US2929769A - Electroplating anode - Google Patents

Description

3 Sheets-Sheet l I. L. NEWELL EI'AL ELECTROPLATING ANODE March 22, 1960 Filed July 7. 1955 IN V EN TORS Isaac L Naw E1 1 Arlhur Zav are 115. BY 7&5. Mow-4140M HTTOBNE 6 March 22, 1960 I. L. NEWELL ETAL ELECTROPLATING ANODE Filed July 7, 1955 5 Sheets-Sheet 2 IN V EN TORS Isaac. L- Newall Arlhur ZavarE 11H.

17 TTOENE. Y5

March 22, 1960 NEWELL ETAL ELECTROPLATING ANODE 3 Sheets-Sheet 3 IEL Filed July 7, 1955 IN V EN TORS Isaac. L NawE11 BY Arihur Z-av ar Ella A T OENE Y5 to provide a uniform deposit. 4 Accordingly, it is an object of this invention to provide ELECTROPLATING ANQDE Isaac L. Newell, Wethersfieid, Coma, and Arthur Zavarella, Agawam, Mesa, assignors to the United States I of America as represented by the Secretary of the Army Application July 7, 1955, Serial No. 520,637 3 Claims. c1. see-2st This. invention relatesv to the electroplating of the interior surfaces of generally tubular articles and is more particularly directed to improvements in the anodes utilized in plating the bores of firearm barrels.

In plating the bores of elongated tubes such as firearm barrels with erosion and corrosion resisting metals, ,considerable difiiculty has been experienced in obtaining a deposit of uniform thickness longitudinally along the surface to be plated. For one thing, the current-carrying capacity of the plating solution or electrolyte is appreciably, reduced by the gaseous products which are evolved during the release of the plating metal from the electrolyte. Since the firearm barrel to be plated is generally disposed in an upright position, these gaseous products progressively accumulate in the upper regions of the electrolyte and consequently produce a corresponding decrease in the rate of plating deposit. The resulting variation in the thickness of the plate cannot be eliminated merely by increasing the supply of current since the additionl amperage will be delivered equally to boththe upper and lower regions of the electrolyte. In fact, the heat produced by such increase in current tends to soften and weaken the anode to such extent as to result in the deposition of an imperfect coating on the bore surface being plated. Moreover, the heat produced in the electrolyte actually adds to the quantities of objectionable gases generated therein and, therefore, merely increases the burning tendency of the anode.

Furthermore, when the metal being plate is chromium, the rate of deposit'varies substantially with any change in the temperature of the electrolyte and, as a result,th e

cooler regions at the bottom produce a thinner. plate than do the hotter regions at the top. While this temperature differential and the accumulation of the gaseous products may be prevented'by adequate circulation of the electrolyte, those few attempts which did meet with some measure of success required relatively complicated and time-consuming procedures because of the elaborate and expensive apparatus involved.

However, even if the foregoing deterrents to a plate of uniform thickness could be successfully eliminated, the voltage drop produced by the electrical resistance along the length of the anode would still detract from the desired uniformity of plate by decreasing the rate of metal deposit along the length of the anode. Although this reduction in the thickness of plate is inverse to that produced by'the accumulatinggaseous products, the compensating effect thereof'is much less an improved anode adapted to electrolytically plate the than that required bore surfaces of firearm barrels with a uniform thickness of depo'sit along the entire length thereof. I I It is a furtherobject of the present invention 'to provide a chromium plating anodewhich' will. compensate for the variations in the rate of deposit normally en. countered in plating the bores of firearm barrels.

While a uniform thickness of chromium plate provides 2.25s Patented Mar. 22, 1960 plating an inversely tapering deposit of chromium designed to maintain the desired uniformity of bore diameter throughout the length of the barrel. It has also been discovered that the accuracy life of firearm barrels can be appreciably increased by providing a restricted bore which converges forwardly toward the muzzle end of the barrel. Accordingly, the necessary construction has been achieved by depositing a uniformly tapering layer of chromium on a bore of constant diameter.

However, the plating of uniformly tapering deposits of chromium on the interior bore surfaces of firearm barrels presents extremely difiicult problems which have heretofore never been satisfactorily solved. In the smaller caliberbarrels, for example, the obvious solution of tapering the anode itself results in such a small diameter for the thinner end thereof as to be impractical. In the larger caliber firearm barrels, the decreased strength and rigidity of the reduced ends of tapered anodes often permits suflicient bending or warping thereof to adversely afiectthe required concentric relationship between the barrel and anode. In any event, the cost of fabricating the larger anodes would be prohibitive in view of the relatively low endurance life normally expected during theplating of chromium. In view of the difficulties presented by the use of tapered anodes, attempts have been made to control the iio'w of current throughchromium plating electrolytes by the'expeclient of fabricating the anodes with a layer of insulation about one end thereof. While this type of anode construction effects a progressive reduction in the quantity of chromium deposited from'the upper regions of the electrolyte, the resulting taper of the. plate is obtained only at the sacrifice of a substantial decrease in the conductivity along the length of the anode. As a result, the throwing power of the plating current at the lower regions of the electrolyte may be insufiicient to properly plate within the corners formed by the riding lands within the bore of the firearm barrel. It is, therefore, another object of this invention to provide an improved anode capable of reducing the flow of current across selected areas of the plating solution without simultaneously decreasing the flow of current along the anode itself.

A further object; of this invention resides in the provision of an improved anode which will plate the interior bore surface of a firearm barrel with a uniformly tapered deposit of metal. I

Still another object of the present invention is directed toward the provision of a chromium plating anode wherein selected areas on the exterior surface are adapted to form an insoluble nonconducting film in an acid type plating solution containing metal in ionic form.

In many of the modern types of firearms, the muzzle velocities thereof are so high that satisfactory barrel endurance life can only be maintained by providing the interior of the barrel with a hollow sleeve or liner fabricated from a more wear-resistant metal than conventional high alloy steel. In fact, the relatively low accuracy life of high velocity firearm barrels has brought fortht the suggestion that the entire barrel be fabricated from such extremely hard metals as cobalt, tungsten, molybdenum or the alloys thereof. However, the advantages provided by the use of these harder barrel materials. are partially offset by the greatly increased cost of machining the.

grooves therein. Furtherthe design of firearm gbarrels,

ode for plating chromium on the bore surface of a firearm barrel along such restricted paths as to form a plurality of helical rifiing lands.

A specific object of the present invention lies in the provision of an improved electrode for electroplating helical rifling lands on the bore surface of a firearm barrel wherein the pitch of the riding increasesprogressively from the breech end to the muzzle end of the barrel.

The specific nature of the invention as well as other objects and advantages thereof will clearly appear from a description of a preferred embodiment as shown in the accompanying drawings in which:

Fig. l is a fragmentary view of a firearm barrel having a bore of constant diameter and shown in longitudinal section to illustrate one type of anode which will plate the surface of the bore with a layer of chromium of uniform thickness;

Fig. 2 is a transverse cross-section of the anode of Fig. 1 taken along line 2--2 and shown on an enlarged scale;

' Fig. 3 is a view similar to Fig. 2 but showing an alternate anode construction;

Fig. 4 is a fragmentary view of a firearm barrel having a forwardly tapered bore and shown in longitudinal section to illustrate the inversely tapered chromium plate adaptedto be deposited therein by the particular anode shown;

Fig. 5 is a longitudinal view of an alternate anode construction adapted to deposit the tapered chromium plate shown in Fig. 4;

Fig. -6 is a view similar to Fig. 4 but showing an anode adapted to deposit a forwardly tapered layer of chrimium within a barrel having a bore of uniform diameter throughout;

Fig. 7 is a fragmentary longitudinal view of an alternate anode construction adapted to deposit the tapered chromium plate shown in Fig. 6;

present invention generally comprises a cylindrical rod 12 of substantially uniform diameter throughout and a plated band 13 helically disposed about the surface of rod 12. While rod 12 and band 13 are both composed of metals or alloys of high electrical conductivity, the prime constituent of the former is preferably an element which will form an insoluble, nonconductive film in an acid plating solution or electrolyte. For example, where chromium is the plating metal involved, rod 12 is preferably formed of silver which may be alloyed with a small amount of copper in order to provide the rigidity and strength necessary to prevent undesirable bending or warping of the anode during the plating operation. Where copper is to be plated from a sulfuric acid bath, the anode may be fabricated from aluminum.

' The helical configuration of band 13 may be obtained by masking rod 12 with a suitable covering of plastic such as vinyl plastisol in which portions have been cut out to expose the required silver areas and then electrodepositing a layer of lead over the mask which will adhere only to the exposed areas of silver. Alternatively, rod 12 can be dipped into a molten bath consisting of about 94% lead and 6% tin and, when cooled, the lead alloy can be mechanically stripped from the silver in the desired areas. Lead has been selected to transmit the current from the anode into the electrolyte in view of the excellent self-healing properties of the metal when scratched or otherwise marred. In chromium plating, the exposed silver of the anode will immediately react with the chromic acid in the electrolyte to form an insoluble, nonconductive film. The thickness of this film does not ordinarily exceed .001 inch and, therefore, has no diminishing effect on the flow of current along the length of rod 12.

a The anode is, of course, centrally disposed within the bore of a firearm barrel 14 which is in turn properly I suspended in a suitable plating tank 15 filled with a chro- Fig. 8 is a fragmentary longitudinal section of a tire.

arm barrel in which chromium rifling lands have been electroplated by-an anode constructed in accordance with the teachings of the present invention;

Fig. 9 is a transverse cross-sectional view taken along :line 99 in Fig. 8; and

Figs. 10-12 are views similar to Fig. 9 but showing "alternate forms of anode construction.

It should be understood that the barrel and anode dimensions are not necessarily to scale or in the proper proportions.

The anodes of the present invention are formed as cylindrical rods consisting essentially of a core fabricated from an electrically conductive metal and covered with various discrete configurations formed from another electrically conductive but diflerent metal. Inasmuch as one of these two difierent metals is selected from the group which will react in an acid plating bath to form an insoluble, nonconductive film on the surface thereof,

the configuration of the coating on the core will form a areas. Such arrangement makes it possible to regulate the uniformity and thickness of plating deposits on the bore surfaces of various types of rifled firearm barrels through the simple expedient of varying the ratio between the plating and nonplating areas at selected points along the anode;

As illustrated in Figs. 1 and 2, the basic anode of the plurality of alternately disposed plating and nonplating mic acid electrotyte 16. Since the film formed 'on the exposed silver areas of rod 12 prevents the passage of electric current from rod 12 into electrolyte 16, the ex- :posure of considerably more lead than silver at the upper portions of the anode will compensate for the re duction in the rate of chromium deposit ordinarily produced by the normal accumulation of the gaseous products in the upper regions of electrolyte 16. The required decrease in the exposed lead areas may be pro vided by progressively decreasing the width of helical band 13 or alternately by increasing the helical pitch thereof. The concentration of lead to silver at the lower throughout the electrolyte and from the voltage drops 'due to the length of the anode. Thus, as shown in Fig. ril, the configuration of helical band 13 is of maximum width -at the upper end of rod 12 andprogressively decreases toward themiddle portion thereof. Thereafter the width gradually'increases toward the lower end of rod 12 but not to such an extent as the upper end of the anode. Thus, if the normal rates of chromium deposit are known or can be readily ascertained, it is a simple matter to determine the ratio of lead to silver necessary :to compensate for the usual variations in plating thickness and produce a chromium plate 17 of substantially {uniform thickness throughout the bore surface of bar- 'rel 14.

Furthermore, the necessity'for an exceptionally long endurance life in certain firearm barrels demands a chromium deposit which tapers uniformly from the breech to the muzzle end of the bore in order to compensate for the anticipated erosion and corrosion thereof. A more' suitable anode for obtaining this tapered plate is :shown in Fig. 4 and essentially comprises a cylindrical rod 18 of lead or a suitable alloy thereof, having a pluralityof circular silver areas or beads 19 deposited there aggro on. Although beads 19 are randomly distributed, the quantities thereof are progressively increased from one end of the anode to the other. The same type of mask ing procedures as those described in connection with the anode of Fig. 1 may be utilized t'o-form silver beads 19. Thus, if the anode is disposed with the greatest quantity of silver beads 19 at the muzzle end of a barrel 20 having a forwardly converging bore 21, the taper of the resulting chromium deposit shown at 22 will progressively decrease in thickness from the breech end to the muzzle end of the barrel. The ratio of silver to lead on the exterior surface of the anode may also be increased by onlarging the size of the silver spots, instead of increasing the quantity thereof in any given area. In such event, the configuration will be that shown in Fig. wherein 23 represents the circularsilver deposits.

In other firearm barrels, accuracy is considered more important than exceptionally long endurance life and'can be achieved by providing a layer of chromium which tapers along-the bore surface so as to progressively increase in thickness from the breech to the muzzle end thereof. The resulting bore restriction provides the same tight grip on the projectile after the engraving thereof by the rifting lands as existed prior thereto and thereby serves to decrease the wobble and yaw of the projectile immediatley upon emergence from the muzzle end of the barrel. A suitable anode for producing the type of tapering chromium deposit in a firearm barrel 24 which will convert a bore of constant diameter to a forwardly restricted bore is illustrated in Fig.6. Such anode essentially comprises a cylindrical lead rod 25 on which silver is deposited inlongitudinal equally spaced strips 26 which progressively decrease in-width from the breech to'the muzzle end of barrel 24 and thereby provide a progressive forward increase in the thickness of the chromium deposit indicated at 27.

Another anode suitable for depositing chromium in barrel 24 for the aforementioned purpose is illustrated in Fig. 7. Such anode is essentially similar to the one shown in Fig. 6 in that silver is deposited on selected areas of a lead rod 28. However, in this anode the silver consists of a plurality of relatively narrow parallel longitudinal stripes 29 equally spaced apart about the exterior periphery of rod 28 and a plurality of circumferentially disposed stripes 30 intersecting stripes 29 substantially at right angles thereto... Stripes 30 are similar in width to stripes.29 but are individually spaced apart so that the distances, therebetween progressively increase from one end of the anode to the other,

When the foregoing anodes are to be employed in depositing chromium in caliber .50 or smaller bore-s, the cylindrical rods should be fabricated from silver in order to provide a maximum how of current longitudinally therethrough. Accordingly, the arrangement of the an- .odes illustrated in Figs. 4-7 may be reversed to'consistof a silver rod on which the beads of Figs. 4 and 5, the stripes of 6, and the stripes of Fig. 7 consist of lead or the aforementioned lead-tin alloy.

However, in firearm barrels over caliber .50 the dimensions of the cylindrical rod are too large to permit the fabrication thereof from such an expensive metal as silver. In such cases, rod 12 of Fig. 1 may consist of a copper or ferrous core on which a relatively thin silver coating has been deposited by electrochemical means. Although a silver-coated copper core will not conduct current as readily as one fabricated almost entirely of silver, the difference in the flow of current is not significant except in those firearm barrels wherein the diameter is less than a half-inch. Where firearm barrels of still larger calibers are involved, the foregoing anodes may be fabricated with a steel core in place of the copper core in order to provide the rigidity necessary to withstand the relatively large magnetic torque encountered during the initial application of the electric current to the anode.

Another anode construction in which the required amount of silver has been reduced to'a minimum is shown in Fig. 3 wherein the cylindrical core 31 is fabricated of copper and coated with a layer of lead 32. The silver is deposited on the lead in the form of a projecting helical ribbon 33 whose width varies in the same manner as the exposed silver areas shown in Figs. 1 and 2.

While the foregoing anodes serve to vary the thickness of plating deposits along the length of a tube such as a firearm barrel, the present invention also contemplates the use of these anodes for varying the thickness of the plating circumferentially about the bore of the barrel. This can be successfully accomplished by increasing the extent to which the lead configurations project from the exposed silver surfaces and by increasing the diameter or the silver rod to bring the projecting lead configurations closer to the bore surfaces of the barrel. Accordingly, if the projecting lead configurations are designed in the form of helical ribs, it is possible to concentrate the plating deposits so as to form rifling lands on the bore surface of the barrel.

This process of building up plating deposits to form distinct shapes can be accomplished with an anode of the type illustrated in Figs. 8 and 9. Such anode preferably comprises a cylindrical steel corev 34 encircled by a silver layer 35 having a plurality of helical ribs 36 of lead projecting outwardly therefrom. Ribs 36 are preferably of arcuate configuration and are equally spaced about the exterior periphery of silver layer 35. Since the film formed on the exterior surface of silver layer 35 limits the passage of electric current into the plating solution only in the immediatevicinity of ribs 36, the plating deposited upon the bore surface of an unrified firearm barrel 37 will approximate theconfiguration of ribs 32% If continued for a sufiicient interval of time, the deposit of plating metal such as chromium will not only cover the bore surface of barrel 37 as indicated as 38 but will also build up into the form of ridges 39 which can be utilized as rifling lands.

In the event difficulty is experienced in providing ribs 36 with the necessary height, the anode may be constructed as shown in Fig. 10 by encircling steel core 34 with a lead layer 40 and then superimposing a silver layer 41 thereon. A plurality of equally spaced helical channels 42 can then be machined along the length of silver layer 41 to expose corresponding areas of lead and thereby provide an anode construction capable of forming the same rifling lands as if the lead areas projected exteriorly of the silver insteadv of beingrecessed therein.

As previously explained, those anodes which are employed to plate chromium on the bore surfaces of firearm barrels in which the diameter of the bore is less than a half-inch, require a considerably greater flow of current than can be provided by a steel core. Accordingly, in theevent it is desired to plate rifiing lands in caliber .22 and .3tl barrels, the core 43 of the anode can be made entirely of silver, or silver alloyed with copper, and

the helical'lead ribs 46 formed directly thereon as shown in Fig. 11. In addition, the core may be made entirely of lead or an alloy thereof as shown at 45 in Fig, 12 and encircled by a layer-d5 of silver in which the necessary helical channels 47 are formed in the same manner as explained in reference to the construction of Fig. 10.

it is, therefore, apparent that the thickness of chromium plate deposited in the bores of firearm barrels can be readily controlled by utilizing anodes in which the exterioi' surfaces thereof consist of various combinations of silver and lead areas. Since the flow of current from th anode into the plating solution takes place only through the lead areas, those factors which normally tend to reduce the rate at which the chromium will be deposited from the plating solution can be readily overcome by a compensatory increase in the size or quantity of lead areas in the affected regions. As a result, the

thickness of the chromium deposit can be kept uniform throughout the entire length of the barrel. Moreover, the ratio between the lead and silver areas can be progressively varied'from one end of the anode to the other to provide a correspondingly tapered plate along the bore surface of the barrel. The technique of varying the ratio between the exposed silver and lead areas on an electroplating anode can be accomplished far more easily -and less expensively than the prior art methods of providing tapered anodes or effecting relative movement between the anodes and the surfaces to be plated.

' Moreover, even though the nonconductive film formed on the exposed silver areas is of negligible thickness, it effectively bars any flow of current into the plating solution so that the metal deposited on the cathode will assume the general shape of the lead areas on the anode especially if such areas are relatively narrow and project substantially beyond the peripheries of the silver areas. Furthermore, unlike conventional organic insulators whose nonconductive power depends on the total thickness thereof, the negligible thickness of the nonconductive film formed on the silver ensures maximum longitudinal conductivity at any given diameter of the anodes described herein. Thus, by utilizing an anode constructed with helical ribs of lead projecting from a covering of silver, it is possible to plate satisfactory chromium rifiing lands on the bore surface of a firearm barrel. Such process is particularly useful where the metal from which the barrel is fabricated is too hard to permit the conventional machining of rifling lands or where the helical pitch of the lands is not constant.

Although a particular embodiment of the invention has been described in detail herein, it is evident that many variations may be devised within the spirit and scope thereof and the following claims are intended to include such variations.

We claim:

1. An anode for plating a uniform layer of chromium on the bore surface of a firearm barrel, comprising a cylindrical rod of silver adapted to form a nonconductive film on the exterior surface thereof when immersed in an acid plating bath containing chromium in ionic form, and a helical band of lead integrally formed about the exterior surface of said silver rod to expose a helical area of silver between each turn of lead for limiting the flow of electrical current into the plating'bath.

2. The combination defined in claim 1 wherein said helical band of lead is formed with a larger pitch at either end of said rod than along the middle section thereof.

3. The combination defined in claim 1 wherein the width of said helical band of lead is a maximum at one end of said rod and progressively decreases toward the central section, thereafter continuing to progressively increase in width to the other end thereof but to a lesser extent than the maximum width at the opposite end thereof. Y

4. An anode for plating chromium onto the bore surface of a metallic tube, comprising a cylindrical rod, a plurality of silver and lead areas alternately disposed be plated is controlled by the silver-lead ratio along the corresponding length of said rod.

5. The combination defined in claim 4 wherein said silver areas are in the form of beads of substantially uniform size dispersed along the length of said rod in progressively increasing quantities for producing a correspondingly tapered plate on the bore surface of the tube.

6. The combination defined in claim 4 wherein said silver areasare in the form of beads whose size is progressively increased from a minimum at one end of said rod to a maximum at the opposite end thereof for producing a correspondingly tapered plate on the bore surface of the tube.

7. The combination defined in claim 4 wherein said silver areas are in the form of longitudinal strips circumferentially disposed about the exterior surface of said rod and progressively decreasing in width from one end of said rod to the other for producing a correspondingly tapered plate of chromium on the bore surface of the tube.

8. The combination defined in claim 4 wherein said silver areas are in the form of a plurality of relatively narrow longitudinal stripes uniformly spaced about the exterior of said rod and a plurality of similar stripes encircling the exterior of said, rod at right angles to said longitudinal stripes and spaced apart at progressively References Cited in the file of this patent UNITED STATES PATENTS 114,191 Parmelee Apr. 25, 1871 823,671 Dieterich June 19, 1906 891,361 Murphy June 23, 1908 915,846 Friedheim Mar. 23, 1909 1,477,099 Baum Dec. 11, 1923 2,230,868 7 Kuhlman Feb. 4, 1941 2,373,352 Smart Apr. 10, 1945 2,523,253 Smith Sept. 19,1950 2,559,926 Beebe July 10, 1951 2,636,856 Suggs et a1. Apr. 28, 1953 2,689,215 Bart Sept. 14, 1954 2,699,425 Niether Jan. 11, 1955 2,795,541 Muller June 11, 1957 FOREIGN PATENTS 724,379 Great Britain Feb. 16, 1955 738,269 Germany Aug. 9, 1943 145,459 Sweden Mar. 11, 1954 OTHER REFERENCES I Weisselberg et al.: 1. Electro Chem, vol. 90, 1946, pp.

Claims (1)

1. 4. AN ANODE FOR PLATING CHROMIUM ONTO THE BORE SURFACE OF A METALLIC TUBE, COMPRISING A CYLINDRICAL ROD, A PLURALITY OF SILVER AND LEAD AREAS ALTERNATELY DISPOSED ON THE EXTERIOR SURFACE OF SAID ROD AND DISTRIBUTED LONGITUDINALLY THEREALONG IN PREDETERMINED VARYING SILVER-LEAD RATIOS, AND AN INSOLUBLE NONCONDUCTING FILM ON SAID SILVER AREAS FOR LIMITING THE PASSAGE OF ELECTRICAL CURRENT FROM THE ANODE TO SAID LEAD AREAS ALONE WHEREBY THE DEPTH TO WHICH ANY PORTION OF THE BORE SURFACE OF THE TUBE WILL BE PLATED IS CONTROLLED BY THE SILVER-LEAD RATIO ALONG THE CORRESPONDING LENGTH OF SAID ROD.

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