JP4783224B2 - CBN ball end mill - Google Patents

Description

  The present invention relates to a ball end mill in which a ball blade is formed of a CBN (cubic boron nitride) sintered body, and more specifically, CBN capable of obtaining an excellent cutting edge strength and an excellent machined surface quality when machining high-hardness steel. Concerning ball end mill.

  A tool having a cutting edge formed of a CBN sintered body is used for processing hardened steel such as hardened steel. Among such tools, there is a ball end mill in which a ball blade is formed of a CBN sintered body. Such a ball end mill is disclosed in, for example, Patent Documents 1 and 2 below.

  The ball end mill disclosed in Patent Document 1 has a rake angle in the normal direction of the ball blade from the outer peripheral side toward the ball tip in order to reduce local damage at the center (ball tip) of the ball blade. It is gradually changed from −20 ° to 0 °.

In addition, the ball end mill disclosed in Patent Document 2 targets a small-diameter end mill of 3 mm or less, and a blade portion defined by a normal reference line and a first flank in a cross-sectional view in the normal direction of the ball blade A chisel edge with a large wedge angle formed by crossing the first flank faces of the blade at the centrally symmetric position at the tip (center) of the ball blade with a wall angle of 85 ° or more and less than 90 °. Provided.
Japanese Patent Laid-Open No. 10-113809 JP 2005-342835 A

The cutting speed of the rotary cutting tool decreases as it approaches the center of rotation. For this reason, the ball end mill has a poor sharpness at the tip of the ball, and particularly when the cutting edge is formed of a CBN sintered body having both hardness and brittleness, local damage to the tip of the ball tends to occur. In the above-mentioned patent document 1, in order to solve this problem, the rake angle of the ball blade is gradually changed so that the rake angle is 0 ° at the tip of the ball. If the length of the chisel edge formed at the center of the ball is not increased , the strength of the tip of the ball is insufficient and the chisel edge tends to be chipped.

However, when the chisel edge length is increased, the R accuracy of the ball blade is deteriorated and the processing accuracy is adversely affected. Further, when the chisel edge length is long , chips scraped by the chisel edge are difficult to be discharged, and the frequency of occurrence of so-called peeling of the processed surface increases.

  In the ball end mill of Patent Document 2, the chisel edge at the center of the ball blade is formed by the ridge lines of the first flank surfaces of the cutting blades (hereinafter referred to as the left and right cutting blades) at the center symmetrical position. The strength of the tip of the ball blade can be increased without lowering the angle, but the clearance angle of the first flank is as small as 0.5 to 5 °, and the chisel edge formed by the intersecting ridges between the first flank surfaces is reduced. Since the wedge angle is 170 ° or more, the surface near the flat surface is pressed against the work material in the center portion, and good cutting is not performed, and the processing surface is likely to be peeled off.

  Thus, in the structures disclosed in Patent Documents 1 and 2, at least one of the strength of the tip of the ball blade and the quality of the processed surface is sacrificed.

  Therefore, the present invention has an object to devise the shape of the cutting edge of the ball blade to achieve both improvement in the cutting edge strength of the CBN ball end mill and improvement in the quality of the processed surface.

In order to solve the above-described problems, in the present invention, the wedge angle β in the normal direction cross section of each part of the left and right cutting edges forming the centrally symmetrical shape of the ball blade formed by the CBN sintered body is set to 100 to 135 °, set the clearance angle α in 5 to 25 °, with a precise length of the chisel edge which flank the cutting edge of said right and left ball blade center is formed by intersecting a rake face with a negative rake angle rotation center It is assumed that it has penetrated to the position beyond it. The rake angle γ of this ball end mill is determined by the formula γ = (90−α−β), and is preferably −30 ° to −50 °.

Chi Zeruejji length W is a 0.05mm or less. Especially, when the length W was made into the range of 0.005-0.04 mm , the R precision of a ball blade center part was especially high, and it was preferable.

In the ball end mill of the present invention, the wedge angle β of each part of the ball blade is set to 100 to 135 °, so that excellent blade edge strength is ensured. Therefore, it is possible to bring the left and right cutting edges closer to the center of rotation, and the chisel edge length can be significantly reduced compared to conventional products to reduce the cutting resistance at the center of the ball blade, resulting in chip removal. It becomes possible to improve. If the length of the chisel edge is short, the chips are well separated and the chips are not left uncut and the chip discharge performance is improved. In addition, by shortening the chisel edge length , the R accuracy of the center portion is improved, which leads to improvement of processing accuracy.

  In addition, the sharpness of the chisel edge is improved by reducing the ball blade clearance angle from 5 to 25 ° and the wedge angle of the chisel edge is smaller than that of Patent Document 2, thereby reducing the cutting resistance at the center of the ball blade. Can also be expected.

  Furthermore, the ball end mill according to the present invention performs processing so as to scrape off the machining allowance portion of the work material with a large negative rake angle exceeding common sense. For this reason, the polishing effect is obtained and the surface roughness of the processed surface is improved as compared with the conventional product.

  In addition, in order to protect the cutting edge, it is common to make the rake angle negative or to strengthen the cutting edge by honing, but the negative rake angle that is commonly used is about -5 ° on average. The maximum is about −20 °. It is considered that if the negative rake angle is increased beyond that, the cutting resistance becomes too large to perform cutting, and it has not been done so far to make the negative rake angle extremely large like the end mill of the present invention. In this respect, it can be said that the end mill of the present invention breaks the conventional concept.

  The CBN ball end mill is used at a high speed rotation of 20000 to 50000 per minute. It has been found that when used under such conditions, the cutting of the work material, rather than cutting it off, results in less cutting of the cutting edge and more stable machining.

  Embodiments of the present invention will be described below with reference to FIGS. 1 to 6 of the accompanying drawings. In the illustrated CBN ball end mill, a CBN sintered body 2 is joined to the tip of the shank 1, and the CBN sintered body 2 is provided with a ball blade 3 and a gash 4 serving as a chip pocket.

  The ball blade 3 is composed of semicircular left and right cutting edges 3a and 3a having a centrally symmetric shape, and a chisel edge 3b provided between the left and right cutting edges 3a and 3a (ball blade center). The rake face 5 of the portion of the left and right cutting edges 3a, 3a in the angle range of θ from the rotation center to the outer end (θ is about 90 °) is formed as a surface having a negative rake angle γ. The rake angle γ (see FIG. 6) of the rake face 5 is set to −40 ° in the case of the exemplified ball end mill.

  Further, the flank faces of the left and right cutting edges 3a, 3a are formed by the second flank face 6 and the third flank face 7, and the flank angle α (see FIGS. 3 and 6) of the second flank face 6 is 20 °. The clearance angle α1 (see FIG. 3) of the third flank 7 is set to 45 °.

Further, the length W of the chisel edge 3b shown in FIG. 5 is preferably 0.005 to 0.04 mm, and the inner ends of the rake faces 5 and 5 of the left and right cutting edges are centered on the center of rotation as shown in FIG. It is inserted to the position beyond it. The chisel edge 3b is formed by a ridge line formed between the second flank faces 6 and 6 of the left and right cutting edges at the rotation center, and the flank angle α of the second flank face 6 is set to 5 to 25 °. (Preferably 10 to 20 °. The illustrated end mill has α = 20 °.) The wedge angle of the chisel edge 3b becomes 170 ° or less, and the cutting resistance is reduced.

  Other dimensions are set as follows. Now, in the case of the ball end mill of R0.5 (machining diameter φD = 1 mm) shown in FIG. 2, the center-up amount S of the gash 4 shown in FIG. 3 is set to 0.05 to 0.10 mm, and the center-up amount S The rake face 5 is arranged in the range of. Further, the width W1 of the second flank 6 is set to about 0.10 mm, and the effective blade length L in FIG. 2 is set to about 0.7 mm.

Next, based on FIGS. 7-9, the mechanism of the process by the ball end mill of this invention is demonstrated.
The ball end mill disclosed in the above-mentioned Patent Documents 1 and 2 and other existing CBN ball end mills (conventional products) are directly applied to the CBN ball end mill with the sharp edge. This is the image of cutting the work material. This is shown in FIGS. In FIG. 8, the rake face 5 is provided with a negative rake angle γ. However, since the negative angle is generally as small as about −5 °, the cutting edge is weak and easily damaged.
Further, as shown in FIG. 9, honing 8 that eliminates the sharp edge of the cutting edge is applied to strengthen the cutting edge. However, when this method is adopted, the sharpness is reduced and the processing surface of the work material W is reduced. The quality deteriorates.

On the other hand, in the ball end mill of the present invention, as shown in FIG. 7, a cutting edge having a large wedge angle β is pressed against the work material W with a rake angle γ of −30 ° or more, so that the machining allowance of the work material is obtained. Process to scrape off the part. The processing image in this case is close to grinding, and a polishing effect can be obtained at the same time as cutting. For this reason, the surface roughness of the processed surface is improved as compared with the conventional product.
In addition, a cutting edge with a large wedge angle is excellent in strength and is difficult to break. In addition, by increasing the wedge angle of the cutting edge, it is possible to shorten the chisel edge length by bringing the left and right cutting edges closer to the center of rotation, reducing the cutting resistance at the center of the ball blade and reducing chip discharge. Improvements can also be achieved. In addition, by reducing the chisel edge length , the R accuracy at the center of the ball blade is improved.

Tests were conducted to evaluate the performance of the ball end mill of this invention. The contents of the test are described below.
Example 1
Using the following ball end mill, the work material was machined under the following cutting conditions, and the state of damage to the cutting edge at the cutting length of 100 m, the surface roughness of the machined surface, and the presence or absence of burring of the machined surface were examined. In this test, the relief angle of the ball blade was kept constant at 20 °, and the influence of the change in the wedge angle (rake angle) on the evaluation performance was examined. The results are shown in Table 1.

Tool used: R1.0, clearance angle 20 °, chisel edge length 0.02 mm, CBN ball end mill with a CBN content of 52 vol% of the CBN sintered body joined to the tip of the shank Cutting conditions Rotational speed: N20,000 min ⁻¹
Feeding speed: F2,000mm / min, f0.05mm / tooth
Cutting depth: Ad0.05mm
Pick feed: 0.05mm
Work Material SKD11 (60HRC)
Processing form Flat processing, dry cutting Cutting length 100m

  According to this test result, damage to the blade edge is significant when the wedge angle is 100 ° or less, and the quality of the processed surface is not good when the wedge angle is 140 °. Therefore, in the ball end mill of the present invention, the wedge angle of the ball blade is limited to a range of 100 to 135 °.

-Example 2-
Next, the rake angle of the ball blade was made constant at −40 °, and the influence of the change in the clearance angle on the evaluation performance was examined. The tool used is the same CBN ball end mill as in Example 1 except for the rake angle, wedge angle and clearance angle. The cutting conditions, work material, and cutting length were the same as in Example 1. The results are shown in Table 2.

  As can be seen from the test results, when the clearance angle is 3 °, the quality of the machined surface is not good, and when the clearance angle is 30 °, the blade edge damage increases, so the clearance angle is limited to a range of 5 to 25 °.

Example 3
The effect of the chisel edge length at the center of the ball blade on the evaluation performance (R accuracy at the center of the ball blade, roughness of the processed surface, and unevenness of the processed surface) was examined. The tool used was a CBN ball end mill having the same rake angle of −40 ° and a clearance angle of 20 °, and the other specifications as in Example 1. The cutting conditions, work material, and cutting length were the same as in Example 1. The results are shown in Table 3. The R accuracy at the center represents an average value obtained by measuring 10 samples.

  As can be seen from the test results, the chisel edge having a width of 0.04 mm or less has very good R accuracy at the center of the ball blade, and the processing accuracy is increased.

The side view which shows embodiment of the CBN ball end mill of this invention Enlarged side view of the main part of the CBN ball end mill of FIG. Side view in the direction of arrow A in FIG. Enlarged front view of the CBN ball end mill of FIG. The figure which further expands and shows the B section of FIG. Enlarged side view of the tip of the ball blade Sectional drawing which shows the processing condition in the CBN ball end mill of this invention Sectional view showing the processing status of a conventional CBN ball end mill Sectional view showing the processing status of a conventional CBN ball end mill

Explanation of symbols

1 Shank 2 CBN sintered body 3 Ball blade 3a Cutting edge 3b Chisel edge 4 Gash 5 Rake face 6 Second relief face 7 Third relief face 8 Honing γ Rake angle β Wedge angle α Escape angle W Chisel edge length S Gash core up Quantity L Effective blade length

Claims (1)

In the CBN ball end mill in which the ball blade (3) is formed of a CBN sintered body, the wedge angle β in the normal direction cross section of each part of the left and right cutting blades (3a, 3a) forming the center symmetrical shape of the ball blade (3) Is set to 100 to 135 °, the clearance angle α is set to 5 to 25 °, and the rake angle γ obtained by the equation γ = (90−α−β) is set to −30 ° to −50 °. A rake face (5) having a chisel edge (3b) having a length W of 0.005 to 0.04 mm and having a negative rake angle (γ) formed by intersecting the flank faces (6, 6) of the cutting edge. The ball end mill is characterized in that is inserted to a position beyond the center of rotation.

Images