JPS59161269A - Porous vitrified boron nitrified grindstone - Google Patents

Abstract

PURPOSE:To form a porous vitrified boron nitrified grindstone by mixing organic particles of a diameter 2-4 times as large as a grinding particle diameter in a ratio of 3-12pt.wt. against 100pt.wt. of the grinding particle for calcinating after formation. CONSTITUTION:Vitrified bond and caking agent for formation are mixed and stirred with cubic cristalized boron nitrified grinding particles and organic particles sufficiently, and after shaping by pressure into an optional grindstone shape and drying, they are calcinated for 1.5-2.0hr in a nitrogen atmosphere about at 1,200-1,300 deg.C. The porous vitrified boron nitrified grindstone obtained in this method is a vitrified cubic cristalized boron nitrified grindstone with larger unit air holes than the grinding particle diameter inside, and the unit air holes themseleve are made difficult to be choked as tip pockets to make the grindstone excellent for abrasion with low abrasive resistance. At the same time, little abrasion permits to make the grindstone highly economical.

Description

[Detailed Description of the Invention] This invention is difficult to cause clogging even when cylinder efficiency grinding is performed under a small amount of grindstone wear, enables cool grinding, and facilitates dressing. The present invention relates to a porous vitrified boron nitride grindstone that aims to improve the wear of a dresser (redressing tool), significantly extend the life of the grindstone, and achieve economical grinding.

Conventionally, from a practical standpoint, vitrified nitrided material having a normal structure, as already proposed by the applicant of the present invention in Japanese Patent Publication No. 57-49351, has been used for internal grinding using a small-diameter grindstone with a grindstone diameter of about 15 m+n or less. Satisfactory grinding is possible using a boron grindstone, but the grindstone diameter is 25mm.
When grinding the inner surface of a through-hole or arcuate groove surface using a large-diameter grindstone that exceeds mm, or grinding the outer ring groove of a constant-velocity ball joint, it is necessary to increase the contact arc or contact surface between the grindstone and the workpiece. Accordingly, if we try to perform economical grinding with minimal wear on the grindstone, clogging is likely to occur, and welding of metal chips will occur on the surface of the grindstone. on the other hand,
Conventional porous whetstones that use general abrasive grains such as alumina and silicon carbide cannot be used to cut difficult-to-cut tool steels such as carbon tool steel, cylinder speed steel, and alloy tool steel, or difficult-to-cut materials such as aluminum alloys and sintered alloys. Targeting materials, the grinding wheel bonding degree is alphabetical hardness (J15-R6210-1980), and even if it is hard, it is less than J and uses a soft grinding wheel, placing emphasis on grinding performance at the expense of grinding wheel wear. Yes, and the grinding ratio is small at less than 5%. Therefore, if a porous grinding wheel is made using the conventional technology using expensive cubic boron nitride abrasive grains (hereinafter referred to as CBN abrasive grains), economical grinding becomes impossible. It loses all usefulness.

In addition, the vitrified CBN grinding wheel is not suitable for use because the abrasive grains tend to deteriorate at high temperatures of about 1000°C or higher, or because the coefficient of expansion of the abrasive grains is as low as 3.5 x 10-6 cm/cm 7°C. There are various problems such as there are limits to selecting a pound that is compatible with the bonding force and has excellent bonding strength.
By overcoming these problems, the
Like No. 1, compared to the Regimade Bond CBN grindstone, the binder has excellent abrasive grain support, is more rigid, and
Vitrified CB, where the grinding wheel structure is more porous
N grinding wheels have been developed, and such grinding wheels are on the verge of being widely used in the field of precision grinding.

By promoting these characteristics of the vitrified CBN grinding wheel, if we try to achieve high economic efficiency by increasing the amount of binder in order to further reduce the amount of wear on the grinding wheel, the grinding wheel will become severely clogged. The reality is that as the frequency of dressing increases, wear on the grinding wheel and wear on the dresser due to dressing increases significantly, making it impossible to pursue 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, without increasing grinding wheel wear,
Attempts have been made to improve the clogging of the grinding wheel, automatically create a void near the grinding point at the tip of the abrasive grain during grinding, facilitate the discharge of grinding debris, and improve grindability. Among them are the following, for example: That is, ■ Glassy or ceramic C'BN grinding wheels are softer and more abrasive than alumina abrasive grains (aggregates such as zircon or mullite, which are mainly composed of crystals with a modified Mohs hardness of 11 or less and a melting point of 900°C or more). (Special Publication No. 52-27394).

(1) A fine-grained grindstone using a thermosetting resin as a binder, in which organic or inorganic micro hollow bodies act as pores, which are chip pockets of the grinding wheel (Japanese Patent Application Laid-open No. 137887/1987).

■ Graphite, hexagonal boron nitride, and disulfide are added to the ceramic matrix for the purpose of low grinding resistance and high grinding ratio performance in dry grinding of cemented carbide tools and bird speed steel without using liquid lubricants or coolants. Diacene or CBN whetstone containing an appropriate amount of molybdenum, etc. (Special Publication No. 5717/1983).

etc. All of these grindstones contain effective ingredients to reduce the grinding resistance at the grinding point during grinding, to form chip pockets that 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 grindstone.

This invention was made with attention to such a current situation, and in a grindstone in which CBN abrasive grains are bonded with vitrified hont, organic particles having a particle size 2 to 4 times the abrasive grain size are combined into 100% abrasive grains. The object of the present invention is to provide a porous vitrified boron nitride grindstone characterized in that 3 to 12 parts by weight are blended, molded and then fired. The details are described below.

First, the particle size of the CBN abrasive grains used in this invention is as follows:
Although not particularly limited, from the viewpoint of practical grindability, surface roughness, and the size of organic particles described below,
100 to 270/325 mesh (ANSI National Standards Institute) may be selected as appropriate.

Next, the organic particles of this invention are particles that are burned away during grindstone firing and form pores in one structure of the grindstone, and can achieve a large effect with a small amount (weight) of use, that is, those with specific gravity. is around 1.0 to 1.4, and there is little volume change due to pressure or moisture absorption during grindstone forming, that is, elasticity is small and water absorption rate (24 hours) is 0.1% or less, and there is no change during firing. It is desirable to use a material that does not generate poisonous gas, and examples thereof include synthetic resins such as styrene resin, polyester resin, modified polyphenylene oxide, and epoxy resin. Although wood materials are no exception in terms of low specific gravity, they are not preferred because they have high elasticity and water absorption, which can cause cracks to occur after compression molding. However, even such a wood material can be made into a material that can be used satisfactorily if these problematic properties are improved through resin processing or the like.

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 outside the above conditions will shrink during firing, causing deformation or cracking during firing. 1
Not only will this cause difficult manufacturing problems, but the strength of the product will also be low, which may impede safety during rotating operations.

It is et al. Organic particles larger than the abrasive grain size are
Since the whetstone plays the role of aggregate before firing, it does not easily deform or wear out when mixed with other materials, and the shape of the particles does not match the whetstone structure. In order to make it homogeneous, it has a spherical, ellipsoidal, granular, granular or similar shape, and the ratio of the major axis to the minor axis or the diameter to the height is close to 1, for example 1 to 1.
．． 2 is preferable.

Vitrified bond and a binding agent for molding (for example, dextrin powder or aqueous solution) were thoroughly stirred and mixed with the CBN abrasive grains and organic particles described above, and the mixture was pressure molded into an arbitrary grindstone shape and dried. Thereafter, this is fired at 1200 to 1300° C. for about 1.5 to 2.0 hours under a nitrogen atmosphere. The reason for firing in a nitrogen atmosphere is that C
This is to prevent the BN abrasive grains from gradually oxidizing at around a high temperature of about 800° C. or higher and further becoming boron oxide at a high temperature of 1000° C. or higher and losing the original hardness of the CBN abrasive grains. Therefore, the organic particles contained in the whetstone are approximately 400 to 60% in a slightly oxidized or neutral atmosphere.
It is necessary for the vitrified bond to be completely burnt out without affecting the bond strength between the abrasive grains and the bond at a temperature around 0°C.
Needless to say, it is necessary that the melting proceeds sufficiently at a cylinder temperature of 0.5 C and that a sufficiently satisfactory bonding force is generated.

The porous boron nitride grindstone of the present invention obtained in this way is a Bl-IJ Fido CBN grindstone that has unit pores larger than the abrasive grain diameter in the product grindstone, so the unit pores themselves can be seen as chip pockets. To cause a blockage
It has low grinding resistance and excellent grinding performance, and at the same time has very little wear on the grindstone (abrasive grains), so it can be said that it is an economical grindstone that is unparalleled in conventional products.

Examples are shown below.

〔Example〕

Compared to 140/170 mesh CBN abrasive grains, the size of styrene resin grains is JIS-Z8801-1.
Based on the nominal size of the standard sieve specified in 966, approximately 2
(Things that pass through a 210μ sieve and remain on a 177μ sieve,
It is abbreviated as 177/210. (same below), approximately 3 times (29
7/350) and about 4 times (4201500) were selected, and as shown in Table 1, 5 groups with different particle sizes and mixed amounts (parts by weight relative to 100 parts by weight of CBN abrasive grains) were selected.
A variety of sample grindstones and one control product containing no styrene resin particles were prepared. The sample grindstone contains a predetermined amount of CBN abrasive grains,
The styrene resin particles, vitrified bond, and dextrin were thoroughly stirred and mixed, and the values (%) obtained by dividing the volumes of the abrasive grains, honto, and pores by the volume of the whetstone were 37.5 and 37.5, respectively. The bulk specific gravity of a green grindstone with a pound ratio of 25.0 and a porosity of 37.5 is calculated in advance, and each blending ratio and molding pressure are determined based on this bulk specific gravity, and an outer diameter of 25 mm is determined. , inner diameter I Q I
A green grindstone with Tlm and height of 3 Q mm was molded. After forming and drying these green grindstones, the product temperature is maintained from 600°C during heating to a maximum temperature of 1250°C in an electric furnace under a nitrogen gas atmosphere, and the time required for it to drop to 600°C during cooling is Baked under conditions such that 48 hours,
Regarding the obtained whetstone, RM hardness, grinding No. 1
The surface ratio, grinding cycle, etc. were measured, and the results are also listed in Table 1. In addition, each measurement method is as follows. That is, RM hardness: In a rock hardness tester, a steel ball with a diameter of 1/4 inch is used, and the load is 100 kg.

Grinding ratio and grinding cycle: Using a water-soluble grinding oil (50 times diluted) with a Toyo Kogyo internal grinding machine, the inner surface (inner diameter 35 mm) of bearing steel (SUJ class 2, 60762 on the Rockwell hardness C scale) was
, width 25 mm). The main grinding conditions at this time are:
Grinding wheel rotation speed is 29,000 times per minute, workpiece rotation speed is 7 per minute.
Grinding was carried out at a constant pressure depth of cut, with a table oscillation rate of 2 Q mm, an oscillation frequency of 100 cycles per minute, and a front fork gauging device to directly measure changes in the processed inner diameter. Therefore, the grinding cycle (
A grinding wheel with good grinding performance (seconds) changes, and the grinding cycle is short, whereas a grinding wheel with poor grinding performance requires a long time. On the other hand, the dressing is Diamond F'):
The conditions of the grinding surface of the whetstone were made constant using the reducer. After dressing under the above grinding conditions, a grinding test was performed on 30 workpieces in succession for each grinding wheel, and the grinding amount was determined by the cumulative volume (mm”) of the hard material removed by grinding. The value obtained by dividing the value by the grinding wheel wear tt (n1m3) is the grinding ratio.

It is.

As shown in Table 1, the size of the styrene resin particles C
With the grinding wheel of sample 1-1, which is small at about twice the size of the BN abrasive grains and has a small additive amount of 5%, the grinding ratio is suppressed or the grinding performance is poor, and the grinding cycle becomes longer. There is. On the other hand, sample 3-2, which uses styrene resin particles that are about 4 times larger in size and has a larger additive amount of 11%,
Although the grinding ratio is lower, the grinding cycle is significantly shortened.

Essential: The most desirable results are obtained when the size of the styrene resin particles is 2 to 4 times the CBN abrasive particle size, and when the resin particles of less than 2 times the size are added in a small amount of 5%. , the grinding ratio is large, approximating the performance of the control wheel shown in Table 1, but the grindability is inferior, the grinding cycle is longer, and the addition effect is not significant. On the other hand, when the amount of resin grains with larger particles larger than 4 times is increased to more than 11%, the grindability improves and the grinding cycle is shortened, but the amount of wear on the grinding wheel increases and 1. It became clear that the grinding ratio became low and could not be said to be desirable.

Patent applicant: Mizuho (Representative of θF Grinding Stone Co., Ltd.)
sickle 1) sentence 2

Claims (1)

[Claims] In a grinding wheel in which cubic boron nitride abrasive grains are bonded by vitrified bond, 3 to 12 parts by weight of organic particles having a particle size 2 to 4 times the diameter of the abrasive grains are added to 100 parts by weight of the abrasive grains. A porous vitrified boron nitride grindstone characterized by being formed and then fired.