Bonded Graphite Electrodes

As EDM applications become larger and larger, the size of the graphite electrode material must be bigger. This is one reason why electrode bonding has become so important. However, a major concern among many end users is the EDM performance of the bonded electrode assembly.

Electrode bonding can be used for small and large applications.

For most EDM shops, a bonded or pieced graphite electrode is an option. But, it’s a necessity for manufacturers of large plastic injection molds, forging dies, or other large parts. Although some smaller applications of pieced electrodes exist, most of the bonded electrode applications are used in large cavity molds.

Generally, when you bond graphite material to form larger electrodes, you also deal with high-amperage EDM applications. When high amperage is applied (i.e., 16 amps or greater), the ridge left in the workpiece due to the bond joint usually disappears. Eliminating this ridge contributes to the following factors:

 Amount of Overburn. When two pieces of graphite are bonded together with properly prepared surfaces, then the overburn or overcut created by the spark will usually eliminate the ridge generated by the nonconductive bond seam. If the bond gap is wide and low amperage is used, the workpiece will likely have a ridge remaining after a nonorbiting EDM burn. If the EDM has orbital capability, then you don’t have to worry about leaving a ridge in the part, provided the bond was not too wide.

 Wear Resistance. If the actual particle size of the graphite used in the bonded assembly is known, then it is possible to select the correct EDM operating parameters. Also, the pieces of graphite to be bonded together must be of the same manufacturer’s grade. Housebranded grades need to be verified to eliminate the possibility of blocks of material with variance in material properties. If different grades were joined in a bonded assembly, the bonded electrode would perform as two separate electrodes.

Some experiences have shown that the material with the higher electrical resistance also will control the speed of metal removal of the pieced material. This creates variances in surface finish, ridges in the workpiece, and arcing caused by differences in electrical conductivity. If arcing occurs, the material with the highest electrical resistance could cause the bonded assembly to perform erratically in the application.

 Dimension. The width of the bond joint helps determine the success of EDMing with a bonded assembly. Bond joints were tested with dimensions of 0.040" and 0.003". As expected, the bond with the narrow gap gave the best results.

 Surface Preparation. Some applications do not allow the use of high amperage during the EDM burn. The physical size of the electrode or a highly-detailed electrode may prohibit high amperage. In these applications, proper preparation of the graphite blocks to be bonded becomes very critical. Due to nonconductivity of the adhesive in the bond joint, a ridge will be formed in the workpiece. Testing has been done to show how to eliminate this ridge.

Flatness Requirements

For bonding of graphite blocks, electrode manufacturers rough finish the mating surfaces to a flatness of maximum ± 0.020" for a 24"X 40" block. Increased flatness requirements should be utilized, since this reduces or eliminates the ridge left in the part after EDMing.

Engineers at Poco Graphite Inc., Decatur, TX, tested two groups of samples: one group with a bond gap of ± 0.040" and one group with a bond gap of 0.003". The sample group with the ± 0.040" bond gap was prepared by using a large belt sander with a #20-grit abrasive. The sample group with the 0.003" bond gap was a true surface-ground finish on both mating surfaces. The surface was prepared using a normal surface grinder (i.e., the same type of machine that is found in most machine shops).

Generally, a surface-ground finish is too smooth to allow proper bonding. The ground surfaces were roughed up with 120-grit sandpaper on an orbital sander as part of the preparation process.

Care must be taken to produce a finely scratched surface without changing the overall flatness of the block. After sanding, the surfaces may be wiped clean with a dry cloth and then blown clean with compressed air. Alcohol may be used to clean the surfaces. However, it’s important to make sure that all the alcohol is evaporated before applying adhesive. A tack cloth can be used as a final preparation to remove dust without leaving any residue on the graphite surface.

Applying Adhesive

Properly bonded electrode material makes the bond joint very difficult to find.

At this point, blocks are prepared for adhesive application. After mixing, the adhesive is applied to all mating surfaces intended for bonding. Application is done with a common 4" putty knife. The glue must spread easily while completely wetting the graphite surface. All mating surfaces must have sufficient adhesive to wet the entire surface. Excess adhesive will be pushed to the bond edges.

After the pieces are placed together, clamps are applied to the assembly. The clamps should be checked for tightness as the adhesive begins to set. If cured at room temperature, the total curing period would be seven days. However, it’s possible to speed up the curing process with the use of a heating cycle. After the assembly has set for 24 hours, you must raise the ambient temperature to 150°F for approximately six hours. This will provide a properly bonded electrode assembly, ready to machine, in less than 48 hours.

The billets bonded with a flatness of ± 0.020" on a 24" X 40" area were tested in actual applications at various amperage settings under nonorbiting conditions. The assemblies were tested on O-6 tool steel. At low amperage (i.e., +16 amps/17.9 kHz), we discovered a ridge in the workpiece that ranged from 0.006" to 0.004" high and 0.013" to 0.028" wide. A slight ridge in the workpiece was still visible at 64 amps/17.9 kHz.

When tested at 64 amps/8.9 kHz (about 100 microseconds on-time), no visible ridge was found in the workpiece after EDMing. This held true for burns made in negative polarity as well. These tests were conducted on blocks with a lenient flatness tolerance. In actuality, the bond gap could have been as much as 0.040" to 0.050" at any given point.

No Visible Ridges

The EDM testing done on pieced electrode material with true surface flatness, then sanded, produced no visible ridges in the workpiece at any of the tested conditions. The thinner bond joint allowed the pieced electrodes to perform as a single block.

The electrodes need to be configured in a manner that puts the bond line perpendicular to the workpiece. This configuration eliminates any ridge that might be formed by using orbital

EDMing. A bond that flows parallel to the workpiece can be used to extend the length of an electrode. However, preparation to ensure electrical conductivity via dowels or mounting devices may need to be utilized.

If problems arise, it is usually due to the nonconductive property of the epoxy. Testing was done adding graphite and other conductive materials to the epoxy. These additives increased electrical conductivity within the epoxy, but weakened the flexural strength of the bond joint.

When we tested the flexural strengths of the normally bonded epoxy assemblies, we discovered that the joints were often as strong as the graphite itself. This is assuming that the blocks were prepared properly, and the manufacturer’s recommendations on bonding with epoxy were followed.

Bonded graphite electrodes have expanded the use of EDM as a machining process. Size limitations of the graphite electrode materials are no longer a limiting factor. Using a pieced graphite electrode is a more-than-satisfactory solution for builders of large plastic injection molds, forging dies, and other large parts.

About the Author
Steve Slate is an EDM applications engineer at Poco Graphite Inc., Decatur, TX.