Cbn Cutting Tool for High-Quality, High-Efficiency Cutting

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(
a) is a perspective view of a cutting tool according to the present invention;

FIG. 1(
b) is a plan view of the tool of FIG. 1(a);

FIG. 1(
c) is a side view taken along line C-C of FIG. 1(b);

FIG. 1(
d) is a side view taken along line D-D of FIG. 1(b);

FIG. 1(
e) is an enlarged perspective view of a cutting tip of the tool of FIG. 1(a);

FIG. 2 shows a cutting edge having a negative land and a cutting edge having no negative land, as viewed in the direction of the principal cutting force;

FIG. 3(
a) shows loci of cutting edges of the conventional tool and the tool according to the invention, while they are being fed;

FIG. 3(
b) shows feed marks formed by the conventional tool and the tool according to the invention;

FIG. 4 is a perspective view of a tool according to another embodiment of the present invention;

FIG. 5 is a perspective view of a tool according to still another embodiment of the invention;

FIG. 6 is a perspective view of a tool according to a further embodiment of the invention;

FIG. 7 shows how the tool according to the invention is positioned during use;

FIG. 8(
a) is a schematic plan view of the tool according to the present invention during use;

FIG. 8(
b) is a schematic end view of the tool according to the present invention during use;

FIG. 9(
a) is a schematic perspective view of a conventional tool with arcuate noses and having a chamfer at each cutting tip;

FIG. 9(
b) is an enlarged sectional view of one of the cutting tips of the tool of FIG. 9(a); and

FIG. 10 is a schematic perspective view of a conventional wiper tool.

DESCRIPTION OF REFERENCES

1 Cutting tool

2 Substrate of cemented carbide

2
a Top surface

2
b Bottom surface

2
c Side face

2
d Clamp hole

3 Seat

4 Cutting tip

4
a cBN-based sintered body

4
b Back metal layer

5 Arcuate nose

6 Rake face

7 End flank

8 Side flank

9 Negative land

9
a, 9b Surfaces inclined in opposite directions

10 Cutting edge

11 Ridgeline between negative land and rake face

12 Conventional wiper edge

13 Chamfer

20 Workpiece

MODE FOR EMBODYING THE INVENTION

Now referring to FIGS. 1 to 7, CBN cutting tools embodying the present invention are described.

The CBN cutting tool 1 shown in FIG. 1 is a diamond-shaped negative insert including a substrate 2 made of cemented carbide and formed with two seats 3 at each of the two diagonally opposed acute corners thereof. On each seat 3, a cutting tip 4 comprising a cBN-based sintered body is mounted by brazing. The four cutting tips 4 can be used alternately for cutting. Each cutting tip 4 has an arcuate nose 5, rake face 6, end flank 7, side flank 8 and negative land 9. Since each cutting tip 4 is symmetrical with respect to its centerline, the end flank 7 and the side flank 8 may serve as a side flank and a end flank, respectively, depending on how the insert is used.

The substrate 2 has parallel top and bottom surfaces 2a and 2b and four sides 2c that intersect the top and bottom surfaces 2a and 2b at a right angle, and is formed with a clamp hole 2d extending centrally therethrough between the top and bottom surfaces 2a and 2b. The top and bottom surfaces 2a and 2b are of the same structure. Thus, the tool 1 can be used upside down with the surfaces 2a and 2b as bottom and top surfaces, respectively. Each cutting tip 4 shown comprises a cBN-based sintered body 4a, and a back metal layer 4b made of cemented carbide. But the cutting tips according to the present invention may each consist only of a cBN-based sintered body. The corner of any of the four cutting tips 4 can be used for cutting. The cBN-based sintered bodies forming the cutting tips 4 contain 45 to 99.9 percent by volume of cBN so that the cutting tips can cut hardened steel with high efficiency.

The negative land 9 of each cutting tip 4 intersects any of the rake face 6 and end and side flanks 7 and 8 at an obtuse angle, and is arranged such that a ridgeline 10 as an end cutting edge (straight line P-Q1 extending between apex P and point Q1 as viewed from one side of the insert) forms an angle β′ of not less than 20 degrees and not more than 35 degrees with respect to the rake face 6. The ridgeline 10 as the end cutting edge, which is defined by the negative land 9 and the end flank 7, is the lowest at the apex P and rises gradually toward point Q1 at which the rake face 6, negative land 9 and end flank 7 intersect each other. Similarly, the ridgeline 10 as the side cutting edge, which is defined by the negative land 9 and the side flank 8, is the lowest at the apex P and rises gradually toward point Q2 at which the rake face 6, negative land 9 and side flank 8 intersect each other. The ridgeline 11 defined by the negative land 9 and the rake face 6 extends between points Q1 and Q2.

In order to provide a desired economical tool, the cutting tips should be as small as possible. For this purpose, each cutting tip is preferably dimensioned as follows:

Radius of curvature of the arcuate nose 5: not less than 0.4 mm and not more than 1.6 mm
Apex angle α of the arcuate nose 5:55 to 90 degrees
Inclination angle β′: 20 to 35 degrees
Width W of the negative land 9 along the bisector of the apex angle α of the arcuate nose 5: not less than 0.5 mm and not more than 2.0 mm

Under ISO, inserts with alternately usable cutting tips are classified into four types according to the nose apex angle, i.e. classes D, T, C and S (of which the nose apex angles are 55, 60, 80 and 90 degrees, respectively). If the present invention is applied to an insert that is classified in one of these types, its cutting tips are preferably dimensioned such that the arcuate nose 5 has a radius of curvature of not less than 0.8 mm and not more than 1.2 mm, the inclination angle β′ is not less than 25 degrees and not more than 32 degrees, and the width W of the negative land 9 (its maximum width along the bisector of the apex angle α) is not less than 0.5 mm and not more than 1.5 mm.

The cutting tips 4 may consist only of a cBN-based sintered body or may comprise a cBN-based sintered body and a back metal layer of e.g. cemented carbide bonded to the cBN-based sintered body. The cutting tips 4 may be fixed to the substrate 2 of cemented carbide by means other than brazing. For example, if the cutting tips 4 can be stably held by the substrate 2, they may be fixed to the substrate 2 by sintering the cutting tips and the substrate together, press-fitting, shrinkage fitting or expansion fitting or by means of a self-gripping clamp mechanism.

For sufficient durability, the cBN-based sintered bodies of the cutting tips 4 should have a thickness t of not less than 0.8 mm and not more than 1.5 mm. If each cutting tip 4 is brazed to the substrate 2 of cemented carbide, the brazing area is preferably not less than 2.5 mm²and not more than 5 mm²for higher bond strength and smaller size and thus lower cost. The cutting tips may be asymmetrical with respect to the bisector of the nose apex angle as shown in FIG. 4. But cutting tips that are symmetrical with respect to the bisector of the nose apex angle as shown in FIG. 1 are preferable because such cutting tips can be used not only for outer peripheral cutting but also end surface cutting.

FIGS. 5 and 6 show different types of symmetrical cutting tips embodying the present invention. The cutting tip of FIG. 5 has a negative land 19 comprising two surfaces 9a and 9b intersecting each other along the bisector of the nose apex angle a and inclined in opposite directions to each other. The surfaces 9a and 9b are symmetrical with respect to the bisector CL of the nose apex angle α, and both are lowest at the apex P of the arcuate nose 5. The cutting tip of FIG. 6 is a modification of the cutting tip of FIG. 5 and includes a curved surface as a negative land 29 which is symmetrical with respect to the bisector CL of the nose apex angle a of the arcuate nose as viewed from the top of the insert.

Among cutting tools having alternately usable cutting tips are ones in which the cemented carbide substrate 2 has a thickness t1 of 3.18 mm, 4.76 mm or 6.35 mm (03, 04 and 05 under ISO, respectively), a clamp hole 2d is formed in the substrate 2 (though the clamp hole is not an essential element), the substrate 2 is dimensioned such that its reference inscribed circle has a diameter of 6.35 mm, 9.525 mm or 12.7 mm, and the cutting tips, which comprise a cBN-based sintered body, have a nose apex angle α of 80 degrees. The concept of the present invention is applicable to such cutting tools, too.

The negative land, which is provided on the rake face at its portion near the arcuate nose, is not limited to a flat one, but may comprise a plurality of surfaces that intersects each other and inclined relative to each other such as the negative land 19 shown in FIG. 5, may be a curved negative land such as the negative land 29 shown in FIG. 6, or may be a surface which is asymmetrical with respect to the bisector CL of the nose apex angle α, such as the negative land 39 shown in FIG. 4. Further, the concept of the present invention is applicable to an insert of which the flank 7 intersects the rake face 6 at an angle greater than 90 degrees or to a square insert, too.

The CBN cutting tool according to the present invention is used for turning with the cutting tip arranged such that the inclination angle λ of the cutting edge and the side rake angle γn (see FIG. 7) are both negative, the end clearance angle αof, side clearance angle αos, end cutting edge angle κf, and side cutting edge angle κs are all positive, the angle β shown in FIG. 8(b) is 20 to 35 degrees, and the distance h shown in FIG. 8(b) is not less than 0.5 mm and not more than 1.5 mm.

The angle β is the downward inclination angle of the ridge line 10 used as the end cutting edge (portion of the cutting edge extending from the apex P of the arcuate nose to point Q1, i.e. straight line P-Q1 as viewed from one side of the cutting tool), with respect to the rake face 6. If the angle β is less than 20 degrees, a wiper edge cannot be effectively formed. If larger than 35 degrees, the cutting resistance increases, thus making it difficult to improve the resistance to chipping.

The distance h is the downward distance of the apex P of the nose 5 from the plane parallel to the rake face 6 of the insert and containing the rotation axis O of the workpiece 20. By determining the distance h in the range of not less than 0.5 mm and not more than 1.5 mm, both the chipping resistance of the cutting edge and the surface roughness of the workpiece improve. Ordinarily, cutting is not carried out with the apex P located below the abovementioned plane.

The feed rate f of the tool during cutting of this type is preferably set to not less than 0.1 mm/rev and not more than 0.5 mm/rev. More preferably, the feed rate f is set to not less than 0.15 mm/rev and not more than 0.5 mm/rev with the downward distance h, shown in FIG. 8(b), set to not less than 0.7 mm and not more than 1.2 mm. By setting these parameters within the above ranges, the advantages of the invention are achieved to maximum extent.

Detailed examples are now described.

EXAMPLES 1

CBN cutting tool specimen Noss 1 to 39 having cutting portions comprising cBN-based sintered bodies having different shapes were prepared, and evaluated for their cutting abilities.

The cBN-based sintered bodies were formed by mixing together cBN powder and a binder powder comprising TiN and Al in a ball mill made of cemented carbide, and sintering the mixture using an ultrahigh pressure device at 5 GPa and 1500 degrees C. The sintered bodies contained 60 percent by volume of cBN particles having an average particle diameter of 3 micrometers, the balance comprising a Ti compound of which the major component was TiN, Al compounds such as nitrides, borides or oxides of Al, and trace amounts of W and Co compounds.

The cutting tools of Examples 1 are based on insert type tools classified into CNMA120404, CNMA120408, CNMA120412, CNMA120416 and CNMA120420 under ISO, which include cutting tips each comprising a cBN-based sintered body and having a thickness of 1.8 mm, a nose apex angle a of 80 degrees and a bottom length of 4 mm. Some of these cutting tips are directly joined to a steel shank, and the others are joined to a substrate of cemented carbide. The cutting tips have different kinds of conventional chamfers and the negative land that characterizes the present invention at their cutting edges.

Any of the above tools includes cutting tips each comprising a cBN-based sintered body and a back metal layer of cemented carbide and joined to a steel shank or a substrate of cemented carbide. On each cutting tip, after forming a rake face and an arcuate nose by grinding, a chamfer and a negative land were formed by grinding. Before forming the chamber and negative land, the cBN-based sintered bodies of any tool had a thickness of 1.5 mm, the noses of the respective tools had radii of curvature R of 0.8 mm, 1.2 mm, 1.6 mm and 2.0 mm (the last two digits of each type number indicate the radius of curvature of the nose), and the brazing areas of the bottoms of the cutting tips of the respective tools were 3.15 mm², 2.90 mm², 2.55 mm²and 2.10 mm².

The tools were used as shown in FIG. 8 under the same conditions with the cutting edge inclination angle λ, side rake angle γn, end clearance angle, side clearance angle, end cutting edge angle, and side cutting edge angle set to −5 degrees, −5 degrees, 5 degrees, 5 degrees, 5 degrees and −5 degrees, respectively. The downward distance h of the ridgeline which is used as the end cutting edge, the width W of the negative land, and the downward inclination angle β of this cutting edge were set to values shown in Table 2. Using these tools, the following workpiece was cut under the following conditions.

Workpiece: JIS type: SCR415H outer periphery machining (DIN type: equivalent to 15Cr3); Hardness HRc 58 to 60

Cutting velocity: V=150 m/min

Depth of cut: d=0.2 mm

Feed rate: f=0.3 mm/rev

Type of machining: Continuous machining of the outer periphery of round rod, with no coolant

The results of an evaluation test are also shown in Table 1.

TABLE 1

Width

Cutting

Residual stress
Residual stress

Downward
W of

time
Surface
on cut surface
on cut surface

Specimen

inclination
negative
Downward
to tool
roughness
of workpiece
of workpiece

No.
Tool
angle β
land
distance h
breakage
Rz
(circumferential
(axial

No.
shape
°
mm
mm
min
μm
direction) GPa
direction) GPa
Remarks

1
CNMA120404
0
—
0
3
20.8
−0.2
−0.2
γ: 25°, CW: 0.15 mm

2
CNMA120408
0
—
0
9
12.5
−0.2
−0.2
γ: 25°, CW: 0.15 mm

3
CNMA120408
0
—
0.80
9
12.3
−0.2
−0.2
γ: 25°, CW: 0.15 mm

4
CNMA120416
0
—
0
11
7.5
−0.3
−0.2
γ: 25°, CW: 0.15 mm

5
CNMA120420
0
—
0
12
6.5
−0.3
−0.2
γ: 25°, CW: 0.15 mm

6
CNMA120408
0
—
0
8
10.8
−0.3
−0.2
γ: 45°, CW: 0.15 mm

7
CNMA120408
0
—
0
10
10.8
−0.3
−0.2
γ: 25°, CW: 0.4 mm

8
CNMA120408
0
—
0
11
10.9
−0.3
−0.2
γ: 35°, CW: 0.4 mm

9
CNMA120408
0
—
0
11
11.0
−0.3
−0.2
γ: 45°, CW: 0.4 mm

10
CNMA120408
0
—
0.80
11
11.5
−0.2
−0.2
γ: 45°, CW: 0.4 mm

11
CNMA120408
0
—
0
9
2.5
−0.4
−0.3
WW: 0.5, γ: 25°,

CW: 0.15 mm

12
CNMA120408
0
—
0
7
2.7
−0.5
−0.3
WW: 0.5, γ: 45°,

CW: 0.4 mm

13
CNMA120408
17.5
1.70
0.80
10
10.0
−0.5
−0.4

14
CNMA120408
20.0
1.50
0.80
15
6.0
−0.6
−0.5

15
CNMA120408
20.0
0.80
0.80
15
6.0
−0.6
−0.5

16
CNMA120408
25.0
0.80
0.80
18
5.5
−0.6
−0.5

17
CNMA120408
35.0
0.80
0.80
20
2.8
−0.7
−0.5

18
CNMA120408
45.0
0.80
0.80
9
—
—
—
Chattering developed

19
CNMA120408
25.0
1.10
0.80
18
5.5
−0.6
−0.5

20
CNMA120408
25.0
1.50
0.80
18
5.5
−0.6
−0.5

21
CNMA120408
25.0
2.00
0.80
14
5.5
−0.6
−0.5

22
CNMA120408
25.0
3.00
0.80
1
—
—
—

23
CNMA120408
32.0
0.45
0.80
8
10.0
−0.4
−0.4

24
CNMA120408
32.0
0.50
0.80
13
3.1
−0.6
−0.5

25
CNMA120408
32.0
0.70
0.70
25
2.4
−0.7
−0.5

26
CNMA120408
32.0
0.80
0.80
27
2.2
−0.7
−0.5

27
CNMA120408
32.0
0.95
0.80
25
2.5
−0.6
−0.5

28
CNMA120408
35.0
0.75
0.80
20
2.8
−0.7
−0.5

29
CNMA120408
45.0
0.60
0.80
9
—
—
—
Chattering developed

30
CNMA120408
32.0
0.80
0
1
—
—
—

31
CNMA120408
32.0
0.80
0.45
10
9.5
−0.3
−0.3

32
CNMA120408
32.0
0.80
0.50
18
3.5
−0.5
−0.4

33
CNMA120408
32.0
0.80
0.70
24
2.5
−0.7
−0.5

34
CNMA120408
32.0
0.80
1.2
20
2.7
−0.6
−0.5

35
CNMA120408
32.0
0.80
1.5
20
4.5
−0.5
−0.5

36
CNMA120408
32.0
0.80
2.0
2
—
—
—

37
CNMA120404
32.0
0.80
0.80
23
2.3
−0.7
−0.5

38
CNMA120416
32.0
0.80
0.80
24
2.2
−0.7
−0.5

39
Bite type
32.0
0.80
0.80
27
2.2
−0.6
−0.5

Tool Nos. 1 to 10 are inserts including cutting tips each comprising a cBN-based sintered body brazed to a corner thereof (tools with conventional arcuate noses). Each cutting tip has an arcuate nose having a radius of curvature in the range of 0.4 to 2.0 mm, and has, along the cutting edge, a chamfer parallel to the cutting edge and having a constant width CW of 0.15 to 0.4 mm and a chamfer angle γ of 25 to 45 degrees.

Tool Nos. 11 and 12 are inserts including cutting tips each comprising a cBN sintered body brazed to the substrate (conventional wiper tools). Each cutting tip has an arcuate nose having a radius of curvature R of 0.8 mm, and a chamfer identical to the chamfer of any of the tool Nos. 1 to 10, and further includes a wiper edge having a wiper width WW of 0.5 mm.

Tool Nos. 1 to 38 are tools including alternately usable cutting tips comprising cBN-based sintered bodies fixed to a 25 mm square rod-shaped steel holder by a pin-lock type clamp mechanism. Tool No. 39 is a tool including cutting tips comprising cBN-based sintered bodies directly brazed to a 25 mm square rod-shaped steel shank. Any of the tools was mounted on a lathe so that the length of its portion protruding from the lathe turret to its tip is 40 mm.

The cBN-based sintered bodies were brazed to the substrate through activated brazing filler comprising 72 wt % of Ag, 25 wt % of Cu and 3 wt % of Ti.

The surface roughness of the cut surface (hereinafter simply “surface roughness”) and the residual stress on the cut surface (hereinafter simply “residual stress”) were measured for tool specimens which were free from chipping five minutes after the start of cutting.

The surface roughness shown is the ten point height of irregularities (RZ) under JIS B0601, which was measured in the axial direction of the workpiece with the cutoff at 0.8 micrometers and the reference length at 4 mm.

The residual stress is a residual stress value measured at the depth of 20 micrometers from the surface by a microscopic X-ray stress measuring device and by etching using chemical treatment. The symbol “−” in Table 1 indicates that compressive stresses remained.

The results of the test on Examples 1 are shown in Table 1.

Summary Evaluation

Among the tool Nos. 13 to 39, which all have a negative land, CBN cutting tools according to the present invention (Nos. 13-17, 19-21, 24-28, 32-35 and 37-39), which were used with the downward distance h set to not less than 0.5 mm and not more than 1.5 mm, showed superior results in any of the life span to tool breakage, surface roughness and residual stress. Among them, tools which were used with the downward distance h set to 0.7 to 1.2 mm achieved particularly superior surface roughness.

Tool Nos. 30 and 31 achieved poor surface roughness because of a large gap present between a wiper edge formed by a three-dimensionally displaced ridgeline and the cut surface of the workpiece.

From the data in Table 1 regarding tool Nos. 13-17, 19-21, 24-28, 32-35 and 37-39, it is apparent that the CBN cutting tools according to the present invention, of which the downward inclination angle β of the ridgeline as the end cutting edge was not less than 20 degrees and not more than 35 degrees, and particularly tools of which the downward inclination angle β was in the range of 25 to 35 degrees, achieved superior surface roughness. Tool Nos. 18 and 29, of which the downward inclination angle β was 45 degrees, suffered a short life span due to chipping of the cutting edge resulting from large cutting resistance and unstable cutting.

Tools according to the present invention, of which the negative land has a width W of not less than 0.5 mm and not more than 2.0 mm, particularly the tools of which the negative land has a width W of 0.5 to 1.5 mm (No. 14-17, 19, 20, 24-28, 32-34 and 37-39) showed superior results in terms of the life span, surface roughness and residual stress, compared to conventional tool Nos. 1 to 12. In tool No. 22, of which the negative land has a width W of 3.0 mm, because the downward distance from the rake face to the apex P of the arcuate nose is large, the cutting tips, each comprising a cBN-based sintered body, has a reduced thickness, and thus reduced strength, which resulted in immature chipping of the cutting tips. In tool No. 23, of which the negative land has a width W of 0.45 mm, because the downward distance from the rake face to the apex P of the nose is insufficient, cutting was performed even at portions beyond the point Q1, so that the surface roughness was not good, and chipping of the cutting edge occurred at a relatively early stage of use of the tool.

The thicker the cBN-based sintered body, the higher its strength. But if it is thicker than 1.5 mm, the amount of cBN, which is an expensive material, unduly increases. Also, when forming the tool, it is difficult to grind such a thick cBN-based sintered body. Thus, in view of the manufacturing cost, its thickness is preferably not more than 1.5 mm.

The CBN cutting tools according to the present invention produced large residual compressive stresses compared to conventional tool Nos. 1 to 12. In particular, residual compressive stresses in the axial direction (feed direction of the tool) of the cut surface of the workpiece were remarkable. This is presumably because, unlike conventional chamfer shapes and wiper shapes, the cutting tips of the tools according to the present invention are shaped such that they reveal strong burnishing action in the feed direction of the tool.

In the case of conventional tool Nos. 1 to 12, not only the cutting tips comprising cBN-based sintered bodies but also their substrates made of cemented carbide have to be subjected to chamfering and honing. In contrast, in the case of the CBN cutting tools according to the present invention, grinding or honing is necessary only on the cutting tips to form the negative land. Thus, the cutting tools according to the present invention are more advantageous than conventional tools in terms of productivity and manufacturing cost, too.

EXAMPLES 2

Tool Nos. 41 to 56 shown in Table 2 were prepared. The tools used are based on diamond-shaped inserts classified into CNMA120412, CNMA120416 and CNMA120420 under ISO, which carry alternately usable cutting tips and have an apex angle α of 80 degrees. Each cutting tip comprises a cBN-based sintered body (of the same composition as the sintered bodies of Examples 1) having a base length of 4 mm and a thickness of 1.1 mm, and a back metal layer made of cemented carbide and joined to the sintered body by sintering. The cutting tips were joined to a substrate of cemented carbide by brazing their base metal layers to the substrate. Different kinds of chamfers and flat negative lands similar to those in Examples 1 were formed on the cBN-based sintered bodies of the cutting tips. The cutting tips were brazed to the substrate of cemented carbide through activated brazing filler comprising 50 wt % of Cu, 25 wt % of Ti and 25 wt % of Zr.

After brazing the cutting tips to the substrate of each tool, a rake face and an arcuate nose were formed on each cBN-based sintered body by grinding. Then, a chamfer and a negative land were formed to define cutting edges. Before forming the chamfer and the negative land, the cBN-based sintered bodies of tool No. 56 had a thickness of 0.7 mm, and those of all the other tools had a thickness of 0.8 mm. For the tools having noses having four different radii of curvature of 0.8 mm, 1.2 mm, 1.6 mm and 2.0 mm, the brazing areas of the bottoms of the cutting tips were 3.15 mm², 2.90 mm², 2.55 mm²and 2.10 mm², respectively.

The tools were used as shown in FIG. 7 under the same conditions with the cutting edge inclination angle λ, side rake angle γn, end clearance angle αof, side clearance angle αos, end cutting edge angle κf, and side cutting edge angle κs set to −5 degrees, −5 degrees, 5 degrees, 5 degrees, 5 degrees and −5 degrees, respectively. The downward distance h of the ridgeline which is used as the end cutting edge, the width W of the negative land, and the downward inclination angle β of this cutting edge were set to values shown in Table 2. Using these tools, the following workpiece was cut under the following conditions.

Workpiece: JIS type: SCR415H end surface machining (DIN type: equivalent to 15Cr3); Hardness HRc 58 to 60

Cutting velocity: V=120 m/min

Depth of cut: d=0.25 mm

Feed rate: f=0.1-0.6 mm/rev

Type of machining: Continuous machining of the end surface of a disk, with no coolant

The results of an evaluation test are also shown in Table 2.

TABLE 2

Downward

Surface

Specimen

inclination
Width W
Downward
Number of workpieces
roughness

No.

angle β
of negative land
distance h
cut to tool breakage
Rz
Feed

No.
Tool shape
°
mm
mm
due to flaking
μm
mm/rev.
Remarks

41
CNMA120408
0
—
0
3
11.0
0.3
γ: 25°, CW: 0.15 mm

42
CNMA120412
0
—
0
9
10.5
0.3
γ: 25°, CW: 0.15 mm

43
CNMA120412
0
—
0
9
3.6
0.08
γ: 25°, CW: 0.15 mm

44
CNMA120412
0
—
0
9
5.0
0.15
γ: 25°, CW: 0.15 mm

45
CNMA120416
0
—
0
7: Separation
8.0
0.3
γ: 25°, CW: 0.15 mm

of brazing

46
CNMA120420
0
—
0
3: Separation
7.0
0.3
γ: 25°, CW: 0.15 mm

of brazing

47
CNMA120412
0
—
0
0.5
—
0.3
γ: 40°, CW: 0.15 mm

48
CNMA120412
0
—
0
9
—
0.3
WW: 0.5, γ: 25°,

CW: 0.15 mm

49
CNMA120412
26.0
1.0
0.70
—
10.5
0.08
Chattering developed from

the beginning

50
CNMA120412
26.0
1.0
0.70
28
2.8
0.10

51
CNMA120412
26.0
1.0
0.70
35
2.9
0.12

52
CNMA120412
26.0
1.0
0.70
33
3.0
0.15

53
CNMA120412
26.0
1.0
0.70
25
3.5
0.3

54
CNMA120412
26.0
1.0
0.70
20
3.6
0.5

55
CNMA120412
26.0
1.0
0.70
6: Separation
3.8
0.6

of brazing

56
CNMA120412
26.0
1.0
0.70
20
3.0
0.15

Tool Nos. 41 to No. 47 are inserts including cutting tips each comprising a cBN-based sintered body brazed to a corner thereof (tools with conventional arcuate noses). They have arcuate noses having radii of curvature in the range of 0.8 to 2.0 mm, and have, along the cutting edge, a chamfer parallel to the cutting edge and having a constant width CW of 0.15 mm and a chamfer angle γ of 25 degrees.

Tool No. 48 is an insert including cutting tips each comprising a cBN sintered body brazed to the substrate (conventional wiper tool). Each cutting tip has an arcuate nose having a radius of curvature R of 1.2 mm, and a chamfer identical to the chamfer of any of the tool Nos. 41 to 47, and further includes a wiper edge having a wiper width WW of 0.5 mm.

Tool Nos. 49 to 55 are tools including alternately usable cutting tips comprising cBN-based sintered bodies fixed to a 25 mm square rod-shaped steel holder by a pin-lock type clamp mechanism as in Examples 1. Any of the tools was mounted on a lathe so that the length of its portion protruding from the lathe turret to its tip is 40 mm.

The surface roughness of cut surfaces was measured for tools which were free from chipping of the cutting edge after two workpieces had been cut. The surface roughness was measured in terms of Rz as in Examples 1.

Summary Evaluation

When cutting hardened steel having high hardness, extremely large radial force acts on the cutting edge of the tool. Especially when such hardened steel is cut discontinuously at a high feed rate with a large depth of cut as in Examples 2, such large radial force acts impulsively on the cutting edge of the tool, so that the rake face of the tool tends to suffer flaking (seashell-like flakes are produced). Chippings observed in tool Nos. 41 to 56 in Table 2 were all such seashell-like flakes.

It would be possible to extend the life span of even such conventional tools when used in high-feed, high-efficiency cutting by increasing the radius of curvature of the nose of each cutting tip. But if the nose has too large a radius of curvature, the brazing area of the cutting tip has to be correspondingly reduced, which increases the possibility of breakage of the brazing joint portion if the cutting tip is subjected to high cutting resistance during cutting at a high feed rate (see the results for tool Nos. 45 and 46).

Tool No. 47, of which the chamfer angle γ was 40 degrees, suffered chipping of the cutting edge in the initial stage of cutting before the separation of the brazing joint portion, and thus its life was short. This is presumably because due to the large chamfer angle γ, no sharp cutting was possible, so that the cutting resistance increased.

The wiper tool having the wiper edge, i.e. tool No. 48 suffered from flaking as with tool Nos. 41 to 46, though there was no separation of brazing.

In contrast, the tools according to the present invention, i.e. tool Nos. 50-54 and 56 suffered no separation of brazing even when cutting was performed in a wide range of high feed rates of 0.15 mm/rev to 0.5 mm/rev. Also, flaking scarcely occurred. As in Examples 1, the wiper edge formed due to three-dimensional displacement of the cutting edge provided good surface roughness.

The reason why the tools according to the present invention, i.e. tool Nos. 50-54 and 56 scarcely suffered chipping, i.e. flaking is presumably because when large radial force acts impulsively on the cutting edge of the tool, the negative land serves to apply large compressive stresses as a reaction to the formation of chips to the cutting edge both in the direction of the principal cutting force (normal direction of the rake face) and the tool feed direction, and the compressive stresses cancel tensile stresses applied to the brittle cBN-based sintered body in the direction of the principal cutting force due to the impulsive radial force.

Tool No. 55 suffered separation of the brazed cutting tips. Also, it was inferior in surface roughness to tool Nos. 50 to 54. This is because the feed rate was too harsh, i.e. 0.6 mm/rev.

Tool No. 49 was inferior in surface roughness to conventional tools due to chattering that had developed from the beginning of cutting. This is presumably because while the feed rate is low, e.g. 0.08 mm/rev, natural oscillations occurred due to specific shapes of the tools according to the present invention. The tools according to the present invention are therefore preferably used at a feed rate of 0.1 to 0.5 mm/rev, more preferably 0.12 to 0.5 mm/rev, and further preferably 0.15 to 0.5 mm/rev for higher machining efficiency.

Tool No. 56 showed improved resistance to chipping and surface roughness compared to conventional tool No. 44. But tool No. 52, which is higher in rigidity because its cBN-based sintered bodies have a thickness of 0.8 mm, showed further improved resistance to chipping compared to tool No. 56.

EXAMPLES 3

Tool Nos. 61 to 71 shown in Table 3 were prepared. The tools used are based on diamond-shaped inserts classified in classes D (apex angle α=55 degrees), T (apex angle α=60 degrees), C (apex angle α=80 degrees) and S (apex angle α=90 degrees) under ISO, which carry alternately usable cutting tips. Each cutting tip comprises a cBN-based sintered body (of the same composition as the sintered bodies of Examples 1), and a back metal layer made of cemented carbide and joined to the sintered body by sintering. The cutting tips were joined to a substrate of cemented carbide by brazing their base metal layers to the substrate. Different kinds of chamfers and negative lands shown in FIGS. 1, 5 and 6 were formed on the cBN-based sintered bodies of the cutting tips.

On the surface of tool No. 67, including the substrate surface, after forming a negative land along the cutting edge of each cutting tip, a hard film of (Ti_0.5Al_0.5)N was formed to the thickness of 3 micrometers by PVD (arc ion plating).

After brazing the cutting tips to the substrate of each tool, a rake face and an arcuate nose were formed on each cBN-based sintered body by grinding. Then, a chamfer and a negative land were formed to define cutting edges. Before forming the chamfer and the negative land, the cBN-based sintered bodies of any of the tools had a thickness of 1.2 mm. The brazing area of the bottom of the cutting tips was 2.90 mm². The cutting tips were brazed to the respective substrates of cemented carbide through an activated brazing filler of the same composition as the brazing filler used in Examples 2.

The tools were used as shown in FIG. 7 with the cutting edge inclination angle λ, side rake angle γn, end clearance angle αof, side clearance angle αos, end cutting edge angle κf, and side cutting edge angle κs set to (−7 degrees, −5 degrees, 7 degrees, 5 degrees, 32 degrees and −3 degrees), respectively, for the type D tools, (−6 degrees, −6 degrees, 6 degrees, 6 degrees, 15 degrees and −15 degrees), respectively, for the type T tools, (−5 degrees, −5 degrees, 5 degrees, 5 degrees, 5 degrees and −5 degrees), respectively, for the type C tools, and (−5 degrees, −6 degrees, 5 degrees, 6 degrees, 15 degrees and 15 degrees), respectively, for the type S tools. For any of the tools, the noses had a radius of curvature of 0.8 mm. The downward distance h of the ridgeline which is used as the end cutting edge, the width W of the negative land, and the downward inclination angle β of this cutting edge were set to values shown in Table 3. Using these tools, the following workpiece was cut under the following conditions.

Workpiece: JIS type: SCR415H end surface machining (DIN type: equivalent to 15Cr3); Hardness HRc 48 to 60 (the hardness changes (decreases) inwardly from the surface with the hardness at the depth of cut of 1 mm being HRc 48)

Cutting velocity: V=100 m/min

Depth of cut: d=1.0 mm

Feed rate: f=0.2 mm/rev

Type of machining: Composite machining with the area ratio between the radially outer surface and the end surface set to 1:1 (for removing a carburized layer) (It is necessary that the boundary between the radially outer surface and the end surface be an arcuate surface having a radius of curvature of not more than 0.8 mm. An emulsion type coolant diluted to 20 times was used.)

The results of an evaluation test are shown in Table 3.

TABLE 3

Downward

Cutting time
Amount of
Surface

Specimen

inclination
Width W
Downward
to tool
wear
roughness
Chip

No.

Shape of
angle β
of negative
distance h
breakage
of flank
Rz
length

No.
Tool shape
negative land
°
land mm
mm
min
μm
μm
mm
Remarks

61
DNMA150408
—
0
—
0
20
0.090
7.0
300
γ: 25°,

CW: 0.15 mm

62
DNMA150408
Flat
29.0
1.10
0.80
60
0.093
3.5
60

63
TNMA160408
—
0
—
0
18
0.087
8.0
290
γ: 25°,

CW: 0.15 mm

64
TNMA160408
Flat
28.0
1.10
0.80
65
0.086
3.5
60

65
CNMA120408
—
0
—
0
22
0.090
7.2
295
γ: 25°,

CW: 0.15 mm

66
CNMA120408
Flat
24.0
1.10
0.80
70
0.090
3.5
50

67
CNMA120408
Flat
24.0
1.10
0.80
100
0.050
3.5
55
PVD coating

68
CNMA120408
Curved
24.0
1.10
0.80
75
0.092
3.0
45

69
CNMA120408
Multifaceted
24.0
1.10
0.80
80
0.090
3.0
40

70
SNMA120408
—
0
—
0
22
0.085
8.0
310
γ: 25°,

CW: 0.15 mm

71
SNMA120408
Flat
22.0
1.10
0.80
70
0.085
3.5
60

Tool Nos. 61, 63, 65 and 70 are inserts including cutting tips each comprising a cBN-based sintered body of a conventional shape and brazed thereto. Each cutting tip has a nose having a radius of curvature of 0.8 mm, and has, along the cutting edge, a chamfer having a width CW of 0.15 mm and a chamfer angle γ of 25 degrees (conventional tools and Comparative Examples).

Tool No. 67 carries cutting tips having the same specs as the cutting tips of tool No. 66. But on the surface of tool No. 67, including the substrate surface, a hard film of (Ti_0.5Al_0.5)N is formed by PVD.

Tools in Table 3 are tools including alternately usable cutting tips comprising cBN-based sintered bodies fixed to a 25 mm square rod-shaped steel holder by a pin-lock type clamp mechanism. Any of the tools was mounted on a lathe so that the length of its portion protruding from the lathe turret to its tip is 40 mm.

The length of chips is the length of chips collected 5 minutes after the start of cutting.

Summary Evaluation

In Examples 3, due to the specific shape of the workpiece at the boundary between its radially outer surface and end surface, the radius of curvature of the arcuate nose is restricted to not more than 0.8 mm. But because cutting is high-efficiency cutting at a high feed rate, conventional tool Nos. 61, 63, 65 and 70 suffered chipping due to increased feed component of the cutting force. Their life spans were thus short.

In contrast, any of tool Nos. 62, 64, 66-69 and 71 according to the present invention showed marked improvement in the resistance to chipping. This is presumably because, as in Examples 2, the negative land serves to apply large compressive stresses as a reaction to the formation of chips to the cutting edge both in the direction of the principal cutting force and the tool feed direction, and the compressive stresses cancel tensile stresses resulting from the thrust force.

Tool Nos. 68 and 69, of which the negative land comprises two surfaces that are symmetrical with respect to the bisector of the apex α of the nose and inclined in opposite directions to each other so that its central portion protrudes toward the apex of the nose, showed further improved cutting abilities compared to tools having a flat negative land (tool Nos. 62, 64, 66 and 67).

According to the present invention, the negative land prevents thick and continuous chips from tangling with the workpiece or the tool, which was a big problem when turning a hardened steel workpiece at a high feed rate with a large depth of cut as in Examples 3. Specifically, while conventional tool Nos. 61, 63, 65 and 70 produced long chips of about 300 mm, the tools according to the present invention produced much shorter (less than 60 mm) and thus more easily disposable chips. Thus, the tools according to the present invention will serve to improve the operating ratio of facilities for e.g. mass-production of vehicle parts and reduce defectives.

Tool No. 67, which is identical to tool No. 66 except that its surface is coated with a TiAlN film, showed a marked reduction in the wear of the flanks, and thus a prolonged life span.

According to the present invention, each cBN-based sintered body has a negative land formed between the rake face and the arcuate nose to define cutting edges on both sides thereof whose position changes three-dimensionally. This makes it possible to form a high-strength obtuse cutting edge having the burnishing function both in the rotational direction of the workpiece and the tool feed direction while the workpiece is being turned. Further, by arranging the tool such that the end cutting edge forms a downward inclination angle and the apex of the nose is downwardly spaced from the rotation axis of the workpiece, which is generally inconceivable, a wiper edge is formed on the end cutting edge. This makes it possible to perform high-efficiency machining at a high feed rate, and high-precision machining, which was impossible with conventional cutting tools because high-efficiency machining and high-precision machining are generally considered to be contradictory to each other, and also to stably achieve a long life span of the tool. By high-precision machining, the machined surface of the workpiece shows improved fatigue strength, and the fitting portion of the workpiece shows improved sealability.

The tool according to the present invention can be especially advantageously used for high-efficiency turning of a hardened steel workpiece, but can also be advantageously used to cut a sintered alloy, which is one of workpieces that are difficult to cut.

Cbn Cutting Tool for High-Quality, High-Efficiency Cutting

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