Method and device for treating machined surface of workpiece

In a method and a device for treating a machined surface of a workpiece made of a brittle material, a laser beam generated by a laser oscillator is applied to the machined surface of the workpiece while the workpiece is relatively moved with respect to the laser beam. Additiives and impurities exsisting in cracks generated on the machined surface melt due to thermal energy of the laser beam thereby to eliminate the cracks and improve the strength of the workpiece.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method and a device for treating 
machined surfaces of workpieces made of brittle materials to eliminate 
cracks generated thereon. 
2. Description of the Prior Art 
In the case where a workpiece made of a brittle material such as ceramics 
is subjected to the machining work such as the grinding work, cracks are 
generated on the surface of the workpiece due to the machining work to 
lower the strength of the obtained workpiece. 
For preventing the generation of the above-descrived cracks and increasing 
the strength of the workpiece, conventionally, fine machining work such as 
fine grinding work has been performed by decreasing the cutting depth of a 
grinding wheels, and/or by reducing the feeding speed of the workpieces 
during the grinding work, for example. 
By this fine machining work, the number and depth of cracks can be reduced, 
but the generation of cracks cannot be completely prevented. Accordingly, 
the fine-machined workpiece is subjected to the lapping work. But, even 
after the workpieces are lapped by a lapping oil containing abrasive 
grains, the generated cracks are not completely eliminated thereby. 
Moreover, the above-described fine machining work takes a long operation 
time. 
Therefore, this fine machining work causes the increase in production cost 
and time, and does not serve to improve the strength of the machined 
workpiece. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method and a device 
for speedily treating the machined surface of a workpiece made of a 
brittle material to obtain a workpiece of a high strengh, of which the 
machined surface is free from cracks, with good productivity. 
In the method of the present invention, a laser beam is applied to the 
machined surface of the workpiece made of the brittle material for a 
predetermined time, and the laser beam is relatively moved with respect to 
the machined surface for heating the whole of the machined surface. 
And the device of the present invention has laser generating means for 
generating laser, a lens member provided opposite to the machined surface 
for applying the generated laser to the machined surface as a laser beam, 
and moving means for relatively moving one of the lens member and the 
machined surface with respect to the other one. 
According to the present invention, by virtue of the use of the laser beam, 
additives and impurities exsisting in the boundary between grains located 
near the machined surface melt due to thermal energy of the laser beam to 
fill up the cracks thereby to completely eliminate the cracks and improve 
the strength of the machined surface of the workpiece. 
Moreover, according to the present invention, the conventionally performed 
fine-machining work and lapping work become unnecessary. This results in 
the production cost and time being decreased, and accordingly the 
productivity being greatly improved.

DETAILED DESCRIPTION OF THE EMBODIMENT 
Hereinafter, the basic technical idea of the present invention will be 
explained with reference to FIGS. 1 and 2. 
When a workpiece 1, which is made of a brittle material such as ceramics, 
is machined cracks C are generated on a machined surface 11 of the 
workpiece 1 to incur the lowering of the strength of the machined 
workpiece 1. According to the present invention, in order to eliminate 
these cracks, laser beam is applied to the machined surface 11 of the 
workpiece 1 from a laser oscillator 2 through an optical transmission 
medium 3. 
In the case of the laser beam of a short wave length, such as YAG laser 
beam, an optical fiber 31 and two convex lenses 32, 33 respectively 
disposed at laser beam inlet end and outlet end of the optical fiber 31, 
serve as the optical transmission medium 3. The laser beam transmitted 
from the laser oscillator 2 is converged by the convex lens 32 and enters 
the optical fiber 31. After passing the optical fiber 31 while being 
repeatedly reflected on an inner surface of the optical fiber 31, the 
laser beam is converged again by the convex lens 33 and is applied to the 
machined surface 11 of the workpiece 1. The whole machined surface 11 of 
the workpiece 1 is irradiated with the laser beam by relatively moving the 
machined surface 11 with respect to the applied laser beam. 
By applying the laser beam to the machined surface 11 of the workpiece 1, 
additives such as sintering aids and binders, and impurities each existing 
in the boundary between grains located near the machined surface the laser 
beam to fill up the cracks C thereby to completely eliminate the cracks C 
and recover the lowered strength of the workpiece 1. 
In the case of the laser beam of a long wave length such as Co.sub.2 gas 
laser beam, the laser beam generated by the laser oscillator 21 is 
transmitted within a guide tube 34 and is reflected by a mirror disposed 
at a bent portion of the guide tube 34. The reflected Co.sub.2 gas laser 
beam is converged by a convex lens 35 disposed at an outlet end of the 
guide tube 34, and is applied to the machined workpiece 1 to eliminate the 
cracks generated on the machined surface thereof similarly to the case of 
YAG laser beam. 
According to the present invention, any kind of laser other than the 
above-described YAG laser and Co.sub.2 gas laser can be used. 
Hereinafter, the present invention will be explained in accordance with 
several embodiments wherein the present invention is applied to workpieces 
which are subjected to the grinding work. 
FIG. 3 illustrates a first embodiment of a device and a method for treating 
a machined surface of a workpiece of a brittle material according to the 
present invention, wherein laser beam is applied to a workpiece ground by 
means of an external cylindrical grinding machine. 
A bed 40 of an external cylindrical grinding machine 4 is provided with a 
first slideway 41 and a second slideway 42. The first slideway 41 extends 
rightward and leftward in FIG. 3, and slidably mounts a table 43 while the 
second slideway 42 extends backward and frontward in FIG. 3, and slidably 
mounts a wheel head 44. 
On the table 43 are oppositely disposed a headstock 50 and a tailstock 60. 
The headstock 50 rotatably supports a main spindle 51 which is to be 
driven by a motor 52. And a chuck 53 is attached to a tip end of the main 
spindle 51 while a center 61 is attached to the tailstock 60. A 
cylindrical workpiece 1 made of a brittle material such as fine ceramics 
is supported by the chuck 53 and the center 61. 
The wheel head 44 rotatably supports a wheel spindle 45, and on a tip end 
of the wheel spindle 45 is detachably mounted a diamond grinding wheel 46. 
The wheel spindle 45 is driven by a motor 47 through a pulley 48. 
The table 43 and the wheel head 44 are slid on the slideway 41 and 42 by 
means of servomotors 70 and 71 and ball-screw mechanisms (not shown), 
respectively. 
The reference numeral 20 represents a YAG laser oscillator separately 
disposed from the bed 40. Laser beam transmitted from the laser oscillator 
20 is guided to a laser torch 23 through an optical fiber 22. The laser 
torch 23 is attached to the bed 40 on a forward side of the grinding wheel 
46 with respect to the grinding direction of the table 43 so as to be 
directed to an outer surface of the workpiece 1. In FIG. 3, the table 43 
is slide leftward. So, the laser torch 23 is attached to the bed 40 near 
the main spindle 51 by a bracket 24. The laser torch 23 is provided with a 
convex lens(not shown) for converging the laser beam. 
Next, the operation of the device of the first embodiment will be 
explained. 
First, the table 43 is slid to its rightward sliding end and the wheel head 
44 is slid to its backward sliding end. And in this state, the cylindrical 
workpiece 1 is supported by the chuck 53 and the center 61 on the table 
43. Then, the motors 47 and 52 are operated to rotate the grinding wheel 
46 and the main spindle 51, respectively. 
Next, the wheel head 44 is slid forward until the grinding wheel 46 abuts 
on the outer surface of the workpiece 1, and the table 43 on which the 
workpiece 1 is mounted is repeatedly slid leftward and rightward while the 
wheel head 44 is slid forward for grinding, until the outer surface of the 
workpiece 1 becomes smooth. At the start of the leftward sliding of the 
table 43 for the final grinding, the laser oscillator 20 is started 
transmitting laser beam to the laser torch 23 by way of the optical fiber 
22, and the workpiece 1 starts moving relatively with respect to the 
grinding wheel 46 and the adjacent laser torch 23. This results in the 
outer surface of the work piece 1 being ground, and then the ground outer 
surface of the workpiece being irradiated with laser beam. 
After the whole outer surface of the workpiece 1 is irradiated with the 
laser beam, the wheel head 44 is slid backward, the table 43 is slid 
rightward, and the rotations of the grinding wheel 46 and the main spindle 
1 are stopped. 
FIG. 4 is a graph showing the relation between the irradiation time of the 
laser beam and the bending strength of the workpiece in the case of a 
laser output of 721W/cm.sup.2. 
As is apparent from FIG. 4, by applying the laser beam to the machined 
workpiece for a short time (15 sec.), the average bending strength thereof 
is increased as compared with the oases of the machined workpiece being 
not irradiated with the laser beam, and the machined workpiece being 
subjected to the lapping work. And as the irradiation time of the laser 
beam is extended, the average bending strength of the workpiece is much 
increased. 
According to the device and the method of the above-described first 
embodiment, the cracks generated during the grinding work can be 
eliminated due to thermal energy of the laser beam. Therefore, the 
strength of the ground workpiece can be improved. 
Moreover, according to the first embodiment, the final grinding process and 
the cracks-eliminating process can be effected at substantially the same 
time. Therefore, the production time can be greatly decreased. 
In addition, according to the first embodiment, the grinding process and 
the cracks-eliminating process can be effected on a single machine tool. 
So, the production cost is also greatly decreased, accordingly, the 
productivity is remarkably improved. 
In the first embodiment the laser torch is fixed to the bed through the 
bracket. Instead, if a cover is provided for the grinding wheel, the laser 
torch may be fixed to this cover. 
The present embodiment is applied to the external grinding machine. In 
addition, the present embodiment can be applied to the other machine tools 
such as a lathe turning machine, too. 
FIGS. 5 and 6 illustrate a second embodiment of the present invention, 
wherein the present invention is applied to the external cylindrical 
grinding machine, similarly to the first embodiment. 
In FIGS. 5 and 6, the laser torch 23 is attached to an upper end of a 
bracket 231 projecting from the bed 40 so as to be directed to the 
frontside of the workpiece 1 opposite to the grinding wheel 46, and a 
coolant nozzle 25 is provided for injecting coolant toward the outer 
surface of the workpiece 1 to be ground by the grinding wheel 46. 
And a coolant guard plate 26 secured to the headstock 50 is provided at an 
intermediate position between the grinding wheel 46 and the laser torch 23 
along the outer surface of the workpiece 1. To a tip end of the guard 
plate 26 is fixed a scraper 27 made of rubber in watertight contact with 
the outer surface of the workpiece 1. 
The other structure of the device of the second embodiment is substantially 
equal to that of the first embodiment. 
In operation, in the second embodiment, while the workpiece 1 is rotated in 
a direction shown by an arrow in FIG. 6, and is ground by the grinding 
wheel 46, coolant is injected from the coolant nozzle 25 for cooling the 
ground surface of the workpiece 1 of which the temperature is excessively 
raised due to grinding. At this time, the coolant guard plate 26 prevents 
the coolant from flowing into the frontside of the workpiece 1. Therefore, 
the outer surface of the frontside of the workpiece 1 to be irradiated 
with laser beam transmitted from the laser torch 23 can be prevented from 
being cooled by the coolant. Accordingly, the irradiation of laser beam to 
the workpiece 1 can be effected at a high temperature, so the 
cracks-eliminating process can be effectively performed. 
The other operation of the second embodiment is substantially equal to that 
of the first embodiment. 
FIG. 7 illustrates a third embodiment of the present invention wherein the 
present invention is applied to a surface grinding machine. 
In FIG. 7, a table 430 is slidably supported by a bed 400, and a thick 
board-shaped workpiece 10 is mounted on the table 430, and is moved 
rightward and leftward with the table 430. A grinding wheel 460 is 
rotatably supported by a wheel head (not shown) so as to be moved downward 
and abut on the upper surface of the workpiece 10. And a coolant nozzle 
250 is provided on the left side of the grinding wheel 460 so as to be 
directed to the upper surface of the workpiece 10 on which the grinding 
wheel abuts. 
An industrial robot 80 is mounted on the bed 400. The robot 80 is provided 
with a first arm 81 which is rotatably supported by a main body 83 of the 
robot 80, and a second arm 82 which is hinged on a tip end of the first 
arm 81. And a laser torch 230 is attached to a tip end of the second arm 
82 so as to be directed to the upper surface of the workpiece 1 moving on 
the bed 400 with the table 430. A laser oscillator 200 is connected to the 
laser torch 230 through an optical fiber 220. And an air blower 84 is 
provided between the grinding wheel 460 and the laser torch 230. 
In operation, the workpiece 10 mounted on the table 430 is moved rightward 
and leftward on the bed 400 for grinding. The grinding wheel 460 is 
rotated in a direction shown by an arrow in FIG. 7 and grinds the upper 
surface of the workpiece 10 while moving downward. At this time, the 
coolant nozzle 250 injects coolant to the ground surface of the workpiece 
10 for decreasing the temperature thereof. When the workpiece 10 is moved 
rightward for the final grinding, the first arm 81 and the second arm 82 
of the robot 80 are rotated so that the laser torch 230 is moved in 
directions, each being perpendicular to the moving direction of the 
workpiece 10. This results in the laser beam being applied to the whole 
upper surface of the workpiece 10. Before the workpiece 10 is irradiated 
with the laser beam, the coolant remaining on the ground upper surface is 
blown away by means of the air blower 84. 
FIG. 8 illustrates a fourth embodiment of the present invention wherein the 
present invention is applied to an internally ground work piece. 
The reference numeral 100 represents a cylindrical workpiece of which an 
inner surface 101 has been ground by an internal cylindrical grinding 
machine (not shown) or the like. The cylindrical workpiece 100 is mounted 
on a jig 401. A laser torch 232 is provided on an axis of the cylindrical 
work piece 100 outside of one open end thereof. The laser torch 232 ia 
connected to a laser oscillator 201 through an optical fiber 221. The 
reference numeral 90 represents a mirror feeding device which is provided 
outside of the other open end of the workpiece 100. A cone-shaped mirror 
92 is attached to a tip end of a rod-shaped mirror feeding member 91. The 
mirror feeding member 91 is moved leftward and rightward in FIG. 8, on the 
axis of the workpiece 100 by means of a cylinder (not shown) or the like 
for feeding the cone-shaped mirror 92 into an inner bore 102 along the 
axis thereof from one open end to the other open end of the inner bore 
102. 
The laser beam is applied to the inner bore 102 of the workpiece 100 from 
the laser torch 232 while the cone-shaped mirror 92 is fed leftward and 
rightward within the inner bore 102 along the axis thereof by means of the 
mirror feeding member 91. The applied laser beam is reflected by the 
cone-shaped mirror 92 in directions, each being perpendicular to the axis 
of the cylindrical workpiece 100. And due to the movement of the mirror 
feeding member 91, the whole ground inner surface is irradiated with the 
laser beam from one end to the other end thereof. 
According to the present embodiment, the inner surface of the cylindrical 
bore of the workpiece, having such a diameter that the laser torch cannot 
be provided in the bore so as to be directed to the inner surface thereof, 
can be irradiated with laser beam. By virtue of the cone-shaped mirror, an 
inner surface of a large area can be irradiatd with laser beam at a time. 
The present embodiment is effected in an off-line system. The present 
embodiment can be also integrally provided on a single machine tool such 
as an internal griniding machine. 
Moreover, in the present embodiment, the mirror feeding member is moved for 
irradiating the whole inner surface with laser beam. Instead, the 
workpiece may be axially moved while the cone shaped mirror is fixed.