WO2017032958A1

WO2017032958A1 - Ultrasonically vibrated die rings

Info

Publication number: WO2017032958A1
Application number: PCT/GB2015/052483
Authority: WO
Inventors: Miles ASHCROFT
Original assignee: Magnaparva Packaging Limited
Priority date: 2015-08-27
Filing date: 2015-08-27
Publication date: 2017-03-02

Abstract

An ultrasonically vibrated die ring (2) for forming tubular metal workpieces comprises a flat interface (14) for connecting an ultrasonic transducer (22) to the die ring (2). A first rib (20) projects outwards along part of a circumference of the die ring (2) on one side of the interface (14). A similar second rib (18) may be disposed along a circumference on the other side of the interface (14). The first rib (20) and the optional second rib (18) at the circumferential position of the transducer interface (14) help to keep the interface (14) flat and maintain a good connection with the transducer (22) as the die ring (2) vibrates in a radial bending resonant mode.

Description

TITLE

Ultrasonically vibrated die rings Technical field

The invention relates to an apparatus for forming metal workpieces by driving the workpieces into a die. It has particular application to annular workpieces that commonly 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.

Vibrations are applied to the die by an ultrasonic transducer that is interfaced to a flat surface on the circumference of the die. This induces the die to vibrate in one or more resonant modes, depending on the frequency of the vibrations and the shape and material of the die. US patent 5,095,733 (Porucznik et al.) discloses and classifies various possible resonant modes of a ring-shaped die. It teaches that the preferred mode is a pure radial mode termed "R0", in which the die ring expands and contracts radially, centred on the axis of the ring, as the axial length respectively contracts and expands to a lesser extent.

The present inventors have found that the pure radial mode R0 cannot generally be achieved at suitable frequencies and within the typical space constraints of a die in a forming machine. However, the die ring can readily be induced to vibrate in a "radial bending" mode termed "RBO", which is schematically illustrated in Figures 1 A to 1C. Figure 1 A shows a simple, hollow cylinder in its resting state. Because the harmonic number is zero, this mode continues to display circular symmetry about the axis of the ring, whereby in ideal circumstances the contact between the working surface of the die ring and the workpiece is synchronous around any given circumference. The expansion and contraction are also substantially synchronous along the axis of the die ring. However, the amplitude of the vibration is not uniform along the axis. In particular, the component oscillates between an hourglass shape (Figure IB) and a barrel shape (Figure 1C) over a cycle of the vibration, passing through approximately its original cylindrical configuration (Figure 1A) at an intermediate point between each of these two extremes. When the component is viewed at right angles to the axis, the profile of the outer surface cycles between convex and concave.

This flexure of the outer surface of a die ring vibrating in the RBO mode causes a problem at the interface with the transducer, which requires a flat surface to be maintained for the efficient transmission of ultrasonic energy into the die ring. As a gap opens and closes between the flat surface of the transducer and the flexing, oscillating surface of the die ring, waves can be reflected back into the transducer, wasting energy at best and risking damage to the transducer at worst.

Summary of the invention

The invention provides a die ring, comprising:

an outward facing interface providing means for connecting an ultrasonic transducer to the die ring; and

a first rib disposed adjacent to the interface, the first rib extending along part of a first circumference of an outer surface of the die ring and projecting outwards from the first circumference.

The first rib extending around just part of the circumference adjacent to the transducer interface adds mass and/or stiffness to the die ring at this location. This changes the resonant mode sufficiently to reduce the bending of the outer surface of the die ring in the region of the interface and permit a good contact with the transducer to be maintained during vibration of the die ring. The rib introduces a departure from the ideal circular symmetry of the die ring and its resonant mode of vibration at the outer surface. However, it is found that the asymmetry introduced at the inner, working surface of the die ring is manageable and is outweighed by the benefits of the invention.

Preferably the die ring further comprises a second rib disposed adjacent to the interface, the second rib extending along part of a second circumference of an outer surface of the die ring and projecting outwards from the second circumference, wherein the interface lies between the first and second ribs.

Providing a second rib on the opposite side of the interface from the first rib further helps to maintain the interface flat as the die ring vibrates in the radial bending mode. The second rib provides a degree of mirror symmetry along the axial direction. However, this symmetry cannot be perfect because the die ring is typically mounted at one axial end and the workpiece is inserted into the other axial end to engage a working surface that has an asymmetrical longitudinal profile. In view of this the first and second ribs need not be identical to achieve the best results. For example, they can have different thicknesses, thickness in this context being the dimension measured on a line parallel to the axis.

The invention further provides a method of using a die ring as previously described, comprising connecting an ultrasonic transducer to the flat surface and using the transducer to apply ultrasonic vibrations to the die ring at a predetermined frequency, whereby the die ring is induced to vibrate in a radial bending resonant mode.

The drawings

Figures 1 A to 1C are perspective views of a computer model of an annular component undergoing vibration in radial bending mode RBO.

Figure 2 is a perspective view of a die incorporating a die ring in accordance with the invention.

Figure 3 is a longitudinal section of the die of Figure 2.

Figures 4A and 4B are partial side elevations of a computer model of a die ring in accordance with the invention undergoing vibration in radial bending mode RBO. Description of the preferred embodiment

Figures 2 and 3 illustrate a die 1 that incorporates a die ring 2 according to an embodiment of the present invention. The die ring 2 is coupled to a resonant mounting tube 4 that is coaxial with the die ring 2 and extends from one end thereof. Part way along the tube 4 is a radially projecting flange 6, which is used for mounting the die 1 on a forming machine (not shown). As can be seen in Figure 3, the section of the tube 4 between the die ring 2 and the flange 6 is thin-walled so as to be relatively flexible and to minimize the coupling of the vibration of the die ring 2 into the tube 4. Therefore, to a first approximation, the vibration modes of the die ring 2 can be considered independently from the remainder of the die 1. The features of the mounting tube 4 and the flange 6 do not form part of the present invention and will not be described further.

The die ring 2 has a central aperture 8 that opens to the axial end remote from the mounting tube 4. The interior wall of the aperture 8 defines a working surface 10 that is profiled to form a tubular workpiece (not shown) as it is driven into the aperture against the working surface 10. The die ring 2 is vibrated ultrasonically to assist the forming process.

The outer surface 12 of the die ring 2 is generally cylindrical. At one point on its circumference there is formed a planar surface, parallel to the axis, that acts as an interface 14 for an ultrasonic transducer (not shown). The axis of the ultrasonic transducer is thus orthogonal to the axis of the die ring. The interface surface 14 has a threaded bore 16 in its centre for receiving a stud (not shown) that is used to secure the transducer. In this embodiment the interface 14 is a rectangular platform that stands proud of the cylindrical surface 12 but alternatively it could intersect the cylindrical surface and could be of other shapes. A flat interface surface 14 is simplest and conforms to the flat working surface of standard ultrasonic transducers. If required, a different interface profile, complementary to a given transducer, could be adopted and the invention would still operate to reduce the distortion of the interface surface 14 during the preferred mode of vibration of the die ring 2. On two circumferences of the die ring, located on opposite sides of the interface 14, there are respectively formed two ribs 18, 20 in accordance with the present invention. Each rib 18, 20 projects radially outwards from the cylindrical outer surface 12 of the die ring 2. Each rib 18, 20 extends only partially around its respective circumference of the die ring 2, being centred on the circumferential location of the interface 14. In this embodiment of the invention the two ribs 18, 20 are located axially at the respective ends of the die ring 2.

Viewed along the axis, the ribs 18, 20 in this embodiment have profiles in the form of circular arcs, the radii of the arcs being smaller than the outer radius of the die ring 2. Other profiles are possible, for example a central part of the profile could be concentric with the axis of the die ring. There may be a curved transition between each rib 18,20 and the outer surface 12, either to facilitate machining of the die ring 2 or to reduce stress at the junction as the die ring vibrates.

The two ribs 18, 20 may have equal thickness but it can be seen in Figure 3 that in this embodiment they do not. The rib 18 closer to the mounting tube 4 has a smaller thickness than the rib 20 remote from the mounting tube 4. This has been found to compensate for the asymmetries in the die ring 2 due to the attachment of the mounting tube 4 at one end and the asymmetric longitudinal profile of the working surface 10. In some embodiments it might be possible to omit one of the ribs entirely. Each rib need not be uniformly thick and may vary in thickness over its radius or its circumference.

It is possible to provide additional ribs or cut-outs at other circumferential locations around the die ring if this helps to maintain the desired mode of vibration, e.g. by suppressing undesired harmonic modes that might otherwise be encouraged by the presence of the ribs 18, 20.

Figures 4A and 4B show part of a computer model of a die ring 2 in accordance with the invention, this time orientated with its axis horizontally on the page. The die ring 2 is undergoing ultrasonic vibration in radial bending mode RBO, with the amplitude of the vibrations greatly exaggerated to make their characteristics visible. An ultrasonic transducer 22 is attached to the surface of the interface 14 between the two ribs 18,20. In the resting state, the interface surface 14 is planar and parallel to the axis, while the ribs 18,20 extend radially. Figure 4 A shows one extreme of the vibration of the ring, in which the outer surface 12 forms a barrel shape with a convex longitudinal profile and the ribs 18,20 tilt apart from one another. Figure 4B shows the opposite extreme, half a cycle of the vibration later, in which the outer surface 12 forms an hourglass shape with a concave longitudinal profile and the ribs 18,20 tilt towards one another. The effect of the mass and/or stiffness added by the ribs 18,20 is to reduce this changing curvature of the outer surface 12 at the circumferential location of the interface 14. As a result, the interface surface rises and falls but it remains relatively flat so that good contact can be maintained between the interface 14 and the transducer 22 throughout the cycle.

Claims

1. A die ring (2), comprising:

an outward facing interface (14) providing means for connecting an ultrasonic transducer (22) to the die ring (2); and

a first rib (20) disposed adjacent to the interface (14), the first rib (20) extending along part of a first circumference of an outer surface (12) of the die ring (2) and projecting outwards from the first circumference.

2. A die ring (2) according to claim 1, wherein an outer profile of the first rib (20) when viewed along the axis is a circular arc.

3. A die ring (2) according to claim 1 or claim 2, further comprising:

a second rib (18) disposed adjacent to the interface (14), the second rib (18) extending along part of a second circumference of the outer surface (12) of the die ring (2) and projecting outwards from the second circumference, wherein the interface (14) lies between the first and second ribs (18,20).

4. A die ring (2) according to claim 3, wherein the first and second ribs (18,20) have different thicknesses, as measured on a line parallel to the axis.

5. A die ring (2) according to any preceding claim, wherein the interface (14) is a flat surface that is parallel to a central axis of the die ring (2).

6. A method of using a die ring (2) according to any preceding claim, comprising connecting an ultrasonic transducer (22) to the interface (14) and using the transducer to apply ultrasonic vibrations to the die ring (2) at a predetermined frequency, whereby the die ring (2) is induced to vibrate in a radial bending resonant mode (RB0).