GB2184371A

GB2184371A - Manufacture of extrusion dies

Info

Publication number: GB2184371A
Application number: GB08531133A
Authority: GB
Inventors: Edward George Feldcamp
Original assignee: PRESSABRADE Ltd
Current assignee: PRESSABRADE Ltd
Priority date: 1985-12-18
Filing date: 1985-12-18
Publication date: 1987-06-24
Also published as: GB8531133D0; GB2184371B

Abstract

In the manufacture a steel die blank 5 is hardened prior to spark erosion of the die cavity 6a by wire- cutting technique. This is followed by spark erosion of a back clearance 7 using a graphite electrode 8 to provide the maximum bearing length "b" for the cavity. The graphite electrode 8 is then profiled so that it conforms to a determined overall bearing contour of the die to produce even flow through the die, after which the back clearance 7 is spark eroded using the profiled electrode 8 to produce substantially the correct bearing contour for the die. A test extrusion after initial back clearance erosion provides an end profile measured by a probe which supplies a CNC milling machine information to produce the desired electrode profile for the desired bearing contour. <IMAGE>

Description

SPECIFICATION Manufacture of extrusion dies This invention relates to the manufacture of extrusion dies with spark erosion of the die cavity employing the so-called "wire-cutting" technique in which the electrode is a moving small-diameter wire.

In our co-pending Patent Application No 84 18081 (Publication No 2 143 445 A) we have disclosed a method of manufacturing an extrusion die which additionally includes the steps of producing a graphite electrode and using that electrode to spark erode the die blank to produce a back clearance providing the determined maximum bearing length for the cavity, thereafter profiling 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 opposite "front" face is that which faces the metal billet during the metal extrusion process.

According to the present invention a method of manufacturing employs the same steps and it is a feature that the spark erosion of the die cavity with the blank in a hardened state is carried out prior to spark erosion of the back clearance using a graphite electrode, which corresponds to the geometry of the die cavity plus an all round clearance, to provide the determined maximum bearing length for the die cavity.

Determination of the required overall bearing contour, in order to profile the graphite electrode correspondingly, may be achieved by making a test extrusion through the die after said spark erosion to provide the maximum bearing length. 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 as hereinafter described as an interpretation based on a reading of the nose profile.It depends on 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 from wire spark erosion is removed prior to making the test extrusion. This may be done by hand first, using emery cloth, and then 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 using the graphite electrode. Preferably the test extrusion is a partial extrusion in the sense that the extrusion is stopped whilst the nose end thereof is still within the die cavity, which allows it to be removed as an integral projection from the flat wax billet in the extrusion press.This billet may then be attached to a metal backing plate before it is placed on a table and an electronic probe used to map the Z coordinates of the nose profile, these coordinates being input to a computer and then modified to determine the complementary profile to be formed on the graphite electrode prior to the final spark eroding step therewith.

The modification to the Z coordinates of the nose profile of the test extrusion may be determined by means of a "bearing form" computer programme, the computer controlling a milling machine which profiles the graphite electrode accordingly. Alternatively, or additionally, the modification of the nose profile to be applied to the graphite electrode may be determined by a skilled operative based on his reading of the test extrusion and knowledge of the characteristics of the metal to be extruded.

Modification of the nose profile (as compared with an exact reverse image) applied to the graphite electrode will desirably include relative exaggeration of the Z coordinates to compensate for deflection of the die under the metal extrusion pressure as well as for varying flow rates of the metal through the die cavity.

This deflection will be materially influenced by the metal to be extruded as this governs the extrusion pressure.

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: Fig. 1 is a front face view of the finished die with a simple U-shaped cavity; Fig. 2 illustrates, in section on the line Il-Il in Fig. 1, the die blank immediately prior to heat treatment with a pre-chamber milled and a start hole drilled for the wire cutting spark erosion; Fig. 3 illustrates in similar view the die blank after a wire cutting first spark erosion step; Fig. 4 illustrates in similar view the back clearance cut in a second spark erosion step; Fig. 5 illustrates in the direction of arrow V in Fig. 4 the general form of a test extrusion; Fig. 6 illustrates the mapping of the nose profile of the test extrusion, and Fig. 7 illustrates a final spark erosion step.

The die 1 shown in Fig. 1 has a U-shaped cavity 2 with a wide intermediate limb 3 and relatively narrow side limbs 4 to produce a Usection extrusion of similar shape. In the manufacture of this die a circular steel die blank 5 is first provided in its front face with the usual welding pre-chamber 6 which corresponds in shape to the cavity 2. It is machined on an CNC 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 through the blank which is now subjected to heat treatment to provide the required metal hardness.

After heat treatment of the blank 5 the die cavity is 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 Fig. 3 which is given a back clearance 7 formed by spark erosion, using a graphite electrode 8 which is advanced in the direction of the arrow marked A (see Fig. 4). However, this back clearance is smaller than the clearance at the pre-chamber 6 to provide back-ofdie bearing support and is minimized to strengthen the back support.

For maximum support the back clearance may (as shown) be of stepped form and this is preferably achieved by starting with the graphite electrode 8 sized to provide a correspondingly large clearance over a portion 7a, and then reducing the dimensions of the electrode 8 to provide the smaller minimum clearance before spark eroding the inner clearance portion 7b. Alternatively, the stepped form may be produced by orbital "rotation" of the electrode, according to standard spark machining practice, or this procedure may be employed to provide a back angle or taper instead of the stepped form. The electrode 8 is formed, using a reciprocatory filing machine in co-operation with a CNC 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 providing the determined maximum bearing "b" for the cavity 3.

The part-finished die now has the sparkeroded 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 partially extruded through it, to provide a test extrusion 10 with a profiled nose 11. As shown in Fig. 5, this test extrusion is removed as an integral projection from the remainder of the wax billet 12 used in the test extrusion. The nose profile results from the different flow rates of the wax 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 the purpose of illustration this profile is considerably exaggerated in Figs. 5 and 6.

After removal of the test extrusion 10, with the wax billet 12, from the extrusion press a metal backing plate 13 is attached and it is placed on a table T (as shown in Fig. 6) to allow the Z coordinates of the upwardly facing profile 11 are mapped with an electronic probe measuring head 14. The head 14 during the mapping is traversed horizontally in X and Y coordinate directions, the measured Z coordinates determined by deflection of the probe 15 being fed to a computer C. The nose 9 of the graphite electrode 8 is now contoured, using a CNC milling machine controlled by the computer C, so as to be complementary (in the sense hereinbefore discussed) to the nose contour 11 of the test extrusion 10.Modification data for modification of the Z coordinates to be applied to the graphite electrode 8 are obtained partly from a computer programme and partly from an operator input based on an experienced assessment of expected die performance with the metal to be extruded. The modifications applied compensate for the deflection of the die under the metal extrusion pressure which will be used in the production environment.

The contoured electrode 8 is now reintroduced into the back clearance of the die blank 5 as shown by arrow A in Fig. 7. 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 providing substantially the required bearing contour of the die with any final correction being of a minimal nature.

The invention has been more particularly described hereinbefore with reference to the manufacture of so-called "flat" dies as used for the extrusion of solid profiles and open sections in which the complete die cavity can be spark eroded by the wire cutting technique.

It will, however, be appreciated that the invention can also be employed to manufacture dies as used for the extrusion of hollow sections. In this case the female part of the die is wire cut after hardening, and the male part after hardening is separately formed and at least mainly profiled by milling. Thereafter, with the die assembled, the invention is em ployed as has been described for a flat die using a graphite electrode.

Claims

1. A method of manufacturing an extrusion die from a blank including spark erosion of a die cavity, employing wire-cutting technique, with the blank in a hardened state and prior to spark erosion of a back clearance of the die to provide a determined bearing length of the cavity using a graphite electrode which corresponds to the geometry of the die plus an all round clearance, after which the graphite electrode is profiled so that it conforms to a determined overall bearing contour of the die followed by spark erosion of the back clearance using the profiled electrode to produce substantially the correct bearing form for the die.

2. A method according to claim 1, wherein said back clearance provides the maximum bearing length for the die cavity in the finished condition.

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

4. A method according to claim 3, wherein the test extrusion is a partial extrusion and is removed from the test extrusion press as an integral projection from the flat billet remaining in the extrusion press after the extrusion.

5. A method according to claim 3 or claim 4, wherein the die cavity is polished, so that surface roughness from wire spark erosion is removed, prior to making said test extrusion.

6. A method according to claim 5, wherein polishing of the die cavity is done by hand, using emery cloth, and/or mechanically by extruding an abrasive wax compound through the die opening.

7. A method according to any one of claims 3 to 6, wherein the test extrusion is made with wax material on a test extrusion press.

8. A method according to any one of claims 3 to 7, wherein an electronic probe is used to map the Z coordinates of the nose profile of the test extrusion, these coordinates being input to a computer and then modified to determine the complementary profile to be formed on the graphite electrode prior to the final spark eroding step therewith.

9. A method according to claims 4 and 8, wherein said billet is attached to a metal backing plate before it is placed on a table prior to mapping of said Z coordinates by said electronic probe.

10. A method according to claim 8 or claim 9, wherein the modification to the Z coordinates of the nose profile of the test extrusion in order to determine the correct complementary form of the electrode nose profile is determined by means of a "bearing form" computer programme. the computer controlling a milling machine which profiles the graphite electrode accordingly.

11. A method according to claim 8 or claim 9, wherein the modification of the nose profile (as compared with an exact reverse image of the test extrusion) to be applied to the graphite electrode is determined by a skilled operative based on a "reading" of the test extrusion and knowledge of the characteristics of the metal to be extruded through the die.

12. A method according to any one of claims 3 to 8, wherein modification of the nose profile (as compared with an exact reverse image of the test extrusion) applied to the graphite electrode includes relative exaggeration of the Z coordinates to compensate for deflection of the die under the metal extrusion pressure to be used, as well as for varying flow rates of the metal through the die cavity.

13. A method according to any one of the preceding claims, wherein said back clearance is of stepped or tapered form.

14. A method according to claim 13, wherein the stepped form of the back clearance is achieved by starting with the graphite electrode sized to produce a correspondingly large clearance initially, and. then reducing the dimensions of the electrode to provide a smaller minimum clearance over the remaining length of the back clearance.

15. A method according to claim 13, wherein the stepped or tapered form of the back clearance is achieved by orbital motion of the graphite electrode during the spark erosion of the back clearance.

16. A method according to any one of the preceding claims, wherein the extrusion die is a flat die for the extrusion of solid profiles and the complete die cavity is eroded by said wire cutting technique.

17. A method according to any one of the preceding claims, wherein the extrusion die is designed for the extrusion of hollow sections and said blank provides a female part of the die which is eroded after hardening to provide the die cavity, a male part of the die after hardening being separately formed and at least mainly piofiled by machining, the die being assembled prior to the spark erosion of said back clearance with the graphite electrode.

18. A method according to any one of the preceding claims, wherein the blank is machined prior to hardening to provide a welding pre-chamber in the front face of the die.

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

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