US2438837A

US2438837A - Tool and method for dimpling

Info

Publication number: US2438837A
Application number: US562596A
Authority: US
Inventors: James O Archer; Douglas S Salmi
Original assignee: Lockheed Aircraft Corp
Current assignee: Lockheed Corp
Priority date: 1944-11-09
Filing date: 1944-11-09
Publication date: 1948-03-30
Anticipated expiration: 1965-03-30

Description

March 30, 1948. J. o. ARCHER m'AL TOOL AND METHOD FOR DIMPLING Filed Nov. 9, 1944 INVENTORS DOUGLAS 5. SALMl y & JAMES O. ARCIHER Patented Mar. 30, 1948 2,438,837 TOOL AND METHOD FOR DIIHPLING James 0. Archer, Burbank, and Douglas S. Salmi, Glendale, Callf., asslgnors to Lockheed Aircraft Corporation, Burbank, Calif.

Application November 9, 1944, Serial No. 562,596

The invention relates to forming tools and methods, and relates more particularly to a tool and method for forming dimples at the rivet and screw receiving openings of metal sheets, and the like.

In the fabrication of various parts and structures, such as aircraft components, it is the common practice to provide dimples at the rivet receiving openings of sheet metal parts so that the heads of the rivets may lie flush with the surfaces of such parts. The dimpllng is often a very critical operation in which the forming stresses approach the rupture values. Procedures and tools which satisfactorily dimple relatively soft metals or alloys, with only occasional cracking at the dimples, are not suitable where harder I materials are to be dimpled. For example, the conventional tools employed to dimple 24S-T aluminum alloy sheets cannot be used to dimple sheets of high strength aluminum alloys such as S-T and 24S-T86 without excessive cracking pansion of the opening and forming of the dimple. The forging action also reduces stresses at the mouth of the dimple, produced by the dimple forming action, thereby preventing cracking at this point. This forging action is controlled by the substantially equal diameters of the punch and die, and during the final stages of the dimpling action the stresses which are set up during 7 I the initial stages of the forming are counteracted so that sharp, substantially stress-free dimples result. However, the forming of the metal coupled with the forging action obtained by the yieldingly supported ram, causes substantial warpage of the sheet, particularly where the dimpling operations are performed adjacent the margins of the sheets, which is the location of a great majority of dimpled openings. The warp- 16 Claims. (Cl. 15 3--21) 1 cause of the misalignment of the dimpled openings with openings in the substructure, and also causes poor mating of superimposed sheets and irregularities in the assembled sheets. Accordingly, while the tools employing forging rams at the dies form good, sharp dimples substantially devoid of cracks, such tools are not acceptable for general usage because of the sheet distortion caused by the forging action.

It is a general object of the present invention to provide a method and tool operable to form sharp radius, crack-free dimples in high strength aluminum alloy sheets, and the like, without deforming or distorting the sheets. The tool and method of the invention have all the advantages and capabilities of the forging-method tool above referred to, but completely avoid the accompanying distortion and warpage of the sheets.

Another object of the invention is to provide 1 a dimpling tool employing a yieldingly supported ram in the die assembly to forge the metal around the opening in the sheet and thus obtain a true, sharp dimple free of cracks, which tool further embodies means to confine the effects of the forging of the dimple from the remainderof the ;bodying a double action dimpling punch having an annular clamping pr pressure pad which contacts an annular area of the sheet around the zone to be dimpled, and serves to clamp the sheet against an opposing surface of the die prior to and throughout the forming and forging phases of the dimpling operation to thereby prevent distortion of the sheet as a whole by reason of displacement of material from the dimple into the sheet, or from other causes.

Another object of the invention is to provide a method and tool operableto form sharp radius dimples in high strength aluminum alloy sheets, etc., of such character that dimples of superimposed sheets may properly nest or fit one within the other without the necessity of altering the dimensions of the dimples in the subsheets or substructure. The walls of the dimples are uniform and are forged so that an unlimited number of sheets di pled by the same tool may be accurately neste or engaged one upon the ether.

tailed description of typical preferred forms of the invention wherein reference will be made to the accompanying drawings in which:

Figure l is a vertical sectional view of the die and punch assemblies of one form of the invention showing the parts in the idle positions and illustrating certain portions in side elevation;

Figure 2 is a view similar to Figure 1 showing the pressure ring of the punch assembly in engagement with the sheet;

Figure 3 is a view similar to Figure 1 showing the parts in the final actuated positions;

Figure 4 is a fragmentary vertical sectional view of another form of the invention illustrating a means for adjusting the resilient plunger supporting means;

Figure 5 is an enlarged fragmentary detailed sectional View showing the active parts in the final operative positions; and,

Figure 6 is a vertical sectional view of another form of the invention. r

The tool and method of the invention are adapted to dimple the openings of sheets and like parts of various materials and configurations.

In the drawings we have illustrated typical forms of the invention suitable for dimpling openings in aluminum alloy sheets, and the like, it being understood that the tools are capable of modification to adapt them for given uses and that the invention is not to be construed as limited to the particular details herein set forth,

The tool of Figures 1 to 3 inclusive comprises a die assembly In and a punch assembly II. The two units or assemblies may be designed for use in any selected or appropriate machine suitable for carrying out the dimpling operations. The assemblies l and H are related for relative movement so that the active parts may be moved into and out of engagement with the sheet S or other work. It will be assumed that the die assembly ID remains stationary and that the punch assembly H is moved downwardly and then upwardly with respect to the die assembly to perform the dimpling operation and to restore the parts to their original relationship. The die assembly I0 is shown as being the lower unit of the tool and the punch H is illustrated as bein the upper assembly, it being apparent that this relationship may be reversed if desired.

The die assembly It) comprises a. holder I2 provided at its lower end with a reduced shank [3, or other appropriate means for facilitating its mounting or attachment in the machine. A central vertical opening [4 enters the holder ID from its upper end and joins an internal chamber or openin I of larger diameter. The opening 15 is preferably cylindrical and extends longitudinally within the holder [2. The holder I2 may be constructed in two sections to facilitate the provision of the openings l4 and I5, the sections normally being in rigid connection with one another. A central vertical socket I6 is provided in the bottom wall of the opening l5. In the form V engagement by the work or sheet S. While the surfaces l1 and 21 are shown as being normal to the longitudinal axis of the tool, it is to be understood that either one or both of these surfaces may be given a small angle or curvature to be slightly conical or spherical and thus compensate for spring-back of the sheet S following the dimpling operation. The surface I1 may be annular and of substantial area.

The die recess l8 of the tool occurs in the surface H at the mouth of the opening M. The recess I8 is frusto-conical, having an annular wall which slopes downwardly and inwardly from the surface I! to the wall of the central opening.l4. This sloping wall constitutes the die surface for assisting in shaping the dimple. In accordance with the invention, it may not be necessary to provide a radius where the recess I8 joins the flat surface I! because the forging action assures the production of sharp, effective dimples Without cracking at this point. It is to be understood that the dimensions and configuration of the die recess l8 may be varied in different applications of the invention.

The die assembly if) further includesa plunger or ram I9 operable in the openings l4 and i5 and engageable with the sheet S around its opening 0 to coin or forge the material of the dimple. The ram 19 slidably operates in the opening I4 and has a fiat normal upper end for acting on an annular area of the sheet around the opening 0. A reduced stem or pilot 20 extends upwardly from the end of the ram l9 to engage in the opening 0 of the sheet S to assure the correct concentric disposition of the dimple with respect to the opening. The end of the pilot 20 may be rounded to readily enter the opening 0. The ram 19 continues downwardly through the opening l5 and its lower end portion is slidably guided in the socket I6. A flange or piston 2| is provided on the ram 19 to operate in the enlarged opening 15 of the holder [2. The piston 2! is rigid with the ram l9 and is engageable with the upper wall of the opening l5 to limit the upward travel of the ram.

The lower side of the piston 2| is engaged by a resilient supporting or actuating means. While the character of the resilient means may be varied, we prefer to employ rubber, rubbercomposition or synthetic rubber of the required shore hardness as the resilient element of the die assembly. Thus in the drawings a resilient body 22 of rubber, or the like, occupies that portion of the opening l5 below the piston 2 l The resilient body 22 is preferably of such volume that it is under a selected compression between the piston 2| and the bottom wall of the opening I5 when the ram I9 is in its normal unactuated position of Figure 1. Accordingly, the resilient body 22 normally holds the ram IS in its fully raised position. When the ram I9 is in this unactuated position, its upper end occupies substantially the same plane as the upper surface I! of the holder I2. When we employ the term resilient body it is to be construed as contemplating any resilient element or unit yielclingly supporting the ram IQ for its forging operation to be later described. I

The punch assembly ll includes a holder 23 provided at its upper end with a stem 24 of reduced diameter for facilitating its mounting in the machine. A cylindrical socket 25 of substantial diameter enters the lower end of the holder 23 and slidably receives a pressuresleeve or ring 26. The pressure ring extends downwardly beyond the end of the holder 23 and may have a plain flat lower surface 21, normal to the longitudinal axis of the tool, for engaging the sheet S. As above described, the surface 21 may be slightly conical or spherical. The outside and inside diameters of the surface 21 substantially conform to the corresponding diameters of the surface ll/of the die holder. The upper end of the pressure ring 26 is spaced from the upper wall of the socket 25.

The punch assembly I I further includes a punch 28 slidably operable in the opening of the rality of openings 0 may be simultaneously dimpled. Figure 1 illustrates the normal or idle positions of the die and punch assemblies and shows the pressure ring 26 spaced clear of the pilot 20 to allow ready engagement of the sheet S on the die assembly. When the sheet S has been properly positioned the machine, with which the die and punch set is associated, is put into operation to produce relative movement between the die and punch assemblies.

When the punch assembly is in its idle condition the body of fiowable material 32 holds the pressure ring 26 in the extended position. The initial phase of downward movement of the punch holder 23 brings the lower end of the ring 26 into engagement with the sheet S. This engagement limits the downward travel of the ring and as the holder 23 continues to move downwardly, the material 32 is compressed between the upper wall of the socket 25 and the upper end of the ring 26. The material 32, acting under compression, transmits downward pressure axis of the plunger 28, occurs within the punch surface 29. This annular surface acts on the sheet S in a zone immediately surrounding the opening 0 to assist in bringing about the forging action to be later described. A central opening or socket 3| extends upwardly from the surface 30 to receive the'pilot 20.

In accordance with the invention, the punch assembly I I is double acting; that is, the pressure pad or ring 26 is first actuated against the sheet S to confine a zone of the sheet around the area to be dimpled and the punch 28 is then actuated to form the dimple while said area or zone. remains confined. To accomplish this double action a body 32 of fiowable material is provided in the socket 25 to transmit movement and actuating pressures to the ring 26 and punch 28. The upper end of the ring 26 is exposed for full engagement by the material 32 and a head 33 may be provided onthe upper end of the punch 28 to present an extensive area for engagement by the material. The head 33 is in the nature of a piston-like flange slidably operating in a counterbore or socket 34 in the upper end of the pressure ring 26. The nature of the material 32 may vary considerably and practically any selected material capable of flowing under pressure may be employed. A stiff grease or similar semi-liquid may be used, or a mass of flowable comminuted material may be employed. It is desirable to use a material that has good flow characteristics and that will not break down with extended use. Cured rubber of about 40 Shore hardness may be successfully employed as the actuating material 32. Such material has the ability to return to its normal volume and configuration when relieved of pressure. The body of material 32 preferably completely occupies the socket 25 above the ring 23 and punch head 33. The edge or or forms the dimple.

corner of the pressure ring 26 occurring where the wall of the socket 34 joins the end of the upper ring may be rounded off to permit free flow of the material 32 into the socket.

The method of the invention will be described in connection with the operation of the tool disclosed above. The sheet S previously provided from the holder 23 to the ring 23 and the ring is pressed against the sheet S with considerable force. This results in an annular area of the sheet, spaced around the opening 0, being confined or gripped between the end surface 21 of the ring ZB a'ind the end surface I! of the die holder I 2. Figure 2 of the drawings shows the pressure sleeve 26 in its active position in engagement with the sheet S. ontinued-downward movement of the holder 23 increases the pressure exerted on the annular zone of the sheet surrounding the die recess [8 and while this pressure may reach a high value, it never attains the yield point of the sheet S. 1

The body of flowable material 32 transmittin the downward pressure for the actuation of the pressure ring 26, as described above, is partially placed into the socket 34 forces the punch 28 downwardly, serving to transmit the, downward movement and pressure from the holder 23 directly to the punch. Thus, as the holder 23 travels downwardly, the punch 28 is forcibly moved against the sheet S to form the dimple. The opening 3! receives the pilot 20 as the punch moves downwardly. The punch surface 29, acting in conjunction with the upper surface of the sheet S and the wall of the die recess 18, shapes It is to be observed that this forming of the dimple occurs while a substantial annular zone of the sheet S, surrounding the area of the dimple, remains confined or compressed between the pressure ring 23 and the surface ll of the die holder.

During the forming of the dimple,the portion of the sheet material immediately surrounding the opening 0 is engaged by the flat end surface 30 0f the punch and is pressed against the flat upper end of the forging ram 13. This more or less limited portion of the sheet is gripped between the opposite flat surfaces of the punch and ram to be subjected to compression and forging. The body of yieldable material 22 under the ram piston 2| offers a high resistance to downward movement of the ram, and during the dimple forming phase of the operation, the

surrounding the opening 0, forge the material to counteract the stresses set up by the forming action. As the dimple is shaped or formed, there is a drawing action tending to stretch the upper section or layer of the material of the dimple. This stretching sets up tensile stresses which, unless counteracted, weaken the material and cause cracks to develop. The forging action of the ram 19 and surface 30 of the punch counteracts or corrects this drawing or thinning" at the mouth of the dimple and produces a radial outward flow of the material in the dimple to effectively counteract substantially all excessive stresses. The forging action also prevents enlargement of the opening 0, thinning of the dimple wall, and over stressing of the sheet material at the periphery of the dimple.

- The radial outward flow of material caused by the forging of the metal supplies material to prevent over stressing and cracking of the sheet at the angle occurring at the mouth of the dimple.

As shown in Figure 3, the ram l9 retracts to some extent during the dimpling and forging operations, but the yieldable material 22 permits such retraction only after the material of the sheet around the opening has been subjected to the desirable high compression and forging. The tool may be constructed so that the material 22 offers the required degree ofresistance to ram movement to adapt the tool for the dimpling of given materials or classes of materials. The resisted retraction ofthe ram i9 is important as it assures the desirable compression and forging of the material in the dimple and yet allows full action of the die and punch surfaces in the forming of the dimple. The forging action just described is effective in preventing cracking and over stressing of the dimple, even in the case of high strength aluminum alloys which are subject to cracking when conventional dimpling methods are used.

The confining or clamping under pressure of the annular zone of the sheet S immediately surrounding the dimple area prevents stressing of the sheet beyond the dimple itself. It is to be noted that the compressive force exerted on the annular zone of the sheet around the area being dimpled prevents stressing of the sheet itself as a result of the dimple forming action as well as the dimple forgingaction. The confining or clamping pressure increases with the forming and forging pressures, and when the latter are at their maximum values the clamping pressure is also at its greatest value to effectively resist stressing of the'sheet beyond the dimple area. Accordingly, the sheet S is not distorted or warped as a result of the dimpling operation, even in instances where the dimples are formed at openings adjacent the edge of the sheet.

At the completion of the dimpling operation, the punch holder 23 moves upwardly to disengage the punch 28 and pressure ring 26 from the sheet. This frees the sheet S for removal from the tool. Upon such retraction of the punch assembly, the body of elastic material 22 returns to its original configuration restoring the ram iii to its normal idle position. In a like manner, the pressure ring 26 and punch 28 return to their normal relative positions.

The dimples formed by the tool in Figures 1, 2 and 3 are such that it may be necessary to redrill the openings 0 to remove the material within the inner margins of the dimples. The form of the invention illustrated in Figure 4 is such that there is no material remaining within the dimple at the completion of the dimple forming operations and the wall of the opening and'the inner margin of the dimple coincide. The punch assembly of Figure 4 includes the holder 23, the pressure ring 26 and the body 32 of flowable material all as above described. In this form of the invention the punch 48 slidably operates in the opening of the ring 28 and has a head 49 slidably engaged in the counterbore 34 to be acted upon by the material 32. The lower end of the punch 48 has a frusto-conical surface 50 which operates in conjunction with the recess l8 of the die to form the dimple. We have shown a pilot 5| on the lower end of the punch 48, it being understood that the pilot may be carried by the ram of the die assembly as in Figures 1, 2 and 3. Where the pilot 5| is carried by the punch 48 the surface 50 surrounds the root of the pilot.

The die assembly of Figure 4 includes means whereby the forging pressures may be readily adjusted to adapt the tool for the forming of dimples in sheet materials having different physical characteristics. The die holder is made up of two sections 40 and 4| adjustably threaded together at 42. The holder is generally of the same configuration as the above described holder l2 and carries the ram 52 and the resilient material 22. Shims 43 are preferably engaged between opposing surfaces of the threadedly connected holder sections 40 and 4|. The shims 43 may be adjusted in thickness to regulate the initial volume of the resilient body 22 so as to offer the required forging pressure for the particular material being worked upon. A set screw 44, or the equivalent, may be employed to retain the sections 40 and 4| in their/relative adjusted positions.

The ram 52 operates in the opening l4 of the I die holder to have its upper end exposed for the forging of the metal of the dimple. Where the pilot 5| is carried by the punch 48, a vertical opening 53 enters the upper end of the ram 52 to receive the pilot. The forging surface 54 of the ram is annular and is in surrounding relai tion to the mouth of the opening 53.

When the machine carrying or embodying the tool of Figure 4 is operated, the pressure ring 26 presses the sheet S against the surface I! of the die holder to confine or clamp an annular area of the sheet surrounding the zone to be dimpled. As in the previously described form of the invention, this confining of the sheet continues throughout the several phases of the operation and the pressure increases in value as the forming and forging pressures increase. The punch surface 50 and the recess l8 form the dimple and the ram 52 supported or actuated by the yielding material 22 forges the material of the dimple to counteract the effect of the stresses set up therein. It is to be observed that the annular surface 54 of the ram 52 acts upon the material of the dimple immediately surrounding the opening in the sheet. This material is compressed or forged between the ram surface 54 and the lower portion of the punch surface 50. The resultant forgin action prevents enlargement of the openingin the sheet, which would otherwise accompany forming of the dimple, and also counteracts the stresses set up in the inner portion of the dimple which might produce cracks in the dimple. The forging action of the opposed ram surface 54 and punch surface 50 also effects an outward material flow in the dimple to counteract stresses in th peripheral portion of the dimple. The forming of the dimple may set up tensile stresses at or ady embody pressure rings for confining the area of the sheet around the zone to be dimpled. The punch assembly I I may be the same as in Figures 1, 2 and 3 and corresponding reference numerals are applied to its several parts. The holder 50a of the die assembly has a socket 5la in its upper end slidably receiving a pressure ring and die member 52a. The member 52a presents a flat upper surface 53a of the same dimensions as the surface 21 of the pressurering 26 and the die recess 54a is formed in the upper end of the pressure ring and die member. A central vertical opening 55 is provided in the member 52a and is counterbored from its inner end. The forging ram 56 is slidable in the opening 55 and has a flat upper end surface for the forging operation. A suitable pilot .5! may extend fromv the upper end of the ram. The ram has a portion of enlarged diameter slidably operating in the counterbore of the opening 55. A body 58 of elastic flowable material is arranged in the bottom of the socket 5| :1 to support the member 52a and ram 56. The body of material 58 is capable of flowing into the counterbored opening 55 when compressed by downward pressure exerted on the member 52a. 9

In the operation of the tool of Figure 6 the sheet is engaged on the surface 53a to have the pilot 51 pass through its opening 0 and the punch assembly H is moved downwardly by the machine. pressure ring 26 against the sheet so that an annular zone of the sheet is gripped between the ring and the surface 53a of the member 52a. As the punch assembly continues to move downwardly, the pressure ring and die member 52a retracts slightly under downward pressure and the body of material 58 is obliged to partially flow into the counterbored opening 55. This results in upward relative movement of the ram 56. The forging action is substantially the same as above described and serves to supply material radially in the dimple .to counteract stresses therein and to an=aintain-uniform wall thickness throughout the dimple. During the dimple forming and forging phases a substantial annular zone of the sheet S remains confined or gripped between the

surfaces

21 and 53 to prevent stresses This downward movement brings the against said surface to resist flow of the material of the work in the area around the dimple during the forming of the dimple and the forging of said material. 1

2. A tool for forming a dimple around an opening in a work piece comprising two units related for relative movement; one a die having a dimple recess, the other a punch assembly including a dimple forming punch operable in conjunction with said recess to form a dimple in the work piece around said opening, means on the units for clamping an area of the work piece around the dimple toresist flow of the material of the work piece in said area while the dimple is being formed, and means on the die and punch for forging the material of the work piece immediately surrounding said opening during the forming of the dimple, the last named means including a yieldingly supported ram carried by the die for acting on the work opening.

3. A tool for forming a dimple around an opening in a work piece comprising two units related piece around said for relative movement, one a die having a dimple recess, the other a punch assembly including a dimple forming punch operable in conjunction with said recess to forms. dimple in the work piece around said opening, means on the units for clamping an area of the work piece around the dimple to resist flow of the material of the work piece in said area while the dimple is'being formed, and means on the die and punch for forging the material of the work piece immediately surrounding said opening during the forming of the dimple, the last named means including a forging surface on the punch for acting on the work piece, and a yieldingly supported ram carried by the die and opposing said forging surface. i

4. A tool for forming a dimple around an opening in a work piece comprising two units related dimple forming punch operable in conjunction from developing in the sheet S outside of the dimple itself. Accordingly, there can be no warping or deformation of the sheet as a result of the dimpling operation.

Having described only typical forms of the invention, we do not wish to be limited to the speciflc details herein set forth, but wish to reserve to ourselves any variations or modifications that may appear to those skilled in the art or fall within the scope of the following claims.

We claim: 4

1. A dimpling tool comprising two elements, one a die assembly the other a punch assembly, one element being movable toward the other, the die assembly having a work engaging surface and a dimple depression, a punch on the punch assembly for forming a dimple in said depression l and having a forging surface for engaging the work within the dimple, a yieldingly supported with said recess to form a dimple in the work piece spaced around said opening, means for forging the material of the work piece immediately surrounding the opening during the forming of the dimple to counteract the stresses set up in the dimple, said means including a surface on the punch and a yieldingly supported ram carried by the die, said punch surface and ram being engageable with the opposite surfaces of the i work piece around the opening, and a pressure pad on one unit for clamping the zone of the work piece surrounding the area being dimpied against the other unit to prevent stressing of the work piece around the dimple.

5. A dimpling tool for forming a dimple in a work piece having an opening comprising relatively movable die and punch assemblies having surfaces for forming a dimple in the work piece spaced around said opening upon movement of one assembly toward the other, work piece engaging parts on the assemblies for forging the material of the portion of the work piece being dimpled and which immediatelysurrounds said opening during substantially the entire period of forming of the dimple to counteract the effect 11 said forming of the dimple, yieldable means mounting 'at least one of said work piece engaging 7 surfaces for forming azdimple in the work piece around said opening upon movement of one assembly toward the other, parts on the assemblies for acting on and forging the inner portion of the work piece being dimpled which immediately surrounds said opening during substantially the entire action of the die and punch in forming the dimple to counteract the stresses set up in the dimple and to prevent enlargement of the opening as a result of said formin of the dimple, a yieldable element for transmitting force between at least one of said assemblies and its said part, and means on the assemblies for acting on the work piece in a zone surrounding the area being' dimpled to prevent stressing of the work piece as a result of said forming of the dimple and said forging of the material, the last named means comprising a yieldingly supported pressure rin on one assembly for pressing the work piece against the other assembly.

'7. A dimpling tool for forming a dimple in a work piece having an opening comprising relatively movable die and punch assemblies havin surfaces for forming a dimple in the work piece around said opening upon movement of one as-. sembly toward the other, parts on the assembles for acting on and forging the inner portion of the work piece being dimpled which immediately surrounds said opening during substantially the entire action of the die and punch in forming the dimple to counteract the stresses set up in the dimple and to prevent enlargement of the opening as a result'of said forming of the dimple, yieidable elements for transmitting force between said assemblies and their respective forging parts,

' yieldingly resisting movement of said ram relaunease-r and said forging of the material surrounding the opening.

9. A dimpling tool for forming a dimplein awork piece having an opening comprising rela-' tively movable die and punch assemblies having surfaces for forming a dimple in the work piece spaced around said opening upon movement of one assembly toward the other, means on the bly for acting upon the other side of the work piece around said opening, and resilient means tive to said other assembly, and means on the assemblies for acting on the work piece in a zone surrounding the area being dimpled to prevent and means on the assemblies for acting on the work piece in a zone surrounding the area being dimpled to prevent stressing of the work piece as a result of said forming of the dimple and said forging of the material, the last named means comprising opposing yieldingly supported pressure members on the assemblies for pressing.

against the opposite sides of said zone;

8. A dimpling tool for forming a dimple in a work piece having an opening comprising relatively movable die and punch assemblies havin surfaces for forming a dimple in.the work piece around said opening upon movement of one assembly toward the other, means on the assemblies for forging the material of the work piece immediately surrounding said opening to counteract the stresses set up in the dimple and to prevent enlargement of the opening as a result of said forming of the dimple, said means including a surface portion on one assembly for acting upon one side of the work piece around said opening, a ram carried by the other assembly for acting upon the other side of the work piece around said opening, and means yieldingly resisting movement of said ram relative-to said other assembly, and means on the assemblies for acting on the work piece in a zone surrounding the area being dimpled to prevent stressing of the work piece as a. result of said forming of the dimple stressing of the work piece as a, result of said forming of the dimple and said forging of the material surrounding the opening.

10. A dimpling tool for forming a dimple in a work piece having an opening comprising relatively movable die and punch assemblies havin surfaces for forming a dimple in the work piece spaced around said opening upon movement, of one assembly toward the other, means on the assemblies for forging the material of the work piece immediately surrounding said opening to counteract the stresses set up in the dimple and to prevent enlargement of the opening as a result of said forming of the dimple, said means including a surface portion on one assembly for acting upon one side of the work piece around said opening, a ram carried by the other assembly for acting upon the other side of the work piece around said opening, and resilient means yieldingly resisting movement of said ram relative to said other assembly, and means on the assemblies for acting on the work piece in a zone surrounding the area being dimpled to prevent stressing of the work piece as a result of said forming of the dimple and said forging of the material surrounding the opening.

11. A dimpling tool for forming a dimple in a work piece having an opening comprising relatively movable die and punch assemblies having counteract the stresses set up in the dimple and to prevent enlargement of the opening as a result of said forming of the dimple, said means including a movable ram carried by one assembly for acting on one side of the work piece around said opening and opposing the dimple forming surface of the other assembly, yielding means for resisting movement of the ram in a direction away from the work piece, and means for adjusting the resistance offered by the yielding means, and means on the assemblies for acting on the work piece in a zone surrounding the area being dimpled to prevent stressing of the work piece as a result of said forming of the dimple and said forging of the material surrounding the opening.

12. A dimpling tool for forming a dimple in a work piece having an opening comprising relatively movable die and punch assemblies havin amass? surfaces for forming a dimple in the work piece spaced around said opening upon movement of ing on the portion of said areawhich immediately surrounds said opening with forging tools durone assembly toward the other and means on the i assemblies for forging thematerial of the work piece immediately surrounding said opening-to to prevent enlargement of the opening as a result of said forming of the dimple, said means including a movable ram carried by one assembly for acting on one side of the work piece around I said opening and opposing the dimple formin surface of the other assembly, yielding means for resisting movement of the ram in a direction away from the work piece, and means on the assemblies for acting on the work piece ina zone surrounding the area being dimpled to prevent stressing'of the work piece as a result'of, said forming'of the dimple and said forging of the material surrounding the opening.

13. The method of dimplinga work piece which comprises resisting material flow in the work piece in directions outwardly from the areathere- I of tb-be dimpled by clamping an annular zone of the work piece around the area to be dimpled,

then acting on the outer portion of said area to shape a. dimple therein while said zone is thus confined, and acting on the inner portion of said area to forge the material thereof and cause a radially outward material flow in said area during at least the initial and major phases of said shaping of the dimple. e

14. The method of dimpling a work piece which comprises the steps of actingon an annular area of the work piece to shape a dimple therein,re-

ducing. stressing of said annular area by forging the material of the inner portionv of said area during at least the major phases of shaping of the dimple to cause a radially outward material flow in said area, and counteracting stressing of the work piece around said area as a result of said shaping and forging by clamping a zone of the workpiece around said area during said shaping and forging to resist material flow therein.

15. The method of dimpling a work piece having an. opening which comprises dimpling an an- 4 nular area of, the work piece around said openin by'acting on said'area' with forming tools, acting at least the initial and major phases of said I dimpling of said area to cause radially outwardcounteract the stresses set up in the dimple and from the area around the opening and which material flow in the area being dimpled, and resisting material flow in the work piece around said area as a result of said dimpling and forging by subjecting the work piece around saidarea to compressive forces during s-aid'dimpling and forging.

16. The method of dimpling a work piece ha ing an opening which comprises resisting material flow in the work piecein directions outwardly is to be dimpled by subjecting an annular zone of the work piece around said area to compressive forces, then forming a dimple in said area around,

said opening by acting on said area with forming tools while said compressive I forces are maintained on said'annular zone, and acting on the portion of said areaimmediately contiguous to said opening, during substantially the entirezstep of forming the dimple, to forge the material in said portion and cause a radially outward material fiow in said area being dimpled.

JAMES 0. ARCHER. DOUGLAS 8. SALMI.

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

DSTATES PATENTS Great Britain Aug. 24, 1939-