US3272729A - Method of electropolishing small metal objects - Google Patents

Description

Sept. 13, 1966 J, JUMER 3,272,729

METHOD OF ELECTROPOLISHING SMALL METAL OBJECTS Filed April 10, 1963 INVENTOR JOHN F. JUMER BY 8 a ATTORNEY United States Patent C) 3,272,729 METHOD OF ELECTROPOLISHING SMALL METAL QBJEQTS John F. Jurner, 16 W. 131 Timber Trails Drive, Elrnhurst, Ill. Filed Apr. 10, 1963, Ser. No. 272,043 4 Claims. (Cl. 204140.5)

My invention has reference to a method of electropolishing quantities of small metal objects.

The electropolishing of small metal parts, particularly those formed of stainless steel, is frequently an expensive operation due to the need for suspending each individual piece on a rack which is subsequently immersed in a suitable bath. In some instances, the rack must be specifically fabricated to receive the objects being treated and has little application to other work of a similar nature.

One of the principal objects of my invention is to provide a suitable method for electropolishing large numbers of small metal objects in bulk and thereby eliminating the need for racks to support such objects for immersion in an electropolishing bath.

More specifically the important objects of my invention include practicing the new method by means of an electropolishing device including a barrel adapted to contain these parts together with a drive for this barrel adapted for periodic intermittent movement of the barrel to produce tumbling of the contained objects, separated by periods of quiescence during which effective electropolishing occurs.

Another important object of my invention is the provision of a process whereby large quantities of small metal objects may be given a lustrous finish of superior quality at low cost by electropolishing without requiring the racking or individual suspension of each metal object.

Additional important objects of my invention will be apparent from the following detailed description and from the drawings in which:

FIGURE 1 is a partially sectioned elevation of apparaq tus suitable for use in the practice of my invention; and

FIGURE 2 is an end view of the apparatus shown in FIGURE 1.

In brief, my method for electropolishing utilizes a barrel or drum of generally polygonal cross section, preferably having not less than three nor more than eight sides, adapted for rotation about its major axis and immersible in a liquid bath of suitable composition. The small metal parts to be electropolished are placed in the barrel in electrical contact with the positive pole or anode of an electrical circuit. The other pole or cathode of the electrical circuit is axially disposed within the barrel and spaced from the objects to be polished in such manner that current will flow between the parts and the cathode when the drum is immersed in the bath.

Electropolishing metal objects is only superficially similar to plating. One of the principal distinctions between these processes is the apparent mechanism of the process. In plating, the reaction starts virtually immediately upon completion of the circuit but in electropolishing, an appreciable period of time elapses before the desired reaction reaches a reasonable level of efliciency. During this period the formation of a film, presumably gas, on the surface of the objects being electropolished will be observed. Under proper conditions this film will initially be uniform and relatively thin and forms on the surfaces of the objects in line of sight from the cathode. As the reaction progresses, however, bubbles of appreciable and irregular size will be formed in the film. These gradually enlarge and eventually rise to the surface of the bath, frequently traveling along the surface of the object being polished. These bubbles, if alllowed to remain in con- "Ice tact with the surface for an appreciable period of time, or to move along the metal surface, will usually produce a streaked or mottled appearance rather than a desired uniform lustrous finish. Furthermore, they markedly increase the resistance to the flow of current.

These undesirable effects are avoided in my invention by allowing the parts in the barrel to remain motionless for a predetermined period of time. The period of time is of importance and is selected to permit the formation of the desired film without which satisfactory electropolishing cannot be accomplished plus an additional period of time in which the actual electropolishing occurs. However, before the bubbles can grow to an undesirable size or break free from the surface being polished in appreciable quantities, I rotate the drum, thus tumbling the parts, destroying the gas film and discharging the bubbles which have grown within the film without damage to the surface. This is followed by another motionless period and again by tumbling, the process being repeated as described until the desired finish is obtained.

Referring to the drawings, my apparatus includes a frame generally designated 10 suitably formed of a generally rectangular plate 11 having downwardly extending legs 12 and 13 aflixed intermediate its extremities. The lower extremity of the leg 12 is formed to receive and support the upper portion of a bushing 17 which is firmly clamped to the leg 12 by a cap 18 pivotally secured to leg 12 by a pin 19. A thumb screw 21 or other suitable device may be utilized to secure the opposite end of cap 18 to the leg 12. The lower extremity of leg 13 is also formed to receive the upper half of a generally cylindrical bushing 14 of suitable synthetic resin inert to the bath ingredients and of such character to provide a satisfactory bearing surface. Bushing 14 is journaled in a metal cap 16 similar to cap 18 and attached to the leg 13 in a similar manner.

A drum or barrel, generally designated 22, is formed of end members 23 and 24 connected rigidly by a plurality of axially extending bars 26. The bars 26 are formed to receive and support panels 27 containing a plurality of openings somewhat smaller than the smallest metal ob ject intended to be placed in the barrel 22. In order to obtain satisfactory resistance to chemical attack as well as relatively low cost, I prefer to form the abovementioned portions of the barrel of a suitable synthetic resin such as polypropylene. One of the panels 27 is provided with a suitable spring clip 28 adapted to retain it in position, thereby forming a door, designated 29, through which parts may be inserted or removed from the barrel 22 The end member 23 is shaped for engagement with a grove 31 formed in the bushing 17 and is adapted to rotate therein. End member 24 is somewhat larger in diameter than end member 23 and is provided with a grooved periphery for engagement with a V-belt 32. A speed reducer 33 driven by a motor 34 is mounted on the frame 10 and drives a pulley 36 over which the belt 32 extends.

Within the barrel 22 are longitudinally extending strips 37 of a suitable metal such as titanium prefer-ably arranged on the flat inner surfaces of the panels 27 and fixed against movement by engagement with the end member 23. These strips preferably are of such shape as to constitute both flights to insure tumbling of the parts during rotation of the barrel and satisfactory electrical contact for the parts being polished.

A titanium plate 38 is secured to the strips 37 in suitable manner such as by screws 39 and constitutes an additional contact for metal objects within the barrel. The central portion of the plate 38 is bored to receive and support a bearing 41 of suitable synthetic resin such \as polypropylene. I

A shaft 42 having a flange 43 supports the drum for rotation in the bushing 14 which is suitably bored to receive the shaft 42. The bolts 44 extend through the end member 24, the flange 43, the plate 38 and the bearing 41, anchoring the component parts. A copper cathode 46 extends axially through the bushing 17 along the axis of the drum 22 and is supported at its free end by the bearing 41.

Insulators 47 secured to the frame by bolts 48 support grooved metal bars 49 adapted to rest on bus bars (not shown) arranged above a suitable bath in which my device may be immersed. A conductor 51 connects the bar 49, intended to contact the negatively charged bus, with the cathode 46. Conductor 52 connects the other bar 49 with a brush 53 mounted in a holder 54 and urged by a spring 56 into rubbing contact with the shaft 42.

The motor 34 is connected to a suitable timing device, generally designated 57, of conventional design adapted to regulate the duration of the period in which the motor is actuated as well as the interval between actuations of the motor. The timer 57 may for example include a timing device 58 for adjustably regulating the period of time the motor 34 will be actuated. A dwell timing device 59 may also be provided to adjustably regulate the interval between motor operating periods. Since my device is intended to be transported by an overhead hoist, I provide lifting means such as an eyebolt 61 engaging the frame 10 above the center of gravity and a conventional flexible cord and plug 62 for connection with a suitable A.C. circuit after the device has been placed in position in the polishing tank.

In practice the requisite quantity of small parts to be polished is inserted in the barrel 22 through the door 29, the quantity selected being such that contact with the cathode 46 will not occur during rotation of the drum. When fully charged, the small parts form a generally rectangular zone within the drum 22 below the cathode 46, this zone being generally maintained throughout operation of my device. The entire assembly is then transferred to a tank containing a bath of suitable composition and temperature and is supported therein by bus bars which engage bars 49. The plug 62 is connected to a suitable circuit, the timers 58 and 59 set to the desired intervals and the starting button is pressed. The parts within the barrel are in contact with the metal end plate 38 and with the strips 37 and thus constitute the anode of an electrical circuit extending through the bath to the cathode 46. As previously noted, it is necessary that the drum remain motionless for a period of time sufficient to insure the formation of a satisfactory film primarily along the surfaces of the mass of parts exposed to the cathode. This will require a minimum of approximately two seconds and a maximum of not to exceed ten seconds. The film formed will initially be quite uniform in thickness and characterized by the absence of large bubbles. These, however, will form gradually and should be destroyed by actuation of the motor 34 and tumbling of the parts to largely destroy the film before damage to the finish occurs. The length of time the barrel should remain motionless will vary from a minimum of about ten seconds to a maximum of about two hundred seconds. The time is necessarily variable and is influenced not only by the character of the bath but also its temperature and the density of the current flowing between the parts and the cathode. The appropriate intervals may be selected by observation in each instance.

The timer is adjusted so that rotation of the motor will occur before gas bubbles of appreciable size are formed on the surface being polished and before such bubbles break free from the parts and rise to the surface of the bath in appreciable quantity. Actuation of the motor causes rotation of the barrel and the desired tumbling of the contained parts, this action being aided by the strips 37 which serve as flights to insure movement of the small parts in the desired manner. The period of barrel rotation may be quite short and sutficient only to destroy the gas film and expose a new surface to the action of the cathode. This will ordinarily occur in approximately one-quarter of a revolution, but greater movement is not harmful and several revolutions may be utilized if necessary to thoroughly disperse the film. This procedure is, of course, repeated until a satisfactory surface finish is obtained. Upon completion of the polishing operation parts may be removed from the barrel 22, washed and packed for shipment.

It should not be understood from the foregoing that the materials of construction indicated are intended to limit my invention. They constitute satisfactory materials for the use described but other materials may be substituted. Titanium constitutes a preferred material for portions of the electrical circuit constituting the anode but should not be employed as a cathode. The cathode may be formed of copper, carbon or other suitable material, copper being the most generally available. It will also be understood that other driving mechanisms may be utilized as, for example, gears instead of belts.

By following the method of operation herein described, the time required to impart the desired lustrous finish to the parts being treated will be reduced and the charatcer of the finish materially improved. Furthermore, it has been found that small objects such as screws treated as herein described exhibit improved corrosion resistance when tested in a salt spray.

I claim:

1. The method of enhancing the uniformity of lustre of small metal objects capable of being electropolished in an electrolytic bath which includes the steps of arranging a plurality of said objects in contact with each other in a relatively thin generally rectangular zone in an electropolishing electrolytic bath, maintaining said objects motionless while passing electrical current from said parts to a cathode spaced from said zone, thereby forming a gas film on the surfaces of the parts exposed directly to said cathode, maintaining said objects motionless until said gas film consists of a multiplicity of bubbles of irregular size, then destroying said film by tumbling said objects within said zone and thereby exposing a substantially bubble free surface to the cathode, and periodically repeating said steps of holding said objects motionless and tumbling said objects to destroy a film of bubbles of irregular size while continuing the passage of electrical current between the parts and the cathode.

2. The method defined in claim 1 which'includes the step of maintaining said parts motionless for not less than ten seconds.

3. The method defined in claim 1 which includes the steps of maintaining said parts motionless for a period of not more than two hundred seconds.

4. The method defined in claim 1 wherein said parts are maintained motionless for not less than ten seconds nor more than two hundred seconds.

References Cited by the Examiner UNITED STATES PATENTS 1,873,826 8/1932 Devecis 204213 2,390,282 12/1945 Tour et al 204- 2,741,463 4/ 1956 C-olclesser 204213 X 3,058,902 10/1962 Neilson 204213 3,213,008 10/1965 Valentin 204-1405 OTHER REFERENCES Ransohoif, Plating, November 1956, pages 3146-8.

JOHN H. MACK, Primary Examiner.

HOWARD S. WILLIAMS, Examiner.

R. MIHALEK, Assistant Examiner.

Claims (1)

1. THE METHOD OF ENHANCING THE UNIFORMITY OF LUSTRE OF SMALL METAL OBJECTS CAPABLE OF BEING ELECTROPOLISHED IN AN ELECTROLYTIC BATH WHICH INCLUDES THE STEPS OF ARRANGING A PLURALITY OF SAID OBJECTS IN CONTACT WITH EACH OTHER IN A RELATIVELY THIN GENERALLY RECTANGULAR ZONE IN AN ELECTROPOLISHING ELECTROLYTIC BATH, MAINTAINING SAID OBJECTS MOTIONLESS WHILE PASSING ELECTRICAL CURRENT FROM SAID PARTS TO A CATHODE SPACED FROM SAID ZONE, THEREBY FORMING A GAS FILM ON THE SURFACES OF THE PARTS EXPOSED DIRECTLY TO SAID CATHODE, MAINTAINING SAID OBJECTS MOTIONLES UNTIL SAID GAS FILM CONSISTS OF A MULTIPLICITY OF BUBBLES OF IR-

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