JP2761321B2 - Abrasive - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an abrasive for grinding wheels made of polycrystalline cubic boron nitride (cBN). As the grinding wheel to which the abrasive grains of the present invention are applied,
Particularly, a resin bond grindstone, a vitrified grindstone and the like for high quality and high efficiency grinding are exemplified.

[0002]

2. Description of the Related Art Since cBN has hardness and thermal conductivity next to diamond and has characteristics that diamond does not react with iron-based metals, it has been increasingly used as abrasive grains for grinding of iron-based metals. Have been.

[0003] In recent years, the grinding process is in the direction of higher quality and higher efficiency. High-quality processing is processing to improve the surface properties of the workpiece, and specific examples include reducing the surface roughness of the workpiece surface or intentionally leaving compressive stress on the workpiece surface. To increase the mechanical strength of the workpiece. On the other hand, to improve the efficiency means to increase the peripheral speed of the grindstone, for example, in order to increase the amount of processing per unit time. Recently, high-speed grinding at a peripheral speed of 60 m / sec or more has been studied.

[0004] However, conventional cBN abrasive grains cannot be made into a grindstone satisfying both high quality and high efficiency at the same time. This is because no abrasive grains satisfying the following requirements at the same time existed. (1) For the purpose of keeping the surface roughness of the workpiece small and constant, among the abrasive grains contained in the grindstone, some of the grains do not cause blindness and the strength of each grain varies. The need for uniform abrasive grains without blemishes. (2) For the purpose of leaving compressive stress on the workpiece surface, as described in “cBN wheel grinding technology” (issued by the Industrial Research Council in 1988), the sharpness of the grinding wheel is improved. In addition, it is necessary to reduce the frictional heat generated between the workpiece and the workpiece during grinding by reducing the grinding resistance. Therefore, in order to maintain the sharpness, the abrasive grains must be properly broken and fall off during grinding, and new abrasive grains must be exposed on the grinding wheel surface. Is required. (3) For the purpose of increasing the peripheral speed of the grinding wheel and performing high-speed grinding, a large load is applied to the abrasive grains during grinding.
Abrasives with high strength are required.

[0005] Single-crystal type cBN conventionally used
Abrasive grains have a relatively uniform strength because each grain has a self-shaped single crystal shape.Since it is a single crystal, it is easily cleaved and has high friability. It was not something that could withstand high-speed grinding at a speed of 60 m / sec.

On the other hand, there is a polycrystalline type cBN abrasive having high strength. Since the polycrystalline type abrasive has a polycrystalline structure in which fine crystal grains are strongly bonded to each other, a single crystal type cBN in which one particle is composed of a single crystal is used.
Shows high strength without causing large destruction such as cleavage like abrasive grains. However, conventional polycrystalline cBN abrasive grains, on the other hand, are too strong to be appropriately crushed, so that the tip of the cutting edge is worn and rounded during grinding, resulting in poor sharpness. Furthermore,
Since there is no uniform abrasive grain strength, some abrasive grains on the grindstone surface are destroyed, and the surface of the workpiece becomes rough, so there is a problem that frequent dressing and truing must be performed. .

Generally, there is a correlation between the shape and the strength of the abrasive grains, and it has been said that the larger the bulk, the higher the strength. Therefore, JIS R6
As specified in 126 "Test Method for Bulk Specific Gravity of Artificial Abrasive", bulk specific gravity obtained when particles are filled to a certain volume has been adopted. However, this method can only indirectly grasp the shape of the particles, and a good correlation has not been obtained particularly with the grinding characteristics when used as a grindstone.

[0008]

DISCLOSURE OF THE INVENTION The present inventors have solved the above-mentioned problems, and an object of the present invention is to provide abrasive grains for a grinding wheel suitable for high-quality and high-efficiency machining. As a result of various studies on the strength of the grains, as follows:
The inventors have found that the shape of the particle tip greatly affects the strength of the abrasive grains, and have completed the present invention.

(1) Grinding was actually performed with a grindstone using commercially available polycrystalline cBN abrasive grains at a high efficiency of a peripheral speed of 60 m / sec, and the state of the grindstone surface was observed before and after the grinding. Abrasive grains having a shape with large irregularities at the tip of the particles are selectively crushed to form a new sharp edge. (2) Abrasive grains having a shape with few irregularities at the tip of the particle before grinding do not break during grinding, but the tip of the particle is worn and rounded. (3) When high-efficiency grinding at a peripheral speed of 60 m / sec was performed with a grindstone using only abrasive grains having large irregularities at the particle tip, there was no increase in grinding resistance over a long period of time, and the wear amount of the grindstone was small. . Moreover, compressive stress remained on the surface of the workpiece. (4) Furthermore, as a result of various studies on methods for expressing the shape of a particle which is liable to break, a shape factor represented by a reciprocal of a ratio between a two-dimensional projected image of the particle and a circle circumscribed therewith, and a polycrystalline cBN abrasive grain There is a strong correlation with the intensity of
The breaking strength of abrasive grains changes greatly between 1.7 and 1.8.

[0010]

That is, according to the present invention, the average of the shape coefficient represented by the reciprocal of the ratio of the two-dimensional projected image of the particle to the circumscribed circle exceeds 1.7, and the standard deviation thereof is
An abrasive grain comprising a polycrystalline cubic boron nitride of 0.4 or less.

Hereinafter, the present invention will be described in more detail.

In the present invention, the reason for defining the shape of the abrasive grains by using statistics such as the average and the standard deviation of the shape factors is that the polycrystalline cBN abrasive grains have different shapes when viewed one by one. It is a collection of different particles, and the grindstone contains many particles with different shapes, so to express the actual shape of the abrasive grains and define the quality, a group consisting of various particles is This is because it is necessary to use a statistic that can be expressed.

In order to accurately obtain the average and the standard deviation of the shape factors of the abrasive grains, a sampling method by a reduction operation is important, and this is specified in JIS R6003 in "Abrasive Sampling Method". It is better to use the method.

The shape factor in the present invention is defined as the reciprocal of the ratio between a two-dimensional projected image of a particle and a circumscribed circle, and is expressed by the following equation. Shape factor = (area of two-dimensional projected image of particle / area of circumscribed circle) ^-1 As is clear from the above equation, the value of this shape factor is 1.0 when the projected view of the particle is a perfect circle, It becomes larger as the shape becomes random.

The area of a two-dimensional projected image of a particle can be obtained by irradiating the particle with light from one direction, for example, by taking a projection photograph of the particle with a commercially available projection microscope and analyzing it with an image analyzer. You can ask. Some commercially available image analyzers automatically measure the shape factor used in the present invention for a plurality of projections of particles and automatically calculate statistics such as the average and standard deviation thereof. It is convenient to use it.

The area of a circle circumscribing the two-dimensional projected image is expressed as the area of the circle having the smallest diameter among the circles that include the two-dimensional projected image while touching the inside.

In the present invention, the average shape factor is 1.7
It is necessary to exceed Perhaps because abrasive grains with a shape factor of 1.7 or less are unlikely to break, grinding resistance increases when actual grinding is performed with a grinding stone, and compressive stress does not remain on the surface of the work piece. High-quality and high-efficiency grinding cannot be performed.

The standard deviation is expressed as the square root of the variance of the probability function. In the present invention, the standard deviation of the shape factor needs to be 0.4 or less, preferably 0.3 or less. If the standard deviation of the shape factor is large, the shapes of the abrasive grains become irregular, some of them are blinded, the grinding resistance is increased, and the unevenness of the grindstone surface becomes uneven, so that the surface of the workpiece becomes rough. Therefore, the smaller the standard deviation is from the viewpoint of the performance of the abrasive grains, the better the mechanical strength of the entire abrasive grains becomes uniform, which is preferable. However, the yield in the production process of the abrasive grains cannot be increased. As a result of examining this point by changing the standard deviation in various ways, if the standard deviation is 0.4 or less, the surface roughness of the workpiece can be significantly reduced, especially 0.3
Since the surface roughness can be further reduced and stabilized below, the standard deviation of the shape factor is limited to 0.4 or less in the present invention.

The abrasive grains of the present invention can be produced, for example, by the following method. That is, a commercially available polycrystalline cBN
Obtain abrasive grains and select those with an average shape factor of more than 1.7 and a standard deviation of 0.4 or less. Specifically, a polycrystalline cBN abrasive grain having a particle size larger than a desired particle size is obtained in advance, and after adjusting to a particle having a large irregularity at the tip of the particle with an impact crusher such as a hammer mill,
A method of further dividing a material having a desired shape coefficient using a shape separator such as a vibration feeder type is adopted.

Furthermore, instead of commercially available polycrystalline cBN abrasive grains, a polycrystalline cBN sintered body can be synthesized by a direct conversion method, and then pulverized to select a particle size. The direct conversion method is widely known,
For example, as described in JP-B-63-394, it can be obtained by treating pyrolytic boron nitride (PBN) at high temperature and high pressure.

[0021]

[Function] As in the present invention, a grindstone using an abrasive having an average shape factor of more than 1.7 and a standard deviation of 0.4 or less is extremely sharp even under severe grinding conditions such as high efficiency grinding. The following are conceivable reasons why the surface roughness of the work is sharply reduced and compressive stress remains on the surface.

That is, as described above, the breaking behavior of each abrasive grain greatly changes at the boundary of the shape factor of 1.7 to 1.8, and when it is 1.8 or more, the breaking of the entire grain occurs simultaneously with the minute breaking of the tip of the grain. Such a destruction phenomenon may also occur due to collision with a workpiece even when an actual grindstone is used. That is, when the shape factor is 1.0 to 1.7,
Before the entire particle is broken, the tip part selectively breaks and wears out and absorbs the impact energy generated by the action with the workpiece.However, in the case of abrasive grains with a shape factor exceeding 1.7, especially 1.8 or more,
Since the concaves and convexes at the tip are so severe that the recesses penetrate considerably inside the abrasive grains, it is considered that the breaking energy is transmitted to the inside of the abrasive grains, and the entire abrasive grains are appropriately crushed to form a new cutting edge.

Conventionally, commercially available polycrystalline cBN abrasives have an average shape factor and a standard deviation of 1.80, respectively.
0.6, which is different from the standard deviation of the present invention. In a whetstone using the abrasive grains of the present invention, while a conventional whetstone using the abrasive grains as it is wears severely and the surface of the work becomes rough, and a tensile stress remains on the surface. Even in high-efficiency grinding, the sharpness is extremely sharp, and the surface roughness of the work is significantly reduced, and compressive stress remains on the surface.

[0024]

The present invention will be described below more specifically with reference to examples and comparative examples. Comparative Example 1 The only commercially available polycrystalline cBN abrasive (trade name “BOrazon BZ550” manufactured by General Electric): particle size 60/8
0) was obtained, sampled by the method of JIS R6003, a projection photograph of the particles was taken with a projection microscope at a magnification of 50 times, and the shape factor of the abrasive grains was determined using an image analyzer (“LA555” manufactured by Pierce). The average and standard deviation were measured.

Next, 100 carats of the carat were extracted and a vitrified bond grindstone having a diameter of 200 mm, a thickness of 10 mm and a concentration of 100 was prepared. Using the grindstone, a test was performed by a plane plunge cut method, and the grinding resistance, the surface roughness of the work, and the residual stress on the work surface were measured. Table 1 shows the results.

The work material used in the test was bearing steel SUJ2,
Grinding conditions are: grinding wheel peripheral speed 3600m / min, work material feed speed 9m / m
in, the cutting depth of the grindstone is 10 μm. Grinding resistance was measured using a dynamometer manufactured by Nippon Kistler Co., Ltd., and residual stress on the surface of the workpiece was measured using an X-ray residual stress measuring machine manufactured by Rigaku Denki.

Examples 1 to 5 Comparative Examples 2 and 3 Commercially available polycrystalline cBN abrasive grains (trade name “Borazon BZ550” manufactured by General Electric Co., Ltd .: particle size 18/20) were crushed by a hammer mill and then sieved with a stainless steel mesh sieve. The one with a particle size of 60/80 was further divided. This is processed by changing the inclination angle and frequency of the vibrating plate using a vibrating plate type feeder (“HS-10” manufactured by Japan Elise Magnetics Co., Ltd.), and seven types of abrasive grains having different shape factor averages and standard deviations Was manufactured, and a grindstone was prepared in the same manner as in Comparative Example 1 to perform a grinding performance test.
Table 1 shows the results.

Example 6 Polycrystalline cBN sintering by a direct conversion method using a commercially available pyrolytic boron nitride as a raw material at a temperature of 2200 ° C. and 7.7 GPa for 1 hour using a flat belt type ultra-high pressure and high temperature generator. The body was synthesized. After grinding it with a roll crusher, it was classified to separate abrasive grains having a particle size of 60/80. 10 of these abrasive grains
0 particles were extracted and a projection photograph was taken with a stereomicroscope. The shape factor expressed by the reciprocal of the ratio of the two-dimensional projected image of the particles obtained from the photograph and the circle circumscribing the same was measured using an image analyzer ("LA555" manufactured by Pierce Co., Ltd.). The abrasive grains were found to have various shapes with a shape factor of 1.0 to 2.5.

Next, one particle is sandwiched between a plate of cemented carbide and a load is applied by a material testing machine, and a load showing minute destruction at the tip of the particle (a micro-destruction load) and a load causing the entire particle to be destructed (the whole Breaking load) was measured. The result is shown in FIG. From Fig. 1, the fracture mode changes greatly at the boundary between the shape factors of 1.7 and 1.8. When the shape factor is 1.7 or less, the abrasive particles themselves are not crushed even when a load is applied to them, and only the tip of the particles is minutely broken. It can be seen that the micro-fracture load is different from the total fracture load, whereas if it exceeds 1.7, especially 1.8 or more, the whole particle is easily broken at the same time as the micro-fracture at the tip of the particle.

Examples 7 to 13 Comparative Examples 4 to 7 The particles obtained in Example 6 were subjected to a vibration plate type feeder ("HS-10" manufactured by Elise Magnetics Japan) to change the inclination angle and frequency of the vibration plate. To produce 11 types of abrasive grains having different averages and standard deviations of shape factors.
A grindstone was prepared in the same manner as in Example 1 and a grinding performance test was performed. Table 1 shows the results.

[0031]

[Table 1]

As is clear from the above Examples and Comparative Examples, the average of the shape factor represented by the reciprocal of the ratio of the two-dimensional projected image of the particle to the circumscribed circle exceeds 1.7, especially 1.8.
As described above, the grindstone using the polycrystalline cBN abrasive grains having a standard deviation of 0.4 or less has a sharp cutting edge, a small surface roughness of the workpiece, and a low surface roughness of the workpiece even under high-efficiency grinding conditions. It turns out that the compressive stress which raises mechanical strength remains.

[0033]

By using the abrasive grains of the present invention, it is possible to obtain a grinding wheel suitable for high-efficiency and high-quality processing, which has not been obtained conventionally.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the shape of abrasive grains and a micro-breaking load and a total breaking load.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁶ , DB name) C09K 3/14 550 B24D 3/00 C04B 35/583

Claims (1)

(57) [Claims] 1. A polycrystalline cubic boron nitride having an average shape factor represented by the reciprocal of the ratio of a two-dimensional projected image of a particle to a circle circumscribing the particle exceeding 1.7 and having a standard deviation of 0.4 or less. Abrasive grains characterized by comprising: