GB2028208A - Apparatus for shaping a workpiece - Google Patents

Abstract

Apparatus for shaping a workpiece includes a base 15 and a die part 11 which is formed with a die cavity 13 and which is supported on the base 15 by way of at least one pad 16 shaped to engage the die part 11 and the base 15 over a finite area sufficient to allow the pad 16 to withstand the force required to effect shaping of the workpiece, substantially without deformation of the pad 16. The workpiece is shaped by applying a force thereto cause material from the workpiece to flow into or through the die cavity 13 and a plurality of transducers 14 are arranged to subject the walls of the die cavity 13 to vibration in a radial resonant mode during shaping of the workpiece. The material of the pad 16 is such that, when the die part 11 is undergoing radial resonance, the ratio of the vibrational energy reflected by the pad 16 back into the die part 11 to the vibrational energy transmitted through the pad 16 to the base 15 is greater than unity. <IMAGE>

Description

SPECIFICATION Apparatus for shaping a workpiece This invention relates to apparatus for shaping a workpiece,the apparatus being of the kind including a base, a die part supported on the base and formed with a die cavity, means for applying a force to the workpiece to cause material from the workpiece to flow into or through the die cavity to effect shaping of the workpiece, and means for subjecting the wall(s) of the die cavity to vibration in a radial resonant mode during shaping of the workpiece.

Apparatus of the kind specified includes both extrusion apparatus and drawing apparatus and achieves the advantage that causing the wall(s) of the die cavity to undergo radial resonance during shaping of a workpiece lowers the friction opposing the flow of material from the workpiece into or through the die cavity. The force required to effect shaping of the workpiece can therefore be reduced.

There is, however, a problem with apparatus of this kind in providing a satisfactory arrangement for supporting the die part on the base so that the die part is capable of vibration relative to the base without undue energy dissipation, while at the same time the support arrangement is able to withstand the forces necessary to effect shaping of the workpiece. Thus, in one known form of the apparatus, at least part of the required support is provided by locating steel balls or a steel ring between the die part and the base so as to make substantially point or line contact with the die part at positions corresponding to nodes when the die part is vibrating.

However, this arrangement leads to high loadings and rapid wear of the support. A further proposed solution is to provide the die part with radial extensions each of which is then gripped at a nodal position to provide the required support. However, this alternative suffers from the problem of being bulky and expensive. It is therefore an object of the present invention to alleviate the above-mentioned problems experienced in the prior art.

Accordingly, the invention resides in apparatus of the kind specified for shaping a workpiece wherein the die part is supported on the base by way of at least one pad shaped to engage the die part and base over a finite area sufficient to allow the pad to withstand the force required to effect shaping of the workpiece, substantially without deformaton of the pad, the material of the pad being such that, in use, when the die part is undergoing radial resonance, the ratio of the vibrational energy reflected by the pad back into the die part to the vibrational energy transmitted through the pad to the base is greater than unity.

Using the apparatus described in the preceding paragraph it is found that energy losses through the pad can be maintained at an acceptable level, while at the same time a long working life can be realised for the support pad despite the fact that the pad necessarily engages regions of the die part which move relative to the pad when the die part is vibrating. Moreover, it will be appreciated that the pad provides a simple and inexpensive support arrangement for the die part.

The ratio ofthevibrational energy reflected by the pad back into the die part to the energy transmitted through the pad depends upon the ration of the acoustic impedances of the materials of the die part and the pad respectively. The acoustic impedance of and material is proportional to the product of the density of the material, p, and the velocity, c, in the material. However, the velocity of sound in a material obeys the following relationship:

where E is the Youngs Modulus of the material.

Thus the acoustic impedance is proportional to +/and, to ensure a high degree of reflecton of vibrational energy back into the die part it is desirable that the ratio of the acoustic impedance of the die part to the acoustic impedance of the pad should be at least 4. On the other hand, of course, the strength of the pad material must be sufficient to withstand the forces necessary to effect the required shaping of the workpiece.

Example of suitable materials for the die part are steel and titanium which, considering the bulk materials, have acoustic impedances of about 48 and 27 kg/m2s respectively. Examples of suitable materials for the pad and their approximate acoustic impedances measured in bulk (except in the case of wood example) are nylon (2-2.7 kg/m2s), polytetrafluorethylene (3.0 kg/m2s), wood (3-3.5 kg/m2s measured in bar), polystyrene (2.8 kg/m2s), asbestos felt (6.5 kg/m2s), and the laminated asbestos reinforced plastics materials sold as Ferobestos type LA4 (3.8 kg/m2s) and LF10 (5.4 kg/m2s).

The total surface area of the pad or pads engaging the die part should be arranged so that the load supported by the pad (s) divided by the total surface area of the pad(s) is less than or equal to the compressive strength of the pad material or more preferably, in order to ensure a reasonable service life for the pad(s), is less than the compressive fatigue strength of the pad material. It will of course be appreciated that the compressive fatigue strength is less than the compressive strength. In fact, however, little published data is available on the compressive fatigue strengths of many materials and also with certain materials, such as wood, there is considerable variation in compressive fatigue strength between different samples.In view of these factors, and the fluctuating forces generated during vibration-assisted shaping, a reasonable safety factor is in practice supplied to the area of the pad so as to avoid excessive loading of the pad. It is therefore normally desirable to ensure that the load supported by the pad(s) divided by the area ofthe pad(s) is less than half the compressive strength of the pad material. In this respect it is to be appreciated that the load which must be supported by the pad(s) is determined by the resistance to flow of the work piece at the chosen rate of shaping the workpiece.

The accompanying drawing is a sectional view of drawing apparatus according to one example of the invention.

Referring to the drawing, the apparatus includes a circular die part 11 which is formed of steel, or more preferably titanium, and which carries a centrally disposed, tungsten carbide insert 12. The insert 12 is apertured to define a generally cylindrical die cavity 13 through which material from a workpiece can be caused ta flow to effect a drawing operation. Bonded to the circumference of the die part 11 is a single transducer 14, or as in the example shown, a plurality of equi-angularly spaced transducers 14 which, when energised, serve to vibrate the die part 11 and insert 12 in a radial resonant mode and in a direction perpendicular to the cylindrical axis of the die cavity 13.The transducers 14 are energised when material is being drawn through the die cavity 13 and the vibration thereby imparted to the wall of the die cavity lowers the friction opposing the flow of material and hence reduces the force necessary to effect the drawing operation.

The die part 11 is mounted on a fixed base 15 by way af a plurality of angularly spaced pads 16 shaped complementarily with the die part 11 and the base 15 and extending over the majority ofthe area of the die part. Each pad 16 is formed of the laminated asbestos reinforced plastics material sold as Ferobestos LA4 and is shaped so that the compressive forces applied to the pad during drawing are at right angles to the asbestos laminations.

This asbesto material has an acoustic impedance of 3.8 kg/m2s and hence the ratio of the acoustic impedances of the die part 11 and each pad 16 is approximately 12.7, in the case where the die part is formed of steel, and 7 when the die is formed of titanium. In use, therefore, when the die part 11 is undergoing radial resonance, the ratio of the vibrational energy reflected by the pads 16 back into the die part 11 to the vibrational energy transmitted through the pads 16to the base 15 is greater than one. In fact, the amount of vibrational energy reflected by the pads 16 is of the order of 80% when the die part is formed of steel and of the order of 70% when the die part is formed of titanium. As an alternative to the example shown, a single annular pad could be used in place of the plurality of pads 16.

In one practical embodiment, the total surface area ofthe pads 16 engaging the die part 11 was such that the load supported by the pads divided by the pad area was of the order of one twentieth of the compressive strength of the laminated asbestos material Ferobestos LA4 (i.e. 1968 kg/cm2). Despite the relatively large pad area it was found that the operational efficiency of the apparatus was not significatnly affected. Moreover it was found that the pads were able to withstand the forces necessary to effect drawing of a metal workpiece through the die cavity 13 substantially without deformation and hence the pads provided the necessary supportfor the vibrating die part. In addition, it was found that the working life of the pads 16 was relatively long as compared with the steel ball supports employed in the prior art arrangement discussed above.

Claims (6)

1. Apparatus of the kind specificed for shaping a workpiece wherein the die part is supported on the base by way of at least one pad shaped to engage the die part and base over a finite area sufficient to allow the pad to withstand the force required to effect shaping of the workpiece, substantially without deformation of the pad, the material of the pad being such that, in use, when the die part is undergoing radial resonance, the ratio of the vibrational energy reflected by the pad back into the die part to the vibrational energy transmitted through the pad to the base is greater than unity.

2. Apparatus as claimed in claim 1, wherein the ratio of the accoustic impedance of the die part to the acoustic impedance of the pad is at least 4.

3. Apparatus as claimed in claim 1 or claim 2 wherein the die part is formed in steel or titanium.

4. Apparatus as claimed in any preceding claim, wherein the pad is formed of nylon, polytetrafluoroethylene, wood, polystyrene, asbestos felt or an asbestos reinforced plastics material.

5. Apparatus as claimed in claim 4, wherein the pad is formed of an asbesto reinforced plastics material.

6. Apparatus as claimed in claim 1 for shaping a workpiece comprising the combination and arrangement of parts substantially as hereinbefore described with reference to and as shown in the accompanying drawing.