GB2028211A - Trimming E.D.M. electrodes - Google Patents

Abstract

The present invention relates to a method of electrical discharge machining in which the or each electrode 4, located in a cartridge spaced from a nose guide 20 of the apparatus with part of the length thereof supported in the nose guide and extending towards a workpiece to be machined, is withdrawn from said workpiece and advanced towards a means 32 for trimming an operative end of the or each electrode. Suggested trimming means are grinding or milling means or a guillotine. The present invention also provides an electrical discharge machining apparatus for carrying out the method. <IMAGE>

Description

SPECIFICATION Grinder refeed The present invention relates to a method of trimming electrodes.

In our co-pending UK Patent Appiication No.

42164/75 there is described and claimed a method of refeeding one or more electrodes in an electrical discharge machining apparatus in which the or each electrode is located in a cartridge spaced from a nose guide of the apparatus with part of the length thereof supported in the nose guide and extending towards a workpiece to be machined, which comprises advancing said cartridge towards said nose guide whilst retaining frictional contact upon the or each said electrode in such a manner as to allow the or each electrode to be slidable received back in said cartridge upon the tip or tips of the or each electrode striking the workpiece.

This method is a convenient means of feeding the electrode or electrodes when wear takes place, as a result of machining and the method in general is known hereinafter as refeeding on to the workpiece.

The method described in UK Patent Application No. 42164/75 is particularly suitable for electrical discharge machining apparatus having a number of electrodes of small cross-section.

The method relies upon the refeeding of the electrodes on to the surface of the workpiece to unsure that they are aligned, followed by continued machining. The number of times refeeding on to the workpiece is necessary will depend on the rate of erosion of the electrodes themselves.

It has now been found that although the above method is excellent for ensuring alignment of a number of electrodes, erosion often causes thin electrodes to become pointed, which means that there is a corresponding undesirable drop in discharge current when machining.

The present invention provides a method of trimming one or more electrodes in an electrical discharge machining apparatus in which the or each electrode is located in a cartridge spaced from a nose guide of the apparatus with part of the length thereof supported in the nose guide and extending towards a workpiece to be machined, which comprises withdrawing the or each electrode from said workpiece and advancing the or each electrode towards a means for trimming an operative end of the or each electrode.

The above-mentioned method is preferably carried out whilst the or each electrode is clamped in said cartridge, thereby preventing the electrode from being pushed back in the cartridge by abutment with the trimming means.

Where more than one electrode is provided, the additional step of advancing the electrodes towards a refeed member may be inserted prior to and/or subsequent to advancement of the electrodes towards the trimming means. The refeed member may be a workpiece to be machined, the trimming means, or a separate refeed member.

The trimming means is preferably a grinding or cutting means.

It is desirable that the grinding means has a grinding surface configuration similar to that of the surface of the workpiece to be machined. If this is so the refeed member may be the workpiece itself.

If the grinding means has a grinding surface configuration dissimilar to the surface configuration of the workpiece to be machined, then the grinding means itself may be the refeed member. In this case the electrodes are advanced to abut the grinding surface of the grinding means preferably when the grinding means is inoperative (i.e non-grinding) before grinding is carried out.

The electrodes, after trimming, are preferably advanced to abut the workpiece surface to be machined with the clamping means released. Any excess length of electrode or electrodes is then pushed back into the apparatus, the clamping means being locked before recommencing machining.

The invention provides in another of its aspects, electrical discharge machining apparatus having at least one electrode, releasable clamping means for said electrode, a nose guide arranged to support said electrode so that its operating end extends towards a position for a workpiece, and trimming means positionable beneath said operating end of the electrode for trimming the operative end thereof.

The trimming means is preferably a grinding means such as a rotatable grinding wheel.

The invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a part of an electrical discharge machining apparatus electrodes of which have been advanced to abut a workpiece; Figure 2 shows a part of the apparatus of Figure 1 during grinding of electrodes thereof by a grinding wheel; Figure 3 shows a part of the apparatus of Figures 1 and 2 during machining of a component having a stepped surface; and Figure 4 shows a part of the apparatus of Figures 1 to 3, electrodes of which are being advanced to abut a flat surfaced refeed member.

Figure 1 shows apparatus comprising an electrode cartridge shown generally as 2 containing six thin wire electrodes 4. The electrodes pass through an electrode clamp member 6 capable of clamping the electrodes rigidly in the cartridge 2. The clamp member comprises a limb 8 pivoted at one end 10 to the cartridge 2 and movable in the direction of arrow Ato clamp the electrodes between a resilient pad 12 secured to the limb 8 and an abutment on the cartridge (not shown). An end 14 of limb 8 is movable in the direction of arrow A by means of a pneumatic piston-cylinder assembly (not shown) the piston 16 of which is attached to the end 14.

The cartridge 2 is mounted on a chassis 18 for vertical movement with respectto a nose guide 20 through which the electrodes 4 pass. The nose guide is mounted on a main frame 22 which is itself mounted for vertical movement on the machining apparatus proper. A pneumatic cylinder 24 is mounted on the frame 22 and a piston 26 of the cylinder is fixed to a bracket 28 on the chassis 18.

Electrodes 4 are advanced through the nose guide to provide sufficient electrode length for machining by clamping the electrodes in the cartridge and moving the chassis 18 towards the frame 22 by means of the piston 26 and cylinder 24, so pushing a desired length of the electrodes through the nose guide.

The electrodes are then advanced to abut a surface of a workpiece 30 to be machined, any electrodes which are too long being pushed back into the apparatus by abutment with the workpiece. When all the electrodes are in contact with the workpiece surface (i.e. aligned with respect to each other) the electrodes are retracted a small distance by moving the whole apparatus away from the workpiece, and machining is commenced.

After an interval of time the electrodes become worn and may be worn unevenly to needle points.

The length of the interval of time depends upon the rate of machining and the materials from which the electrodes and workpiece are made. When wear of the electrodes has occurred the workpiece is then removed or alternatively the apparatus is moved from above the workpiece, and the electrodes are positioned above a trimming means such as a grinding wheel 32 as shown in Figure 2. If the surface configuration of the grinding wheel is similar to the surface configuration of the surface of the workpiece 30 to be machined, the electrodes are advanced to abut the grinding wheel and the wheel is rotated to grind the ends of the electrode flat.

After grinding the electrodes are repositioned above the workpiece and are advanced to abut the workpiece once more, so aligning them, and machining is then recommenced.

Figure 3 shows the apparatus of Figures 1 and 2 during machining of a workpiece 34 having an uneven (in this example stepped) surface. The electrodes, having been initially advanced to abut the workpiece are then retracted slightly and machining is commenced. When the electrodes become worn they are repositioned over a grinding wheel similar to the grinding wheel 32 of Figure 2 and they are then advanced towards the grinding wheel to abut its surface, preferably whilst the grinding wheel is stationary. When all the electrodes have abutted the grinding wheel surface the arrangement is substantially the same as that shown in Figure 2 and grinding is commenced by rotating the grinding wheel.

After grinding, the electrodes are repositioned above the workpiece surface to be machined, and are advanced to abut the workpiece until they are aligned ready for machining, i.e. each electrode is abutting the desired part of the workpiece surface to be machined. The electrodes are retracted slightly and machining is recommenced.

As an alternative before the electrodes are positioned above the grinding wheel, they may be positioned above a refeed member 36 as shown in Figure 4, the refeed member being conveniently mounted on a pivotable bracket enabling it to be brought into position below the electrodes at a desired time. The refeed member 36 is chosen so as to have the same surface configuration as the grinding wheel, the electrodes being advanced to abut the refeed member and then being positioned with their ends on the grinding wheel surface which is rotated to grind the ends of the electrodes flat.

Again after grinding the electrodes are preferably advanced to abut the workpiece surface before machining is recommended.

The ends of the electrodes need not necessarily be ground, of course. Other examples of trimming means are cutting means such as milling tools and guillotines which may be used to trim the ends of the electrodes in similar manner.

The purpose of either advancing the electrodes to abut the grinding wheel surface before the grinding wheel is rotated, or advancing the electrodes to abut the surface of a refeed member, is to ensure that the electrodes are aligned substantially in the same configuration as the surface of the grinding wheel so that small amounts may be ground off each electrode simultaneously. If the foregoing is not carried out and the electrodes are advanced to abut the grinding wheel when they are in a non-aligned state then large amounts of electrode will be ground from the electrodes which are projecting most from the nose guide, such as those indicated as 4a in Figure 3.

The wearing away of large amounts of electrode may not matter in which case it may be desirable to omit the refeed member in orderto save time.

Claims (12)

1. A method of trimming one or more electrodes in an electrical discharge machining apparatus in which the or each electrode is located in a cartridge spaced from a nose guide of the apparatus with part of the length thereof supported in the nose guide and extending towards a workpiece to be machined, which comprises withdrawing the or each electrode from said workpiece and advancing the or each electrode towards a means for trimming an operative end of the or each electrode.

2. A method as claimed in claim 1 wherein the or each said electrode is clamped in said cartridge prior to the or each electrode contacting said trimming means for trimming of said ends, thereby preventing the electrode from being pushed back in the cartridge by abutment with the trimming means.

3. A method as claimed in claim 1 or 2 wherein at least two electrodes are located in the cartridge and further comprising advancing the electrodes towards a refeed member to align the operative ends of the electrodes.

4. A method as claimed in claim 3 wherein said workpiece serves as said refeed member.

5. A method as claimed in claim 3 wherein said trimming means serves as said refeed member.

6. A method as claimed in claim 3,4 or 5 wherein the electrodes are advanced towards the refeed member prior to advancement towards the trimming means for trimming the operative ends thereof.

7. A method as claimed in any of claims 3 to 6 wherein the electrodes are advanced towards the refeed member subsequent to being trimmed.

8. A method as claimed in any of claims 1 to 7 wherein the trimming means is a grinding means.

9. A method as claimed in claim 8 wherein the grinding means has a grinding surface configuration similar to that of the surFace oftheworkpieceto be machined.

10. A method as claimed in any of claims 1 to 7 wherein the trimming means is a cutting means,

11. An electrical discharge machining apparatus having at least one electrode, releasable clamping means for said electrode, a nose guide arranged to support said electrode so that its operating end extends towards a position for a workpiece, and trimming means positionable beneath said operating end of the electrode for trimming the operative end thereof.

12. An electrical discharge machining apparatus wherein the trimming means is a grinding means.