JPH0138628B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a superabrasive vitrified grinding wheel in which superabrasive grains such as cubic boron nitride (CBN) or diamond and aggregates such as mullite or zircon are bonded together with a glassy or ceramic bond. Vitrified superabrasive grinding wheels that contain only superabrasive grains are commonly used, but a variety of effects have been proposed for grinding wheels that mix superabrasive grains with normal abrasive grains (with some exceptions). However, it has not been put into practical use due to various difficulties. For example, cubic boron nitride (hereinafter
When manufacturing a vitrified grinding wheel by bonding CBN (abbreviated as CBN) with a glassy, crystalline, or ceramic bond, the grinding speed is approximately 10,000 times that of general abrasive grains.
In order to reduce the high cost of CBN abrasive grains, as stated in Japanese Patent Publication No. 52-3147, instead of using CBN alone, it is also used together with alumina abrasive grains to lower the concentration of CBN abrasive grains and reduce the concentration of CBN abrasive grains. It is being done to provide a whetstone for concentration. In addition, in Japanese Patent Publication No. 52-27394, aggregates mainly composed of crystals such as zircon, mullite, cordierite, spodiumene, β-eucryptite, and petalite with a modified Mohs hardness of 11 or less and a melting point of 900℃ or higher are used. There is. Furthermore, in Japanese Patent Publication No. 55-20826, a small amount of CBN is mixed with a general abrasive material such as fused alumina or silicon carbide to make a whetstone. Vitrified CBN grinding wheels manufactured using these technologies are attracting attention in the fields of grinding and precision grinding of difficult-to-cut materials that cannot be ground with conventional abrasive grains. When the concentration of CBN is lowered, the holding power of the abrasive grains decreases significantly, and the grinding performance is often much lower than that corresponding to the CBN content.Inexpensive low concentration CBN grinding wheels are often used for grinding general steel materials. , it was almost impossible to apply it economically. For example, alumina abrasive grains and CBN abrasive grains were mixed 1:1 and the concentration was 100.
The grinding ratio of CBN grinding wheel is as follows: CBN grinding wheel alone (concentration
200), it can be said that performance proportional to the CBN abrasive grain percentage was obtained, but in reality, the grinding ratio was often only a fraction of that of a grindstone with a concentration of 200. That is, it has been impossible to obtain grinding performance commensurate with the concentration of superabrasive grains. For this reason, it has been desired to develop an economical CBN grinding wheel with low concentration and excellent grinding ratio. The present invention aims to eliminate the decrease in abrasive grain retention (bonding strength) caused by lowering the degree of concentration, and to utilize the characteristics of superabrasive grains to provide a high-performance grindstone that exhibits grinding performance equivalent to or higher than the superabrasive grain content. It is something. That is, in the present invention, in a vitrified grinding wheel using superabrasive grains such as CBN or diamond, the coefficient of thermal expansion is ±2.0 with respect to the coefficient of thermal expansion of the superabrasive grains.
A super-abrasive grinding wheel made by blending aggregate consisting of oxide particles with × ^{10 -6} K ^-1 in a super-abrasive/aggregate ratio of 90/10 to 10/90 by volume and sintering it with a vitrified bond. I will provide a. The present invention will be explained in detail below. As a result of research into the drawbacks of the conventional method described above, the following facts were discovered. That is, the bitrified bond is melted during the heating process of firing and forms bond bridges between the abrasive grains as shown in FIG. Then, when the cooling process begins
Shear stress due to the difference in thermal expansion between CBN grains and aggregate grains such as alumina abrasive grains occurs parallel to the grain surfaces, that is, in the direction of dividing the bond bridge. If the stress is large, cracks parallel to the grain plane will occur in the bond bridge, reducing the abrasive grain retention.
When super abrasive grains such as CBN are mixed with normal abrasive grains such as alumina abrasive grains, the blending ratio of super abrasive grains/normal abrasive grains is 50/
It has become clear that as the value approaches 50, the number of cracks increases and the abrasive grain retention of the whetstone decreases as a whole. Furthermore, a factor that affects the abrasive grain retention is the affinity of the abrasive grains and aggregate with the vitrified bond. The better the affinity, the higher the abrasive retention.Generally, vitrified bond has good affinity for oxides and borides, but poor affinity for carbides such as SiC. On the other hand, the particle size of the aggregate also affects the wear pattern of the grinding wheel. For example, if aggregate with a larger particle size than CBN abrasive grains is used, it is thought that the CBN abrasive grains will also fall off as the aggregate falls off, and in fact the grindstone will easily wear out, which is not good. The present inventor focused on the coefficient of thermal expansion of aggregate, and also took into account affinity and particle size.As a result, the thermal expansion coefficient of mullite, zircon, etc. is higher than that of CBN, diamond, etc. By using oxide particles whose thermal expansion coefficient is within ±2.0×10 ^-6 K ^-1 and whose particle size is 20 to 100% of the CBN abrasive particle size, abrasive grain retention due to low concentration is achieved. We have developed a high-performance whetstone that eliminates the drop in power and has performance equivalent to or higher than that of concentration. For example, a high-performance grindstone with a concentration of 100 has a grinding performance of 60% to 100% of a grindstone with a concentration of 200. CBN and/or diamond abrasive grains are used as superabrasive grains, and CBN abrasive grains are generally preferred for grinding iron-based materials. The difference between the coefficient of thermal expansion of CBN or diamond abrasive grains between room temperature and 500℃ of the oxide particle aggregate to be added is within ±2.0×10 ^-6 K ^-1 , preferably ±1.5×10 ^-6 Use one within K ^-1 . If the difference exceeds this range, bond cracks will form and increase and the abrasive retention will decrease. Within this difference, the minimum value due to deterioration of holding force near the superabrasive concentration of 100, which was observed in the past, is no longer observed, and the problem is that a grinding ratio that is lower than that corresponding to the conventional superabrasive content can be obtained. Not only has the problem been solved, but a high-performance grinding wheel with a grinding ratio greater than that commensurate with the amount of superabrasive grains contained can be obtained. Compounding ratio in the whetstone, super abrasive grain/aggregate grain ratio 90/10~
The 10/90 capacity ratio means that something similar to superabrasive grains alone or a mixture ratio other than a very small amount of superabrasive grains exhibits a remarkable effect in terms of performance and economy. Even outside this range, it is of course possible to improve the conventional method by selecting the difference in thermal expansion coefficient, the difference in particle size, and the affinity. Within the above blending ratio range, the blending ratio of aggregate and superabrasive grains is selected depending on the object to be ground, the cost of the grinding wheel, etc. The total amount of superabrasive grains and aggregate in the grinding wheel is usually about 30 to 60% by volume, preferably 40% by volume.
~55% by volume, but may be determined depending on the purpose. As the oxide particle aggregate, use ceramic particles that have a fire resistance of 1200°C or higher and are not dissolved by bitrified bond during grindstone firing, such as molten mullite, zircon, or consisting essentially of mullite crystals. Alumina-silica ceramics, cordierite, or a mixture thereof can be used. The thermal expansion coefficient of CBN is 3.5×10 ^-6 K ^-1 (room temperature ~ 500
℃), and diamond has a temperature of 2.7×10 ^-6 K ^-1 (room temperature to 500°C), but the above-mentioned aggregate can satisfy the condition of differential thermal expansion. Furthermore, the aggregate may also satisfy conditions regarding affinity with bond. The thermal expansion coefficients of various aggregates are illustrated in Table 1.

[Table] Select from items
(choose)
The fire resistance of the aggregate should be 1200°C or higher, in order to avoid deformation, cracking, melting, melting of the aggregate, etc. during sintering as much as possible. A fire resistance lower than this cannot withstand firing in which glassy or crystalline bonds that soften and flow coexist. In addition, it is a simple oxide and has a fire resistance of 1200.
Strongly basic substances such as MgO and CaO are unsuitable as aggregates even at temperatures above ℃ because they easily melt into vitrified bond. The particle size of the oxide particle aggregate is 20~
The particle size should be 100%, preferably 40-80%. By limiting the particle size of the aggregate, the desired effect as an aggregate can be achieved without impairing the effect of the superabrasive grains. When the aggregate particle size exceeds 100%, the grinding ratio decreases relative to the superabrasive grain blending ratio. Furthermore, if the particle size is smaller than 20%, it will not be able to perform the desired function as an aggregate, and the grinding ratio will also be lower than the proportion corresponding to the superabrasive grain content. These relationships are CBN/fused mullite 50/
This is clear from FIG. 3, which illustrates the case of a 50 capacity ratio. In addition, when approximately spherical mullite particles are used as the aggregate, the bulk specific gravity is similar to that of CBN abrasive grains, so there is little change in firing shrinkage due to the degree of concentration. This makes it easy to apply the holding body part. In addition, from the viewpoint of compatibility with bond, SiC, B ₄ C
Carbide systems such as these are not preferred even if they satisfy the thermal expansion coefficient conditions. Note that carbides are generally not preferred for the purpose of grinding iron materials, also from the viewpoint of reactivity with iron. There are some boride materials that meet the requirements of the present invention, but these are generally expensive and are not suitable for providing an inexpensive grindstone by blending them as aggregate. Note that the bitrified bond used in the present invention is generally selected to be suitable for a grindstone using CBN abrasive grains or diamond abrasive grains. The blending ratio of vitrified bond in the grindstone is almost the same as in the conventional method, and is usually about 10 to 30% by volume, but it is determined as appropriate depending on the purpose. According to the above conditions, by arranging appropriate aggregates on superabrasive grains, forming and firing a vitrified grinding wheel, the grinding performance of a grinding wheel with a concentration of 200 obtained by using CBN or diamond alone can be reduced to 60%. A grinding performance of ~100% can be obtained in a wheel with a superabrasive concentration of 100. Therefore, the amount of superabrasive required for a given grinding performance can be significantly reduced. Further, according to the present invention, it is possible to freely design a grindstone with a superabrasive/aggregate blend ratio depending on the purpose. For special purposes, a mixture of CBN and diamond abrasive grains is also available. A grinding wheel containing a mixture of alumina and silicon carbide abrasive grains has a practical problem as the bond cracks and its strength is significantly weakened due to the significant difference in the expansion coefficients of the two abrasive grains, but CBN and diamond are mixed because their expansion coefficients are similar. A grindstone is possible. Also, by using a mixture of CBN and diamond,
When reducing the content of superabrasive grains by using aggregate, it is possible to manufacture a superabrasive combination grindstone with a low concentration and good grinding performance by using aggregates such as fused mullite described in this patent. It is possible. Examples are shown below. Comparative example 1

【table】 Comparative example 2

【table】 Example 1

【table】 Example 2

[Table] Each of the formulations of Comparative Examples 1 and 2 and Examples 1 and 2 was press-molded into a prismatic shape with a length of 40 mm, a width of 4 mm, and a thickness of 6 mm, and fired at 950° C. for 5 hours to obtain a grindstone. The strength of these grindstones was measured by three-point bending under conditions of a span of 30 mm and a crosshead speed of 1 mm/min. The results are shown in FIG. The bending strength is shown as a relative value, with the case of CBN 100 parts being 1. The thermal expansion coefficient of alumina abrasive grains is (7.4×10 ^-6 K ^-1 )
It is thought that cracks have occurred in the bond bridge due to the large difference between CBN's coefficient of thermal expansion (between room temperature and 500℃, the same applies hereinafter) (3.5×10 ^-6 K ^-1 ).
This is clear from the fact that in Figure 2, the lowest strength is shown at the mixing ratio of 50:50, where the number of bond bridges between CBN and aggregate is considered to be the highest. On the other hand, the coefficient of thermal expansion of silicon carbide abrasive grains is 4.2×
Since the thermal expansion coefficient is close to that of CBN at 10 ^-6 K ^-1 , the bending strength shows a linear relationship with the mixing ratio without causing any cracks in the bond bridge.
However, since it lacks compatibility with bitrified bond, the strength decreases as the silicon carbide abrasive grains increase. Mullite according to the present invention has a coefficient of thermal expansion of 4.5×
±2.0× with respect to CBN thermal expansion coefficient at 10 ^-6 K ^-1
10 ^-6 K ^-1 and has good affinity with vitrified bond, so it shows a constant high level of strength regardless of the mixing ratio. In addition, cordierite also exhibits a certain level of strength, similar to mullite. As mentioned above, if the thermal expansion coefficient of the aggregate is ±2.0×10 ^-6 K ^-1 relative to the thermal expansion coefficient of CBN, cracks in the bond bridge due to stress between different particles will not occur.
It shows a linear intensity change with respect to concentration. on the other hand,
Compatibility with bitrified bond is good when the aggregate is an oxide such as alumina or mullite, and exhibits high strength. However, the present invention can be applied to particles other than oxides if affinity with the vitrified bond is improved by particle surface treatment or other compounds formed on the particle surfaces. Example 3 Compound CBN#80/100 50 parts by volume Electrofused mullite 50 〃 Vitrified bond * 32 〃 * (Same as Example 1) Thickening agent 20 〃 The particle size of fused mullite is 0.2 of the particle size of CBN, 0.4,
Using particle sizes of 0.6, 0.8, 1.0, 1.2 and double, the compound was press-molded into a disk shape with an outer diameter of 25 mm, a thickness of 14 mm, and a hole diameter of 11 mm, and was fired at 1000°C for 3 hours to obtain a grindstone. . Comparing these whetstones with a whetstone with a concentration of 200,
Figure 3 shows the results of internal grinding. The grinding conditions are as follows. Grinding conditions Grinding wheel peripheral speed 2700m/min Workpiece peripheral speed 75m/min Cutting speed φ1mm/min Oscillation frequency 160cpm Oscillation speed 42m/min Machining allowance φ0.5mm Work material SNCM-420H (HRc42) Work material dimensions Diameter 50 mm x thickness 13 mm x hole diameter 30 mm As is clear from Figure 3, this grindstone with a concentration of 100 is 100% of the grinding wheel with a concentration of 200 when the particle size of the aggregate is 20 to 100% of the particle size of the CBN abrasive grains. Grinding performance of /2 or higher was obtained. In particular, when the aggregate particle size was 60%, a grindstone with a concentration of 100 achieved the same high performance as a grindstone with a concentration of 200. Comparative example 3 Mixed CBN#80/100 50 parts by volume Aggregate 50 〃 Bitrified bond * 32 〃 * (same as Example 1) Sizing material 20 〃 WA #150 and GC #150 were used as aggregates, A grindstone was manufactured in the same manner as in Example 3, and a grinding test was conducted. The results are shown in FIG. Example 4 Compound <CBN#80/100 electrode mullite> 100 parts by weight Vitrified bond 32 〃 Glue 20 〃 The blending amount of CBN was varied to give a concentration of 50, 100, 150, and 200. A grindstone was manufactured in the same manner as in Example 3, and a grinding test was conducted. The results are shown in FIG. As is clear from Figure 4, when a grinding wheel with a concentration of 100 is made using molten alumina or silicon carbide as an aggregate, a grinding ratio of only about 30 to 40% less than that of a grinding wheel with a concentration of 200 can be obtained. When fused mullite was used as the aggregate, the best one had a concentration of 100 and a grinding ratio equivalent to that of a concentration of 200. The performance is higher than that of the straight line). As described above, by selecting the coefficient of thermal expansion, affinity, and particle size of the aggregate, it is possible to fully utilize the characteristics of CBN and provide a higher-performance, lower-cost vitrified CBN grinding wheel. It is. Comparative examples and examples will be shown below regarding the case where diamond abrasive grains are used as superabrasive grains. Comparative example 4

【table】 Example 5

[Table] The above compound was added to a length of 40 mm as in the case of CBN.
It was press-molded into a prismatic shape with a width of 4 mm and a thickness of 6 mm, and fired at 900° C. for 3 hours in a nitrogen atmosphere to obtain a grindstone, and its three-point bending strength was measured. The results are shown in FIG.
Bending strength is 100 parts by volume of diamond (concentration 200)
It is shown as a relative value with the case of 1 being 1. From Figure 5, when white alundum (WA) is mixed, the bending strength decreases as in the case of CBN. The coefficient of thermal expansion of WA is 7.4×10 ⁻⁶ K ⁻¹ , which is outside the condition of a difference of ±2.0×10 ⁻⁶ K ⁻¹ , and therefore the conventional result is obtained. Comparative Example 5 Compound Diamond #140/170 50 parts by volume Green Carborundum GC #230/270
50 〃 Vitrified bond ^* 32 〃 Sizing material 20 〃 (Production and testing are described in Example 4) Example 6 Compound diamond #140/170 50 parts by volume Fused mullite #230/270 50 parts by volume Vitrified Bond ^* 32 〃 Glue 20 〃 * Same as Example 5 After sintering, the above compounds (Comparative Example 5, Example 6) were press-formed into a ring shape with an outer diameter of 150 mm x thickness of 8 mm x hole diameter of 140 mm. Then, baked at 900℃ in a nitrogen atmosphere for 5 hours, left to cool, and finished with an outer diameter of 140 mm x thickness of 8 mm.
It was glued to the outer circumferential surface of a general grindstone with a hole diameter of 76.2 mm. Table 2 shows the results of surface grinding using these grindstones under the following conditions. Grinding conditions Grinding wheel peripheral speed 1600m/min Cross feed 2mm/Pass Table feed speed 10m/min Depth of cut 0.03mm Work material Cemented carbide K10 (HR _A 90±1) Work material dimensions Length 50 x Width 100

[Table] As is clear from Table 2, the grindstone using fused mullite as an aggregate had less wear and 1.6 times higher performance than the grindstone using silicon carbide as an aggregate.

[Brief explanation of drawings]

Figure 1 is a schematic diagram of the bond bridge and its cracks formed between CBN and aggregate. Second
The figure shows the relationship between the mixing ratio of various aggregates and the bending strength. FIG. 3 shows the relationship between the particle size ratio of the aggregate (electrofused mullite) and the grinding ratio (relative value). Figure 4 shows the relationship between the degree of concentration and grinding ratio (relative value) when using aggregate (electrofused mullite) and the degree of concentration when using GC and WA.
Shows the grinding ratio (relative value) of a grinding wheel of 100. FIG. 5 shows the relationship between the mixing ratio of diamond abrasive grains and aggregate and bending strength. 1... CBN abrasive grain, 2... aggregate, 3... pore, 4... crack, 5... bond.

Claims (1)

[Claims] 1. A vitrified grinding wheel using superabrasive grains such as cubic boron nitride and diamond,
±2.0 for the thermal expansion coefficient of superabrasive grains between room temperature and 500℃
Superabrasive grain ^/ aggregate ratio of 90/ ¹⁰ ~
A super abrasive grindstone characterized by a 10/90 volume ratio blend. 2. The whetstone according to claim 1, wherein the oxide particles are ceramic particles that have a refractoriness of 1200° C. or higher and are not dissolved by bitrified bond during firing of the whetstone. 3. The grindstone according to claim 1 or 2, wherein the oxide particles have a particle size of 20 to 100% of the superabrasive particles. 4. The oxide particles are comprised of molten mullite, zircon, alumina-silica ceramics consisting essentially of mullite crystals, cordierite, or a mixture thereof.
The whetstone described in item 1. 5. The grindstone according to claim 1, wherein the superabrasive grains are cubic boron nitride abrasive grains and/or diamond abrasive grains. 6. The whetstone according to claim 1, wherein the total amount of superabrasive grains and aggregate in the whetstone is approximately 30 to 60% by volume.