US 2924261 A
Description (OCR text may contain errors)
Feb. 9, 1960 z. s. FREDERICKS 2,924,261
STRETCH FORMING MACHINE WITH TENSION CONTROL Filed Dec. 30, 1957 4 Sheets-Sheet 1 INVENTQR.
' ATTOENEK 1960 z. s. FREDERICKS 2,924,261
STRETCH FORMING MACHINE WITH TENSION CONTROL I Filed Dec. 30, 1957 4 Sheets-Sheet 2 1 24 rm/MEX Feb. 9, 1960 Z. S. FREDERICKS STRETCH FORMING MACHINE WITH TENSION CONTROL Filed Dec; :50. 1957 4 Sheets-Sheet 3 ATTORNEY Feb. 9, 1960 z. s. FREDERICKS 2,924,261
STRETCH FORMING MACHINE WITH TENSION CONTROL Filed Dec. 30. 1957 4 Sheets-Sheet 4 WRAP 5 INVENTQR.
STRETCH FORMING MACHINE TENSION CONTROL Zygmuut S. Fredericks, Chesterland, Ohio, assignor to Tthglgyril Bath Company, Solon, Ohio, a corporation o Application December 30,1957, Serial No. 705,940
8 Claims. (Cl. 153-40) This invention relates to a stretch forming machine of the general type disclosed in US. Letters Patent No. 2,514,830, issued July 11, 1950, to'Cyril Bath, and particularly to a new and improved stretch forming-mac'hine'and control. combinationby virtue of whi'ch'the tension applied to the stock during the formation thereof can be controlled and varied as desired as the forming of the stoc'k'progresses.
Heretofore, in stretch forming metal stock, the-custoin'has been to anchoro'ne portion of a length of stock in fixed position relative'to the operating 'face-of-a side face die, to grip the stock at another portion by asuitable tensioning assemblage, usually a piston and cylinder assemblage, and to form the stock by m'oving-th'edie and assemblage in a direction to wrap the steak-progressively onto the side face of the die while-maintaining thestock under tension near to the elas'ticlimit of theistok. Herei pressure. However, constant pressure supplied the piston and-cylinder ass-emblagedoes not produce a constant tension nor a constant elongation'due to a l-'arge number of variables inherent in Lie assemblage itself, These variables, such as the changes in fr-iction and ibinding stresses as the assemblage extends and retracts and-shifts k v in alignment laterally from its-starting position a re complicated further by changes in the cross section and physicals of the metal. in fact, to obtain constant-tension on the stock, the exact reverse 'i's-true, variablepressure, not constant, must be provided in the assemblage.
One prior attempt to obtain prope'r control 'w'a's to circumvent the use, for control, of variable pressure and its inherent difficulties and, instead, to provide continuous'ly sufiicient pressure inthepistonand cylinder assemblage to stretch the stock'far be'yond its elastic limit, or even to its breaking point, and-tl1en tomontrol the movement of the gripping head of the stretch forming assemblage by a positioning device. This-type secontrol means operates on the principle thatfregardless of the higher pressure applied by the head to thestoek, the head is stopped in position as soon as thes took has been stretched a predetermined amount. The positioning device is related in operation to the forming of the stock by means of a tape which is wrapped about-a-template having a curvilinear face substantially the same incontour' as, and coaxial with, theface of thedi'e. "'Onee'n'd of the tape" is "secured for movement withthe template and the other is secured to' the'positioning device located on the part of the stretch' forminguasse'mblage to-which the stretching head is secured. The iposit ioning fdevice operates a servo-valve which controls the directionand rate of flow of pressurefluid to and from the sides of the piston to 'cause the-piston and cylinder assemblage to extend and retract in relation to the extension and retractionof the positioning device as operated by the tape. solution requires a -templatewhich -is pro.
' fastened in fixed-relation to'one-end-ofrthe temp1ate,.and.
2,924,261 Patented Feb. :9, .1960
cisely shaped relative to the die, so that for each unit length of tape which is wrapped onto the template, an equal unit length of stretched stock is wrapped onto the die face. Thus, regardless of the stretching force applied by the piston and cylinder assemblage and the resistance of the stock being formed, the stock is elongated a very definite and positive amount by linear dimensional control only. The relative movement of the stretch forming head follows a predetermined accurate" dimensional path with respect to the die regardless of the tonnage applied, of the cross sectional area of the stock, of the characteristics of the material, or of the personal characteristics of the machine operator. This positive position control-has certain advantages and limitations.
Another control is one in which the elongation of'the stock 'is controlled by controlling the tensioning force applied to the stock, the measurements of the tension applied being measured between the stock and the-stretch forming assemblage so as to eliminate the various frictional and binding'stresses and the like inherent in the assemblage itself. In this type of control, examples of which are described in copending applications of Henry W. Hein and Zygmunt S. Fredericks, Serial No. 568,553., now abandoned, filedFebruary 29, 1956, and of Richard L. .Curtner, SerialtNo. 667,376, now Patent No. 2,849,048 filed June 24, 71957, all ofthe extraneous binding stresses and .thelike inherent in the stretch forming assemblage and its position are combined with ;the;force 'due :toxthe pressure fluid and only the resultant (single force .and stress due .to these: forces and that caused by the applied pressure fluid pressure is measured and controlled. .Accordingly, the stress applied toithe stock canbe main? taihed .constant or'can be varied, depending upon the desires of the operator.
In the .Curtneriapplication, the itension applied .to the stockis controlled by varyingthe pressure in :the high pressure side ofjtherstretch forming :assernblage by variable venting of the pressure fluid in -.the high ;pressure side of the piston-and.cylinderassemblage. The variable venting is by means .ofa'relief alve discharging itoa sump, and under the control of a motor device which, in turn,;is-operated in responsetothechanges insaiload cellstrain produced on the load cell by'the single tre-v sultant force applied by the stockand assemblage, *the load cell being located between theassemblage and its stock grippinghead .forthispurpose.
iln accordance with the present invention, the .elonga tion and tensioningof the stookiscontrolled also by varying the. pressure in the high pressure side :of the stretch forming piston and cylinder assemblage, but, instead of this .-pressure being varied inresponse'toiaload cellxsubje'cted tozthe tension applied 'tothe stock itself, it *is varied in response to ya loaducell operated-hyaitapei which, in turn, is-wrapped-about atemplate concurrently with the wrapping of the stock about the :sideface vof the die.
In general, in case the .stretchforming machine is of the type disc'losedinzthe above identified Bath paten t,. a rotatable die is mounted on a turntable and-the template is mounted with its axis coincident with the axis of the side face of 'the die. One ndof a length oftape is the, jother end is fastened in I fixed relation *endwise of zthe stretch forming assemblage relative Ito the gripping ahead of the assemblage which grips the stock. Thedoad cell isinterpose'd betwee t-the 'latter-i endaof the:tapeand'the stretch forming assemblage, -so -"that it is strained-due to,
e r a g 2,924,261.
Thusthe present invention distinguishes from that of load cell is effected by means of tension applied by a tape and a template or other means movable in fixed relation to the side face forming die,r rather than by the tension on the stock itself. 1 i r The present invention has'an advantage in that, by changing the template,'the tension applied .to the stock at different points along the length of thestock are being progressively laid onto the side face die, may be varied so that the tension at any instantaneous pointjof tangency of the stock and side face die can be readily preselected. 1 Further, it has an advantage in that, when the stock is laid onto the die and it is found that the finished piece does not quite conform to that desired, thetension can be varied by modifying the template slightly until exactly the right stress is obtained'for the particular piece.
The present invention also has great advantageswhen a wipe forming shoe is used inconnection with the stretch forming, or when a suitable clamp such as described in the above Bath patent, is mounted on the table and holds the stock firmly into engagement with the side face of the stretch forming die, after a predetermined portion of the length of the stock has been formed. In such instances, it is apparent that by holding theformedportion of the stock firmly against the side face die at the instantaneous line of tangency, greater pressures can be applied at the unformed portion of the stock. Again, the tension can be increased or decreased where desired, as the stock is progressively laid onto the side face of the die, depending upon the cross section of the metal, degree of curvature, and the like. 1
Further objects and advantages will become apparent from the following description whereinreference is made to the drawings which illustate a preferred embodiment of my invention, and in which: I i
"and wipe form mg concurrently, and it has been the Curtnerapplication primarily in that the strain of the a Fig. l isa perspective view of a machine illustrating base 1 on which is mounted a rotatable turntable 2, a
stretch forming pistonand cylinder assemblage 3, and' a wipe forming piston and cylinder assemblage '4. A
carriage 5 is mounted on slideways 6 on the frame 1 for movement toward and away from the table 2 to different adjusted positions. is a support 7 which is rotatable about an upright axis Referring to the drawings, the machine comprises a- Mounted on the carriage 5,
desirable that a wipe forming piston and cylinder assemblage be arranged for reciprocation alongside the face of the stock being formed on the die, in addition to its being'operative to force awipe forming shoe toward the face of the die. For this purpose, the base or frame \1 has a lateral extension 20Twhich is located alongside the turntab1e 2,.and which .has guideways ,21--.-.on which a suitable saddle or carriage-22 is, mounted for reciprocation alongside the table.- "'Ihewipe forming piston and cylinder assemblage t4 isamounted on .the carriage i and has a cylinder 23 in ifixed'positionon the carriage 22. The assemblage has a piston 24 having 'a rod 25, which is connected to a pedestal26ofafslide 27. The
wipe forming shoe or'toot 28 is connected to the pee estal in a position so 'asto be pressed laterally against the stock as it is being wrapped along side, face die. If desired, the wipe forming cylinder may be controlled by separate power means and fluid pressure means. in-
:by the stretch forming assemblage.3, with which the present invention is particularly concerned, a load cell 32 is connected to the pedestal 13 atone end so as to be in fixed relation thereto. The other end of the load cell 32 is connected to a tape 33the opposite end of which is connected by a clamp 34 toa template 35. The templateis preferably arranged so as to be supported above the die D with its operating face as near 'as may be par.- allel to the operating face of the die D but offset therefrom towards the axis of the turntablexZ. Thus, as the assemblage 3 and die D are moved relative to each other so as to wrap the stock progressively about the dieface,
: the tape 33 is concurrently progressively wrapped about and which supportsa saddle 8 on which the assemblage it 3 is carried. The assemblage 3 comprises a cylinder 10 which is mounted in fixed position on the saddle 8 with its axis extending generally toward and away from the table in a plane normal to the table axis; A piston 11 having a piston rod 12 is mounted for reciprocation in the cylinder 10, and is fixedly connected at its outer end to a pedestal 13 of a slide 14 which, in turn, is mounted in guideways 15 of the saddle for movement parallel to the axis of the cylinder 10. Mounted in fixed position endwise of the assemblage 10 on the pedestalone end ofa length of stock S. The head 16 is mounted on the pedestal for rocking movement about an-upright desirable to subject the, stock to both stretch forming the side face of the template 35.
In the type of machine illustrated, this relative movement is effected by rotation of the turntable 2. Since the pressure maintained in=the piston and cylinder asi semblage 3 is sufficient tostretch the stock, it is, of
course, greater than that which would'be necessary. to
elongate the load cell 32' by the relativelyweak tape.
However, as the tension on the loadcell 32 increases,
the means responsive to the. load cell are operative to. reduce the tension-and, as the tension decreases beyond connectedbetween the. pressure side of the. pump39 and the directional valve 37. The directional valve is connected to opposite ends of the cylinder 10 by lines 41 and 42. a t t In one. position of the valve 37, the controlled high pressure fluid is supplied to the rod end of the cylinder 10 through the line 41 .and concurrently the low pressure fluid is. supplied to the head end through the line42. Thus, the pull exerted on the piston by the stoclcis yield-.
ably resisted by the high' pressure fluid, the unit pressure of which is controlled by the loadcell device 32, diminished to a' slight'extent .by the. pressure exerted on the headwend of the piston by the low pressurefluid, As the piston is extended, the excess volume of high pressure fluid is exhausted to sump from the rod end of the cylinder 10 through the-1ine 41 and the high pressure relief valve 38,. :As the piston is retracted the excess low pressure fluid exhausted to sumpfrom the headend and i 5 of the cylinder through the line 42 audible-Impressure relief valve 40. a a
If it is desired to extend the piston the directional valve is reversed so that the low pressure fluid is directed through line 41 to the rod end of the cylinder '10 and the high pressure fluid is directed through line 42 to the head end, thus extending the piston. The excess fluid in'the rod end ofthe cylinder 10 vents through the low pressure relief valve.
The differential between'high and low pressure is sutfieiently great at all times to effect this proper direction of movement selected.
The control of the pressure to the high pressure or rod end of the cylinder 10 is preferably effected by varying the setting of the high pressure relief valve 38. This valve is of the type described in the above identified copending application, and is driven by a reversible motor 43 to different settings to increase and decrease the pressure at which the valve vents to sump. The motor '43 is controlled, in turn, by a suitable amplifier and control 44, which may be such as described in the above application, tl1e signals therefor'being supplied throughsuitable electric lines 45 from strain gauges 46 arranged on the'load cell 32. The load cell 32 is preferably of the I-beam construction but relatively light so as to be readilystrained by the tension applied by-the relatively Weak-metal tape 33. Thus, when the tension of the tape increases unduly,
the motor 43 operates the valve 31% to increase its venting and thus reduce its pressure, and, as the pressure decreases unduly, it cl'oses down the venting ofthe valve 38 so as to increase the pressure. The pressure supplied by the pump 36 is always considerablyabove that required for the stretch forming operation. Accordingly, the tension applied to the stock is controlled by varying the pressure supplied to the rod end of the cylinder 3 in response-to the tension exerted by'the tape 33 on the load cell 32, and the resultant strain of the load cell.
The permanent stretching of the stock adds a substan tial amount to its final length, but the tension on the tape is insufiicient to cause a corresponding elongation thereof. Consequently, the len th or" the face of the template 35 is made less than the length of the face of the die, inasmuch as it is required that a tension be continuously maintained on the tape as the stock is laid on the'die, and to allow for the stretching and elongation of the stock-as it is laid on the die. 2 I
in order to understand the operation and how it distinguishes from a positional controL-reference is made to Fig. 5, wherein the relative lengths of the tape and stock and the distance of the stock driving-head and the load cell from their starting points, measured along the 'tem-' plate and die faces, respectively, we shown by diagrams in a number of operating positions.
The first position, indicated by diagram 50, shows the tape-33 as connected at its right end to the template 35 and the stock S as connected at its right end'to the die D. It is-to-be notedthat the length L of the tape 33 is greater than the length of the stock S, as the stock will be stretched prior to any rotation of the table 2. The amount of elongation which the stock must have to make the finished part can be determined readily in advance withone 'or two test pieces. This elongation is constant for subsequent lengths of stock for the same article.
Accordingly, the length of the stock measured between 7 its -point of connection with the 'die and thestretching head-is equal to L minus a constant, or L minus c, in the starting position. However, before rotation of thetable, when the stretching fluid pressure is introduced into the rod end of the cylinder :10 for 'jprestr'etching the stock,- the stock length is increased by an amount s, as indicated in diagram 51. When the amount of stretch s is sufficient so that the length of the stock and the tape are equalLthe stretching force is reduced by the control action of the tape, so as not to cause further stretching after the initial stretch takes the slack out of vth'extape and unwrapped portion of Lx. Since the length of the die face is greater than thelength of the template, in that the template face is offset toward the axis of the turntable I from'the die face'at all points along its length, the wrapping of an amount of tape equalto length x results in the wrapping of an amount of stock equal to a length x-I-Ax, wherein Ax is the incrementby which the stockS- is elongated over and above its initial stretch or elonga tion s.- This leaves, as the unwrapped remainder of -the stock, a length equalto Lx, as illustrated in diagram 52. As the wrapping continues and the additional length y of the tape 33 is wrapped on the template, leaving an un- Wrapped length of -Lxy, the length of the stock wrappedon the die is equal to -x-l-y-l-Ax-l-Ay, 'Ay being the additional stretch of the piece of stock leaving an unwrapped portion of the stock equal to L-x y, the same asfor the tape, as indicated in diagram '53.
Attheend of the wrapping operatiorn'as indicated in diagram '54, all of the tape of length L having been wrapped, as indicated successively by the lengths x-l-y-l-r, the stock also has been fully wrapped and the wrapped portion is equal to 'x-l nx+ v+Ay+z+Az, in which or and Ay and or arethe increments of-stretch imparted to the stock S in addition to the prestretch s originally im-' parted to the StQC-kyZS indicated at s in Fig. 1.
Sometimesa constant tension is maintained on the stock throughout the forming operation. However, 'in some instances the stock, at different locations along its length,: has difierent cross sections or different degrees-of bend or indentation both later-ally of the die face and endwise of the die face. In other instances, the stock is wider at one portion than at another. As aresult, it is desirable to vary the total stretching force to maintain the desired unit force for forming the particular portions.
Accordingly, the face of the template does not 'neces-' sarily have to remain at all times parallel to,'or offset the same distance inwardly from the face of the 'dieto ward the axis of the turntable or in the direction in which the'stock is moved to approach the die face. "Instead, its contour'may besuchas to'approachandrecede" from the face of the die different amounts at different portions along its length so "that the pressure can be varied in accordance with-the demands of stock.
.For example, it may be desired in Fig. 5 that the por-- tion x be stressed at a degree different fromtheportion'y] of the stock. If, for a unit length along the die "face, the length of the template corresponding to that'uni t' length is increased, more tape is wound-onto .the'template and this increases the strain on the load cell, causing a reduction in pressure fluid pressure.
apparent that, with this arrangemenhthetension applied" to the stock can range from the maximum of whichthe' hydraulic system is capable down to zero.
These variations in pressure are particularly desirable when the stock is concurrently wipe formed and stretch formed, as the wipe shoe itself affects'the tensionon the 7 stock. 1 r
If extreme cases so require, one or more holding piston and cylinder assemblages may be provided on -the table, as described in the above identified Bath-patent As illustrated in Fig. 3; a holding pistonand cylinder On the other hand, if, for a unit length of the die face, 'theface of the template is decreased correspondingly, then less tape" 81 a a that the length of the template face is less than'the length of the die face. i i 3. The structure according to claim 1 characterized in that the ratio of the length of the template face, measured fromthe" starting positionof the tape to the instantaneous linejof tangencyof the tape and template, and the length of the die face; measured from the starting position of to which different portions of the length of stock are subjected. .For example, beginning with the stock at the right-handend of the die being stretched under light pressure, it may be desirable beyond the point at which the clamp 58 .is positioned to greatly increase the pressure. In such a case, the clamp 58 is applied just beyond the instantaneous point of tangency in a direction toward the starting end of the die, and is held firmly clamped to the die during continued rotation of the table. During this continued rotation, the tension is greatly increased between the clamp 58 and the stretch forming head 16. In such an instance, the face of the template is farther removed from the face of the dieD at the location of the clamp 58 so that for a given unit length of the die, the length of the template is still further decreased than normal. This results in slackening the tape and therefore a demand for greater tension. Thetension is increased until the stock is stretched enough to remove the slack in the tape. This increase continues to the point desired. If it is.desired to reduce the tension,
the face of the template is shaped to approachrnore closely to the face of the die, thus increasing the tension;
on the tape and calling for a reduction in pressure on the stock which would thenbe subjected to lesstension. It is apparent that, with the present structure, a wide the stock to the instantaneous line of tangency of the stock and die, is different for different locations of the point Qf tangencyof the die and stock. p e
4.,The structure according to claim 1 characterized in thatthe distance between the faces ofthe die and template, in said directionof relativemovement, is different at ditferentjlocations endwise of the die face, whereby the stock is subjected to different preselected tension at dif erent-portions along its length as it is wrapped onto the die. a t
;5 The structure'according to claim .1 characterized in that said load :cell, is strainable for operation at tension lessthan thatrequired to impart a permanent stretch to the'stock.
t 6. The structure according toclaim 1 characterized in.
that the tensioning means is a piston and cylinder assema blage connected in a circuit to a source of pressure fluid, the settable meansisa variable relief valve and is arranged in said circuit between the source and the pressure side. ofthe cylinder so as tochangethe tension applied variety of effects in combined wipe and stretchforming can be obtained and all are within ent invention. j M Having thus ,described my invention, I, claimzw t 1. A stretch forming machine .for stretch forming a length of metal stock against a curved side facedie pro gressively from one, location to another on the face the scopeof thepres:
whilethe stock is held under tension endwise, including.
a curved side face die, means adapted to securethe stock at one portion in fixed position relative to the die, tool means. adapted to be connectedto; the stockfat; another portionfor transmitting'endwise tensioning force to the. stock, tensioning means connected to the tool for applying said tensioning force to the tool, settable'm'eans settable in different operating positions for controlling:
the tensioning means and thereby the tension, applied to the stock; means supporting the die and tensioning means forrelative movement in adirection transversely of the die for wrapping .the stock onto the diezface progressively endwise from one'location on the face .to. another, power means for eifecting said relativemovement, a load cell, having substantialresistance to. elongation by tension 1 applied thereto, connected to the: as-
semblage for endwise movement therewithin fixed endwise relation to the tool, reversible motor means connected to the settable means and operative, when" energized, for setting the settable means, strain sensing means carried by the load cell so that changes in the; strain'of the load all produce related changesin the conditioripf the sensing means, and said sensing means beingiopera-u tively connectedto the motor for etfectingqoperation of thegmotor in opposite directions as the strain on the: cell increases and decreases, respectively, a side face tern-z;
' by'the cylinder.
7. A stretch forming machine for stretch forming a length .of metal stock-against a curved side face die progressively from onelocation toanother on the face while the" stock. is held under tension endwise, including a curved side face die, means adapted to secure the stock at, one portionin fixed position relative to the die, tool means adapted to be connected to the stock at another portion for transmitting endwise tensioning force to the, stock, a fluid pressure operated piston and cylinder assemblage connected to the tool for applying said tensioning force to. the tool in relationqto the pressure ,of working pressure fluid supplied to the assemblage, a fluid circuit for supplying said pressure fluid from a source to the. assemblage, a variable relief valve means in the. circuit and settable in different operating positions for controlling the pressure of the operating fluid supplied to the assemblage, means supporting the die and assemblage for relative'movement in a direction transversely-of the die forwrapping the stock onto the die face progressively endwise from one location on the face to another, power.
meansffor effecting said relative movement, a load cell,
havinga substantial resistance to elongation by tension applied thereto, connected to. the assemblage for endwise. movementtherewith in fixed endwise relation. to the tool, reversiblemotor means connected to thevalve and operative; when energized, for setting the: valve,,strain gauge 'means mechanically connected to the load cell so that changes in the strain of the load produce related changes in the condition ofthe strain gauges, and oper- ,atively connected to the motor for effecting operation;
of the motor in oppositedirections. as the strain increases and decreases, respectively,- atension tape, capable of exerting sufiicienttension to elongate said load cell, connected at one portion to theload cell, means connected to the tape at another portion and movable relative to the load cell and in predetermined relation ,to the. die
for causing the tape toapply tension to the load cell V in a-predetermined relation to the tension applied to plate in-fixed position relative to the die, a tensionutape,
capable of'exerting suflicient tension to elongate said'load cell, connected atone portion in fixedrelation to the template and at another portion'in fixed relation to the, load: cell for wrapping of the tape onto the template face during.
said relative movement of the 'dieand assemblage,;so;as
applied by the tape.
- 2;. The structure according to. claim; 1 characterized in to strain the'load celland in proportion to the tension l the stoek by the assemblage.
8. A stretch forming machine for-stretch forming a length of metal'stock against a curved side face die apro-,
gressively from one location to another onthe face while the stock isiheldmnder tension endwise, and including a ourvedside: face. die; means adapted to secure the stock at one portion in fixed position relative to the die, tool" means adapted to be connected. to :the stock at another portion fortransmitting endwise tensioning force to the stock, tensioning means connected to the tool for applying said tensioning force to the tool, settable means for controlling the tensioning means and thereby the tension applied to the stock, means supporting the die and tensioning means for relative movement in a direction transversely of the die for wrapping the stock onto the die face progressively endwise from one location on the face to another, power means for effecting said relative movement, reversible motor means connected to the settable means and operative, when energized, for setting the settable means, tension sensing means, having substantial resistance to elongation by tension applied thereto, connected to the assemblage for endwise movement therewith in fixed endwise relation to the tool, and connected to the motor for effecting operation of the motor in opposite directions as the tension on the tension sensing means increases and decreases, respectively, clamping means movable with the die and operable for clamping the formed stock against the die in fixed position at preselected locations, respectively, so as to isolate the tension applied to the unforrned portion of the stock from the formed portion, a side face template in fixed position relative to the die, a tension tape, capable of exerting sufficient tension to elongate said load cell, connected at one portion in fixed relation to the template and at another portion in fixed relation to the tension sensing means for wrapping the tape onto the template face during said relative movement of the die and assemblage, the length of the template face being less than the length of the die face so as to strain the tension sensing means by, and in proportion to the tension applied by the tape during stretch forming, and the ratio of the length of the template face, measured from the starting position of the tape to the instantaneous line of tangency of the tape and template and the length of the die face measured from the starting position of the stock to its instantaneous line of tangency, being different for different locations of the point of tangency of the die and stock.
References Cited in the file of this patent UNITED STATES PATENTS Gray Feb. 25, 1958 Bath July 11, 1950