US 3793169 A
Electrochemical machining of small deep holes (several hundredths of an inch in diameter and less) is achieved at higher feed rates utilizing a constant current source and constant feed rate, allowing the voltage and power to vary with depth. Small holes with profiled cross section are achieved by programming the controlled current for different current/feed rate ratios as a function of hole depth.
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Description (OCR text may contain errors)
United States Patent 1 Joslin [4 1 Feb. 19, 1974 SMALL HOLE ECM DRILLING WITH CONTROLLED CURRENT Inventor: Frederick R. Joslin, Lebanon, Conn.
United Aircraft Corporation, East Hartford, Conn.
Jan. 10, 1972 Assignee:
US. Cl. 204/129.55, 204/224 M Int. Cl B23p l/02, B23p l/l2, B23p l/16 Field of Search 204/224 M, 129.25, 129.55
 References Cited UNITED STATES PATENTS 12/1971 Koire et a1. 204/224 M /1962 Williams 204/224 M 6/1968 Shibasaki 204/l29.75
OTHER PUBLICATIONS et a1. Electrochemical Machining, 1968 De Barr Amer. Elsenier Pub. Co., NY, NY, pages -47, -72, -82 and 191-192.
Uhlir, Jr., The Review of Scientific Instruments Vol. 26, No. 10 October 1955 pp. 965-968.
Primary Examiner-F. C. Edmundson Attorney, Agent, or FirmMelvin Pearson Williams [5 7] ABSTRACT 1 Claim, 7 Drawing figures SMALL HOLE ECM DRILLING WITII CONTROLLED CURRENT BACKGROUND OF THE INVENTION l. Field of Invention This invention relates to electrochemical machining, (ECM), and more particularly to improved ECM drilling of small holes.
2. Description of the Prior Art As is known in the ECM art, the presence of an electrolyte in a suitably confined gap between an advancing hollow cathode tool with outer surfaces electrically insulated except at its frontal (gap) area and a workpiece anode, with a suitable voltage impressed across the gap, comprises an electrolytic cell in which an electrochemical process will cause controlled erosion of the metal of the workpiece. Employing a suitable slightly alkaline saline electrolyte produces an insoluble metal hydroxide sludge which is flushed away and hydrogen is evolved at the cathode.
In the ECM drilling of holes, a thin hollow cathode conducts the electrolyte to the work area through its central bore, and the effluent electrolyte flows away from the work area in the annular space created between the surface of the hole being drilled and the hollow cathode. Electrical power, equal to the product of the voltage across the gap times the current through the gap, accomplishes the useful work of metal removal and also creates heat in the work area, which warms the electrolyte. The effluent electrolyte exiting from the drilled hole warms the incoming electrolyte through the walls of the hollow-cathode, in a heat exchange type of action. In the drilling of holes on the order of one hundredth of an inch in diameter, the rate of flow of electrolyte is extremely small, being on the order of one drop every 5 seconds. Because of the heat exchange action and the low flow rate, the temperature of electrolyte in the gap can rise considerably, and tends to become increasingly higher as the hole depth increases.
In ECM processes known to the prior art, a constant voltage is employed acrossthe gap. As the temperature increases, the conductivity of the electrolyte increases so that the current in the gap increases. With a constant voltage, an increase in current means an increase in the power generated in the gap, which power is taken up in the form of heat within the electrolyte, causing a further increase in the temperature. If the temperature becomes sufficiently high, the electrolyte will boil, causing vapor pockets in the gap; the vapor pockets insulate the anode from the cathode and thus preclude the electrochemical reaction from occurring; and, as the hollow cathode continues to advance, without erosion of the workpiece, the gap becomes smaller. If not corrected, this process may culminate in an unintentional contact between the cathode tip and the workpiece, which is commonly referred to as a spark-out.
Devices known to the prior art have obviated the situation by utilizing reduced feed rates. As an example, prior art feed rates for ECMdrilling of small holes are typically limited to below one tenth of an inch per minute. For a given hole diameter, the rate of metal re} moval (feed rate) is proportional to the current. The achievement of high feed rates requires the ability to utilize higher currents without spark-outs.
SUMMARY OF INVENTION The object of the present invention is to provide improved ECM drilling of small holes.
According to the present invention, holes less than three hundredths of an inch in diameter and with a ratio of depth to diameter in excess of ten, are drilled utilizing an electrochemical machining process in which the current is completely controlled. In accordance with one embodiment of the invention, drilling of substantially uniform straight holes is achieved employing a constant current in an ECM process. In accordance with another embodiment of the invention, drilling of contoured holes is accomplished by programming the current to feed rate ratio as a function of hole depth.
The present invention provides an order of magnitude increase in the rate at which small holes can be drilled using ECM processes. The invention also permits the drilling of holes having desired contours, which are not otherwise readily obtainable. The invention provides a very practical method of producing small holes in ordinary metals as well as super alloys.
In drilling holes employing the process of the present invention (with the voltage being a dependent variable resulting from fixed current and other parameters), as the depth of the hole progresses, the heat builds up and the conductivity of the electrolyte increases; the voltage necessarily decreases, so the thermal power input to the electrolyte goes down. Thus there is a tendency for the process to thermally limit itself. This in turn necessarily results in mitigating the preheating effect which the effluent electrolyte has on the incoming electrolyte. Metal removal rate is a function of current which is held constant. Therefore the hole diameter remains essentially constant with a fixed feed rate despite increases of electrolyte temperature in the gap producing corresponding decreases in gap voltage. In a typical case the gap electrolyte has exhibited alinear negative temperature coefficient of resistance of 0.33%/F from 1 10 to .170F.
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.
BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a simplified schematic illustration of electrochemical machining apparatus which may be employed in the practice of the present invention;
FIGS. 2, 4 and 6 are sectioned, side elevations of contoured holes; and v I FIGS. 3, 5 and 7 are current versus depth plots relating respectively to the contours of FIGS. 2, 4 and 6.
DESCRIPTION OF THE PREFERRED EMBODIMENT 26. The feed mechanism 26 includes a ram 28 on which the collet 24 or other attaching means is suitably disposed. The ram 28 may be journaled in a precision ball guide assembly comprising a plurality of balls (such as the balls 30) confined by a suitable plate (such as the plate 32). In fact, the entire surface of the ram 28 may preferably be guided by additional balls and plates (not shown). The ram 28 may be advanced by a rack 34 and pinion 36 driven by a shaft 38 connected to a synchronous motor 40. Alternatively a precision ground ball screw or other suitable precise feed mechanism may be utilized, as is known in the art. A controlled current DC source 42 is connected by a suitable lead 44 to the workpiece 10, and by a suitable lead 46 to the collet 24, thereby to impress a voltage between the hollow cathode 14 and the workpiece 10.
In operation, the electrolyte under a suitable pressure is driven through the hollow cathode 14 and out the end thereof in the vicinity of the gap formed between the end of the hollow cathode 14 and the workpiece metal in an area having a diameter slightly larger than the diameter of the hollow cathode 14 is eroded by electrochemical action, the metal forming a metallic hydroxide sludge which is flushed out of the hole as the effluent electrolyte passes upwardly in an annular passage formed between the surface of the hole '12 andthe hollow cathode 14. The effluent electrolyte is collected in a suitable sump (not shown), as is known in the art.
In accordance with one embodiment of the invention, substantially uniform deep holesare drilled at a relatively high feed rate.
As an example, consider a beryllium copper hollow cathode 14 having a 5 mil ID and a ten mil OD covered with a third of a mil parylene C insulation, used in drilling a high temperature nickel base alloy. An electrolyte consisting of a saline solution (two pounds per galdrilled. The initial volta e was approximately ten volts and the voltage decaye toabout 6% volts as the drilling proceeded. In a similar example, a hole was drilled at a rate of 0.3 inches per minute utilizing 400 rnilliamps constant current.
The examples just described, utilizing constant current, produce a substantially uniform hole at a relatively high rate of feed. Depending on the material being worked, and particularly in cases where higher feed rates are used, it is possible that constant current ECM drilling may result in a hole having a slight taper (that is an increase in diameter as a function of depth of approximately 0.001 per inch). In accordance with another aspect of the invention, this can be eliminated by programming the power supply as a function of depth to decrease the current slightly as the drilling depth increases. This is illustrated in FIG. 1 by a depth sensor 48 which is connected by a lead 50 to the controlled current DC source 42. The depth sensor 48 may comprise a variable resistance having a wiper arm at tached to the ram 28, or any other suitable position sensor capable of response to the position of the ram 28, the collet 24, or the hollow cathode 14. In accordance with teachings known to the art, a variable resistance or voltage derived therefrom may be utilized so as to adjust the current setting of the control current DC source 42 as a function of depth.
In accordance with another aspect of the invention, the controlled current DC source 42 may be programmed to provide current (I) as a linear or nonlinear function 'of depth (D) so as to provide any desired hole contour as illustrated in FIGS. 2-7. A jug-like shape is achieved as seen in FIG. 2 by first decreasing the current and then increasing it as illustrated in FIG. 3. A wasp-waist configuration as seen in FIG. 4 is provided by a concave upward current curve as shown in FIG. 5. In a hole with an increasing diameter (suitable for use with cast or deformable rivets) as seen in FIG. 6, may be achieved by providing an increase in current with depth as illustrated in FIG. 7.
Although the invention has been shown and described with respect to preferred embodiments thereof, it should be understoodby those skilled in the art that various changes and omissions in the form and detail thereof may be made therein without departing from the spirit and the scope of the invention.
Having thus described typical embodiments of my invention, that which I claim as new and desire to secure by Letters Patent of the United States is:
1. In the process of drilling a small hole, having a diameter less'than thirty mils and and a ratio of depth to diameter in excess of ten, in a metallic workpiece, the steps of:
positioning an exteranlly insulated hollow electrode,
having an'outside diameter of less than 30 mils, adjacent a surface of a workpiece in which said small hole is to be drilled; impressing a voltage of between 6.5 and ten volts from a constant current DC source of between 275 and 400 milliamperes between said workpiece and said electrode, poled to cause said electrode to act as a cathode and said'workpiece to act as an anode;
flowing a saline electrolyte through said electrode at a pressure on the order of 50 PSI; and advancing'said electrodein a direction toward said workpiece at a rate of between 0.18 and 0.30
l inches per minute.