GB2529844A

GB2529844A - Forming butted tubes

Info

Publication number: GB2529844A
Application number: GB1415595.6A
Authority: GB
Inventors: Keith T Noronha
Original assignee: REYNOLDS TECHNOLOGY Ltd
Current assignee: REYNOLDS TECHNOLOGY Ltd
Priority date: 2014-09-03
Filing date: 2014-09-03
Publication date: 2016-03-09
Anticipated expiration: 2034-09-03
Also published as: GB2529844B; GB201415595D0

Abstract

A method of forming butted tubing comprises drawing a tube through a die 52 held in a holder and over a plug 54 attached to a bar. The die holder and plug bar are each independently moveable in the direction in which the tube is drawn through the die and in the opposite direction. Movement of the holder and bar occurs while the tube is being drawn to vary spacing between them and so vary a wall thickness of the tube. In one aspect the holder is moved in the same direction that the tube is drawn through the die, increasing tube wall thickness. In another aspect, movement of the die holder and of the plug bar are at different speeds in the same direction while the tube is being drawn to vary spacing between the die and plug, so varying a wall thickness. Since the plug and die may be moved in the same direction as the movement of the tube, the large friction forces associated with tube drawing do not have to be overcome to vary the spacing between the die and plug bar which allows for lower powered actuators such as stepper motors (62, 72, Figures 5 and 6).

Description

Forming Butted Tubes

Technical Field

The present invention concerns a method and apparatus for forming butted tubes. More particularly butted metal tubes.

Background

A butted tube has a constant outer diameter along the length of the tube and a varying inner diameter and wall thickness. In particular, though not exclusively, butted tubes may have thicker walls near the tube ends and a thinner wall section along the length between the butted ends. An example of a butted tube 10 is shown in Figure 1. The tube has a length L made up of a thinner wall section C, having wall thickness T2, between thicker wall section butted ends Bi and B2, having thicknesses Ti and T3 and lengths Li and L4 respectively.

There are lengths [2 and [3 having a gradual change of wall thickness between the thinner wall length C and each of the thicker wall butted ends Bi and B2. Some butted tubes may have one or more intermediate thicker wall sections at locations along the length of the tube.

Butted tubes are typically used in applications where additional strength and material thickness is required for making fabricated joints, e.g. by welding, but where a thinner wall section has sufficient strength for most of the length in between, and where a lighter weight of tube is advantageous. A good example of where butted tubes are used is in the fabrication of bicycle frames.

Tube drawing is a well-known and long used technique for forming of metal tubes. Figure 2 illustrates the principal forming components of a tube drawing machine 20 that include a die 22 and a plug 24, which is supported on the end of a plug bar 25. A larger diameter tube 26 is drawn down to a thinner walled tube 28 by drawing the tube through the die 22 and over the plug 24, as shown. This may involve pulling the tube from the front end, or (as in the case of drawing over a mandrel as described below), pushing the tube/mandiel through the die. The drawing process involves the plastic deformation of the tube metal.

Known methods of forming butted tubes involve the use of a shaped mandrel. The mandrel is formed of a metal material having suitable properties (i.e. strength and hardness) with an outer surface piofile that mirrors the inner tube surface profile of the butted tube to be formed. Figures 3a to 3e illustrate the steps in the forming process. First, as shown in Figure 3a, a mandrel 30 is placed inside a length of tube 32 of the material to be formed into the butted tube, but having larger internal and external diameters than the finished butted 1.

tube. The combination of tube 32 and mandrel 30 is then drawn through a die 34, as shown in figure 3b, so that the inner surface of the tube 32 is drawn down onto the mandrel 30 and adopts an inner surface profile of the same shape as the profile of the mandrel 30, as shown in figure 3c, with butted ends 36a, 36b. Next, as shown in Figure 3d, the mandrel 30 is extracted from inside the tube 32 through one end 38 of the tube 32. Because the mandrel is effectively trapped inside the tube 32 in the condition shown in figure 3c, the extraction of the mandrel 30 out of the tube 32 forces the thicker-walled butted section 36a at the end 38 to be pushed outwards thereby increasing the outer diameter of the tube at the end 38, as shown in figure 3d. In the final stage, as shown in Figure 3e, the tube is drawn through a die (but without any plug as shown in Figure 1), to return the tube 32 to one having a constant outer diameter. In doing this the thicker wall sections of the butted end 36a are compressed inwards such that the inner wall profile of the tube adopts the butted tube profile of the mandrel 30.

It will be appreciated that the method described above with reference to figures 3a to 3e can only produce a butted tube having the particular dimensions set by the mandrel. The mandrel itself must be made of special materials and machined to precise dimensions. Also the method involves three separate drawing operations.

The methods and apparatus described below have been conceived with the foregoing in mind.

Summary

According to a first aspect of the present invention there is provided a method of forming butted tubing. The method comprises drawing a tube through a die and over a plug, the die being held in a die holder and the plug being aftached to a plug bar. The die holder and plug bar are each independently moveable in the direction in which the tube is drawn through the die and in the opposite direction. The method further comprises controlling movement of the die holder and of the plug bar while the tube is being drawn to vary a spacing between the die and plug and thereby vary a wall thickness of the drawn tube.

The die holder may be moved in the same direction that the tube is drawn through the die in order to increase the thickness of the tube wall. The plug bar is moved in an opposite direction to that in which the tube is drawn, or in the same direction but at a lower speed than the die holder.

The plug bar may be moved in the same direction that the tube is drawn through the die in order to decrease the thickness of the tube wall. The die holder may be moved in an opposite direction to that in which the tube is drawn, in order to decrease the thickness of the tube wall.

It is an advantage that in having both a moving die holder and a moving rear plug bar mounting it is possible to use the frictional forces that arise in the process to best effect, and to achieve a more consistent wall thickness change than would not otherwise be possible with a fixed die holder.

The die holder may be moved by action of a motor controlled by a controller such as a programmable logic controller (PLC) or a computer. The plug bar is moved by action of a motor, which is also controlled by a controller (e.g. PLC or computer). The speed and distance of movement of the die holder and/or of the plug bar may be controlled by the controller based on a speed at which the tube is being drawn to obtain a consistent wall thickness profile for a given tube diameter and material. It is an advantage that, by combining the relative movements of the die holder and plugbar, it is possible to increase the accuracy and consistency of the operation, or vary the rate of wall thickness change for the tube without the need for a fixed and defined mandrel.

The controller may be programmed to control movement of the die holder and/or of the plug bar in accordance with a predetermined sequence of movements. Alternatively, or additionally, the controller may be programmed to actively adjust a speed and distance of movement of the die holder and/or of the plug bar in response to signals received from sensors and/or relating to parameters of the tube drawing process. The sensors may include one or more of: strain gauges mounted on one or both the front die holder and rear plug bar, position sensors on the die holder and/or plug bar, tube positioning sensors, and speed sensors monitoring physical movement of the drawn tube.

According to a second aspect of the invention there is provided an apparatus for forming a butted tube. The apparatus comprises a die for drawing the tube through; a die holder supporting the die and moveable in the direction in which the tube is drawn through the die and in the opposite direction; a plug attached to a plug bar, wherein the plug bar is moveable in the direction in which the tube is drawn through the die and in the opposite direction; and a controller controlling movement of the die holder and the plug bar while the tube is being drawn to vary a spacing between the die and plug and thereby vary a wall thickness of the drawn tube.

The apparatus may further comprise a linear actuation mechanism for providing movement of the die holder. The apparatus may further comprise a linear actuation mechanism or providing movement of the plug bar. The linear actuation mechanism or mechanisms may comprise a motor providing a rotational drive and a mechanism to transfer the rotational drive into a linear movement. The motor may be a stepper motor.

The controller may comprise a programmable logic controller or a computer. The controller may be programmed to control movement of the die holder and/or of the plug bar in accordance with a predetermined sequence of movements.

The apparatus may further comprise one or more sensors, wherein the controller is programmed to actively adjust a speed and distance of movement of the die holder and/or of the plug bar in response to signals received from sensors. The sensors may comprise one or more sensors for detecting a physical dimension of a tube being drawn. The sensors may comprise one or more sensors for detecting a level of force exerted on the plug and/or on the die.

Embodiments of the invention have the advantage that a butted tube can be produced using a single drawings operation. In addition, the butted tube may be of any length that can be accommodated on a tube drawing machine equipped to carry out the process. A further advantage is that no mandrel is required, thereby making a significant saving in terms of the cost and materials required to form a mandrel.

Other benefits and advantages may become apparent from consideration of the present

disclosure.

Brief Description of the Drawings

Figure 1 is an illustration of a butted tube.

Figure 2 is an illustration showing the principles of a tube drawing operation.

Figures 3a to 3c 2 illustrate the stages in a known process for forming a butted tube.

Figure 4 is an illustration of an apparatus for forming butted tubes in accordance with an embodiment of the invention.

Figure 5 is an enlarged view of a moveable die holder and die forming part of the apparatus of figure 4.

Figure 6 is an enlarged view of a moveable plug bar mounting forming part of the apparatus of figure 4.

Figures 7a and 7b illustrate two relative positions of a plug and die that provide different wall thicknesses of a drawn tube.

Detailed Description

Figure 4 illustrates an embodiment of an apparatus in the form of a tube drawing machine 40. The tube drawing machine 40 has a "back" section 42 and a front" section 44. A plug bar (not shown) is mounted at the rear end 48 of the back section 42 and extends along the length of the back section 42. The plug bar and plug bar mounting have been omitted from the drawing of Figure 4 or clarity, but these are described in more detail below with reference to Figure 6. A counterweight disc 49 is used to prevent sudden changes in the large forces that are exerted on the plug bar. A plug (also not shown) is attached to the other end of the plug bar and is located adjacent and in-line with a die 52 mounted in a die-holder 54. The front section 44 of the tube drawing machine 40 extends from the other side of the die holder 54 to the front end of the machine. The draw bench 40 includes rails 57, 58, to which supports (not shown) can be mounted to support a tube that is being drawn through the die 52. Note that the front section 44 is longer than the back section 42 because as a tube is drawn through the die to reduce the tube diameter the length of the tube increases.

In the apparatus shown in Figure 4, the die holder 54 is mounted on a carriage 56. This is shown in more detail in Figure 5. The die holder 54 is moveable backwards or forwards in a direction along the length of the machine. The carriage 56 is mounted to one side (in this case to the rear) of a buffer plate 59a, which is held against buffers 59 that are fixed to the machine 40. An actuating mechanism 60 is used to provide linear movement of the die holder 54 towards or away from the buffer plate 59a. In the example shown, the linear actuation is provided by a pair of screw-threaded shafts 60a, 60b are held against the buffer plate 59a and pass through threaded nuts (not visible) which are fixed to the die holder 54.

A motor 62, such as a stepper motor, drives an actuating shaft 64, rotation of which actuates rotation of the screw threaded shafts 60a, 60b to move the die holder 54 towards or away from the buffers 58 depending on the direction of rotation. The die holder 54 is attached to the carriage 56, which is supported on tracks 65 that allow movement of the carriage in the forwards and backwards directions. Other actuation mechanisms may be used to provide linear movement of the die holder 54 towards and away from the buffer plate 59a.

Figure 6 shows the plug bar mounting 48 at the back end of the machine 40 of figure 4. A linear actuation mechanism is used to provide linear movement of the plug bar towards and away from the die holder 54 (see figure 4). The linear actuation mechanism shown includes a bearing 66 fixed to the back end of the plug bar 46 by way of a coupling 67. The bearing 66 engages an end of a rotatable shaft 68. The shaft 68 passes through a female threaded bore of a cylindrical member 70, which is fixed to the plug bar mounting 48. The rear end of the shaft 68 is coupled to a rotational drive, which in this example is a motor 72, such as a stepper motor. The motor 72 applies rotation to the shaft 68, which causes the plug bar 46 to move axially towards or away from the mounting 48, depending on the direction of rotation.

In use, a tube to be drawn is placed on the back section 42 with the plug bar passing along the inside of the tube. The end of the tube is offered up to the die 52 and the plug 54 is advanced into the throat of the die. The tube is then drawn by being forced through the die 52 in similar manner to the conventional tube drawing process described above with reference to figure 2. The outer diameter of the tube is determined by the inner diameter of the throat of the die, and this does not change for the drawing operation. However, the inner diameter of the tube, and thus the wall thickness, are determined by the position of the plug relative to the throat of the die. As shown in figure 7a, where the plug 54 is held in a retracted position, the wall thickness of the tube is only reduced by a small amount from that of the undrawn tube 70a and the drawn tube 7Db, meaning that inner diameter of the drawn tube is relatively small. As shown in figure 7b, if the plug 54 is advanced further into the throat of the die 52 there is a greater degree of reduction of wall thickness, leading to a drawn tube 70c having a relatively larger internal diameter. Thus, by moving the relative positions of the plug 54 and the die 52 during the drawing operation, the wall thickness can be varied along the length of the drawn tube. As a result a butted tube of any specified length and required wall thickness dimensions can be formed in a single drawing operation, and one that does not require the use of a mandrel.

An important consideration in making this principle work is in the control of movement of the die holder 52 and the plug 54. In the embodiment shown this is achieved by control of the motors 62, 72 as shown in figures 5 and 6. The drawing process generates large frictional forces on the die and plug and these need to be allowed for in actuating the movement of one or the other of the die holder 52 and plug 54. The frictional forces act in the direction in which the tube is being drawn and tend to pull the plug 54 into the throat of the die 52.

These forces also produce a large tensile load in the plug bar 46, resulting in a corresponding strain extension of the plug bar that needs to be controlled. For example, moving the plug away from the die in order to produce an increase in wall thickness, requires movement in the opposite direction to the frictional force caused by the tube being drawn over the plug. This means that the frictional force has first to be overcome before any movement of the plug takes place. Overcoming or breaking' the friction can be unpredictable, making it difficult or impossible to accurately control the change in wall thickness. The material of the tube being drawn, and also of the die and plug, affect the frictional forces, and therefore also the control of movement of the die and plug.

The apparatus and methods described herein involve movement of the die holder instead of, or in addition to, movement of the plug. Moving the die holder so that the die is moved in the forward direction, away from the plug, in order to produce an increase in the wall thickness has the advantage that the frictional forces are acting in the same direction as the direction of movement of the tube. This makes it easier to control the amount of movement required and does not require breaking of the friction. Reducing the wall thickness involves moving the die holder and die in the backwards direction towards the plug. As this will involve an increase in the friction on the plug and die, the movement can be relatively easily controlled.

Another factor that needs to be taken into consideration is that simply moving the plug away from the die, even only an initial small distance, will reduce the friction on the plug, resulting in a reduction in the tensile load in the plug bar and a corresponding reduction in strain/elongation. In other words the plug will move further away from the die than the initial small distance, due to the reduction in the friction. One way to counteract this is to control movement of both the die holder and the plug at the same time. Another possibility is to move both the die holder and the plug in the same direction, but at different speeds.

Accordingly, the table below shows the options for increasing and decreasing wall thickness.

To increase wall thickness To decrease wall thickness Movement of die Movement of plug Movement of die Movement of plug holder holder none backwards none forwards forwards backwards backwards forwards forwards none backwards none forwards Forwards at lower backwards Backwards at lower speed speed The control of movement of the die and plug is performed by the motor controller or controllers. In some cases separate controllers may be employed for each of the motors 62, 72, although both may be controlled by the same controller or a computer. In some cases the controllers may be pre-programmed simply to control the motors to actuate movement in accordance with a predetermined sequence. However, in other cases, sensors may be employed on the machine to measure any of various physical parameters, such as dimensions of the tube as it is drawn or the forces acting on the plug and die. Signals from these sensors may be fed back to the controller, which may be programmed to actively adjust the speeds of the motors to compensate for any measured variations so that the dimensions of the drawn tube are kept within certain tolerances. Examples of the sensors that may be employed include: strain gauges mounted on the front die holder and/or rear plug bar, position sensors on the die holder and/or plug bar, tube positioning sensors, and speed sensors monitoring physical movement of the drawn tube. In addition parameters such as the current drawn by the drawbench motor (used to pull the drawn tube through the die) may be fed back to the controller and used for controlling the process.

Claims

Claims 1. A method of forming a butted tube, comprising: drawing a tube through a die and over a plug, the die being held in a die holder and the plug being attached to a plug bar, wherein the die holder and plug bar are each independently moveable in the direction in which the tube is drawn through the die and in the opposite direction; controlling movement of the die holder and of the plug bar while the tube is being drawn to vary a spacing between the die and plug and thereby vary a wall thickness of the drawn tube.
2. The method of claim 1 wherein the die holder is moved in the same direction that the tube is drawn through the die in order to increase the thickness of the tube wall.
3. The method of claim 2 wherein the plug bar is moved in an opposite direction to that in which the tube is drawn, or in the same direction but at a lower speed than the die holder.
4. The method of any preceding claim wherein the plug bar is moved in the same direction that the tube is drawn through the die in order to decrease the thickness of the tube wall.
5. The method of any preceding claim wherein the die holder is moved in an opposite direction to that in which the tube is drawn, in order to decrease the thickness of the tube wall.
6. The method of any preceding claim wherein the die holder is moved by action of a motor controlled by a controller.
7. The method of any preceding claim wherein the plug bar is moved by action of a motor controlled by a controller.
8. The method of claim 6 or claim 7, wherein a speed and a distance of movement of the die holder and/or of the plug bar is controlled by the controller based on a speed at which the tube is being drawn to obtain a wall thickness profile for a given tube diameter and material.
9. The method of any of claims 6 to 8 wherein the controller is programmed to control movement of the die holder and/or of the plug bar in accordance with a predetermined sequence of movements.
10. The method of any of claims 6 to 8, wherein the controller is programmed to actively adjust a speed and distance of movement of the die holder and/or of the plug bar in response to signals received from sensors and/or relating to parameters of the tube drawing process.
11. Apparatus for forming a butted tube, comprising: a die for drawing the tube through; a die holder supporting the die and moveable in the direction in which the tube is drawn through the die and in the opposite direction; a plug attached to a plug bar, wherein the plug bar is moveable in the direction in which the tube is drawn through the die and in the opposite direction; and a controller controlling movement of the die holder and the plug bar while the tube is being drawn to vary a spacing between the die and plug and thereby vary a wall thickness of the drawn tube.
12. The apparatus of claim 11 further comprising a linear actuation mechanism for providing movement of the die holder.
13. The apparatus of claim 11 or claim 12 further comprising a linear actuation mechanism or providing movement of the plug bar.
14. The apparatus of claim 12 or claim 13, wherein the linear actuation mechanism or mechanisms comprise a motor providing a rotational drive and a mechanism to transfer the rotational drive into a linear movement.
15. The apparatus of claim 14 wherein the motor is a stepper motor.
16. The apparatus of any of claims 11 to 15, wherein the controller comprises a programmable logic controller or a computer.
17. The apparatus of any of claims 11 to 16 wherein the controller is programmed to control movement of the die holder and/or of the plug bar in accordance with a predetermined sequence of movements.
18. The apparatus of any of claims 11 to 16, further comprising one or more sensors, wherein the controller is programmed to actively adjust a speed and distance of movement of the die holder and/or of the plug bar in response to signals received from sensors.
19. The apparatus of claim 18, wherein the sensors comprise one or more sensors for detecting a physical dimension of a tube being drawn.
20. The apparatus of claim 18 or claim 19, wherein the sensors comprise one or more sensors for detecting a level of force exerted on the plug and/or on the die.
21. The apparatus of claim 18 wherein the sensors include one or more of: strain gauges mounted on one or both the front die holder and rear plug bar, position sensors on the die holder and/or plug bar, tube positioning sensors, and speed sensors monitoring physical movement of the drawn tube.
22. Apparatus substantially as hereinbefore described with reference to the drawings of Figures 4 to6.Amendments to the Claims have been filed as follows Claims 1.. .A method of forming a butted tube, comprising: drawing a tube through a die and over a plug, the die being held in a die holder and the plug being attached to a plug bar,. wherSn the die holder and plUg bar are each noependently moveable n the direction in which the tube is dawn through the die and in the opposite direction; controlling movement of the die holder and of the piijg bar while the tube is being drawn to vary a spacing between the die and plug and thereby vary a wall thickness of the drewn.tube, wherein the die holder is moved hi the same direction that the tube is drawn through the die in order to increase the tNckness of the tube walk 2 The method of clam I whei en the plug bai is moved n an opposite direct on to that in which the tube is drawn., or in the same direction but at.a lowEr speed than the die holder, IC) 3. The method of any preceding claim wherein the plug bar is moved in the same direction that the tube is drawn through the: die in order to decrease the r thickness of the tube wall ci) 4. The method of any preceding daim whetn the die. hoider s moved in an opposite direction to that in which the tube is drawn, n order to decrease the thickness of the tube wall.5. The method of any preceding claim wherein the die holder is moved by action of a motor controlled by a controller.5. The method of any preceding claim wherein the plug bar is moved by action of a motor controlled by a controller.7. The method of claimS or claim 6, wherein a speed and a distance of movement of the die holder and/or of the plug bar is controlled by the controller based on a speed t which the tube is being drawn to obtain a wall thickness profUé for a given tube diameter and material, 8. The method of:any of Sims 5 to 7 wherein the controllers programmed to control movement of the. die holder and/or of the plug bar in accordance with a predetermined sequence of movements.9 The method of any of clams 5 to 7, wherein the contro'ler is programmed to actively adjust a speed and distance of movement of the die mlder and/or of the plug bar in response to signals received from sensors. and/Or relating to parameters of the.tube drawing process.10.. Apparatus for forming a butted tube, comprising: a die for drawing the tube through; a die holder supporting the die and moveable in the drection in which the tube is drawn through the die and En the opposite direction; a plug attached to a plug bar, wherein the plug bar is rnoveab'e in the direction in which the tube is drawn through the die and in the opposite direction, and UD a controller controlhng movement of the die holder and the plug bar while the tube is being drawn to vary a spacing between the die ano plug and thereby vary a wall thickness of the drawn tube, wherein the controller is configured to move the die r holder h the same direction that the tube is drawn through the dis in order to increase the thickness of th.e tube wall. ci)ii The apparatus of ck:im 10, wherein the contmfler is further configured to move the die holder and of the plug bar at different speeds in the same directicn while tne tube is being drawn to vary a spacing between the die and plug and thereby vary a wall thickness of the drawn tube.12. the apparatus of claim. 10 or.daim 1.1 further comprising a linear actuation mechanism for providing movement of the die holder.13. The apparatus of any of claims 10 to 12 further comprising, a linear actuation mechanism or providing movement of the plug bar.14. The apparatus of claim 12 or claim 13, wherein the linear actuation mechanism or mechanisms comprise a motor providing a rotational drive and a rrechanism to transfer the rotational drive into a linearmovemnent.15. The apparatus of claim 14 wherein the motor is a stepper motor.16... The apparatus of. any of daims 10 to 15, wherein the. controller comprises a programmable log it controller or.a computer.17. The apparatus f any of claims 10 to 16 wherein the controller is p rogrammed to contro] movement of the die hOlder and/or of the plug bar in accordance with a predetermined sequence of movements..16. The apparatus of any of daims 10 to 16, further comprising one or more sensors, wherein the controller i$ p:ro:gramrned to actively adjust a speed and distance of movement of the the holder andlot of the plug bai n response to sqnals received from sensors.19. The apparatus pfclaim IS, wherein the. sensors comprise one or more sensors for detecting a. physical dimension of atube being drawn.20. The apparatus of claim 18 or claim 19, wherein the sensors comprise one or more sensors for detecting a level of force exerted on the plug and/cr on the die, r r 21. Th apparatus. of claim 1.8 wherein the sehSors include one or more of: strain o gauges mounted on one or both the front die holder and rear plug bar, position sensors on the die holder and/or plug bar, tube positioning sensors, and speed sensors monitoring physical movement of the drawn tube.22. Apparatua substantially as hereinbefore described with reference to the drawings of Fiçures 4 to E.