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Emerald Group Publishing Limited
Copyright © 2004, Emerald Group Publishing Limited
Time to wipe the slate clean?
Time to wipe the slate clean?
Keywords: Intech, Graphite, Electrodes, Manufacturing
Intech, part of Agie Charmilles and the EDM consumables and wear parts specialist, is urging EDM die-sink users to consider the significant advantages that graphite has over copper when used for the manufacture of electrodes.
Viewed by many as a dirty and dust-producing material, graphite produces negative reactions amongst some EDM users – and yet the performance of the material in terms of ease of machining, weight, heat resistance, etc. suggest that graphite can provide real productivity benefits for those prepared to see beyond the popular myths surrounding the materials' dust-producing properties.
As part of Intech's “Consumability” campaign – an initiative designed to ensure that EDM users in the UK know how to get the most from their consumables – and which was launched in January 2004 – the company is on a mission to educate EDM users about the pros and cons of using graphite within their operations. There are no hard and fast rules – except to say that the advantages would appear to outweigh the disadvantages.
The advantages of using graphite as an electrode material can be summarised as follows.
Easy to machine
A typical electrode can take 16h to machine using copper and only 4h using graphite.
Easy to hand finish
A machined electrode will usually retain machining marks which, if not removed, will be reproduced on the machined workpiece. These marks, caused by rotating cutters, can be removed easily on graphite by using fine emery paper.
Higher current density means faster cutting and less wear
Graphite electrodes are capable of carrying a higher current density than copper. Because of this more power can be used and, as a consequence, roughing out times can be reduced significantly. The volumetric wear on graphite electrodes is also reduced in roughing operations by as much as 30 per cent in some instances.
Graphite weighs less than copper and clearly this becomes an important consideration when large electrodes are being manufactured. Large and heavy electrodes can affect machine wear and lead to poorer erosion quality.
Low co-efficient of expansion
Graphite expands slowly when heat is applied (0.25 times copper). In all EDM processes heat is generated despite the availability of chiller units. Any increase in electrode size caused by thermal expansion will affect the size of the spark cavity ... and thus overall EDM performance. Greater precision can be achieved with graphite.
High heat resistance
Copper melts at 1,083° and graphite at 3,650°C. During erosion graphite is unaffected by the heat generated and will not reach its sublimation point.
There are some disadvantages in using graphite as an electrode material – these are as follows.
When graphite is machined dust is created and this makes electrode manufacture a dirty process. However, many milling machining centres can be equipped or come supplied with powerful dust extraction units that remove the dust during the machining process thereby eliminating the problem.
Graphite is abrasive and when mixed with suds or oil can create a paste that could have a wearing effect on the slideways of the machine. However, the dust extraction facilities mentioned above will reduce any abrasive effect and simple maintenance procedures can be employed to remove thin layers of film or dust that might remain.
Graphite is easily damaged. However, graphite will break rather than dent – enabling quick and easy identification of any damage. The key considerations when using graphite are quite clearly – care and attention.
The cost of graphite depends on the quality and grade of the material chosen and this is an important cost consideration. Large uncomplicated electrodes do not need to be made from the more expensive fine grain graphite.
Graphite's performance in the EDM process
Graphite electrodes can be used on all modern “pulse” generators. The performance of the electrode is inextricably linked to the grade of graphite used. Large grain sizes are suitable for roughing operations and small grains are more suited for fine detail and finishing operations. Graphite is not used for super-finishing (polishing) operations. Generally speaking for finishes down to CH24 (1.6 μm Ra) – graphite is an ideal material.
During roughing operations a higher power level can be used when using graphite. This means that larger sparks are used and that metal removal rates are higher and electrode wear is lower.
The performance of graphite will be influenced by a number of different factors – workpiece material, dielectric, machine type, maintenance of the machine and operator knowledge. These variables can be influenced and changed so that the true performance benefits of graphite can be realised.
Says Martin Spencer, sales and marketing manager at Intech:
There's a lot of misinformation that surrounds the use of graphite in the EDM process. As part of our “Consumability” initiative we want EDM users to explore the full potential of graphite as an electrode material. There are obvious productivity benefits to be gained.