ELECTRONIC DISCHARGE SAWING

A new metal cutting process called electronic discharge sawing (EDS) incorporates spark erosion – the basic operating principle of wire EDM – with a rotating mild steel belt to promise less material waste, less environmental impact, greater productivity and low operating costs.

In EDS, a mild steel belt is stretched across an automatic tensioning system. The only reason mild steel is used for the blade, instead of some other material, is that mild steel is very accessible and really inexpensive to use. Theoretically, any electrically-conductive material blade will work. As the rotating band is lowered onto the workpiece, sodium silicate dielectric is sprayed across the band and the spark-erosion process begins, cutting at an average of 2.5 to 3 square inches per minute.

Notice that the average cutting speed of EDS is not measured in inches per minute, but is instead measured in square inches per minute. This method of measuring the cutting speed is similar to wire EDM. When referring to the maximum cutting speed of 1.5 to 2.5 square inches per minute of EDS, this signifies that a 2.5 in long job can be cut by EDS with a speed of 1 inch per minute. The work piece width does not affect the cutting speed significantly, only if the width is sufficient to handle the maximum current. Otherwise, it may vary marginally. Also, because the cut width is around 1.5 mm, which is 1 mm more than the actual belt width of 0.5 mm, there is less material waste using this process.

The electronic discharge saw, developed by Electronica Machine Tools Limited (Maharashtra, India), will cut any conductive metal, irrespective of hardness. Using spark erosion technology, it will cut through exotic alloys as much as 70 percent faster than wire EDM or conventional band saw cutting. The speed lies in the machine's proprietary electronics. The blade system can cut through hard material faster by the discharge of sparking energy taking place between the blade and the material being cut, where the sparking energy melts through the material.

For example, one extremely dense metal composite that had previously taken four hours to cut by wire EDM can now be sawed by this process in 26.5 minutes. The heat affected zone is about the same as wire EDM at less than .05 mm, and the cut is very straight. EDS has an extremely low wave tolerance, which signifies that the cutting is quite straight, without any waviness. The EDS tolerance on the straightness is typically ± 1 mm. Cuts can be made on round, straight or angled surfaces with a simple pneumatic clamping system.

Due to the small sawing force, an EDS machine is easily operated and maintained. Setup of new jobs includes the entry of standard Z-axis feed programming, which is very simple and quick on a touch panel. Once a design is set, it can be stored and reused. Pre-programmed macros for standard recipes, such as shapes, are included. The typical saw includes a programmable infeed and can be fully automated. An automatic change of cutting parameters according to Z depth, such as current, on-time of the pulse, sparking voltage and Z-axis feed rate, allows efficient cutting of uneven shapes.

Trying to compare the EDS process with wire EDM is rather inappropriate because each have totally different applications. The similarities of EDS to wire EDM lie in their basic operating principle of spark erosion and their common end use of metal cutting. The application areas of wire EDM are extrusion dies, wire drawing dies and other similar mold making, while EDS has its main application in cutting raw material ingots or blanks. As such, the EDS process primarily replaces conventional band sawing. As a matter of fact, it may replace any metal cutting process, which usually is very difficult or not possible, due to the hardness of the material that is to be cut.

EDS differs from wire EDM in end results, cutting speed and surface finishes. The wire EDM process basically uses lower cutting currents as compared to EDS and hence lacks in cutting speed. Wire EDM has a good surface finish and very high accuracy levels, but can take a long time to cut through extremely hardened composites. EDS uses very high cutting currents allowing for the same accuracy, but at a much faster cutting rate. EDS will cut materials such as super composites in a fraction of the time of wire EDM. While the surface finish may be slightly less smooth than EDM, the heat affected zone remains less than .05 mm.

One common question about the EDS process asks why not just have a fixed belt acting like a ram electrode? Is the rotation speed constant or adjusted for material? Instead of being a fixed electrode, the rotating belt used in EDS ensures even wear over the entire length of the belt. The rotation speed is usually constant for all materials. This movement of the belt also helps to effectively remove debris from the cutting area and improve cutting efficiency.

The consumables in EDS include the mild steel used for the blade, which is readily available and must be changed after three cuts of 300 mm diameter, and sodium silicate, a water-based dielectric that is available by the drum. The machine is self-lubricating and rarely needs to be refilled. Inexpensive nozzles and ceramic guides are the parts most replaced.

The ceramic guides keep the belt straight (hence, it wears). These ceramic guides can be overturned and reused again. As a result, the ceramic guides should be replaced after around 300 hours of sparking. The carbon brushes are used for energizing the belt. The wheels, over which the belt runs, are connected to 2 slip ring assemblies. Carbon brushes are used to energize the slip ring assemblies which, in turn, energize the belt.

An EDS machine is inexpensive to operate. It typically uses about $4.00 per hour of electricity and the mild steel blade used as the electrode averages about four cuts, takes under two minutes to replace and costs about $1.40 per blade (a strip butt welder is provided for making the bands). It uses 200 gallons of non-toxic, non-corrosive sodium silicate dielectric that is easily disposable by truck at approximately 35 cents a gallon. The EPA defines the sodium silicate used in this application as non-hazardous. Due to small amounts of metal in the silicate, screening and flushing could be done, but it would require a filtration system that is likely to be more expensive to maintain.

While EDS is new to the North American market, India has been using this process over the last eight years. The markets for EDS include aerospace, defense, nuclear and solar energy, medical devices and virtually any supplier and R&D companies working with the fast changing metals market.

Stephen Ross is the vice president of Ross Group Equipment Technologies, 51445 Oro Drive, Shelby Township, MI 48315, 810-441-5908, ross@rossgroupglobal.com, www.RossGroupGlobal.com.