VITRIFIED BONDED GRINDING STONE

Abstract

The vitrified bonded grinding stone includes a vitrified bond in which super abrasives such as cubic crystal boron nitride grains or diamond grains and an aggregate are contained. The aggregate is formed by porous ceramics and a substance which closes pores of the porous ceramics and the substance is weaker in strength than the porous ceramics and has a physical property that the substance does not dissolve under a burning temperature of the vitrified bond.

Description

INCORPORATION BY REFERENCE

This application is based on and claims priority under 35 U.S.C. 119 with respect to Japanese Application No. 2012-157172 filed on Jul. 13, 2012, the entire content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a vitrified bonded grinding stone which has a low grinding resistance by mixing aggregate which has strength lower than the abrasive grain with the grinding stone.

2. Description of Related Art

As an example of the related arts, a grinding stone which has a large abrasive grain distance between the grains by mixing aggregate which has strength lower than the abrasive grain with the binder in order to decrease the grinding resistance (in other words, reduced the degree of concentration of the abrasive grains) has been known. As an example of the aggregate disclosed in the Patent Document 1, porous ceramics are used as the aggregate which is easy to be fractured and difficult to be abraded with a ground material or material to be ground.

DOCUMENT LIST OF RELATED ART

Patent Document

Patent Document 1: JP2009-83036 A

DISCLOSURE OF INVENTION

However, according to the above grinding stone disclosed in the Patent Document 1, at the time of burning the binder, a melted binder may be invaded into the pores of the porous ceramics depending on the type of the binder which binds the abrasive grains and the aggregate. This may lead to the hardening of the binder invaded into the pores of the porous ceramics due to cooling and further, the porous ceramics are becoming difficult to be fractured and abrasion with the material to be ground occurs which leads to the burning due to the grinding and may result in the interference of originally aimed reducing of the grinding resistance.

This invention was made considering the above issues of the related art and it is an object of the invention to provide a vitrified bonded grinding stone which can prevent the vitrified bond from the invasion into the pores of the porous ceramics and further can advance the fracturing of the porous ceramics by closing the pores of the porous ceramics by a substance which is weaker than the porous ceramics and yet which does not dissolve even under the burning temperature for burning the vitrified bond.

Means to Solve the Problems

The vitrified bonded grinding stone associated with the invention according to claim 1 is characterized in that a vitrified bonded grinding stone includes a vitrified bond in which super abrasives such as cubic crystal boron nitride grains or diamond grains and an aggregate are contained and the aggregate is formed by porous ceramics, and a substance which closes pores of the porous ceramics and wherein the strength of the substance is weaker than that of the porous ceramics and the substance has a physical property that the substance does not dissolve under the burning temperature of the vitrified bond.

Further, according to the invention associated with claim 2, in claim 1, the substance is filled in the pores of the porous ceramics to close the pores thereof.

Effects of the Invention

According to the invention associated with claim 1, since the aggregate is mixed into the vitrified bonded grinding stone, a greater grain distance between the grains of the super abrasives can be obtained thereby to reduce the grinding resistance. Thus, a longer duration of life of the grinding stone can be expected. Further, according to the invention, the aggregate of the grinding stone is formed by porous ceramics which are easily fractured when brought into contact with the material to be ground and the substance which is formed by a material, strength of which is weaker than the porous ceramics for closing the pores of the porous ceramics. Accordingly, a heat generated upon abrasion between the aggregate and the material to be ground can be suppressed and grinding burning of the material to be ground can be prevented. At the same time, even the vitrified bond is melted during the manufacturing of the grinding stone, such molten vitrified bond would not enter into the pores of the porous ceramics and the fracturing property of the porous ceramics cannot be influenced thereby.

According to the invention associated with claim 2, the pores of the porous ceramics are closed by the substance by filling the pores of the ceramics with the substance. Thus, the invasion of the vitrified bond into the pores of the porous ceramics can be easily prevented.

BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS

Various aspects of this invention will become more apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings, in which:

FIG. 1 is an overall view of the vitrified bond grinding stone according to an embodiment of the invention;

FIG. 2 is an enlarged view of the grinding stone layer of the vitrified bonded grinding stone; and

FIG. 3 is an enlarged view of grinding stone layer of the grinding stone upon grinding process.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

The embodiments of the invention will be explained hereinafter with reference to the attached drawings. FIG. 1 shows an overall view of the vitrified bonded grinding stone and FIG. 2 is an enlarged view of the structure of the vitrified bonded grinding stone at grinding surface area thereof.

As shown in FIG. 1, the vitrified bonded grinding stone 10 is formed by a disc shaped core 21 and a ring shaped grinding stone layer 22 fixed to an outer peripheral surface of the core 21 by bonding agent or by sintering. The core 21 is formed by a metal material, such as for example, copper, aluminum or titan, FRP (fiber reinforced plastic) material or a ceramics (normal grinding stone). The grinding stone layer 22 is formed by fixing the ring shaped burnt grinding stone layer onto the outer peripheral surface of the core 21. Alternately, the grinding stone layer 22 may be formed by adhering a plurality of grinding stone segments to the outer peripheral surface of the core 21 arranged to be a ring shaped profile.

A central bore 23 is provided at the center of the core 21 and is engaged with a boss for centering projecting to an axial end of the grinding stone shaft of a grinding stone base (not shown). A plurality of bolt bores 24 is provided around the central bore 23 and the corresponding number of bolts is inserted onto the bolt bores 24. The bolts are threaded into screwed bores which are open to the axial end of the grinding stone shaft. The vitrified bonded grinding stone 10 is fixed to the grinding stone shaft by inserting the bolts into the bolt bores 24 and threading the bolts with the screwed bores.

The grinding stone base and table are provided on a grinding machine (not shown) and are slidably guided in a mutually perpendicularly intersecting with each other. The vitrified bonded grinding stone 10 is to be installed into the grinding machine. The grinding stone shaft is rotatably mounted on the grinding stone base in an axial line which is in parallel with a longitudinal direction of a material to be ground (work W) which is to be ground by the vitrified bonded grinding stone 10. The grinding stone shaft is rotatably driven by a motor provided on the grinding stone base. A main shaft base and a tail stock are also provided on the grinding stone base and between the main shaft base and the tail stock, the material to be ground (work W) is rotatably supported in an axial line which is in parallel with the moving direction of the table.

As shown in FIG. 2, the grinding stone layer 22 is formed by a super abrasives 12 composed by CBN (cubic boron nitride) grains, an aggregate 14 the strength of which is weaker than that of the super abrasives 12 and a vitrified bon 16 which connects the super abrasives 12 and the aggregate 14. The aggregate 14 is formed by a base material of porous ceramics 14a and a substance 14b which closes the openings of the pores of the porous ceramics 14a to prevent invasion of the vitrified bond 16 into the pores of the porous ceramics 14a. as the substance 14b which closes the pores of the porous ceramics 14a, a substance, such as for example, a glassy carbon, which is weaker than the porous ceramics 14a and which does not dissolve at the burning temperature for burning the vitrified bond 16 is used.

The aggregate 14 can be easily obtained by filling the substance 14b in the pores of the porous ceramics 14a under the vacuum atmosphere after manufacturing the porous ceramics. Alternatively, upon manufacturing the aggregate 14, the porous ceramics 14a is formed by burning by mixing the porous ceramics 14a with the substance 14b to close the opening of the pores of the porous ceramics 14a.

The vitrified bond 16 connects the neighboring super abrasives 12, neighboring aggregates 14 and neighboring super abrasives 12 and aggregates 14 by bridging therebetween. Thus an air hole 18 is formed between the bridging portions 20. The porosity of the porous ceramics 14a is for example usually between 10% and 80% and preferably between 30% and 60% and in this range fracturing upon grinding can be effectively generated and maintain the strength which can maintain the structure of the grinding stone. The average grain diameter of the super abrasives 12 formed by CBN is for example, 115 μm (#170) and the average grain diameter of the porous ceramics 14a is, for example, 100 μm (#200). In this case, the average grain diameter of the porous ceramics is about 87% of the average grain diameter of the super abrasives 12. Thus, it is experimentally confirmed that the structural strength of the grinding stone as the aggregate can be maintained by setting the grain diameter of the porous ceramics as the aggregate relative to the grain diameter of the super abrasives 12 to be in the range between 70% and 150%. It is considered that the porous ceramics 14a does not weaken the bridging portions 20 formed by the vitrified bond 16. Instead of using CBS, a diamond grinding grains can be used.

Next, the manufacturing method of the vitrified bonded grinding stone 10 according to the embodiment of the invention will be explained hereinafter. First, the grinding stone layer 22 is formed by the CBN grains. In this case, the super abrasives 12 of the CBN, the porous ceramics 14a filled with the substance 14b such as glassy carbon (which is weaker in strength than the porous ceramics 14a and which does not dissolve under the burning temperature of the vitrified bond 16) and the vitrified bond 16 are mixed with a predetermined proportion which is defined in advance and kneaded.

For example, the amount of the porous ceramics 14a to be used is equal to or less than 50% of the entire volume of the grinding stone layer. If the amount of vitrified bond 16 in volume relative to the amounts Of the super abrasives 12 of CBN and the porous ceramics 14a is excessively used, the bridging of the binder (bridging portion 20) between the neighboring porous ceramics 14a, between the porous ceramics 14a and neighboring super abrasives 12 is difficult to be formed and on the other hand, if the amount of porous ceramics 14a relative to the vitrified bond 16 in volume is too small, the concentration degree of the super abrasives 12 increases, and accordingly the grinding resistance become big. Therefore, the proportion among the composites is defined in advance. Thus mixed mixture is filled in the formwork forming the space corresponding to the ring shaped grinding stone layer 22 and formed by press.

The ring shaped grinding stone layer 22 which has been press-formed is then drawn from the formwork and burnt under the appropriate burning temperature of the vitrified bond 16 to manufacture the ring shaped grinding stone layer 22. At this time, the vitrified bond 16 forms the bridging portions 20 and the air holes 18 between the neighboring super abrasives 12 due to the melting during the burning process. However, since the pores of the porous ceramics 14a are filled with the substance 14b such as glassy carbon which does not dissolve under the burning temperature of the vitrified bond 16, the invasion of the molten vitrified bond 16 into the pores of the porous ceramics 14a can be prevented. Thereafter, thus burnt formed grinding stone layer 22 is fixed to the outer peripheral surface of the core 21 by bonding agent to complete the vitrified bonded grinding stone 10.

Thus manufactured vitrified bonded grinding stone 10 has the grain diameter of the porous ceramics 14a as the aggregate is approximately equal to the grain diameter of the super abrasives 12 and accordingly, the porous ceramics 14a becomes the core of the mesh which the vitrified bond 16 forms, and effectively forms the bridging portions 20 between the neighboring porous ceramics 14a or the neighboring super abrasives and the porous ceramics 14a. This can strengthen the structure of the vitrified bonded grinding stone 10 and at the same time prevent the super abrasives 12 from dropping off by abrasion and elongates the duration of life.

Next, the operation at the grinding machining using the vitrified bonded grinding stone 10 according to the embodiment of the invention will be explained hereinafter. First the thus manufactured vitrified bonded grinding stone 10 is fixed to the grinding stone shaft of the grinding stone base then the grinding stone shaft is driven by the motor to rotate the vitrified bonded grinding stone 10. Then the work W to be ground supported between the main shaft base and the tail stock (not shown) is rotated in an axial line direction by rotating the main shaft of the main shaft base. Thus, the grinding stone base is advanced in a direction perpendicular to the axial line of the work W to grind the work W.

The position of the aggregate 14 and the position of the outer peripheral surface of the grinding stone (projecting position of the super abrasives 12) of the grinding stone layer 22 of the vitrified bonded grinding stone 10 are approximately the same as shown in FIG. 2, before the grinding process begins. However, since the aggregate 14 is formed by a porous ceramics 14a which material is weak and is formed with many pores, the aggregate 14 is fractured by the contact with the surface of the work W when grinding operation and as shown in FIG. 3, and retreats from the front end position of the super abrasives 12 which oppose to the work S and serve as the cutting blade. It is noted that the glassy carbon substance 14b is filled in the pores of the porous ceramics 14a. however, since the substance 14b is weaker than the ceramics 14a in strength and accordingly, the fracturing property of the porous ceramics 14a will not be influenced by the grinding operation.

According to the embodiment, since the aggregate 14 is mixed into the grinding stone layer 22 of the vitrified bonded grinding stone 10, the distance between the grains of the super abrasives 12 can be widened to reduce the grinding resistance. Further, since the aggregate 14 is structured by the porous ceramics 14a which is easy to be fractured by the contact with the material (work W) to be ground, the porous ceramics 14a is fractured when the ceramics 14a is brought into contact with the work W upon grinding and further the fractured ceramics retreats from the grain position, which is the cutting blade position, to prevent heat generated when the aggregate 14 and the work W are abraded each other. This can further prevents grinding bum. Still further, the grinding efficiency can be improved by forming a chip pocket by the fractured ceramics, in which cutting waste is kept and wasted, and by accelerating circulation of cooling liquid.

Further, since the substance 14b which strength is weaker than the ceramics 14a is filled in the pores of the porous ceramics 14a as the aggregate 14, the melted vitrified bond 16 melted upon manufacturing the grinding stone would not invade into the pores the porous ceramics 14a. Accordingly, the fracturing performance of the porous ceramics 14a cannot be impaired by the vitrified bond 16 which normally invaded into the pores of the porous ceramics 14a and hardens therein after burning.

According to the embodiment of the invention, as the substance, which is to be filled in the pores of the porous ceramics 14a, glassy carbon is exampled. However, any substance may be used as long as the strength of the substance is weaker than the porous ceramics 14a and the substance does not dissolve under the burning temperature of the vitrified bond 16.

The invention is explained with the above embodiments, however, the invention is not limited to the embodiments explained above, but various other embodiments or modifications are within the scope of the invention as far as such are not deviated from the subject matter described in the claims.

Claims

1. A vitrified bonded grinding stone comprising:

a vitrified bond in which super abrasives such as cubic crystal boron nitride grains or diamond grains and an aggregate are contained, wherein

the aggregate is formed by porous ceramics and a substance which closes pores of the porous ceramics;

the substance is weaker in strength than the porous ceramics and has a physical property that the substance does not dissolve under a burning temperature of the vitrified bond.

2. The vitrified bonded grinding stone according to claim 1, wherein

the substance is filled in the pores of the porous ceramics to close the pores thereof.