Electrode assembly and machine for small-hole electrochemical drilling

The present invention discloses an improved electrode assembly and electrochemical drilling machine for drilling small diameter holes. The electrode assembly includes a metallic working electrode tube having an inlet end for electrolyte entry and an outlet end for drilling, the tube having preselected dimensions to provide the desired diameter and depth of hole. The assembly also includes a filter tube one end portion of which defines a plurality of individual access openings for electrolyte entry from an electrolyte source, each opening having a smaller cross section than the bore of the working electrode tube to filter out particles in the electrolyte which could clog the electrode tube and the other end of which is in fluid communication with the inlet end of the electrode tube for delivering filtered electrolyte thereto. Connector means are provided for releasably and sealably connecting the electrode tube and filter tube, enabling the electrode tube to be removed, if damaged or otherwise defective, without disconnecting the filter tube from the electrolyte source.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to electrochemical drilling and, more 
particularly, to an improved electrode assembly and machine for use in 
drilling small diameter holes. 
2. Description of the Prior Art 
Electrochemical drilling is a widely used technique for providing holes and 
the like in difficult to machine metal and alloy articles, such as nickel 
or cobalt base superalloy turbine blades or vanes. U.S. Pat. No. 3,647,674 
discusses several factors which limit the minimum diameter of holes which 
can be drilled with such processes. 
One limiting factor which has been especially troublesome is the propensity 
for blockage of the hollow electrode tube by dirt particles or other 
matter in the electrolyte being pumped therethrough. Of course, if 
electrolyte flow through the tube is blocked, drilling cannot proceed and 
the tube must be removed and cleaned before drilling can resume. In some 
cases, this necessitates dismantling major parts of the electrochemical 
machine, such as, for example, the electrolyte manifold which distributes 
electrolyte to the individual electrode tubes. 
As the diameter of the hole to be drilled decreases, the internal diameter 
of the hollow electrode tube in general must also decrease. As a result, 
clogging and blocking of the tube is considerably more frequent in 
drilling smaller diameter holes than larger ones and this increased 
clogging has somewhat limited the use of electrochemical drilling in the 
commercial production of large numbers of parts having a plurality of 
small diameter holes therein. The problem is aggravated when the holes are 
not only small in diameter but also of considerable depth. 
Another problem associated with small hole drilling is that the thin 
walled, small diameter electrode tubes are prone to bend and flex during 
drilling and thereby deviate from the desired drilling path. This bending 
produces misaligned holes and is aggravated as the depth of the hole 
increases. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved electrode assembly and drilling machine especially useful for 
drilling small diameter holes, such as holes of a diameter of about 10 
mils to 20 mils and above. 
It is another object of the invention to significantly reduce, if not 
eliminate, clogging or blocking of the electrode tube during drilling of 
small diameter holes. 
It is another object of the invention to allow a damaged or otherwise 
defective electrode tube to be removed without dismantling major 
components of the drilling machine. 
It is still another object of the invention to minimize bending and flexing 
of the electrode tube during drilling, thereby reducing the number of 
misaligned holes and number of workpieces rejected therefor. 
In a typical preferred embodiment of the present invention, the electrode 
assembly includes a metallic working electrode tube having an inlet end 
for receiving electrolyte and an outlet end for directing the electrolyte 
against the workpiece for drilling, the dimensions of the tube being 
selected to produce the desired diameter and depth of hole. The assembly 
also includes a filter tube, one end portion of which defines a plurality 
of individual access openings for electrolyte entry from an electrolyte 
source, the cross-section of the access openings being smaller than the 
bore of the working electrode tube to prevent particles capable of 
clogging the electrode tube from entering the assembly, and the other end 
of which is in fluid communication with the inlet end of the electrode 
tube for delivering filtered electrolyte thereto. Preferably, connector 
means, such as a plastic-jacket, are provided for releasably and sealably 
connecting the inlet end of the electrode tube with the electrolyte 
delivery end of the filter tube. Such connector means enables the working 
electrode tube to be removed, if defective, without disconnecting the 
filter tube from the electrolyte source. 
These and other objects and advantages of the present invention will appear 
more fully from the following drawings and description of the preferred 
embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1a, a preferred electrode assembly of the invention is 
shown as including a metallic working electrode tube 4, a metallic filter 
tube 6 and connector means in the form of plastic jacket 8 for releasably 
and sealably joining the tubes. As is usual in the prior art practice, the 
electrode tube has an electrical insulating layer 4a thereon. By way of 
example only, the electrode assembly will be described as one for use in 
drilling a hole 13.5 mils in diameter. 
In drilling such a hole, the electrode tube 4 has an inner and outer 
diameter of 6.5 mils and 12 mils, respectively, the outer diameter 
including the thickness of insulation layer 4a. The tube includes inlet 
end 4b for receiving filtered electrolyte from the filter tube and outlet 
end 4c for directing the electrolyte against a workpiece (not shown) for 
drilling. For optimum drilling, the outer tip of the electrode tube has a 
frustoconical surface contour, as shown in FIG. 1a. In drilling nickel 
base superalloy parts, such as gas turbine blades and vanes, the electrode 
tube is made of titanium tubing to resist the corrosive effects of the 
electrolyte utilized, for example, 8% nitric acid. Of course, the inner 
and outer diameter of the tube are selected to produce the desired 
diameter hole in the workpiece. Contrary to the prior art, the length of 
the electrode tube is small as compared to the total length of the 
electrode assembly. The electrode tube length required is dictated solely 
by the actual depth of hole to be produced, not by the dimensions of the 
drilling machine. For example, in the prior art, the electrode tube 4 
would be of such a length to extend from the workpiece 14 to the 
electrolyte manifold 10, see FIG. 2. In contrast, in the present 
invention, the electrode tube is of a length just sufficient to form the 
desired depth of hole, as discussed hereinafter. The net result is that a 
much shorter electrode tube is used, thereby reducing bending and flexing 
of the tube during drilling and the cost of the electrode assembly. 
According to the present invention, filter tube 6 is connected to electrode 
tube 4, as shown, to deliver filtered electrolyte thereto. The filter tube 
is preferably a metallic tube, for example, titanium to allow completion 
of the electrochemical circuit through the electrode assembly and includes 
an end portion 6a defining a plurality of individual access openings 6b 
for receiving electrolyte from an electrolyte source, such as manifold 10 
in FIGS. 2 and 3. As illustrated, the end portion 6a of the filter tube 
includes the actual end of the tube, which end may be fully closed (as 
shown) or partially closed, and also the tube walls adjacent thereto. In 
FIG. 1a, the end of the tube is fully closed for convenience and ease of 
fabrication. Although it may be possible to only partially close the tube 
end to provide an additional access opening, this would require an 
additional somewhat costly step as compared to simply crimping and welding 
the end shut and laser drilling the access openings in the tube walls. In 
order to effect the desired filtering action, the cross-section of each 
access opening in end portion 6a must be smaller than the bore (inner 
diameter) of the working electrode tube, for example, access openings 4.5 
mils in diameter have been satisfactorily used with the electrode tube of 
6.5 mils inner diameter. The number of access openings can be varied as 
desired to ensure adequate electrolyte flow rate and filtering action. 
However, it too few access openings are provided, there is a danger that 
the openings themselves will become clogged in a short time and block 
electrolyte flow into the assembly. 
In certain situations, it may be desirable and preferred to increase the 
wall thickness of the filter tube to provide sufficient rigidity to 
further reduce or minimize flexing and bending of the electrode tube 
during drilling. This results in even fewer misaligned holes and a 
decrease in the number of drilled parts rejected. For example, a filter 
tube having inner and outer diameters of 13 mils and 25 mils, 
respectively, has been found suitable to further minimize flexing of the 
electrode tube of dimensions given hereinabove. 
As shown in FIG. 1a, the inlet end of the working electrode tube is 
inserted in the open end of the filter tube to provide fluid communication 
therebetween, the portion of the electrode tube inserted into the filter 
tube preferably being provided with bends or kinks to insure electrical 
contact is made between the outer wall of the electrode tube and inner 
wall of the filter tube. Thus, the filtered electrolyte can pass through 
the filter tube into the electrode tube for direction against the 
workpiece. To prevent leakage at the area of insertion and to allow the 
electrode tube to be removed, if damaged or otherwise defective, without 
dismantling the manifold 10, the tubes are preferably joined together by 
plastic jacket 8 which is heat-shrunk or otherwise formed therearound. A 
heat-shrinkable polyolefin jacket has been satisfactorily used. Of course, 
those skilled in the art will recognize that other releasable connector 
means may be used. Those skilled in the art will also recognize that the 
electrode tube and filter tube may also be permanently and sealably 
connected together, such as by brazing material 5 as in FIG. 1b, 
conductive epoxy binder and the like. Or, the tube ends may be flared, 
swaged and the like to effect such a permanent and sealed connection. 
In FIG. 2, a plurality of electrode assemblies are shown positioned in a 
conventional electrochemical drilling machine which machine includes an 
electrolyte manifold 10 mounted on a movable ram (not shown) and a guide 
member 12 for positioning the electrode assemblies in precise relation to 
the workpiece 14. As illustrated, the working electrode tubes 4 are 
positioned against the workpiece for commencement of drilling. It should 
be noted that the length of the electrode tubes is just sufficient to 
produce the desired depth of hole (dotted line) as the ram moves the tubes 
downwardly toward and into the workpiece. The filter tubes 6 extend from 
the electrode tubes through the guide member and into the manifold. In the 
manifold, electrolyte enters the access openings provided in the end 
portion of the filter tube, as shown in FIG. 3. 
It will now be apparent that the present invention provides several 
advantages over prior art electrodes and drilling machines. An especially 
important advantage of the invention is that the electrolyte is filtered 
before it enters the electrode assembly. Filtering insures that drilling 
will not have to be prematurely terminated or interrupted to change a 
clogged electrode tube. If one or more electrode tubes should be damaged 
or otherwise become inoperative, the malfunctioning tubes can be removed 
and replaced simply and easily without dismantling the manifold, guide 
member of other machine components. The use of the short electrode tubes 
and/or the relatively thick walled filter tubes minimizes flexing and 
bending of the electrode tubes and greatly reduces the number of 
misaligned holes and rejected parts. 
Although the invention has been shown and described with respect to 
illustrative embodiments thereof, it will be understood by those skilled 
in the art that changes and additions in the form and detail thereof may 
be made without departing from the spirit and scope of the invention.