Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
2000-07-12
2003-10-21
Dunn, Tom (Department: 1725)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S453000, C164S454000, C164S433000, C164S434000, C164S155100, C164S151200, C072S262000, C072S468000
Reexamination Certificate
active
06634415
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is concerned with a continuous extrusion machine, and a method of operation for continuously extruding non-ferrous metals such as aluminium and copper.
In general a continuous extrusion machine comprises a chassis a wheel and tooling. The tooling consists principally of a shoe and a die. The chassis supports the wheel for rotation by a motor. An endless groove is formed in the periphery of the wheel into which is entrained a feedstock which is commonly a bar of a non-ferrous metal such as aluminium or copper but may comprise metal particles or molten metal. Part of the periphery of the wheel is closely enveloped by the shoe so that the groove cooperates with the shoe to form a passage in use feedstock entrained in the groove enters the passage at an open end as the wheel rotates. The other end of the passage is obstructed by an abutment which is mounted on the shoe and intrudes into the passage. Because the feedstock is confined in the passage and the wheel continues to rotate, the feedstock is heated by friction with the groove. A die is mounted in a chamber formed in the shoe immediately upstream of the abutment. Eventually the thermal and other stresses imposed on the feedstock cause the feedstock to extrude through the die.
The continuous extrusion machine is capable of continuously extruding a wide range of sections of non-ferrous metal, for so long as feedstock is delivered to the groove.
In order to operate successfully it is necessary to have a small gap between the periphery of the wheel and the shoe. This gap permits a small quantity of the feedstock known as the flash, to extrude out of the passage onto the periphery of the wheel and into the gap. The size of the gap has a significant effect on the performance of the machine in terms of the speed, quality and type of extrusion which can be produced. Conventionally the gap is set before starting the machine. However, when the machine is in operation heat causes thermal expansion of the machine components and pressure on the wheel and chassis causes elastic deformation so that the gap size changes.
Thermal expansion typically alters the gap by up to 0.7 mm while elastic deformation alters the gap by between 0.3 and 0.5 mm. The effects of thermal expansion and extrusion pressures are non-uniform, will vary during start up, and may vary during operation and conventionally cannot be measured accurately.
The elastic deformation is relieved when feedstock ceases to enter the machine, as at shut down, and it is essential that the shoe does not impinge on the wheel or serious damage will occur. It is consequently not possible to pre-set the machine to run with a gap of less than the elastic deformation. It is also disadvantageous that the gap cannot be varied and accurately measured during machine operation in order to test the performance of various clearances in the production of an extrusion.
Accordingly the present invention provides a continuous extrusion machine having a chassis supporting a wheel for rotation and a shoe enveloping a span of the periphery of the wheel and co-operating with a groove formed in the periphery of the wheel to form a passage, a support mechanism supporting said shoe and/or wheel to be relatively displaceable in a direction perpendicular to the axis of rotation of the wheel during use, a gap sensor system able to sense the size of a gap between the wheel periphery and the shoe when the machine is operating, and control means responsive to the gap sensor to adjust the support mechanism to displace the shoe relative to the wheel.
The gap sensor system may also sense the shape of the gap.
In practice it is preferable to support the shoe via the support mechanism. However, the fundamental objective is to be able to accurately control the gap size and shape and so the displacement of the wheel relative to the chassis is deemed within the broad concept of this invention. Also within the scope of this invention is the displacement of the shoe and the wheel relative to the chassis particularly where it may be convenient to displace the shoe on one axis and the wheel on another.
A preferred support mechanism comprises a hydraulic wedge assembly having a wedge longitudinally displaceable against a complementary ramp. The ramp engages and supports the shoe and is constrained to move in a direction towards or away from the wheel. By mounting such a support mechanism at a tangent to the wheel so that shoe displacement is radial it is possible to control the gap size. However, a unidirectional active shoe positioning system is less than wholly satisfactory at least in part because of difficulties in adapting different shoe types used for radial and tangential mode extrusion and because it is desired to control the shape of the gap in addition to its size. To completely control both the size and the shape of the gap, as independent variables, it is preferred to provide the support mechanism with a first and a second wedge assembly. The first wedge assembly is disposed to displace the shoe in a first direction perpendicular to the axis of rotation of the wheel and the second wedge assembly is disposed to displace the shoe in a direction perpendicular to the rotary axis of the wheel and the first wedge assembly. The directions will ordinarily be the vertical and horizontal.
It is preferred that each wedge assembly includes an hydraulic ram to longitudinally displace the wedge.
Although wedges, ramps and rams are thought to be the best way of implementing the support mechanism at this time it is conceived that the use of hydraulic rams alone or ball screw driven rams may be capable of providing a support mechanism.
Means such as Poly-Tetra-Fluoro-Ethylene (PTFE) surfaces may be provided to reduce the friction between the wedge and the wedge bearing.
Preferably, where two wedge assemblies are provided to implement a bi-directional dynamic or active shoe positioning process, it is preferred to provide a gap sensor system having three gap sensors each located peripherally spaced from the other, to sense the size and shape of the gap.
An alternative arrangement would be for the shoe to be supported in the chassis by means of a pivot and swung into position to set the gap size. By supporting the pivot to be displaceable radially via the operation of a first actuator such as an hydraulic ram, and arranging for a second actuator such as a second hydraulic ram to be capable of swinging the shoe around the pivot, the size and shape of the gap may be dynamically adjusted during machine operation in accordance with the size and shape of the gap sensed by the gap sensor.
In order to sense both the size and shape of the gap the gap sensor system will preferably comprise a plurality of gap sensors deployed to detect the gap size at positions spaced circumferentially around the wheel.
Preferably the gap sensor system comprises gap sensors which sense the gap size directly to avoid the corrections required if the gap size and shape is sensed indirectly. To this end each gap sensor must tolerate the hostile environment at the interface between the wheel and the shoe while continuing to measure with accuracy of the order of 0.1 mm, so that a gap size of 0.2 mm can be accurately set. The sensor range will preferably exceed 0.5 mm to facilitate starting the machine and ideally will exceed 1 mm. The preferred form of sensor is a sonic gap sensor.
A sonic gap sensor relies on the principle that fluid flow through an orifice will choke when a fluid pressure upstream of the orifice reaches a critical pressure at which the flow through the orifice is sonic. In this condition the fluid condition downstream of the orifice has no influence on the conditions upstream of the orifice. When the orifice is choked the fluid condition upstream of the orifice correlates with the size of the orifice. By making the gap the orifice the size of the gap can be measured. Thus the gap . sensor of the present invention consists of at least one port located in the shoe adjacent the gap and a gas delivery
Maddock Brian
Smith Martin James
Dunn Tom
Fay Sharpe Fagan Minnich & McKee LLP
Holton Machinery
Lin I.-H.
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