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Electronica Machine Tools Ltd.EDM Division

Electronica Machine Tools Ltd.EDM Division

Application hints

Selecting Electrode Materials

Important EDM Machining Parameters and their Explanations.

Typical Applications and Commonly Faced Problems

Case Studies

Selecting Electrode Materials

The mechanical properties of the electrode have negligible effect on machining performance. However, the thermo physical properties of the electrode like thermal and electrical conductivity, thermal expansion, and heat to vaporize from room temperate, melting and boiling temperatures have considerable influence on the EDM process performance in terms of material removal rate, electrode wear and surface integrity of the workpiece.

There is a wide range of materials used to manufacture electrodes, for instance, brass, tungsten carbides, copper, copper-tungsten , copper- graphite , graphite etc. In respect to the application, performance of copper and graphite is summarized below.

COPPER:

This electrode material is widely used when smooth workpiece surface finishes are required. This material can be machined by all conventional methods such as drilling, turning, milling, grinding etc. But machining can be sometimes difficult because copper has a trend to drag on the edge of the cutting tool and the grinding wheel. However, copper machines on Wire EDM better than graphite. Very complex shapes can be obtained by Wire EDM onto copper electrodes. Another advantage of copper in comparison to graphite is its ability to be coined and then to be a very good material for engraving electrodes. For certain applications, such as electrodes to be used in medicine engineering field, copper is the best choice because of its facility to be highly polished.

GRAPHITE:

This material is available in many different grades from large grain sizes (20 m), used in rough EDM operations, to very fine grains (1 m) for finish EDM operations, particularly in steel. The costs of graphite vary from inexpensive, for coarse-grain sizes, to very expensive for fine-grain sizes. Graphite provides a high material removal rate and low electrode wear - depending on the EDM parameter settings - as compared to copper. Graphite has a much lower density than copper, which makes it the best material for large electrodes. Although graphite is very abrasive it is relatively easy to be machined by all the conventional machining processes like milling, drilling, turning and grinding. The major drawback of graphite is the fine dust it produces during its machining. It is able to settle on the guides of the machine tool and when mixed with the machine's cutting fluid it will act like a lapping compound, which eventually reduces the accuracy of the machine. This EDM dust can also settle on electrical parts of the machine causing local short circuits and can impose breakdowns. Glass scales can give false measurements due to the settling of dust. Precautions must be taken when machining graphite. Most EDM jobs that can be done with copper can also be executed with graphite. The end result might be the same, but the cost to accomplish the job can be vastly different. In practical terms selection of the electrode material will depend mainly on the electrode size, type of EDM machine, methods of making the electrodes and most importantly the cost.

Important EDM Machining Parameters and their Explanations.

Standard workpiece and electrode materials are used traditionally by machine manufacturers to establish the EDM parameter settings. However, this is not the usual situation of the tooling industry, where many different grades of workpiece and tool electrode materials are used. Consequently, the customers are required to develop their own process parameters, which normally demand many experimental tests.

In order to enable the user carry out such experiments and develop the proper settings it is required that the user knows the significance of each and every EDM parameter used.

IP Sparking current

It is the power used for cutting material, more power more cutting rate but there is a limitation. For faster cutting speed higher values of Ip are recommended, but with higher values you get rough surface finish and also larger spark gap. To achieve good quality finish lower values of Ip are recommended.

Recommended Ip range for different material combination are as follows

Ton Pulse on time

It is pulse sparking time in EDM operation. The current Ip flows during this time. More the value of Ton less is the wear of electrode, but there is a limitation when sparking different materials. For carbide machining higher values of Ton can result into micro cracking. Also to reduce wear in Rib or thin and sharp profile job you can use SAFE mode. This mode is not recommended in finishing. For machining using graphite electrodes and also for carbide machining SAFE mode is not recommended.

Recommended Ton range for different material combination are as follows

Material combination

Ton Range

Roughing

Semi finish

Finish

Copper to Steel

200 to 400

75 to 150

5 to 20

Graphite to steel

150 to 300

50 to 100

5 to 20

Copper to carbide

50 to 75

10 to 30

2 to 5

Tau Factor related to Pulse off time

After every pulse on time there is a pulse off time. Current Ip does not flow during this time. A off time period helps the gap to recover in between 2 on times. More the value of tau less is the pulse off time.

Lower values of Tau can result in better efficiency at the cost of higher electrode wear.

Recommended Tau range for different material combination are as follows

Material combination

Tau Range

Roughing

Semi finish

Finish

Copper to Steel

9 to 10

7 to 8

6 to 7

Graphite to steel

8 to 10

6 to 7

5 to 6

Copper to carbide

6 to 7

4 to 5

3 to 4

Vg Sparking voltage

Sparking voltage or gap voltage is the voltage between electrode and the work piece during sparking. It also represents the physical gap between the electrode and the workpiece. More the value of Vg more is the gap between electrode and work piece. Bigger gap helps clearing the debris during finishing operation.

Recommended Vg range for different material combination are as follows

Material combination

Vg Range

Roughing

Semi finish

Finish

Copper to Steel

60 to 65

70 to 75

80 to 90

Graphite to steel

60 to 65

70 to 75

80 to 90

Copper to carbide

40 to 50

50 to 60

50 to 60

SEN Sensitivity

It is the speed of Z axis during sparking. More the value more is the speed. Maximum speed at value 10 in open gap condition is around 80 mm/min. During cutting process the SEN parameter decides the stability of operation.

Recommended SEN range for different material combination are as follows

Material combination

SEN Range

Roughing

Semi finish

Finish

Copper to Steel

7 to 8

6 to 7

5 to 6

Graphite to steel

7 to 8

6 to 7

5 to 6

Copper to carbide

7 to 8

6 to 7

5 to 6

ASEN Anti arc Sensitivity

This parameter decides sensitivity of antiarcing circuitry. This sensitivity decreases with the value of ASEN and the antiarc action gets bypassed at ASEN=1.

Recommended ASEN range for different material combination are as follows

Material combination

ASEN Range

Roughing

Semi finish

Finish

Copper to Steel

2 to 3

3 to 4

3 to 4

Graphite to steel

2 to 3

3 to 4

3 to 4

Copper to carbide

2 to 3

2 to 3

2 to 3

Typical Applications and Commonly faced Problems

Rib machining.

If length to width ratio exceeds 25 then that cross section falls under rib category.

e.g. 25 X 1mm, 50 X 2mm, 75 X 3mm, 100 X 4mm etc.

Copper electrode

Recommended to use oxygen free copper for best results. Inferior grade of copper can result in more electrode wear. Generally copper electrodes are used for sections which are more than 2mm thick. For thin sections copper electrodes can warp due to heat. Also electrodes wear can be more as the current density is more. In such cases use of SAFE mode is recommended to give ultra low electrode wear.

Word of caution Switch off SAFE mode in finishing.

Graphite electrode

Recommended to use fine grain structure graphite (1 to 4 Microns).Coarse grain graphite may disintegrate into the cavity causing contamination of gap resulting into instability. For sections which are less than 2 mm thick graphite electrodes are preferred over copper as graphite has good thermal stability as compared to copper. Hence warpage of electrodes due to heat during sparking is avoided.

Word of caution Switch off SAFE mode during use of graphite electrodes.

General guidelines for rib machining

1. Use Jump function with lower working time (TW=0.4 to 0.6) and depth proportionate RD values to avoid arcing problems.

2. To avoid arcing lower values of Tau than the standard recommended are to be used.

3. Continuous supply of clean dielectric should be ensured for arc free rib machining.

4. Side jet flushing should be along the length of the rib and not across.

5. Excessively high flushing pressure can cause deviation of very thin electrodes. Minimum required flushing is advised.

Fine Finishing.

Large Area

Contact area more than 5000 Sq.mm is considered as large area in EDM process.

Copper electrode

Recommended to use oxygen free copper as it wears out less.

Graphite electrode

Recommended to use 1 micron grain structure graphite for fine finish.General guidelines for large area finishing.

1. In EDM achieving fine finish on large area has a limitation as compared to smaller area. This is because the capacitance value of the gap increases with the electrode area, which in turn increases the power of spark resulting in rough surface finish

2. Current density should be maintained as per the guidelines given above.

3. The settings used for roughing operation depend on the number of electrodes planned to be used.

If the cavity is to be finished using single electrode then the roughing settings should be necessarily of low wear type. For multiple electrodes efficient settings for roughing can be used.

4. Matching of finishing electrode in roughed cavity becomes very critical in order to achieve uniform fine finish. Quick fixing precise tooling systems for electrode holding are recommended in such cases.

5. For large area it is very important that enough amount of flushing should be provided all over the contact area.

6. Frequent cleaning of cavity in final stages of finishing takes away the accumulated carbon film from the cavity. This can make the process unstable after resumption and prone to arcing or uneven finishing.

7. For large areas, a pulling force is exerted on the electrodes due to vacuum created in the gap. As the spark gap decreases with EDM parameter settings, this pulling force increases. Hence if high speed jump is used there are chances of electrode getting shifted. To avoid this phenomenon it is strongly recommended to use soft jump function along with high speed jump.

Small Area

Contact area less than 100 Sq.mm is considered as small area in EDM process.

Copper electrode

Recommended to use oxygen free copper for best results.

Graphite electrode

Recommended to use 1 micron grain structure graphite for fine finish.

General guidelines for small area finishing.

1. Most important prerequisite to achieve fine finish is to have electrode surface finish better than the required finish in the workpiece.

2. Thin carbon layer formed during finishing in the cavity helps to dissipate energy and thus achieve uniform fine finish in comparatively less time. Hence feeble flushing or no flushing is recommended during erosion.

3. For engraved electrodes having intricate details like coining dies it is recommended use high speed jump with very small working time (TW) and practically no side jet flushing.

4. Use of reverse polarity (electrode negative) in last finishing setting gives mirror finish to the job, the quality of mirror finish depends upon workpiece material, grade and its hardness. Special materials like Stavax, NAK 80 etc. are some of the popularly used tools steels from different manufactures due their high polishability.

Carbide machining.

Tungsten carbide has two different grades one is relatively softer and the other one is hard. Best material to machine carbide on EDM is copper tungsten. Copper graphite can also be used which has comparatively higher wear resistant properties.

General guidelines for carbide machining.

1. Use lower on and off times. That means carbide machining needs high frequency.

2. Electrode wear is nearly 30 to 50%.

3. Carbide machining is very slow as compared to steel machining.

4. Use of through job flushing (most of the carbide dies have hole in the centre) can eliminate the need of jumping operation. This makes the machining process much faster.

5. Positive polarity may work for softer grades but for harder grades of carbide negative polarity is mandatory.

Forging

In general as a concept, forging dies are considered to be low accuracy requirement dies. Erosion of forging dies asks for more material removal rate than any other result like surface finish or electrode wear. The electrode sizes in forging applications are bigger as compared to other applications like plastic moulding. Generally graphite electrodes are preferred because of their lower density.

General guidelines for forging dies.

1. Since the surface finish requirement is not so important, even coarse grain structured graphite can also be used for this application. In such case the flushing becomes very important as the coarse grain structured graphite disintegrates and this graphite dust interferes with the sparking process causing instability.

2. It is advised to have holes in the electrode for flushing. Through electrode flushing can accelerate the EDM process and save time. Also arcing tendency is reduced substantially due to more efficient flushing.

3. Use of reverse polarity (electrode negative) increases the material removal tremendously.

4. Copper electrodes can also be used in case the electrode size is not very big.

Case Studies

Description:

Sparking Pre machined mobile case die.

Work-piece:

Material : Stavax

Hardened : Yes

Pre machining : Yes

Electrode:

Material : Oxygen free copper

Roughing settings :

Ton

100

75

50

20

10

Tau

10

8

8

8

8

Ip

6

3.0

3

2

2

IB

Auto

Auto

Auto

Auto

Auto

Vg

60

60

60

60

60

SEN

6

6

6

6

6

Asen

3

3

3

3

3

AF/ JUMP

06

06

06

06

06

Tw

0.6

0.6

0.6

0.6

0.4

RD

3.0

3.0

3.0

3.0

3.0

POL

Normal

Normal

Normal

Normal

Normal

Finishing Settings:

Ton

20

20

20

10

5

4

2

Tau

8

8

7

7

7

6

6

Ip

0.5

0.5

0.5

0.5

0.5

0

0

IB

Auto

Auto

Auto

Auto

Auto

Auto

Auto

Vg

90

100

125

150

150

150

150

SEN

6

6

6

6

6

6

6

Asen

3

3

3

3

3

3

3

AF/ JUMP

06

06

04

04

04

04

04

Tw

0.6

0.6

0.6

0.4

0.4

0.4

0.4

RD

3

3

3

4

4

4

4

Polarity

Normal

Normal

Normal

Normal

Rev

Rev

Rev

Description:

Erosion bunch of 7 copper electrodes in steel and Nickel alloy.

Work piece:

Material : Steel

Electrode:

Material : Electrolytic copper

Ton

75

Tau

10

Ip

20

Sv

60

Ig

7

Vg

60

SEN

6

Asen

3

JUMP

06

Tw

1.0

RD

2.0

Flushing Type

Side flushing

SAFE

On

Work piece:

Material : Nickel alloy

Electrode:

Material : Electrolytic copper

Ton

20

Tau

10

Ip

10

Sv

65

Ig

5

Vg

60

SF

7

Asen

2

JUMP

06

Tw

1.0

RD

2.0

Flushing Type

Side flushing

SAFE

Off

Description :

Erosion of a cavity using single copper electrode of 53 mm X 70 mm to get good surface finish.

Work-piece:

Material : Stavax

Hardened : Yes

Pre machining : No

Electrode:

Material : Oxygen free copper

Settings used:

Ton

50

20

10

5

5

4

2

t

8

8

8

8

8

8

8

Ip

0

0

0

0

0

0

0

Ib

5

5

3

1

0

0

0

Vg

75

80

100

125

150

175

175

Sen

6

6

6

6

6

6

6

Asen

3

3

3

3

3

3

3

Jump

06

06

06

06

06

06

06

Tw

0.6

0.6

0.6

0.6

0.4

0.4

0.4

Rd

5.0

5.0

5.0

5.0

5.0

5.0

5.0

Depth

-0.075

-0.100

-0.125

-0.150

-0.180

-0.190

-0.200

Polarity

Normal

Normal

Normal

Normal

Normal

Normal

Normal

Description:

Erosion of fastener dies in carbide and steel.

Work piece:

Material : Carbide.

Electrode:

Material : Copper tungsten

FINISHING

ROUGHING

Ton (sec)

15

Ton (sec)

50

Tau

8

Tau

8

Ip

3

Ip

6

Vg

60

Vg

60

SF

7

SF

7

Asen

3

Asen

2

Jump

00

Jump

00

Tw

00

Tw

00

Rd

00

Rd

00

Flushing

Type

Through

flushing

Flushing

type

Through flushing

SAFE

OFF

SAFE

OFF

Work piece:

Material : Hardened Steel (SKD 11)

Electrode:

Material : Copper tungsten

FINISHING

ROUGHING

Ton (sec)

50

Ton (sec)

150

Tau

10

Tau

10

Ip

2

Ip

4

Vg

75

Vg

60

Sen

6

Sen

7

Asen

4

Asen

3

Jump

00

Jump

00

Tw

00

Tw

00

Rd

00

Rd

00

Flushing

Type

Through

flushing

Flushing Type

Through flushing

SAFE

ON

SAFE

ON

Description:

Sinking of aluminium die casting cavity of automobile component

Work piece:

Material : Tool Steel

Electrode:

Material : Electrolytic copper

Roughing

Finishing

Ton (sec)

200

Ton (sec)

50

Tau

10

Tau

10

Ip

20

Ip

6

Vg

60

Vg

70

Sen

6

Sen

6

Asen

4

Asen

4

Jump

06

Jump

06

Tw

1.0

Tw

1.0

Rd

3

Rd

3

Flushing

Type

Side jet

flushing

Flushing Type

Side jet flushing

SAFE

OFF

SAFE

OFF

Description:

Erosion of wrist watch cavity

Work piece:

Material : Tool steel

Electrode:

Material : Oxygen free copper

Settings used for roughing:

Ton (sec)

200

150

100

50

20

tau

10

10

10

10

8

Ip (Amps)

10

8

6

3

2

Vg

50

50

50

60

65

Sen

8

8

8

8

8

Asen

2

2

2

2

2

Jump

00

00

00

00

00

Tw

NA

NA

NA

NA

NA

Rd

NA

NA

NA

NA

NA

Flushing Type

Through work piece

Through work piece

Through

work piece

Through

work piece

Through

work piece

Polarity

Normal

Normal

Normal

Normal

Normal

Settings used for finishing:

Ton (sec)

50

20

10

5

tau

10

10

8

8

Ip (Amps)

3

2

1

0.5

Vg

60

65

75

85

Sen

7

7

7

6

Asen

2

2

2

2

Jump

00

00

00

00

Tw

NA

NA

NA

NA

Rd

NA

NA

NA

NA

Flushing Type

Through

work piece

Through

work piece

Through

work piece

Through

work piece

Polarity

Normal

Normal

Normal

Normal

Description:

Template mould cavity

Work piece:

Material : Tool steel

Electrode:

Material : Oxygen free copper

Settings used :

Parameters

Roughing

Semi finish

Finish

Ton

150

50

20

Tau

10

10

8

Ip

8

3

2

Vg

50

75

80

Sen

6

6

6

Asen

4

4

4

Jump

06

06

06

Tw

0.8

0.8

06

Rd

2.0

2.0

3.0

Flushing Type

Side

flushing

Side

flushing

Side

flushing

Polarity

Normal

Normal

Normal

Description:

Center support 4 cavities

Work piece:

Material : Hardened Steel

Electrode:

Material : Oxygen free copper

Settings used:

Parameters

Roughing

Semi finish

Finish

Ton

20

20

10

Tau

10

8

6

Ip

2

1

1

Vg

75

85

90

Sen

6

6

6

Asen

4

4

4

Jump

06

06

06

Tw

0.8

0.8

0.8

Rd

2.0

2.0

2.0

Flushing Type

Side

flushing

Side

flushing

Side

flushing

Polarity

Normal

Normal

Normal

Description:

Finishing in carbide

Work piece:

Material : Carbide

Electrode:

Material : Oxygen free copper

Parameters

Rough

Semi finish

Finish

Ton (sec)

50

20

5

2

2

tau

6

6

4

4

4

Ip (Amps)

5

2

0.5

0

0

Vg

50

50

60

80

90

Sen

7

7

7

7

7

Asen

3

3

3

3

3

Jump

06

06

06

06

06

Tw

0.4

0.4

0.6

0.6

0.8

Rd

2.0

2.0

2.5

2.5

2.5

Flushing Type

Side jet

Side jet

Side jet

Side jet

Side jet

Polarity

Rev

Rev

Rev

Rev

Rev

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