Ultrasonically Vibrated Die Rings

Abstract

An ultrasonically vibrated die comprises a generally cylindrical die ring supported by a coaxial resonant mounting tube. The die ring is vibrated in a radial bending mode of vibration, in which an end surface of the die ring oscillates between a concave and a convex state. The mounting tube joins the end surface of the die ring at a radius R where the amplitude of the oscillation of the end surface is at a minimum, in order to reduce transmission of the vibration into the mounting tube.

Description

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.

U.S. Pat. No. 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 die ring is mounted on a forming machine via a mounting tube that is coaxial with the die ring.

The die ring needs to be mounted firmly enough to withstand the high forces exerted on it during the forming of a metal workpiece, while allowing it to vibrate as freely as possible at the applied frequency. It is desirable to minimize the transmission of vibrations from the die ring into the mounting tube, both because this causes energy to be lost from the die ring and because it may interfere with the desired mode of vibration of the die ring.

U.S. Pat. No. 5,095,733 (also 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 “RO”, 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 RO 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 “RB0”, which is schematically illustrated in FIGS. 1A to 1C. FIG. 1A 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 (FIG. 1B) and a barrel shape (FIG. 1C) over a cycle of the vibration, passing through approximately its original cylindrical configuration (FIG. 1A) at the midpoint between each of these two extremes. It can be seen that in the “hourglass” configuration of FIG. 1B, the annular end surface of the component bulges outwards in a convex cone, while in the “barrel” configuration of FIG. 1C, the annular end surface of the component sinks inwards in a concave cone. Note that the shape of the end surface in these configurations is not necessarily a true cone, i.e. a plane containing the axis may intersect the end surface in a curved line rather than a straight line.

SUMMARY OF THE INVENTION

The invention provides a die, comprising:

a generally cylindrical die ring comprising an end surface and having a radial bending (RB0) mode of vibration in which the end surface oscillates between a concave and a convex state; and

a mounting tube coaxial with the die ring and extending from the end surface of the die ring;

characterized in that the mounting tube joins the end surface of the die ring at a radius where the amplitude of the oscillation of the end surface is at a minimum.

The invention also provides a method of operating a die that comprises a generally cylindrical die ring having an end surface; and a mounting tube coaxial with the die ring and extending from the end surface. The method comprises vibrating the die ring in a radial bending (RB0) mode, in which the end surface of the die ring oscillates between a concave and a convex state, characterized in that the minimum amplitude of the oscillation of the end surface occurs at a radius where the mounting tube joins the end surface.

By making the radius of the mounting tube join the end surface of the die ring at a radius where the amplitude of the oscillation is at a minimum, the undesired transmission of vibrational energy from the die ring into the mounting tube can be reduced. This is an unforeseen advantage compared with the prior art suggestion that the die ring should be vibrated in a pure radial mode RO, because in the RO mode all points on the die ring oscillate in phase and there does not exist a radius at which the amplitude of oscillation reaches a minimum.

THE DRAWINGS

FIGS. 1A to 1C are perspective views of a computer model of an annular component undergoing vibration in radial bending mode RB0.

FIG. 1D is a schematic sectional view of the end wall of the component of FIG. 1A, shown at the two extremes of its vibration.

FIGS. 2A and 2B are perspective views in different orientations of a die in accordance with the invention.

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

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1D schematically shows the end wall of the component of FIG. 1A at the two extremes of its vibration in the radial bending mode RB0. Dotted lines 30 show the component in its “hourglass” configuration, corresponding to FIG. 1B. Solid lines 32 show the component in its “barrel” configuration, corresponding to FIG. 1C. It can be seen that the movement of any point on the surface of the end wall between the two extremes is principally in a direction parallel to the axis 34. A point P on the radially outer part of the end surface moves between a greater axial elongation in the barrel configuration and a smaller axial elongation in the hourglass configuration, while a point Q on the radially inner part of the end surface does the opposite, oscillating 180° out of phase with the outer part. At a point in between, at an intermediate radius R, the amplitude of the oscillation of the end surface must be at a minimum. In fact, the movement of the points on the end surface is not in general purely axial—there is also a radial component—but it is still true that at an intermediate radius there exists a circle of points on the end surface where the amplitude of the oscillation of the points is at a minimum.

The amplitude may be defined in various ways. Preferably, it is the straight-line distance between the corresponding points at the two extremes of the oscillation. Alternatively, the amplitude may be measured along the path that a point on the surface follows between those two extremes. Another possibility is to measure only the component of the movement parallel to the axis. If preferred, the amplitude may be defined as one half of any of the aforementioned values, to conform to the conventional definition for a waveform; this makes no difference to identifying the radius at which the minimum value occurs.

FIGS. 2A, 2B and 3 illustrate a die 1 according to an embodiment of the present invention. The die 1 incorporates a die ring 2 that defines a central axis 3. The die ring 2 is formed integrally with a resonant mounting tube 4. The mounting tube 4 is coaxial with the die ring 2 and extends axially from an end surface 5 of the die ring 2. 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) to support the die ring in use. As can be seen in FIG. 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.

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 interface surface 14 has a threaded bore 16 in its centre for receiving a stud (not shown) that is used to secure the transducer.

The shape and material of the die ring 2 are chosen such that, when an ultrasonic transducer is coupled to the interface 14 and introduces energy at a predetermined frequency, the die ring 2 vibrates in the previously described radial bending mode RB0. During this vibration, the end surface 5 oscillates between a convex and a concave configuration as illustrated in FIG. 1D. The radius R of the mounting tube 4 where it joins the end surface 5 is equal to the radius where the amplitude of this oscillation of the end surface 5 is at a minimum. More precisely, the circle of points on the end surface where the oscillation is a minimum lies within the thickness of the wall of the mounting tube.

Because the mounting tube 4 is thin-walled and flexible, to a first approximation the vibration modes of the die ring 2 can be considered independently from those of the mounting tube 4. The mounting tube 4 joins the end surface 5 of the die ring 2 where the amplitude of vibration is at a minimum, so it is desirable to design the mounting tube 4 such that at the operating frequency the vibration of the mounting tube 4 is also at a minimum at that junction. The mounting tube 4 typically vibrates in an axisymmetric mode with nodes and antinodes of vibration distributed along its length. At the frequency of the radial bending mode (RB0) of the die ring 2, a node of the mounting tube preferably coincides with the junction of the mounting tube and the die ring so that the amplitude of vibration is at a local minimum there.

Claims

1. A die, comprising: a generally cylindrical die ring comprising an end surface and having a radial bending mode of vibration in which the end surface oscillates between a concave and a convex state; anda mounting tube coaxial with the die ring and extending from the end surface of the die ring;characterized in that the mounting tube joins the end surface of the die ring at a radius where the amplitude of the oscillation of the end surface is at a minimum.

2. The die according to claim 1, wherein the end surface is annular.

3. The die according to claim 1, wherein, at the frequency of the radial bending mode of the die ring, the mounting tube vibrates in a mode in which the amplitude of vibration is a local minimum at the junction of the mounting tube and the die ring.

4. A method of operating a die that comprises a generally cylindrical die ring having an end surface, and a mounting tube coaxial with the die ring and extending from the end surface, the method comprising vibrating the die ring in a radial bending mode, in which the end surface of the die ring oscillates between a concave and a convex state, characterized in that the minimum amplitude of the oscillation of the end surface occurs at a radius where the mounting tube joins the end surface.

5. The die according to claim 2, wherein, at the frequency of the radial bending mode of the die ring, the mounting tube vibrates in a mode in which the amplitude of vibration is a local minimum at the junction of the mounting tube and the die ring.