CA1102671A - Tube forming process - Google Patents

Abstract

TUBE FORMING PROCESS

ABSTRACT OF THE DISCLOSURE

A method of forming a tube by pushing it through a tilted die to form a bend or change circumferential variations in wall thickness.

Description

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` BACKGROUND OF T~E INVENTION
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This invention rela-tes to the manufacture oE tubular sections of ductile materials and, more particularly, to a novel method of forming bends or of changing wall eccentricity by pushing the tubular section through a tilted die causing a greater diameter reduction on one portion of the tube circum~erence than on the opposite portion.
Numerous bending processes have been developed over the years, but generally speaking,most such methods are variations of a few basic processes. No single process can be sucaessfully applied to all bending situations where vàriations of tubular section size, diameter-to-wall thickness ratio, material or angle of bend are considered. For instance, the -press method, wherein the tube is laid across a plurality of ~ -wiper dies and then subjected to the pressure exerted by a form die, is useful when some flattening of the tubing can be permitted. The roll method of bending employs three or more triangularly arranged rolls, the center one of which is adjustable. `~
The workpiece is fed between the fixed driven rolls and the adjustable roll to form the bend. The draw method bends the tube by clamping it against a rotating form and drawing it through a pressure die~ In all of these methods, thinning of the tube wall, especially on the extrados, and loss of section circularity occur. The thinner the tube wall and/or the tighter the bend sections, the more severe these problems become. ;~
In attempting to eliminate loss of cross section circularity, the use of various types of mandrels or other mean-s of internal support has been employed with varying degrees of success. In some instances, the use of internal tools has led to process complications or given birth to new problems such as scarring of the inner wall or non-uniform wall thinning.
U.S. Patent 3,354,681 discloses a method and apparatus .
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forming elbows from tubular sections by pushing through a forming die. A portion of this apparatus consists of a "tapered land"
which the inventor claims to cause bending by differential ~riction, the friction force being greater on the inside radius ;
of the bent tubular section than on the outside radius, which is in direct contradiction to the findings of our invention.
Another problem pervasive in the tubing industry i s that of tube wall eccentricity. Eccentricity may be loosely defir~ed as the distance between the center of the tube cross section with 10 respect to its inner diameter and the center with respect to its outer diameter. When such centers do not coincide, the member is ~
eccentric. Eccentricity correction is concerned with reducing ;
differences in wall thickness. U.S. Patent 3,095,083 discloses a method and apparatus for correcting eccentricity by drawing (pulling) the member through a tilted die without the use of internal tools. However, not only is the amount of eccentricity correction obtainable limited but it has been found that the die -~ ;
will in some instances produce wall thickening and in other instances produce wall thinning. This same technique to effect 20 eccentricity correction is employed in U.S. Patent 3,131,803 wherein the tilted die is used in combination with an internal mandrel. Other approaches to eccentricity correction are also employed, for example: U.S. Patent 3,167,176 uses a swivel mandrel and U.S. Patent 3,698,229 uses metal removal from the heavy wall portion of the tube.
The present invention provides a method of bending tubing in a die having a truncated cone shaped passage terminating in a throat, formed with a steep section and a shallow section directly opposite the steep section, and proportioned and arranged so that 30 the maximum die inlet angle Ix is no greater than about 40 and the die tilt angle T is no greater than about 20 and greater than 0 and less than the cone angle C, where Ix is e~ual to C+T, T is the angle between the die centerline and the entering tubing 7~
centerline, C is the angle between the surface of the cone and the die centerline, the method comprising pushing the tubing through the die passage to subject it to circumferential swaging forces within the die, causing the tube to be reduced in ou-tside diameter, varying from a maximum where it encounters the steepest section and to subject it to an offset of die forces producing a couple or force moment, to a minimum where it encounters the shallow section to cause bending of the tubing about the shallow section, and allowing the tubing to bend without restraint beyond the throat.
The application of a tilted die in a composite die for forming tubular fittings is disclosed in co-pending application Serial No. 324,609.
For a better understanding of the invention, its operating advantages and specific objects obtained by its use, reference -~
should be had to the accompanying drawings and descriptive matter in which a preerred embodiment of the invention is illustrated and described.
BRIEF DESCRIPTION _F THE DRAWINGS
Fig~ 1 generally depicts a suitable arrangement employed for carrying out the forming process;
Fig. 2 shows a cutaway view of the tubular member being forced through the tilted die of Fig. l; ;~
Fig. 3 shows a cross section of a tubular member before being subjected to the eccentricity correction procedure; and Fig. 4 shows the tube cross section after having undergone the eccentricity correction procedure.
_ESCRIPTION OF THE PREFERRED EMBODIMEMT
The present invention is generally directed a-t a process for selectively changing various dimensional aspects of already formed tubular members to produce high quality bends, or to correct undesirable eccentricity characteristics3 or to create desirable eccentri-city charac~elistics. The invention is applicable ~o tubular members which are constructed of flowable (ductile) materials such as ferTous and non-ferrous metals as well as plastics and related ~lowable materials.

I_TUBE BENDING
Referring to Fig. 1, tubular membeT 10, the outside surface of ~ich ~ay be ~reated with a commercial lubricant, is operatively positioned at the entrance section of tilted die 12. hn mtroductory guidance sec~ion (not shown~ may be desirable. Die 12 rests on or is firmly attached to support fixture 14. Press platen 16 separately contacts OT, in some manner, fixes with the free end of the tubl~laT
member 10 and pushes the member into and through die 12. The tube does n~t necessarily have to be pushed on its end, for example, it can be pushed with grips wh~ch clamp the tube ahead of the die entrance. The pushing foTce can be provided by a press or any other pushing device.
Fixture 14 supports the forming die 12 and pro~ides an exit path for the formed tubular membeT 26 through opening 18.
Referring to Fig. 2, the combir~tion o~ the tilted die 12 with tubular member 10 having been pushed ~herein is characte~ized by ~0 certain geome*ric considerations related thereto. Member 10 starts with an orig}nal outside diameter O~s. ~Note, for convenience o~ illustration, Fig. 2 sho~s a particular form of a bilaterally symmetric die (or tilted die~ composed of circular conical sections.) For purposes of ~urther e~planation, it is helpful to locate the centerline ( ~ ) of the entering tube 10 as it enters the die 12~ Tilted die 12 may be thought of as a shape fashioned from an entrance cone 20 and a relief cone 22. Cone 20 is a first truncated hollowed conical section, and cone 22 is a second truncated hollowed conic~l sectionO Note that these sections need not necessarily be circular cones, although for most practical processes circular cones would be used. ~he conical sections 20 and 22 meet at the plane o truncation commonly called a land or throat 24 such that when the 7~

unbent tubular member 10 is forced through cone 20, i~ passes land 24 as a bent tube 26 into section 22. Tubular member 10~ which started with an original outside diameter ODs is deformed by passage through the die to a formed tubular member 26 exhibiting an outer diameter ODf. The entrance cone 20 may be further described with respect to the starting member 10 and the formed meJnber 26 by reference to the follo~ing 5ymbols:
C = the die cone angle (often called the semi-cone angle) which is the angular relationship between the surface of the cone and -~
the centerline of the cone.
T = die tilt angle which is the angular relationship between the die or cone centeTline and the entering tube centerline.
Ix = maximum die inlet angle, equal to C + T.
Ii ~ minimNm die inlet angle, equal to C - T.
Rc = inner radius of curvature of the bent tube.
Shown in Fig. 2 is a tilted die whose die exit plane 27 is norm21 to ~he die or cone centeTline. Although this is desirable for most prac-tical processes, ~his exit plane 27 need ~ot necessarily be normal to the die centerline. Instead, the exit plane 27 could be canted to either side of this normal orientation, and tube bending would still result.
It will be observed that Ix and Ii define oppositely located steep and shallow sections9 respectively, of the entrance cone 20 with respect to the centerline of member 10. As member 10 is pushed through die 12, one portion of its ci~cumference, which encounters the steepest portion of the die experiences a larger swage ~diameter reduction) than the opposite portion, the largest swage and accompanying swaging force occurring at ~hat portion of the cone associa~ed with the maxim~m inlet angle Ix. Well-established metal fo~ming principles dictate the maximum practical angles which can be utilized wi~hout causing excessive "~edundant work" tha~
crea~es high pushing forces which in turn promote tube buckling or irregu ~ ~L~L~ 73L

lar bending. ~Ye have found that Ix has a ~ritical upper limit ~f about 40; and the tilt angle has a c~itical upper limit of 20~ and should be greatel than 0 and e~ual to or less than the cone angle. The critical limit of Ix varies somewhat depending upon the ODs/t ~atio (wherein t is the thickness of the original tube wall), upon the diameter redu~ti~n, and frictional characteristics. When these limits are exceeded, *he the entering tubing will tend to buckle or the member exiting/die will have unpredic*able irregular bending and a non-uniform ~adius of curvature.
These limits define a transition zone and, when not exceeded, Tesult in predic~able, uniform bending of the tubing ha~ing a unifoIm radius of curvatuTe. Beyond this transition zone, the member exiting the die exhibits unpredictable behavior with a surprising decrease in bending and an erratic radius of curvature.
The differential swaging results in material flow proportio~al thereto causing greater elongation at ~hat portion of the tubular member experiencing the larger swage, the differential elongation resulting in bending. It will be noted that during pushing ~f the tubular member 10 through die 12~ a portion of the member's circumference closest to the Ii element 25 of the entrance cone contacts the die prior to the opposed portion co~tacting the Ix elemen~ 23 of the cone. This offset of initial contact in the entrance zone 2Q results in an offset of die forces normal to the tube 10~ thus ploducing a couple ~or moment~ which in turn promotes further tube bending. It should be noted that, even in the ext~eme case of no diameter reduction ~that is, when the tube ODs equals the diameter of the die throat 24), a tube which is pushed thrDugh a tilted die will experience this offset of die fo~ces and thus will bend; ~his phenomenon can be proven geonetrically. Sume finite amount of permanent bending will occur so long as the ~ilt ~ngle is large enough to cause some finite amount of plastic deormation of the tube.
It has also been found that the above approach results in the ~a~ 7~

overall tubular cross section Temaining substan~ially round, and generally in wall thickening around the entire cross section. When properly practiced, the process ~irtually eliminates the possibility of tube wall collapse which has hampered so many prior art bending processes, but does so without requiring use of a mandrel OT othe~
~ypes of internal support. The i~ventive process also displays an extremely desirable range of application with respect to ODs/t ratios in comparison with those prior art processes without internal support mechanisns, with slight variations with respect to the par-ticular m2terial. Bends well beyond 180 can be routinely made, the limitation being only bent tube clearance of the equipment. The process is applicable to any malleable or ductile material. By pro-viding suyport to either the outside or the inside surface of the straight tube 10, buckling could be retarded. ~y performing the entire process under a sufficiently high envirDnmental hydrostatic pressure ~e.g., in a high pressure chamber), normally brittle ~dif-ficult-to-deform without fractuTe) materials could be bent. The tube can be formed cold, warm, or hot.
The following Table 1 summarizes test results obtaine~ in the beN~ing of particular carbon steel tubing experiencing a ~.3%
re~uction of outer diameter.

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As a result of a comprehensive analysis ~f many tests we have discovered ~hat the radius of curvature of the bent tubing is strongly influenced by the tilt angle and to a lesser degree by the outside diameter reduction and the original diameter-to-thickness ratio. The required pushing force on *he tubing within the die is a strong function of the outside diEmeter reduction and a weaX function of the tilt angle, the cone angle, and the original diameter-to-thickness ~atio. We have also found that maximum bending occurs when the tilt angle a~proaches 18 and the cone angle is a minimum in excess of the tilt angle, in the order of O to 2. The ~est Tesults further indicate that naximum bending occurs when the ~ercent reduction of outside diameter of the tubing is equal to a~proximatel)r one-half the value of the original diameter-to-thickness ratio.

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II TUBE ECC~NTRICITY CO~RECTION
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The pushing of a tubular member through tilted die 12 sets up forces resulting in material flow proportional to the s~aging angle that the particular portion of the tube "sees". In all cases, pushing the mber 10 through die 12 results in increased wall thickness completely aIound the circumference. The maxlmum ~hickness increase occurs at that portion of ~he tube seeing the maximum swage ~at Ix), and the minImum thickness increase corresponds to the minimum swage ~at Ii).
Fig. 3 shows a cross sectional view of tubular member 10 ~with a minimum wall thickness 28, a ~EuCLmUm wall *hickness 30, and an inside diameter 32) prior to its entry into tilted die 12. Eccentricity is shown in exaggerated form for easier viel~Lng.
Tubular member 10 is pushed thr~ugh dle 12 in accordance with the procedure heretofore described. Howe~er, when the process i5 being used for eccentricity correction purposes, the member's orientation is quite important. Since pushing the member through the die always results in ~all thickening about the member's circumference, the minimu~ wall thickness 28 sh~uld "see" the maximum swage portion 20 of the die. The ~aximum swage angle can be selected based on the amount of eccentricity correction required. Of course, bending accompanies the eccentricity corTection, and the tube may require a straightening operation depending on the application requirements.
Fig. 4 shows the cross section of member 26 after exiting relief cone 22 of die 12. The member is shown as having a wall 34 uniform in cross section about the member's cîrcumference, an inside diEmeter 36 reduced from original inside dizmeter 32, and an outside diameteT ODf reduced from orig~nal outside diameter ODs.
Table II compares the change in percent eccentricity (after ; straightening) obtai~able by the present process as compared to the ~ ~L~L~3~6'73L

prior ar~ method of dra~ng the tube through the die. As is readily ~pparent, a significant increase m the change in percent eccentricity characterizes the present inventi~e method.
In some instances it mAy be desired to change but not necessarily to correct the eccentricity. In these cases the entering tube is properly oriented with respect to the die to e ff ect the desired change in wall thickness about the tube circumference in accordance writh the principles previously described.

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Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of bending tubing in a die having a truncated cone shaped passage terminating in a throat, formed with a steep section and a shallow section directly opposite the steep section, and proportioned and arranged so that the maximum die inlet angle Ix is no greater than about 40° and the die tilt angle T
is no greater than about 20° and greater than 0° and less than the cone angle C, where Ix is equal to C+T, T is the angle between the die centerline and the entering tubing centerline, C is the angle between the surface of the cone and the die centerline, the method comprising pushing the tubing through the die passage to subject it to circumferential swaging forces within the die, causing the tube to be reduced in outside diameter, varying from a maximum where it encounters the steepest section and to subject it to an offset of die forces producing a couple or force moment, to a minimum where it encounters the shallow section to cause bending of the tubing about the shallow section, and allowing the tubing to bend without restraint beyond the throat.

2. A method of bending tubing as in claim 1 wherein the die has a tilt angle of 18° and a cone angle of 18° to 20° for maximum bending of the tubing.

3. A method as in claim 1 wherein the entire wall section of the tube increases in thickness in passing through the die.