Repair of airfoil edges

Abstract: The present invention provides a method and apparatus (30) to repair gouged out and damaged leading and trailing edges (18,19) of gas turbine engine blades (12), particularly fan blades, by cutting away a curved section (X) including the damaged area and forming a blend radius (RM) along the repaired edge. The apparatus (30) includes a unique machine tool guide having an adjustable cutting depth guide (42) and mountable to a hand held machine tool driver (34), such as a 90 degree angle head air motor, that drives a rotary cutter (60).

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a hand held machine tool for use in the repair of gas turbine engine airfoils without removing the airfoil from the engine, and more particularly to the repair of BLISK fan blades. Description of Related Art Leading and trailing edges of aircraft gas turbine engine rotor blade airfoils are subject to wear and damage that forms gouges and requires repair with a minimum of cost and difficulty. Repair of the airfoils conventionally requires their removal. However in combat as well as under other extenuating circumstances there is a need to field repair damaged airfoils without removing the blade or the engine from the aircraft. With the advent of blade integral disks or BLISKs there is an even greater need to repair the leading and trailing edges of the blade airfoils in the field while the engine is installed on the aircraft, either on the wing or in the fuselage, because blisk blades cannot easily be disassembled. One form of hand held machine tool used for on-site milling of window frames is know from DE-A-3008357. According to the invention there is provided a hand held machine tool including a rotary cutter for effecting repair of a damaged airfoil extending radially outward from a rotor without removing the airfoil or rotor from a surrounding case and working through an access window in the case, said machine tool including a guide and an adjustable sliding base characterised by: said guide having a pair of parallel rails extending in a direction normal to the axis of the cutter, one on either side of the cutter, to provide a platform for sliding engagement with the airfoil side during a machining operation and said base being slidably retained within the guide parallel rails and not extending outwardly of the guide in the direction of the axis of the cutter; said base further including an elongated slot through which the rotary cutter axis extends for allowing the base to slide in a direction normal to the axis of the rotary cutter; and said base further including two pin stops which extend outwardly of the guide parallel to the axis of the rotary cutter for engaging an airfoil edge at the desired depth of cut. A preferred method of airfoil edge repair employs different tool guides, including a tool guide in accordance with the present invention, and different rotary tools for cutting and blending the damaged areas of the airfoil. ADVANTAGES Among the advantages provided by the apparatus of the present invention is a savings of time required to remove the damaged blades or in the case of BLISKs disassemble the engine, remove and fixture the rotor to allow the airfoil to be machined, and reassemble the rotor in the engine. Another advantage provided by the present invention is that it allows damaged blades to be simply and effectively repaired whereby an operator does not require a great deal of training. Yet another advantage provided by the present invention is a field level repair of damage to leading and trailing edges of rotor airfoils and particularly to BLISK type fan blades. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing aspects and other features of the invention are explained in the following description, taken in connection with the drawing, the single figure of which is a perspective view of a machine tool guide having an adjustable cutting depth means in accordance with the present invention mounted on a hand held 90 degree angle head air motor inserted into a portion of the fan section of an engine having damaged BLISK airfoil leading and trailing edges. DETAILED DESCRIPTION OF THE INVENTION Referring to the drawing, a portion of a gas turbine engine fan section 6 having a fan case 8 circumscribing a BLISK type fan rotor 10 having axially disposed circumferential rows of fan blade airfoils integrally formed thereon including damaged airfoils 12. Also shown is a damaged area 14 having gouges 16 located on a leading edge 18 of a damaged airfoil 12 and a similarly damaged trailing edge 19 on another damaged airfoil 12. One embodiment of the invention illustrated in FIG. 1 is an adjustable cutting depth machine tool guide 30 mountable, by a mounting means such as mounting clamp 32, to a hand held machine tool driver, such as a 90 degree angle head air motor 34 available from ARO. Air motor 34 drives a straight edge rotary cutter 60 for rough cutting or machining away the damaged area of the leading and trailing edges 18 a d 19 of airfoil 12. Adjustable tool guide 30, which has a pair of parallel rails (51), provides a flat platform 36 operable to rest against the side of airfoil 12 to allow machining of the edge. An adjustable stop means generally shown at 37 has two pins 42 mounted on a slidable base 40 operable to be translated by adjusting screw 46, which is located by a tab (50) extending from the guide (30) and positioned between said parallel rails (51), and to be set by set screw 48. A slot 38 through which straight rotary cutter 60 is disposed allows the pins to set the depth of the cut of rotary cutter 60. The drawing also depicts part of an airfoil repair method wherein first access to the airfoils is gained through a fan case access window 9 on engine case 8, next airfoils 12 are visually inspected for damage, and the maximum depth D of the damaged area is measured, usually corresponding to the deepest gouge 20. Then the damaged airfoil is cleaned, preferably wiped clean using a suitable solvent such as Methyl Ethyl Keytone and a lint free cloth, and a curved line L is drawn around the damaged area wherein the marked off area to be removed X preferably has a spanwise length eight times the depth D of the deepest gouge 20 and is equally spaced from the point of the deepest gouge 20. Curved line L is preferably marked with a laboratory certified high purity marker and highlighted with masking tape. A mask (not shown) is inserted into the access window on the fan case to protect adjoining airfoils from damage while machining and the delineated area to be removed X is rough machined away by the operator working through the access window, using an adjustable depth tool guide 30, of the present invention, mounted on a hand held air motor 34 driving straight rotary cutting tool 60, as described previously. The depth of the cut is adjusted by turning adjusting screw 46 and set with set screw 48 according to the condition of area X. Area X is then removed with the machining apparatus previously described using smooth motions to prevent gouging and burning of the airfoil. This produces a repaired leading edge 18R (or alternatively a repaired trailing edge) with a new spanwise contour. After the damaged area is rough cut away, at least one rotary radius cutter having a concave contour to produce an airfoil edge blend radius RM, which is only slightly different than the original radius R, is set up in air motor 34 in conjunction with a non-adjustable tool guide. Blend radius RM may required two or more concave contour rotary radius cutters of narrowing sizes to form the final blend radius in gradual steps. Using this tooling an airfoil edge blend radius RM is machined on the airfoil edge. The surfaces along the airfoil edge having blend radius RM are radiused and polished to form smooth blends using rubberized abrasive grinding wheels having a grinding surface contoured to form an acceptable airfoil edge radius. Finally the entire reworked area is cleaned using a shop vacuum and wiped using a suitable solvent such as Methyl Ethyl Keytone. This procedure is repeated for all damaged airfoils. Note that the airfoil edge repair process is essentially the same for leading and trailing edges.

Claims: A hand held machine tool including a rotary cutter (60) for effecting repair of a damaged airfoil (16) extending radially outward from a rotor (10) without removing the airfoil or rotor from a surrounding case (8) and working through an access window (9) in the case, said machine tool including a guide (30) and an adjustable sliding base (40) characterised by: said guide (30) having a pair of parallel rails (51) extending in a direction normal to the axis of the cutter (60), one on either side of the cutter (60), to provide a platform (36) for sliding engagement with the airfoil side during a machining operation and said base (40) being slidably retained within the guide parallel rails and not extending outwardly of the guide (30) in the direction of the axis of the cutter (60); said base (40) further including an elongated slot (38) through which the rotary cutter (60) axis extends for allowing the base (40) to slide in a direction normal to the axis of the rotary cutter (60); and said base (40) further including two pin stops (42) which extend outwardly of the guide (30) parallel to the axis of the rotary cutter (60) for engaging an airfoil edge (18,19) at the desired depth of cut. The machine tool recited in claim 1 further characterized by a tab (50) extending from the guide and positioned between said parallel rails (51) for locating an adjusting screw (46) threaded in the direction in which the base (40) is slidable and a set screw (48) for locking the position of the base. The tool recited in claim 1 further characterized by the drive axis of the driving motor (34) being at substantially a right-angle to the rotary cutter (60) axis.

Inventors: BRACKO PETRA, DUNKMAN DEWEY DUANE, GROSSKLAUS WARREN DAVIS JR, BRACKO, PETRA, DUNKMAN, DEWEY DUANE, GROSSKLAUS, WARREN DAVIS, JR.
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1991
Main classification: B23Q35
IPC codes: B24B23, B23Q35, B23C1, B23P6, B23C3, F01D5