Abrasive tool making process – material – or composition – With inorganic material
Reexamination Certificate
2002-09-10
2004-03-09
Marcheschi, Michael (Department: 1755)
Abrasive tool making process, material, or composition
With inorganic material
C051S309000, C051S293000, C051S308000
Reexamination Certificate
active
06702867
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to vitrified bonded abrasive tools made with a high strength, low temperature bond, comprising phosphorous oxide and boron oxide in amounts sufficient to improve the performance of an abrasive tool containing sintered sol gel alumina abrasive grain. As a result of the bond selection, sintered sol gel alumina abrasive grain (or other thermally labile abrasive grain) may be used effectively, without loss of grinding performance in the abrasive tool.
The invention further includes a vitrified bond composition suitable for firing at relatively low temperatures such as 700-1,100° C., comprising at least two amorphous, immiscible, glass phases. Abrasive tools comprising superabrasives (diamond or cubic boron nitride (CBN)), or seeded or unseeded sintered sol gel alumina abrasive grain, also referred to microcrystalline alpha-alumina (MCA) abrasive grain, are known to provide superior grinding performance on a variety of materials. The manufacture and characteristics of these MCA grains and the performance of these MCA grains in various applications are described in, for example, U.S. Pat. Nos. 4,623,364, 4,314,827, 4,744,802, 4,898,597 and 4,543,107, the contents of which are hereby incorporated by reference.
Vitrified or glass bonded abrasive tools containing MCA grain and superabrasive grain are commercially useful for grinding precision metal parts and other industrial components requiring consistent and improved grinding performance. To produce these types of abrasive tools with consistent quality, reactions between glass bond components and the abrasive grain must be avoided. Reactivity is a particular problem at typical temperatures encountered during firing of the bond, e.g., 1100-1400° C. Controlling these reactions minimizes damage to the critical microcrystalline structure of the MCA grain, preserving the grain sharpness and performance.
To reduce the amount of reaction between MCA grain and vitrified bond, U.S. Pat. No. 4,543,107 discloses a bond composition suitable for firing at a temperature as low as about 900° C. In an alternate approach, U.S. Pat. No. 4,898,597 discloses a bond composition comprising at least 40% fritted materials suitable for firing at a temperature as low as about 900° C. However, in certain grinding applications these low temperature bonds have demonstrated insufficient mechanical strength to meet commercial objectives prompting development of stronger bonds.
Vitrified bonds characterized by improved mechanical strength have been disclosed for use with either conventional fused alumina oxide or MCA (also referred to as sintered sol gel alpha-alumina) abrasive grits in manufacturing grinding wheels having improved form holding properties. Such bonds are described in U.S. Pat. No. 5,203,886, U.S. Pat. No. 5,401,284 and U.S. Pat. No. 5,536,283, which are hereby incorporated by reference. These vitrified bonds may be fired at relatively low temperatures (e.g., about 900-1100° C.) to avoid reaction with high performance, sintered sol gel alpha-alumina abrasive grain. The wheels made with these bonds and MCA grain have shown excellent performance in finishing precision moving parts, particularly ferrous metal parts. Other vitrified bonds suitable for use with MCA abrasive grain may be fired at temperatures below about 875° C. These bonds are disclosed in U.S. Pat. No. 5,863,308, which is hereby incorporated by reference.
It has now been discovered that by selecting appropriate material components, improved high strength, tough bonds may be made and fired at about 700 to 1,100° C., preferably 750 to 950° C. In particular, by selecting appropriate contents of phosphorous oxide, boron oxide, silica, aluminum oxide, alkali oxides and alkaline earth oxides, and by maintaining the correct ratios of oxides, one may achieve a high strength, tough (e.g., resistant to crack propagation), low temperature bond. These bonds are characterized by a 25% or larger increase in modulus of rupture value relative to comparative bonds of the prior art. In certain embodiments, bonds comprising at least two amorphous, immiscible, glass phases may be used with MCA grain to yield greater mechanical strength. While the appropriate selection of raw materials having the desired oxide ratios upon firing can achieve a glass with immiscible phases, fritted glasses are preferred for this purpose. A fritted glass is a glass formed by firing initially to temperatures of at least 1,200° C., cooling, crushing and sizing to yield a powdered material (“a frit”). The frit then may be melted at a temperature well below the initial firing temperature used to make the glass from the raw materials, such as silica and clays.
When formulating an abrasive tool, such as an abrasive wheel or hone, the use of these vitrified bonds with superabrasive or MCA grain, yields abrasive tools having improved grinding performance with reduced power draw. When used to grind or finish a workpiece, these abrasive tools yield very acceptable workpiece surface finishes. These tools offer improvements over the low temperature fired, vitrified bonded superabrasive or MCA grain tools previously known in the art.
The invention is an abrasive tool comprising at least 1%, by volume, MCA abrasive grain and 3 to 30%, by volume, vitrified bond, wherein the vitrified bond comprises after firing of the abrasive tool, 40 to 60% SiO
2
, 10 to 18% Al
2
O
3
, 12 to 25% alkali oxides, 5 to 20% B
2
O
3
, and 1 to 8% P
2
O
5
, on a mole percent basis, and whereby the abrasive tool is characterized by at least a 30% increase in modulus of rupture relative to an comparable abrasive tool made with a vitrified bond comprising less than 1 mole % P
2
O
5
The commonly used hardness grades of abrasive tools containing MCA grain (e.g., K grade and harder on the Norton Company scale) are characterized by having a modulus of rupture of at least 6,000 psi when made according to the invention.
The alkali oxides of the bond are selected from the group consisting of sodium oxide, lithium oxide and potassium oxide.
The abrasive tool preferably comprises 5 to 25 volume % vitrified bond and 10 to 56 volume % MCA abrasive grain, and may comprise about 0.1 to about 60 volume % of additional components selected from the group consisting of secondary abrasive grains, fillers and adjuncts. The vitrified bond after firing may comprise alkaline earth oxides, and the molar ratio of SiO
2
to the combined contents of Na
2
O, alkali oxides other than Na
2
O and alkaline earth oxides is at least 1.2:1.0.
The invention further is an abrasive tool comprising at least 1%, by volume, MCA abrasive grain and 3 to 30%, by volume, of a vitrified bond, wherein the vitrified bond, during firing of the abrasive tool at about 700 to 1,100° C., comprises at least two immiscible phases, and whereby the abrasive tool is characterized by at least a 30% increase in modulus of rupture relative to an comparable abrasive tool having a single phase vitrified bond.
The vitrified bond having at least two immiscible phases preferably comprises a maximum of 12 mole % Al2O3.
Either bond may further comprise fluorine, TiO
2
, ZnO, ZrO
2
, CaO, MgO, CoO, MnO
2
, BaO, Bi
2
O
3
, and Fe
2
O
3
, and combinations thereof.
The invention also includes a method for making an abrasive tool comprising the steps of:
a) mixing about 70 to 95 weight % abrasive grain selected from the group consisting of MCA grain, silicon carbide grain, diamond grain, and cubic boron nitride grain, and mixtures thereof, and about 5 to 30 weight % bond mixture, wherein the vitrified bond comprises after firing of the abrasive tool, 40 to 60% SiO
2
, 10 to 18% Al
2
O
3
, 12 to 25% alkali oxides, 5 to 20% B
2
O
3
, and 1 to 8% P
2
O
5
, on a mole percent basis;
b) molding the mixture into a green composite; and
c) firing the green composite at a temperature in the range of 700 to 1,100° C. to form the abrasive tool; and whereby the abrasive tool is characterized by at least a 30% increase in modulus of rupture relative to an comparable abrasive tool made with a vitrifi
Carman Lee A.
Havens Irvin F.
King Wesley A.
Marcheschi Michael
Porter Mary E.
Saint-Gobain Abrasives Technology Company
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