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

A process for machining edge portions of a ceramic article preform without chipping includes grinding in such consecutive machining steps and machining directions that a chipped portion which results in a machined part of the edge portion in a machining step, is removed by any succeeding. Further, a lastly remaining part of the edge portions of the 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.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an improvement on a process for machining 
an edge portion of a ceramic article preform, particularly, an edge 
portion of a ceramic article preform having a rotationary symmetrical 
edge. 
2. Related Art Statement 
When a workpiece such as 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 preform is conveyed to a station 1 from a feed side 
by means of a chuck C1-1, 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 C1-1, 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 C2-1, 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 1. 
FIG. 2 shows portions of the ceramic valve preform W chucked by the chuck 
C1-2 in the station 1, the 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 number "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. 4. 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 "1" 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 trace 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. 
SUMMARY OF THE INVENTION 
The present inventors decreased the machining speed or 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 a 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 characterized in 
that 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 convexed 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.

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 VP 
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 20 (See 
FIG. 2), the valve head portion 22 and the rear end portion 24 of the 
shaft are ground by the grinding tool T (See FIGS. 7 and 8). As to the 
valve head portion 22, 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 
stone T is first moved from left to right as shown by "4", thereby 
grinding the peripheral face 26 of the valve head portion. 
Chipping occurs at an edge portion 9 through which the grinding stone 
leaves the valve head portion 22. 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 9 through which the grinding stone leaves the 
valve head portion 22 has a shape concaved (28) 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 or inclined face 30 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 
"4", 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 32 of the valve head portion is ground. As shown in FIG. 
7, the end face 32 extends substantially perpendicularly to the central 
longitudinal axis X thereof. Since the valve preform is rotated, the end 
face 32 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 32 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 32 in "7" because the end face 32 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 or concave face 28 of the 
shaft end portion 24 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 end face in "15". In this manner, the ceramic view 
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 22 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 
preforms 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 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 32, a peripheral face 26 continuing to the end face 32, an 
inclined face 30 continuing to the peripheral face 26, and a concaved face 
28 continuing to the inclined face 30, and the consecutive machining steps 
and the 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.