GB2143445A

GB2143445A - Manufacture of extrusion dies

Info

Publication number: GB2143445A
Application number: GB08418081A
Authority: GB
Inventors: Edward George Feldcamp
Original assignee: PRESSABRADE Ltd
Current assignee: PRESSABRADE Ltd
Priority date: 1983-07-16
Filing date: 1984-07-16
Publication date: 1985-02-13
Also published as: GB2143445B; GB8418081D0

Abstract

A method of manufacturing an extrusion die includes the production of a graphite electrode 8 which corresponds to the geometry of the die cavity plus an all round clearance, and spark erosion of a die blank 5 using the graphite electrode to produce a back clearance 7 which provides the determined maximum bearing length t for the cavity. The die cavity is spark eroded from an initial hole 6a employing a wire cutting technique with the blank 5 in a hardened state. After profiling the end face of the graphite electrode 8 so that it conforms to a determined required overall bearing contour, it is re-introduced to spark erode the back clearance and thus to produce substantially the correct bearing form for the die. To reprofile the electrode, an initial test extrusion is taken, the electrode profile being complementary thereto; alternatively a computer-controlled milling machine is used. <IMAGE>

Description

SPECIFICATION Manufacture of extrusion dies This invention relates to the manufacture of extrusion dies.

In the manufacture of extrusion dies it is normal practice to cut out the die aperture or "cavity" using a spark erosion technique. In order to achieve a uniform flow velocity as a metal billet (for example of aluminium alloy) is extruded through the die the wall of the cavity requires a varying depth or "bearing", dependent at any point on the width of the cavity at that point and the position thereof relative to the centre part of the billet. A uniform flow velocity through the die cavity is essential for the production of a straight and undistorted extrusion.

Until recently the spark erosion cutting of the cavity employed a profiled electrode, the result being a cavity the walls of which were not parallel but tapered in the direction of cut. Furthermore, with a multi-cavity die the cavity side walls and the gap dimensions are not the same in each cavity. Thus after the machining operation appropriate "correction" of the die has been necessary, usually involving skilled hand filing, in order to achieve the uniform metal flow. The bearing contour or socalled "form" of the die, which is dependent as already mentioned on the varying width of the cavity and distance from centre, can be determined by a computerised calculation on the basis of established parameters plus the experience of the operator to produce a die which approximates to the correct form.

The final correction of the die is carried out by the extruder making test extrusions on the actual machine (extrusion press) with which the die is to be used. If the extrusion equipment is not well maintained the die must be proved to the press, and also to the effective die diameter and thickness when mounted in the press. In the past considerable correction has been required to compensate not only for the non-parallelism of the cavity walls but also to a lesser degree for the approximate nature of the bearing form which is usually achieved by machining a back clearance in the die blank on a numerically-controlled milling machine.

Recent developments in the spark erosion field have resulted in so-called "wire cutting" of the cavity, using as an electrode a moving smalidiameter wire which is employed in a manner analogous to a bandsaw. This provides a cavity with substantially parallel walls and uniform gap dimensions so that the correction time is much reduced whilst that associated with the bearing form is relatively much greater, so that the provision of an accurate bearing form has become much more desirable with the aim of reducing the necessary final correction time to the minimum.

The object of the invention is to provide a method of die manufacture which reduces correction time to a minimum, thereby reducing expensive hand correction on the part of the extruder.

According to the invention a method of manufacturing an extrusion die includes the steps of - producing a graphite electrode which corresponds to the geometry of the die cavity plus an all round clearance spark eroding the die blank, using the graphite electrode, to produce a back clearance to a depth which will provide the determined maximum bearing length for the die cavity spark eroding the die cavity employing a wire cutting technique with the blank in a hardened state - profiling the end face of the graphite electrode so that it conforms to a determined required overall bearing contour, and spark eroding the back clearance using the profiled electrode to produce substantially the correct bearing form.

In accordance with the usual terminology of the art, the "back" face of the die is on that side thereof which faces in the extrusion direction, so that the "front" face faces the metal billet during the extrusion process.

The heat treatment of the blank to provide the hardened state at the wire cutting stage may be effected at any time prior to that stage and after any initiai machining of the front face to provide the usual welding pre-chamber. However, it is preferably effected prior to the first spark erosion step with the graphite electrode.

Determination of the required overall bearing contour, in order to profile the graphite electrode correspondingly, may be achieved by making atest extrusion through the die after said spark erosion of the die cavity. The end face of the graphite electrode is then profiled so as to be "complementary" to the nose profile of the test extrusion. The term "complementary" as used herein with reference to the profiling of the graphite electrode is not to be taken as implying an exact reverse image of the nose profile of the test extrusion, the actual form of the complementary profile being determined by a skilled operative as an interpretation based on his reading of the nose profile.It depends on experience and a knowledge of the relative extrusion profile of the alloy to be extruded and the test extrusion material, and will differ materially for example with a soft aluminium alloy such as HE-9 and a hard alloy such as HE-30.

Preferably the die opening is polished so that surface roughness is removed prior to making the test extrusion. This may be done by hand, using emery cloth, or mechanically by extruding an abrasive wax compound through the die opening.

The test extrusion is preferably made with wax material on a test extrusion press, and it will be appreciated that the nose profile of this extrusion will correspond to the flow velocity profile of the wax through the die cavity as formed with the constant bearing produced by the first spark erosion step. Thus spark eroding the back die clearance, using the electrode profiled so as to be complementary to the nose profile of the extrusion, will result in a bearing form which provides a substantially uniform flow velocity through the die.

As a result the minimum amount of correction work is required on the part of the extruder.

As an alternative to making a test extrusion, the required profile of the graphite electrode may be determined by means of a "bearing form" computer programme which, in dependence on the form of the die cavity, controls a milling machine which profiles the graphite electrode. This programme determines the bearing form in dependence on factors such as distance from the centre of the die, the material to be extruded, and the local "mass" flow through the die cavity. The wire cutting spark erosion of the die cavity will normally have been computer controlled according to a wire cutting programme, and the bearing form programme may be grafted on to this wire cutting programme for control of the profiling of the graphite electrode during which only vertical control of the milling cutter is required.

Although the first spark erosion step may as described provide a uniform bearing for the die cavity subsequently cut prior to the test extrusion, in some cases it may be desirable to provide initially a more approximate bearing form. Thus in this case the electrode may be withdrawn after spark erosion of the constant bearing clearance, then profiled to an approximate or intermediate bearing form and re-introduced to provide this form for the back clearance before the wire-cutting of the die cavity.

This has an advantage in that less wire-cutting of metal is required, which is a relatively expensive procedure, and less final cutting of the bearing form based on the test extrusion is required. With some die geometry this may provide a more accurate final result based on a single test extrusion when the latter is employed to determine the graphite electrode profile.

It will be appreciated from the foregoing paragraph that when the profiling of the graphite electrode is determined by a bearing form computer programme, which may be grafted on to the wire cutting programme, the spark erosion step using the profiled graphite electrode can be carried out either before or after the wire cutting of the die cavity.

The graphite electrode may be produced on a special-purpose machine as commercially available for this purpose. However, it can be produced more cheaply and avoiding the need to purchase such a machine by employing an NC (numericallycontrolled) milling machine in co-operation with a reciprocating filing machine using a thin circular file of 'Abråfile' type. The necessary variation of die clearance at different points of the die opening, dependent on the form of the latter, can be achieved with an appropriate NC programme. The die manufacturer will normally already have a suitable milling machine used for clearance machining of the die blanks. Dies manufactured in accordance with the present invention will in most cases utilise such a machine to provide the conventional front welding pre-chamber which faces the metal billet when fitted to an extrusion machine.In this chamber re-welding of the front face of a new billet to the metal of the previous billet occurs, to produce continuous extrusion of the profile which is necessary in modern press handling practice.

The invention will now be further described with reference to the accompanying drawings which illustrate, diagrammatically and by way of example, the manufacture of a typical die by a method in accordance with the invention. In the drawings: Figure 1 is a front face view of the finished die with a simple U-shaped cavity; Figure 2 illustrates, in section on the line Il-Il in Figure 1, the die blank Immediately prior to heat treatment with the pre-chamber milled and a start hole drilled for the subsequent wire cutting spark erosion; Figure3 illustrates in similarviewthe back clearance cut in a first spark erosion step; Figure 4 illustrates in similar view the die after a second wire cutting spark erosion step; Figure 5 illustrates in the direction of arrow V in Figure 4 the general form of a test extrusion; and Figure 6 illustrates in similar view a final spark erosion step.

The die 1 shown in Figure 1 has a U-shaped cavity 2 with a wide intermediate limb 3 and relatively narrow side limbs 4 to produce a U-section extrusion of similar shape. In the manufacture of this die a circular steel die blank 5 is first provided in its front face with a U-shaped welding pre-chamber 6 which corresponds in shape to the cavity 2. It is machined on an NC milling machine to the geometry of the cavity 3 plus a constant or varying offset to provide the pre-chamber for re-weld and, if desired, to effect some control on the metal flow before the die bearing. A start hole 6a for the subsequent wire cutting step is drilled to an appropriate depth determined by the maximum bearing which will exist at the wire cutting stage.

The blank is now subjected to the usual heat treatment to provide the required metal hardness.

After the heat treatment the blank 5 is given a similar back clearance 7 formed by spark erosion, using a graphite electrode 8 which is advanced in the direction of the arrow marked A. However, this back clearance is smallerthan the clearance at the pre-chamber to provide back-of-die bearing support and is minimized to strengthen the back support.

For maximum support it may be of stepped form and orbital "rotation" of the electrode according to standard spark machining practice may be employed to provide such a stepped form or the more usual back angle or taper. The electrode 8 is formed, using a reciprocatoryfiling machine in cooperation with the numerically controlled milling machine, to the geometry of the cavity 3 plus a clearance offset. The electrode 8 has a flat nose 9 and the back clearance 7 is cut in the blank 5 to a constant depth leaving a thickness of metal t equal to the determined maximum bearing which is required for the cavity 3. This cavity is now spark eroded by the wire-cutting technique previously referred to, starting with the wire passed through the hole 6a, resulting in the part-finished die shown in section in Figure 4.

This part-finished die now has the spark-eroded bearing surface polished to a suitable surface finish, either by hand with emery cloth or by extruding an abrasive wax compound through it which is a known polishing technique. It is then mounted on a test extrusion press and wax material extruded through it, in the direction of arrow B in Figure 5, to provide a test extrusion 10 with a profiled nose 11.

This nose profile results from the different flow rates through the cavity 2, there being a greater restriction to flow through the narrow side limbs 4 than through the intermediate limb 3 as the cavity has at this time a uniform bearing. For purpose of illustration this profile is considerably exaggerated in Figure 5.

The nose 9 of the graphite electrode 8 is now contoured so as to be complementary (in the sense hereinbefore discussed) to the nose contour 11 of the test extrusion 10, and it is reintroduced into the back clearance of the die blank 5 as shown in Figure 6. It is then used in a final spark erosion step which cuts the back clearance 7 to conform to the profile of the electrode nose 9, resulting in the finished die with a back clearance which provides a bearing contour giving a substantially uniform flow rate through the die. Any final correction required on the part of the extruder will be of a minimal nature, considerably less than is normally now required.

Claims

1. A method of manufacturing an extrusion die, including the steps of - producing a graphite electrode which corresponds to the geometry of the die cavity plus an all round clearance spark eroding the die blank, using the graphite electrode, to produce a back clearance to a depth which will provide the determined maximum bearing length for the die cavity spark eroding the die cavity employing a wire cutting technique with the blank in a hardened state - profiling the end face of the graphite electrode so that it conforms to a determined required overall bearing contour, and spark eroding the back clearance using the profiled electrode to produce substantially the correct bearing form.

2. A method of manufacturing an extrusion die according to claim 1, wherein heat treatment of the blank to provide a hardened state at the wire cutting stage is effected prior to the first spark erosion step with the graphite electrode.

3. A method of manufacturing an extrusion die according to claim 1 or claim 2, wherein determination of the required overall bearing contour is achieved by making a test extrusion through the die after said spark erosion of the die cavity, the end face of the graphite electrode then being profiled so as to be "complementary" (as that term is hereinbefore defined) to the nose profile of the test extrusion.

4. A method of manufacturing an extrusion die according to claim 3, wherein the die opening is polished so that surface roughness is removed prior to making said test extrusion.

5. A method of manufacturing an extrusion die according to claim 3 or claim 4, wherein said test extrusion is made with wax material on a test extrusion press.

6. A method of manufacturing an extrusion die according to claim 1 or claim 2, wherein the required profile of the graphite electrode is determined by means of a bearing form computer programme which, in dependence on the form of the die cavity, controls a milling machine which profiles the graphite electrode.

7. A method of manufacturing an extrusion die according to claim 6, wherein the wire cutting spark erosion of the die cavity is computer controlled according to a wire cutting programme, and the bearing form programme is grafted on to this wire cutting programme for control of the profiling of the graphite electrode during which vertical positioning control of the milling cutter is effected.

8. A method of manufacturing an extrusion die according to claim 6 or claim 7, wherein the step of spark erosion of the back clearance using the profiled graphite electrode is effected prior to the step of spark erosion of the die cavity.

9. A method of manufacturing an extrusion die according to any one of claims 1 to 7, wherein the graphite electrode is withdrawn after the spark erosion of the constant bearing clearance, then profiled to an approximate or intermediate bearing form and re-introduced to provide this form for the back clearance before the spark erosion of the die cavity.

10. A method of manufacturing an extrusion die according to any one of the preceding claims, wherein the graphite electrode is produced on an NC milling machine in co-operation with a reciprocating filing machine using a thin circular file of 'Abråfile' type and employing an appropriate NC programme.

11. A method of manufacturing an extrusion die employing a profiled graphite electrode for spark erosion of the back clearance of the die, substantially as herein described.

12. An extrusion die manufactured by a method in accordance with any one of the preceding claims.

13. A method of manufacturing an extrusion die, substantially as herein particularly described with reference to the accompanying drawings.