Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Post sintering operation
Reexamination Certificate
2000-10-03
2003-07-15
Jenkins, Daniel J. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Post sintering operation
C419S038000
Reexamination Certificate
active
06592809
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for producing powder metal gears, and more particularly, to a method for producing fully dense powder metal helical gears.
2. Description of the Background
The production of powder metal articles, including gears, is well-known in the art. One type of alloyed or unalloyed powder metal is selected or different types can be blended together as is known in the art, and additives also may be included with the powder or powder blend. The powder is disposed in a mold cavity which may be a simple cylindrical preform or may have the profile of the finished product. Next, pressure is applied to create the preform. The preform can then be removed and sintered to produce the part.
Apparatus for forming helical gears are also known in the art, wherein portions of the mold rotate when the preform is impacted to cause the preform to take the shape of the helical gear. In this apparatus, the preform has the shape of the actual helical gear to be produced, in contrast to first forming a cylindrical preform that is later transformed into a helical gear.
Another conventional apparatus for making a fully dense powder metal helical gear generally produces the helical gear by first molding a cylindrical preform and then by sintering the preform. The preform is then heated and placed in the forming mold where it is axially impacted to both impact the helical toothed shape and also to densify the gear. A disadvantage of this type of method can be that when the preform is impacted significant flashing can result as the preform is forced into the shape of the helical gear. Consequently, additional finishing processes can be required to deflash the gear before it is acceptable to a customer.
The above-described apparatus utilize mechanically created pressure to form the gear. However, it is also known to utilize isostatic pressure to form a helical powder metal gear. For example, one such method of manufacturing a helical gear from powder metal uses hot and cold isostatic pressure. This method employs a first mold to create a simple cylindrical preform having only the general geometry of the intended gear. A second mold is provided having the specific geometry of the gear and is slightly larger than the preform. The preform is placed inside the second mold, wherein additional powder metal is provided adjacent the preform to produce a second preform having a helical gear shape. Cold isostatic pressure is used to create both the simple preform and the helical gear preform. After the helical gear preform is made, hot isostatic pressure and/or sintering is employed to create the densified helical gear.
Isostatic pressure forming can generally involve placing a gear preform within a mold cavity having the specific geometry of the helical gear. For an outer diameter helical gear, a rubber bladder is inserted thorough a center bore in the gear. Fluid is pumped into the rubber bladder at extremely high pressures, thus radially expanding the preform against the walls of the mold cavity, and causing it to take on the helical gear shape. If an inner diameter helical gear is desired, a solid core rod having a helical gear profile engages the inner diameter of the preform and inward pressure is applied to the outer diameter of the preform resulting in the inner diameter taking on the helical gear profile. A disadvantage with isostatic forming is that it can take much longer for the process to fully densify the gear. In hot forming, enormous amounts of pressure can be generated in an instant by impacting the gear axially. In contrast, with isostatic pressure it may take time to build up sufficient pressure and it may be preferable to keep the gear subjected to the pressure for a relatively long time to ensure that the preform fully conforms to the specific geometry of the helical gear. Also, for example, obtaining accurate dimensions can be difficult when using isostatic pressure forming. There is generally no mold abutting the axial ends of the gear. Thus, the axial dimension can be difficult to accurately control. Consequently, more finishing steps can be required to obtain final dimensions having the desired accuracy. Moreover, besides controlling the length of the gear, the lack of control over the axial dimension can also make it more difficult to fully density the gear. This is because without control over the axial dimension, the gear can experience some undesirable axial expansion in addition to the radial expansion.
Yet another method for producing helical gears utilizes cutting. Generally, a piece of steel bar stock is chosen and cut to the desired length. The gear profile is then cut into the preform. The disadvantage of this type of method is that the equipment used in this method is slow, expensive and labor intensive, resulting in an expensive gear.
Accordingly, there is a need for a method of producing fully dense powder metal helical gears that can eliminate the step of creating a cylindrical preform and that can control both the axial and radial dimensions of the gear to create a helical gear with greater density, more accurate axial dimensions and less flashing. Consequently, less finishing steps may be necessary to obtain a final product.
The need also exists for a method of producing fully dense powder metal helical gears that does not form the gear profile by cutting and thus, decreases the cost of producing a high quality gear profile.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a method for making a powder metal helical gear including providing powder metal in a preform mold having a rotating die member and a desired gear profile. The metallurgical powder metal may include, for example, any alloyed or unalloyed single metallurgical powder or combination of powders and metallurgical and non-metallurgical additives. The method further includes axially impacting the powder metal with the rotating die member to create a gear preform with the desired gear profile, sintering the gear preform, heating the preform, placing the preform in a hot forming mold having a rotating die member and a desired gear profile, and axially impacting the heated gear preform with the rotating die member to create a densified gear. Alternatively, the heating step may be eliminated and the preform can be removed from the sintering step and while still hot can be placed into the hot forming mold.
The present invention further provides a method for forming helical gears, wherein the helical gear teeth can be either on the inner diameter or the outer diameter of the gear.
A method for producing a fully dense powder metal helical gear according to the invention can include placing a desired powder metal composition into a first preform die. The metallurgical powder metal may include, for example, any single alloyed or unalloyed metallurgical powder or combination of powders and metallurgical and non-metallurgical additives. Preferably, the preform die has the specific shape and approximate dimensions of the desired finished article, for example, a helical gear. The powder metal can then be axially compacted by punches with enough force to generate sufficient pressure to create a helical gear preform. Next, the helical gear preform is sintered. The sintered preform can then be lubricated, heated, and delivered to a hot forming press. In the hot forming press, the sintered preform can be axially impacted by punches with sufficient force to generate enough pressure to fully densify the gear. As used herein, “fully densify” and “fully dense” refer to a gear having a density of greater than 96% of the theoretical density. The hot forming press may have a core rod and/or punches that may rotate. After the hot forming process the densified helical gear can be slowly cooled to room temperature at a rate so as to obtain a hardness less than Rockwell B 100, and preferably less than Rockwe
Anderson Gary L.
Muroski Jerome E.
Jenkins Daniel J.
Keystone Investment Corporation
Kirkpatrick & Lockhart LLP
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