Metal working – Method of mechanical manufacture – Impeller making
Reexamination Certificate
2002-07-15
2004-07-13
Rosenbaum, I Cuda (Department: 3726)
Metal working
Method of mechanical manufacture
Impeller making
C029S889000, C029S558000
Reexamination Certificate
active
06760971
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a gas turbine engine diffuser, and more particularly to a method of machining a gas turbine engine diffuser.
BACKGROUND OF THE INVENTION
The compressor section of a gas turbine engine includes a diffuser downstream of the compressor. The function of the diffuser is to reduce the velocity of the compressed air and simultaneously increase the static pressure, thereby preparing the air for entry into the combustor at a lower velocity. Presenting high-pressure and low-velocity air to the combustor section is essential for proper fuel mixing and efficient combustion.
A centrifugal compressor impeller draws air axially, and rotation of the impeller increases the velocity of the air flow as the input air is directed over impeller vanes to flow in a radially outward direction under centrifugal forces. In order to redirect the radial flow of air exiting the impeller to an annular axial flow for presentation to the combustor, a diffuser assembly is provided which redirects the flow as it also reduces the velocity and increases static pressure of the air flow.
A conventional diffuser assembly of this type, sometimes known as a fishtail diffuser, generally comprises a machined ring which surrounds the periphery of the impeller for capturing the radial flow of air and redirecting it through generally tangential orifices into an array of diffuser tubes. The orifices in the diffuser ring are circumferentially spaced apart, each one being intersected by two adjacent bores in an asymmetrical configuration. The diffuser tubes are generally brazed or mechanically connected to the ring and have an expanding cross-section rearwardly.
In general, the design of diffusers requires a compromise between the desired aerodynamic properties and the practical limits of manufacturing procedures. For example, the orifices in the impeller surrounding ring are typically cylindrical bores or conical bores due to the limitations of economical drilling procedures. To provide elliptical holes for example, would involve prohibitively high costs in preparation and quality control.
Engine performance is directly affected by the quality of the tangential diffuser bores. For good performance, a very accurate diameter and true position of these bores, a sharp edge of the bore intersection area and a very good surface finish of these bores are all required. This makes the diffuser one of the most costly and difficult parts of the gas turbine engine to manufacture.
The manufacturing process for the diffuser typically includes both roughing and finishing operations on its various surfaces. It is common practice to complete the roughing operation for all surfaces before beginning the finishing operation. This is done for convenience of changing tools, etc., and more importantly to prevent damage to the finished surfaces by completing the roughing first. Conventionally, diffuser bores in a diffuser ring are machined with a gun drilling machine which performs the roughing process for all bores in the diffuser ring, and then the finishing process is performed with a cylindrical and/or taper reamer.
Because of the configuration of the intersecting bores in a roughed-out diffuser, the finishing tool is always between the two intersections of the adjacent bores when finishing the bores. The two intersections of adjacent bores are not symmetrical, and therefore, the radial cutting force on the finishing tool is unbalanced, creating undesirable tool deflection, which results in poor quality of both position and diameter.
Furthermore, the unbalanced radial cutting force and the tool deflection inhibit the use of carbide tools which are adapted for high speed cutting but are too brittle to handle tool deflections normal in this type of operation. Thus, productivity of the diffuser bore machining process is limited. The conventional process also cannot provide a superior quality of surface finishing of the diffuser bores because the asymmetrical intersections of each diffuser bore limits the use of super-finishing tools such as burnishing tools.
Therefore, an improved process for machining the bores in the diffuser ring with better quality control and better productivity is desired.
SUMMARY OF THE INVENTION
One object of the present invention is to provide an improved method of machining diffuser bores in a gas turbine engine diffuser ring which minimizes tool deflection.
Another object of the present invention is to provide a method of machining diffuser bores in a gas turbine engine diffuser ring which improves the surface finish of the diffuser bores.
In general terms, a method in accordance with one aspect of the present invention is to provide for machining a plurality of circumferentially spaced bores in an object, each of the bores extending generally tangentially and inwardly and being positioned in the object so as to generally asymmetrically intersect two adjacent bores. The method comprises steps of (a) providing the object; (b) determining a plurality of bore positions generally around a circumference of the object; (c) machining a first bore; (d) performing at least one subsequent machining operation on the first bore to complete a machining process of the first bore; (e) machining a second bore immediately adjacent to the completed first bore, wherein the second bore is machined so as to intersect the completed first bore; (f) performing at least one subsequent machining operation on the second bore to complete a machining process of the second bore; (g) sequentially machining a remaining plurality of bores except a final bore, wherein each bore is machined so as to intersect an immediately adjacent completed bore; and (h) machining the final bore positioned at one side thereof immediately adjacent to the first bore and at the other side thereof immediately adjacent the bore previously completed, wherein the final bore is machined so as to intersect the two immediately adjacent completed bores positioned at opposite sides thereof.
It is preferable that when each of the second bore to the final bore is to be machined, a bore position is selected such that an intersection of the bore and a previously completed adjacent bore will occur at an end of the bore while the bore is being machined. Thus, a cutting tool in each bore except the first and final bores is affected by only one of the two intersections. By avoiding the intersection that is relatively closer to the bore entry, the tool will work properly for a longer portion of the bore, without any deflection.
It is also preferable that a plug is inserted into a previously completed adjacent bore before machining the next bore, except for the machining of the first bore.
The method according to the present invention, when being used to machine diffuser bores in a gas turbine engine diffuser ring, advantageously reduces manufacturing costs by providing improved quality of position and diameter, thereby eliminating scraps and deviations. Manufacturing costs are further reduced by the reduction in machining time and lead-time, which increases productivity. Furthermore, the method of machining diffuser bores in a gas turbine engine diffuser ring according to the present invention provides a better surface finish of the diffuser bores and a better repeatability of the turbine engine diffuser rings, which both improve turbine engine performance.
Other advantages and features of the present invention will be better understood with reference to a preferred embodiment of the present invention described below.
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Gendreau Bertrand
Lanoue Robert Francois
Sasu Ioan
(Ogilvy Renault)
Cuda Rosenbaum I
Pratt & Whitney Canada Corp.
Van Wayne H.
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