JPH0428502B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method of manufacturing a porous vitrified boron nitride grinding wheel using a porcelain binder, vitrified. Conventionally, in order to perform internal grinding using a small-diameter grindstone, the applicant of this invention has already developed
Satisfactory grinding can be achieved using a vitrified boron nitride grindstone with a normal structure as proposed in No. 49351 and a grindstone diameter of approximately 15 mm or less. However, grinding of through holes or internal grinding of arcuate groove surfaces with a large-diameter grindstone with a grindstone diameter exceeding 25 mm, or
When grinding the outer ring groove of a constant velocity ball joint, etc.
Since the contact arc or contact surface between the grindstone and the workpiece increases, clogging is likely to occur. Furthermore, if we try to increase the hardness of the grinding wheel and perform economical grinding with minimal wear on the grinding wheel, welding of metal chips will occur on the surface of the grinding wheel, making it impossible to obtain a suitable surface for grinding. . On the other hand, conventional porous grinding wheels that use general abrasive grains such as alumina and silicon carbide are made of tool steels such as carbon tool steel, high-speed steel, and alloy tool steel, or of tool steels such as aluminum alloys and sintered alloys. Targeting difficult-to-cut materials, we used a soft grindstone with an alpha-average hardness (JIS-R6210-1980) and less than J even when hardened, placing emphasis on grindability at the expense of grinding wheel wear. The grinding ratio is also small at less than 5%. Therefore, using expensive cubic boron nitride abrasive grains (hereinafter referred to as CBN abrasive grains),
If a porous grinding wheel were to be produced using conventional techniques, economical grinding would soon become impossible and its usefulness would be completely lost. In addition, the Vitrified CBN grinding wheel has a temperature of approximately 1000℃.
Because the abrasive grains tend to deteriorate at higher temperatures, or because the coefficient of expansion of the abrasive grains is as low as 3.5×10 ^-6 cm/cm/°C, a bond with excellent bonding strength that is compatible with this is used. There are various problems, such as the fact that there are limits to the choices that can be made. In order to overcome such problems,
As described in the above-mentioned Japanese Patent Publication No. 57-49351, a vitrified CBN grinding wheel has a binder that has excellent abrasive grain support, is more rigid, and has a more porous structure than a resinoid bonded CBN grinding wheel. This type of grinding wheel has been developed and is on the way to being widely used in the precision grinding field. However, in order to promote these characteristics of the vitrified CBN grinding wheel and further reduce the wear amount of the grinding wheel, if we try to achieve high economic efficiency by increasing the amount of binder, etc., the grinding wheel will become severely clogged. The reality is that the frequency of dressing increases, and the wear of the grindstone and wear of the dresser due to dressing increases significantly, making it impossible to pursue higher economic efficiency beyond a certain limit. This phenomenon is particularly noticeable when grinding a grindstone with a large contact arc or contact surface between the grindstone and the workpiece, or when grinding difficult-to-cut materials such as tool steel. On the other hand, we are trying to improve grinding performance by improving grinding wheel clogging without increasing grinding wheel wear, automatically creating a void near the grinding point at the tip of the abrasive grain during grinding, and facilitating the discharge of grinding debris. Measures are being tried to do so. Among them are the following, for example: In other words, a vitreous or ceramic CBN grinding wheel that uses an aggregate such as zircon or mullite, which is softer and more abrasive than alumina abrasive grains and whose main component is crystals with a modified Mohs hardness of 11 or less and a melting point of 900°C or more. (Tokuko Showa 52-27394
issue). A fine-grained grindstone in which organic or inorganic micro hollow bodies act as pores, which are chip pockets of the grinding wheel, as a binder (Japanese Patent Laid-Open No. 137887/1987). Graphite, hexagonal boron nitride, and disulfide are added to ceramic matrices for the purpose of low grinding resistance and high grinding ratio performance in dry grinding of cemented carbide tools and high-speed steel without using liquid lubricants or coolants. A diamond or CBN whetstone containing an appropriate amount of molybdenum, etc. (Special Publication No. 5717, 1973). etc. All of these grindstones contain effective ingredients to reduce the grinding resistance at the grinding point during grinding and to form chip pockets, which are thought to greatly contribute to cool grinding, or to provide lubricity. However, we have not yet reached a satisfactory stage as to whether highly economical grinding can be performed with less wear on the grinding wheel. Focusing on the above-mentioned current situation, this invention has been developed to provide a system that is difficult to clog even when performing high-efficiency grinding with a small amount of grindstone wear, allows cool grinding, and is easy to dress (reface). Furthermore, it is an object of the present invention to provide a method for manufacturing a grindstone that can improve the wear of the dresser (redressing tool), significantly extend the life of the grindstone, and achieve economical grinding. In order to achieve the above object, the present invention provides a method for manufacturing a grinding wheel in which CBN abrasive grains are bonded with a vitrified bond, the particle size is 2 to 4 times the abrasive grain size, and the specific gravity is around 1.0 to 1.4. 3 to 12 parts by weight of organic particles having a water absorption rate of 0.1% or less per 24 hours are mixed with 100 parts by weight of abrasive grains, pressure molded, dried, and then fired. The details are described below. First, the particle size of the CBN abrasive grains used in this invention is not particularly limited, but is determined from 80/100 to 270/ 325 (ANSI National Standards Institute) as appropriate. Next, the organic particles of this invention are those that are burned away during grindstone firing and form pores in the structure of the grindstone, and are those that can achieve a large effect with a small amount (weight) of use, that is, those that have a specific gravity. 1.0 to 1.4, with little volume change due to pressure or moisture absorption during grindstone forming, that is, elasticity is small (for example, elastic modulus 200 to 300 Kgf/mm ² ), water absorption rate (24 hours) is 0.1% or less, and It is desirable to use a material that does not generate toxic gas during firing, and examples thereof include synthetic resins such as styrene resin, polyester resin, modified polyphenylene oxide, and epoxy resin. In general, plastics having a polar group in their structure have a large water absorption rate, and therefore the elastic modulus decreases greatly due to water absorption, making them unsuitable. Furthermore, although wood materials are no exception in that they have a low specific gravity, they are undesirable because they have high elasticity and water absorption, which can cause cracks to occur after compression molding.
However, even with such wood materials, if these problematic properties are improved through resin processing, etc.
It can be a material that is durable enough to withstand use. The size of such organic particles is preferably 2 to 4 times the size of the abrasive grains used, and the amount added is preferably 3 to 12 parts by weight per 100 parts by weight of the abrasive grains. This is because when making a whetstone using organic particles that are larger than the abrasive grain size, particles and blending ratios that are outside the above conditions will shrink significantly during firing and may deform or crack during firing. This is because not only difficult problems arise in manufacturing, but also the strength of the product is low, which impedes safety during rotational operation. Organic particles larger than the abrasive grain size play the role of aggregate before firing the whetstone, so they are easily deformed or worn out when mixed with other materials. In addition, in order to make the grinding wheel structure homogeneous, the shape of the particles is spherical, ellipsoidal, granular, or similar, and the ratio of the major axis to the minor axis or the diameter to the height is close to 1. A value of, for example, 1 to 1.2 is preferred. In addition to the CBN abrasive grains and organic particles described above, vitrified bond and a binding agent for molding (for example, dextrin powder or aqueous solution)
After thoroughly stirring and mixing, press-forming into any desired shape and drying, it is heated to 1200 to 1300 in a nitriding atmosphere.
Bake for approximately 1.5-2.0 hours at °C. The reason for firing in a nitrogen atmosphere is that CBN abrasive grains begin to oxidize gradually at a high temperature of approximately 800°C or higher, and then
This is to prevent CBN abrasive grains from losing their original height due to becoming boron oxide at high temperatures above ℃. Therefore, the organic particles contained in the grindstone are
It is necessary to completely burn out without affecting the bonding force between the abrasive grains and the bond at a temperature of around 400 to 600℃ in a slightly oxidized or neutral atmosphere. 1200〜
It goes without saying that melting must proceed sufficiently at a high temperature of 1300° C. and a sufficiently satisfactory bonding force must be produced. According to the manufacturing method of such a porous vitrified CBN grindstone, the water absorption rate of the organic particles dispersed and held together with the abrasive grains in the vitrified bond is low, and therefore the rate of increase in volume and decrease in elasticity due to swelling is low. Since it is extremely small and its specific gravity is favorable for efficient dispersion, during pressure molding, the organic particles are uniformly dispersed and maintain the particle size at the time of blending. After pressure molding, the above-mentioned organic particles are more evenly distributed under a high density environment in which excess moisture, air, etc. are removed from the inside of the molded product, and then these ideal organic particles are Drying and firing are performed while maintaining the dispersed state of the particles. Also,
The organic particles that are burnt away during firing have low water absorption and do not release moisture into the surrounding area.Therefore, there is no problem of water vapor rapidly dissipating from the pores and causing cracks around the pores. . In the present invention, the unit pores themselves formed in this way act as chip pockets to prevent clogging, and have low grinding resistance and excellent grinding performance. At the same time, the wear of the grinding wheel (abrasive grains) is extremely low, so it is better than conventional products. You can obtain a highly economical whetstone like no other. Examples are shown below. [Example] Compared to 140/170 mesh CBN abrasive grains, the size of the styrene resin particles is approximately twice that of the standard sieve size specified in JIS-Z8801-1966 (passing through a 210μ sieve). and remains on the 177μ sieve.
Abbreviated as 177/210. (same below), approximately 3 times (297/
350) and about 4 times (420/500),
As shown in Table 1, five types of sample grindstones with different particle sizes and mixed amounts (parts by weight relative to 100 parts by weight of CBN abrasive grains) and one control product containing no styrene resin particles were prepared. The sample grindstone is made by thoroughly stirring and mixing the specified materials of CBN abrasive grains, styrene resin grains, vitrified bond, and dextrin.
Bulk specific gravity of a raw whetstone after firing, such that the values (%) obtained by subtracting the volumes of abrasive grains, bonds, and pores by the volume of the whetstone are abrasive grain ratio of 37.5, bond ratio of 25.0, and porosity of 37.5, respectively. Calculate in advance,
Based on this bulk specific gravity, each compounding ratio and molding pressure were determined, and a green grindstone with an outer diameter of 25 mm, an inner diameter of 10 mm, and a height of 30 mm was molded. After forming and drying these green grindstones, the product temperature is maintained from 600℃ during heating to a maximum temperature of 1250℃ using an electric furnace under a nitrogen gas atmosphere, and the time required for it to drop to 600℃ during cooling. The grinding wheel was fired under conditions such that the grinding time was 48 hours, and the RM hardness, grinding ratio, grinding cycle, etc. were measured, and the results are shown in Table 1.

It is also listed in [Table]. In addition, each measurement method is as follows. That is, RM hardness: A steel ball with a diameter of 1/4 inch is used on the Rockwell hardness tester, and the load is 100 kg. Grinding ratio and grinding cycle: Bearing steel (SUJ2 type,
Grind the inner surface (inner diameter 35 mm, width 25 mm) of 60/62 on the Rockwell hardness C scale. The main grinding conditions in this case were a grinding wheel rotation speed of 29,000 times per minute, a workpiece rotation speed of 770 times per minute, a table oscillation amount of 20 mm, an oscillation frequency of 100 cycles per minute, and a front fork gauging device that directly machined the inner diameter. Grinding was carried out at a constant cutting pressure while measuring the change in . Therefore, the grinding cycle (seconds) will change depending on the difference in the grinding performance of the sample grindstone for a workpiece prepared so that the grinding allowance is 0.16 mm in diameter in the pre-processing dimensions, and the grinding cycle (seconds) will change depending on the difference in the grinding performance of the sample grindstone. ,
The grinding cycle becomes shorter, and on the other hand, when the grindability is poor, a long time is required. On the other hand, for dressing, the conditions of the grinding surface of the grindstone were kept constant using a diamond dresser. After dressing under the above grinding conditions, a grinding test was performed on 30 workpieces in succession for each grinding wheel, and the amount of grinding was calculated using the cumulative volume (mm ³ ) of the workpiece removed by grinding. The value obtained by dividing the amount by the grinding wheel wear amount (mm ³ ) is the cutting ratio. It is. As shown in Table 1, in the grinding wheel of sample 1-1, the size of the styrene resin grains is small, about twice the size of the CBN abrasive grains, and the amount added is small at 5%, although the grinding ratio is high. , inferior in grindability,
Grinding cycles are getting longer. On the contrary, sample 3-2
The size is about 4 times larger, and the amount added is 11%.
Products that use a large number of styrene resin particles have a lower grinding ratio, but the grinding cycle is significantly shorter. In short, the size of the styrene resin particles is
The most desirable results were obtained even with 2 to 4 times the CBN abrasive grains, and when a small amount of resin grains less than 2 times the size was added, less than 5%, the performance approximated that of the control wheel shown in Table 1. Although the grinding ratio increases, the grindability is poor, the grinding cycle becomes longer, and the effect of addition is not significant. On the other hand, when adding more than 11% of the resin grains, which are more than 4 times larger, the grindability improves and the grinding cycle is shortened, but the amount of grinding wheel wear increases and the grinding ratio becomes low. It became clear that this was not a desirable result. As explained above, this invention adds organic particles with a predetermined particle size, specific gravity, and water absorption rate to abrasive grains at a predetermined blending ratio, and by firing the organic particles, the organic particles are evenly distributed within a vitrified CBN grindstone. The manufactured grindstone can perform highly efficient and precise grinding with a small amount of grindstone wear, and is less prone to clogging. Therefore, it is possible to produce a high-performance porous bitrified CBN grinding wheel that requires less dressing, has a long life, can achieve economical grinding, and is also capable of cool gliding, and this invention has industrial utility value. can be said to be high.

Claims (1)

[Claims] 1 In a method for manufacturing a grinding wheel that combines cubic boron nitride abrasive grains with a vitrified bond,
3 to 12 parts by weight of organic particles with a particle diameter of ~4 times as large, a specific gravity of around 1.0 to 1.4, and a water absorption rate of 0.1% or less per 24 hours are mixed with 100 parts by weight of abrasive grains, and then pressure molded. A method for producing a porous vitrified boron nitride grindstone, which comprises drying the grindstone and then firing it.