GB2203360A - Manufacture of spark erosion electrodes - Google Patents

Abstract

A method of making precision articles by spark erosion, in which a dimensioned model 23 of the article is used to make a multiplicity of negative copies of the model, rendered conductive for use as the electrodes 25 ,27. The model is sized to provide a predetermined gap between the resultant electrodes and the work to be machined. The model 23 provides a first electrode, 24 which is used to produce more electrodes. This can involve producing a conducting layer 29 which has a backing 30 attached, so that the layer and backing can be stripped from the master 28.

Description

Manufacture of precision articles This invention relates to the manufacture of articles which are required to be precisely dimensioned, and more especially to articles of this nature which are to be made in quantity.

In industry there frequently arises a need for the production in quantity of an article which for some reason, such as the intricacy of its shape, cannot be made by conventional automatic production methods to the desired dimensional tolerances. Examples of such products include metal engineering components of many kinds, and while small quantities of such articles can be made by tool-room methods, production is slow and costly, and subject to small variations from one sample to the next inevitable with what is in effect individual manufacture or finishing of the samples.

One process that can be used to improve the production rate whilst permitting the desired accuracy of dimensions to be obtained is that of spark erosion. In this process the article is made in a first stage by a method which allows the articles to be produced in quantity, but not to the desired dimensional tolerances; so made, the articles are oversize by a small amount. In the second stage of manufacture an electrode accurately complementing the desired final shape and dimensions of the article is made and is positioned closely against the surface of the article, but separated from it by a precise distance. A high voltage is then impressed between the article and the electrode, suitably while the article and the electrode are immersed in quenching liquid, with the result that metal is removed from the article until its surface accurately conforms to the desired shape of the article.

The advantage of this spark erosion method is that while the electrode has to be made in the first place to the desired accuracy of shape and dimensions it can be used a number of times, so that the cost of making the electrode is spread over the production of a number of articles. There is also the advantage that the articles made from the one electrode are all very closely the same in shape and dimensions.

In many cases, where quantity production is involved, it is necessary to make many such electrodes and to use them simultaneously each in a separate spark erosion apparatus, to achieve an adequate throughput of articles. The time taken to make the electrodes, and the cost, is very reluctantly borne, of necessity, by the manufacturing industry. There is also the disadvantage that the electrodes, individually made, will differ slightly from one another in dimensions, and these differences will be reflected in the dimensions of the finished articles.

The present invention relates to a method for the production of an original or master electrode, and the relatively rapid, accurate production of copy or duplicate electrodes of precision matching that of the master electrode, the copy electrodes being made at substantially less cost than that of making copy electrodes by conventional methods.

In the method according to the invention there is first made a model of the article to be produced; this model is made slightly but accurately oversize by a dimension which is equal to the spacing between the electrode and an article to be finished by spark erosion. From the model is made by casting or a similar precision copying process a negative replica of the shape of the model. This negative replica of the shape of the model is made conductive, either by direct treatment of the surface but preferably by a further process by which is produced from the negative replica a substantial layer of conductive material having precisely the shape of the negative replica of the model. The first negative replica can be used as a finishing electrode; preferably the second generation copy is used.

When it is desired to increase production by setting up a further line of spark erosion e-quipment, it is possible, with the method described, to make further electrodes from the first model, thereby avoiding the time and expense of making models for each line. S uch further electrodes will be very close copies of the first.

The invention has been found very advantageous in the manufacture of pistons for internal combustion engines.

Modern pistons have to be made to very close tolerances not only in the exterior cylindrical bearing surfaces, but also in the shaping of the piston crown and especially the interior of the piston, which is intricately shaped with many differing curvatures. A piston blank can be made by die-casting in a suitable alloy, but not to the desired tolerances. With the invention, using the method of spark erosion in conjunction with a plurality of electrodes made by the method described it is possible to produce, from a series of different spark erosion apparatus, finished pistons which are closely similar in dimensions. Such uniformity of a product is most desirable in engineering manufacture.

Features and advantages of the invention will also appear from the following description of embodiments thereof, given by way of example, in conjunction with the accompanying drawings, in which: Figure 1 is a flow diagram showing the successive steps in the production of an article, and Figure 2 is a diagram showing stages in the production of an article.

Figure 1 is a flow diagram showing the basic steps in the production of an article by spark erosion finishing.

In the first step, indicated at 10, there is produced by any suitable process, for example by die casting, a blank of the article. The blank is oversize on all surface dimensions.

A model of the article is produced at 11; this model can be made by any suitable methods, but tool room methods will usually be most appropriate. The model is made oversize on its surface dimensions by an amount which corresponds to the spacing between the surface of the blank and the surface of the finishing electrode appropriate to the final finishing stage. In a practical case the spacing might be 0.002 inch.

In most cases it will be necessary or convenient to make the model in a number of parts which fit together to make the comlete model.

In stage 12, from the model is made a casting, using a suitable casting resin, so that there is produced a negative replica of the surface of the model; this negative surface will be the complement of, but slightly larger than, the surface of the finished article.

In stage 13 there is made a copy of the surface of the casting from stage 12, but slightly larger, leaving the still negative surface larger than the surface of the finished article by an amount appropriate to the clearance between a spark erosion electrode and the surface of the blank in an initial roughing operation. In a practical case this clearance might be 0.010 to 0.015 inch. A convenient method of making this oversize copy of the surface is by a pantographic method. The surface is made conductive by a suitable method, such as plating, at 14, or the copy can be made in a sufficiently conductive material, such as graphite.

In stage 15 the original blank from stage 10 is subjected to spark erosion finishing, using the body from stage 14 as the electrode. The voltage and current used in the erosion process may vary widely in accordance with the conditions and the article being finished, with voltages up to 600 or 800 volts and currents up to 600 amperes. The current is pulsed, with a duty cycle as high as 0.999 or as low as 0.001, again depending upon circumstances.

When the roughing erosion is completed a further, finishing, spark erosion step is carried out in stage 16. It would be possible to use as an electrode in this stage the first negative replica body produced in stage 12, the surface being made conductive as by plating at 17 if necessary, but where a number of articles are to be made it is preferred to make at 18 an electrode of the same surface shape as the body from stage 12 but with a substantial conductive surface. A method of producing such an electrode is described hereinafter.

As an alternative to the use of the pantographic method to produce the roughing electrode from the copy from stage 12, in some cases it may be more economic to make a second model of which the dimensions differ from the dimensions of the article by an amount corresponding to the electrode clearance in the roughing stage of the spark erosion. From the two models the two electrodes can then be made by similar processes. For example, from the roughing electrode model at 19 there is produced a negative replica by casting at 20, made conductive on its surface at stage 21 if it is not already conductive, and used as the electrode in stage 14. A similar method, starting from the first model at 11, is used to produce the finishing electrode. In both cases there may be introduced steps to produce electrodes having heavy conductive coatings, for example by a method decribed hereinafter.

Where it is desired to greatly increase the output of finished articles, it would be possible to duplicate stages 11 to 16 or 20, using a new model such as 11 or 11 and 19 for each production line, but with the system described it is possible to duplicate stages 12 to 16 or 20, as at 22, making the cast in stage 12 from the same model from stage 11; if the second model 19 is used the cast in stage 20 can be made from the model at 19, thus avoiding the time and cost of making new models for each line.

In industry, it is very often the practice to call for large numbers of products at short notice, and in such cases the time taken to produce the models for each line imposes a limitation on the speed with which large orders can be met.

In addition, the electrodes in each finishing stage, made from the same original model, will be very closely similar in all respects, giving'high uniformity in the finished product. It will be understood that where the electrodes are assembled from parts, these parts will be made separately, as in stages 11, 12, 13 and 14, and appropriately assembled for use in stages 15 and 16.

By way of illustration, Figure 2 shows diagrammatically the successive steps in the production of accurately shaped electrodes.

First, a model 23 of the article is prepared; this is an accurate model but on its exterior dimensions is slightly oversize, for example by 0.002 inch.

Next a cast 24 is made from the model, using a suitable casting resin. This cast is used to make, by a pantographic method, a copy 25 which is slightly larger, for example 0.010 inch larger, on its interior dimensions, as described above. Copy 25 can be used as a roughing electrode, in which case its surface is made conductive, as at 26, or it can itself be copied into another material, such as graphite, as at 24.

Copy 25 is also used to make a positive copy 28 of the model, which is then heavily copper plated at 2a96. The exposed surface of the copper is roughened to give a good key with backing resin 30 which is cast onto it. The copy portion 28 is then removed, leaving the heavily copper coated electrode to be used for the roughing spark erosion.

Incidental advantages of the process described lie in the fact that so perfectly is the finish on the article a copy of the final electrode that it is possible to produce on the article an identifying marking which is sufficiently subtle or intricate to make copyright piracy difficult.

Claims (12)

1 A method of making a precision article by spark erosion of a blank, which includes the steps of making a model of the article differing in dimension from the dimension of the article by a predetermined amount related to the separation of an electrode from the blank in the erosion process, making from said model for use as a spark erosion electrode a negative form of the model said negative form having a conductive surface, making from said model for use as a further electrode at least one further negative form of said model, said further negative form having a conductive surface.

2 A method as claimed in claim 1, and comprising the steps of making from said negative form a positive form of said model, providing a conductive layer on the surface of said positive form and removing said conductive coating for use as said electrode.

3 A method as claimed in claims 1 or 2, and comprising the steps of making from said further negative form a further positive form of said model, providing a conductive layer on the surface of said further positive form and removing said conductive coating for use as said further electrode.

4 A method as claimed in claims 2 or 3 wherein said conductive layer is provided by plating.

5 A method as claimed in claims 2, 3 or 4, wherein said layer is applied to the surface of said positive or further positive form, and comprising the steps of applying a backing material to said layer, and stripping said layer with said backing material from the positive form to provide said electrode or further electrode.

6 A method as claimed in any of the preceding claims wherein the said electrode or said further electrode or both are appropriate for the finishing stage of the spark erosion process.

7 A method as claimed in any of claims 1 to 6 wherein the said electrode or said further electrode or both are appropriate for the roughing stage of the spark erosion process.

8 A method as claimed in any of the preceding claims wherein the negative copy or the said further negative copy of the model is made by a casting process from said model.

9 A method as claimed in claim 7, and comprising the steps of making from said negative form by a pantographic method an additional negative form appropriate for the roughing stage of the spark erosion process.

10 A method as claimed in any of the preceding claims, and comprising making a first model the dimensions of which are appropriate for the production of a finishing electrode, making a copy of said model and making from said copy a finishing electrode; making a second model the dimensions of which are appropriate for the production of a roughing electrode, making a copy of said second model and making from said copy a roughing electrode.

11 A method as claimed in any of the preceding claims, and including the step of forming the article from said blank by spark erosion.

12 A precision article made by a process in accordance with any of the preceding claims.