GB2621370A

GB2621370A - Method of manufacture of an electric discharge machining tool

Info

Publication number: GB2621370A
Application number: GB2211683.4A
Authority: GB
Inventors: Maithri Weerasinghe Vijitha
Original assignee: Individual
Current assignee: Individual
Priority date: 2022-08-10
Filing date: 2022-08-10
Publication date: 2024-02-14
Also published as: GB202211683D0

Abstract

A method of manufacturing an electrode for making cutting inserts using EDM comprising: providing a test specimen having a surface with the same complex topography as the surfaces to be machined using the tool; providing an electrode blank made of a material having a lower melting point than the test specimen and an initially flat surface; and electrical discharge machining the flat surface of the blank using the test specimen as an electrode. The flat surface of the blank is selectively eroded to form an electrode tool to have a complementary surface to the test specimen. The electrical discharge machining is carried out with reverse polarity, that is the electrode blank is made positive and the test insert negative. Preferably, the electrode blank has a machined end that has the same (or slightly smaller) shape and area as the rake face of the test specimen. Preferably, after the blank has been eroded to a desired depth, the current is reduced to achieve a desired surface finish on the tool electrode. Preferably, the blank comprises aluminium.

Description

Method of Manufacture of an Electric Discharge Machining Tool

Field of the invention

The present invention is concerned with a method of manufacture of a tool for electric discharge machining (EDM) of a surface having a complex topography. By way of example, the surface having a complex surface topography may be the rake face of a metal cutting carbide insert.

Background to the invention

Electric Discharge Machining (EDM) removes material with repetitive spark discharges from a pulsating dc power supply, with a dielectric fluid flowing between the workpiece and the tool. Each spark discharge removes a small amount of material both from the workpiece and from the tool. As the material is removed by repetitive spark discharges, the tool is fed toward the workpiece by a servo-controlled feed mechanism. In this way, the shape of the tool is imprinted on the surface of the workpiece. The machined surface has a characteristic crater-like non-directional topography.

The workpiece is normally made the positive electrode and the tool the negative electrode. This is termed the 'normal polarity' and it results in a higher material removal rate from the workpiece and a minimal removal rate from the tool. Nevertheless, the tool does gradually wear during the machining process. Furthermore, the appropriate polarity can sometimes depend on the tool and work materials. The term 'reverse polarity' refers to the workpiece being made the negative electrode and the tool the positive electrode. This mode results in a higher material removal rate from the tool and a minimal removal rate from the workpiece. The tool materials that are normally used have a high melting point and a high electrical conductivity, graphite and copper-tungsten being examples.

The background to the present invention is a previous invention which was granted patent in the US and other countries, US8435624B2. In this invention, the inventive aspect was a novel surface modification process which was applied to the rake face of a metal cutting carbide insert. The surface modification process involved first machining the surface of the insert to a shallow depth using an Electric Discharge Machine (EDM) to produce a characteristic crater-like surface topography, then applying a wear-resistant coating, such as titanium nitride (TiN), to the crater-like surface, the coating thickness -2 -being typically of the order of 4 microns. Fig. 1 of the accompanying drawings shows a section through such a modified EDM surface. It is to be noted that the coating sits conformal with the crater-like EDM surface.

The surface modified cutting inserts showed much enhanced cutting characteristics within a narrow window of surface roughness. In these studies, the cutting inserts that were used had no chip breakers. Therefore, the rake face to which the EDM surface modification was applied was a plain flat surface. However, commonly used inserts, as shown in Fig. 2, are formed with chip breakers that serve to break up the swarf produced 1() during machining operations.

Fig. 2 of the accompanying drawings shows some currently available inserts with chip breakers and it will be seen that their rake faces have a complex surface topography. In order to apply the EDM surface modification to such inserts, a tool having the complimentary (i.e. negative) imprint of the chip breaker topography is required. It would be possible, for example, to micro mill the negative imprint on the EDM tool. However, as a consequent of tool wear, costly frequent 'dressing' or re-milling will be needed, and this requirement is likely to make the process economically unsuited to mass production.

Object of the invention The present invention seeks therefore to provide an economical and convenient method of producing a tool for electrical discharge machining of surfaces having a complex topography, such as surfaces of cutting inserts, moulds and dies.

Summary of the invention

According to the present invention, there is provided a method of manufacture of an electrode tool for electrical discharge machining of surfaces having a complex topography, as hereinafter set forth in Claim 1 of the appended claims.

Features of embodiments of the invention are as set forth in the dependent claims.

Brief description of the photographs

The invention will now be described further, by way of example, with reference to the accompanying photographs, in which: Fig.1 is a cross sectional view of the EDM modified surface with a 4 micron thick TiN coating, Fig. 2 shows examples of cutting inserts with chip breakers, Fig. 3 shows three aluminium electrode blanks (right), two electrode tools with chip breaker profile imprinted on them (left), and a tool electrode installed on to a tool 1() holder, Fig. 4 shows the chip breaker topography of a cutting insert, Figs. 5a and 5b show magnified views of a cutting corner of the insert shown in Fig.4, before and after EDM surface modification.

is Detailed description of the invention

The manufacture of an electrode tool for electrical discharge machining of inserts having a complex surface topography is carried out by the following method: a) Select a test insert that is identical to the inserts to be machined in terms of general shape and chip breaker topography.

b) The electrode tool commences as an aluminum blank having a machined end same as the rake (top) face of the test insert in terms of its shape and area or slightly smaller. Note that the end face of the tool blank will be a plain flat surface at this stage, Fig. 3.

c) The test insert and the electrode blank are installed in an EDM machine and the polarity of the machine is reversed, that is the electrode blank is made positive and the test insert negative.

d) The electrode blank is brought down on to the test insert and machining commences with an appropriate high current amplitude for rough machining. Machining continues for a pro-longed time (approx. 5 min) until all of chip breaker features of the test insert are imprinted by way of a wear profile on to the surface of the aluminium blank.

e) Thereafter, the EDM machining may be continued still with reverse polarity but with a lower current to achieve an improved surface finish on the imprint. The aluminium blank is now ready to use as an EDM tool. -4 -

The EDM tool can be used to machine production inserts as follows: a) The polarity of the machine is switched back to normal if rough machining is required or kept as reversed if finishing is required b) The test insert is removed and a new production insert is located at the same position as the test insert and without removing the tool to maintain positional accuracy. The production insert will be the same as the test insert in all aspects.

c) The machining process is repeated using appropriate parameters to achieve the desired surface finish and depth. It is to be noted that in order to achieve to enhanced cutting characteristics, the production insert will have to be subsequently coated with an appropriate coating material and application method.

Aluminium, which has a substantially lower melting point than the carbide insert, was chosen by way of example only. Other conductive materials having a lower melting point than the specimen may alternatively be used. Reversed polarity and the choice of the electrode material were aimed to maximise the wear on the tool whilst minimising the wear to the test insert. In this way, the chip breaker profile is readily imprinted on to the aluminium blank with minimal damage to the test insert. If a conventional tool material is used such as for example copper, copper tungsten or graphite, a much longer machining time will be required to fully 'burn' or imprint the chip breaker profile. As a consequence, substantial damage is likely to the test insert with possible distortion of the chip beaker profile imprint. A further key feature is that the aluminium electrode can be used many times without the need to 'dress' because the chip breaker profile keeps re-generating during machining. However, at some point, after using the electrode a certain number of times, dressing is likely to be required. This is because of the inherent dimensional tolerances of the insert specimens which will cause progressive deviations to the imprint geometry. However, when this happens, the imprint will have to be machined off and the manufacturing method described above will have to be repeated. This dressing process will be less costly than re-milling a conventional tool material.

While the invention has been described above by reference to the manufacture of a tool for machining cutting inserts, it will be clear that it is equally applicable to other surfaces of complex topography, such as the surfaces of moulds and dies.

Fig. 3 of the drawings shows electrode tools, and blanks from which they are made, as well a tool holder that form part of an EDM machine and positions the electrode -5 -tool opposite an insert to machined or the test insert to be used in the production of the electrode tool itself Fig.4 shows the chip breaker topography of a cutting insert, while Figs 5a and 5b show magnified views of a cutting corner of the insert shown in Fig 4, before and after EDM surface modification.

Claims

-6 -CLAIMS A method of manufacture of an electrode tool for electrical discharge machining of surfaces having a complex topography, which method comprises: providing a test specimen having a surface with the same complex topography as the surfaces to be machined using the tool, providing an electrode blank made of a material having a lower melting point than the test specimen and having an initially flat surface, and electrical discharge machining the flat surface of the blank using the test specimen fo as an electrode, in order to erode the flat surface of the blank selectively and thereby form an electrode tool to have a surface topography that is complementary to the topography of the surface of the test specimen, the electrical discharge machining being carried out with reverse polarity of the machine, that is the electrode blank is made positive and the test insert negative.
2. A method as claimed in Claim 1, wherein the electrode blank has a machined end that is the same as the rake face of the test specimen in terms of its shape and area or slightly smaller.
3 A method as claimed in claim 1 or 2, wherein after application of sufficient current for a sufficient time to erode the blank to a desired depth, the current is reduced to achieve a desired surface finish on the tool electrode
4. A method as claimed in any preceding claim, wherein the blank comprises aluminium