US2516930A - Filament forming method - Google Patents

Description

Aug. 1, 1950 s. F. VARIAN FILAMENT FORMING METHOD Filed April 14, 1944 INVENTOR 5'. E'VAR/AI/V Patented Aug. 1, 1950 FILAMENT FORMING METHOD Sigurd F. Varian, Garden City, N. Y., minor to The S ware perry Corporatiomn corporation of Dela- Application April 14, 1944, Serial No. 531,100

Claims. (01. 29-2548) This invention is concerned with methods for forming filamentary materials into selected patterns or shapes and is particularly related to an improved method of producing spirally wound filaments for electronic apparatus which are capable of maintaining their original size, shape and spacing between turns under actual conditions of use, including heating to normal temperatures of emission.

Prior art coils, grids and filaments have been characterized by two major defects, namely lack of uniform spacing between turns, and distortion from the desired size and shape when electric current is caused to flow through them. These .defects of presently known filaments have made it extremely difiicult to obtain uniform electron emission over the face of the filament, and to produce electronic tubes having the desired uniformity of performance. a

While the present invention is aimed at overcoming the above described problems oi the'electron tube manufacturing art it is also intended to facilitate the manufacture of precision coils, precision springs and similar forms from materials of many kinds where uniformity of spacing and permanence of shape are among the requirements to be met by a satisfactory finished product.

With the previously used methods of manufacturing spiral tungsten filaments, for example,

it has been customary to wind the filament on a steep conically shaped mold having a special groove on its surface into which the filament is guided as it is wound. The filament is usually heated while still on the mold and when removed from the mold naturally retains a conical shape. In an attempt to change the conically shaped coil to a fiat spiral coil, the filament is compressed between two fiat molybdenum plates and fired at high heat to remove the bending stresses from the tungsten and force it to take a permanently flat set. The major defect of this method is that the bending stresses in the highly elastic tungsten wire are not entirely removed and the residual stresses which remain within the filament cause it to resume a semi-conical or v z The above described-winding method has the further disadvantage that the spacing between turns of the filaments loses its uniformity during the process of transferring the coils from the conical mold to the flat molybdenum pressing plates since during this step the filamentary coil .is not held against movement caused either by internal stress, handling. or misalignment of process equipment. There also exists the further drawback that any desired change in the. spacing between turns of the filament requiresthe construction of anew forming mold or jig.

If an attempt is made to wind the tungsten wire into a flat spiral precision coil without properly guiding and supporting the wire duringthe process, the wires which are usually circular in cross section will not hold the proper alignment but will roll and slip over one anothercausing otherwise distorted shape when installed as the cathode of an electron tube and heated to normal emission temperatures. In order to remove all distorting stresses from tungsten, for example, it would be necessary to heat the wire to approximately 1200" C. but when heated to this high temperature and then allowed to cool it would cease to be flexible, becoming instead very brittle and therefore easily destructible.

the spiral either to collapse or to become greatly distorted. V

From the point of view 01' the tube manufacturer it is highly desirable to be able to construct a flat coiled filament which will remain free of distortion during actual operation, in order to prevent variation in tube characteristics resulting from change in shape of the electron source.

An ideal spirally wound filament complies with the following major requirements:

(a) The turns must all lie in the same plane. (b) The spacing between turns must be uniform.

(c) The coil must be of the correct size.

(d) It must have a form which-is easy to manufacture.

(e) It must be readily duplicable.

(f) It must be protected against damage durcorrect size: and

The present novel forming method makes posi- 1 sible a high degree of success in meeting the above requirements in the actual manufacture of filamentary coils and grids of various types.

One object of the invention is to provide an easily performed method of winding uniform, ac-

curately sized, electron tube filaments or oath:- odes.

Another and important object of this invention is to provide a method of manufacturing coils or other selected shapes from relatively elastic filamentary and other slender substances such that the residual distorting stresses remain;-

ing within the material at the end of the process will be as small as it is possible to make them.

It is a further object of this invention to provide a method of bending filaments and slender material into fiat shapes in which the residual stresses unavoidably left in the filaments are of such a nature that any unwinding or movement caused thereby will occur only in the plane of the fiat coil or shape.

It is also an object of this invention to provide a winding or forming method for use with filamentary substances which will provide complete protection of the processed material from nicking or other damage during the winding thereof.

A further object of the invention is to provide a method and apparatus for shaping filamentary materials which is readily adaptable to the production of coils or forms of different sizes and shapes.

It is likewise an object of the invention to provide a method of manufacture broadly applicable in accurately and permanently bending elastic filamentary and elongated thin substances of many kinds into a wide variety of forms and shapes.

Referring now to the drawing wherein for illustrative purposes the method of the invention is shown applied to the formation of fiat spiral filaments:

Fig. 1 is an isometric view of a tungsten or other filament extending through a tube of metal or other readily bendable substance.

Fig. 2 is an isometric view of the tube and filament after the tube has been rolled fiat on two opposite sides.

Fig. 3 is a plan view of the tube and inclosed filament in the process of being formed on a winding jig or forming machine.

Fig. 4 is a cross section of the partly wound coil taken along the line 4-4 of Fig. 3.

Fig. 5 is a plan view showing the completely wound coil encased in a fusible composition or solder and held in shape by a metallic strip bent and tightly crimped about the coil.

Fig. 6 is a sectional view along line 6-6 of Fi 5.

Fig. 7 is a plan view of the filament in its'final form after annealing and after the surrounding tube and solder have been etched away.

Fig. 8 illustrates an alternative method of holding the coil to size and shape during the annealing and etching steps.

While the invention has been used with particular success in forming tungsten filaments with the aid of medium-hard-drawn copper tubing and will therefore be described primarily against the background of this specific application, it should be understood that the invention is primarily in a method that may be employed for forming a wide variety of substances into many different shapes and patterns. Flexible plastic materials for example, could be shaped to desired patterns by the method of this invention.

In place of tungsten any other suitable slender and filamentary substance, either elastic or non-elastic, may be used provided the substance is flexible and can be readily bent. In place of copper, the tube may be composed of other materials, metallic or non-metallic, provided they have sufflcient strength to bend the inclosed filament, are themselves sufficiently flexible to per mit bending into the desired shapes and will Withstand the annealing temperatures which must be used with the chosen filamentary material. The material used for the tube also 4 a should be such as can be removed from the completed coil without damage to the filament itself as by corrosive chemical action or otherwise.

The formation of a flat spiral filament or heater coil from tungsten wire provides a typical illustration of the use of the method of this invention. For this purpose the method of the invention is carried out in the following manner.

A piece of copper tubing or conduit of a length determined by the size of the filament which it is desired to produce is selected along with a. piece of tungsten wire, the tungsten wire being about an inch longer than the tubing, and the inside diameter of the tubing preferably being just large enough to permit the tungsten wire Next the copper tubing or sheath is rolled or compressed to produce two flat surfaces on opposite sides thereof as shown in Fig. 2, this procedure producing two surfaces which are suitable for rolling or winding one upon the other. Obviously if square or rectangular copper duct of a size proper to accommodate the tungsten wire is available for use, the rolling process above mentioned may be dispensed with. To a certain extent the spacing between the turns of the completed filament can be varied by controlling the extent of the rolling to which the copper tubing is subjected, more closely spaced turns being obtained if the copper tubing is rolled relatively thin. Where wider spacing between turns of the cathode is desired than can be obtained by controlling the rolling procedure in the manner described it becomes necessary to start with tubing having a greater wall thickness initially.

If the copper tubing is sufllciently flexible to bend readily it may now be placed in a winding jig for forming to the desired shape. However if the tubing is too stiff to bend easily it must be annealed before forming and this annealing should preferably be done in a hydrogen furnace to avoid oxidation. Unless the tubing and the inclosed wire can be bent readily the forming apparatus may not be able to withstand the strains occasioned by the bending; and breakage or failure of this apparatus will occur frequently.

Having annealed the copper tube, if necessary, we may now place the tube and the inclosed filament on a winding or forming jig or mold, shown in Fig. 3 as a winding table II having two tube-holding pins 18 and I9, this apparatus being suitable for forming a fiat spiral non-inductive coil when the copper tubing is wound upon it, fiat side to fiat side, as illustrated. The winding jig is so constructed that the distance between pins l8 and [9 may be adjusted to meet the varying space requirements of coils having diilerent spacing between turns. As the coil is wound the turns are forced and held tightly against one another by restraining the free ends of the tubing. The winding is continued until the entire tube and filament except short portions at the ends suitable for terminal projections have been brought within the spiral to form a heater elemeat of the desired size. The disposition of the tubing and the tungsten wire at this stage of the process isshown in Fig. 4 where the function of the tubing in controlling the spacing between theannealing, and immersed in a corrosive chemical solution for etching away the sheath.

While the coil is still on the forming Jig molten solder or a similar fusible alloy or composition is deposited on the coil and allowed to solidify into a temporary protective casing around the same. In Fig.3 this protective casing 20 is shown on the right hand portion of the coil. Thus encased in solder the coil may be removed from the winding jig and immediately placed upon a sheet of nickel or other suitable heat-resistive material which is rigid enough to firmly hold the coil during subsequent annealing operations. The hickel sheet 2! is then bent tightly up around the coil in its final form and shape and the corners 2i" are then tightly crimped over the adjacent turns ofthe coil or grid to hold it securely against unwinding and to help protect it from damage by nicking or abrading. The condition of the coil at this stage of the process is shown in Figs. 5 and 6. Pure indium has been successfully used asthe fusible alloy in the above described step of the process and may be preferable since it does not damage a hydrogen furnace and readily decomposes w'ith the copper sheath when exposed to nitric acid.

If preferred, the use of solder in the manner above described may be avoided by the adoption of the following alternative procedure. Nickel sheet or strip 2! may be provided with two holes large enough for pins l8 and IQ of the winding -jifg' to pass through. Strip 2| may thus be placed flat on the table of winding jig I! with the coil thereafter being wound on top of strip 2!, the edges of which may then be crimped over the coil in the manner previously described upon completion of the winding and before removing the coil from the winding jig.

In forming a coil from tungsten or other elastic material it becomes necessary at this stage of the process to subject the coil to a second and final annealing to the end that bending stresses created in the winding of the co l and which would result in deformation thereof if not eliminated may be reduced to the lowest practical magnitude before removing the copper tubing and the nickel holding strip. At this point then the coil and its supporting strip are placed in a hydrogen annealing furnace and subjected to full annealing temperature for 10 minutes or longer. The heat of the furnace naturally disintegrates the solder or fusible alloy in which the coil is encased during the annealing process. However the nickel strip which was crimped about the coil in the manner shown in Fig. 5 will continue to hold the coil against radial expansion during annealing, even after the solder has vaporized and the coil has been removed from the annealing furnace.

The final step in carrying out the method of the invention consists of removing the copper tube or sheathing and the holding strip which surround the tungsten wire, leaving the finished filament free and unencumbered by any supporting structures or substances. One method by which this final step may be accomplished is to subject the coil to the action of some chemical agent, nitric acid for example, which will etch away or disintegrate the copper tubing and any of the soldering compound which may still be adhering to the coil, without aflecting the tungsten or other filamentary material being shaped. As the copper tubing is disintegrated by the selectedchemical agent the nickel holding or supporting strip will become loosened from the coil and may be removed to be set aside for use in forming further coilsby the method of the invention.

The completed coil in its final form after the removal of the copper tubing and the temporary supporting means is shown in Fig. 7 with the uniform spacing 24' between turns clearly evident.

Fig. 8 shows an alternative method of supporting the coil during the annealing and etching steps of the process. A rigid bar or support, spot welded or brazed to the copper tubing after formation of the coil, is substituted for the soldering compound and the nickel crimping strip specified in the method as originally described. This supporting bar may be readily removed from the coil after the copper has been etched away or if preferred the bar itself may be removed by chemical corrosion.

It is of course obvious that if the filamentary material being shaped to size and pattern is nonelastic, a flexible plastic material for example,

the second annealing step mentioned in the above described procedure would be unnecessary and could therefore be dispensed with. If the method of the invention were being employed to produce springs or similar resilient forms the second annealing step might also .be eliminated from the forming process.

In forming coiled filaments from elastic materials similar to tungsten it is not desirable to remove all elasticity from the coil by annealing it completely as this would render the coil brittle and unworkable. One of the advantages of this invention is that the coil or form is shaped or bent in the same plane in which it is later to be used so that unwinding or relief of the small residual stresses still remaining in the coil at the completion of the forming process takes place in the plane of the coil and therefore does not materially affect the spacing between cathode and plate of the tube in which the filament is installed. In actual practice the method of this invention has been used to produce filaments having an entirely satisfactory degree of uniformity.

As many changes could be made in the details of the steps by whieh'the method of this invention is carried out without departing from the scope thereof, it is intended that all matter contained in the specification and claims or shown in the accompanying drawing be interpreted in an illustrative and not in a limiting sense.

Having particularly described and illustrated the method of the invention and its use, what is claimed is as follows:

1. A method of forming uniformly sized precision coils from relatively hard, springy filamentary material, comprising encasing a flexible filament of said material in an inclosing tube formed with a bore closely fitting the filament for initially straightening the same, flattening said tube on at least two opposite sides, winding said tube and the, inclosed filament fiat side to flat side upon a supporting surface to form a coil in which the adjacent turns of the filament are equally and accurately spaced and supported .by the flattened walls of the tube, temporarily supporting said coil by encasing it in a fusible composition, surrounding said coil with a stifl binding strip, applying heat to the tube to anneal the enclosed filament and thereafter removing said inclosing tube, binding strip and said fusible composition by immersion in an etching bath to which said filament is impervious to leave a uniformly spaced coil consisting only of the original filamentary material.

2. A forming method as claimed in claim 1 in which indium employed as the fusible composition.

3. A method of forming hard, resilient fila mentary material according to selected patterns having uniform spacing between adjacent elements thereof, comprising encasing a filament of said material in an inclosing tube, flattening said tube on at least two opposite sides, annealing said tube and inclosed filament, forming said tube and inclosed filament into the shape of the selected pattern by bending said tube and inclosed filament fiat side to fiat side upon a supporting surface, temporarily binding said form by encasing it in a fusible composition, surrounding said form with a stiff band, annealing said filament and said inclosing tube, and removing said inclosing tube and said fusible composition by immersion in a solution corrosive only to said tube, composition and said band to leave a uniformly spaced filamentary form composed only of the original filamentary material.

4. A method of forming precision-coiled electron tube filaments having uniformly spaced turns from tungsten wire, comprising thrusting a strand of said wire into a soft metal tube, flattening said tube on at least two opposite sides, winding said tube and the inclosed strand fiat side to fiat side to form a coil, temporarily supporting said coil by encasing it in a fusible alloy and reinforcing it with a binding strip, annealing said coil and the enclomd tungsten wire, and disintegrating said metal tube and said fusible alloy to leave a uniformly spaced flat tungsten filament the binding strip being readily detachable after the partial disintegration of the binding strip.

5. A method of producing uniformly sized tungsten wire coils having uniformly spaced turns, comprising encasing a filament of tungsten within a copper sheath, flattening said sheath on two opposite sides, annealing said sheath in an atmosphere of hydrogen, winding said sheath and the inclosed tungsten filament fiat side to fiat side to form a fiat spiral coil, reinforcing said soil by encasing it in indium, temporarily binding said coil by crimping a nickel strip around it;

annealing sad filament and said inclosing sheath" in an atmosphere of hydrogen, and disintegrsting said encasing sheath and said reinforcing indium by exposing said coil to the corrosive' action of nitric acid thus producing a uniformly spaced fiat spiral filament composed only of tungsten the nickel strip being loosened with the disintegration of the encasing sheath, per-' mitting the same to be removed easily during the disintegration process.

SIGURD F. VARIAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,209,247 Bastian Dec. 19, 1916 1,921,600 Babcock Aug. 8, 1933 1,947,449 Anderson et al Feb. 20, 1934 2,047,555 Gardner July 14, 1938 2,115,855 Holman May 3, 1938 2,286,759 Patnode June 16, 1942 2,425,113 Miller Aug. 5, 1947

Images