Automatic blocked hole identification

A method for hole formation using an electrical discharge machining tool configured to penetrate metal. The method includes the steps of: penetrating the metal using an electrode to form a hole by moving the electrode away from a starting point; verifying that the hole is complete using the electrical discharge machine; and wherein the verifying step includes the step of probing the hole with the electrode.

BACKGROUND OF THE INVENTION

The present invention relates to hole formation in a machine component and more specifically to a method for determining whether a hole is blocked or is completely formed through a turbo-machinery component wall.

Airfoils in a turbine engine often include cooling holes for discharging a film of cooling air along the outer surface of the airfoil to affect film cooling. These may be referred to as “film cooling holes” or “film holes.”

Generally, cooling holes extend through a wall in an aircraft component from an entry end to an exit end. It is critical that these holes be formed completely through the respective component. In order for proper cooling, all cooling holes should be formed completely through the wall of the component. Blocked holes will result in overheated areas or “hotspots.” Blocked holes can be the result of incomplete hole formation or of debris in the hole.

One problem with current methods for producing cooling holes is that it is very difficult and time-consuming to determine whether a cooling hole is properly formed through the wall of the component. If one or more holes are not formed properly, the component is reworked and the blocked holes are re-drilled. In order to re-drill a hole, the equipment must be repositioned over the hole.

Conventional methods of verifying whether a cooling hole is properly formed include pin checks and water checks. Pin checks require an operator to manually insert a pin into the hole to determine whether the pin passes completely through the associated wall. In water checks, water pressure is applied within an airfoil and the hole is observed to determine whether water passes through it. Both of these methods have associated problems. The pin check method is manual and can result in repetitive motion injuries. The water check method can be inaccurate. Therefore there is a need for a method for forming cooling holes through aircraft components which incorporates a step of verifying whether a cooling hole is properly formed through the component or is blocked and for reworking such a hole before another hole is started.

BRIEF DESCRIPTION OF THE INVENTION

This need is addressed by a method for verifying an initial indication that a hole is complete as part of the hole formation process.

According to one aspect of the technology described herein, there is provided a method for hole formation using an electrical discharge machining tool configured to penetrate metal. The method includes the steps of: penetrating the metal using an electrode to form a hole by moving the electrode away from a starting point; verifying that the hole is complete using the electrical discharge machine; and wherein the verifying step includes the step of probing the hole with the electrode.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,FIG. 1illustrates an exemplary turbine blade10. The turbine blade10includes a conventional dovetail12, which may have any suitable form, including tangs that engage complementary tangs of a dovetail slot in a rotor disk (not shown) for radially retaining the blade10to the disk as it rotates during operation. A blade shank14extends radially upwardly from the dovetail12and terminates in a platform16that projects laterally outwardly from and surrounds the shank14. A hollow airfoil18extends radially outwardly from the platform16and into the hot gas stream. The airfoil has a root17at the junction of the platform16and the airfoil18, and a tip22at its radially outer end. The airfoil18has a concave pressure side wall24and a convex suction side wall26joined together at a leading edge28and at a trailing edge31.

The airfoil18may take any configuration suitable for extracting energy from the hot gas stream and causing rotation of the rotor disk. The airfoil18may incorporate a plurality of trailing edge bleed slots32on the pressure side wall24of the airfoil18, or it may incorporate a plurality of trailing edge cooling holes (not shown). The tip22of the airfoil18is closed off by a tip cap34which may be integral to the airfoil18or separately formed and attached to the airfoil18. An upstanding squealer tip36extends radially outwardly from the tip cap34and is disposed in close proximity to a stationary shroud (not shown) in the assembled engine, in order to minimize airflow losses past the tip22. The squealer tip36comprises a pressure side tip wall38disposed in a spaced-apart relationship to a suction side tip wall39. The tip walls38and39are integral to the airfoil18and form extensions of the pressure and suction side walls24and26, respectively. The outer surfaces of the pressure and suction side tip walls38and39respectively form continuous surfaces with the outer surfaces of the pressure and suction side walls24and26.

A plurality of film cooling holes100pass through the pressure side wall24. The film cooling holes100of the airfoil18communicate with an interior space19as shown inFIG. 2. The interior space19may include a complex arrangement of cooling passageways defined by internal walls. By way of example and not limitation, the cooling passageways can include one of the following characteristics: serpentine, intertwined, intersecting, non-intersecting, and a combination thereof. It should be appreciated that airfoil18may be made from a material such as a nickel-based or cobalt-based alloy having good high-temperature creep resistance, known conventionally as “superalloys.”

Referring now toFIGS. 2-7, the present invention provides a method for forming a hole100through a wall. In the illustrated embodiment, a hole is formed through the pressure wall24using an electrical discharge machining tool40. The electrical discharge machining process is referred to as EDM herein. It should be appreciated that the machine40includes a computer processor (not shown) configured to store, manipulate, compared, and monitor data such as travel distances, applied force, hydraulic pressure, electrical properties, and the like as indicated below. The EDM tool40also includes a tubular electrode42that extends to a tip45. Electrode42includes a conduit43formed there through for flushing debris from the hole being formed and for providing an electrolyte solution. In an EDM process, fluid is pushed in through conduit43of the electrode42for lubrication.

In a hole forming, or penetrating, step; voltage is applied to the electrode such that the electrode42erodes an in-process hole120into the wall24as shown inFIG. 2. According to the thinking step, the electrode42away from a starting point. Once the tip45is through the wall24as shown inFIG. 3, the electrode42must stop to avoid crossing the interior space19and penetrating the wall26of the airfoil18.

There is a monitoring status step to determine whether the EDM tool40has completely formed the in-process hole120through the wall24. In this regard, the EDM tool40is configured to monitor a predetermined property during the penetrating step. The property can be an electrical property such as voltage or current. Alternatively, the property can be a hydraulic property associated with the electrolyte such as flow or pressure. When the predetermined property reaches a predetermined value, the machine40stops the penetrating step. Stated another way, the computer processor of machine40determines during the monitoring status step that the hole is complete. According to conventional processes, the machine40would then be moved to another location to form another hole.

In contrast to conventional processes, the present invention provides a verifying step that occurs before the EDM tool40is relocated. The verifying step includes a probing steps and a retracting step. In the probing step, a travel first distance Y is determined as described below. Referring now toFIG. 4, in the retracting step the electrode42used to form the in-process hole120is retracted a predetermined second distance X. During the probing step the electrode42is reinserted into the hole120until it stops. During the probing step, the electrode is stopped by contact with the bottom of an incompletely formed hole120, an indication of monitored properties as described above, or because the probe has reached the maximum travel distance. The maximum travel distance can be determined by a second physical machine stop, or by a measurement such as a predetermined maximum third distance. The predetermined maximum third distance is greater than the predetermined second distance X and is determined such that the tip45of the probe42can extend from an unblocked and completely formed hole120into the space19. The predetermined maximum third distance can be fixed by the mechanical limits of machine40. In this regard, the machine40can be configured such that, by moving the predetermined maximum third distance, the electrode42can travel into the in-process hole120such that the electrode tip45of the electrode42extends through the wall24. The actual travel first distance is indicated as Y as shown inFIG. 5and is monitored by the computer processor.

When the electrode42stops traveling during the probing step, the travel first distance Y is noted. If the travel first distance Y is greater than the predetermined second distance X, and about equal to predetermined maximum third distance, it is concluded that the in-process hole120is open. It should be appreciated that the during the penetrating step, the hole120might be formed such that a space exists between the tip45of the electrode and the bottom of the hole120. It should be appreciated that the predetermined maximum third distance is chosen such that the space, if any, is accounted for. In addition, a deadband value can be added to the predetermined second distance X to account for his process variability space. The deadband may be determined by process understanding. The travel first distance Y would be compared to predetermined second distance X plus the deadband value. In a retracting step, the electrode42is withdrawn from the in-process hole120and the EDM tool40is relocated to form another in-process hole120at a new predetermined location.

FIG. 6illustrates an example wherein the monitored predetermined property incorrectly indicated that the in-process hole120was complete. In this situation, the electrode42is withdrawn in the retracting step the predetermined second distance X as indicated above. The probing step is initiated and travel first distance Y of the electrode42is measured as described above. As shown inFIG. 6, the electrode42is prevented from further travel during the probing step because the hole is blocked. The travel first distance Y is not about equal to the predetermined maximum third distance and is not substantially greater than the predetermined second distance X. Thus it is determined that the hole is blocked. At this time, the hole forming or penetrating step is reinitiated. The progress of the in-process hole120is verified as described above by the computer processor based upon the monitored property. In this regard, when the property reaches a predetermined value, the hole forming step is stopped and a verifying step is initiated.

The cycle of hole formation and verifying can continue until the hole is determined to be properly formed through the component wall.

According to an alternative by, electrode42stopped during the monitoring step thus defining a stopping point. In the probing step, electrode42is then moved further away from the starting point and past the stopping point without first being retracted toward the starting point.

The foregoing has described a method for forming cooling holes in the airfoil. The method includes steps for verifying and confirming whether the hole was satisfactorily formed prior to formation of another hole. This method can reduce or eliminate the need for off-line quality checks such as pin checks and water checks for hole formation. Thus the process for forming holes is more efficient and less labor-intensive according to the present invention.