Abstract:
A method for finishing a part having excess material includes generating one or more surfaces on a model of the part to be machined, creating a machining tool path drive geometry, using the machine tool path drive geometry and one or more surfaces on the model to be machined to generate machining tool paths on the surfaces; and running tool paths on the part in a fixture.

Description:
This application is a divisional of U.S. patent application Ser. No. 10/852,828 filed May 25, 2004, which issued as U.S. Pat. No. 7,377,037 on May 27, 2008 which is hereby incorporated by reference and is assigned to assignee of the present invention. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to machining of surfaces and to part having machined surfaces, and more particularly, to methods and computer-aided machining of surfaces without the requirement of adaptive probing and to parts produced thereby. 
     Forging processes used to create some surfaces, for example, airfoil and platform surfaces, can leave excess material at intersections of edges. For example the forging process used to create airfoil and platform surfaces leaves extra material at an intersection between the airfoil lead edge (LE, or thick edge), trail edge (TE, or thin edge) and the platform. In at least one known production operation, a manual process is used to remove this excess material. In this process, referred to as “benching” or “bench blend,” a highly skilled operator grinds away material using a rotating abrasive wheel. The operator must be dedicated to the task and cannot complete any other tasks while conducting the benching operation. The operator must be highly skilled because of the proximity of the operator&#39;s hands to the grinding wheel and the high volume of parts handled in production operations. Furthermore, the blending operation must produce a smooth transition between the root fillet, airfoil and platform surfaces. No undercuts of the airfoil or platform surface are permitted since this will create stress concentrations. 
     Besides manual bench blending, another known technique for removing the excess material utilizes a milling machine. The region around the root fillet is probed with a Renishaw-type probe and the geometric data thus obtained is used to adjust a computer numerical control (CNC) program to adapt to the airfoil and platform surfaces. Such probes are available from Renishaw PLC, New Mills, Wooton-Under-Edge, Gloucestershire, U.K. This technique usually requires a second control unit to make essential calculations and to modify the CNC program. For high volume, short cycle time parts, the extra time required has an adverse effect on productivity. 
     Another known technique for removing the excess material is profile milling. Profile milling uses a ball nose end mill of the same radius as the desired fillet to generate the correct geometry, A single curve is described as the drive geometry, and the CNC program drives the end mill along this path. However, a drawback of this technique is that the end mill can be fully engaged (i.e., the entire radius of the ball is cutting), which usually increases the chance of machining chatter. 
     BRIEF DESCRIPTION OF THE INVENTION 
     There is therefore provided, in some aspects of the present invention, a method for finishing a part having excess material. The method includes generating one or more surfaces on a model of the part to be machined, creating a machining tool path drive geometry, using the machine tool path drive geometry and one or more surfaces on the model to be machined to generate machining tool paths on the surfaces; and running tool paths on the part in a fixture. 
     In other aspects, the present invention provides a method for finishing a part having excess material. The method includes generating one or more surfaces on a model of the part to be machined, creating a machining tool path drive geometry, using the machine tool path drive geometry and the one or more surfaces on the model to be machined to create a computer numerical control path for a machining tool on the surfaces, and running tool paths on the part in a fixture. 
     In still other aspects, the present invention provides a forged part having airfoil and platform surfaces. The forged part is finished by a method that includes generating one or more surfaces on a model of the part, creating a machining tool path drive geometry, using the machine tool path drive geometry and the one or more surfaces on the model to create a computer numerical control path for a machining tool on the surfaces, and running tool paths on the part in a fixture. 
     Configurations of the present invention provide flexibility in the shape of a machined surface. This, in turn, facilitates a more continuous mating with, for example, an air foil and platform. In general, a true arc is not required at the LE and TE platform fillet. In various configurations, an improved surface finish is achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a drawing of a partial view of a typical compressor blade from two angles showing excess material from a forging operation. 
         FIG. 2  is a radial view along and showing a portion of the airfoil leading edge of the compressor blade shown in  FIG. 1 , where an area to be machined to remove excess forging stock is shown. 
         FIG. 3  is a partial side view of blade surfaces of the compressor blade of  FIG. 1 , showing a tapered ball nose end mill to be used for a milling operation. 
         FIG. 4  is an illustration of a drive surface produced when generating a milling tool path, wherein the drive surface is created by offsetting and extending an actual fillet surface. 
         FIG. 5  is a flow chart showing a method representative of various configurations of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As a technical effect of the present invention, flexibility in the shape of a machined surface is provided. Other technical effects include facilitating a more continuous mating with, for example, an air foil and platform. In various configurations, another technical effect of the present invention is that an improved surface finish is achieved. 
     In some configurations of the present invention, a single point milling technique is provided that uses a smaller ball end mill than is used in root fillet milling. The drive geometry is a surface instead of a curve, and the final shape of the machined surface is controlled by the shape and location of the drive surface rather than the radius of the end mill. These configurations of the present invention provide flexibility in the shape of the machined surface, which in turn facilitates a more continuous mating with the air foil and platform. In general, a true arc is not required at the LE and TE platform fillet. The end mill also has a smaller percentage of its periphery cutting at any one time than is the case with root fillet milling, thereby providing an improved surface finish. 
     A typical example of excess material  100  left by a forging operation at the intersection of leading edge  102  of an airfoil  104  and a platform surface  106  is shown in  FIG. 1 . In some configurations of the present invention and referring to  FIG. 2 , a drive surface used for the single point milling is created. Surface  202  represents an approximate region that is machined to remove the excess material. Surface  202  is created by trimming fillet surface  204  at 45 and 55% of its chord-wise length, thereby producing surface  202 . 
       FIG. 3  is a side view illustrating blade surfaces. Cylinder  302  represents a tapered ball nose end mill used for the milling operation. Cylinder  302  has a tapered ball nose end mill  304  that is used for the milling operation. In some configurations of the present invention, ball nose end mill has a 0.1 inch (0.254 cm) radius ball geometry that is used to create a 0.125 inch (0.3175 cm) radius fillet. More generally, some configurations of the present invention utilize a ball nose end mill  304  that is between about 15 to 30 percent smaller than the radius size of the fillet to minimize or reduce the number of passes required. Such configurations also advantageously avoid overloading the cutter and reduce cycle time. 
     An offset surface is created from the surface  202 . For example, for one particular blade useful for practicing a configuration of the present invention, the offset is 0.008 inch (0.02032 cm). This offset surface is then extended 10 to 20% in directions A and B indicated in  FIG. 4 . These extensions enlarge the surface linearly. The new surface  402  is the drive surface for a CNC program. By offsetting and extending drive surface  402 , configurations of the present invention make it possible to create a tool path that will not undercut airfoil  104 , the platform  106 , or a forged portion of the root fillet under excess material  100 . This technique also can accommodate natural manufacturing variation produced by the forging operation. Fixtures used in this operation reduce or minimize effects of positional variation, and programming techniques can be used to reduce or minimize the effects of airfoil shape and thickness variation. More particularly, some non-exclusive examples of fixture configurations useful for practicing the present invention are designed to locate parts accurately to reduce or eliminate variation using three points on a platform, with two of the three points located near an area being machined. These three points establish a plane. Two additional points arc located on a convex or concave side of the airfoil to orient the airfoil edge and to lock rotation. Another point is on the airfoil edge being machined, to locate the edge being machined. This fixture configuration compensates for variation of the airfoil and the platform of the blades and vanes, and locates the intersection of these surfaces (i.e., the machined area), thereby minimizing variation. Configurations of the present invention that use the fixture configurations described herein along with configurations of tool path generation methods and machining methods described herein advantageously produce acceptable surfaces with no undercuts. Such configurations are also able to acceptably deal with variations in parts resulting from forging processes. 
     Thus, in some configurations and referring to flow chart  500  of  FIG. 5 , a technical effect of the present invention is achieved by defining a machining fixture at  502  and building this fixture at  504  for a part. In some configurations, the part includes airfoil and platform surfaces, and/or the part is produced by forging. A surface or surfaces are generated at  506  on a model to be machined, the surface(s) being drive surface(s) to be used for single point milling. The surface(s) represent an approximate region that is to be machined to remove excess material. The surface or surfaces are created in some configurations by trimming fillet surface(s) at 45 to 55% of their chordwise length. At  508 , the machining tool path drive geometry is created or determined. Machining tool paths are generated on surface(s) to be machined at  510 , and tool paths are run on the part in the fixture at  512  to finish the part being machined. 
     As used herein, the terms “trim” and “offset” refer to operations conducted in UNIGRAPHICS® CAD/CAM software that is used commercially for creating CNC programs. (UNIGRAPHICS® is a trademark of UGS PLM Solutions, Inc. of Plano, Tex., and UNIGRAPHICS® CAD/CAM software is available from UGS PLM Solutions Inc., Maryland Heights, Mo.) Although some configurations of the present invention utilize UNIGRAPHICS® CAD/CAM software, it will be understood that this particular brand of CAD/CAM software is cited only by way of non-limiting example. The present invention is not limited to the use of particular brands of software. 
     Exemplary dimensions are provided above for some configurations of the present invention. The selection of appropriate dimensions for use in other configurations is considered to be a design choice that can be made by one of ordinary skill in the art, after such a person has gained an understanding of the present invention. 
     It will thus be appreciated that configurations of the present invention provide flexibility in the shape of a machined surface. This, in turn, facilitates a more continuous mating with, for example, an air foil and platform. In general, a true arc is not required at the LE and TE platform fillet. In some configurations, an end mill is used that has a smaller percentage of its periphery cutting at any one time than is the case with root fillet milling, thereby providing an improved surface finish. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.