US2412731A

US2412731A - Stretch forming machine

Info

Publication number: US2412731A
Application number: US534977A
Authority: US
Inventors: Frederick C Hoffman
Original assignee: Lockheed Aircraft Corp
Current assignee: Lockheed Corp
Priority date: 1944-05-10
Filing date: 1944-05-10
Publication date: 1946-12-17
Anticipated expiration: 1963-12-17

Description

Dec. H7, 394%.

F. c. H FFMAM EAEZJSI STRETCH FORMING MACHINE Filed May 10, 1944 4 Sheets-Sheet 1 wilu AGENT 0 17,. R946. F. c. HOFFMAN STRETCH FORMING MACHINE Filed May 10, 1944 4 Sheets-Sheet 2 IN VEN TOR. P32235105" LT. 1%FFMN I AGENT F. C. HUFFMAN STRETCH FORMING MACHINE Ema 4 Sheets-Sheet 3 Filed May 10, 1944 l N VEN TDR.

AGENT Dec. 17, 1946. F. c. HOFFMAN STRETCH FORMING MACHINE Filed May 10, 1944 4 Sheets-Sheet 4 INVEN TOR. 2 Fkzzzfimzr L. HUFFW AGENT Patented Dec. 17, 1946 STRETCH FORMING MACHINE Frederick C. Hoffman, North Hollywood, Calif., assignor to Lockheed, Aircraft Corporation,

Burbank, Calif.

Application May 10, 19

16 Claims. 1

This invention relates to devices for shaping or forming metal parts, and. has particular reference to machines for stretch forming preformed sections of aluminum alloys such as channels and extrusions and other elongate parts.

Various devices and methods have been pro:

posed for stretch forming elongate metal parts. Difficulty has been experienced in obtaining satisfactory uniform results with such machines owing to the fact that the parts spring back to a,

considerable extent following the forming operations. To compensate for this spring-back, it has been necessary to scale or proportion the forming surfaces of the machine in such a manner that the formed part will have the desired dimensions and contour subsequent to the springback. This scaling of the forming surfaces complicates the designing of the machine and materially increases its cost.

, It is an important object of the invention to provide a machine for stretch forming parts which eliminates or minimizes the spring-back of the parts. In the machine of th present invention, the spring-back is overcome or is reducedtothe extent that it is unnecessary to compensate for it in most cases in the design of the forming block. As a result, a substantial saving is effected in the designing and manufacturing of the machine.

Another object of the invention is to. provide a machine of the character referred to embodying means for effecting a predetermined progressive rate of increase of stretch of the Work part as the bending r forming occurs. This increased stretching puts the entire cross-sectional area of the part under a tension exceeding theelastic limit, during the forming, so that substantially all the residual elastic stresses are eliminated and the resultant spring-back avoided. The tensioning of the entire cross-sectional area of the part also eliminates the tendency for the inner side 44, Serial No. 534,977

' of the machine illustrated in Figure 1;

of the part to distort or buckle as a result of be readily adjusted to handle work parts having wide ranges of proportions, curvatures, etc.

Other objects and features of the invention will be readily understood from the following detailed description of typical preferredforms of the invention wherein reference will be made to the accompanying drawings in which:

Figure 1 is a plan view of a stretch forming machine of the invention;

Figure 2 is a diagrammatic or schematic view Figures 3 and 4 are enlarged transverse detailed sectional views taken as indicated by lines 3-:3 and 44 respectively on Figure 1;

Figure 5 is an enlarged fragmentary vertical section of a portion of the form block showing the pressure member in elevation;

Figure 6 is a fragmentary horizontal sectional view taken as indicated by line B6 on Figure 5;

Figure 7 is an enlarged fragmentary vertical detailed sectional view taken as indicated by line 1--'| on Figure 1; T

Figure 8 is an enlarged fragmentary vertica sectional view taken substantially as indicated by line 8-8 on Figure 1; v

Figure 9 is a fragmentary vertical detailed sectional viewtaken as indicated by line 9-9 on Fig,- ure 8; and

Figure 10 is a diagrammatic plan view of another form of the invention.

The embodiment of the invention illustrated in Figures 1 to 9 inclusivemay be said to com prise generally a supporting structure H), a rotatable form block H, means l2 for associating the workpiece W with the form blocli I I to be stretch formed upon rotation thereof, means l3 for rotating theform block H, and'means I 4 for imposing an initial tension onthe workpiece.

The frame or supporting structure It may vary considerably in machines of different design, proportion, etc and the invention is not to be considered as limited to the particular frame structure illustrated. Asfshown, the supporting structure ha's an elongate track or .bed comprised of a pair of horizontal beams l5 carried by longitudinally spacedpedestals or supports it. The beams [5 may be in the nature of I beams arranged with their intermediate webs in spaced vertical planes and their lower flanges may be suitably secured to the supports it. The upper flanges of the beams I5 present broad supporting or workingsurfaces. The form block H is arranged adjacent oneend of the elongate bed just described. In the particular machine illustrated, the form. block II is horizontally disposed and 3 is positioned at one side of the bed. The supporting frame includes one or more beam members ll projecting horizontally from the elongate bed adjacent one of its ends. The beam H is suitably secured to the adjacent beam l5 of the bed. and is supported by spaced legs IS. A supporting pedestal i9 is.- arranged at the opposite side of the elongate bedi'n alignmentwith the beam member II. The pedestal l9 may be constructed of pipe or tubular stock and has a lower horizontal arm fixed to a beam l5 of the bed. A second arm 2! is fixed to the top of the standard or pedestal I9 and extends toward the formblock H to project over the bed. The purposeof'the pedestal.

l9 will be later described.

The form block ll is the active work-shaping or forming element of the machine. Theblock ll may be horizontally arranged as illustrated, or may, if desired, be mounted to occupy a vertical or inclined plane. In accordance with the invention, the form block I I is supported for free ings 24 in the member [-1 receives the lower portion of the trunnionZZand the plate 23 is secured to the beam member around the opening. The form block has a hub 25 fixed to an elongate cup-like bearing member 26free1y rotatable on the trunnion 22. The end of the bearing member has a thrust'bearing Z'I'engaging with'the mounting plate 23; If desired, suitable anti-friction bearing means maybe'associated with the'trunnion 22 and bearing member 26. The body 28 of the, forn'r block H is a flat disc-like element or assembly reinforced at its under side-by a channel sectioned rim member 29. The form block ll may be large in diameter and it is desirable to support it against tipping or deflection. One or more rollers 30 may be provided on the beam member IT for engagement by the-under surface of therim member-29 to aid in -supporting the form block and to prevent such tipping.'

The body 28 of the form block ll has-a multiplicity'of circumferentially spaeedradial slots 3!. The slots 3'! extend inwardly from adjacent the periphery of the form block, and are preferably, equally spaced around the block: Adjustable strips or-blocks 32 carry spaced rods or bolts 33 and 34 which pass vertically through the slots 3|. As shown in Figures 4 and 5, the two bolts 33'and 34 of eachblock 32 extend through the respective slot 3| of the block. Tubular spacers or bushingsSE extend through the'vertical openings in the blocks 32yand washers 36, are provided under'the heads and nuts 31 of the bolts; It will be seen that when: the nuts-3l are loosened the blocks S'Zmay' be adjustedradially ontheform block H As shown'in' Figures 4 and'5, theouter edges of 'the'adjustableblocks' .32 are adjacent the rim or periphery of the form block; it being under-- stood'that the blocks may be. adjusted inwardly as required in the particular stretchforming operation.

The formblock H isfurther,providedwith segments 38 for receiving or acting on the work: piece W. Thesegmentstii arecarried on the above described blocks-32 and: are' capablejof limited radial adjustment with the block's; As'illustrated, each segment SBBis sup-ported ;by thr.ee adjacent blocks 32. Openings 39 in the segments 38 receive the outer bolts 34 and their bushings 35 whereby the segments are secured to the form block [I by the bolts 34. The outer edge surfaces of the segments 38 are arcuate or curved so that the series of segments presents a continuous surface which is curved concentrically with the axis of rotation, of the form block I I. The Opposite ends of -the: segments 38 may be radial so that they may have flush engagement one with the other. The number of segments 38 employed depends upon the length of the curvature to be formed in theworkpiece'W. In the case illustrated, the series. of segments I 38 extends well over 200 aroundi'the-formmlock II. It is to be understood'thatthesegments 38 are removable and are adapted to be'replaced by segments shaped to form.a-.lcurvature: of any selected radius in the workpiece.

The curved or arcuate active faces of the segments 38 are preferably prepared to receive the workpiece-W. The formation or configuration of thesegments 38 willdepend uponxthe character of the workpiece tobestretch formed. The particu+ lar workpiece W illustrated is substantially. z shaped in transverse cross section having a broad Web and edgeflangesextending inopposite directions therefrom. Slotsor grooves 46 are pro-- vided in the active edges of the segments 38 to. re-. ceive theworkpiece W. The grooves 4050f the severalsegments 38Lare aligned-and register to constitute acontinuous peripheral'groove extending throughout the. series. of. segments. grooves so are formed and proportioned sothat the web of the workpiece may lie againstone side wall, with the inner workpieceflange err-e gaged with the bottomwall of the groove. The. outer flange. of the workpiece cooperates with the arcuate outer faces of the segments 38 and the faces of the segments may bestepped'at M to receive the bead onthe outerworkpiece flange. The inner or bottomwall of the groove tllis curved concentrically With'th-e outer active..surfaces of the segments38; Figure5 clearly illustrates the mannerin which: the workpiece W en.- gages in the grooves '40 :and :conformsto. the outer,

surfaces. of the segments 38.

Work partsof the configuration illustrated; and

workpieces of other. cross sectional. configurations:

may tend to distort or assume irregular cross sectionalshapes as a result of the stretchforming operation; For example, in the particular case illustrated, the broad web oftthe. workpiece W may tend to bulge, buckle or wringlecwhen the workis' stretch formed into-the groove. Ml. In order to prevent this, I prefer to employ a's'pacer or filler 22. The filler 42. isadapted to'occupythat portion of the groove 48 which" is notiilled or occupied by the workpieceW.

rial having considerable flexibility. Multiplicie ties of spacednotches or slots 43=of substantial depth are formed in the inneredge of the filler The e In practice. the filler dzmay be a strip; of wood'or other mate'- with-the surface of the outer edge flange on the workpiece. This is clearly illustrated in Figure 5.

Itiwill be observed that the surface of the workpiece flange is free and exposed at the face of the series of segments 38.

The particular segments 38 illustrated are proportioned and arranged to impart a curvature of maximum radius to the stretch formed work. In the event that it is desired to give the work a curvature of small radius the form block II is adjusted or moved to have its axis of rotation closer to the elongate bed of the supporting means and suitable segments of the required contour are secured in the correct positions on the form block: The bIOkBnrlinBS in Figure 1 show the form" block ll arranged with its axis of rotation relatively close to the elongate bed of the supporting structure, and illustrates a series of segments on the :block presenting a forming surface of relatively small radius. The supporting bed is cut away as at 8 to allow such adjustment of the formblock l I. There may be a plurality of form block mounting stations comprising spaced openings24 in the beam member ii and the machine may be equipped with several sets or series of segments 38 of graduated radii whereby the machine is adapted for the-formation of any stretch formed curvature within its extreme ranges of operation. In this connection, it is to be understood that the segments t8 are radially adjustable to some extent on the form block H, to give the machine a substantial flexibility of adjustment.

i The means !2 for associating the workpiece W with the form block I l for stretch forming thereby includes a holding device orgrip 46 for releasably securing the workpiece to the block. This grip .45 includes a housing having upper and lower plates 4! which engage about the end portion' of the segment 38 that is foremost relative to the direction of active rotation of the form block II. The grip housing plates 47 may be secured to the form block l I by the endmost bolts 33 and 34 of the segment. A pair of blocks 48 having opposing convergent surfaces 49 is fixed in the grip housing. Gripping wedges 5B conform to and slide on the surfaces 49, and have opposing surfaces 5! shaped to conform to the workpiece W. Thesurfa'ces 5| may be serrated or=roughened to effectively grip the same. The

workpiece W is readily engagedbetween the wedges 5B, and when tension is applied to the work the wedges 56 are automatically actuated by the convergent surfaces 49 to grip the work. While I have shown automatically energized mechanical grip means, it is to be understood that other suitable forms of grips may be employed if desired. As shown in Figure 8 of the drawings, the grip 46 is located at the forward end of the groove it relative to the active direction of rotation of the form block ii, and arranged so that the work extending from it immediately enters the groove 40.

Theend of the workpiece W which is remote from the form block I l, prior to the stretch forming' operation, is operatively connected with a carriage or traveler 52 movable along the elongate horizontal bed of the supporting structure Ill. "The carriage 52 is capable of riding or moving along the upper flange of one of the beams l5 and comprises a horizontally disposed body equipped with axles 53 carrying freely rotatable wheels 55. The pairsofwheels 54 roll along the top of the beam l5. Lugs or ears 55 depend fro the opposite sides of the carriage 52 and carry rotatable wheels or rollers 56 which engage the under side of the upper beam flange to pre vent displacement of the carriage from the beam. A plate 51 is secured to the upper side of the carriage 52 and projects forwardly from the carriage. The forward end of the plate 51 carries a grip 58 for releasably connecting the workpiece W with the movable carriage 52. The grip 58 may be of the self-energizing type, and may be similar to the above described grip 46 having a pair of wedges 59 for gripping the workpiece when the assembly is put under tension.

The means for actively associating the workpiece W with the form block I I, further includes a flexible element secured at one end to the form block, to be wrapped thereon when the block is rotated, and secured at its other end to the carriage 52, which in turn is connected with the free end of the workpiece W. This flexible element may be a rope, wire, cable or chain. In the particular embodiment of the invention illustrated, the flexible element is in the nature of a chain 60. The chain 60 has one end anchored or fixed to the form lblock II at El adjacent or beyond the trailing end of the series of segments 38 relative to the direction of active rotation of the form block as indicated by the arrow A in Fig-ure l. The chain 60 is wrapped or trained about the form block I! to cooperate with theforward edges of its radially disposed plates or blocks 32.

The forward edges of the plates or blocks 32 are shaped to have effective cooperation with the chain 6!]. Chain locating notches 62 are provided in the edges of the blocks 32 and lugs 53 on the walls of the notches are operable to fit within the links of the chain 60; see Figures 4 and 5. The engagement of the multiplicity of the spaced blocks 32 with the chain 60 assures an adequate traction or drive relation between the form block H and the chain 68. It is important to note that I have shown the effective chainengaging radius of the blocks 32 as being shorter than the efiective radius of the Work-receiving groove 40 of the segments 38. In Figure 2 the line B designates the effective chain-engaging radius of the blocks 32 and the line C represents the efiective work-engaging radius of the segments 38. The differential in radii as indicated in Fig ure 2 results in the portion of the chain 66 leaving the form block H being made increasingly shorter in relation to the portion of the work W I .being drawn onto the form block and gradually increases the tension on the workpiece as the stretch forming operation progresses. This action and its purpose will be more fully described. It is to be understood that the difierential in the radii B and C may be variedto suit different operations, and in some instances the radii may be identical.

- .The chain 60 passes tangentially from the form block II at or adjacent the. elongate bed of the supporting structure IE! and extends along the top adjacent the opposite end of the supporting bed.

The chain 60 passes substantially arouncl end-"257m .7 the-.sprocket 81 toreturn over the top :of the supporting bed to the above describedtrcarriage 52. The chain til is suitably secured to the carriage at 63. The sprocket 8'! is? supported by the means M to rotate. aboutv a generall vertical axis as will be hereinafter described. It is to beiobserved that the chain 68, the workpiece W and the intervening carriage 52 constitute. acontinuous flexibleassembly havingits opposite ends securedto the rotatable form :block H.- Upon imparting axial movement to this loop-like assembly, the form block H is rotated in the direction-A to draw the work W onto it for forming thereby. The means It, to be later described,

imparts the longitudinalmovement to the chain and workpiece linkage .or assembly.

:In order to assure proper contact of the. workpiece W with the form block segments 38' and to assure correct forming of the work to the contour of. the segments, it is desirable to employ a pressure foot or pressure member l'il for acting on the work as it moves onto the segments. The pressure member is carried by 'a block H, which in turn is guided for horizontal movement toward and away from the form lblock H; see Figure 4. The shiftable .block'll is directed. for horizontal movement in .a direction radially with respect tothe form block H by a suitable guide '12 on the under side of the pedestal arm 2 l. The

.pressuremember iii is a generally cylindrical roller-like part engaged in a correspondingly shapednotch or recess T3 in the forwardend of the. block ii. The member "if? is supported. and retained for partial rotation in the'recess. Discs M are. secured to the block H at the ends of the recess 73. and carrya shaft 5 for retaining the pressure member 13. .The cylindrical surface of the pressure member 19 may directly bear on the wall of the recess 13 and the shaft l need not assume the heavy active loads. The pressure memlber it has a chordal surface '56 for contacting the workpiece W. The. surface 16 is capable of automatically conforming to theexposed flange of:the workpiece W even in situations where the flange is pitched or irregular. Rollers 9-may be rotatably mounted in the member 18 to be ex posed at the surface 1'5 to engage the work part. The rollers 9 will'reduce the friction between the pressure member '58 and the work part W.

Means is provided for maintaining the pressure member .78 in effective pressure engagement with the workpiece W as it moves onto the form block ll. This means comprises'a cylinder and piston mechanism 'l'i secured to the standard I9 under the arm iii. The piston rod 58 of the mechanism 17 is operatively connected with the block H. Fluid pressure lines, 19 connect with the cylinder of the mechanism Ti and the mechanism is under the control of a suitable manually operable valve 88. The mechanism 71 forces the pressure member '10 against the workpiece under a constant or uniform pressure during the stretch forming op- V eration.

The means l3maycompriseany-selectedor suit- 8 able 'form'ofprime' mover or power means. 'In the-particular form of the invention illustrated, this power-means comprise an electric motor 8| mounted on a suitablesupport 82 on one side of the elongate supportingbed. The motor shaft carriesordrives a worm 83. The-worm drives a sprocket carried by a'vertical shaft 85 mounted on the support 82. The protective housing 66 for the chain 60 is interrupted at the sprocket 84 and the sprocket has meshing or driving engagement with the chain. Spaced idler sprockets 86 are rotatablymounted on the supporting bed to cooperate"with the chain 60 at longitudinally spaced points to hold it in proper engagement with the driving sprocket84. It will be understood how the motor driven sprocket 84 operates "to advance the chain 88 in the direction D to rotate the form block H so that the work W is drawn onto the block. In this connection it will be noted that the differential in the radii .B s and C results -in the work W being drawn onto the form block H more rapidly than the chain 68 leaves theblock. This differential in movement results in a gradual increase in'the tension on the workpiece W, this action continuing throughout the stretch forming. operation. 1

The" means I4 is operable to impose an initial tension on the :work W preparatory to the actual stretch forming operation and. is preferably-operable. to maintain the initial tension or vary the tension as conditions may require. during the formingof'the work. The means I4 includes a bracket or slide 81 carrying the above referred to sprocket 6'1; see Figure '7. The vertical shaft -88 of the sprocket-81 has its opposite end portiOlls supported-by the slide 81 and the shaft projects beyond the lower'side of the slide. The .bracketor slide 81 is an open sided box-like structure containing the sprocket-61 and the chain=60 enters and leaves its opposite open sides. A plate .89 is fixed to the beams 15 of the supporting bed and the slide 81 is engaged on the upper surface of the plate for free sliding movement longitudinally of the bed. A longitudinal slot 90 inthe plate 89 receives the projecting lower portion of the sprocket shaft 88 to assist inguiding the slide 8'5- and to limit its axial movement.

The means 14 further includes screw thread means for moving or advancing the slide 81 to tension the workpiece'W. This means includes a threaded stud or screw 9| fixed to the slide 81 as by welding. The screw 9| projects from'the end of the slide 8! and passes freely through an opening 92 in=-a flange or block 93 fixed on the end of the supporting bed. .A nut 94 is threadedly engaged on the screw 91 to react orbear against the block 93. In practice a washer 95 maybe interposed betweenthe nut 94 and the block 93. Radially projecting handles or levers 96 are provided on the nut to facilitate its manual operation. It will be seen that when the nut .94 is rotated in one direction the slide 81 is drawn The 'tensioning means [4' also preferably 111- I eludes a fluid pressure actuated mechan'lsmt'l for imposing tension on theworkpiece"W,."where.

by the tensile strain may be conveniently varied at will during the stretch forming of the work. The cylinder and piston mechanism 9'! is suitably fixed to the plate 89, or to other structure on the supporting bed l9, and. its piston or ram 98 may act directly on the inner end of the slide 81. A pipe or line 99 carries the actuating fluid pressure to the inner end of the cylinder and piston mechanism 91. A manually operable valve me controls the line 99 and is preferably such that the pressure exerted on the ram 98 may be readily and precisely controlled. A pressure gauge llJi may be connected in the line 59 to indicate the pressure exerted on the ram 98 and thus disclose the tension being imposed on the workpiece W.

The invention preferably includes means for restoring or returning the form block ll, chain 50 and carriage 52 to their initial p sitions subsequent to the work forming operations. This means comprises a pullback line or cable IE2 attached to the form block H. Notches I03 in the edges of the blocks 32 receive and guide the cable as it is wrapped on the form block during stretch forming of the workpiece; see Figure 5. The cable Hi2 extends from the form block I! and its outer end is connected to the movable carriage 52. The cable is readily accessible at the top of the supporting bed to be grasped and pulled to restore the form block H, chain 69 and carriage 52. A prime function of the cable M2 is to serve as a safety means to prevent sudden rotation of the form block it in the event the work part W breaks while under tension.

To prepare for the stretch forming operation, the ends of the workpiece W are engaged in the grips 46 and 58 in the position to correctly enter the groove 40 of the form block segments 38. At this time the pressure member "I0 is in the retracted position. The filler 42 is associated with the workpiece and started in the groove 49. At this stage of the operation. the grip 46 on the form block I i is adjacent the pressure member 10. With the workpiece engaged in the grips 46 and 58, the pressure member 10 is actuated against the workpiece by supplying fluid pressure to the mechanism 11 and the nut 94 is rotated to impose the initial tension on the workpiece W. The automatic grips 46 and 58 are actuated by this initial tensioning of the work. The actuation of the nut 94 may apply the full initial tension to the workpiece or the mechanism 97 may be utilized to assist in imposing the initial tension. In either case, this initial tension while relatively low is suflicent to put the metal of the workpiece in the plastic condition for ready forming to th contour of the form block segments 38.

With the workpiece W conditioned as above described, the motor 8| is put into operation to advance the chain 69 and rotate the block II in the direction A. The tensioned work is drawn onto the rotating form block H to be shaped or formed to the contour of the groove 40, which in the instance illustrated, has a cylindrical curvature. It is to be understood that all fibers of the workpiece are in tension at the commencement of the forming operation. The combination of the bending and tensile forces effectively forms the work to the active shaping surfaces of the form block segments 38. Owing to the configuration of the workpiece ins reference to the axis of rotation of the form block H. The invention automatically provides for sufi'icient tensioning of the fibers of the inner zones of the workpiece without tensioning the outer fibers beyond the maximum limit, thereby eliminating wrinkling or other distortion of the inner sides of the workpiece, and more important, completely eliminating or reducing to a minimum, the spring-back that is often present in other stretch forming procedures.

In accordance with the invention, it may be preferred that the tension on the workpiece be accelerated or continuously increased asthe operation progresses; thus continuously increasing tension acting to maintain the fibers in the inner zones of the workpiece under tension without unduly tensioning the fibers in the o-uterzones. The continuously increased tension imposed on the workpiece throughout theforming operation assures the maintaining of the fibers in the inner zone of the workpiece under sufficient tension to eliminate or minimize springeback. The increasing elongation of the workpiece and the accelerated stretching of its inner fibers are produced by releasing the chain 69 from the smaller radius B of the form block while simultaneously drawing the workpiece onto the larger radius C. The chain releasing radius B, being shorter than the work forming radius C, causes the portion of the chain leaving the form block to be increasingly shorter in relation to the unformed portion of the workpiece W being drawn onto the block. Further, the portion of the tension'ed workpiece between the form block H and the outer grip 52 is increasingly shorter as the operation progresses and this'further results in increasing the tensile strain on the workpiece. These two-factors or actions assure the application of sufficient tension to the inner fibers of the workpiece throughout the entire stretch forming operation to maintain these fibers under suiiicient tension to eliminate spring-back, or to reduce spring-back to an absolute minimum. In practice, I have found that by employing a proper relation between radii B and C no, or only negligible, spring-back occurs in many cases, and as a result it is unnecessary to take spring-back into consideration when designing the form block II. This materially reduces the cost of designing and fabricating the block. The machine of the invention assures uniform results even when handling work parts that necessitate designing or proportioning the form block to compensate for spring-back. The mechanism 91 and/or the nut 94 may be employed to vary thetensile strain on the workpiece W at any phase of the operation. When the form block H has been rotated to draw substantially the entire workpiece W onto the segments 38, the motor 8i is deenergized and the formed workpiece is removed from the machine. The'cable H32 may then be employedto restore the parts to their original positions for the neXt forming operation. The machine when employed as just described, is operable to uniformly stretch form parts with no residual spring-back or with only negligible spring-back, and uniform results are consistently obtainable.

Figure 10 schematically illustrates another form of the invention in which the continued increase in the tension on the workpiece W during the forming operation is obtained by an automatic means supplementing or'replacin'g thedifferential radii B and C of the form block. The differential in the radii B and C of the work and chain receiving surfaces of the block I i may be maintained as Figures 1 to 9, maybe reduced, or may be eliminated sothat the radii are equal; The general construction of the machine may be the same as above described. and corresponding reference numerals are applied to corresponding parts. In this form of the invention, the screw 9! at the outer end of the slide, together with its actuating nut 94, may be retained and the cylinder and piston mechanism 91 may also be included in the machine. A screw I I is held against rotation by a sliding connection with the ram 98 and its opposite end is slidably associated with the'slide 81. A nut Ill threadedl engages the screw H9 and is engaged between spaced flanges l2 secured to the pulley supporting slide 81. Upon rotation of the nut III, the screw Hi2 and slide 81 are moved longitudinally'of the supporting bed. The ram 98 of the mechanism 9? may act against the inner end of the screw HE] so that the mechanism may advance the pulley slide 81 through the medium of the screw. The periphery of the nut l l l is in the form of a gear which meshes with a pinion 13.

Any suitable or selected means maybe employed to drive the pinion H3 at a given rate relative to the rate of advancement of the work W onto the form block ll. Ihave shown the pinion 1 l3 keyed or fixed toone'end' of a shaft H4. A beveled gear ll5'is fixed to the opposite end of the'shaft to mesh with 'amat'i'ng gear H6 formed on or rotating with the sprocket 84'. With atselected pitch of the threads on the screw Ill and a selected ratio in the gearing, the slide 8'! may be moved at a speed to produce the desired gradual'increase in the tension in the workpiece W. When the sprocket 84 is motor driven during thestretch forming operation, the nut l l i is rotated'through the medium ofthe gearing, and rotation of the nut produces translation of the slide 81 in an outward direction. This outward travel of the slide 87 increases the tension on the workpiece. The unformed portion of the workpiece between the form block II and the grip 58 becomes increasingly shorter as the operation progresses. This factor, coupled with the automatic outward movement of the tensioning sprocket 61 and the optional actuation of" the mites and mechanism 91 amplifies the tensile strain imposed on the workpiece and maintains theiinner fiber 0f the workpiece under tension throughout the entire forming operation to prevent spring-back. The cylinder and piston mechanism ii? may be employed to supplement the action of the gear and screw thread means or may be utilized independently of the ear and screw thread means to obtain a constant, variable or increasing tension on the workpiece during the stretch forming operation. The mechanism 9'! may-be readily operated or governed by the valve IE0 or other suitable valve mechanism.

During operation of both forms of the invention herein disclosed sufficient tension may be imposed onthe part being formed or bent to maintain its entire cross-sectional area under a tensile strain exceeding the elastic limit of the material to bring about the permanent stretch forming ofthe part and: the accelerated or increasing tension operating, as above described eliminates orv reduces to a minimumthe. undesir-- able spring-back;

Having described only typical formsof the in- Vention, I do not wish to be limited" to the specific details herein set forth, but wish. to reserve to myself any-variations ormodifications. that mayappear to those skilled 'in the art. or fal within the scopeof the following claims.

' I claim:

' 1; A stretch forming machine comprising a rotatable form block having a curved formingsurface and a second surface means closer to the axis of rotation of-the block than the forming surface, means for associating an elongated workpiece with the'form block to be drawn onto said forrning surface and formed thereto when the block is rotated, and means for rotating the form block and 'tensioning the workpiece as the block rotates, including "an element connected between theblock and a part of the workpiece which is remote from the block and trained over said second surface means to be unreeled therefrom as the block rotates.

2. A machine of "the character described comprising a rotatable form block having a forming surface and a second surface means closer to the axis of rotation of the block than the forming surface, means for engaging an elongated workpiece to be formed with the forming surface, the block being rotatable to form the part, and means for tensioning the part operable to increase the tension as the workpiece is formed by said sur face, the last named means comprising a flexible element wrapped on said second surface means and connected with the form block and a point on the workpiece remote from the block to be unwrapped from said second surface means as the block rotates.

3. A machine of the character described comprising a form block having a forming surface, means for engaging an elongated workpiece with the block, the block being rotatable to cause said surface to form the workpiece, and means for tcnsioning the workpiece and operable to increase the tension as the workpiece is formed by said surface upon rotation of the block, the last named means comprising a second surface on the block substantially coaxial with the forming surface and closer to the axis of rotation of the block than theforming surface, and a flexible element connected between the block and a portion of the workpiece remote from the block and wrapped on the second. surface to unwrap therefrom at a rate slower than the rate at which the Workpiece is formed on said surface.

4.. A machine of the character. described comprising a rotatabl form block having a curved forming surface and a second curved surface, said surfaces being substantially concentric with the axis of rotation of the block, the radius of curvatureof the second surface being shorter than the radius of curvature of the forming surface, means for connecting an elongated part to be formed with the-block to be drawn onto and formed by thev forming surface upon rotation of theiblock, and'means for tensioning the part and operable to. increase the tension thereonas the block ro-- tates to form the part, the last namedmeansineluding a flexible element connected with the portion of said part remote from the block, secured to the block and wrapped on the second surface of the block to be unwrapped therefrom as the part is drawn onto the forming surface.

5. A machine of the characterdescribed comprising arotatableform :block having a curved forming surface and a seco-ndcurved surface, said surface being substantially concentric with the axis of rotation of the block, the radius of cur-vature of the second surface being shorter than. the radius of curvature of the forming surfaceim'eans for connecting an elongated part to beformed withthe block to be drawn onto and formed by the forming surface upon rotation of the block, and means for tensioning the part and operable to increase the tension thereon as the block rotates to form' the part, the last named means including a flexible element connected with the portion of said part remote from the block, connected with the block and wrapped on the second surface of the block to be unwrapped therefrom as the part is drawn onto the forming surface; and means for acting on said element to impose an initial tension on the part.

i 6. IAmachine of the character described comprising a rotatable form block having a curved forming surfaceand a second curved surface, said surfaces being substantially concentric with the axis of rotation of the block, the radius of curvature of the second surface being shorter than the radius of curvature of the forming surface, means for connecting an elongated part to be formed with the block to be drawn onto and formed by the forming surface upon rotation of the block, and means for tensioning the part and operable to increase the tension thereon as the block romesa form the part, the last named means including a flexible element connected with the portion of said part remote from the block, connected with the block and wrapped on the second surface of the block to be unwrapped therefrom as the part is drawn onto the forming surface,

and means for acting through the medium of the element to impose an initial tension on the part and operable to vary the tension on the part as it is drawn onto the forming surface.

'7 A machine of the character described comprising a rotatable form block having a curved forming surface and a second curved surface, said surfaces being substantially concentric'with the axis of rotation of the block, the radius of curvature of the second surface being shorter than the radius of curvature of the forming surface, means for connecting an elongated part to be formed with the block to be drawn onto and formed by the forming surface upon rotation of the block, and means for tensioning the part and operable to increase the tension thereon as the block rotates to form the part, the last named means including a flexible element connected with the portion of said part remote from the block, connected with the block and wrapped on the second surface of the block to be unwrapped therefrom as the part is drawn onto the forming surface, and screw thread means for exertin an initial tension on the part.

8. A machine of the character described comprising a rotatable form block having a curved forming surface and a second curved surface, said surfaces being substantially concentric with the axis of rotation of the block, the radius of curvature of the second surface being shorter than the radius of curvature of the formin surface, means for connecting an elongated part to be formed with the block to be drawn onto and formed by the forming surface upon rotation of the block, and means for tensioning the part and operable to increase the tension thereon as the block rotates to form the part, the last named means including a flexible element connected with the portion of said part remote from the block, connected with the block and wrapped on the second surface of the block to be unwrapped therefrom as the part is drawn onto the forming surface, and screw thread means acting through the medium of said element for imposing an initial tension on the part.

9, Amachine of the character described comprising a rotatable form block having a curved forming surface substantially concentric with the axis of rotation, means for connecting an elongated part to be formed with the block to bedrawn onto and formed by said surface upon rotation of the block and means for rotating the block and for tensioning the partduring such rotation including a flexible element connected between the block and the portion of the part remote from the block, means for exerting a longitudinal pull on the element to rotate the block, and means acting on the element to tension the part comprising surface presenting means on the block substantially concentric with the axis of rotation of the block on which the element is wrapped and having an effective radius which is shorter than the effective radius of said forming surface, the element being unwrapped from said surface means as a result of said pull:

10. A machine of the character described comprising a rotatable form block having a forming surface curved generally about the axis of rotation of the block, means for connecting an elongated part to be formed with the block to be drawn onto and formed by saidsurface upon rotation of the block and means for rotating the block and for tensioning the part during such rotation including a second surface means on the block closer to the axis of rotation of the block than the forming surface and curved generally about said axis, a flexible element connected between the block and the portion of the part remote from the block and reeled on said second surfac means to unreeltherefrom upon rotation of the block in a direction to draw said part onto the forming surface, means acting on the element to rotate the block in said direction, and means acting on the element to tension the part comprising a fluid pressure actuated part movable to tension the element and part.

11. A machine of the character described comprising a rotatable form block having a curved forming surface curved generally about the axis of rotation of the block, means for connecting an elongated part to be formed with the block to be drawn onto and formed by said surface upon roi tation of the block and means for rotating the block and for tensioning the part during such rotation including a second surface means on the block closer to the axis of rotation of the block than the forming surface and curved generally about said axis, a flexible element connected between the block and the portion of the part remote from the block and reeled on said second surface means to unreel therefrom upon rotation of the block in a direction to draw said part onto the forming surface, means acting on the element to rotate the block in said direction, and means acting on the element to tension the part comprising a sprocket for driving the element and means for moving the sprocket.

12. A machine of the character described comprising a, rotatable form block having a curved forming surfacecurved generally about the axis of rotation of the block, means for connecting an elongated part to be formed with th block to be drawn onto and formed by said surface upon rotation of the block and means for rotating the block and for tensioning the part during such rotation including a second surface means on the block closer to the axis of rotation of the block than the forming surface and curved generally about said axis, a flexible element connected between the block and the portion of the part reaeiarsl mote fromthe block and reeled on; said second surface means to unreel therefrom upon rotation of the block in a direction to draw. said partonto theifor-ming surface,-means acting on theelement to'rotatel-the block in said direction,- andmeans driven by the last named means and actingon the element to tension the part.

13. ;A machine of the character described com.- prising' a rotatable form block having a curved forming surface curved generally about the axis of rotation of. the block, means for connecting an elongated. part to beformed with the block to be drawn onto and formed by said surface upon rotationofthe block and means for rotating the blockand for tensioning the part duringsuch r0- tationincluding a second surface means on the block closer to the axis of rotation of the block than the forming surface and curved generally about said axis, a flexible element connected between the block and the portion of the-part remote from the block and reeled on said second surfacemeans. to unreel therefrom upon rotation: of the-block in a direction to draw said part onto the forming surface, means acting on the element-torotate the block. in said direction, and

means acting. on the element to tension th part ment,. and screw thread means actuated by' the block: rotating means for moving the sprocket.

1.4;; A'machine of the character described comprising a rotatable form: block having a forming surface curvedv generally about the axis of rotation. of the block, means :for connecting an comprising a movable sprocket engaging the ele- 16 V and means acting on the element totension the workpiece comprising, surface presenting means on the block curved generally about. the-axis of rotation. of the block onwhich the element is wrapped and having an effective radius which is shorter than the effective radius of said forming surface, and means acting onthe element for varying the tension on the workpiece during rotation of the block.

V 15. A machin of the character described comprising a rotatable form block having a forming surface and a second surface, the surfaces being curved about the-axis of rotation of the block, a flexible element having'one end connected with the form block and being wrapped on said second surface, means for connecting the other end of the element with one end portion of an elongated workpiece, means for connecting the other end portion of the workpiece with the form block-so that the workpiece is drawn onto the forming surface upon rotation of the block, and means associated with the element for rotating the block to cause the workpiece to be formed by theblock.

16. A machine of .the character described com-. prising a rotatable form block having a forming surface and a second surface, the surfaces being curved about the axis of rotation of the block, a flexible element having one end connected with the form block and being wrapped on said second surface, means for connecting the other end of the element with one endportion of an elongated workpiece, means for connecting the other end portion of the workpiece with the form block so that the workpiece is drawn ontothe forming surface upon rotation of the block, means for ro tating the block to draw the workpiece onto the forming surface, and means associated with the element for tensioning the workpiece as it is formed.

FREDERICK C; HOFFMAN.