CA1075505A - Method and means for relieving stresses in die assemblies - Google Patents
Method and means for relieving stresses in die assembliesInfo
- Publication number
- CA1075505A CA1075505A CA283,199A CA283199A CA1075505A CA 1075505 A CA1075505 A CA 1075505A CA 283199 A CA283199 A CA 283199A CA 1075505 A CA1075505 A CA 1075505A
- Authority
- CA
- Canada
- Prior art keywords
- die
- die cavity
- improvement
- supporting means
- contact
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21J—FORGING; HAMMERING; PRESSING METAL; RIVETING; FORGE FURNACES
- B21J13/00—Details of machines for forging, pressing, or hammering
- B21J13/02—Dies or mountings therefor
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21K—MAKING FORGED OR PRESSED METAL PRODUCTS, e.g. HORSE-SHOES, RIVETS, BOLTS OR WHEELS
- B21K1/00—Making machine elements
- B21K1/28—Making machine elements wheels; discs
- B21K1/30—Making machine elements wheels; discs with gear-teeth
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B30—PRESSES
- B30B—PRESSES IN GENERAL
- B30B15/00—Details of, or accessories for, presses; Auxiliary measures in connection with pressing
- B30B15/02—Dies; Inserts therefor; Mounting thereof; Moulds
Landscapes
- Engineering & Computer Science (AREA)
- Mechanical Engineering (AREA)
- Forging (AREA)
Abstract
METHOD AND MEANS FOR REDUCING STRESSES IN DIE ASSEMBLIES
Abstract of the Disclosure A method and means for relieving unwanted tensile stresses in a die are disclosed. The method involves steps of determining level of stress in critical regions of the die and then supporting the die in a way to induce counter-stresses that substantially eliminate unwanted tensile stresses therein during a forming operation. A supporting means for reacting to the axial component of loads applied to the die cavity during the forming operation includes a supporting surface which is geometrically related to a contacting surface of the die to effectively reduce or eliminate tensile stresses during the forming operation.
Abstract of the Disclosure A method and means for relieving unwanted tensile stresses in a die are disclosed. The method involves steps of determining level of stress in critical regions of the die and then supporting the die in a way to induce counter-stresses that substantially eliminate unwanted tensile stresses therein during a forming operation. A supporting means for reacting to the axial component of loads applied to the die cavity during the forming operation includes a supporting surface which is geometrically related to a contacting surface of the die to effectively reduce or eliminate tensile stresses during the forming operation.
Description
1~7SS05 Brief Summary of the Invention ; The present invention relates to a method and structural designs which improve die life in certain types of dies used in metal forming operations. More specifically, the invention is concerned with providing greatly improved die life for die designs having relatively deep cavities and relatively sharp angular changes in their internal profiles. Dies of this type include deep cavity designs for forging or forming certain gear shapes from billets or sintered powder metal preforms.
Die assemblies of the type contemplated by this invention are typically mounted in a forming or forging press, and the press operates a reciproacting punch for closing the die and exerting an axial force on whatever material is contained within the die.
Dies used for such purposes are designed to withstand relatively high normal loads exerted on the die cavity wall by the punch via the mateeial being formed, and these loads occur during each forming cycle. Although the art of die design is relatively ;
- sphisicated and understood, it has been found that certain dies which are designed and manufactured in accordance with state-of-the-art knowledge do not live up to predicted performance in terms of the number of parts that can be manufactured in the die before a crack or fracture forms in a critical region of the die. Because of the nature of the specific cavity profiles of these dies, especially those which are shaped to include tooth forms therein for the manufacture of gears, it is not practical to merely split the die in the critical region as has been done with certain other die designs suffering from premature failure. Thus, it became apparent that a different approach to die design would be required for the deep cavity type dies contemplated herein.
In accordance with the present invention, it was discovered !
Die assemblies of the type contemplated by this invention are typically mounted in a forming or forging press, and the press operates a reciproacting punch for closing the die and exerting an axial force on whatever material is contained within the die.
Dies used for such purposes are designed to withstand relatively high normal loads exerted on the die cavity wall by the punch via the mateeial being formed, and these loads occur during each forming cycle. Although the art of die design is relatively ;
- sphisicated and understood, it has been found that certain dies which are designed and manufactured in accordance with state-of-the-art knowledge do not live up to predicted performance in terms of the number of parts that can be manufactured in the die before a crack or fracture forms in a critical region of the die. Because of the nature of the specific cavity profiles of these dies, especially those which are shaped to include tooth forms therein for the manufacture of gears, it is not practical to merely split the die in the critical region as has been done with certain other die designs suffering from premature failure. Thus, it became apparent that a different approach to die design would be required for the deep cavity type dies contemplated herein.
In accordance with the present invention, it was discovered !
-2-1075s05 .. ;~
that certain die cavity geometries can result in the development of very high axial tensile stresses in critical regions of the die during loading of the die cavity wall during a forming or forging operation. Since there has been substantial effort in the past to fully contain and reinforce die structures to minimize the effects of periodic loading of the die cavity, it was somewhat of a surprise to discover that relatively high tensile stresses could occur in any part of the structure during typical forming operations. Furthermore, it was found that the unwanted tensile stresses are extremely localized and in close proximity to compressive stresses within the same die body, and thus, they were easily overlooked and not noticed in prior art effoets to analyze die stresses. Once the existence and location of these relatively high tensile stresses was determined, it was necessary to solve the problem of reducing or eliminating such stresses with available material and equipment in present day usage. The present invention accomplishes this through an unusual approach of superimposing a counter-stress field upon the tensile stress region of the die during the forming operation so that the unwanted tensile stresses in the die are effectively mullified as they develop and grow during loading of the die cavity. Unexpectedly, it was found that a geometric relationship between a supporting structure and a contacting surface of the die itself could provide for substantial reduction or elimination of the unwanted tensile stress conditions in the die without changing the basic geometry of the die cavity and without adding additional reinforcement to the die assembly. Additionally, this reduction in unwanted tensile stresses can be accomplished without pre-stressing of the die (at least for the purpose of overcoming the unwanted axial tensile stresses in the die), and this means that the die can be fabricated and used in an un-stressed condition except for whatever pre-stressing is required for containing the radial component of loads on the die cavity.
As a result of this characteristic, the unwanted axial tensile stresses which develop and reach a peak during loading of the die cavity are matched on a time scale basis with a counter-stress field which serves to substantially reduce or eliminate the unwanted tensile stress condition.
In accordance with a specific method of the present invention, a first step is carried out for determining the stress field in a die during loading thereof in a forming operation. This step is followed by a next step of establishing a geometric relationship between the die and a supporting surface to control deflections in the die and to thereby induce a counter-stress field in the die which effectively reduces or eliminates the unwanted axial tensile stresses.
Such supporting of a die can be achieved with a supporting surface which is geometrically related to a contacting surface of the die to control deflections which are experienced by the die during a forming operation. In one sense, the supporting means is shaped to resist bending or deflection of the die in an area that normally experiences such bending during loading thereof, and in another sense, the supporting means is designed to induce bending or deflection of another portion of the die to thereby create a counter-stress field in the region of tensile stressing of the die.
In a specific embodiment of the invention, the supporting means is shaped to provide initial contact (and therefore a resistance to bending) in a central area of the die and in axial alignment with the forming load applied to the die cavity. This ~075505 ~
area of initial contact corresponds generally to the central portion of the die which is otherwise deflected during loading of the die cavity, and thus, the normal tendency to deflect is ; resisted by the geometric relationship which is established between the die body and the supporting surface. The supporting surface may be further shaped to provide for progressive or later contac~ between an area toward the periphery of the die and the supporting surface to thereby induce a deflection in the periphery - of the die as the die is loaded during a forming operation.
In an alternative embodiment, the contacting surface of the die itself can be shaped to make similar initial contact at the central area of the die with later contact toward the periphery of the die, and this alternative also effectively reduces or -~
eliminates high tensile stresses during loading of the die.
In either case the thrust of the invention is one of con-trolling deflections of a die in such a way that unwanted stresses are reduced or nullified as the die cavity is loaded during a forming operation. One of the remarkable characteristics of this approach is that the bottom portion of a die can be substantially thinner than is conventionally calculated for a given cavity geometry, provided it is supported in a way to induce counter-stressing in certain critical regions. Also, the invention is remarkable in its simplicity and reliability. No elaborate constructional features or precise manufacturing processes are required and the counter-stresses which are produced are developed in proportion to and counter to the tensile stresses.
These and other features and advantages of the present invention will become apparent in the detailed description which follows. In that description reference will be made to the accompanying drawings as briefly described below.
107550S ~ -Brief Description of Drawings Fig. 1 illustrates general relationships between a die and its conventional supporting structures as seen in an elevational ~ -view taken through a section which passes through the center of the die; -Fig. 2 is a graph which displays information regarding axial compressive and tensile stresses at various points along a die cavity profile, as would be experienced during a forming operation which applies a load to a billet or other part contained within ~ -the die which is supported as depicted in Fig. l; -Fig. 3 is a graph similar to that of Fig. 2 for depicting relative stresses in a die of slightly different design than that shown in Fig. 2;
Fig. 4 is a graph similar to that of Figs. 2 and 3 for depicting stresses in a die designed for a spur gear having a hub; and Fig. 5 is a sectional view in elevation of a die assembly incorporating the design features of the present invention in a die designed to form bevel side gears of a type used in present day automobile differentials.
Detailed Description of Invention Fig. 1 illustrates basic relationships in a typical die assembly and its associated supporting structures. As illustrated, a die 10 is contained within a sleeve 12 and supported on a known supporting means 14. The supporting means 14 is illustrated in Fig. 1 as a solid disk-shaped member, but in assemblies which include a core rod or an e~ector member passing through the bottom of the die, a bore or opening mus~ be provided through the central axis of the supporting means 14, and in such cases the supporting means 14 takes the form of a ring member. In the illustrated , ~ . , . .. , ~ ' ~. I, .. ' ' ! '.' ',' ' ' ' , . ' ' . ' . . ' '.', ':.'. " ' . .' " ' ' ' , '. ' . . ' ' ,',' '" ': ' \~
iO75505 assembly, a load is applied to a billet 18 contained within the ; die 10 by a punch 16 moving in the direction of the arrow indi-cated thereon. The die cavity, into which the billet 18 is placed for forming, includes a bottom portion 20, a substantially frusto-conical portion 22, and a substantially cylindrical portion 24 adjacent to the frusto-conical portion 22. During loading of the die assembly, it has been considered by many that the die 10 is under compressive stresses in the radial and axial direc-tions throughout the areas which define the die cavity and against which the billet or part is pressed during a forming operation.
This theory was understandable because the billet 18 moves radially outwardly as the punch 16 advances downwardly into the die cavity to form a part, and the very high forces which are created by this action press downwardly and radially outwardly in all direc-tions to apply compressive loads to all surfaces of the die cavity in contact with the part. However, actual experience with deep cavity die designs of the type generally depicted in Fig. 1 has shown that there is a tendency for die failure through the formation of cracks in the positions indicated at 26 in Fig. 1. These cracks typically developed in the region of the die cavity where the frusto-conical portion 22 of the cavity joins the cylindrical portion 24. Although this type of failure was not understood completely at first, it became very apparent that there were economic limits to choices of material or methods of reinforcing the walls of the die to attempt to overcome this defect.
Unlike many prior approaches to a solution to this problem, I did not assume that the die cavity of the die 10 was everywhere compressively stressed during a forming operation, and I raised the additional question of the possibility of tensile axial stresses existing in certain portions of deep cavity dies of the :.. .:: .. .. ~ . . , ~ 1075505 ;. ..
specific type shown. In order to test these questions, three-dimensional finite element analyses were carried out for several different die cavity geometries, and compressive and tensile axial stresses were plotted for points along the die cavity under simulated load conditions. Figs. 2-4 illustrate the results of three such analyses as carried out for three somewhat different gear geometries. In these illustrations, only portions of the die cavities and associated punches are shown, and stress determinations are plotted along a zero axis. The zero axis is a linear representation of the die cavity profile. The stress determinations were based upon forming loads developed when a sintered powder metal compact is formed into a fully dense high strength part.
As shown in Fig. 2 the die cavity geometry being analyzed included a flat bottom portion 20, a relatively steep frusto-conical portion 22, and a cylindrical por~ion 24 at right angles to the bottom portion 20 (and at approximately 140 to the portion 22), as might be used for forming a bevel gear. Axial stresses under simulated load conditions were determined for the die design shown in Fig. 2, and these stresses were plotted along the vertical zero axis of the Fig. 2 graph to indicate the degree of relative stress and whether the stress was compressive (negative) or tensile (positive) in nature. Although the stresses ~ !
were plotted for the entire die assembly, the critical points for our consideration are between point 30 (representing the bottom of the die cavity) and point 32 (representing the top of the cylindrical portion 24 of the die cavity). Looking at the plotted graph between these two points it can be seen that the lower parts of the die cavity are under cons$derable compressive stress but that the stress field becomes tensile in the - . . . : . ~: : . : , . : ;.. ,., - . . :: I
` 1075505 approximate region where the frusto-c¢nical portion 22 of thP
die cavity joins the cylindrical portion 24 of the die cavity (point 34 on the graph). The stress gradient in this region is quite severe, but the lev~l of tensile str~ss obtained is not so high as to render the design impractical.
Fig. 3 graphically portrays axial stress conditions in a somewhat different die design from that shown in Fig. 2, and it can be seen that a very sharp tensile stress "spike" exists near the point 34 on the ver$ical axis of this graph. Again, this point corresponds to the region in the die cavity where the frusto-conical portion 22 joints the cylindrical portion 24.
Regions immediately above and below this critical region are under fairly high compressive stresses, and therefore, this seems to explain the tendency for cracking that has been experi-enced with die designs of the specific type shown in Fig. 3.
Basically the difference between th~ die cavity shape shown in Fig. 3 as compared to that of Fig. 2 is one of a lesser included angle between the frusto-conical portion 22 and the cylindrical portion 24. Thus, it would appear that as the frusto-conical por-tion 22 becom s less obtuse in its relationship to the cylindricalportion 24 (as is the design shown in Fig. 3, for example), there is a higher degree of tensile stressing in the critical region where these two portions join one another. However, there are apparent exceptions to this general relationship as shown for example in the spur gear die design of Fig. 4. In that design the portion 22 of the die cavity, which corresponds to the frusto-conical portions of bevel gear die designs, is ev~n less obtuse in its relationship to the cylindrical portion 24, and therefore an even greater tensilP stress condition should b~ predicted ; 30 for that critical region. That was not the case though in the ` ` ~075505 particular dssign analyzGd, and it is th~eorized that othar factors (such as the exis~ence of a hub and a relativ~ly narrow "shelf" for the portion 22) accounted for a different stressing of the die. However, it is believed that othsr spur g~ar die designs (and other two-level part designs) would exhibit tensile stressing in the region 34 if the othsr factors were changed.
What is really indicated by the data of Fig. 4 is that th2 str~ss analysis of die design requires consideration of many factors, some of which have a cancelling effect on others, but the basic principles of this invention can be applied to many types of die designs which experience the type of tensile stress failure dis-cussed ahove.
Once the type of information shown in Figs. 2 and 3 was developed, the next problem was one of creating a new die design that would overcome the tensile stress condition without upsetting other stress relationships in the die cavity. My solution to this problem was one of doing just the opposite of what might be considered a sound and traditional engineering practice.
Instead of attempting to furthsr reinforce the die in the ragion of stress rP~arsal, I have provided for a control of deflections of the die in certain areas to induce counter-stressing of the critical region of the die as it is loaded during a forming operation. The counter-stress is calculated and set to effec-tively reducs or eliminate the high axial tensil stress "spiken which otherwise develops during loading of th~ die.
Fig. 5 illustrates a typical embodiment of this invention as applied to a die assembly designed to manufacture bevel side gears from sintered powder metal pr~forms. In such manufac ure, a powder ~etal preform (having a bor~ thPrethrough) is plac~d in the die, in the same manner as with forging processes which 1~075505 utilize solid matal billats, and a punch member 16 is advanced into the dia to rashape and fully densify the powder metal pra-form. Fig. 5 illustrates one-half of such a die assembly in a sectional view showing relationships of parts at the end of a forming operation with the preform 18 fully densified and formed into the shape of a bevel side gear. This particular arrangement also includas a core rod 40 and ejector 42 passing through the supporting means 14 and the bottom of the die 10. Thus, a bore must be provided through the supporting means 14 and through the bottom of the die 10 to accommodate the core rod and ejector assemhlies operating theroin. The double-headed arrow which is depicted on the die 10 in the Fig. 5 view is intended to represent the critical region in which tsnsile strssses normally tend to cause a premature cracking or failure of the di2. The double-tailed arrow represents the cancelling stresses created by the specific structural arrangements shown in the Fig. 5 embodiment.
As previously indicated, I have redesigned the die 10 and its supporting means 14 to permit a control of deflections of the die during a forming operation, and it is this control that creates counter-stresses for effectively reducing or eliminating the high tensile stresses that otherwise occur in the critical region of the die between the frusto-conical portion 22 and the cylindrical portion 24. This is accomplished by establishing a certain geometric relaticnship between a bottom surface 44 of the die 10 and a top surface 46 of a supporting structura which it contacts. As shown in Fig. 5, the top surface 46 of the supporting ring 14 has baen shaped to provide a slight taper from its central area of greatest thickness to a peripheral area of lessar thickness. In other words, the supporting means 14, ~075~05 whether in th2 form of a solid disk or a ring, can be crowned so that an unloaded die 10 will mQke initial contact only with its central area of maximum thickness. In this r2gard, the relationship b~tween the bottom 44 surfacP of die 10 and thP
top surface 46 of the supporting ring 14 is depicted as it would appear in an unloaded condition (even though the punch 16 is shown in a final forming position in the Fig. 5 view). Thus, as the die cavity wall is loaded by the punch 16 via the material being form~d during a forming operation, the bottom surface 44 of the die 10 makes initial contact with the central area of the top surface 46 of the supporting means 14. Contact continues until the bottom of thP die is in substantially increased contact with th2 supporting means. This gsometric relationship results in a control of deflection of the die during loading. In one sense, the relationship is such ~hat "bending" of the die is resisted (as indicated by the series of arrows depicted in sup-porting m2ans 14) in the central area which is axially aligned with the bottom 20 of the die cavity. In another sense, the die is encouraged to deflect in its outer peripheral areas as it is loaded. The nat effect is one of creating a counter-stress field in the critical region where the wall portion 24 joins the frusto-conical portion 22. In order to assure control of die deflections, it is necessary to establish a maximum thickness of the lower portion of the die ~hat will permit controlled deflection. In this r~gard, the thickness between the bottom portion 20 of the di2 cavi~y and the bottom surface 44 of the die is relatively thinn~r than would be the design for state-of-ths-art diPs of this typs. Thus, I have required a struc.ure which might be considered a weakening, rather than a reinforcing, of the die 10 in order to produce the desired control of deflec-_12 ~075505 tions which effectively cancel unwanted tensile stresses ina critical region of the die. Fig. 5 also illustratas the relative geometric r~lationships between sleeve 12 (which contains the die 10) and th~ supporting means 14 as well as adjacent supporting structuras 50 so as to provide for limited movement of the sleeve 12 with the die 10 rsl2tive to the supporting structures 50 as the die 10 is loaded during a forming operation.
Of course, the geomstric relationship which has been dis-cussed above with reference to the bottom surface 44 of die 10 and the top surface 46 of the supporting means 14 can be provided in other ways. For example, the bottom surface 44 of the die 10 (and coextensive bottom portions of an associated sleeve 12) can be shaped to provide the sam progressive contact between thesa surfaces and a flat-topped supporting means 14.
Another specific structure (which may be preferred for practical production of geometric r~lationship that permit a degree of variance from theoretical designs) provides for a steppad con-figuration of either the surface 46 or the bottom 44 of the die so that contact is provided by a ~step" of calculated width in the central araa of the die and with no actual peripheral contact provided at all. Such a structure may be formed by initially shaping the surface 46 as shown in Fig. 5, followed by a relieving of a peripheral area of the surface radially beyond a central area where maximum contact with the die is prefPrred. In any case, the objective is one of providing for contact (between the dia and its supporting structures) which is biased toward the central area of ths die so as to permit a control of deflec-tions of the die as the die cavity is loaded during a forming operation. This control tends to "bend" the die compr_ssively in the ragion in which t2nsila str~ss~s are developed, and thus, _13-'1075505 the t~nsila stresses are effectively reduced or eliminated.
Having described structural fea~ures of a specific embodiment of this invention, it can be appreciated that fully equivalent -structural designs and relationships can be thought of and used by those skilled in this art without substantially departing from the basic teachings of the present invention. For example, the principles of this invention can be applied to die cavity configurations which provide for less than a 90 angular rela-tionship between a conical portion 22 and a cylindrical portion 24, All such equivalant subst.itutions are int~nded to be included wlthin the scope of this invention as claimed below.
that certain die cavity geometries can result in the development of very high axial tensile stresses in critical regions of the die during loading of the die cavity wall during a forming or forging operation. Since there has been substantial effort in the past to fully contain and reinforce die structures to minimize the effects of periodic loading of the die cavity, it was somewhat of a surprise to discover that relatively high tensile stresses could occur in any part of the structure during typical forming operations. Furthermore, it was found that the unwanted tensile stresses are extremely localized and in close proximity to compressive stresses within the same die body, and thus, they were easily overlooked and not noticed in prior art effoets to analyze die stresses. Once the existence and location of these relatively high tensile stresses was determined, it was necessary to solve the problem of reducing or eliminating such stresses with available material and equipment in present day usage. The present invention accomplishes this through an unusual approach of superimposing a counter-stress field upon the tensile stress region of the die during the forming operation so that the unwanted tensile stresses in the die are effectively mullified as they develop and grow during loading of the die cavity. Unexpectedly, it was found that a geometric relationship between a supporting structure and a contacting surface of the die itself could provide for substantial reduction or elimination of the unwanted tensile stress conditions in the die without changing the basic geometry of the die cavity and without adding additional reinforcement to the die assembly. Additionally, this reduction in unwanted tensile stresses can be accomplished without pre-stressing of the die (at least for the purpose of overcoming the unwanted axial tensile stresses in the die), and this means that the die can be fabricated and used in an un-stressed condition except for whatever pre-stressing is required for containing the radial component of loads on the die cavity.
As a result of this characteristic, the unwanted axial tensile stresses which develop and reach a peak during loading of the die cavity are matched on a time scale basis with a counter-stress field which serves to substantially reduce or eliminate the unwanted tensile stress condition.
In accordance with a specific method of the present invention, a first step is carried out for determining the stress field in a die during loading thereof in a forming operation. This step is followed by a next step of establishing a geometric relationship between the die and a supporting surface to control deflections in the die and to thereby induce a counter-stress field in the die which effectively reduces or eliminates the unwanted axial tensile stresses.
Such supporting of a die can be achieved with a supporting surface which is geometrically related to a contacting surface of the die to control deflections which are experienced by the die during a forming operation. In one sense, the supporting means is shaped to resist bending or deflection of the die in an area that normally experiences such bending during loading thereof, and in another sense, the supporting means is designed to induce bending or deflection of another portion of the die to thereby create a counter-stress field in the region of tensile stressing of the die.
In a specific embodiment of the invention, the supporting means is shaped to provide initial contact (and therefore a resistance to bending) in a central area of the die and in axial alignment with the forming load applied to the die cavity. This ~075505 ~
area of initial contact corresponds generally to the central portion of the die which is otherwise deflected during loading of the die cavity, and thus, the normal tendency to deflect is ; resisted by the geometric relationship which is established between the die body and the supporting surface. The supporting surface may be further shaped to provide for progressive or later contac~ between an area toward the periphery of the die and the supporting surface to thereby induce a deflection in the periphery - of the die as the die is loaded during a forming operation.
In an alternative embodiment, the contacting surface of the die itself can be shaped to make similar initial contact at the central area of the die with later contact toward the periphery of the die, and this alternative also effectively reduces or -~
eliminates high tensile stresses during loading of the die.
In either case the thrust of the invention is one of con-trolling deflections of a die in such a way that unwanted stresses are reduced or nullified as the die cavity is loaded during a forming operation. One of the remarkable characteristics of this approach is that the bottom portion of a die can be substantially thinner than is conventionally calculated for a given cavity geometry, provided it is supported in a way to induce counter-stressing in certain critical regions. Also, the invention is remarkable in its simplicity and reliability. No elaborate constructional features or precise manufacturing processes are required and the counter-stresses which are produced are developed in proportion to and counter to the tensile stresses.
These and other features and advantages of the present invention will become apparent in the detailed description which follows. In that description reference will be made to the accompanying drawings as briefly described below.
107550S ~ -Brief Description of Drawings Fig. 1 illustrates general relationships between a die and its conventional supporting structures as seen in an elevational ~ -view taken through a section which passes through the center of the die; -Fig. 2 is a graph which displays information regarding axial compressive and tensile stresses at various points along a die cavity profile, as would be experienced during a forming operation which applies a load to a billet or other part contained within ~ -the die which is supported as depicted in Fig. l; -Fig. 3 is a graph similar to that of Fig. 2 for depicting relative stresses in a die of slightly different design than that shown in Fig. 2;
Fig. 4 is a graph similar to that of Figs. 2 and 3 for depicting stresses in a die designed for a spur gear having a hub; and Fig. 5 is a sectional view in elevation of a die assembly incorporating the design features of the present invention in a die designed to form bevel side gears of a type used in present day automobile differentials.
Detailed Description of Invention Fig. 1 illustrates basic relationships in a typical die assembly and its associated supporting structures. As illustrated, a die 10 is contained within a sleeve 12 and supported on a known supporting means 14. The supporting means 14 is illustrated in Fig. 1 as a solid disk-shaped member, but in assemblies which include a core rod or an e~ector member passing through the bottom of the die, a bore or opening mus~ be provided through the central axis of the supporting means 14, and in such cases the supporting means 14 takes the form of a ring member. In the illustrated , ~ . , . .. , ~ ' ~. I, .. ' ' ! '.' ',' ' ' ' , . ' ' . ' . . ' '.', ':.'. " ' . .' " ' ' ' , '. ' . . ' ' ,',' '" ': ' \~
iO75505 assembly, a load is applied to a billet 18 contained within the ; die 10 by a punch 16 moving in the direction of the arrow indi-cated thereon. The die cavity, into which the billet 18 is placed for forming, includes a bottom portion 20, a substantially frusto-conical portion 22, and a substantially cylindrical portion 24 adjacent to the frusto-conical portion 22. During loading of the die assembly, it has been considered by many that the die 10 is under compressive stresses in the radial and axial direc-tions throughout the areas which define the die cavity and against which the billet or part is pressed during a forming operation.
This theory was understandable because the billet 18 moves radially outwardly as the punch 16 advances downwardly into the die cavity to form a part, and the very high forces which are created by this action press downwardly and radially outwardly in all direc-tions to apply compressive loads to all surfaces of the die cavity in contact with the part. However, actual experience with deep cavity die designs of the type generally depicted in Fig. 1 has shown that there is a tendency for die failure through the formation of cracks in the positions indicated at 26 in Fig. 1. These cracks typically developed in the region of the die cavity where the frusto-conical portion 22 of the cavity joins the cylindrical portion 24. Although this type of failure was not understood completely at first, it became very apparent that there were economic limits to choices of material or methods of reinforcing the walls of the die to attempt to overcome this defect.
Unlike many prior approaches to a solution to this problem, I did not assume that the die cavity of the die 10 was everywhere compressively stressed during a forming operation, and I raised the additional question of the possibility of tensile axial stresses existing in certain portions of deep cavity dies of the :.. .:: .. .. ~ . . , ~ 1075505 ;. ..
specific type shown. In order to test these questions, three-dimensional finite element analyses were carried out for several different die cavity geometries, and compressive and tensile axial stresses were plotted for points along the die cavity under simulated load conditions. Figs. 2-4 illustrate the results of three such analyses as carried out for three somewhat different gear geometries. In these illustrations, only portions of the die cavities and associated punches are shown, and stress determinations are plotted along a zero axis. The zero axis is a linear representation of the die cavity profile. The stress determinations were based upon forming loads developed when a sintered powder metal compact is formed into a fully dense high strength part.
As shown in Fig. 2 the die cavity geometry being analyzed included a flat bottom portion 20, a relatively steep frusto-conical portion 22, and a cylindrical por~ion 24 at right angles to the bottom portion 20 (and at approximately 140 to the portion 22), as might be used for forming a bevel gear. Axial stresses under simulated load conditions were determined for the die design shown in Fig. 2, and these stresses were plotted along the vertical zero axis of the Fig. 2 graph to indicate the degree of relative stress and whether the stress was compressive (negative) or tensile (positive) in nature. Although the stresses ~ !
were plotted for the entire die assembly, the critical points for our consideration are between point 30 (representing the bottom of the die cavity) and point 32 (representing the top of the cylindrical portion 24 of the die cavity). Looking at the plotted graph between these two points it can be seen that the lower parts of the die cavity are under cons$derable compressive stress but that the stress field becomes tensile in the - . . . : . ~: : . : , . : ;.. ,., - . . :: I
` 1075505 approximate region where the frusto-c¢nical portion 22 of thP
die cavity joins the cylindrical portion 24 of the die cavity (point 34 on the graph). The stress gradient in this region is quite severe, but the lev~l of tensile str~ss obtained is not so high as to render the design impractical.
Fig. 3 graphically portrays axial stress conditions in a somewhat different die design from that shown in Fig. 2, and it can be seen that a very sharp tensile stress "spike" exists near the point 34 on the ver$ical axis of this graph. Again, this point corresponds to the region in the die cavity where the frusto-conical portion 22 joints the cylindrical portion 24.
Regions immediately above and below this critical region are under fairly high compressive stresses, and therefore, this seems to explain the tendency for cracking that has been experi-enced with die designs of the specific type shown in Fig. 3.
Basically the difference between th~ die cavity shape shown in Fig. 3 as compared to that of Fig. 2 is one of a lesser included angle between the frusto-conical portion 22 and the cylindrical portion 24. Thus, it would appear that as the frusto-conical por-tion 22 becom s less obtuse in its relationship to the cylindricalportion 24 (as is the design shown in Fig. 3, for example), there is a higher degree of tensile stressing in the critical region where these two portions join one another. However, there are apparent exceptions to this general relationship as shown for example in the spur gear die design of Fig. 4. In that design the portion 22 of the die cavity, which corresponds to the frusto-conical portions of bevel gear die designs, is ev~n less obtuse in its relationship to the cylindrical portion 24, and therefore an even greater tensilP stress condition should b~ predicted ; 30 for that critical region. That was not the case though in the ` ` ~075505 particular dssign analyzGd, and it is th~eorized that othar factors (such as the exis~ence of a hub and a relativ~ly narrow "shelf" for the portion 22) accounted for a different stressing of the die. However, it is believed that othsr spur g~ar die designs (and other two-level part designs) would exhibit tensile stressing in the region 34 if the othsr factors were changed.
What is really indicated by the data of Fig. 4 is that th2 str~ss analysis of die design requires consideration of many factors, some of which have a cancelling effect on others, but the basic principles of this invention can be applied to many types of die designs which experience the type of tensile stress failure dis-cussed ahove.
Once the type of information shown in Figs. 2 and 3 was developed, the next problem was one of creating a new die design that would overcome the tensile stress condition without upsetting other stress relationships in the die cavity. My solution to this problem was one of doing just the opposite of what might be considered a sound and traditional engineering practice.
Instead of attempting to furthsr reinforce the die in the ragion of stress rP~arsal, I have provided for a control of deflections of the die in certain areas to induce counter-stressing of the critical region of the die as it is loaded during a forming operation. The counter-stress is calculated and set to effec-tively reducs or eliminate the high axial tensil stress "spiken which otherwise develops during loading of th~ die.
Fig. 5 illustrates a typical embodiment of this invention as applied to a die assembly designed to manufacture bevel side gears from sintered powder metal pr~forms. In such manufac ure, a powder ~etal preform (having a bor~ thPrethrough) is plac~d in the die, in the same manner as with forging processes which 1~075505 utilize solid matal billats, and a punch member 16 is advanced into the dia to rashape and fully densify the powder metal pra-form. Fig. 5 illustrates one-half of such a die assembly in a sectional view showing relationships of parts at the end of a forming operation with the preform 18 fully densified and formed into the shape of a bevel side gear. This particular arrangement also includas a core rod 40 and ejector 42 passing through the supporting means 14 and the bottom of the die 10. Thus, a bore must be provided through the supporting means 14 and through the bottom of the die 10 to accommodate the core rod and ejector assemhlies operating theroin. The double-headed arrow which is depicted on the die 10 in the Fig. 5 view is intended to represent the critical region in which tsnsile strssses normally tend to cause a premature cracking or failure of the di2. The double-tailed arrow represents the cancelling stresses created by the specific structural arrangements shown in the Fig. 5 embodiment.
As previously indicated, I have redesigned the die 10 and its supporting means 14 to permit a control of deflections of the die during a forming operation, and it is this control that creates counter-stresses for effectively reducing or eliminating the high tensile stresses that otherwise occur in the critical region of the die between the frusto-conical portion 22 and the cylindrical portion 24. This is accomplished by establishing a certain geometric relaticnship between a bottom surface 44 of the die 10 and a top surface 46 of a supporting structura which it contacts. As shown in Fig. 5, the top surface 46 of the supporting ring 14 has baen shaped to provide a slight taper from its central area of greatest thickness to a peripheral area of lessar thickness. In other words, the supporting means 14, ~075~05 whether in th2 form of a solid disk or a ring, can be crowned so that an unloaded die 10 will mQke initial contact only with its central area of maximum thickness. In this r2gard, the relationship b~tween the bottom 44 surfacP of die 10 and thP
top surface 46 of the supporting ring 14 is depicted as it would appear in an unloaded condition (even though the punch 16 is shown in a final forming position in the Fig. 5 view). Thus, as the die cavity wall is loaded by the punch 16 via the material being form~d during a forming operation, the bottom surface 44 of the die 10 makes initial contact with the central area of the top surface 46 of the supporting means 14. Contact continues until the bottom of thP die is in substantially increased contact with th2 supporting means. This gsometric relationship results in a control of deflection of the die during loading. In one sense, the relationship is such ~hat "bending" of the die is resisted (as indicated by the series of arrows depicted in sup-porting m2ans 14) in the central area which is axially aligned with the bottom 20 of the die cavity. In another sense, the die is encouraged to deflect in its outer peripheral areas as it is loaded. The nat effect is one of creating a counter-stress field in the critical region where the wall portion 24 joins the frusto-conical portion 22. In order to assure control of die deflections, it is necessary to establish a maximum thickness of the lower portion of the die ~hat will permit controlled deflection. In this r~gard, the thickness between the bottom portion 20 of the di2 cavi~y and the bottom surface 44 of the die is relatively thinn~r than would be the design for state-of-ths-art diPs of this typs. Thus, I have required a struc.ure which might be considered a weakening, rather than a reinforcing, of the die 10 in order to produce the desired control of deflec-_12 ~075505 tions which effectively cancel unwanted tensile stresses ina critical region of the die. Fig. 5 also illustratas the relative geometric r~lationships between sleeve 12 (which contains the die 10) and th~ supporting means 14 as well as adjacent supporting structuras 50 so as to provide for limited movement of the sleeve 12 with the die 10 rsl2tive to the supporting structures 50 as the die 10 is loaded during a forming operation.
Of course, the geomstric relationship which has been dis-cussed above with reference to the bottom surface 44 of die 10 and the top surface 46 of the supporting means 14 can be provided in other ways. For example, the bottom surface 44 of the die 10 (and coextensive bottom portions of an associated sleeve 12) can be shaped to provide the sam progressive contact between thesa surfaces and a flat-topped supporting means 14.
Another specific structure (which may be preferred for practical production of geometric r~lationship that permit a degree of variance from theoretical designs) provides for a steppad con-figuration of either the surface 46 or the bottom 44 of the die so that contact is provided by a ~step" of calculated width in the central araa of the die and with no actual peripheral contact provided at all. Such a structure may be formed by initially shaping the surface 46 as shown in Fig. 5, followed by a relieving of a peripheral area of the surface radially beyond a central area where maximum contact with the die is prefPrred. In any case, the objective is one of providing for contact (between the dia and its supporting structures) which is biased toward the central area of ths die so as to permit a control of deflec-tions of the die as the die cavity is loaded during a forming operation. This control tends to "bend" the die compr_ssively in the ragion in which t2nsila str~ss~s are developed, and thus, _13-'1075505 the t~nsila stresses are effectively reduced or eliminated.
Having described structural fea~ures of a specific embodiment of this invention, it can be appreciated that fully equivalent -structural designs and relationships can be thought of and used by those skilled in this art without substantially departing from the basic teachings of the present invention. For example, the principles of this invention can be applied to die cavity configurations which provide for less than a 90 angular rela-tionship between a conical portion 22 and a cylindrical portion 24, All such equivalant subst.itutions are int~nded to be included wlthin the scope of this invention as claimed below.
Claims (8)
1. In a die in which there is a die cavity having a geometry such that high tensile stresses are formed in a critical region of the die during loading of a wall of the die cavity during a forming operation, the improvement comprising supporting means for reacting to axial components of the loads applied to said die cavity during a forming operation, said supporting means having a supporting surface which makes contact in the unloaded state with a contacting surface of said die only in the central area of the die to resist axial movement of said central area of the die and to permit axial movement of a peripheral area of the die during loading of the die cavity, to thereby deflect and counterstress said die such as to substantially reduce or eliminate said high tensile stresses.
2. The improvement of claim 1 wherein said supporting surface of said supporting means is shaped to produce said contact in the unloaded state only in the central area of the die.
3. The improvement of claim 1 wherein a surface of said die which contacts the supporting surface of said supporting means is shaped to produce said contact in the unloaded state only in the central area of the die.
4. The improvement of claim 1 wherein said die cavity is of a design for forming a bevel gear product and wherein said supporting means comprises a ring member.
5. The improvement of claim 4 wherein said ring member is thicker in a central portion thereof so that loading of the die cavity results in progressive contact between the die and the ring member from their central areas toward their outer peripheral areas.
6. The improvement of claim 1 wherein the geometry of said die cavity includes a bottom portion, a substantially frusto-conical portion adjacent to said bottom portion, and a substantially cylindrical portion adjacent to said frusto-conical portion, and wherein counter-stresses are developed in the region where said frusto-conical portion joins said cylindrical portion.
7. The improvement of claim 6 wherein said supporting means and said die provide for progressive contact therebetween during loading of the die cavity, with initial contact taking place near the central longitudinal axis of the die and with final contact taking place toward the periphery thereof.
8. The improvement of claim 6 wherein said supporting means and said die assembly provide for contact only in an area near the central longitudinal axis of the die assembly and with no contact taking place near the periphery thereof during loading of the die cavity.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US72591776A | 1976-09-22 | 1976-09-22 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1075505A true CA1075505A (en) | 1980-04-15 |
Family
ID=24916462
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA283,199A Expired CA1075505A (en) | 1976-09-22 | 1977-07-20 | Method and means for relieving stresses in die assemblies |
Country Status (5)
| Country | Link |
|---|---|
| JP (1) | JPS5346321U (en) |
| CA (1) | CA1075505A (en) |
| DE (2) | DE2742310A1 (en) |
| FR (1) | FR2365387A1 (en) |
| GB (1) | GB1559042A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US9248493B2 (en) * | 2011-06-30 | 2016-02-02 | United Technologies Corporation | Forge press, die and tooling design with distributed loading |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE2927135B1 (en) * | 1979-07-05 | 1980-09-11 | Kabel Metallwerke Ghh | Method and device for producing bevel gears |
| DE2927192C2 (en) * | 1979-07-05 | 1981-04-23 | Kabel- und Metallwerke Gutehoffnungshütte AG, 3000 Hannover | Device for the seamless production of metallic bevel gears |
-
1977
- 1977-07-20 CA CA283,199A patent/CA1075505A/en not_active Expired
- 1977-08-08 GB GB3315077A patent/GB1559042A/en not_active Expired
- 1977-09-01 FR FR7726629A patent/FR2365387A1/en active Granted
- 1977-09-20 DE DE19772742310 patent/DE2742310A1/en not_active Withdrawn
- 1977-09-20 DE DE19777729127 patent/DE7729127U1/en not_active Expired
- 1977-09-22 JP JP12699477U patent/JPS5346321U/ja active Pending
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US9248493B2 (en) * | 2011-06-30 | 2016-02-02 | United Technologies Corporation | Forge press, die and tooling design with distributed loading |
Also Published As
| Publication number | Publication date |
|---|---|
| FR2365387B3 (en) | 1980-06-20 |
| FR2365387A1 (en) | 1978-04-21 |
| DE7729127U1 (en) | 1978-02-02 |
| JPS5346321U (en) | 1978-04-20 |
| GB1559042A (en) | 1980-01-16 |
| DE2742310A1 (en) | 1978-03-23 |
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