an engine valve preform made of a ceramic material as a ceramic article, without chipping, according to the present invention, respectively; Figs. 13 and 14 show a further example of the process for grinding a valve head portion and a rear edge portion of a shaft portion of an engine valve preform made of a ceramic material as a ceramic article, without chipping, according to the present invention, respectively; and Fig. 15 shows a still further example of the process for grinding a valve head portion of an engine valve preform made of a ceramic material as a ceramic article, without chipping, according to the present invention, respectively. Detailed Description of the Invention In the following, a specific embodiment of the present invention will be explained. Figs. 7 and 8 show the process for machining the engine valve preform made of a ceramic material as a ceramic article without chipping according to the present invention. This machining process is fundamentally the same as in the case of the conventional machining process explained in Figs. 1 to 4 except for the machining order and the machining directions. First, the ceramic engine valve preform is chucked at the shaft portion (See Fig. 2), the valve head portion and the rear end portion of the shaft are ground by the grinding tool T (See Figs. 7 and 8). As to the valve head portion, it is ground by means of the grinding tool T in the machining order and directions as shown by "4", "5", "6" and "7", while the valve preform is being rotated around its axis. That is, the grinding alone T is first moved from left to right as shown by "4", thereby grinding the peripheral face of the valve head portion. Chipping occurs at a portion 9 through which the grinding stone leaves the valve head portion. Next, a valve seat surface at which the valve head seats a valve seat is formed through grinding by moving the grinding tool downwardly from right to left as shown by "5". It may be considered that chipping occurs at a portion 10 by the above grinding. However, since the portion 10 through which the grinding stone leaves the valve head portion has a shape concaved toward the axis, chipping is unlikely to occur. Even if chipping occurs, it is very limited. Therefore, such a very limited chipped portion can be easily removed by ordinary finishing performed next in the station 2. Then, in the station 1, a chamfered face is formed as a chucking face by moving the grinding stone downwardly from left to right as shown by "6". At that time, the chipping at the portion 9 is removed by the grinding in the step "6", whereas chipping occurs in a right lower edge portion 11 through which the grinding stone leaves the head portion. Finally, as shown by "7", the end face of the valve head portion is ground. Since the valve preform is rotated, the end face of the valve head portion needs not be machined by moving the grinding tool through the entire end face, different from the machining in the case of "4", "5" and "6", but ordinarily up to a portion slightly beyond the center of the end face of the valve head has only to be ground in a contacted manner. By so doing, the chipping occurred at "9" by the grinding in "4" is removed, whereas no chipping occurs in the grinding of the end face in "7" because the end face is ground not through the entire face from one side to the other in "7". Similarly, as to the end portion of the valve shaft, the peripheral face of the shaft end portion is ground by moving the grinding stone T under rotation of the valve preform over a given length as shown by "13" (In this case, no chipping occurs). Then, after a corner of the shaft edge portion is chamfered as shown by "14", and a chucking face is formed in the station 1 (In this case, chipping occurs in a portion 16 of the end face of the shaft portion through which the grinding stone leaves it). Finally, the chipped portion formed at 16 in the grinding of "14" is removed by grinding the end face in "15". No chipping occurs in the case of the grinding of the edge face in "15". In this manner, the ceramic valve fundamentally free from chipping can be obtained by the rough grinding. This ceramic valve is finish ground by an ordinary method in the station 2, thereby obtaining a finish ground ceramic valve (See shapes shown by dotted lines in Figs. 7 and 8). In Figs. 7 and 8, so long as the effects aimed at by the chipping-free machining process according to the present invention, the machining order and/or the machining directions may be appropriately changed, for example, the grinding in "4" of Fig. 7 may be effected from right to left. Other examples of the process for grinding a valve head portion and a rear edge portion of a shaft portion of an engine valve preform made of a ceramic material as a ceramic article, without chipping, according to the present invention are shown in Figs. 9 through 15. The engine valve performs are machined in the same manner as in Figs. 7 and 8 (chucking, grinding, grinding tool, grinding stone and machining under rotation) except for the machining order and the machining directions. The machining order and the machining directions are indicated in the same manner as in Figs. 7 and 8. As to the grinding process in Fig. 15, a rear edge portion of a shaft portion of the engine valve preform is machined according to the any one of the methods in Figs. 8, 10, 12 and 14. As easily understood, the examples of the grinding process shown in Figs. 9 through 15 fall in the scope of the claimed invention. The process for machining the edge portion of the ceramic article without chipping according to the present invention has the following effects. (1) The consecutive machining steps and machining directions are selected such that a chipped portion occurring in a machined part of said edge portion in a certain machining step is removed by any succeeding step, and a lastly remaining part of the edge portion of ceramic article is machined in a final step without chipping, thereby enabling the edge portions of the ceramic article to be ground without chipping. Therefore, any chipped portion needs not be removed in the post machining treatment. Thus, the ceramic article can be effectively machined. Further, since any chipped portion needs not be removed in the post machining treatment, the ceramic article finished at a necessary minimum level, thereby enhancing the yield of the products. (2) When the above edge portion of the ceramic article is included in any of an end face, a peripheral face continuing to the end face, an inclined face continuing to the peripheral face, and a concaved face continuing to the inclined face, and said consecutive machining steps and said machining directions are selected such that a side of a machined part of the ceramic article through which a grinding tool left in a certain machining step is set at a side of a part of the ceramic article through which the grinding tool enters in any succeeding step, the same effects mentioned in (1) may be obtained. (3) When the ceramic article preform is a valve preform having a rotationary symmetrical shape, made of a ceramic material and comprising a head portion and a shaft portion, and said machining is to grind edge portions of the head portion and the shaft portion of the valve preform while the valve preform is being rotated around an axis of the valve preform. Such a ceramic valve preform free from chipping can be easily produced without necessitating excess amount of the finishing. Further, in this case, when the machining of said lastly remaining part of the edge portion of ceramic article preform in said final step without chipping is to machine an end face of the ceramic valve preform, said end face extending in a direction substantially perpendicular to the axis in said rotation or being convexed in an axially outward direction, no chipping occurs in the final machining step. Therefore, the ground ceramic valve free from chipping can be obtained, which can be easily finished in the post machining.

Claims: A process for machining edge portions of a ceramic article preform, said process comprising a plurality of consecutive machining steps, each said machining step having a linear machining direction and forming a face at an angle to each other face formed in said machining steps, wherein said consecutive machining steps and each machining direction thereof are selected such that (i) a grinding tool in each said machining step after the first enters the article through a face that has been machined in a preceding said machining step and (ii) in at least one of said machining steps a chip formed in a preceding said machining step is removed, and wherein the final one of said machining steps and its machining direction are selected such that no chipping occurs in said final step, and wherein in the process each chip formed in said machining steps is removed either by a subsequent one of said machining steps or by a finishing step performed after said machining steps. The machining process set forth in claim 1, wherein said face formed in each machining step is selected from an end face, a peripheral face extending to an end face, an inclined face extending to a peripheral face, and a concave face extending to an inclined face. The machining process set forth in claim 1 or 2, wherein said ceramic article is a valve preform having a rotationary symmetrical shape, made of a ceramic material and comprising a head portion and a shaft portion, and said machining is to grind edge portions of the head portion and the shaft portion of the valve preform while the valve preform is being rotated around its axis. The machining process set forth in claim 3, wherein said final machining step involves machining an end face of the ceramic valve preform, said end face extending in a direction substantially perpendicular to the axis of rotation of the pre-form or being concave.

Inventors: DEWITTE CARINNE ANNE-MARIE CLA, NISHIO AKIFUMI, PATTIMORE JEFFREY PHILIP JOHN, DEWITTE, CARINNE ANNE-MARIE CLAUDINE, NISHIO, AKIFUMI, PATTIMORE, JEFFREY PHILIP JOHN
Applicants: NGK CERAMICS EUROP SA, NGK INSULATORS LTD, NGK CERAMICS EUROPE S.A., NGK INSULATORS, LTD.
Application year: 1997
Main classification: B24B9
IPC codes: B24B9, B24B5, B24B15