United States Patent (19) (11) 3,849,271

Joslin (45) Nov. 19, 1974

54 ELECTROCHEMICAL DRILLING OTHER PUBLICATIONS
75 Inventor: Frederick R. Joslin, Lebanon, Conn. t Machining by DeBarr et al., pp. 46,
73 Assignee: United Aircraft Corporation, East 1968. 193, pub.
p byy Amer.r. Elseview,
Elseview, NNew York,
Hartford, Conn.
22 Filed: Dec. 13, 1973 Primary Examiner-F. C. Edmundson
21 Appl. No.: 424,482 Attorney, Agent, or Firm--M. P. Williams
- -- Related U.S. Application Data (57) ABSTRACT
I63) Continuation of Ser. No. 278,135, Aug. 4, 1972, - -
abandoned. In drilling small diameter holes through workpieces by
-- electrochemical means, a constant voltage is imposed
52 U.S. Cl.......................... 204/129.2, 204/224 M during the drilling operation and the operation is in
51 int. Cl......... B23p 1100, B23p 1102, B23p 1/12 terrupted after breakthrough when the current
58) Field of Search....................... 204/129.2, 129.1 reaches a selected value thereby determining the con
figuration of the hole at the exit end.
56) References Cited
UNITED STATES PATENTS 1 Claim, 9 Drawing Figures
3,385,947 5/1968 Inoue........................... 204/129.2 X
PATENT:CY 191974 3,849,27l
SHEET 1 OF 2

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ELECTROCHEMICAL DRILLING FIG. 6b is a hole configuration drilled by the opera
tion plotted in FIG. 6a.
This is a continuation of application Ser. No.
278,135, filed Aug. 4, 1972, now abandoned. DESCRIPTION OF THE PREFERRED
EMBODIMENT
BACKGROUND OF THE INVENTION
Referring first to FIGS. 1 and 2, the workpiece shown
When electrochemically drilling a plurality of holes is an impingement tube 2 for use as an insert in a blade
through a workpiece or workpieces of uniform thick for a gas turbine engine. This impingement tube is air
ness, the drilling operation is usually stopped when a foil shape in cross section as shown in FIG. 2 and has
preselected depth of movement of the drilling elec 10 a plurality of cooling holes 4 formed in the leading edge
trode has occurred. This is effective when there is a by electrochemical drilling. The operation is performed
precise location of all the workpieces in exactly the by an electrode assembly 6 FIG. 1, the head 8 support
same position or when the exit surface for the electrode ing a plurality of hollow electrodes 10. The head 8 is
is always in exactly the same position with respect to fed in a vertical direction to bring the electrodes to
the feed of the electrode. However, differences in di 15 ward and into the workpiece as the holes 4 are formed
mension of workpieces resulting from permitted toler therein. It will be understood that an electrical poten
ances cause the breakthrough surface not always to be tial is provided between the electrodes and the work
at the desired location for the electrode to form the ap piece and that an electrolyte is pumped through the
propriate surface at the exit end of the hole being several electrodes against the workpiece. The tooling
drilled. This problem is especially made difficult in dril 20 may be similar to that shown in the copending applica
ling holes through a hollow body such as a turbine tion of Shaw, Ser. No. 89,862 filed Oct. 18, 1971 and
blade or vane as the tolerance buildup is doubled by the having a common assignee with this application.
varying dimension of the outer surface of the blade on The latter is supported in a fixture 12 having a base
which the blade is supported during the drilling opera 14 on which are mounted two locating blocks 16 and
tion and the tolerance in wall thickness of the wall to 25 18. Each of these blocks has two locating surfaces
be drilled. against which the opposite ends of the workpiece are
SUMMARY OF THE INVENTION held. The spaced workpiece engaging surfaces on block
16 are shown in FIG.3 as the spaced points 20 and 22
A feature of the present invention is the termination on a plate 24, FIG. 1, attached to the block 16.
of the drilling operation after breakthrough at a prede 30 The impingement tube is held against the four locat
termined current level. It has been found that the cur ing surfaces by a clamp 26 which engages the side of
rent varies after breakthrough and that a properly the impingement tube and is held in position by a suit
shaped exit contour for the hole occurs at a predeter able clamping screw 28. The impingement tube is man
mined current level. ufactured to certain dimensions and those dimensions
According to the invention the workpiece is sup 35
have tolerances thereon as is customary. The tolerance
ported in predetermined relation to the electrode so buildup may be such that the leading edge of the im
that the latter may be moved toward and through the pingement tube is not always in the same location and
workpiece as it is being drilled, a predetermined fixed may be as far out of the approved or accepted position
voltage value is imposed between the electrode and the 40
as the extent of the deviation represented by the dotted
workpiece and an electrolyte is pumped through the lines 30 in FIG. 2. That is to say, the portion of the tube
hollow electrode and against the workpiece with the where the holes are to be drilled is not always in pre
electrochemical drilling operation continuing, the elec cisely the same relationship to the electrodes or in fact
trochemical current increasing to a peak value, then to the base of the machine with respect to which the
decreasing at breakthrough until a predetermined cur 45 electrodes are fed into the impingement tube. Thus, in
rent value is indicated at which time the drilling opera drilling the holes with the electrodes 10 if the set of
tion is terminated. electrodes is stopped in the same predetermined point
BRIEF DESCRIPTION OF THE DRAWINGS at the end of each drilling operation, the hole will not
necessarily be finished in some cases or in other cases
FIG. 1 is a schematic showing of an electrochemical 50 the electrode may have progressed through the work
drilling operation. piece and be fed beyond the necessary position for dril
FIG. 2 is a sectional view through the workpiece ling the desired hole. The dimension of the impinge
showing the deviation from the normal in successive ment tube is such that it is difficult to determine pre
workpieces. cisely the location of the successive impingement tubes
FIG. 3 is a fragmentary sectional view along line 55 when they are mounted in the fixture and located as
3-3, not shown, of FIG. 1 showing the location of one shown and it therefore becomes necessary to interrupt
end of the workpiece. the electrochemical drilling operation by some device
FIG. 4a is a plot of drilling time against electrical cur-. that determines the actual relation of the electrode to
rent for drilling a hole of the type shown in FIG. 4b. the inner surface of the workpiece itself, that is to say,
FIG. 4b is a fragmentary sectional view showing the 60 the location of the inner end of the hole being drilled
desired type of electrochemically drilled hole. by the electrode.
FIG. 5a is a plot of drilling time against current where In accordance with the present invention, it has been
the drilling operation is not interrupted until a hole of found that interruption of the drilling operation at a
the shape of FIG. 5b is formed. predetermined current value in the electrical potential
FIG. 5b is the shape hole resulting from a drilling op 65 applied between the electrodes and the workpiece will
eration as plotted in FIG. 5a. permit the proper interruption of the drilling operation
FIG. 6a is a plot of current against time in drilling the so that the electrode is in a predetermined relation to
hole of the shape shown in FIG. 6b. the inner end of the hole and the hole will be formed
 3,849,271
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to the desired shape. Referring now to FIGS. 4, 5 and sive workpieces, the particular vertical location of the
6, the configuration of the hole in FIG. 4b is a result of portion of the impingement tube through which the
termination of the drilling operation when the current holes will be made did not affect the configuration of
reaches a selected value 32 FIG. 4a. During the electro the holes drilled so long as the operation was stopped
chemical drilling operation the current starts at a value at the amperage found empirically to be suitable for
34 and gradually builds up to a peak and then after the making holes the shape of those in FIG. 4b. Accord
electrodes break through on the inner end of the holes, ingly, the automatic interruption of the drilling opera
the current reaches the value 32. It has been found that tion at a predetermined amperage makes possible the
at this particular value the resulting hole shape is sub production of uniformly sized and configured holes in
stantially cylindrical as shown in FIG. 4b and has O successive workpieces in spite of the tolerance changes
rounded ends both at the outer end 38 and at the inner that affect the position of the workpiece with respect
end 40 of the hole. to the head that carries the electrodes.
If the operation is allowed to continue until a lower It should be noted that this process relates to produc
current value is reached as represented by the point 42 tion drilling, and that the empirical determination, by
in FIG. 5a, the hole will be excessively bell-mouthed at 5 trial and error on a few workpieces, of the predeter
the inner end as shown at 44 FIG.5b. On the contrary, mined value at which operations are to be terminated
if the drilling operation is interrupted at a relatively comprises an insignificant amount of preparation in
large current value as represented by the point 46 in that the same current value can thereafter be used for
FIG. 6a, the inner end of the hole will not be finished drilling any number of workpieces of the same general
to the desired diameter and will have an inwardly pro 20 type, such as the impingement tubes 2 referred to with
jecting rim 48 thereon as shown in FIG. 6b. The point respect to FIGS. 1 and 2 hereinbefore.
of breakthrough of the electrodes is represented by a I claim:
particular point in the plot of current against time as 1. In the electrochemical drilling of a hole completely
will be apparent. Breakthrough is indicated when the through a workpiece, the steps of:
current starts to fall after it peaks. 25 positioning a hollow electrode adjacent to a work
in drilling the several holes represented by the show piece;
ing of FIGS. 1 and 2, the thickness of the wall to be flowing an electrolyte through the electrode and
drilled is from 0.010 inches to 0.03 inches and the di against the workpiece;
ameter of the hole to be drilled is 0.036 inches. At the applying a constant electrical voltage between the
start of the operation, the voltage applied is l2.5 volts 30 electrode and the workpiece across the electrolyte;
and the current between the workpiece and the elec
trodes at the start of the operation is 1.0 amps. The op advancing the electrode at a constant rate toward the
cration continues to a peak of about 10 amps, and the workpiece for the drilling of the hole while main
cutoff point in this particular operation was 4.0 amps. taining the electrolyte flow and voltage substan
This produced a hole of the configuration shown in tially constant;
FIG. 4b. The electrolyte used was sodium chloride 2.25 measuring the magnitude of electric current flow be
lbs./gal, with sodium hydroxide added to a pH of about tween the electrode and the workpiece through the
0. The operation terminates when the electrode electrolyte as the electrode is advanced toward the
projects 0.005 inches beyond the inner wall. The feed workpiece and drilling proceeds, the electric cur
rate for the electrodes was 0.150 inches per minute and 4) rent first increasing as the drilling proceeds until
the starting gap varied from 0.005 to 0.015 depending electric current reaches a maximum value at initial
on the tolerance buildup. The cathode pressure was 55 perforation, and thereafter decreasing as drilling
pS1. proceeds until it reaches a predetermined value
If the operation were terminated sooner as in FIG. 6 corresponding to the point of advancement of the
at an amperage of 8 amps, the hole had the contour electrode at which a uniform hole is completed
shown in FIG. 6b. If the operation was allowed to con through the workpiece; and automatically
tinue until the amperage reached 2 amps as in FIG. 5a, terminating the drilling operation when the electric
the hole became bell-mouth as in FIG. 5h. current reaches said predetermined current value.
It was found that in electrochemically drilling succes k: : , k k :

50

55

60

65