US3493710A - Electroerosive machining - Google Patents

Description

Feb. 3, 1970 L. M. MOORE I 3,493,710

' ELEGTROEROSIVE MACHINING Filed May 27, 1966 3 Sheets-Sheet l I? A M'V I8 i o P I k I [6 ,0 7 OXYGEN G o I I5 8 a 0 F I G. 2. mvszv'roa LLOYD M. MOORE BY 1W0. 72.x

ATTORNEY Feb. 3, 1970 L. M. MOORE 3,493,710

7 ELECTROEROSIVE MACHINING Filed llay 27, 1966 3 Sheets-Sheet 2 l Q h w I "ll 4 OXYGEN -OXY6EN FIG. 3.

v a V B c U k b w d e g (i f g I h F I G. INVENTOR.

- LLOYD M. MOORE ATTORNEY Feb. 3, 1970 Filed May 27, 1966 RATE OF METAL REMOVAL,M|L MIN.

0 I0 20 3040 50 6O TO 8090 I00 0 l VOLUME OF OXYGEN RATE OF METAL REMOVAL, FAIL /MIN.

L. n. MOORE 3,493,710

ELECTROEROS IVE MACHINING 3 Sheets-Sheet 3 OXYGEN RATE OF METAL REMOVAL, MIL3/ MIN.

2 3 4 5 6 7 8 GAS FLOW RATE, LITERS/ MIN.

FIG. 5. FIG. 6.

0 IO 20 3O 4O 5O 6O 7O 8O 9O IOOHOIZO I30l40 [5O WIRE ELECTRODE SPEED, FT./ MIN.

FIG. 7.

INVENTOR. LLOYD M. MOORE ATTORNEY United States Patent 3,493,710 ELECTROEROSIVE MACHINING Lloyd M. Moore, Pensacola, Fla., assignor to Monsanto Company, St. Louis, Mo., a corporation of Delaware Filed May 27, 1966, Ser. No. 553,369 Int. Cl. B23k 9/16 US. Cl. 219-69 6 Claims ABSTRACT OF THE DISCLOSURE An electrically erosively cutting wire, desirably made of tungsten metal, is connected to the negative side of a source of direct current electrical energy. The positive side of the electrical energy is connected to a workpiece. The apparatus includes means for axially reciprocating the wire in proximity to the workpiece. A spark eroding discharge is set up between the workpiece and the reciprocating wire. A free-oxygen-containing gas is preferably discharged at the spark.

This invention concerns an electroerosive method of cutting small passages through a workpiece of electrically conductive materials such as metals. In particular, the invention relates to an improved method of electroerosively generating non-circular orifices in spinnerets useful for the production of man-made strands having non-circular cross-sections.

The physical characteristics of man-made filaments may be greatly altered by changing the cross-sectional shape thereof. The luster, handle, bulk, porosity, and many other properties of fabrics are modified by selecting textile filaments having a specific form of cross-section, such as Y-section, star-section, ribbon section, hollow section, etc.

The shape of a synthetic filament cross-section, particularly of a filament made by melt-spinning or by dryspinning, is determined primarily by the shape of the orifices in the spinneret through which the molten polymer or polymer solution is extruded. Under practical spinning conditions the filament section resembles the cross-section of the orifice, although sharp corners or intersections in the orifice tend to be rounded off or merged together in the filament section. To produce filaments having uniformly controlled cross-sections it is necessary, therefore, to provide spinneret capillaries having accurately controlled shapes.

In common parlance, the terms capillary, orifice, and hole are used interchangeably to describe the opening in the spinneret plate that the polymer flows through. These terms are so used in the following discussion, but for descriptive accuracy the following distinctions may be noted: capillary refers to the entire passage through the spinneret plate; that is, the three-dimensional form of the passage including both its axial length and its crosssectional shape. Orifice refers to the opening at each end of the capillary and strictly is two-dimensional in connotation.

Numerous methods of forming non-circular capillaries in spinnerets have been proposed in the prior art. Perhaps the oldest method of forming non-circular holes is by punching. A punch having the desired cross-section is forced through the spinneret plate, shearing out metal to form the hole. This method has been limited to comparatively large capillaries of short length and of relatively simple configuration. The punch, whether single-pass or multiple-pass, tends to deflect bordering areas of metal as it enters and to turn up a burr as it passes through the plate. Both of these defects are difficult to correct so that simply punched capillaries usually exhibit poor operational performance, particularly in melt spinning.

Another method involves the use of conventional twist is u 3 ,493 ,7 l0 Patented Feb. 3, 1970 drills of microdimensions. To form a slot, for example, a row of circular holes are drilled first, and the webs between holes are then punched out or milled out, leaving an open slot. Ordinarily, the small circular holes can not be drilled to a depth greater than 5 to 10 times the diameter of the drill so that maximum capillary length is limited thereby. A single hole might extend to the extreme limit of length but duplication of many such holes requires uncommon skill and attention of the worker.

Although well made non-circular capillaries of short capillary length have been made for experimental use by the twist-drill method, the cost is so great that such spinnerets do not provide a practical basis for large scale commercial yarn production. A single Y-section capillary comprising three radial slots 0.005 inch by 0.040 inch long, for example, normally requires the drilling of not less than about 15 accurately aligned circular holes followed by milling and/or punching operations; both of these operations are very time-consuming so that labor costs are prohibitively high.

Another method of forming orifices utilizes the burning or melting of holes through the spinneret plate by means of a focused beam of accelerated electrons that traces out the contour of the capillary. This method has proven deficient practically. Capillary walls converge (or diverge) from the inlet to the outlet end, and the walls tend to have irregular serrated margins; lateral dimensions also tend to vary excessively from one capillary to another. Because of these uncontrolled dimensional variations and wall irregularities, such spinnerets have poor operating characteristics and resultant yarn properties are too highly variable for general commercial acceptance.

A method somewhat analogous to the preceding has been proposed recently, in which an intense beam of coherent electromagnetic radiation from a laser burns or melts a capillary through a spinneret plate. Satisfactory spinnerets made by this method are not known to have been demonstrated experimentally, but it is likely that they will have dimensional and wall finish deficiencies similar to spinnerets made by electron beams.

Another hole-forming method depends upon electrolytic corrosion. The metal spinneret plate is insulated with a thin coating of dielectric material leaving exposed metal in the shape of the desired orifice cross-section. The spinneret plate forms one electrode (usually the anode) in an electrolytic cell containing the appropriate electrolyte. With an applied direct current, the exposed metal is gradually removed as ions that enter the electrolyte solution. Poor dimensional control limits this method to large capillaries, or to other holes in which large dimensional variations can be tolerated.

Somewhat similar to the preceding is a method that depends upon a photochemical reaction which has been applicable to non-metallic spinneret materials of special composition. A beam of light having the shape of the desired orifice cross-section is projected on the spinneret plate, photochemically activating the illuminated area of the plate. Application of the appropriate corrosion agent, such as a strong acid, causes the photoactivated material to be dissolved away forming a shaped hole through the plate. Dimensional control by this method is somewhat erratic and resultant capillaries converge or diverge considerably from inlet to outlet end, making such spinnerets unsuited to melt spinning even if the structural material were adequate.

Another method depends upon a tool piece having the exact cross-section desired in the orifice; a rod having three radial ribs, for example, constitutes the tool for Y-section orifices. The spinneret blank is submerged in a liquid slurry of abrasive material, and the end of the shaped tool piece is brought into contact with the spinneret plate as the tool is vibrated at ultrasonic frequency. High frequency impact of abrasive particles erodes the metal away as the tool is advanced into the resultant shaped crater until the hole passes completely through the plate. The capillary converges in the direction of tool advance, one end of the capillary differing from the other by as much as 50-75 percent in cross-sectional area. Any nonperpendicularity of the tool to the spinneret plate leads to erosion predominating on one side of the tool and to other dimensional variations in the orifice. Sharp corners or apexes in the orifice shape tend to be rounded out irregularly and wall finish is quite rough. Such spinnerets are quite unacceptable practically without additional shaping operations, such as slow and expensive broaching. Making the shaped tools is a formidable task, and the tool itself is rapidly eroded in the drilling process.

Another method of forming capillaries also depends upon a shaped tool piece having the cross section desired in the orifice. The tool comprises one electrode and the spinneret plate the other electrode in an electrical circuit. The spinneret blank mounted on an electrically insulated holder is submerged in a dielectric liquid. One lead from a source of is connected to the spinneret plate and the other lead is connected to the shaped electrode tool. The end of the tool is advanced against the spinneret plate until dielectric breakdown occurs with spark flash that melts out a microscopic crater. Such electroerosion occurs repeatedly until the tool erodes a hole completely through the plate. The shape of the holes is similar to the shape of the electrode, but the capillary converges in the direction of electrode travel and wall finish is very rough unless the cutting rate is uneconomically low. At this stage, the spinneret capillary is quite unusable commercially. By repeated broaching and deburring of feathered edges produced by the broaching operation, the capillary dimensions and wall finish may be brought to an acceptable state. In common with several of the previously mentioned techniques, these multiple finishing operations make the method unduly expensive. Moreover, all of the methods dependent upon shaped tools and broaching are limited principally to combinations of straight slots and to relatively large finished dimensions, slots about 0.003 inch wide being a practical lower limit for commercially useful spinnerets; another undesirable characteristic of these methods is the difiiculty of centering the capillary in the counterbore, off-center errors often ranging up to 0.010 inch or more.

Yet another electroerosion method is known in the prior art. In this process the active electrode is in the form of a metal wire held under tension by weights. The electrode wire is threaded through a small pilot hole in the spinneret blank that is the anode of the circuit. A film of dielectric fluid is spread on the spinneret surrounding the electrode, and the wrie is moved axially back and forth as it bears against the wall of the pilot hole. Spark erosion gradually erodes away the metal of the spinneret at the contacting area to form a slot slightly wider than the diameter of the electrode wire. The wire may be shifted in a direction normal to its axis, or the same relative motion may be obtained by moving the spinneret in the opposite direction, the wire being held at a fixed position. Thus, the reciprocating wire is caused to traverse the path required to generate the desired slot or other opening. Basically, this method provides the most versatile method ofmanufacturing noncircular capillaries since either straight or curved boundaries may be accurately formed. With care, dimensional accuracy and reproducibility may be maintained with negligible differences between the inlet and outlet ends of the capillary. Major drawbacks of the method are a slow rate of cutting and a slightly rough, pebble-grain wall finish. High costs due to the slow production rate is in practice the principal factor limiting utility of this prior art method. The present invention is an improvement in the basic wi e el ctro e m h of cutt ag c pi laries.

It is an object of the present invention to provide an improved apparatus for electroerosively machining a surface of aworkpiece.

Another object of the present invention is to provide an improved apparatus for electroerosively machining a capillary in a spinneret to provide orifices therein having a non-circular cross-section.

Yet another object of the present invention is to provide a method of electroerosively cutting branches in a spinneret capillary.

Other objects may become apparent herein.

In accordance with the present invention apparatus for electroerosively machining a surface of a workpiece is provided. The apparatus utilizes a source of direct current electrical energy. An electrically conductive cutting wire is connected to the negative side of the electrical energy. The connection is made to provide a parallel supply of electrical current to each of the ends of the wire. This parallel supply minimizes current flow variations due to changes in resistance set up as the wire engages in the electroerosion operation. The apparatus includes means for reciprocating the wire in proximity to a workpiece having a surface to be machined. The positive side of the electrical energy is connected to the workpiece. A spark eroding discharge is set up between the workpiece and the wire as the latter reciprocates. Means is used for discharging a free oxygencontaining gas at the point of spark discharge. To provide for progression of the electroerosive machining means is included for moving the workpiece perpendicularly relative to the axis of the wire.

The apparatus can advantageously be used to machine a capillary in a spinneret plate. A specific embodiment for doing this employs a pair of vertically aligned freely rotatable pulleys. An electrically conductive wire is vertically reciprocatably arranged for engagement with and between the pulleys. The spinneret plate has a small pilot hole through which the wire normally extends. Means which may include a variable resistance connects the positive terminal of a direct current supply in electrical serial relationship to the spinneret plate. A positively driven pulley having an eccentric juncture to which one end of the wire is mounted is used to impart the reciprocation of the wire. Means which may include two springs mounted to a frame is used to connect the negative terminal of the direct current supply in electrical parallel relationship to opposite ends of the wire to provide a spark eroding discharge between the spinneret and the wire. At least one small bore tube or the like is employed for discharging a free oxygen-containing gas at the place of spark erosion. In another embodiment the provision of parallel current flow to ends of the wire can be made through the freely rotatable pulleys.

The method herein provides for cutting branches in a spinneret capillary. This is done by placing a small pilot hole in a spinneret and extending an electrically conductive wire through the hole. Each of the ends of the wire is connected in parallel to the negative side of a direct current electrical energy source. The spinneret is connected to the positive side of the energy source. The wire is reciprocated close to a portion of the inside surface of the pilot hole to provide a spark eroding discharge therebetween. A stream of gas containing at least about 14 percent by volume oxygen is established and directed to the point of spark erosion. The spinneret is moved relative to the axis of the wire to provide for progression of the spark erosion, thereby to cut a branch in the spinneret extending from the pilot hole.

The improvements disclosed herein can be most readily understood by reference to the drawing.

FIGURE 1 shows schematically apparatus suitable for carrying out the method.

FIGURE 2 shows schematically certain desirable modifications in the apparatus illustrated in FIGURE 1.

FIGURE 3 illustrates another modification of the apparatus.

FIGURE 4 illustrates some shapes of various orifices that are readily made by the method of the invention.

FIGURE 5 is a graphical representation of certain data given in Table 1, which illustrates the effect of oxygen concentration in the local atmosphere upon the rate of removal of metal by the method of the invention.

FIGURE 6 is a graph illustrating the effect of local atmospheric gas flow rate upon the rate of metal removal according to the method of the invention.

FIGURE 7 is a graphical representation of data illustrating the effect of the axial speed of the wire electrode upon the rate of removal of metal by the method of the invention.

In FIGURE 1 are shown two freely rotatable electrically nonconductive pulleys 1 and 3; the shafts of the pulleys are supported by a frame (not shown). Passing over the two pulleys 1 and 3 is a fine wire 2 which forms one electrode (cathode) in an electrical circuit. One end of the wire is fastened at 4 to one end of a helically coiled spring 5 that is anchored to a frame at 6, and the other end of the wire is attached to a juncture or clamp 7 which pivots about a pin attached to drive wheel 8. The drive wheel 8, pulleys 1 and 3, and anchor pin 6 are all electrically insulated from the frame; this requirement is most easily achieved by making each of these members of nonconductive plastic material such as nylon, polystyrene, polymethacrylate, etc. Numeral 9 indicates a schematic cross-section of a metal spinneret blank having a conventional counterbore from the back side and a pilot hole through which wire electrode 2 passes. The spinneret is fastened by clamps (not shown) to a conventional precision table 10 such as commonly used with micro drill presses or milling machines; by means 10a, such as micrometer screws and a rotational axis, table 10 may be shifted accurately in any direction and with it the spinneret. Of course, provision can be made for moving only the spinneret. A direct curernt source (DC) is provided and connected with polarities as indicated. An ammeter A and a rheostat or potentiometer P are in series so that curent flow in the circuit may be set at any desired level; a voltmeter may also be included. It is to be noted that the negative electrode or cathode of the current source is electrically connected to both ends of the wire electrode through spring 5 and the loosely coiled wire 11. A short length of small-bore tubing 12, such as hypodermic tubing, is held to the frame by an adjustable clamp such that the longitudinal axis of the tube is aimed at the intersection of the wire and the face of the spinncret. Oxygen-containing gas under pressure supplied to the hypodermic tubing forms a high velocity jet that impinges directly on the wire at the region of contact between the wire and the spinneret blank.

The mechanical operation of the appaartus shown in FIGURE 1 can be seen from the drawing. As positively driven wheel 8 rotates, pivoted clamp 7 describes a circular path; and wire 2 is moved axially between the two pulleys a distance equivalent to the diameter of the circular path. For each complete revolution of the drive wheel the wire moves first in one direction and then an equal distance in the opposite direction, or a total travel of twice the diameter of the circle of rotation of pivot 7. As the wire reciprocates and arcing occurs between the wire and the spinneret blank, metal is eroded away. A template 13 of electrically nonconducting material resting on the spinneret and in contact with the wire may be used to guide the path of the spinneret relative to the wire as it is moved by precision table 10 to generate the desired orifice shape. The jet of gas from tube 12 assists mechanically by blowing eroded particles away from the active area. Precision movements and observations of the spinneret can be made with the aid of a conventional low power microscope having long working distance focused on the intersection of the wire and spinneret face.

Except as stated otherwise, all illustrative operational data given below were obtained with apparatus such as illustrated in FIGURE 1.

In prior art electroerosion processes the electrical circuit usually includes a blank of capacitors intended to provide a large surge current at sparking discharge. The electrode was either submerged in or coated with a dielectric liquid which presumably served as a coolant and to assure complete charging of capacitors, while also excluding air from the erosion area. By the present device both the capacitors and the dielectric liquid are eliminated from the circuit, resulting in a positive improvement in performance. Rate of metal removal is not sacrificed and dimensional variation in slot width is reduced.

To note the effect of excluding air but of assuring supplementary cooling of the electrode, jets of inert gas were directed on to the electrode through the hypodermic tubing 12 shown in FIGURE 1. Each of two diatomic inert gases, nitrogen and carbon dioxide, and one monatomic gas, argon, were tested. Somewhat surprisingly, it was discovered that when air was excluded by means of the inert gases the rate of metal removal became virtually nil; some sparking occurred but erosion of metal became negligible. The opposite approach of using air enriched with oxygen was tried which proved to be the key to a significant improvement in the electroerosion process: With supplemental oxygen in the local atmosphere surrounding the active area of the wire electrode, rate of metal removal was increased by -200 percent, dimensional accuracy of the slot width was much improved, the wall finish of the capillary was significantly improved.

In prior art wire electrode electroerosion methods copper wire was used, but it was found that much better accuracy is obtained when the wire is under appreciable tension that is usually beyond the tensile strength of copper so that copper wire tends to Wear very rapidly and to break. Molybdenum wire, Perma-Nickel wire, and above all tungsten wire are the preferred electrode materials in practicing the method of the invention. Tungsten wire has been shown to last indefinitely with no evidence of wear or functional change, and provides a somewhat higher cutting rate than other wire materials. Presently, each of the three previously mentioned types of wire can be bought commercially in virtually any size.

Since spinneret orifices are relatively small, in order to avoid inconvenient decimal fractions the mil (one onethousandth of an inch) is frequently used for linear measure in the illustrations given below. By cycle is meant one complete stroke of the wire, while stroke length is the distance through which the wire moves in one direction during a cycle. To bring all data to a common basis, the metal removal rate is expressed in terms of the volume of metal removed per unit of time, specifically the number of cubic mils of metal removed per minute.

Commercial cylinders of nitrogen and oxygen were each connected to small ball-float rotameters which were in turn connected by plastic tubing to a T fitting; a line from the T was connected to hypodermic tube 12 in FIG- URE 1. By controlling the relative flow rates of oxygen and nitrogen the proportion of oxygen in the mixed gases was controlled. A tungsten wire 4.8 mils in diameter comprised the electrode with the rheostat set for a current of 600 milliamperes indicated by ammeter A. The spinneret material was Type 430 stainless steel 20 mils thick. The stroke length was 3.5 inches and frequency was 200 cycles per minute. The combined gas flow rate was held constant at 7 liters per minute measured at 25 C. and one atmosphere of pressure. Straight slots were cut by electroerosion for 5 minutes, and the length of cut was measured; two 5-minute cuts were made at each gas composition and the average values were used to calculate the rates of metal removal presented in Table I and shown graphically in FIGURE 5.

7 TABLE I Rate of metal removal,

volume percent oxygen: cubic mils/ min. 30

As these data indicate, the metal removal rate increases rapidly as the oxygen concentration in the local atmosphere increases. The cutting rate becomes quite significantly increased at about 3540 percent oxygen concentration and higher. Atmospheric air nominally contains 21 percent oxygen by volume, and rather than diluent such as nitrogen being used, air supplemented by 15-20 percent additional oxygen would be preferred if a moderate cutting rate increase were desired. In the practice of the invention the local atmosphere at the active portion of the electrode should contain at least 35 percent (volume) of free oxygen and preferably 50-100 percent oxygen.

The actual quantity of gaseous oxygen supplied to the active area of the electrode affects the cutting rate. Only a very small fraction of the total quantity of oxygen supplied is actually effective, but a large excess is necessary to assure that oxygen is available at the spark site. This is demonstrated by the data presented in Table II and shown graphically in FIGURE 6, which were obtained under these conditions:

Pure oxygen was blown through the hypodermic tube in one series and compressed air was used in the other series of tests.

TABLE II Rate of metal removal, cubic mils/min.

Air

Gas flow rate, liters/min. (25 0., one atmos.)

8 (max. oxygen) These data show that cutting rate increased with gas flow rate up to about 7 liters/min, and that at all flow rates the metal removal rate is at least 100 percent higher when oxygen is used instead of air.

The rate of cutting by the method of the invention also depends upon the stroke length and speed of axial movement of the wire. In one extreme case, the stroke length might be less than the thickness of the metal being cut, while there appears to be no upper limit except as restricted by physical limitations. It has been found, however, that for practical use in spinneret production the stroke length should be no shorter than about one inch and preferably no longer than about 6 inches, the lower limit being the more important. The speed of axial movement of the wire is of great importance and is, of course, dependent upon the stroke length such that a higher frequency of reciprocation is required to achieve a given speed as the stroke length is decreased. The significance of these factors is illustrated by the data shown graphically in FIGURE 7. These data were obtained by operating apparatus as shown in FIGURE 1 under these general conditions:

Wire electrode: Tungsten, 4.8 mils dia.

Current: 6 0-0 ma.

Oxygen fiow rate: 7 liters/ min.

Spinneret material: Type 430 5/8, 21 mils thick.

Stroke lengths were varied from 0.1875 inch to 4.5 inches and frequency was varied from 50 to 800 cycles/min. in this series of tests. Stroke lengths are shown on the appropriate graphs in FIGURE 7. To bring the various data to a common basis, electrode speed is expressed as feet per minute of axial travel.

As FIGURE 7 clearly illustrates, for a given stroke length a maximum cutting rate may be attained and further increase in electrode speed does not significantly increase the rate of removal of metal. It is also seen that a significant metal removal rate is not attained unless an electrode speed of about 40 feet/minute is reached. The actual positions of the curves plotted are shifted upward or downward in FIGURE 6 depending upon wire size and other factors but there is little shift along the axis of electrode speed. In the particular examples shown in FIGURE 7 it is noted that the maximum rate of removal of metal is achieved with a stroke length of 3.5 inches and electrode speed of about feet/minute. Accord ing to the method of the invention a stroke length in the range of 1 to 6 inches, preferably 2-5 inches, is to be used with an axial wire speed of at least 40 feet/minute and preferably in the range of 50 to 250 feet/minute.

As previously stated, the presence of supplemental free oxygen in the local atmosphere surrounding the active electrode wire not only increases the rate of metal removal but, surprisingly, improves dimensional accuracy and wall finish of the capillary. The actual slot width cut by a wire of given diameter depends significantly upon the current flow in the circuit. As an example, a tungsten wire electrode of 2.8 mils diameter was used under similar conditions except that oxygen was blown on the electrode in one case and air in the other, while cutting Type 430 S/S stock 20 mils thick:

Slot width, mils Current, ma. Air Oxygen Metal removal rate also increases with current, but note that slot width is constant over an appreciable range of current flow when oxygen is used instead of air. The difference is even more pronounced when a wire composed of Perma-Nickel is used; with air and 600 ma. current, the slot width is about 3.7 mils. A similar situation occurs with wires of other sizes; a tungsten wire 4.8 mils in diameter characteristically cuts a slot 5 .0' mils wide when used with oxygen. By proper choice of wire diameter, slot width can be chosen within very close limits and be reproduced repeatedly.

When air is used the wall finish of the capillary becomes progressively rougher as the metal removal rate is increased either by increasing the current or by increasing the electrode speed. It was therefore the more surprising to find capillary wall finish greatly improved when metal removal rate was increased by the presence of supplemental free oxygen. The resultant wall is so smooth that no further treatment is necessary prior to putting the spinneret into service. The improved uniformity of polymer flow in melt spinning is directly observable at the spinneret and is confirmed by the reduced interfilarnent denier variation in the spun yarn.

Hypothetically, the improved capillary wall finish may be rationalized by consideration of at least two factors. Electroerosion as normally performed under a dielectric liquid or in the presence of a predominantly inert gas probably involves removal of particles of free metal; these particles are actually torn from the metal mass by the action of the electric spark and the moving electrode drags these relatively hard particles across the surface of the bulk metal so that the net result is a relatively pitted or pebbly Wall finish. On the other hand, with free oxygen available at the spark spite, it is probable that the metal surface oxidizes rapidly and only the softer friable metallic oxides, rather than metal particles, are eroded away. This general idea seems to be supported by the fact that aluminum does not erode appreciably when treated according to the method of the invention; very little sparking occurs and the erosion rate is negligible. It is well known that metallic aluminum oxidizes almost instantly upon contact with free oxygen to form a very thin, hard transparent film of alumina; since this oxide film is nonconductive, the electroerosion process cannot proceed unless a prohibitively high voltage is employed and even then the process would be extremely erratic. Metal oxides, predominantly iron oxide, formed in practicing the method of the invention, are natural fine abrasives; red iron oxide, for example, is a well known abrasive used to polish gemstones, optical glass, and metal reflective surfaces. It is probable that the metal oxide particles formed in the electroerosion process actually contribute a polishing action to the wall of the capillary as the electrode wire reciprocates moving the oxide particles across the capillary wall as the electrode passes along the slot.

FIGURE 4 illustrates a number of cross sections of spinneret orifices that are readily made by the method of the invention. The dotted circle represents the position at which the pilot hole may be located for the initial stringup of the wire electrode through the spinneret blank. Curved boundaries of any arbitrary shape, as well as straight boundaries, are practicable. In general, one pilot hole must be drilled for any group of interconnected slots. A guide template of nonconductive material such as ceramic or, preferably, synthetic sapphire (alumina) may be used to assist the operative controlling movement of the spinneret relative to the electrode. For the Y-section capillary shown in FIGURE 4-d, for example, a straight guide would be placed on top of the spinneret blank as indicated by numeral 13 in FIGURE 1; the operator then moves the spinneret periodically so that the electrode wire moves parallel to and lightly touching against the straight edge of the guide; the position of the guide is shifted to a new position as each of the three straight slots comprising the orifice are completed. A curved guide is used in like manner to make a curved boundary such as illustrated in FIGURE 4-1. If a very large number of capillaries of the same type are to be made, it is preferable to omit the direct guide template and to use a mechanical movement duplication method, such as pantographic linkages, to move the spinneret in the desired path relative to the wire electrode.

A new phenomenon of practical significance is observed in the action of the wire electrode used according to the invention. When an oxygen-enriched local atmosphere is provided, the wire acts as if it were attracted by the actively eroding area of the spinneret blank; that is, the wire appears to be drawn into the spark site by some force acting normal to the axis of the wire. The practical importance of this effect is that the wire now has a natural" tendency to follow the guided path once electroerosion is initiated. With direct guide templates the wire tends to maintain snug contact with the guide edge, and with spinnerets guided by remote linkage the wire follows very accurately the imposed changes in direction without the tendency to drift off course that occurs when air alone is used. This stabilization of the active electrode by the attraction phenomenon is also believed to be a contributing factor in the improved dimensional control achieved by the method of the invention.

The explanation of the electrode attraction-stabilization phenomenon is not known. Initial belief of magnetic attraction between the tungsten wire and the Type 430 S/ S blank had to be abandoned when the same phenomenon was observed with tungsten wire and a Type 316 S/ S blank, neither of which materials are magnetic. Changing the terminal connections from one end of the wire to the opposite end indicated that any force due solely to electromagnetic interaction of the magnetic field surrounding the wire and that accompanying the current flow through the spinneret blank must be negligible compared with the effect of the presence or absence of supplemental free oxygen in the surroundings. Tentatively, it is believed that the attraction is primarily due to the consumption of oxygen in the actively eroding area, since the phenomenon is just barely discernible when air alone is used. When oxygen in the spark area combines with the metal to form an oxide the volume of the gaseous oxygen comparatively shrinks to zero, and other free oxygen must flow into the area; for each cubic mil of stainless steel removed as oxides about 2000 cubic mils of oxygen must flow into the erosion area. Therefore, a low pressure region should exist in the spark area and the higher pressure of the bulk oxygen atmosphere pushes the wire into the spark region. Irrespective of the true explanation, however, the phenomenon is real and of practical benefit as noted above.

Additional changes in the apparatus as indicated in FIGURE 2 effectively reduce variations in metal cutting rate, improve oxygen utilization efficiency, and substantially increase further the metal removal rate. In comparison with FIGURE 1, the two pulleys 1 and 3 are now made of electrically conducting material, preferably aluminum or silver-plated copper; pulley bearings are insulated from the frame and each pulley is connected to the negative pole of the DC source by means of spring brushes 19 and 20 bearing directly against the pulleys, or by other common expedients for making electrical connection with rotating members. Current flows through the wire to the pulleys so that the effective length of wire in the electrical circuit is constant at all parts of the cycle. Oxygen is brought to the spark area by means of a tube 14 having a T branch to admit metered stream of oxygen. A small O-ring 16 seals the tube 14 to the bottom surface of the spinneret. Wire electrode 2 passes through a small nonconductive abrasion resistant eyelet 15 that has a hole just large enough to freely pass the wire. Some oxygen delivered to tube 14 escapes at eyelet 15 but the much greater fraction is forced to pass through the capillary being cut by the wire electrode, assuring an oxygensaturated atmosphere and high efficiency. Experience shows that as the cutting operation proceeds and the capillary becomes progressively larger, a slight drop in metal removal rate occurs, presumably due to more oxygen flowing through the open area adjacent the electrode. Such channeling is reduced by placing around the wire a light, loose-fitting flanged plastic tube 17 held down by a metal ring 18 that simply rests on the flange of tube 17; this device normally is not used when a direct guide template is employed on top of the spinneret. With the arrangement shown in FIGURE 2 moderately skilled trained operators attain a high degree of proficiency in aligning spinneret and electrode and are able to operate two machines concurrently.

FIGURE 3 illustrates schematically an alternate arrangement for improving oxygen utilization efficiency that in practice is slightly superior to the method illustrated in FIGURE 2 but does require more care and patience of the operator. A second hypodermic tube 21 delivering a metered stream of oxygen is mounted under the precision table 10 to direct a jet of oxygen upward into the counterbore of the capillary being cut by the electrode 2. Preferably, the two oxygen streams to hyperdermic tubes 12 and 21 are metered independently, tube 21 generally requiring only about one-half as much oxygen as tube 12 for efficient operation.

As indicated in FIGURES l and 2, according to the method of the invention, both ends of the electrode wire are connected to the negative terminal of the DC. power source. This parallel path for current flow eliminates the gross current and voltage fluctuations revealed by ordinary damped ammeters and voltmeters when only one end of the wire is connected. Uniformity of cutting is improved, and a small but significant increase in cutting rate is also gained as illustrated by these data:

Wire electrode: Tungsten, 2.4 mils dia. (slot width 2.6

mils).

Electrode speed: 80.2 feet/minute (3.25 inch stroke).

Current: 300 ma.

Oxygen rate: liters/ minute.

Spinneret material: Type 316 S/ S, 30 mils thick.

Metal removal rate Electrode condition: cubic mils/ min.

Both ends connected 195 Bottom end only connected 152 Top end only connected 156 Both ends reconnected 195 Polarity reversed, both ends No erosion The actual metal removal rate, as already noted, depends upon many controllable factors such as current, electrode speed, oxygen flow rate and concentration, etc. The rate, of course, also depends upon the type of material being cut; aluminum or Incoloy, for example, practically are not eroded by the method of the invention. The base metals utilized almost universally for meltspinning spinnerets, namely, stainless steel alloys, are readily cut by the method of the invention. Stainless steels Type 316 and Type 430 are most commonly used for spinnerets. Other more uncommon materials, such as titanium, are also easily out according to the method of the invention. A tungsten wire electrode 4.8 mils diameter with 600 ma. current and 7 liters/minute of oxygen flow was used to obtain the data in Table III presented to illustrate differences in the cutting rates of different materials.

It is noted that Type 316 S/ S is cut much more rapidly than Type 430 8/8, and that all three of the materials are removed at much greater rate than even 316 S/ S when it is electroeroded in the absence of supplementary oxygen.

Any of the common direct current sources may be used in the circuit according to the invention, such as a primary chemical cell, a direct current generator, or an alternating current source with rectifier. The actual voltage required depends upon the length of wire and physical size chosen for the electroerosion machine. In prior art machines the electrode guide pulleys 1 and 3 of FIGURE 1 were spaced several feet apart vertically. It now has been found that to 20 inches spacing is entirely adequate for normal spinneret and extrusion die manufacture. With such dimensions very compact but highly versatile machines are practicable, and a range of voltage from 5 to 25 is entirely adequate. An impressed of 9-15 volts is normally sufiicient to provide a reasonably optimum maximum current of 600 ma. Occasionally it is desirable to be able to raise the current to a higher level to compensate some particular factor such as very long capillaries, but current much in excess of one ampere may be accompanied by a rougher wall finish that is undesirable.

The general eifect of current on the cutting rate may be seen in these data obtained with Type 430 S/ S cut with a tungsten wire electrode 4.8 mils diameter, and with 7 liters/minute oxygen fiow. The DC. source was a common battery eliminator rectifier with slightly pulsating output D.C.:

Rate of metal removal Current, ma. cubic mils/min.

Although emphasis has been placed upon the forming of spinneret capillaries, the method of the invention is applicable to cutting surfaces of various workpieces but finds its greatest advantage when slots are not wider than about 25 mils and a high order of accuracy is required.

There is provided herein a novel method and apparatus for practical production of spinnerets with noncircular capillaries. Simultaneously with a great reduction in labor costs, a substantial improvement in quality of workmanship is achieved. Several hundred capillaries somewhat similar to the shape illustrated in FIGURE 4 but having only three curved slots, 3.5 mils wide by 35 mils long, had been made under the most favorable electroerosion conditions known in the prior art so that accurate production rates were known. By the improved method of the invention using the arrangement illustrated in FIG- URE 2, the same type of capillary may be cut in one-third the time required previously, with an overall reduction in production costs of about percent. Concomitantly the quality of the spinnerets is improved and every capillary in the spinneret is virtually identical in dimensions, shape, and finish.

Since many different embodiments of the invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited by the specific illustration except to the extent defined in the following claims.

What is claimed is:

1. Apparatus for machining a capillary in a spinneret plate comprising:

(a) a source of direct current electrical energy having a positive terminal and a negative terminal;

(b) a pair of vertically aligned freely rotatable pulleys;

(c) an electrically conductive wire vertically reciprocatively arranged between said pulleys;

(d) a spinneret plate having a pilot hole through which the wire normally extends;

(e) means including a variable resistance connecting the positive terminal in electrical serial relationship to the spinneret plate;

(f) a-positively driven pulley having an eccentric juncture to which one end of the wire is mounted;

(g) means including two springs mounted to a frame for connecting the negative terminal in electrical parallel relationship to opposite ends of the wire to provide a spark eroding discharge between the spinneret and the wire as the latter reciprocates;

(h) at least one small bore tube for discharging a free oxygen-containing gas at the point of spark discharge, and

(i) means for moving the spinneret relative to the axis of the wire to provide for progression of the spark erosion.

2. Apparatus for machining a capillary in a spinneret plate comprising:

(a) a source of direct current electrical energy having a positive terminal and a negative terminal;

(b) a pair of vertically aligned freely rotatable pulleys;

(c) an electrically conductive wire vertically reciprocatively arranged between said pulleys;

(d) a spinneret plate positioned on a supporting frame approximately equally spaced between said pulleys and having a pilot hole through which the wire normally extends;

(e) means including a variable resistance connecting the positive terminal in electrical serial relationship to the spinneret plate;

(f) a spring attached to a frame at one end and to the wire at the other;

(g) a positively driven pulley having an eccentric juncture to which an end of the wire is mounted to provide for the reciprocative motion of the wire;

(h) means for connecting the negative terminal in electrical parallel relationship through each of the freely rotatable pulleys to the wire to provide a spark eroding discharge between the spinneret and the wire as the latter reciprocates;

(i) at least one small bore tube for discharging a free oxygen-containing gas at the point of spark discharge, and

(j) means for moving the spinneret relative to the axis of the wire to provide for progression of the spark erosion.

3. Apparatus for electroerosive machining comprising:

(a) a source of direct current electrical energy having a positive and a negative terminal;

(b) a pair of substantially vertically aligned rotatable pulleys;

(c) an electrically conductive wire vertically reciprocatively arranged between said pulleys;

(d) means for supporting a workpiece juxtaposed to said wire and to be machined along a surface extending between two faces;

(e) means including a variable impedance connecting in electrical serial relationship said source, wire and workpiece;

(f) a positively driven rotatable member having an ecmachining.

5. The apparatus of claim 4 wherein the wire is made 15 of tungsten metal.

6. The apparatus of claim 5 including means for discharging an electroerosion supporting fluid at the point of spark discharge.

References Cited UNITED STATES PATENTS Hinternesch.

Combs 143-70 Holslag.

Sibley.

Imalis et al.

Hofer.

Jackson et a1.

JOSEPH V. TRUHE, Primary Examiner R. F. STAUBLY, Assistant Examiner U.S. Cl. X.R.

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