Electrode for electrical discharge machining film cooling passages in an airfoil

A sheet metal electrode for forming a shaped, diffusing film coolant passage has a straight, longitudinally extending forward section forming an acute angle with a flat middle section, which, in turn, extends from a rear section which is the base of the electrode. The middle section includes a pair of edges rearwardly diverging from the longitudinal direction and from which extend side walls substantially perpendicular to the middle section. Longitudinally extending edges of the side walls are in the same plane as a lower, flat surface of the forward section such that the surface of the passage formed by such lower surface of the forward section is in the same plane as a surface of the diffusing portion of the passage which is formed by the longitudinally extending edges of the side walls.

DESCRIPTION 
1. Technical Field 
This invention relates to electrodes for electro-discharge machining. 
2. Background Art 
It is well known the external surface of airfoils may be cooled by 
conducting cooling air from an internal cavity to the external surface via 
a plurality of small passages. It is desired that the air exiting the 
passages remain entrained in the boundary layer on the surface of the 
airfoil for as long a distance as possible downstream of the passage to 
provide a protective film of cool air between the hot mainstream gas and 
the airfoil surface. The angle which the axis of the passage makes with 
the airfoil surface and its relation to the direction of hot gas flow over 
the airfoil surface at the passage breakout are important factors which 
influence film cooling effectiveness. Film cooling effectiveness E is 
defined as the difference between the temperature of the main gas stream 
(Tg) and the temperature of the coolant film (T.sub.f)at a distance x 
downstream of the passage outlet, divided by the temperature difference 
between the temperature of the main gas stream and the coolant temperature 
(T.sub.c) at the passage outlet (i.e., at x=0) thus, E=(T.sub.g 
-T.sub.f)/(T.sub.g -T.sub.c). Film cooling effectiveness decreases rapidly 
with distance x from the passage outlet. Maintaining high film cooling 
effectiveness for as long a distance as possible over as large a surface 
area as possible is the main goal of airfoil film cooling. 
It is well known in the art, that the engine airfoils must be cooled using 
a minimum amount of cooling air, since the cooling air is working fluid 
which has been extracted from the compressor and its loss from the gas 
flow path rapidly reduces engine efficiency. Airfoil designers are faced 
with the problem of cooling all the engine airfoils using a specified, 
maximum cooling fluid flow rate. The amount of fluid which flows through 
each individual cooling passage from an internal cavity into the gas path 
is controlled by the minimum cross-sectional area (metering area) of the 
cooling passage. The metering area is typically located where the passage 
intersects the internal cavity. The total of the metering areas for all 
the cooling passages and orifices leading from the airfoil controls the 
total flow rate of coolant from the airfoil, assuming internal and 
external pressures are fixed or at least beyond the designer's control. 
The designer has the job of specifying the passage size and the spacing 
between passages, as well as the shape and orientation of the passages, 
such that all areas of the airfoil are maintained below critical design 
temperature limits determined by the airfoil material capability, maximum 
stress, and life requirement considerations. 
Ideally, it is desired to bathe 100% of the airfoil surface with a film of 
cooling air; however, the air leaving the passage exit generally forms a 
cooling film stripe no wider than or hardly wider than the dimension of 
the passage exit perpendicular to the gas flow. Limitations on the number, 
size, and spacing of cooling passages results in gaps in the protective 
film and/or areas of low film cooling effectiveness which may produce 
localized hot spots. Airfoil hot spots are one factor which limits the 
operating temperature of the engine. 
U.S. Pat. No. 3,527,543 to Howald uses divergently tapered passages of 
circular cross section to increase the entrainment of coolant in the 
boundary layer from a given passage. The passages are also preferably 
oriented in a plane extending in the longitudinal direction or partially 
toward the gas flow direction to spread the coolant longitudinally upon 
its exit from the passage as it moves downstream. Despite these features, 
it has been determined by smoke flow visualization tests and engine 
hardware inspection that the longitudinal width of the coolant film from 
an eliptical passage breakout (i.e. Howald) continues to expand 
longitudinally only about a maximum of one passage exit minor diameter 
after the coolant is ejected on the airfoil surface. This fact, coupled 
with typical longitudinal spacing of three to six diameters between 
passages, result in areas of airfoil surface between and downstream of 
longitudinally spaced passages which receive no cooling fluid from that 
row of passages. Conical, angled passages as described in Howald U.S. Pat. 
No. 3,527,543 provide at best probably no more than 70% coverage 
(percentage of the distance between the centers of adjacent hole breakouts 
which is covered by coolant). 
The velocity of the air leaving the cooling passage is dependent on the 
ratio of its pressure at the passage inlet to the pressure of the gas 
stream at the passage outlet. In general the higher the pressure ratio, 
the higher the exit velocity. Too high an exit velocity results in the 
cooling air penetrating into the gas stream and being carried away without 
providing effective film cooling. Too low a pressure ratio will result in 
gas stream ingestion into the cooling passage causing a complete loss of 
local airfoil cooling. Total loss of airfoil cooling usually has 
disastrous results, and because of this a margin of safety is usually 
maintained. This extra pressure for the safety margin drives the design 
toward the high pressure ratios. Tolerance of high pressure ratios is a 
desirable feature of film cooling designs. Diffusion of the cooling air 
flow by tapering the passage, as in the Howald patent discussed above is 
beneficial in providing this tolerance, but the narrow diffusion angles 
taught therein (12.degree. maximum included angle) require long passages 
and, therefore, thick airfoil walls to obtain the reductions in exit 
velocities often deemed most desirable to reduce the sensitivity of the 
film cooling design to pressure ratio. The same limitation exists with 
respect to the trapezoidally shaped diffusion passages described in 
Sidenstick, U.S. Pat. No. 4,197,443. The maximum included diffusion angles 
taught therein in two mutually perpendicular planes are 7.degree. and 
14.degree., respectively, in order to assure that separation of the 
cooling fluid from the tapered walls does not occur and the cooling fluid 
entirely fills the passage as it exits into the hot gas stream. With such 
limits on the diffusing angles, only thicker airfoil walls and angling of 
the passages in the airfoil spanwise direction can produce wider passage 
outlets and smaller gaps between passages in the longitudinal direction. 
Wide diffusion angles would be preferred instead, but cannot be achieved 
using prior art teachings. This is particularly true with respect to the 
Sidenstick patent which describes a sheet metal electrode for 
electro-discharge machining divergently tapered film cooling holes have 
trapezoidally shaped cross sections. Such a prior art electrode is shown 
herein in FIGS. 1, 2, and 2a, which are reproductions of FIGS. 4, 6, and 
6a, respectively, of Sidenstick. Although FIG. 2a purports to show, in a 
"gun barrel" view, the shape of the passage formed by the sheet metal 
electrode of FIGS. 1 and 2, we have machined passages in curved surfaces, 
like airfoil surfaces, using electrodes having such shape, but found them 
to produce passages like that shown in FIG. 2b rather than FIG. 2a. This 
passage has a notch 5 along the length of the upstream surface 7; and 
because of this notch the coolant does not diffuse and completely fill the 
diffusing section. Instead it remains as a cohesive jet. This reduces the 
film spreading and produces a coolant film narrower than the passage 
outlet. 
DISCLOSURE OF INVENTION 
One object of the present invention is an improved electrode for forming 
shaped passages through the wall of a work piece. 
Another object of the present invention is a sheet metal electrode for 
forming divergently tapered cooling air passages through the external wall 
of an airfoil. 
According to the present invention, a sheet metal electrode has a straight, 
longitudinally extending forward end section, a flat middle section, and a 
rear section, the forward section having a flat first surface and an 
oppositely facing flat second surface, the middle section having a flat 
first surface integral with the rear edge of the first surface and forming 
an obtuse angle therewith, the middle section having side edges diverging 
from each other away from the forward section, including a pair of side 
walls, each integral with one of the side edges, each side wall having a 
longitudinally extending edge in the plane of the second surface and 
extending rearwardly therefrom, the rear section being integral with a 
rear edge of the middle section and extending rearwardly therefrom for 
attachment to an electro-discharge machine. 
The electrode of the present invention differs from the Sidenstick 
electrode described in U.S. Pat. No. 4,197,443 in that the middle section 
of the present electrode, which forms the tapered surfaces of the passage, 
includes side walls along the length of its diverging edges. The side 
walls assure that the passage formed by the electrode has a flat surface 
extending from the external surface of the airfoil wall (and which is a 
part of the diffusing section of the passage) through the metering section 
to the internal surface of the wall. 
The foregoing and other objects, features and advantages of the present 
invention will become more apparent in the light of the following detailed 
description of preferred embodiments thereof as illustrated in the 
accompanying drawing.

BEST MODE FOR CARRYING OUT THE INVENTION 
With reference to FIG. 3, a blade 100 for use in the turbine section of a 
gas turbine engine is shown in side elevation view. The blade 100 includes 
a hollow airfoil 102 which extends in a spanwise or longitudinal direction 
from a root 104. A platform 106 is disposed at the base of the airfoil 
102. The airfoil 102 is hollow and includes a plurality of film cooling 
passages 108 extending through the airfoil wall 110 (FIG. 4). For purposes 
of simplicity and clarity, only two longitudinally extending rows of 
passages 108 are shown in the drawing. A typical turbine section airfoil 
will have many more rows of passages, some rows being on the pressure side 
of the airfoil, and others being disposed along the leading edge and 
suction side of the airfoil. In all cases the passages 108 communicate 
with a compartment within the airfoil, which compartment is adapted to 
receive pressurized coolant fluid through channels 112 through the root 
104, which channels communicate with the compartments. The pressurized 
fluid flows out of the compartments through the wall 110 via the passages 
108, cooling the wall and preferably forming a film of coolant on the 
outer surface 114 of the airfoil downstream (i.e., in the direction of the 
mainstream hot gas flow over the airfoil surface) of the passage outlet. 
The shape of the coolant passages is best described with respect to FIGS. 
4-6. Essentially, each passage 108 includes a straight metering section of 
constant cross section along its length, and a diffusing section 118 in 
series flow relation therewith. The metering section 116 includes a pair 
of first and second flat, spaced apart, parallel side walls 120, 122 
interconnected by a pair of flat, spaced apart, parallel end walls 124, 
126. These walls intersect the internal wall surface 128 of a compartment 
on the inside of the airfoil and define an inlet 130 to the passage 108 
and to the metering section 116 for receiving a controlled flow of coolant 
fluid from the airfoil compartment. The outlet 132 of the metering section 
is coincident with the inlet of the diffusing section. The diffusing 
section comprises a pair of spaced apart, facing side walls 134, 136 
interconnected by a pair of facing, spaced apart, end walls 138, 140. The 
side walls and end walls of the diffusing section intersect the outer 
surface 114 of the wall 110 to define an outlet 141. The side surface 134 
of the diffusing section 118 is coplanar with the side surface 120 of the 
metering section 116. The side surface 136 of the diffusing section 118 
diverges from the opposing side surface 134 toward the outlet 140 at an 
angle herein designated by the letter A. The end surfaces 138, 140 diverge 
from each other by an included angle B. 
FIGS. 7-11 show a sheet metal electrode 200 for electro-discharge machining 
passages having a shape like that of the passages 108. Each electrode 
includes a plurality of teeth 202. Each tooth 202 includes a front section 
206, a middle section 208, and a rear section 210. The rear sections 210 
of the teeth 202 are coextensive and form a common base (hereinafter also 
referred to by the reference numeral 210) for the electrode 200 which, 
during use, is secured to a tool holder (not shown). The holder is 
connected to a negative terminal of a DC power source; and the airfoil 102 
into which the passages are to be machined is connected to a positive 
terminal. The electrode is moved toward the wall of the work piece to be 
machined, such as the airfoil wall 110, and when the gap between the 
electrode and the surface of the airfoil becomes small enough there will 
be an electric discharge thereacross which removes particles of material 
from the airfoil. The electrode continues to be moved into the airfoil 
until the front sections 206 of the teeth 202 pass entirely through the 
wall 110, but to a predetermined depth. In FIG. 4 the phantom line shows 
the position of the electrode 200 at its full depth. In this embodiment 
the base 210 of the electrode 200 does not penetrate the airfoil wall 110. 
Thus, the shape of the passages formed by the electrode are determined 
solely by the shape of the front and middle sections 206, 208 of the 
electrode teeth 202 and the direction of movement of the electrode into 
the airfoil. 
In accordance with the present invention, the front section 206 is flat and 
elongated in what is herein referred to as the longitudinal direction, 
which is along an axis 212 of the tooth 202. The front section 206 has a 
constant cross-sectional area perpendicular to the longitudinal and 
includes an upper surface 214, the upper surface having a pair of 
straight, parallel side edges 218 extending in the longitudinal direction, 
and a rear edge 220 interconnecting the side edges. The middle section 208 
includes a flat upper surface 222 lying in a plane which forms an obtuse 
interior angle E with the upper surface 214 and an acute angle C with an 
extension of the plane of the upper surface 214. The upper surface 222 has 
a forward edge coincident with and the same length as the rear edge 220 of 
the upper surface 214. The upper surface 222 also has a pair of side edges 
224 on opposite sides of the axis 212, each edge diverging therefrom by an 
angle herein designated by the letter D (FIG. 11). Each edge 224 also 
includes a side wall 226 integral therewith along the length of the edge, 
the side walls, in this preferred embodiment, being perpendicular to the 
plane of both upper surfaces 214, 222. Each side wall includes an outer 
surface 228 facing away from the axis 212, and a straight lower edge 230 
extending rearwardly from the rear edge 220 of the front section to the 
base 210. The edges 230 lie in the plane of the lower surface 216 of the 
front section. The base 210 has a front edge 232 which is contiguous with 
a rear edge of each of the middle sections 208. In this embodiment the 
base 210 is parallel to the front section 206. 
From the drawing it can be seen that the outwardly facing surfaces 228 of 
the side walls 226 of each electrode tooth form the end surfaces 138, 140 
of the diffusing section 118 of the coolant passage 108. In this regard, 
the angle D (FIG. 11) is one-half the desired included angle B of the 
coolant passage. Similarly, the upper surface 222 of the middle section 
208 forms the side surface 136, of the diffusing section 118; and the 
lower edges 230 of the side walls 226, along with the lower surface 216 of 
the front section 206, form the side surface 134 of the diffusing section 
118. Thus, the angle C of each electrode tooth is selected to be 
substantially the angle A of the coolant passage. 
FIG. 12 shows a top view of a portion of a piece of sheet metal from which 
the electrode of FIGS. 7-11 may be cut. The solid lines and that portion 
of the piece of sheet metal which is unshaded is the shape of the flat 
sheet metal (i.e., blank) which may be cut from the larger piece and 
formed to the electrode of the present invention. The shaded areas of the 
piece of sheet metal are discarded. The dashed lines are the lines along 
which the sheet metal is bent to form the electrode shown in FIGS. 7-11. 
Actually, the sheet metal is formed to its finished shape by stamping the 
blank in a die having the desired finished shape. 
FIGS. 13 and 14 show an alternate configuration for the electrode of the 
present invention wherein the side walls 300 blend as a smooth curve with 
the flat upper surface 302 of the middle section. Such an electrode 
produces a coolant passage having smoothly curved corners which increase 
in radii to the passage outlet. Preferably the curved corners are the 
shape of oblique cones. Passages of this shape are more fully described in 
commonly owned U.S. patent application Ser. No. 812104 titled "Improved 
Film Cooling Passages with Curved Corners" by Robert E. Field filed on 
even date herewith and incorporated herein by reference. 
Although the invention has been shown and described with respect to a 
preferred embodiment thereof, it should be understood by those skilled in 
the art that other various changes and omissions in the form and detail of 
the invention may be made without departing from the spirit and scope 
thereof.