Cutting tool of polycrystalline diamond and method of manufacturing the same

A polycrystalline diamond cutting tool is prepared by employing polycrystalline diamond which is synthesized on a mirror-finished surface of a base material by a low-pressure vapor phase method, as a tool material. A surface, which has been in contact with the base material, of the polycrystalline diamond layer is utilized as a tool rake face. A flank of the tool is formed by laser processing. The flank is covered with a graphite coating layer in one embodiment, while such a graphite coating layer is removed by acid treatment or the like in another embodiment. In still another embodiment, a flank is formed by grinding, and a cutting edge portion is honed by laser processing.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a cutting tool of polycrystalline diamond 
having excellent edge sharpness, which is optimum for finishing a work 
piece of a nonferrous metal or a nonmetal, and a method of manufacturing 
the same. 
2. Background Information 
Diamond, which has high hardness as well as high heat conductivity, is 
applied to a cutting tool or a wear resisting tool. However, monocrystal 
diamond is disadvantageously cloven. In order to suppress such a 
disadvantage, there has been developed a diamond sintered body which is 
obtained by sintering diamond by a very high pressure sintering technique, 
as described in Japanese Patent Publication No. 52-12126 (1977). 
As to commercially available diamond sintered bodies, it is known that 
particularly a fine-grained sintered body having a grain size of not more 
than several 10 .mu.m is not subject to cleavage as observed in the 
aforementioned monocrystal diamond, and exhibits excellent wear 
resistance. 
However, since such a diamond sintered body contains several percent to 
several 10 percent of a binder, chipping is inevitably caused in units of 
the grains forming the same. Such a chipping phenomenon is remarkably 
increased as a wedge angle of the tool cutting edge is reduced, and hence 
it is extremely difficult to manufacture a tool having a sharp cutting 
edge. Although it is possible to grind the cutting edge with a fine-grain 
diamond grindstone of #3000 to #5000, for example, in order to suppress 
chipping of the cutting edge in a manufacturing process of the cutting 
tool, such chipping of the cutting edge cannot be suppressed to below 5 
.mu.m even if this method is employed. In the method of grinding the 
cutting edge with a fine-grain diamond grindstone, a new problem is caused 
in that the working efficiency is reduced as the grain size of the 
grindstone becomes smaller. 
In a known method of working a diamond sintered body, electrical discharge 
machining or laser beam machining is employed in place of the 
aforementioned grinding. In the existing circumstances, however, it is 
impossible to obtain a tool having excellent edge sharpness by such 
machining, since the diamond sintered body contains a binder. 
The inventors have devised a cutting tool, which is prepared from a tool 
material of polycrystalline diamond containing no binder, i.e., 
polycrystalline diamond synthesized by a low-pressure vapor phase method, 
in place of a diamond sintered body. Japanese Patent Laying-Open Gazette 
No. 1-212767 (1989) discloses an exemplary cutting tool which is prepared 
from a material of such polycrystalline diamond. 
However, in such a cutting tool of polycrystalline diamond, the cutting 
edge is chipped by about 5 .mu.m when the same is formed by grinding, and 
it is impossible to suppress such chipping. 
SUMMARY OF THE INVENTION 
An object of the present invention is to improve the edge sharpness of a 
polycrystalline diamond cutting tool. 
Another object of the present invention is to reduce the amount of chipping 
in a cutting edge of a polycrystalline diamond cutting tool. 
Still another object of the present invention is to suppress initial 
chipping of a polycrystalline diamond cutting tool. 
A further object of the present invention is to provide a method of 
manufacturing a polycrystalline diamond cutting tool which has an 
excellent edge sharpness. 
In a first aspect of the present invention, a polycrystalline diamond 
cutting tool is prepared from a tool material of polycrystalline diamond 
which is synthesized by a low-pressure vapor phase method. The tool 
material has a cutting edge flank which is covered with a graphite layer 
formed by laser processing and a rake face which has a surface finish of 
not more than 0.2 .mu.m in surface roughness at the maximum height of 
irregularities R.sub.max. Another polycrystalline diamond cutting tool is 
prepared from a tool material of polycrystalline diamond which is 
synthesized by a low-pressure vapor phase method. The tool material is 
connected with a tool holder and has a cutting edge flank which is covered 
with a graphite layer formed by laser processing, as well as a rake face 
which is in a mirror of not more than 0.2 .mu.m in surface roughness at 
the maximum height of irregularities R.sub.max. 
In a second aspect of the present invention, a polycrystalline diamond 
cutting tool is prepared from a tool material of polycrystalline diamond 
which is synthesized by a low-pressure vapor phase method. The 
polycrystalline diamond cutting tool has a rake face of not more than 0.2 
.mu.m in surface roughness at the maximum height of irregularities 
R.sub.max and a flank exposing a surface of polycrystalline diamond, and 
is so structured that the size of cutting edge chipping along the 
intersection between the rake face and the flank is at least 0.5 .mu., and 
not more than 5 .mu.m. This polycrystalline diamond cutting tool also 
includes a structure of an edge-worked polycrystalline diamond tool 
material which is directly mounted on a cutting machine, or a structure of 
such a polycrystalline diamond tool material which is connected with a 
tool holder. 
In a third aspect of the present invention, a polycrystalline diamond 
cutting tool is prepared from a tool material of polycrystalline diamond 
which is formed by a low-pressure vapor phase method. The tool material of 
polycrystalline diamond is provided with a rake face and a flank, while a 
cutting edge portion which is formed along the intersection between the 
rake face and the flank has a honed curved surface. 
In a fourth aspect of the present invention, a polycrystalline diamond 
cutting tool is manufactured through the following steps: First, 
polycrystalline diamond is deposited on a surface of a base material of 
not more than 0.2 .mu.m in surface roughness at the maximum height of 
irregularities R.sub.max by a low-pressure vapor phase method. Then, the 
polycrystalline diamond, which is deposited on the base material, is cut 
into a prescribed tip configuration, and thereafter the base material is 
removed from the polycrystalline diamond tip. Then, a surface of the 
polycrystalline diamond, which intersects with another surface having been 
in contact with the base material, is subjected to laser beam processing, 
thereby forming a cutting edge flank. 
In a polycrystalline diamond cutting tool of such a structure that a 
polycrystalline diamond tip is connected with a tool holder, the tip is so 
connected with the tool holder that a surface, having been in contact with 
the base material, of the polycrystalline diamond tip, from which the base 
material is removed, defines a rake face of the tool, and thereafter a 
cutting edge flank is formed by laser processing. 
In a fifth aspect of the present invention, a polycrystalline diamond 
cutting tool is manufactured by the following steps: 
First, polycrystalline diamond is deposited on a surface of a base material 
having surface roughness of not more than 0.2 .mu.m at the maximum height 
of irregularities R.sub.max by a low-pressure vapor phase method. Then, 
the polycrystalline diamond, which is deposited on the base material, is 
cut into a prescribed tip configuration, and thereafter the base material 
is removed from the polycrystalline diamond tip. A surface of the 
polycrystalline diamond tip, which intersects with another surface having 
been in contact with the base material, is subjected to laser processing, 
to form a cutting edge flank. Thereafter a graphite coating layer which is 
formed on the cutting edge flank by the laser processing is removed. 
In a polycrystalline diamond cutting tool of such a structure that a 
polycrystalline diamond tip is connected with a tool holder, the tip is so 
connected with the tool holder that a surface, having been in contact with 
the base material, of the polycrystalline diamond tip, from which the base 
material is removed, defines a rake face of the tool, and thereafter a 
cutting edge flank is formed by laser processing. 
In a sixth aspect of the present invention, a polycrystalline diamond 
cutting tool is manufactured as follows: First, polycrystalline diamond is 
deposited on a surface of a base material by a low-pressure vapor phase 
method. Then, the polycrystalline diamond, which is deposited on the base 
material, is cut into a prescribed tip configuration, and thereafter the 
base material is removed from the polycrystalline diamond tip. A surface 
of the polycrystalline diamond, which has been in contact with the base 
material, is used to define a tool rake face, and a flank is formed at a 
prescribed angle with respect to the rake face, thereby forming a cutting 
edge portion. The cutting edge portion is then honed to form a curved 
surface. 
The foregoing and other objects, features, aspects and advantages of the 
present invention will become more apparent from the following detailed 
description of the present invention when taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 6 showing a first embodiment of the present invention, a 
polycrystalline diamond tip 3 is strongly attached to a tool holder 4 of 
cemented carbide, steel or the like through a brazing portion 5. The upper 
surface of the polycrystalline diamond tip 3 forms a tool rake face 6, 
while a flank 7 is formed at a prescribed angle with respect to the rake 
face 6. A cutting edge portion is formed along the intersection between 
the cutting face 6 and the flank 7. 
FIGS. 1 to 5 illustrate steps of manufacturing the polycrystalline diamond 
cutting tool shown in FIG. 6. 
Referring to FIG. 1, a polycrystalline diamond layer 2 is formed on a 
surface of a substrate 1 of a metal or an alloy by a low-pressure vapor 
phase method. The surface of the substrate 1 is mirror-finished with 
surface roughness of not more than 0.1 .mu.m at the maximum height of 
irregularities R.sub.max. Polycrystalline diamond is deposited on this 
surface by a low-pressure vapor phase method of decomposing and exciting 
raw material gas through thermoionic emission or plasma discharge, or a 
film forming method employing combustion flame. The raw material gas is 
generally prepared from mixed gas which is mainly composed of an organic 
carbon compound of hydrocarbon such as methane, ethane or propane, alcohol 
such as methanol or ethanol, ester etc. and hydrogen. In addition, the raw 
material gas may further contain inert gas such as argon, carbon dioxide, 
water etc. in a range exerting no influence on synthesis of diamond and 
its characteristics. The polycrystalline diamond is synthesized on the 
surface of the substrate 1 by such a low-pressure vapor phase method so 
that its mean crystal grain size is 0.5 to 15 .mu.m. The crystal grain 
size of the polycrystalline diamond is defined in the above range since 
wear resistance of the cutting tool is reduced if the mean crystal grain 
size is smaller than 0.5 .mu.m, while the cutting tool is easily chipped 
if the mean crystal grain size exceeds 15 .mu.m. In order to reduce 
internal stress of the polycrystalline diamond, the substrate 1 is 
preferably prepared from a material such as molybdenum (Mo), tungsten (W), 
silicon (Si) or the like, whose coefficient of thermal expansion is close 
to that of diamond. In this specification and claims, silicon is treated 
as metal. 
Referring to FIG. 2, cutting plane lines are formed on the polycrystalline 
diamond layer 2, which is provided on the substrate 1, with a laser beam 
10 along a prescribed tool material configuration. 
Referring to FIG. 3, chemical processing is performed with hydrochloric 
acid, sulfuric acid, nitric acid, hydrofluoric acid or a mixed solution 
thereof, to dissolve the substrate 1 and remove the same from the 
polycrystalline diamond. Each of the as-formed polycrystalline diamond 
tool materials 3 is directly subjected to processing for forming a cutting 
edge if the thickness thereof is in excess of 1 mm in the direction of 
lamination, and clamped to a holder so that the same can be applied to a 
tool. If the thickness is in a range of 0.05 to 1 mm, preferrably 0.1 to 1 
mm, on the other hand, the tool material 3 can be applied to a cutting 
tool which is connected with a tool holder of cemented carbide, steel or 
the like. If a polycrystalline diamond tool material 3 having a thickness 
smaller than 0.05 mm is applied to a cutting tool which is connected with 
a tool holder, chipping is easily caused due to insufficient strength of 
the tool material 3. In such a connection type cutting tool, the thickness 
of the tool material 3 may sufficiently be about 1 mm for a general use. 
As shown in FIG. 4, it is requisite that the polycrystalline diamond tool 
material 3 is connected with the tool holder 4 after the former is brazed 
to the latter at the portion 5, so that a surface, which has been in 
contact with the substrate 1, of the tool material 3 defines the cutting 
face 6. 
Referring to FIG. 5, laser beam machining is applied to formation of the 
cutting edge under properly selected conditions, thereby manufacturing a 
cutting tool having an excellent edge sharpness, which has been hard to 
attain by grinding. This sharpness is possible because damage is reduced 
as compared to grinding, which is a mechanical removing method, since the 
polycrystalline diamond serving as the tool material contains no binder 
and the laser beam machining is accompanied by a thermal reaction. A YAG 
laser is preferably employed for the laser beam machining, in 
consideration of working efficiency and quality. A laser beam 15 is 
applied from the cutting face 6 side of the tool, or from an opposite 
side. At this time, an angle .theta. between the laser beam 15 and the 
cutting face 6 is set at a prescribed value so that a predetermined 
clearance angle can be formed. The tool is inclined with respect to the 
laser beam 15 and moved, in order to set the angle .theta.. Alternatively, 
the laser beam 15 may be adjusted while fixing the tool. The laser 
processing conditions are so selected that the size of cutting edge 
chipping is 0.5 to 5 .mu.m. Preferable the conditions are: average power 
of 2 to 5 W, a laser repetition rate of 1 to 5 KHz, and a laser beam 
operation rate of 0.1 to 5 mm/sec. 
The size of chipping is now defined with reference to FIG. 7. FIG. 7 
illustrates the cutting edge portion of the polycrystalline diamond tip 
shown in FIG. 6 in an enlarged manner, for typically showing a state of 
chipping 9 caused along the intersection between the rake face 6 and the 
flank 7. The size of such chipping 9 is defined by a larger value of 
lengths l.sub.1 and l.sub.2 measured toward the rake face 6 and toward the 
flank 7 respectively with reference to the intersection between the rake 
face 6 and the flank 7. It is possible to further reduce such chipping 
beyond the lower limit of 0.5 .mu.m, which has been estimated from the 
results of experiments made by the inventors, by changing the laser beam 
processing conditions. 
Referring again to FIG. 5, a graphite coating layer 8 is formed on the 
surface of the flank 7 after the cutting edge is formed by laser 
processing. The thickness of this graphite coating layer 8 is about 0.5 to 
10 .mu.m. 
Experimental Examples of the present invention are now described in detail. 
EXPERIMENTAL EXAMPLE 1 
Polycrystalline diamond was synthesized for 10 hours on an Si substrate 
having a mirror surface of 0.05 .mu.m in surface roughness at the maximum 
height of irregularities R.sub.max by microwave plasma CVD, under the 
following conditions: 
Raw Material Gas (Flow Rate): H.sub.2 : 200 sccm, CH.sub.4 : 10 sccm. 
Gas Pressure: 120 Torr. 
Microwave Oscillation Power: 650 W 
After the synthesis, the substance was dipped in hydrofluoric-nitric acid 
to dissolve and remove only the Si substrate, thereby recovering 
polycrystalline diamond of 5 .mu.m in mean crystal grain size and 0.2 mm 
in thickness. The growth plane side of this polycrystalline diamond was 
connected with a shank of cemented carbide by brazing. Then this connected 
body was maintained in an inclined state so that a laser beam for 
processing was at an angle .theta. of 101.degree. with respect to a rake 
face of the connected body to form a cutting edge, thereby manufacturing a 
throw-away tip (type No. SPGN120304). The cutting edge was formed by a YAG 
laser of a continuous oscillation mode with laser power of 3 W. The 
as-formed throw-away tip (hereinafter referred to as "sample A") was 
examined with a microscope, whereby it was found that the size of chipping 
was 1 .mu.m and the flank was covered with a graphite layer of 2 .mu.m in 
thickness. 
Comparative samples were prepared by employing a tool material of sintered 
diamond of 5 .mu.m in grain size containing 12 percent by volume of Co as 
a binder and forming a cutting edge by laser processing similarly to the 
above (sample B), forming a cutting edge by grinding with a diamond 
grindstone of #1500 (sample C), and employing the aforementioned tool 
material of polycrystalline diamond and forming a cutting edge by grinding 
similarly to the sample C (sample D). These samples B, C and D exhibited 
amounts of chipping of 30 .mu.m, 20 .mu.m and 15 .mu.m respectively, which 
were larger as compared with the sample A. 
These throw-away tips, serving as finishing tools, were subjected to 
performance evaluation tests under the following conditions: 
Cutting Conditions 
Workpiece: round bar of AC4C-T8 (Al--7%Si) 
Cutting Speed: 500 m/min. 
Depth of Cut: 0.2 mm 
Feed Rate: 0.1 mm/rev. 
Coolant: water-soluble oil solution 
Evaluation Method 
Comparison of work surface roughness values after cutting for 5 minutes and 
60 minutes 
Table 1 shows the results. 
TABLE 1 
______________________________________ 
Roughness of Work Surface 
After 5 min. 
After 60 min. 
Sample No. Cutting Cutting 
______________________________________ 
Inventive Sample 
A 1.8 1.7 
Comparative Sample 
B 5.3 6.7 
Comparative Sample 
C 3.2 3.5 
Comparative Sample 
D 2.3 2.5 
______________________________________ 
It has been clarified from the above results that the inventive tool 
maintained a sharp cutting edge for a long time, to obtain excellent work 
surfaces. 
EXPERIMENTAL EXAMPLE 2 
Polycrystalline diamond was synthesized for 20 hours on Mo substrates 
having surface roughness of 0.03 .mu.m at the maximum height of 
irregularities R.sub.max by thermal CVD using a linear tungsten filament 
of 0.5 mm in diameter and 100 mm in length as a thermoionic emission 
material under the following conditions: 
Raw Material Gas (Flow Rate): H.sub.2 : 300 sccm, C.sub.2 H.sub.2 : 15 
sccm. 
Gas Pressure: 80 Torr. 
Filament Temperature: 2150.degree. C. 
Filament-to-Substrate Distance: 6 mm. 
Substrate Temperature: 920.degree.. 
After the synthesis, the substances were dipped in heated aqua regia to 
dissolve and remove only the Mo substrates, thereby recovering 
polycrystalline diamond members of 3 .mu.m in mean crystal grain size and 
0.15 mm in thickness. Surfaces of the polycrystalline diamond members 
which had been in contact with the substrates were in mirror states of 
0.03 .mu.m in surface roughness at the maximum height of irregularities 
R.sub.max. The growth plane sides of these polycrystalline diamond members 
were connected with shanks of cemented carbide by brazing. These connected 
bodies were maintained in inclined states so that rake face sides thereof 
were irradiated with a laser beam emitted from a YAG laser which was 
continuously oscillated with power of 3 W to form cutting edges, thereby 
manufacturing throw-away tips of different wedge angles. The flanks of the 
tools were covered with graphite layers of 3 .mu.m in thickness. 
Comparative samples were manufactured by employing the aforementioned tool 
materials of polycrystalline diamond and forming cutting edges by grinding 
with a of a diamond grindstone of #2000, and employing a tool material of 
a diamond sintered body of 3 mm in grain size containing 15 percent by 
volume of Co as a binder and forming cutting edges by similar grinding. 
Table 2 shows the amounts of cutting edge chipping of these tools. 
TABLE 2 
______________________________________ 
Amount of Cutting Edge 
Chipping (.mu.m) 
Tool Working Wedge Angle (degree) 
Material 
Method 90.degree. 
85.degree. 
80.degree. 
75.degree. 
______________________________________ 
Inventive 
Polycry- Laser 1 3 2 3 
Sample stalline Beam 
Diamond Processing 
Comparative 
Polycry- Grinding 8 10 8 12 
Sample stalline 
Diamond 
Comparative 
Sintered Grinding 15 18 20 20 
Sample Diamond 
______________________________________ 
It has been clarified from the above results that it is possible to easily 
manufacture a tool having excellent edge sharpness, which has been hard to 
manufacture by general grinding. 
EXPERIMENTAL EXAMPLE 3 
Mixed gas containing H.sub.2, C.sub.2 H.sub.6 and Ar gas in ratios of 8:1:1 
was supplied onto a mirror-finished tungsten substrate of 0.06 .mu.m in 
surface roughness at the maximum height of irregularities R.sub.max, which 
was placed in a reaction tube, at a flow rate of 500 sccm, and the 
pressure was adjusted to 150 Torr. Then, a high frequency of 13.56 MHz was 
applied from a high-frequency oscillator to excite the mixed gas for 
generating plasma, thereby synthesizing polycrystalline diamond for 30 
hours. The high-frequency output was selected in a range of 700 to 900 W 
every synthesizing experiment. 
After each synthesizing experiment was terminated, the substrate was 
treated with heated aqua regia to recover the polycrystalline diamond. 
Mean crystal grain sizes of the recovered polycrystalline diamond members 
were varied with the experiments in a range of 5 to 30 .mu.m, while every 
member exhibited a thickness of 1.6 mm and had a mirror surface of 0.06 
.mu.m in surface roughness at the maximum height of irregularities 
R.sub.max on the substrate side. In order to manufacture throw-away tips 
(type No. TPGN060104-B) from these polycrystalline diamond members, a YAG 
laser was employed with various power conditions for laser processing. The 
as-formed throw-away tips were subjected to observation of machined states 
of the cutting edges, and thereafter subjected to cutting tests under the 
following conditions, for performance evaluation through measurement of 
work surface roughness values: 
Cutting Conditions 
Workpiece: round bar of AC4A-T6 (Al--10%Si) 
Cutting Speed: 300 m/min. 
Depth of Cut: 0.15 mm 
Feed Rate: 0.08 mm/rev. 
Cutting Time: 90 min. 
Coolant: water-soluble oil solution 
Table 3 shows the results. 
TABLE 3 
______________________________________ 
Tool No. 
E F G H I J K 
______________________________________ 
Grain Size of Polycry- 
5 30 12 8 10 15 7 
stalline Diamond 
Laser Power (W) 
1.0 1.5 2.5 18.5 0.2 
3.0 2.0 
Amount of Cutting Edge 
2 30 4 20 * 5 3 
Chipping 
Thickness of Graphite 
2 5 7 30 * 9 6 
Layer Covering Cutting 
Edge 
Roughness of Work 
1.2 3.5 1.5 2.8 * 1.6 1.4 
Surface 
______________________________________ 
*The tool No. I was unmachinable since the laser power was too small. 
It is conceivable from the above results that the tools Nos. F and H were 
remarkably chipped due to an excessive grain size of the polycrystalline 
diamond and improper laser processing conditions respectively. On the 
other hand, the tools Nos. E, G, J and K according to the present 
invention exhibited excellent edge sharpness, and attained excellent 
surface roughness. 
A second embodiment of the present invention is now described. Referring to 
FIG. 8 showing a polycrystalline diamond cutting tool according to the 
second embodiment, a surface of polycrystalline diamond is exposed on the 
surface of a flank 7 which is formed by laser processing. Namely, after 
the manufacturing steps of the first embodiment as shown in FIGS. 1 to 5 
are carried out, the graphite coating layer 8 formed on the flank 7 etc. 
is removed by a chemical method of dissolving the same in acid or alkali 
fused salt for removal. The acid is preferably prepared from dichromic 
acid, or a mixed solution of sulfuric acid and nitric acid. When a mixed 
solution of sulfuric acid and nitric acid is employed, it is important to 
mix the materials in a ratio of 1:9 to 9:1, in order to efficiently 
dissolve the graphite layer. The alkali fused salt may be prepared form 
potassium hydroxide, potassium nitrate, sodium hydroxide, sodium nitrate 
or a mixture thereof. When the polycrystalline diamond cutting tool is 
connected with a tool holder 4 as shown in FIG. 8, the alkali fused salt 
is preferably used in the step of removing the graphite coating layer 8. 
This is because a brazing filler metal employed for connection or the tool 
holder 4 may be damaged if acid is employed. 
The polycrystalline diamond cutting tool manufactured in the aforementioned 
manner has the rake face 6 which is in a mirror state of not more than 0.1 
.mu.m in surface roughness at the maximum height of irregularities 
R.sub.max, a flank 7 which is not covered with graphite but exposes 
polycrystalline diamond, and a cutting edge having excellent edge 
sharpness with chipping suppressed in a range of 0.5 to 5 .mu.m. 
Experimental Examples are now described in detail. 
EXPERIMENTAL EXAMPLE 4 
Polycrystalline diamond was synthesized for 10 hours on an Si substrate 
having a mirror surface of not more than 0.05 .mu.m in surface roughness 
at the maximum height of irregulaties R.sub.max by microwave plasma CVD 
under the following conditions: 
Raw Material Gas (Flow Rate): H.sub.2 : 200 sccm, CH.sub.4 : 10 sccm. 
Gas Pressure: 120 Torr. 
Microwave Oscillation Power: 650 W. 
After the synthesis, the substance was dipped in hydrofluoric-nitric acid 
to dissolve and remove only the Si substrate, thereby recovering 
polycrystalline diamond of 5 .mu.m in mean crystal grain size and 0.2 
.mu.m in thickness. A surface of this polycrystalline diamond on the 
substrate side exhibited surface roughness of 0.05 .mu.m at the maximum 
height of irregularities R.sub.max. The growth plane side of this 
polycrystalline diamond was connected with a shank of cemented carbide by 
brazing. This connected body was maintained in an inclined state to be 
irradiated with a laser beam at an angle of 101.degree. with respect to a 
cutting face thereof for forming a rake edge, thereby manufacturing a 
throw-away tip No. A (type No. SPGN120304). A YAG laser of a continuous 
oscillation mode was employed for such formation of the cutting edge with 
power of 3 W. The as-formed throw-away tip No. A chipping of 1 .mu.m, 
while its flank was covered with a graphite layer of 2 .mu.m in thickness. 
Then, another tip was prepared by laser processing under the same 
conditions as the throw-away tip No. A, and dipped in a mixture of equal 
volumes of potassium nitrate and sodium nitrate, which was heated to 
500.degree. C., for 30 minutes, to dissolve and remove a graphite layer. 
In the as-recovered tip No. B, a brazing filler metal and a shank were not 
damaged and the graphite layer was completely removed from the flank, 
while cutting edge chipping remained 1 .mu.m. 
These throw-away tips Nos. A and B, serving as finishing tools, were 
subjected to performance evaluation tests under the following conditions: 
Cutting Conditions 
Workpiece: round bar of AC4C-T6 (Al--7%Si) 
Cutting Speed: 500 m/min. 
Depth of Cut: 0.2 mm 
Feed Rate: 0.1 mm/rev. 
Coolant: water-soluble oil solution 
Evaluation Method 
Comparison of work surface roughness values after cutting for 5 minutes and 
60 minutes 
As the result, it has been clarified that the inventive tip No. B 
maintained a sharp cutting edge for a longer time as compared with the 
comparative tip No. A, and attained excellent work surfaces, as shown in 
Table 4. 
TABLE 4 
______________________________________ 
Roughness of Work Surface 
Tool After 5 min. 
After 60 min. 
No. Cutting Cutting 
______________________________________ 
Comparative 
A 1.6 1.7 
Sample 
Inventive B 1.2 1.2 
Sample 
______________________________________ 
EXPERIMENTAL EXAMPLE 5 
Polycrystalline diamond was synthesized for 20 hours on an Mo substrate 
having surface roughness of 0.03 .mu.m at the maximum height of 
irregularities R.sub.max by thermal CVD employing a linear tungsten 
filament of 0.05 mm in diameter and 100 mm in length as a thermoionic 
emission material under the following conditions: 
Raw Material Gas (Flow Rate): H.sub.2 : 300 sccm, C.sub.2 H.sub.6 : 15 
sccm. 
Gas Pressure: 80 Torr. 
Filament Temperature: 2150.degree. C. 
Filament-to-Substrate Distance: 6 mm. 
Substrate Temperature: 920.degree. C. 
After the synthesis, the substance was dipped in heated aqua regia to 
dissolve and remove only the Mo substrate, thereby recovering 
polycrystalline diamond of 3 .mu.m in mean crystal grain size and 0.15 mm 
in thickness. This polycrystalline diamond has a mirror surface of 0.03 
.mu.m in surface roughness at the maximum height of irregularities 
R.sub.max on the substrate side. The growth plane side of this 
polycrystalline diamond was connected with a shank of cemented carbide by 
brazing. Then, this connected body was maintained in an inclined state, so 
that its cutting face was irradiated with a laser beam emitted from a YAG 
laser which was continuously oscillated with power of 3 W to form a 
cutting edge, thereby manufacturing a throw-away tip No. C having a wedge 
angle of 80.degree.. The as-formed tip No. C exhibited chipping of 2 
.mu.m, while its flank was covered with a graphite layer of 3 .mu.m in 
thickness. 
Then, another tip No. D of the same type as the above was dipped in a 
mixture containing potassium nitrate and sodium hydroxide in a ratio of 
2:1 in volume, which was heated to 500.degree. C., for 3 minutes, to 
dissolve and remove the graphite layer. A brazing filler metal and a shank 
of the as-recovered tip No. D were not damaged, while cutting edge 
chipping was 2 .mu.m and the graphite layer was completely removed from a 
flank. 
These throw-away tips, serving as finishing tools, were subjected to 
performance evaluation tests under the following conditions: 
Cutting Conditions 
Workpiece: round bar of AC8A-T6 (Al--12%Si) 
Cutting Speed: 600 m/min. 
Depth of Cut: 0.3 mm. 
Feed Rate: 0.08 mm/rev. 
Coolant: water-soluble oil solution 
Evaluation Method 
Comparison of amounts of cutting edge wear (chipping sizes) after cutting 
for 5 minutes and 60 minutes 
Table 5 shows the results of the performance evaluation tests. 
TABLE 5 
______________________________________ 
Removal of Result 
Graphite Item of After 5 mn. 
After 60 min. 
Tool No. 
Layer Evaluation Cutting Cutting 
______________________________________ 
Compar- 
No Cutting Edge 
20 .mu.m 
25 .mu.m 
ative C Chipping 
Roughness of 
1.8 .mu.m 
2.1 .mu.m 
Work Surface 
Inventive 
Yes Cutting Edge 
5 .mu.m 
6 .mu.m 
Sample Chipping 
D Roughness of 
1.1 .mu.m 
1.2 .mu.m 
Work Surface 
______________________________________ 
It is estimated that the comparative tip No. C was initially chipped by 
influence of remarkable workpiece welding to the cutting edge in an 
initial cutting stage, dissimilarly to the inventive tip No. D. On the 
other hand, substantially no workpiece welding to the cutting edge was 
caused in the inventive tip No. D since its flank was covered with no 
graphite layer. Thus, it has been clarified that a sharp cutting edge was 
maintained for a long time in the inventive tip No. D. 
EXPERIMENTAL EXAMPLE 6 
Mixed gas containing H.sub.2, C.sub.2 H.sub.6 and Ar in ratios of 8:1:1 was 
supplied onto a mirror-finished tungsten substrate of 0.06 .mu.m in 
surface roughness at the maximum height of irregularities R.sub.max, which 
was placed in a reaction tube, with a flow rate of 500 sccm, and the 
pressure was adjusted to 150 Torr. Then, a high frequency of 13.56 MHz was 
applied from a high frequency oscillator to excite the mixed gas for 
generating plasma, thereby synthesizing polycrystalline diamond for 30 
hours. The output of the high frequency was selected in a range of 700 to 
900 W every synthesizing experiment. 
After each synthesizing experiment, the substrate was treated with heated 
aqua regia, to recover the polycrystalline diamond. The as-recovered 
polycrystalline diamond members exhibited mean crystal grain sizes of 5 to 
30 .mu.m, which were varied with the experiments, while all samples had 
thicknesses of 1.6 mm and mirror surface of 0.06 .mu.m in surface 
roughness at the maximum height of irregularities R.sub.max on the 
substrate sides. Throw-away tips Nos. E, F and G (type No. TPGN060104-B) 
were prepared from these polycrystalline diamond members, using a YAG 
laser under various power conditions. Further, tips Nos. H, I and J were 
prepared under the same conditions as the aforementioned throw-away tips 
Nos. E, F and G, and dipped in dichromic acid, which was heated to 
100.degree. C., to remove graphite layers from flanks thereof. As to these 
tips Nos. H, I and J and the aforementioned tips Nos. E, F and G, which 
were not subjected to acid treatment, states of cutting edges were 
observed. Thereafter cutting experiments were made under the following 
conditions for performance evaluation with measurement of work surface 
roughness values. 
Cutting Conditions 
Workpiece: round bar of AC4A-T6 (Al--10%Si) 
Cutting Speed: 300 m/min. 
Depth of Cut: 0.15 mm 
Feed Rate: 0.08 mm/rev. 
Cutting Time: 90 min. 
Coolant: water-soluble oil solution 
Table 6 shows the results of the performance evaluation tests. 
TABLE 6 
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Tool No. 
E F G H I J 
______________________________________ 
Grain Size of Polycrystalline 
5 30 12 5 30 12 
Diamond (.mu.m) 
Laser Power (W) 1.0 1.5 2.5 1.0 1.5 2.5 
Amount of Cutting Edge 
2 30 4 2 30 4 
Chipping (.mu.m) 
Thickness of Graphite Layer 
2 5 7 0 0 0 
Covering Cutting Edge 
Roughness of Work Surface 
1.2 3.5 1.5 0.8 3.2 1.0 
______________________________________ 
Referring to Table 6, it is conceivable that the tips Nos. F and I caused 
remarkable chipping and could not attain excellent work surfaces since the 
polycrystalline diamond members had excessive grain sizes. On the other 
hand, it has been clarified that both of the inventive tips Nos. H and J 
exhibited excellent edge sharpness and attained excellent surface 
roughness as compared with the comparative tips Nos. E and G since flanks 
thereof were covered with no graphite layers. 
A third embodiment of the present invention is now described. Referring to 
FIG. 9, a polycrystalline diamond tip 3 is strongly attached to a tool 
holder 4 of cemented carbide or steel through a brazing portion 5. The 
upper surface of the polycrystalline diamond tip 3 forms a tool rake face 
6, while a flank 7 is formed at a prescribed angle with respect to the 
rake face 6. A cutting edge portion 11 is formed along the intersection 
between the rake face 6 and the flank 7. The cutting edge portion 11 is 
formed on a honed curved surface which has prescribed curvature, while its 
surface is covered with a graphite coating layer 8, which has been formed 
during honing. The amount of honing as mentioned in claims is now defined. 
Referring to FIG. 9 showing the cutting edge portion 11 in an enlarged 
manner, the amount of honing is shown by a length L between the 
intersection point of extension lines of the tool rake face 6 and the 
flank 7 and an end portion of the rake face 6 or the flank 7. This amount 
L of honing is preferably in a range of 5 to 20 .mu.m, since no 
improvement of initial chipping resistance is attained if the amount L is 
less than 5 .mu.m, while the cutting edge is damaged and roughness of the 
finished surface is reduced if the amount L exceeds 20 .mu.m. 
Manufacturing steps are now described. The steps of the first embodiment as 
shown in FIGS. 1 to 4 are similarly employed for manufacturing the 
polycrystalline diamond cutting tool according to the third embodiment. 
Referring to FIG. 10, a flank 7 is formed with a fine-grain diamond 
grindstone or the like, at a prescribed angle with respect to a cutting 
face 6. Thereafter a cutting edge portion is honed with a laser beam. 
Since the laser beam machining is accompanied with thermal reaction, the 
cutting edge portion is less damaged as compared with a case of grinding, 
which is a mechanical removing method. The cutting edge portion 11 is 
covered with a graphite coating layer 8 by this laser beam machining. This 
graphite coating layer 8 is preferably 0.5 to 10 .mu.m thick. It is 
difficult to form the graphite coating layer 8 in a thickness of not more 
than 0.5 .mu.m for the time being, while the cutting edge portion 11 is 
remarkably damaged and chipping resistance is reduced if the thickness of 
the graphite coating layer 8 exceeds 10 .mu.m. 
EXPERIMENTAL EXAMPLE 7 
Polycrystalline diamond was synthesized for 10 hours on an Si substrate 
having a mirror surface of 0.05 .mu.m in surface roughness at the maximum 
height of irregularities R.sub.max by microwave plasma CVD, under the 
following conditions: 
Raw material Gas (Flow Rate): H.sub.2 : 200 sccm, CH.sub.4 : 10 sccm. 
Gas Pressure: 120 Torr. 
Microwave Oscillation Power: 650 W. 
After the synthesis, the substance was dipped in hydrofluoric-nitric acid 
to dissolve and remove only the Si substrate, thereby recovering 
polycrystalline diamond of 5 .mu.m in mean crystal grain size and 0.2 mm 
in thickness. The surface of the polycrystalline diamond was 0.05 .mu.m in 
surface roughness at the maximum height R.sub.max on the substrate side. 
The growth plane side of this polycrystalline diamond was connected with a 
shank of cemented carbide by brazing. Then, this connected body was ground 
with a diamond grindstone of #1500, to prepare a throw-away tip No. A 
(type No. SPGN120304). The as-formed throw-away tip No. A exhibited 
cutting edge chipping of 10 .mu.m. 
Another throw-away tip No. B was prepared in the same method as the above, 
and its cutting edge was honed with a YAG laser in an amount L of honing 
of 10 .mu.m. A graphite coating layer of 3 .mu.m in thickness was formed 
on the honed portion of the as-formed throw-away tip No. B following the 
laser beam machining, while the size of cutting edge chipping was 2 .mu.m. 
10 samples each of these throw-away tips Nos. A and B were prepared and 
subjected to performance evaluation tests under the following conditions: 
Cutting Conditions 
Workpiece: round bar of A390-T6 (Al--17%Si) axially provided with four 
V-shaped grooves 
Cutting Speed: 500 m/min. 
Depth of Cut: 0.2 mm. 
Feed Rate: 0.1 mm/rev. 
Coolant: water-soluble oil solution. 
As the result, 5 samples, 3 samples and 2 samples of the throw-away tip No. 
A were chipped in 5 minutes, 8 minutes and 20 minutes respectively after 
cutting. On the other hand, all samples of the inventive tip No. B caused 
no chipping in cutting edges after cutting for 60 minutes, and attained 
excellent work surfaces. It, has been clarified from these results that 
the inventive tip serves as a cutting tool which has high toughness. 
Thus, it is possible to obtain a polycrystalline diamond cutting tool 
having excellent chipping resistance and high toughness by honing its 
cutting edge portion by laser beam machining. 
Although the present invention has been described and illustrate in detail, 
it is clearly understood that the same is by way of illustration and 
example only and is not to be taken by way of limitation, the spirit and 
scope of the present invention being limited only by the terms of the 
appended claims.