JP2008200780A - Mixed abrasive grain grinding wheel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a grinding wheel improved in a grinding ratio, and extended in service life. <P>SOLUTION: The grinding wheel has an abrasive grain layer in which abrasive grains of cubic boronitride and abrasive grains of diamond are dispersed and fixed in a binder phase. The abrasive grains of cubic boronitride and the abrasive grains of diamond are mixed at a volume ratio of 80:20-97:3. The concentration rate of the abrasive grains is 100-200, and the porosity of the abrasive grain layer is 20-40%. The binder phase may preferably comprise vitrified bond. The grinding wheel is favorably usable for inside surface grinders, centerless grinders, camshaft grinders, cylindrical grinders, etc. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a mixed abrasive grindstone provided with an abrasive layer containing cubic boron nitride abrasive grains and diamond abrasive grains.

  Since diamond abrasive grains chemically react with elements such as iron, cobalt, and nickel during grinding, they are generally unsuitable for grinding ferrous materials. Therefore, cubic boron nitride abrasive grains are used for such grinding applications instead of diamond abrasive grains. With a cutting edge made of cubic boron nitride abrasive grains, sharp cutting edges are generated by fine crushing of the abrasive grains during grinding, and the state in which the sharpness is improved lasts for a relatively long period of time.

  A grindstone having an abrasive grain layer containing cubic boron nitride abrasive grains and diamond abrasive grains in various mixing ratios is known. For example, a grindstone (see Patent Documents 1 and 2) in which the mixing ratio of both is diamond abrasive grains: cubic boron nitride abrasive grains = 70: 30 to 40:60, or diamond abrasive grains: cubic boron nitride abrasive grains = 8. A grindstone (see Patent Document 3) having a ratio of 5: 1.5 to 9.5: 0.5 has been proposed. In these patent documents, it has been common to use a relatively small amount of cubic boron nitride abrasive grains in combination with a relatively large amount of diamond abrasive grains.

  On the other hand, a technique using a relatively large amount of cubic boron nitride abrasive grains in combination with a relatively small amount of diamond abrasive grains has also been proposed. For example, a grindstone having a mixing ratio of cubic boron nitride abrasive grains and diamond abrasive grains in the range of 50:50 to 90:10 has been proposed (see Patent Document 4). However, Patent Document 4 discloses that the mixing ratio of cubic boron nitride abrasive grains and diamond abrasive grains is within the above range, whereby the grinding ratio is improved and the surface roughness is maintained for a long period of time. It is not described that lifetime is achieved.

JP-A-6-262527 JP 2000-94336 A JP 2002-178265 A Japanese Patent Laid-Open No. 11-277440

  An object of the present invention is to provide a grindstone capable of further improving the grinding ratio, maintaining the surface roughness for a long period of time, and extending the service life.

  As a result of intensive studies, the inventors of the present invention, in a grindstone containing cubic boron nitride abrasive grains and diamond abrasive grains, unlike the prior art, the amount of cubic boron nitride abrasive grains is relatively larger than the amount of diamond abrasive grains. It has been found that the above-mentioned object can be achieved by increasing the amount of both of them to be within a specific range.

The present invention was made on the basis of the above knowledge, and in the mixed abrasive grindstone having an abrasive layer in which cubic boron nitride abrasive grains and diamond abrasive grains are dispersed and fixed in a binder phase,
Cubic boron nitride abrasive grains and diamond abrasive grains are mixed in a volume ratio of 80:20 to 97: 3, the degree of concentration of the abrasive grains is 100 to 200, and the porosity of the abrasive grain layer is 20 to 20 The object is achieved by providing a mixed abrasive wheel that is 40%.

  According to the mixed abrasive grindstone of the present invention, the grinding ratio is improved and the life of the grindstone is extended as compared with the conventional grindstone. Furthermore, the surface roughness is maintained for a long time.

  Hereinafter, the present invention will be described based on preferred embodiments thereof. The mixed abrasive grindstone of the present invention (hereinafter also simply referred to as a grindstone) is generally formed by forming an abrasive layer on an abrasive layer-forming surface of a base metal having a conventionally known shape such as a wheel shape or a cup shape. It is. The abrasive layer is configured by dispersing and fixing abrasive grains in a binder phase.

  In the present embodiment, mixed abrasive grains made of cubic boron nitride abrasive grains and diamond abrasive grains are used as the abrasive grains. In the following description, cubic boron nitride is represented as cBN. The grindstone of this embodiment has one of the characteristics in the mixing ratio of these two abrasive grains. Specifically, cBN abrasive grains and diamond abrasive grains are mixed at a volume ratio of 80:20 to 97: 3, preferably 85:15 to 95: 5, more preferably 90:10 to 93: 7. Is mixed in a ratio. Thus, in this embodiment, the usage amount of cBN abrasive grains is relatively increased with respect to the usage amount of diamond abrasive grains. As described above, diamond abrasive grains are not suitable for use in grinding ferrous materials. However, by using diamond abrasive grains in combination with a relatively large amount of cBN, setting the concentration of abrasive grains in a specific range, and setting the porosity of the abrasive layer in a specific range, The present inventors have found that the grinding ratio is improved and the life of the grindstone is extended even when grinding. The reason for this is as follows.

  There are initial wear and steady wear in the wear of the grinding wheel as the grinding proceeds. The initial wear is the wear of the grindstone until grinding starts after dressing and reaches a steady state. Dressing refers to the work of scraping off the abrasive grains on the grindstone work surface that has become dull and forming a new cutting edge to restore the sharpness. In the initial wear, the abrasive grains damaged by the dressing are crushed and dropped and wear. In addition, in diamond abrasive grains, a sharp abrasive cutting edge immediately after dressing reacts with a work material such as iron to cause wear. For this reason, a grindstone having 100% diamond abrasive grains has a large initial wear. Conversely, a grindstone with 100% cBN abrasive grains reacts with a work material such as iron and does not cause wear and wear, so initial wear is small compared to a grindstone with 100% diamond. In the case of a mixed grindstone of diamond abrasive grains and cBN abrasive grains, the magnitude of initial wear varies linearly according to the mixing ratio. This state is shown in FIG.

  The phenomenon in which the initial wear stage ends and the grindstone gradually wears as the grinding progresses is called steady wear. In the case of diamond abrasive grains, abrasion wear progresses when the cutting edge immediately after dressing is sharp, but abrasion wear proceeds when the tip of the cutting edge is rounded and the flat area of the abrasive grain cutting edge increases to some extent. Disappear. For this reason, a diamond wheel having 100% diamond abrasive grains has a large initial wear but a small steady wear (however, the grinding resistance increases and a grinding abnormality tends to occur). On the other hand, in the case of cBN abrasive grains, grinding progresses while the cutting edge is finely crushed, so steady wear is large. Therefore, a cBN abrasive grain 100% grindstone has a small initial wear but a large steady wear. In the case of a mixed grindstone of diamond abrasive grains and cBN abrasive grains, the steady wear varies linearly depending on the mixing ratio, but unexpectedly, when the volume ratio of cBN abrasive grains to diamond abrasive grains is in the above range. On the other hand, it was found that the progress of steady wear of the grindstone is extremely reduced. This state is shown in FIG.

  Thus, since grinding wheel wear = initial wear + steady wear, it is considered comprehensively, and by mixing cBN abrasive grains and diamond abrasive grains in a volume ratio of 80:20 to 97: 3, initial wear and steady wear. It is possible to suppress the progress of wear of the grindstone at any stage of wear. On the other hand, a grindstone with a large proportion of diamond abrasive grains moves to a steady wear stage, and after a while, grinding abnormality occurs and the grindstone life is remarkably shortened.

  Further, when paying attention to the finished surface roughness of the work material, a crushing wheel with 100% cBN abrasive grains causes minute crushing of the abrasive grain cutting edge, so that scratches are generated on the finished surface. On the other hand, when cBN abrasive grains and diamond abrasive grains are mixed at a volume ratio of 80:20 to 97: 3, the worn diamond abrasive grains flatten the finished surface, which is better than 100% cBN abrasive grains. A smooth surface can be obtained.

  As for the particle size of the cBN abrasive and the diamond abrasive used in the present embodiment, the particle size specified in JIS B4130 is preferably # 80/100 to # 230/270, particularly # 120/140 to # 170/200. . In this case, the particle sizes of the cBN abrasive grains and the diamond abrasive grains may be the same or different.

In the abrasive grain layer in which the mixed abrasive grains of cBN abrasive grains and diamond abrasive grains are dispersed and fixed in the binder phase, the degree of concentration (concentration) is 880 mg (4.4 carats) of abrasive grains in 1 cm ^3. When 100 is included, it is 100 to 200, preferably 150 to 200, and more preferably 180 to 200. If the concentration of the abrasive layer is less than 100, the number of abrasive grains contributing to grinding is reduced, and the grinding ratio cannot be improved. Further, the amount of diamond abrasive grains responsible for flattening the work material finish surface is reduced, and the effect of flattening the finish surface during grinding cannot be obtained. Even if the degree of concentration is set to less than 100, the number of diamond abrasive grains appearing on the surface of the abrasive grain layer can be made the same by changing the mixing ratio of cBN abrasive grains and diamond abrasive grains. However, in that case, the mixing ratio of the cBN abrasive grains and the diamond abrasive grains changes greatly, so that the above-mentioned range is not satisfied, and the effect of the present invention is not achieved. On the other hand, if the concentration of the abrasive layer exceeds 200, it becomes difficult to form pores, and grinding abnormalities are likely to occur.

The mixed abrasive grains of cBN abrasive grains and diamond abrasive grains are dispersed and fixed in the binder phase. In the grindstone of this embodiment, for example, vitrified bond is used as the binder phase. Vitrified bonds generally contain SiO ₂ , Al ₂ O ₃ and B ₂ O ₃ as main components. The vitrified bond used in the present embodiment preferably has the following composition in view of improving the grinding ratio and extending the life of the grindstone.
· SiO _2: 35~55 weight%
· Al ₂ O _3: 10~30 wt%
· B ₂ O _3: 10~30 wt%
Other oxides: 1 to 10% by weight

  As the binder phase, a resinoid bond can be used instead of the above-described vitrified bond. However, resinoid bonds generally have a low abrasive grain retention and tend to fall off easily during grinding. Moreover, since the rigidity is low, high-precision machining cannot be expected. On the other hand, the grindstone of this embodiment in which cBN abrasive grains and diamond abrasive grains are mixed at a specific ratio and these abrasive grains are dispersed and fixed by vitrified bonds having a specific composition does not have such a defect. It becomes possible to perform stable grinding.

  A large number of pores are formed in the abrasive layer, and in this embodiment, the porosity is set to 20 to 40%, preferably 25 to 35%, and more preferably 28 to 32%. The porosity is a value obtained by dividing the total volume of pores in the abrasive layer by the apparent volume of the abrasive layer and multiplying this by 100. When the porosity is less than 20%, grinding resistance increases and grinding abnormality occurs. On the other hand, when the porosity exceeds 40%, the amount of bonds holding the abrasive grains is small, the abrasive grains fall off, and the grinding wheel wear increases. The porosity of the abrasive layer is measured according to JIS R2205.

  The porosity is a volume fraction (%) of pores contained in the abrasive layer. Accordingly, the sum of the porosity, the abrasive grain ratio, and the binder phase ratio is 100%. The porosity of the abrasive layer can be controlled by the conditions during the formation of the abrasive layer. For example, when a predetermined amount of abrasive grains and a binder phase are put into a mold for forming an abrasive layer and pressed into a predetermined shape, the amount of the abrasive grains and the binder phase put into the mold is controlled. Thus, the porosity can be controlled.

  Further, in order to make the porosity of the abrasive layer within the above range, the maximum particle size of vitrified bond as a binder phase is 1/5 or less, particularly 1/10 or less, compared to the particle size of the abrasive particles. It is also preferable that there is.

  In forming the abrasive layer, it is preferable to uniformly coat the binder phase on the surface of the abrasive. As a coating method, a method similar to a method usually used in the technical field can be employed. In particular, it is preferable to use the method described in Japanese Patent Application Laid-Open No. 2002-294221 relating to the earlier application of the present applicant. Specifically, a slurry in which the binder phase is suspended in a solvent is sprayed on the abrasive grains, dried and the abrasive grains are coated with the binder phase, and then the abrasive grains are fired.

  In the coating method, it is preferable to employ a method of coating the binder phase while rolling the abrasive grains. For this purpose, for example, a centrifugal rolling granulation coating device can be used. In this case, a method in which the abrasive grain surface is previously wetted with a binder and a vitrified bond that is a binder phase is sprayed, or a method in which a binder is dissolved in a solvent and a slurry in which the binder phase that is a vitrified bond is suspended is sprayed onto the abrasive grains. Can be used. In order to provide a uniform coating on the surface of the abrasive grains and with the abrasive grains not agglomerated, the slurry in which the vitrified bond as the binder phase is suspended is ground using a rolling granulation coating apparatus or the like. It is preferable to employ a method of spraying the grains.

  The abrasive grains coated with the binder phase by the above method are formed into a predetermined shape by a press and then fired to form a target abrasive layer.

  The diamond abrasive grains used in this embodiment preferably have an average crushing load of 30 to 120 N, particularly 40 to 60 N. By using diamond abrasive grains having an average crushing load in this range, wear and wear are moderately produced, and the effect of suppressing grinding wheel wear during steady grinding is exhibited. The average crushing load can be obtained by applying a compressive stress to the abrasive grains using a precision tensile tester and measuring the stress at the time of fracture.

Moreover, it is preferable that the diamond abrasive grain used in this embodiment has a tensile fracture stress of 2.4 to 8 GPa, particularly 2.8 to 4 GPa. By using diamond abrasive grains having a tensile fracture stress in this range, wear and wear are moderately produced, and the effect of suppressing grinding wheel wear during steady grinding is exhibited. The tensile fracture stress σ is determined by measuring the average projected cross-sectional area A (m ² ) of the abrasive grains from the scanning electron microscope image, and using the average crushing load W (N), σ (Pa) = W (N) / It is calculated from 0.32 A (m ² ) and Pa = N / m ² .

  The average crushing load and tensile fracture stress of the diamond abrasive grains are related to the average crushing load and tensile fracture stress of the cBN abrasive grains used in combination with the diamond abrasive grains. Specifically, when the average crushing load of cBN abrasive grains is 1, the average crushing load of diamond abrasive grains is 1.1 to 7, particularly 2.5 to 5, for the same reason as described above. Is more preferable. Further, the load applied to the cBN abrasive grains and the diamond abrasive grains are such that the tensile fracture stress of the diamond abrasive grains is 1.1 to 7, particularly 2.5 to 5, when the tensile fracture stress of the cBN abrasive grains is 1. It is preferable from the viewpoint of balancing the load applied to the. Outside these numerical ranges, any one type of abrasive grains breaks or falls off, and the balance of the number of abrasive cutting edges appearing on the grindstone surface tends to be lost. In that case, the micro destruction of the cBN abrasive grains and the abrasion wear of the diamond abrasive grains do not occur at the same time, making it difficult to take advantage of the features of the present invention.

  The ratio of the average crushing load and tensile fracture stress of diamond abrasive grains to the average crushing load and tensile fracture stress of cBN abrasive grains is as described above, and the average crushing load of cBN abrasive grains is 4.3 to 109 N, particularly 8 to 24N is preferred. The tensile fracture stress of cBN is preferably 0.3 to 7.3 GPa, particularly preferably 0.6 to 1.6 GPa.

  CBN abrasive grains and diamond abrasive grains whose average crushing load and tensile fracture stress satisfy the above-mentioned ranges are commercially available.

  The grindstone of this embodiment is, for example, carbon steel such as SC, bearing steel such as SUJ, alloy steel such as SCM, SCR, and SNC, tool steel such as SK, SKH, SKS, SKD, and SKT, and stainless steel such as SUS. It is particularly suitable for grinding various steel materials. For such steel materials, the grindstone of this embodiment is, for example, an internal grinder, a centerless grinder, a cam grinder, a cylindrical grinder, a crankpin grinder, a crankshaft grinder, a surface grinder, a gear grinder It is particularly suitable to be used for a tool grinder, a roll grinder, a profile grinder and the like.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the scope of the present invention is not limited to such examples. Unless otherwise specified, “%” means “volume%”.

[Example 1]
(1) Abrasive grains and cBN abrasive grains used: grain size # 120/140, average crush load 19 N, tensile fracture stress 1.2 GPa
Diamond diamond: particle size # 120/140, average crushing load 53N, tensile fracture stress 3.2 GPa

(2) Method for producing a grindstone First, cBN abrasive grains and diamond abrasive grains are mixed so as to be 45% and 5% with respect to the volume of the grindstone to be produced (90:10 in terms of abrasive volume ratio). A wetting agent such as a liquid phenol resin is added to the mixed abrasive and stirred to sufficiently wet the abrasive surface. After that, a vitrified bond powder with an average particle size of 3 μm (maximum particle size of 7 μm) is added as a binder so as to be 20% with respect to the grindstone to be produced, and stirred to adhere the vitrified bond powder to the surface of the abrasive grains. Get things. The blend was passed through a sieve and then molded into a predetermined shape. Further, after drying, firing was performed at 690 ° C. in a nitrogen atmosphere to produce a straight cBN / diamond vitrified grinding wheel (shape: length 305 mm, width 16 mm, height 127 mm, thickness 3.5 mm). The concentration of the obtained grindstone was 200, and the porosity was 30%. The composition of the vitrified bond was as follows.
・ SiO ₂ : 55% by weight
・ Al ₂ O ₃ : 30% by weight
・ B ₂ O ₃ : 10% by weight
・ Other oxides: 5% by weight

[Comparative Example 1]
A grindstone was obtained in the same manner as in Example 1 except that the volume ratio of cBN abrasive grains to diamond abrasive grains was set to 70:30. The concentration of the obtained grindstone was 200, and the porosity was 30%.

[Comparative Example 2]
A grindstone was obtained in the same manner as in Example 1 except that only cBN abrasive grains were used without using diamond abrasive grains. The concentration of the obtained grindstone was 200, and the porosity was 30%.

[Evaluation]
A cylindrical plunge grinding test (upward grinding) was performed using the grindstones obtained in the examples and comparative examples. As the work material, SKH51 (quenched, hardness HRC65, shape: 80D × 5T) was used. The grinding conditions were a grinding wheel peripheral speed of 60 m / s, a workpiece speed of 0.67 to 0.52 m / s (161 rpm), a plunge speed of 8.94 μm / s, and a wet grinding method. The grindstone dress is a rotary dresser with an AE sensor (dresser wheel: SD40Q75M). The measurement was performed under the condition of an amount of 2.5 μm × 4 times (spark out 20 times). The grinding ratio and workpiece finish surface roughness (maximum height roughness Rz) when processed under the above conditions were measured. Further, the life until the occurrence of abnormal grinding (the amount of grinding until abnormal noise occurs or the amount of grinding until the finished surface roughness exceeds a predetermined amount Rz = 2 μm) was evaluated. These results are shown in Table 1.

  As is apparent from the results shown in Table 1, it can be seen that the grindstone of the example has a higher grinding ratio, lower finished surface roughness, and longer life compared to the grindstone of the comparative example.

It is a graph which shows the relationship between the mixing ratio of an abrasive grain, and the grade of wear.

Claims (7)

In a mixed abrasive grindstone having an abrasive layer in which cubic boron nitride abrasive grains and diamond abrasive grains are dispersed and fixed in a binder phase,
Cubic boron nitride abrasive grains and diamond abrasive grains are mixed in a volume ratio of 80:20 to 97: 3, the degree of concentration of the abrasive grains is 100 to 200, and the porosity of the abrasive grain layer is 20 to 20 40% mixed abrasive grindstone.   The mixed abrasive wheel according to claim 1, wherein the binder phase is made of vitrified bond.   The mixed abrasive grindstone according to claim 1 or 2, wherein an average crushing load of the diamond abrasive grains is 30 to 120N. The mixed abrasive grindstone according to any one of claims 1 to 3, wherein the diamond abrasive grains have a tensile fracture stress of 2.4 to 8 GPa.   The mixed abrasive wheel according to claim 3, wherein the average crushing load of the diamond abrasive grains is 1.1 to 7 when the average crushing load of the cubic boron nitride abrasive grains is 1.   The mixed abrasive wheel according to claim 4, wherein the tensile fracture stress of the diamond abrasive grains is 1.1 to 7 when the tensile fracture stress of the cubic boron nitride abrasive grains is 1.   The mixed abrasive wheel according to any one of claims 1 to 6, which is used for an internal grinder, a centerless grinder, a cam grinder, or a cylindrical grinder.

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