Porous grinding stone and method of production thereof

Details Porous grinding stone and method of production thereof Porous grinding stone and method of production thereof

2000-07-31

2002-11-26

Marcheschi, Michael (Department: 1755)

Abrasive tool making process, material, or composition

With inorganic material

C051S309000, C051S293000, C051S296000

Reexamination Certificate

active

06485533

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a porous super-abrasive grinder or whetstone for use in the field of precision machining. More particularly, the present invention relates to a porous super-abrasive grinder that ensures highly efficient work and has superior strength, and a method of manufacturing the grinder.
BACKGROUND ART
Grinding (abrasive) particles of diamond and cubic boron nitride (hereinafter also referred to as “cBN”) are called “super-abrasive particles” because of having very high hardness, and are often used in precision grinding of steel, very hard metals, glass, ceramics, and stone materials. A super-abrasive grinder (hereinafter simply referred to as “grinder”) using such super-abrasive particles is generally manufactured by binding the super-abrasive particles together by a binder and molding them into a desired shape. Depending on types of binders used, there are a resin bond grinder using a synthetic resin, a vitrified bond grinder using a vitreous material, and a metal bond grinder using a metal. These grinders are selectively employed in accordance with characteristics of works to be ground. Recently, with an increased density of devices and more widespread use of those devices as represented by integrated circuits employing thin film processes, it has been required from the economical reason to precisely grind a work to such an extent that a width of grinding allowance for a substrate is, e.g., not larger than 0.3 mm. A thin-edge grinding wheel capable of achieving the above grinding has been demanded correspondingly.
Of the above grinders, the metal bond grinder is manufactured by putting metal powder including abrasive particles scattered uniformly therein into a mold together with a metal base, and subjecting it to pressing and sintering (or hot pressing) processes. The binder of metal used in the metal bond grinder uses, for example, a Cu—Sn system, a Cu—Sn—Co system, a Cu—Sn—Fe—Co system, a Cu—Sn—Ni system or a Cu—Sn—Fe—Ni system or any of these systems to which phosphorus is added. Such a conventional metal bond grinder has an extremely strong binding strength as compared with conventional resinoid and vitrified bond grinders, and is therefore advantageous in exerting a sufficient abrasive-particle retention force required to perform strong grinding by means of super-abrasive particles. In the metal bond grinder, however, the strength and stickness of the binder itself are so high that the binder is not worn during the grinding process. Even when abrasive particles are worn, the abrasive particles cannot fall from the binder. This means that the dressing interval must be shortened and highly sufficient grinding is impossible. Accordingly, the conventional metal bond grinder has the following disadvantages. Since discharging of chips is deteriorated and loading occurs easily, the grinding resistance increases and the grinding quality deteriorates, so that the heat generated is increased. Further, the grinder has a tendency to unsuccessfully finish the surface of a work. It is therefore very difficult to perform grinding with high efficiency by increasing the infeed or increasing the contact area of the grinder and the work. In addition, the metal bond is softened to cause plastic deformation upon grinding, and loading takes place in the surface of the grinder.
Heretofore, most of thin-edge grinders for use in the precision grinding have been metal bond grinders from the viewpoint of strength. The metal bond grinder is manufactured by the electro-forming or sintering method using, as a binder, a Ni- or bronze-base alloy. However, the structure of a binder phase is dense and a difficulty is encountered in dressing the metal bond grinder. An intricate and expensive technique and apparatus employing the electrolytic method, etc. have been therefore required. To activate a grinder, it is required to project an edge of super-abrasive particles from the surface of the binder phase. Generally, a grinder just after being formed has a condition where the super-abrasive particles and the binder phase are at the same level in the surface of the grinder. To project an edge of the super-abrasive particles from such a condition, a surface layer of the binder phase must be removed to a certain depth while leaving the super-abrasive particles. This operation is called “dressing”. If the surface layer of the binder phase is flat, it is very difficult to remove only the surface layer of the binder phase by a scraping or similar method, for example, while leaving the super-abrasive particles. This means the necessity of an intricate and expensive method, such as the electrolytic method, for ablating the surface layer of the binder phase.
On the other hand, a vitrified bond grinder is usually manufactured by molding a mixture of ceramic particles as a binder and super-abrasive particles, and sintering the molded mixture under pressure. Since a binder phase is porous and has a coarse structure, special dressing is not required. Also, since grinding chips generated during the grinding work are captured in pockets formed by pores and then discharged, loading does not easily occur. Further, even when an edge of the super-abrasive particles is worn, the binder phase is so coarse and brittle as to fall off in an appropriate manner. As a result, a new edge appears and glazing does not also easily occur. In the vitrified bond grinder, however, the binder phase is brittle and the bonding force between the binder and the super-abrasive particles is weak. Accordingly, the vitrified bond grinder cannot be formed into a grinder having a thin edge with a thickness of, for example, not greater than 0.3 mm, and the edge is easily susceptible to dulling. The vitrified bond grinder is therefore not economical when used to grind a difficult-to-grind work having high hardness under a strong pressure, because of serious wear.
In order to eliminate the above defects, a continuous porous metal bond grinder is proposed (Japanese Unexamined Patent Application Publication No. 59(1984)-182064). However, this metal bond grinder does not utilize the powder sintering method. More particularly, the Publication discloses a manufacturing method as follows. An inorganic compound that is melted by a solvent is sintered into a desired shape. Thereafter, voids in the sintered body are filled with abrasive particles and the sintered body having voids filled with abrasive particles is preheated. A melted metal or alloy is pressed into the voids of the sintered body filled with the abrasive particles and is then solidified. Subsequently, the inorganic compound is liquated out by a solvent. Thus, the disclosed method is to add, as filler, a pore forming agent and to form pores in a layer of the abrasive particles. Further, various measures for preventing a reduction in grinding quality have been proposed. In one example, many layers of metal coatings are formed on abrasive particles, and the coated abrasive particles are sintered by hot pressing so as to have a structure that is like a vitrified bond and includes pores formed therein (Japanese Examined Patent Application Publication No. 54(1979)-31727). Furthermore, a grinder using cast iron for the purpose of preventing loading of the grinder has been proposed (Japanese Unexamined Patent Application Publication No. 3(1991)-264263). The grinder using cast iron as a bond advantageously has great strength and high rigidity, enables heavy grinding to be performed at a high infeed, and is worn in the brittle fracture manner without the occurrence of plastic deformation, so that loading is less likely to occur. However, the bond of this grinder is too strong and accordingly the dressing property is deteriorated as compared with the bond of the copper system. Additionally, because of the high rigidity, it is difficult at the present to practically employ this grinder with the existing grinding machines and methods. By forming a large number of pores within the layer of the abrasive particles, a grinding liquid can be impregnated into the pores to enh

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