Technical Field

This invention relates to the controlled abrasive polishing of planar and non-planar surfaces and more particularly to the polishing of the gas path surfaces of gas turbine engine hollow fan blades.

Background Art

A gas turbine engine, commonly used for aircraft propulsion, generates power by compressing incoming air in the front portion of the engine, burning fuel which is mixed with the compressed air, and extracting energy from the expanding combustion gases in a turbine in the rear portion of the engine. A turbofan engine commonly uses a ducted propulsive fan mounted to the forward end of the turbine shaft. The blades of the fan are longer nan the compressor blades, with the extra blade length propelling a greater volume of air than is required for combustion of the fuel. The excess air bypasses the core of the engine, with the fan blades acting much in the manner of propellers. Thus the propulsive forces for powering the aircraft are provided by the combined thrust of the turbine exhaust gases and the propulsive fan. As engines become larger and more powerful, longer wider fan blades are incorporated in order to move an even greater volume of air through the engine. The longer, wider fan blades result in a larger mass of blade material rotating at a greater distance from the rotational center of the engine than in earlier, lower power engines. The greater centrifugal forces generate as the engine rotates generate larger stresses in the

disks in which the blades are mounted. One technique for reducing the stresses generated in the disks is to make the fan blades from high-strength, low-density composite materials. Another technique, as shown in U.S. Patent No. 5,063,662 issued to Porter, et al., and of common assignee with the present application, is to reduce the mass of the fan blade by bonding together preformed blade halves to form a hollow metal blade.

In order to meet the aerodynamic requirements for the flow of air over the surface of a blade, a very smooth surface is required. The necessary surface finish cannot be obtained as a result of the ordinary manufacturing operations but is obtainable by polishing the surface of the blade in a separate finishing operation. Traditionally, large fan blades have been polished by hand, using a form of belt sander mounted on a pedestal. The operator polishes the blade by holding the blade against the moving belt sander so that material is removed from the surface of the blade. The speed and accuracy of the removal process are largely a function of operator technique.

While the material removal can generally be held within acceptable tolerances for a solid fan blade, the thin walls surrounding the cavity in a hollow fan blade require a much more accurate removal process to ensure the structural integrity of the blade.

What is needed is a process capable of quickly and consistently removing a uniform amount of material from the surface of a workpiece while providing a satisfactory surface finish. In the particular case of a hollow fan blade, this means removing 0.001-0.002 inches of material from the airfoil surfaces, while providing a surface finish which will satisfy airflow requirements.

Summary of the invention

The surface of a hollow fan blade is polished by using a computer numerically controlled machining system to pass a rotating flexible abrasive polishing device over the airfoil surfaces while holding the p .ishing device essentially perpendicular to the surfaces as the surface contour changes. While one skilled in the art will recognize that a variety of polishing devices may be used, e.g., buffing wheels or flexible abrasive flap wheels, the description of the invention will be restricted to the use of a flap wheel. By moving a flexible abrasive flap wheel at a predetermined rate across the surface and maintaining a predetermined interference between the flap wheel and the workpiece, material is removed from the surface at a predictable rate. The individual flaps of the flap wheel are flexible enough that they conform to the curved contours of the airfoil surface and assure uniform removal as the flap wheel traverses the workpiece in discrete paths. By performing a series of operations using progressively finer grit flap wheels, enough material can be removed to achieve the desired surface finish while avoiding excessive material removal.

As material is removed from the airfoil surfaces by the flap wheel, the tips of the individual flaps wear away, resulting in a decrease in the diameter of the flap wheel. Since a predetermined interference between the flap wheel and the airfoil surface is critical to the removal rate, compensation must be made for the changing diameter of the flap wheel. This is done by measuring the diameter of the flap wheel continuously during the polishing operation using a light source and a detector in the form of a fiber optic light sensor array mounted on the protective housing surrounding the flap wheel, and sending the data to the control computer

- 4 - which adjusts the distance of the wheel from the workpiece to maintain the required interference.

The foregoing and other features and advantages of the present invention will become more apparent from the following description and the accompanying drawings.

Brief Description of the Drawings

Figure 1 is a perspective view of a gas turbine engine hollow fan blade which is to be polished in accordance with the method of the present invention. Figure 2 is a sectional view taken along the line

2-2 of Figure 1 showing the hollow center and the thin walls of the airfoil surfaces.

Figure 3 is a perspective view of a computer numerically controlled machining system used for polishing non-planar surfaces.

Figure 4 is a block diagram illustrating the interrelationships between the various portions of the machining system of the present invention.

Best Mode for Carrying out the Invention Fig. 1 shows a gas turbine engine fan blade 10 designed for use in a modern high power engine. Although this invention is described with regard to a process for the fabrication of hollow fan blades solely for purposes of illustration, but not limitation, it is to be understood that the present invention may as well be applied to any other polishing operation where it is important to remove a carefully controlled amount of material from a surface while producing a smooth surface finish. Blades of this type are typically up to about 50 inches in overall length. The fan blade 10 consists of a leading edge 12, a trailing edge 14, a pressure surface 16, a suction surface 18, a blade tip 20 and a

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- 5 - blade root 22. The blade root 22 slides into a blade slot in a fan disk (not shown) which positions the blade in the gas path and secures the blade as the engine rotates during operation. Fig. 2 shows a cross section of the blade 10 taken at Section 2-2 in Fig. 1. The cross section shows the cavity 24 and ribs 26 which serve to stiffen the blade and provide support for the walls 28, 30 of the pressure 16 and suction 18 surfaces, respectively. One of the last steps in the production of the fan blade 10 is polishing of the pressure 16 and suction 18 surfaces to achieve a surface finish which provides minimum resistance to airflow through the engine. The walls 28, 30 are typically a minimum of 0.030-0.035 inches thick. Since the structural integrity of the blades is largely dependent on the uniformity of the wall thickness, it is obvious that excessive thinning of the walls during a polishing operation cannot be tolerated. Fig. 3 shows a computer numerically controlled machining system 50 for the polishing of fan blades. A fixture 52 is mounte" on a work table 54. A robot 58, also mounted on the work table 54, includes a movable arm 60. Mounted on the end of the movable arm 60 is a bracket 62, to which is attached an electric motor 64 which drives an arbor 66 by means of a belt inside the protective cover 68. A flexible abrasive flap wheel 70 is mounted on the arbor 66. A protective shield 72 partially surrounds the flap wheel 70. A singular light source 74 is situated so as to project a beam of light through slots 76 in the shield 72. Tfc light beam passes through the slots in the shield and impinges on a fiber optic detector array 78, providing a continuous measurement of the diameter of the flap wheel 70.

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Referring now to Fig. 4, the machining system 50 is depicted schematically as encompassing the machining hardware as well as the electronic hardware which controls the operation of the machining hardware. The box 58 represents the machining hardware and is labeled "machine tool." A machine control 82 sends a variety of signals 84 to the machine tool 58 to move the hardware in a particular manner. The system 50 also includes a computer 80, which contains means for storing data, and the light sensor array 78 used for measuring the diameter of the flexible abrasive flap wheel. For discussion purposes the computer and storage means are shown as separate from the machine controls; however, they may also be considered part of the machine control. .. The storage means is simply a memory which is accessible by the computer 80. In the method of the present invention a computer program, which is also referred to herein as the machine program, is stored in the computer 80. The machine program includes certain preselected nominal engineering design dimensions of the blade to be polished. Further, each time the sensor 78 measures the diameter of the flap wheel 70, data indicative of the diameter of the flap wheel at that instant are sent to the computer 80. During operation the computer accesses the machine program and selected data stored in the computer, performs certain calculations on the stored data, and either stores the newly calculated data for later use or sends instructions to the machine control 82 which operates the machine tool 58 according to those instructions.

Referring again to Fig. 3, after the blade 10 has been mounted in the fixtures 52, 56, the computer 80 sends instructions to the robot 58 to position the flap wheel 70 such that a radius of the flap wheel 70 is

coincident ;.ith a line which is perpendicular to the surface of the blade 10. With the blade 10 mounted essentially vertically, the flap wheel 70 is moved in a generally horizontal direction across the width of the blade. Since the orientation in space of the line perpendicular to the blade 10 is different for different points on the blade surface, the flap wheel 70 is continually being reoriented by the robot arm 60 as the flap wheel 70 passes over the surface of the blade 10 in order to keep the radius of the flap wheel 70 coincident with the perpendicular to the surface.

In order to remove the appropriate amount of material from the surface of the blade 10, the flap wheel 70 is positioned such that there is an interference between the flap wheel and the blade surface of 0.080 - 0.120 inches. This interference is established by positioning the center point of the flap wheel relative to the position on the blade at which the perpendicular line emerges from the surface of the blade.

During a polishing operation abrasive particles are continually being lost from the tips of the abrasive flap material. Although the flap wheel may be dressed to remove the cloth backup material on the ends of the flaps where the abrasive has been lost by passing the rotating flap wheel across a highly abrasive material, it has been determined that the cloth backup material does not generally remain after the abrasive has been removed. Thus, a measurement of the decreasing diameter of the flap wheel as wear occurs is not adversely affected by nonabrasive backup material remaining on the ends of the flaps.

The light source 74 projects a beam of light through the slots 76 in the protective shield 72. As the light beam impinges on the flap wheel 70, a

significant portion of the light striking the flap wheel is absorbed by the wheel. The individual fiber optic elements in the light sensor array 78 detect the position at which the full strength of the light beam passes the flap wheel 70. This provides a measurement of the diameter of the flap wheel, which has been found to be accurate within about plus or minus 0.002 inches. The signal representing the measured diameter of the flap wheel 70 is sent to the data storage means 86. The measurement of the flap wheel diameter is then used by the computer 80 which sends to the machine control 82 instructions for manipulating the flap wheel 70 which are compensated for the changing diameter of the flap wheel. In this manner the machining system 50 is able to maintain the desired interference between the flap wheel 70 and the airfoil surfaces and thus maintain an essentially constant material removal rate from the blade surfaces.

While the tips of the abrasive flaps in the flap wheel 70 are nominally rectangular, their flexible nature enables them to conform to any surface contours on the blade surface. Thus, as the flap wheel traverses across the surface of the blade, the removal of material from the blade is essentially uniform over the width of the flap with no discontinuities at the edges of the flaps.

The width of the abrasive flap wheel used can be varied according to the degree of curvature in the blade. For example, a wide flap wheel can be used on a blade which has a gentle contour, while a narrower flap wheel may be much more suitable for a blade which has sharply contoured surfaces. The process of the invention may be better understood through reference to the following illustrative example.

Example A hollow fan blade about 42 inches long, having a maximum width of about 18 inches, was polished in a series of steps using flexible abrasive flap wheels having grit sizes of 120, 240 and 320. The flap wheel was positioned to provide an interference with the airfoil surfaces of 0.080 - 0.120 inches, and was moved across the surface of the blade at a rate of approximately 2.36 inches per second (60 mm per second). Adjustments of the position of the flap wheel were made only when the diameter of the wheel changed by 0.006 - 0.008 inches, because the robot, with its articulated arm, is adjustable only in increments of about 0.004 - 0.006 inches. One skilled in the art will understand that this control system, when used with a traditional computer numerically controlled machine tool with a smaller resolution of movement, can be used to update the position of the material removal tool at a frequency consistent with the resolution of the machine. The suction and the pressure surfaces of the blade were polished in separate operations, with the blade being rotated approximately 180" after polishing one side in order to polish the other side.

In order to avoid excessive polishing of the leading and trailing edges of the blade, individual passes of the flap wheels were begun near the center of the blade with polishing being done toward one edge from the tip to the root of the blade, and then toward the other edge on the same side of the blade from the tip to the root of the blade. This permits polishing "off of" the edge of the ^J blade, rather than "onto" the edges, which controls the amount of material removed at the edges of the blades. This dual polishing technique requires that, to polish the opposite edges of one side of the blade, the direction of rotation of the flap

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- 10 - wheel must be reversed, and that the flap wheel must be inverted to properly expose the abrasive particles on the surface of the flaps to the blade surface.

This polishing technique resulted in a reduction in surface roughness from approximately 80 Ra to 5 - 8 Ra after polishing. Hand polishing operations on similar blades typically results in a surface finish of up to 20 Ra after polishing.

It has been determined independently that it is necessary to achieve a surface finish of 10 Ra or better in order to produce finished airfoils with the required airflow resistance levels. Ceramic bead peening of the robot polished blade resulted in a surface finish of 20 Ra, which satisfied surface finish requirements for the finished blade. Similar peening operations on hand polished blades typically results in a surface finish of 40 Ra, which is greater than the maximum allowable surface roughness.

A media finishing operation, in which the fan blade is tumbled in a bed of specially shaped pieces of solid abrasive material, is required to improve the surface of the hand polished blades to an acceptable level. With the robot polished blades, however, the need for the media finishing operation is eliminated. The time required for robot polishing of this fan blade was approximately three hours, while the time required for hand polishing of the same blade is estimated to be approximately 12 hours (this estimate is based on an extrapolation from smaller fan blades, since no blade of this size has been hand polished) .

While this invention has been described for the polishing of non-planar surfaces, in particular gas turbine engine fan blades, one skilled in the art will realize that the invention is applicable to the polishing of objects of virtually any configuration in

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- 11 - which a carefully controlled amount of material must be removed in a polishing operation.

Although this invention has been shown and described with respect to a best mode embodiment thereof, it will be understood by those skilled in the art that the foregoing and various other changes, omissions and additions in the form and detail thereof may be made without departing from the spirit and scope of the claimed invention.