WO2017021671A1

WO2017021671A1 - Ultrasonically vibrated die and method of its operation

Info

Publication number: WO2017021671A1
Application number: PCT/GB2015/052227
Authority: WO
Inventors: Lara BUSTO; Alejandro CADARSO; Miles ASHCROFT
Original assignee: Magnaparva Packaging Limited
Priority date: 2015-07-31
Filing date: 2015-07-31
Publication date: 2017-02-09

Abstract

An ultrasonically vibrated die (1) for forming tubular metal workpieces comprises a die ring (2) supported coaxially on a resonant tube (4). A transducer (16) applies ultrasonic vibrations to the die ring (2). The resonant tube (4) is mounted on a forming machine via a radial flange (6). The location of the mounting flange (6) along the resonant tube (4) is chosen to be at an antinode, where the amplitude of the radial vibration of the tube (4) is at a local maximum. This avoids axial and twisting movement of the flange (6), which would tend to couple the vibrations of the die (1) to the machine.

Description

TITLE

ULTRASONICALLY VIBRATED DIE AND METHOD OF ITS OPERATION

DESCRIPTION

Technical field

The invention relates to an apparatus and process for forming metal workpieces by driving the workpieces into a die. It has particular application to annular workpieces that have circular symmetry about the axis of movement, whereby the forming process changes the longitudinal profile of the workpiece, for example to form a neck of reduced radius and predetermined shape.

Background of the invention

It has long been known to change the longitudinal profile of an annular or tubular workpiece by driving the workpiece along its axis of symmetry into a die of suitable shape to form the desired profile - or into a succession of dies that are respectively shaped to create the desired profile in a sequence of smaller steps. It is also known that vibrating the die at ultrasonic frequencies can assist the forming process by reducing the friction between the die and the workpiece and/or by enhancing the way the working surface of the die acts on the workpiece to deform it.

US patent 4,854, 149 (Porucznik et al.) illustrates examples of such an ultrasonically- assisted forming process. The end of the workpiece to be formed is inserted coaxially into the profiled aperture of a die ring. A transducer is attached to the die ring at a location on its circumference and delivers ultrasonic energy into the die ring. The transducer vibrates along its own longitudinal axis, which is aligned with a radius of the die ring. The radial application of ultrasonic vibrations to the die ring induces resonant modes of vibration, depending on the shape and material of the die ring and the frequency applied. The preferred mode of vibration is the fundamental mode, which has circular symmetry about the axis of the die ring, whereby the contact between the die ring and the workpiece is ideally synchronized at all points around the circumference. US patent 4,854, 149 also discloses the mounting of the die ring on a forming machine via a mounting tube that is coaxial with the die ring. A circumferential flange midway along the mounting tube provides means for clamping the mounting tube and supporting the die ring. Vibration of the die ring induces vibrations in the mounting tube, at least the fundamental frequency of which forms a standing wave along the length of the tube, with an axial pattern of alternate nodes where the radial vibration of the tube is a local minimum and antinodes where the radial vibration of the tube is a local maximum. The patent teaches that the mounting flange of the mounting tube should be located at one of the nodes, so that it can be firmly clamped where the radial vibration is at a minimum. It also teaches that the mounting tube should be connected to the die ring at one of the antinodes.

The present inventors have found that mounting the die ring as taught in patent US 4,854,149 gives rise to certain disadvantages. In particular, the mounting fails to isolate the die effectively from the supporting structure of the forming machine. As a result, vibrational energy is lost through the mounting flange so that the use of the ultrasonic transducer becomes inefficient. Also, because of the coupling between the mounting flange and the supporting structure, the structure has an undesired effect on the vibrational modes of the die and the mounting tube. This has adverse consequences for retro-fitting the ultrasonically-assisted die to existing forming machines, which might have variable or unknown vibrational characteristics.

Summary of the invention

The invention provides an ultrasonically vibrated die, comprising:

a generally cylindrical die ring defining an axis;

a transducer interface on the die ring, whereby ultrasonic vibration can be imparted to the die at a predetermined frequency;

a resonant tube coaxial with the die ring and extending from the die ring along an axial direction; and

mounting means on the resonant tube;

wherein: the shape and material of the die are such that, when the die is mounted via the mounting means and vibrated at the predetermined frequency, the die ring and the resonant tube each vibrate in a substantially axisymmetric mode; and

the axial position of the mounting means on the resonant tube is substantially at a local maximum, with respect to distance along the axis, of the radial amplitude of the axisymmetric mode of vibration of the resonant tube.

The invention also provides a metal forming machine comprising such a die supported on the machine via the mounting means; and an ultrasonic transducer attached to the transducer interface of the die for imparting ultrasonic vibration to the die at the predetermined frequency.

The invention further provides a method of operating such a die, comprising supporting the die via the mounting means; and imparting ultrasonic vibration at the predetermined frequency to the transducer interface, thereby causing the die ring and the resonant tube to vibrate in a substantially axisymmetric mode, such that the axial position of the mounting means on the resonant tube is substantially at a local maximum, with respect to distance along the axis, of the radial amplitude of the axisymmetric mode of vibration of the resonant tube.

In this specification, the term "axisymmetric" is used to describe a mode of vibration that has circular symmetry about the axis of the part concerned, so that at any instant all points on a given circumference are moving in the same way relative to the axis. It does not imply translational symmetry along the axis.

Figure 1 illustrates what is understood to be the cause of the problems associated with mounting the die at a node in the prior art. The figure shows the lower half of a computer model of a die 1, seen in side view. A die ring 2 is supported coaxially at one end of a resonant mounting tube 4. Midway along the resonant tube 4 is a mounting flange 6 that projects radially outwards from the tube around its circumference. In Figure 1 square teeth are shown around the circumference of the flange but they are not relevant to the present discussion. An axisymmetric mode of vibration of the die 1 is illustrated with greatly exaggerated amplitude. Figure 1(a) shows one extreme of the vibrational motion and Figure 1(b) shows the other extreme, half a cycle later. It can be seen that a standing wave is set up along the length of the resonant tube, with the surface of the tube oscillating substantially radially to different extents depending on the axial position of the point in question. At some axial positions, such as the free end 8 of the tube 4 remote from the die 2, there are antinodes (A) where the surface of the tube forms peaks and troughs and the radial amplitude of the motion is at a local maximum. At axial positions between the antinodes, there are nodes (N) where the radial amplitude of the motion is substantially zero.

The die 1 is mounted by clamping the mounting flange 6 to a supporting structure (not shown) and it seems natural, as was done in the prior art, to locate the mounting flange at the axial position of a node so that the radial movement of the flange at the mounting location will be minimized. However, the present inventors have observed that at a radial node, the "twisting" movement of the surface is at its greatest as the die 1 vibrates. In Figure 1(a), as a peak forms on one side of the flange 6 and a trough forms on the other side, the disc-like flange is deformed into a cone pointing towards the free end 8 of the tube 4. In Figure 1 (b) the positions of the peak and trough are reversed and the flange is deformed the other way into a cone pointing towards the die ring 2. This twisting movement of the flange is believed to be the reason why the prior art clamping led to undesirable vibrational coupling between the flange and the support structure. Figure 2 illustrates the solution provided by the present invention, namely to locate the mounting flange 6 at an antinode of the resonant tube 4. Then, although the radial movement of the flange 6 is maximized, its twisting movement is effectively zero so the movement of the flange becomes purely a radial expansion and contraction. This is relatively easy to accommodate by clamping the flange 6 between a pair of rings of the supporting structure (not shown in Figure 2) so that the mating surfaces of the flange and the rings lie parallel to the direction of the radial movement and that movement is not strongly coupled into the supporting structure. Locating the flange 6 at an antinode may provide other advantages. For example, if the axial length of the resonant tube 4 is limited by external considerations such as the need to retro-fit in the space provided by an existing forming machine, then the option of locating the flange at an antinode may allow a more compact design to be adopted. Such a design may also prove more resistant to fatigue.

The vibrational mode of the resonant tube 4 may not be purely radial, as illustrated in Figures 1 and 2, but may also include an axial component. In that case it is preferred that, at the axial position of the mounting means, the amplitude of the axial movement should be designed to be at a local minimum with respect to distance along the axis.

The resonant tube may further comprise slots or pits distributed around a circumference of the tube between the die ring and the mounting means. By forming slots (i.e. through holes) or pits (blind holes) in the tube, mass can be removed and/or the stiffness of the tube can be reduced in ways that are useful for controlling the pattern of vibration in the resonant mode.

A die according to the invention will typically be designed using a computer model, using tools such as finite element methods. The die is initially modelled in free vibration, meaning that its modes of vibration are not constrained by any support. The shape, dimensions and material properties of this initial model can be adjusted to produce the desired mode axisymmetric mode of vibration of the die ring and the resonant tube, at the desired frequency (preferably the frequency of a commercially available ultrasonic transducer) and subject to any further conditions such as the envelope within which the die has to fit on a pre-existing machine. This model can be used to determine the positions of the nodes and antinodes along the resonant tube in the axisymmetric mode so that the designer can select an antinode to be the axial position of the mounting means (which is also included in the model).

In a second design stage, the model then needs to be revised to take into account the mounting of the die via the mounting means. The revised model includes a boundary condition to represent the constraint applied by the mounting, which can slightly change the pattern and frequency of vibration of the die. For increased accuracy, the revised model may include details of further components of the mounting and of the cartridge of the forming machine to which the die is attached. With the revised model, the shape of the die can be adjusted as desired to ensure that the die vibrates at the desired frequency and in the desired mode when supported via the mounting means.

Although the pattern of vibration changes slightly in the revised model that incorporates the mounting, and the axial position of the mounting means may be one of the parameters that is adjusted to take account of this, nevertheless these changes are small and do not fundamentally change the relationship between the mounting means and the position of the associated antinode. Thus, for any given die, it is possible to model it in free vibration - or to test it in conditions approximating free vibration - in order to determine the frequency at which it vibrates in an axisymmetric mode and to identify whether the axial position of the mounting means substantially coincides with an antinode of the resonance tube in accordance with the present invention. For this determination it is not necessary to know the details of the mounting. Alternatively, based on the mounting means that are present on the tube, the skilled person can make reasonable assumptions about the manner in which the die is intended to be mounted and can test the die while applying the appropriate boundary conditions.

Drawings

Figure 1 is a side view of a computer model of a vibrating die with a mounting flange located in accordance with the prior art.

Figure 2 is a side view of a computer model of a vibrating die with a mounting flange located in accordance with the present invention.

Figure 3 is a perspective view of a die according to a first embodiment of the present invention.

Figure 4 is a perspective view of a locking nut for use with the die of Figure 4.

Figure 5 is a perspective view of a toothed washer for use with the die of Figure 4.

Figure 6 is a sectional, perspective view of the die of Figure 3 when mounted in a forming machine using the components of Figures 4 and 5. Figure 7 is a perspective view, similar to Figure 3, of a die according to a second embodiment of the present invention.

Description of the preferred embodiment

Figure 3 shows a die 1 according to a preferred embodiment of the present invention, with its axis orientated vertically. Figure 6 shows the same die 1 when mounted in a forming machine.

The die 1 is integrally formed and comprises a die ring 2 and a resonance tube 4 extending from the die ring 2 and coaxial with it. Midway along the resonance tube 4 between the die ring 2 and a free end of the tube 8 is a radially projecting, circumferential mounting flange 6. The position of the flange 6 along the tube 4 is chosen as described above such that, when the die 1 is vibrated freely at a predetermined frequency, the axial position of the flange 6 is at an antinode where the radial amplitude of the vibration is at a local maximum with respect to distance along the axis but any axial or twisting movement is at a minimum.

The die ring 2 has a central aperture for receiving a cylindrical workpiece (not shown). Within the aperture is a working surface 10, which is profiled to form the workpiece into a desired shape as it is driven into the aperture. The die ring 2 has an outer surface 12 that is generally cylindrical except for a transducer interface 14 at one circumferential position. The transducer interface 14 provides a flat surface against which an ultrasonic transducer 16 can be brought to bear for transmitting ultrasonic vibrations into the die ring 2. The interface 14 may be provided with a threaded bore 18 for receiving a stud 20 that can be used to couple the transducer 16 to the interface 14.

The purpose of the resonant tube 4 is to support the die ring 2, while allowing it to vibrate as freely as possible in the desired resonant mode induced by the ultrasonic transducer 16 applied to the interface 14. Accordingly, the tube 4 preferably has low stiffness to reduce the coupling of vibrations from the die ring 2 into the tube 4 which can cause a loss of vibrational energy and adversely affect the desired vibration of the die ring 2. On the other hand, the tube 4 must be able to withstand the large axial forces exerted on it when a workpiece is driven into the aperture of the die ring 2. The resonant tube 4 comprises a thin, cylindrical wall having a smaller external radius than that of the die ring 2. The thinness of the wall makes the tube 4 much less stiff than the die ring 2 but the stiffness can be reduced further, while maintaining axial strength, by forming longitudinal slots 22 in the portion of the tube between the die ring 2 and the flange 6. The tail 24 of the resonant tube 4, between the mounting flange 6 and the free end 8, has a less significant role to play and is shown as a simple cylinder. However, its thickness and/or stiffness can be varied as necessary to determine the pattern of resonance along the tube and the axial position of the flange 6, within any overall length constraints for the die 1.

In the present invention the die ring 2 and the resonant tube 4 are preferably formed as a single, integral component. The material of the die 1 must be chosen to satisfy the various demands on the component, including good acoustical properties at the predetermined frequency, a suitable density and stiffness for the desired modes of vibration, sufficient strength to resist the forces applied to it and sufficient wear resistance at the working surface 10. Tool steel is commonly used for metal -forming dies and may be suitable for the present invention if its poor acoustic properties can be overcome. Titanium and aluminium have good acoustic properties but would require treatment or coating at the working surface to improve their wear resistance. It is possible to use inserts in the die ring to provide a hard-wearing working surface but they are not preferred because the change in acoustic properties at the material boundary can cause poor transmission of the ultrasonic vibrations and a build-up of heat. The die 1 may be designed using computer modelling (e.g. finite element analysis) to have the desired modes of vibration in the die ring 2 and the resonant tube 4 at the predetermined frequency. Preferably the frequency should be one that can be generated by readily available ultrasonic transducers, e.g. in the range 20 to 40 kHz. Higher frequencies are likely to permit the use of smaller dies if the space in which they are to operate is constrained. The design should encourage mode isolation, i.e. it should avoid the creation of alternative, undesired modes of vibration at frequencies close to that applied by the transducer 16. Figures 4 to 6 illustrate how the die 1 of Figure 3 may be mounted in a cartridge 26 of a forming machine (not shown). The cartridge 26 has a cylindrical aperture that receives the tail 24 of the resonant tube 4. The mounting flange 6 butts against an annular end wall 28 of the cartridge 26. In the prior art, the flange 6 would normally be locked in this position using a locking ring (not shown) that slides over the die 1 from the lower end (as viewed in Figure 6) and screws onto a thread 30 on the outer wall of the cartridge 26. In the present embodiment of the invention, the outer radius of the die ring 2 has been enlarged to ensure that the desired axisymmetric mode of vibration occurs at the predetermined frequency with the result that the radius of the die ring 2 is larger than that of the mounting flange 6 and a locking ring that will pass over the die ring 2 cannot act against the flange 6. This further problem has been solved with the use of a locking nut 32 and toothed washer 34 as shown in Figures 4 and 5 respectively.

The locking nut 32 comprises a generally cylindrical wall 36 and the toothed washer 34 just fits inside this wall. A flange 38 projects inwards from the cylindrical wall 36 at the lower end of the locking nut 32 to form a seat for the toothed washer 34. The inner diameter of the flange 38 is sufficiently large that the locking nut 32 can just pass over the mounting flange 6 of the die 1. The mounting flange 6 comprises outwards-facing teeth 40 disposed around its circumference, with gaps between them. The toothed washer 34 comprises an equal number of inwards-facing teeth 42 disposed around its circumference, with gaps between them. The respective radii of the flange 6 and the washer 34 are such that the washer 34 can pass over the flange 6 when the teeth 42 of the washer 34 are aligned with the gaps of the flange 6; but the washer 34 cannot pass over the flange 6 when the teeth 42 of the washer 34 are aligned with the teeth 40 of the flange 6.

The die 1 is attached to the cartridge in the following manner. First the toothed washer 34 is introduced into the locking nut 32 to rest against the flange 38. Then the subassembly of the locking nut 32 with the toothed washer 34 inside is lowered over the flange 6 of the resonant mounting tube 4, the toothed washer 34 being suitably orientated during this operation so that the teeth 42 of the washer 34 are aligned with the gaps between the teeth 40 of the flange 6. Next the toothed washer 34 is rotated through a small angle so that its teeth 42 become aligned with the teeth 40 of the flange 6, thereby preventing the washer 34 from being withdrawn. A small pin (not shown) or other means may be provided to limit the rotation of the toothed nut 34 relative to the flange 6 and maintain their desired relative orientation. Now the assembly of die 1, locking nut 32 and toothed washer can be offered up to the cartridge 26 in the conventional manner, with the tail 24 of the resonant tube being received inside the cartridge 26 and the flange 6 of the resonant tube butting against the end wall 28 of the cartridge. The assembly can be locked in place by rotating the locking nut 32 to engage an internal thread 44 of the locking nut 32 with the external thread 30 of the cartridge. The mounting flange 6 of the resonant tube is then clamped between the annular end wall 28 of the cartridge 26 and the upper end surfaces 46 of the teeth 42 of the toothed washer 34.

Figure 7 is a perspective view, similar to Figure 3, of a die 1 according to a second embodiment of the present invention, in which pits are formed in the outer surface of the tube 4 between the slots 22 of the first embodiment. The pits 48 are a further way of reducing the mass and stiffness of the tube 4 between the die ring 2 and the flange 6. Any suitable pattern of slots 22 and/or pits 48 can be used to create the desired mode of vibration, while maintaining sufficient axial strength.

Claims

1. An ultrasonically vibrated die (1), comprising:

a generally cylindrical die ring (2) defining an axis;

a transducer interface (14) on the die ring (2), whereby ultrasonic vibration can be imparted to the die (1) at a predetermined frequency;

a resonant tube (4) coaxial with the die ring (2) and extending from the die ring (2) along an axial direction; and

mounting means (6) on the resonant tube (4);

wherein the shape and material of the die (1) are such that, when the die (1) is mounted via the mounting means and vibrated at the predetermined frequency, the die ring (2) and the resonant tube (4) each vibrate in a substantially axisymmetric mode; and

characterized in that the axial position of the mounting means (6) on the resonant tube (4) is substantially at a local maximum, with respect to distance along the axis, of the radial amplitude of the axisymmetric mode of vibration of the resonant tube (4).

2. A die according to claim 1, wherein:

when the die (1) is in free vibration at the predetermined frequency, the vibrational mode of the resonant tube (4) includes an axial component, of which the axial amplitude at the axial position of the mounting means (6) is substantially at a local minimum with respect to distance along the axis.

3. A die according to claim 1 or claim 2, wherein the resonant tube (4) further comprises slots (22) distributed around a circumference of the tube (4) between the die ring (2) and the mounting means (6).

4. A die according to any preceding claim, wherein the resonant tube (4) further comprises pits (48) distributed around a circumference of the tube (4) between the die ring (2) and the mounting means (6).

5. A die according to any preceding claim, wherein the mounting means (6) comprises a flange.

6. A die according to any preceding claim, wherein the mounting means (6) comprises a set of outwards-facing teeth (40) disposed about a circumference of the resonant tube (4);

further comprising:

a toothed washer (34) comprising a set of inwards-facing teeth (42) disposed about a circumference of the washer (34) such that the toothed washer (34) can pass over the mounting means (6) when the respective sets of teeth (40,42) are out of alignment but cannot pass over the mounting means (6) when the respective sets of teeth (40,42) are in alignment; and

a locking nut (32) comprising a central aperture, the aperture being sized to pass over the mounting means (6) but to receive and seat the toothed washer (34).

7. A metal forming machine comprising:

a die (1) according to any preceding claim, the die (1) being supported on the machine via the mounting means (6); and

an ultrasonic transducer (16) attached to the transducer interface (14) of the die ring (2) for imparting ultrasonic vibration to the die ring (2) at the predetermined frequency.

8. A method of operating an ultrasonically vibrated die, the die (1) comprising:

a generally cylindrical die ring (2) defining an axis;

a transducer interface (14) on the die ring;

mounting means (6) on the resonant tube (4);

and the method comprising:

supporting the die (1) via the mounting means (6); and imparting ultrasonic vibration at a predetermined frequency to the transducer interface (14), thereby causing the die ring (2) and the resonant tube (4) to vibrate in a substantially axisymmetric mode, such that the axial position of the mounting means (6) on the resonant tube (4) is substantially at a local maximum, with respect to distance along the axis, of the radial amplitude of the axisymmetric mode of vibration of the resonant tube (4).

9. A method according to claim 8, wherein the step of supporting the die (1) comprises clamping the mounting means (6) against axial movement.