METHOD AND APPARATUS FOR BENDING TUBING

Abstract: The bending of a tube (T) is achieved by rotation of a bend die (34) relative to a co-operating pressure die (60). The bend die (34) is U-shaped with two generally parallel tube grooves (48, 49) having the same diameter as the tube (T) interconnected by a bend section (46). The bend section (46) has a bottom surface portion having a radius of curvature (A) less than one-half the outside diameter of the tube (T) being bent and opposite side surfaces having a radius of curvature (B) greater than one-half of the outside diameter of the tube (T). This resists any tendency of the tube (T) to buckle or wrinkle on bending.

Description: This invention relates to a novel and improved method and apparatus for bending of tubing and pipe. It has been customary practice to bend tubing with the use of an internal mandrel so as to minimize wrinkling or crimping of the tube as it is bent; and, while the use of internal mandrels is accepted commercial practice for larger sized tubing in excess of 3" in diameter, it has not been found satisfactory in the bending of smaller tubing. In U.S. Patent Number 4063441 there is described a tube bending machine including a generally heart shaped die. A bending operation is initiated by causing the die initially to operate in a draw bending mode, followed by a compression bending mode with the tube to be bent clamped between the bend die and a clamping die. The pressure die serves to force the tube against the bending die, and initially to advance with the tube so as to exert little or no axial restraint in the initial bending, and then to apply increasing pressure while interrupting the movement of the pressure die, so as to exert increasing axial restraint upon the tube in compression bending to complete the bending operation. FR-A-1602951 which forms the base for the pre-characterising part of the claims discloses a tube bending machine in which the tube to be bent is clamped between a circular bend die and a cooperating pressure die, the tube to be bent being clamped to the circular bend die and the pressure die being advanced in a linear direction as the bend die is rotated, the bend die and pressure die each having confronting faces with a tube groove in each of the faces for embracing diametrically opposed sides of the tube to be bent. BE-A-530611 discloses a tube bending machine including two circular dies having opposing grooves effective to engage opposite sides of a tube to be bent by the dies, such that the radius of the tube is changed by pressure exerted by the dies. It is an object of the present invention to provide a tube bending machine and method of bending a tube in which undesirable bulging, collapse or wrinkling of the tube is avoided so as to maintain a substantially uniform tube diameter throughout the tube. According to a first aspect of the invention, there is provided a tube bending machine in which the tube to be bent is clamped between a bend die and a cooperating pressure die, the pressure die being advanced in a linear direction as said bend die is rotated, said bend die and pressure die having confronting faces with a tube groove in each of said faces for embracing diametrically opposed sides of said tube to be bent, characterised in that said bend die is generally U-shaped and said tube groove on said bend die is generally U-shaped having a bend section of generally semi-circular configuration and opposite sides extending in tangential directions substantially parallel to one another to define tangential grooves along said opposite sides of said bend die, said tangential grooves having a cross-sectional radius of curvature substantially corresponding to that of said tube to be bent, said tube groove along said bend section having a bottom surface portion with a cross-sectional radius of curvature less than one-half of the outside diameter of said tube to be bent and opposite side surfaces of said bend section each having a cross-sectional radius of curvature greater than one-half of the outside diameter of said tube to be bent, and said opposite side surfaces undergoing a smooth gradual transition into said bottom surface portion, said tube groove on said pressure die having a bottom surface portion with a cross-sectional radius of curvature less than one half of the outside diameter of said tube to be bent, and opposite side surfaces of said tube groove on said pressure die having a cross-sectional radius of curvature greater than one-half of the outside diameter of said tube to be bent. Thus the form of the dies, in particular the changes in the radii of the grooves in which the tube is clamped, enables a smooth transition to be achieved between a radius equal to that of the tube to a radius which is less than that of the tube. According to a second aspect of the invention, there is provided a method of bending a tube into a predetermined curvature characterised in that it comprises the steps of positioning the tube against a straight section of a first semi-circular groove in a bend die wherein the first groove has a radius of curvature corresponding to that of the tube, bending said tube around a curved section of a second groove in said bend die which forms a continuation of the first groove, forcing said tube successively into said first and second grooves of said bend die whereby said tube assumes the configuration of said second groove as the tube is bent around said curved section of said bend die wherein said second groove has a bottom surface portion with a cross-sectional radius of curvature less than one-half of the outside diameter of the tube to be bent and opposite side surfaces in said second groove have a cross-sectional radius of curvature greater than one-half of the outside diameter of the tube to be bent, said tube groove on said pressure die having a bottom surface portion with a cross-sectional radius of curvature less than one half of the outside diameter of said tube to be bent, and opposite side surfaces of said tube groove on said pressure die having a cross-sectional radius of curvature greater than one-half of the outside diameter of said tube to be bent. The following is a more detailed description of one embodiment of the invention, by way of example, reference being made to the accompanying drawing in which:- Figure 1 is a perspective view illustrating a tube bending machine and control panels therefor, with a die assembly; Figure 2 is a top plan view illustrating the interrelationship between bend die, pressure die and clamping die of the die assembly of Figure 1 at the initiation of a bend; Figure 3 is a top plan view illustrating the interrelationship between bend die, pressure die and clamping die of the die assembly of Figure 1 at the completion of a bend; Figure 4 is a side view of elevation of a preferred form of pressure die of the die assembly of Figures 1 to 3; Figure 5 is an end view of the preferred form of the pressure die shown in Figure 4; Figure 6 is an end view taken from the rearward end of a preferred form of bend die of the die assembly of Figures 1 to 3; Figure 7 is a top plan view illustrating the relative distances in different radii of curvature employed in the formation of the tube groove of a preferred form of the bend die of Figures 1 to 6; Figure 8 is a cross-sectional view taken about lines 8-8 of Figure 7; Figure 9 is a cross-sectional view illustrating the radii of curvature of the bend die and pressure die of the die assembly of Figures 1 to 8 with respect to a tube to be bent; and Figure 10 is a horizontal sectional view taken through a tube at the completion of a bending operation using the die assembly of Figures 1 to 9 and illustrating the variation in wall thickness of the tube. In Figure 1 a conventional form of bending machine 10 is shown having a main upper bed or table surface 12, a hydraulic cylinder 14 including a plunger 16, the forward and rearward ends of the clyinder 14 mounted in guideways 18 and 19. The machine includes a spindle 20 with optical encoder 21 projecting downwardly from attachment to a swing arm 22 in order to control rotational movement of the swing arm 22 around overhanging portion 24 of the machine. Typically, the machine 10 also includes a degree of bend adjustment 25 which is regulated by a control line C5 from panel P into a distance control unit 26 to determine the distance of forward advancement of the plunger 16 based on the degree of band desired. The main control panel P also includes control lines C1 and C2 to the opposite ends of the cylinder 14 in order to control the speed and pressure applied by the plunger 16. Control lines C3 and C4 are connected into the spindle 20 and auxiliary control panel P′ to regulate rotational movement of the swing arm 22 and associated clamping die 30. Panel P₁ regulates through the control line C4 the speed and pressure of the plunger with respect to the rotational speed of the swing arm 22 in relation to the size and degree of bend of the tubing to be bent, a typical piece of tubing designated at T in Figure 1. One typical form of machine is the MIIC bender sold by Chiro Electric Manufacturing Co., Ltd. of Southfield, Michigan. The clamping die 30 includes a clamping block 31 adjustably mounted in a guideway 33 for slidable movement toward and away from the bend die 34. The guideway 33 is affixed to the upper extremity of the swing arm 22 and includes a mounting plate 36 keyed to the upper end of the spindle 20. The clamp block 31 has a semi-circular tube groove 31′ conforming to the size and configuration of the tube T so as to encircle one-half of the tube. The tube groove 31′ has serrations, not shown, and projecting ribs 38 are positioned in diametrically opposed slots on the bend die 34. A locking screw 29 in the clamp die holder 30 tightens the clamp die and block 31 against one side of the bend die with the tube T between them. Referring to Figures 2, 3, 6 und 7, the preferred form of bend die 34 is a generally U-shaped block having spaced, parallel top and bottom surfaces 40, 41 with a die cavity circumscribing outer peripheral wall 43 of the die, except for a squared end portion 44. The groove, Figure 3, includes a semi-circular portion 46 transversing a rounded end 47 of the die and merging into a tangential, straight groove portion 48 along one side 42 of the die and tangential portion 49 along opposite side 44 of the die to the tangential portion 48. Semi-circular end portion 46 extends just beyond 180° in merging into the tangential side 49 so that the side portion 49 converges rearwardly toward the side portion 48. The tangential side portions 48 and 49 each extends a distance greater than the circumferential extent of the end groove portion 46, and a central opening 50 extends through the thickness of the die with its axis coinciding with the center of the semi-circular portion 46. The central opening 50 receives the upper end of the spindle 20; and the bottom surface 41 of the die, as shown in Figure 6, is recessed at 52 to define a keyway alinged with a corresponding keyway in the upper surface of block 36 for the purpose of receiving a key 54. The upper end of the spindle 20 is threaded to receive a lock nut 55 to clamp the bend die 34 securely against the block 36 and to cause the die to be rotated with the block 36 when the swing arm 22 is rotated by the spindle. In Figures 1 to 5, a pressure die 60 takes the form of an elongated rectangular or oblong block having flat, parallel top and bottom surfaces 61 and 62, opposite squared end portions including a leading end 63 and a trailing end 64, and spaced parallel sides 65 and 66. Opposite sides 65 and 66 are flat surfaces parallel to one another, and a die cavity or groove 68 in the side 66 traverses the entire length of the pressure die 60. As shown in Figure 1, the pressure die 60 is positioned on the table surface 12 such that its groove 68 is aligned on a common axis with the groove 31′ of the clamping die, and the upper surface 61 of the die 60 is aligned flush with the upper surface of the clamping die 30. The pressure die 60 is maintained in axial alignment with the clamping die groove 31′ by generally I-shaped backing member 70 which is disposed on a second guideway 72 directly behind the guideway 30 for the clamping die. One portion of the backing member 60 extends rearwardly as at 74 to be engaged by the leading or forward end of the plunger 16 of the hydraulic cylinder 14. Preferably, the pressure die is of a length such that it will remain in constant engagement with the tube T when bent around the entire peripheral wall surface of the die 34. An important feature is the radius of curvature of the bend die 34 and the pressure die 60 so as to avoid wrinkling or collapse of tubing and pipe when bent through different angles. Reference is made to the following Table I illustrating the selection of representative radii of curvature for the bend die and pressure die and the factors in selecting the optimum radii. Factors .422 .6667 .1787 .9916 .5267 .455 Tube OD A B C E H K 2.000 .844 1.333 .358 1.983 1.053 .910 1.500 .633 1.000 .269 1.475 .790 .683 1.000 .422 .667 .179 .992 .527 .455 .500 .211 .333 .089 .496 .263 .228 .250 .106 .167 .044 .248 .132 .114 As noted from Figures 4 to 7, for a tube having an outside diameter of 2″, the diameter of the tube groove 46 at E along the end of the bend die across the entrance to the groove is 1,983″, or just less than the diameter of the tube in a vertical direction, or direction normal to the bend. The radius of curvature A at the bottom of the groove along the bend area is 0.844″ or less than one-half the diameter of the tube while the radius of curvature B along opposite sides of the groove is greater than one-half of the diameter of the tube. It is important that the center of the radius of curvature B for each side of the groove be located at center points B′ which are disposed eccentrically with respect to the center A′ for the bottom of the groove in order to establish a smooth transition between the radius of curvature A from the bottom into the side radii B. Moreover, the side radii B should continue circumferentially beyond a center A′. Thus, the depth of groove H is established as being 0.053″ greater than the radius or one-half the outside diameter of the tube, and the location of the eccentric centers B′ established at distance C from the center A. The transition or blend between radii A and B is completed by grinding to a uniform gradation in curvature between the two radii A and B so as to eliminate any irregularity in the groove. The multi-radius configuration A and B as described continues throughout the bend area 46 and undergoes a transition into semi-circular grooves along the tangential sections 48 and 49 of the bend die. Preferably, this transition is accomplished by causing the bottom of the groove A to slope upwardly at a gradual angle on the order of 5° into the bottom of the semi-ciurcular or single radius groove along the tangential sections 48 and 49. This sloping will occur at points beginning at each bend area and continue through a limited distance depending on the diameter of the tube to be bent. Similarly, the pressure die 60 is formed with a groove 68 which is a multi-radius groove having the same dimensions or radii A, B as the bend area 46 of the bend die. However, the depth of the groove is just less than one-half the outside diameter or 0.910″, as designated at K, and the opening size at the entrance is approximately 0.050″ less than the opening size E of the bend die, as a result of the groove being shallower. The leading edge 63 of the pressure die which is aligned opposite to one side of the bend area 46 of the bend die is given a semi-circular configuration corresponding to that of the tangent sections 48 and 49 but slopes or merges from that semi-circular groove into the multi-radius groove which traverses the remaining length of the pressure die. As earlier described with reference to the bend die, the transition or slope S′ from the semi-circular groove into the multi-radius groove cross-section is preferably on the order of 5°. Both in the case of the bend die and the pressure die, the transition or slope between grooves in each die can be accomplished by grinding along a gradual angle as described so as to avoid corners or irregularities which could cause scarring of the outer surface of the tube, particularly in softer metal, such as copper. In practice, as shown in Figures 1 and 2, the tube T has one end clamped between the bend die 34 and clamping die 30 which is aligned with the semi-circular groove on the bend die leading onto the tangent section 48. The pressure die 60 is disposed in abutting relation to the end of the clamping die 30 such that the leading end 63 has its semi-circular groove section as designated at S′ aligned with the end of the tangential section 48 at its transition into the bend area 46. As described, the depth of the pressure die groove 66 is shallower than that of the bend die groove 46 and 48 so as to move into firm, clamping engagement with the tubing with a slight clearance or gap left between the confronting surfaces of the pressure die and the bend die. The bend die 34 is rotated by the swing arm 22 at a constant rate of speed typically on the order of 5 to 6 rpm. Simultaneously, the pressure die is advanced by the plunger 16 in a linear direction so as to maintain bending pressure on the tube T as the bend die is rotated, for example, from the position illustrated in Figure 2 to that illustrated in Figure 3. Initially, a reduced hydraulic force is applied to the pressure die through a limited range of movement on the order of 5% to 10% of the total range or distance of movement through which the tubing is to be advanced. The hydraulic force of advancement on the pressure die is then increased to minimize the stretching or elongation to the outer wall of the tube engaged by the pressure die 60 as the tube is being bent along the bend area 46 of the bend die 34. As the pressure die 60 approaches the end of its advancement, the hydraulic force is reduced to that of the initial 5%-to-10%. Throughout the bending cycle, clamping pressure exerted by the pressure die and bend die squeezes the tube T between upper and lower entrance edges of the grooves 46 and 66; however, the increased depth of bottom areas H and K of the grooves will accommodate the increase in effective diameter of th tube in a radial direction extending from the bend die axis of rotation at 20. Figure 9 shows the slight squeezing or inward bending of the tube T between the entrances to the grooves, or in a vertical direction as viewed in Figure 9. To compensate for this inward squeezing, the sides of the tube area free to bow or expand outwardly into the bottoms of the grooves. In Figure 10, the tube upon completion of a 180° bend will accumulate slightly increased wall thickness along the inner surface of the bend 80, but the outer wall 82 will stretch and be reduced in wall thickness. The slight expansion of the tube as described in a horizontal direction, Figure 9, will resist any tendency of the tube to buckle or wrinkle. Upon completion of the bending operation, the tube will return to its original diameter, and the tubing tends to spring outwardly to a position less than the degree of bend. For this reason, it is important to form at least one side of the bend die, such as, along the tangential section 49 at an angle greater than 180° to that of the section 48 so that the tubing will be bent through an angle greater than 180° sufficient to compensate for its tendency to spring outwardly to a 180° bend angle, Figure 10. 90° bends may be performed with the bend die and pressure die by controlling the advancement of the dies through 90°, or slightly greater than 90°. Again, the hydraulic advancement of the pressure die 60 with respect to the bend die 34 will cause some increase in thickness of the tube Tl as at 84 and not as much reduction in thickness along the outer wall 86. Bends may be formed at different desired angles by suitable regulation of the bend adjustment control 26 as described. It will therefore be evident that the bend die 34 and pressure die 60 undergo a change in radius in a smooth transition from a semi-circular groove to a multi-radius groove through out the bending area, initially advancing the pressure die at the same rate as the bend die along the straight sectionof the bend die, then increasing the hydraulic force of advancement along the bend area followed by reducing this force at the completion of the bend. This is true whether carrying out 180° bends or less than 180° bends. The factors as indicated in Table I are arrived at by trial and error, once a determination is made that a tube is of a diameter and wall thickness which is susceptible to bending without an internal mandrel in the manner described. This determination may be best made by calculating the wall factor of the tube (WF) as follows: The bend radius (D/B) is determined as follows: Finally, If the (E/B) factor is less than 10, the tube is one that would qualify for bending in the manner described; if (E/B) is greater than 13, generally speaking it may not qualify for tube bending. This is a rule of thumb determination arrived at only to avoid trial and error in determining whether a given size and wall thickness of tube can be bent as described. This determination is influenced too by other factors, such as, composition of material and extreme differences in wall thickness. Various modifications and changes may be made in the preferred method and apparatus of the present invention without departing from the scope of the invention as defined by the appended claims.

Claims: A tube bending machine in which the tube (T) to be bent is clamped between a bend die (34) and a cooperating pressure die (60), the pressue die (60) being advanced in a linear direction as said bend die (34) is rotated, said bend die (34) and pressure die (60) having confronting faces with a tube groove (46,48,49; 68) in each of said faces for embracing diametrically opposed sides of said tube (T) to be bent, characterised in that said bend die is generally U-shaped and said tube groove on said bend die (34) is generally U-shaped having a bend section (46) of generally semi-circular configuration and opposite sides (44) extending in tangential directions substantially parallel to one another to define tangential grooves (48,49) along said opposite sides of said bend die (34), said tangential grooves (48,49) having a cross-sectional radius of curvature substantially corresponding to that of said tube (T) to be bent, said tube groove along said bend section (46) having a bottom surface portion with a cross-sectional radius of curvature (A) less than one-half of the outside diameter of said tube (T) to be bent and opposite side surfaces of said bend section (46) each having a cross- sectional radius of curvature (B) greater than one-half of the outside diameter of said tube (T) to be bent, and said opposite side surfaces undergoing a smooth gradual transition into said bottom surface portion, said tube groove (68) on said pressure die (60) having a bottom surface portion with a cross-sectional radius of curvature (A) less than one half of the outside diameter of said tube (T) to be bent, and opposite side surfaces of said tube groove (68) on said pressure die (60) having a cross-sectional radius of curvature (B) greater than one-half of the outside diameter of said tube (T) to be bent. A tube bending machine according to claim 1, characterised in that means for clamping (30) a leading end of said tube (T) to be bent to said bend die (34) are provided in leading relation to said pressure die (60). A tube bending machine according to claim 1 or claim 2, characterised in that said tube groove (68) on said pressure die (60) has cross-sectional radii of curvature (A,B) corresponding to that of said bend section (46) of said bend die (34). A tube bending machine according to any one of claims 1 to 3 characterised in that the radius of curvature of said tangential grooves on said bend die undergoing a gradual transition into said bend section (46), and the radius of curvature (A) along the bottom surface of said bend section is on the order of 85% of the radius of curvature of said tube (T) to be bent. A tube bending machine according to any one of claims 1 to 4 characterised in that said bend section (46) extends for just greater than 180°, and said tangential grooves (48,49) of said bend die (34) converge away from said bend section (34) for a distance substantially equal to the circumferential length of said bend section (46). A method of bending a tube into a predetermined curvature characterised in that it comprises the steps of positioning the tube (T) against a straight section of a first semi-circular groove (48) in a bend die (34) wherein the first groove (48) has a radius of curvature corresponding to that of the tube (T), bending said tube (T) around a curved section of a second groove in said bend die (34) which forms a continuation of the first groove (48), forcing said tube (T) successively into said first and second grooves (48,46) of said bend die (34) whereby said tube (T) assumes the configuration of said second groove (46) as the tube (T) is bent around said curved section (46) of said bend die (34) wherein said second groove (46) has a bottom surface portion with a cross-sectional radius of curvature (A) less than one-half of the outside diameter of the tube (T) to be bent and opposite side surfaces in said second groove (46) have a cross-sectional radius of curvature (B) greater than one-half of the outside diameter of the tube (T) to be bent, and said tube groove (68) on said pressure die having a bottom surface portion with a cross-sectional radius of curvature (A) less than one half of the outside diameter of said tube (T) to be bent, and opposite side surfaces of said tube groove (68) on said pressure die (60) having a cross-sectional radius of curvature (B) greater than one-half of the outside diameter of said tube (T) to be bent. A method according to claim 6, characterised in that there is provided an initial step of clamping said tube (T) against the straight section (48) of said bend die (34), and rotating said bend die (34) while maintaining said tube (T) in clamped relation to said bend die (34).

Inventors: DEMARTELAERE, GARY E., STANGE, RONALD R.
Applicants: TOOLS FOR BENDING, INC.
Application year: 1988
Main classification: B21D7
IPC codes: B21D7