GB2045134A - Methods of forming tubing - Google Patents

Abstract

A tube 10 is formed by pushing it through a tilted die 12 having a truncated cone shaped passage 20 terminating in a throat 24 and formed with a steep angled section 23 directly opposite a shallow angled section 25, to form a bend and may cause circumferential variations in wall thickness of the tube 10. <IMAGE>

Description

SPECIFICATION Methods of forming tubing This invention relates to methods of forming tubing and more particularly to methods of bending and bending and reducing tubing.

Numerous bending methods have been developed over the years, but generally speaking, most such methods are variations of a few basic processes. No single process can be successfully applied to all bending situations where variations of tubular section size, diameter-to-wall thickness ratio, material or angle of bend are considered. For example, 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 centre 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 cross-sectional 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-sectional circularity, the use of various types of mandrels or other means of internal support have 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 No.3354681 discloses a method and apparatus for bending-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 friction, 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 is that of tube wall eccentricity. Eccentricity may be loosely defined as the distance between the centre of the tube cross-section with respect to the inner diameter and the centre with respect to the outer diameter. When such centres do not coincide, the member is eccentric. Eccentricity correction is concerned with reducing differences in wall thickness. U.S. Patent No 3095083 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 eccentricity correction is employed in U.S.Patent No.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 No.3 167 176 uses a swivel mandrel, and U.S. No. Patent 3698 229 uses metal removal from the heavy wall portion of the tube.

According to the present invention there is provided a method of bending tubing in a die having a truncated cone shaped passage terminating in a throat, said passage being formed with a relatively steep section and a relatively shallow section directly opposite the steep section, and proportioned and arranged so that the maximum die inlet angle lx is no greater than about 40 and the die tilt angle T is no greater than about 20 and greater than n and less than the cone angle C, where lx is equal to C + T, T is the angle between the die centreline and the entering tubing centreline, C is the angle between the surface of the cone and the die centreline, the method comprising pushing the tubing through the die passage to subject itto circumferential swaging forces within the die varying from a maximum where it encounters the steep 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.

Embodiments of the present invention to a large extent overcome the problems of the prior art processes relating to tube bending and eccentricity correction. In that aspect, a tube of ductile material is pushed through a tilted die defined by certain angular relationships with respect to the longitudinal axis of the tube.

As used in this disclosure, a tilted die is a die having bilateral symmetry about the incoming tube axis, that is, a unique plane of symmetry contains the straight incoming tube axis. When pushed through such a die, the tube is subjected to differential swaging and to a displacement of forces acting normal to the tube axis, thus causing the tube to bend. Unlike prior art practices, the tube experiences wall thickening around the complete circumference.

A second aspect involves pushing the tube through a tilted die to bring about eccentricity correction by proper orientation of the originally eccentric tube with respect to the tilt angle of the die.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 depicts generally an arrangement employed for carrying out a tube forming process; Figure 2 shows a cutaway view of a tubular member being forced through a tilted die of the arrangement of Figure 1; Figure 3 shows a cross-section of a tubular member before being subjected to the eccentricity correction procedure; and Figure 4 shows the cross-section of a tubular member after having undergone the eccentricity correction procedure.

Embodiments of the invention are concerned with methods of selectively changing various dimensions of already formed tubular members to produce high quality bends, orto correct undesirable eccentricity characteristics, or to create desirable eccentricity characteristics. The invention is applicable to tubular members which are constructed of flowable (ductile) materials such as ferrous and non-ferrous metals as well as plastics and related flowable materials.

Example 1 Tube Bending Referring to Figure 1, a tubular member 10, hereinafter referred to as the tube 10, the outside surface of which may be treated with a commercial lubricant, is operatively positioned at the entrance section of a tilted forming die 12. An introductory guidance section (not shown) may be desirable. The die 12 rests on or is firmly attached to a support fixture 14. A press platen 16 separately contacts or, in some manner, fixes with the free end of the tube 10 and pushes the tube 10 into and through the die 12. The tube 10 does not necessarily have to be pushed at the end, for example, it can be pushed with grips which clamp the tube 10 ahead of the die entrance. The pushing force can be provided by a press or any other pushing device.The fixture 14 supports the die 12 and provides an exit path fortheformed tube 26 through an opening 18.

Referring to Figure 2, the combination of the tilted die 12 with the tube 10 being pushed therein is characterised by certain geometric considerations related thereto. The tube 10 starts with an original outside diameter ODs. (For convenience of illustration Figure 2 shows a particular form of a bilaterally symmetric die, or tilted die, composed of circular conical sections.) For purposes of further explanation, it is helpful to locate the centreline ( of the entering tube 10 as it enters the die 12. The die 12 may be thought of as a shape fashioned from an entrance cone 20 and a relief cone 22. The cone 20 is a first truncated hollowed conical section, and the cone 22 is a second truncated hollowed conical section. Note that the cones 20 and 22 need not necessarily be circular cones, although for most practical processes circular cones would be used.The cones 20 and 22 meet at the plane of truncation commonly called a land or throat 24, such that when the unbent tube 10 is forced through the cone 20, it passes the throat 24 as a bent tube 26 into the cone 22. The tube 10, which started with an original outside diameter ODs is deformed by passage through the die 12 to a formed tube 26 with outer diameter ODf. The entrance cone 20 may be further described with respect to the entering tube 10 and the formed tube 26 by reference to the following symbols: C = the die cone angle (often called the semi-cone angle) which is the angle between the surface of the cone 20 and the centreline of the cone 20.

T = die tilt angle which is the angle between the die or cone centreline and the entering tube centreline.

lx = maximum die inlet angle, equal to C + T.

i, = minimum die inlet angle, equal to C - T.

Rc = inner radius of curvature of the formed tube 26.

Shown in Figure 2 is a tilted die 12 whose die exit plane 27 is normal to the die or cone centreline. Although this is desirable for most practical processes, this exit plane 27 need not necessarily be normal to the die centreline. 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 lx and li define oppositely located steep and shallow angled sections 23 and 25, respectively, of the entrance cone 20 with respect to the centreline of the tube 10. As the tube 10 is pushed through the die 12, one portion of the circumference, which encounters the steep portion 23 of the die 12 experiences a larger swage (diameter reduction) than the opposite portion, the largest swage and accompanying swageforceoccuring at that portion of the cone 20 associated with the maximum inlet angle Ix. Well-established metal forming principles dictate the maximum practical angles which can be utilised without causing excessive redundant work' that creates high pushing forces which in turn promote tube buckling or irregular bending.We have found that lx has a critical upper limit of about 40-, and the tilt angle has a critical upper limit of 20 and should be greater than 0 and equal to or less than the cone angle C. The critical limit of 1x varies somewhat depending upon the ODs t ratio (wherein t is the thickness of the original tube wall), upon the diameter reduction, and frictional characteristics. When these limits are exceeded, the entering tube 10 will tend to buckle or the formed tube 26 exiting the die 12 will have unpredictable irregular bending and a non-uniform radius of curvature. These limits define a transition zone and, when not exceeded, result in predictable, uniform bending of the tube 10 having a uniform radius of curvature.Beyond this transition zone, formed tube 26 exiting the die 12 exhibits unpredictable behaviour with a surprising decrease in bending and an erratic radius of curvature.

The differential swaging results in material flow proportional thereto causing greater elongation at that portion of the tube 10 experiencing the large swage, the differential elongation resulting in bending. It will be noted that during pushing of the tube 1 0 through the die 12, a portion ofthe circumference of the tube 10 closest to the shallow section 25 of the entrance cone 20 contacts the die 12 prior to the opposed portion contacting the steep section 23 of the entrance cone 20. This offset of initial contact in the entrance cone 20 results in an offset of die forces normal to the tube 10, thus producing a couple (or moment) which in turn promotes further tube bending. It should be noted that, even in the extreme case of no diameter reduction (that is, when the tube ODs equals the diameter of the throat 24), a tube which is pushed through a tilted die will experience this offset of die forces and thus will bend; this phenomenon can be proved geometrically.

Some finite amount of permanent bending will occur so long as the tilt angle is large enough to cause som finite amount of plastic deformation of the tube.

It has also been found that the above approach results in the overall tubular cross-section remaining substantially round, and generally in wall thickening around the entire cross-section. When properly practiced, the method virtually eliminates the possibility of tube wall collapse which has hampered so man prior art bending methods, but does so without requiring use of a mandrel or other types of internal support The method also displays an extremely desirable range of application with respect to ODs/t ratios in comparison with those prior art methods without internal support mechanisms, with slight variations with respect to the particular material. Bends well beyond 1800 can be routinely made, the limitation being only bent tube clearance of the equipment. The method is applicable to any malleable or ductile material.By providing support to either the outside or the inside surface of the entering tube 10, buckling could be retarded. By performing the entire method under a sufficiently high environmental hydrostatic pressure (fa example in a high pressure chambers, normally brittle (difficult-to-deform without fracture) materials could be bent. The tube can be formed cold, warm, or hot.

The following Table I summarises test results obtained in the bending of particular carbon steel tubing experiencing a 5.3% reduction of outer diameter.

TABLE I - Bending of 2 & 7 mm ODs Carbon Steel Tubes. 5.3% OD Reduction STARTING TUBE FORMED TUBE DIE Dia- Wall Thick Out- meter- ness Increase OD Inner side Wall to- Out- Radius Required Dia- thick- thick- Inner Outer of- of Cone Tilt Pushing meter ness ness Radius Radius Round- Curv- Angle Angle Force (ODs) (t) mm ratio ness ature (C) (T) kg mm (ODs/ mm (Rc) t) cm 28.7 2.16 13.3 4.6% 4.7% 0.05 145.0 8 30 1045-1180 4.6 4.6 4.7 0.1 99.8 20 6 1360 5.9 " 5.9 5.8 0.05 82.0 15 6 1545-1590 3.3 " 3.3 5.6 0.1 56.1 8 6 1360-1405 " " " 7.0 8.2 0.48 46.7 28 12 1680 ,, " 7.0 0.33 40.9 20 12 1590 " " " 2.3 8.2 0.33 34.5 15 12 1635-1725 " " " 7.0 19.8 0.74 56.4 28 18 3040 " " " 3.5 9.2 0.66 26.7 20 18 1860-1905 " " " 3.5 19.5 0.86 63.0 20 20 3135-4090 " 5.8 19.5 0.99 57.7 24 22 3180-3540 28.7 2.95 9.7 4.2% 3.4% 0.08 138.2 8 30 1315-1455 " " " 4.3 3.4 0.08 94.0 20 6 1635-1680 " " 5.2 5.1 0.08 65.0 15 6 1950-2045 3.4 4.2 0.05 55.1 8 6 1635-1770 6.0 6.0 0.38 40.6 28 12 2360 " " 6.0 5.1 0.3 33.5 20 12 2045-2090 3.4 5.9 0.23 29.7 15 12 1860-1952 " " 8.5 20.3 0.79 121.1 28 18 4630 " " 5.1 8.6 0.64 23.4 20 18 2450-2495 4.2 20.5 0.1 70.3 22 20 4315-4765 " " " 5.0 21.8 1.12 77.0 24 22 4675-5765 " 3.66 7.8 4.2 2.7 0.05 115.8 8 3 1455-1545 3.5 2.7 0.1 71.4 20 6 2000-2090 4.8 4.9 4:9 0.05 54.4 15 6 2180-2410 3.4 " 3.4 4.2 0.05 48.5 8 6 1725-1860 6.2 6.0 0.41 32.5 28 12 2860-2995 6.9 " 6.9 4.1 0.41 27.4 2b 12 2315-2405 " " " 3.4 4.8 0.3 25.9 15 12 2135-2270 n ,, a Tube Buckled 28 18 4.8 " 4.8 7.5 0.79 20.6 20 18 3040-3085 " " " 4.1 16.9 0.81 89.7 22 20 5900-6265 Tube Bent Irregularly 24 22 8625-9670 As a result of a comprehensive analysis of many tests we have discovered that the radius of curvature of the bent tube 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 the tube within the die is a strong function of the outside diameter reduction and a weak function of the tilt angle, the cone angle, and the original diameter-to-thickness ratio. We have also found that maximum bending occurs when the tilt angle approaches 18 and the cone angle is a minimum in excess of the tilt angle, aboutO to 2". The test results further indicate that maximum bending occurs when the percentage reduction of outside diameter of the tube is equal to approximately one-half the value of the original diameter-to-thickness ratio.

Example 11 Tube Eccentricity Correction The pushing of a tube 10 through tilted die 12 sets up forces resulting in material flow proportional to the swaging angle that the particular portion of the tube 10 'sees'. in all cases, pushing the tube 10 through the die 12 results in increased wall thickness completely around the circumference. The maximum thickness increase occurs at that portion of the tube 10 seeing the maximum swage (at lx), and the minimum thickness increase corresponds to the minimum swage (at Is).

Figure 3 shows a cross-sectional view ofthetube 10 (with a minimum wall thickness 28, a maximum wall thickness 30, and an inside diameter 32) priorto entry into the die 12. Eccentricity is shown in exaggerated form for easier viewing.

The tube 10 is pushed through the die 12 in accordance with the procedure heretofore described. However, when the method is being used for eccentricity correction purposes, the orientation of the tube 10 is quite important. Since pushing the tube 10 through the die 12 always results in wall thickening about the circumference of the tube 10, the minimum wall thickness 28 should 'see' the steep section 23 of the die 12.

The maximum swage angle can be selected based on the amount of eccentricity correction required. Of course, bending accompanies the eccentricity correction, and the tube 10 may require a straightening operation depending on the application requirements.

Figure 4 shows the cross-section of the formed tube 26 after exiting the relief cone 22 of the die 12. The tube 10 is shown as having a wall 34 uniform in cross-section about the circumference of the tube 10, an inside diameter 36 reduced from original inside diameter 32, and an outside diameter ODf reduced from original outside diameter ODs.

Table II compares the change in percent eccentricity (after straightening) obtainable by the present method as compared with the prior art method of drawing a tube through a die. As is readily apparent, a significant increase in the change in percent eccentricity characterises the present 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 effect the desired change in wall thickness about the tube circumference in accordance with the principles previously described.

TABLE 11 - Eccentricity Correction of Carbon Steel Tubes Diameter- Change in Percent Tilt Cone to- Initial Eccentricity Angle Angle thickness Eccentricity (T) (C) ratio (E%)* (ODsit Present Prior Process Art 6" 82 10.5 3.15% 4.09% 2.4% 12 15 10.5 3.87 6.43 4.3 12 15 14.7 4.34 7.09 5.1 E % = tmax - tmin x 100 tmax + tmin where Tmax and Tmin are the maximum and minimum wall thicknesses, respectively.

**Absolute value of the percent change from the initial condiction.

Claims (8)

1. A method of bending tubing in a die having a truncated cone shaped passage terminating in a throat, said passage being formed with a relatively steep section and a relatively shallow section directly opposite the steep section, and proportioned and arranged so that the maximum die inlet angle 1x is no greater than about 40 and the die tilt angle T is no greater than about 209 and greater than 0" and less than the cone angle C, where lx is equal to C + T, T is the angle between the die centreline and the entering tubing centreline, C is the angle between the surface of the cone and the die centreline, the method comprising pushing the tubing through the die passage to subject it to circumferential swaging forces within the die varying from a maximum where it encounters the steep 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 according to claim 1 wherein T is 18 and C is 18 to 20 for maximum bending of the tubing.

3. A method according to claim 1 or claim 2 wherein the tubing undergoes bending while being reduced in outside diameter.

4. A method according to claim 1 or claims 2 wherein the entire wall section of the tube increases in thickness in passing through the die.

5. A method according to claim 3 wherein the wall of the original tubing varies in thickness and the tubing is so pushed through the die passage that the thinnest wall part experiences the maximum swage causing that part to increase in thickness by more than other wall parts experiencing lesser swages.

6. A method according to claim 5 wherein the thinnest wall part of the tubing does not experience the maximum swage.

7. A method of bending tubing, the method being substantially as hereinbefore described with reference to Example 1 and the accompanying drawings.

8. A method of bending and reducing tubing, the method being substantially as hereinbefore described with reference to Example 2 and the accompanying drawings.