(1) Field of the Invention
The present invention relates to method to fabricate non-separating diffuser holes formed by way of a two step process and an electrode comb used in the method.
(2) Description of Related Art
Cooling hole efficiency is of great consequence for High Pressure Turbine Blades in high performance aircraft gas turbine engines. To be achieve maximum efficiency, each hole ideally exhibits a small meter section to limit cooling air flow and a diffuser to slow down the air flow and spread it over the surface of the airfoil in a thin film.
One common hole used to provide such cooling characteristics is the “Vehr” hole (disclosed in U.S. Pat. No. 4,653,983) which permits the cooling air flow to be discharged at an angle to the main gas path flow. This “cross flow” helps to create a more uniform layer of film cooling to insulate the blade from the high temperature gas path air. The Vehr hole uses two diffusion surfaces for holes on the pressure and suction surfaces, and three diffusers for holes along the tip and platform of the blade.
A critical requirement of such a hole is that there be no step between the meter section and the diffuser surfaces. This is not a problem when both features are produced from a single electrode, but is a major problem if the hole is produced by one process and the diffuser is produced by a different process and/or operation.
There are several reasons to want to use one process to produce the metering hole and another process or operation to produce the diffuser, specifically, cost and difficulty of fabrication. For example, it is more efficacious, and hence cheaper, to use a more efficient LASER process to produce the  hole, while using an EDM process to machine the diffuser. With regards to difficulty, drilling a “Vehr type hole” into a small cavity in modern “super-cooled airfoils” is very difficult with a single shaped electrode and is very costly if not impossible without back-striking the inner wall which causes a stress concentration factor in the already highly stressed inner wall. This problem is greatly reduced if the hole is produced in a two step process.
Regardless of the processes used, a two step hole process using a combination of LASER, electro discharge machining (EDM) or Electron beam drilling will create a step between the hole and the diffuser surfaces which will degrade the cooling ability of the “Vehr” holes produced. The step is caused by the location tolerance of the diffuser relative to the centerline of the hole.
What is therefore needed is a modified “Vehr” type diffuser, and a method for producing it, which can accommodate a certain amount of location tolerance of the diffuser relative to the hole without causing “separation” of the flow from the diffuser surface.