Superior Surface Finishes in EDM
Over the years, EDM technology has undergone countless improvements, and the driving force behind many of these improvements has always been the search for finer surface finishes at faster speeds. Vast improvements in power control sophistication, machine reaction time, electrode materials and dielectric fluids, all have played a part. However, the quest for an even finer finish at a faster speed never ceases.
Various machine tool builders have collectively raised the bar at different times in terms of speed, finish and surface integrity. A new technology recently introduced by Makino, called High Quality Surface Finish (HQSF), has raised the bar once again.
Makino’s HQSF process is a new EDM technology that utilizes a mixture of conductive powder in the dielectric fluid. The technology relies on this advanced additive, special filtration systems, controller software and other enhancements to produce superior surface finishes in dramatically shorter times.
Fine surface finishes can be achieved with existing EDM technologies and processes, but with major limitations. For instance, using copper as an electrode material produces excellent surface finish over small areas. Likewise, other additive technologies besides HQSF produce similar results even in large work pieces. But both processes pose major problems in other areas that make them less attractive and not widely accepted.
EDMing with Copper
Copper electrodes can achieve mirror finishes on small work pieces for upto 2 sq. inches area and no more. As the workpiece area gets larger, and amperage is sent through the copper electrode, the electrode cannot be contained. This is due to copper’s density and expanding characteristics. As a result, a fine finish becomes increasingly difficult to achieve for larger cavities.
Machining of a copper electrode is also very difficult, because it run’s up a burr or what is sometimes called a “rolling edge”, which requires additional secondary operations to remove. The HQSF process can also be used with a copper electrode, but it does not eliminate the disadvantages of machining copper or substantially reducing roughing time. However, it does enhance copper’s ability to achieve superior finishes in larger work pieces.
Limitations of the EDM Process with Graphite Electrodes
With conventional EDM processes, the reason fine finish cannot be achieved is primarily due to the grain structure of the electrode material. In the EDM process, the spark gap controls your ability to achieve good finishes. For a manufacturer who would like to achieve a finer finish, of say 5 mRmax, the graphite electrode limits his ability to achieve such a finish because it interferes with the machine’s ability to maintain the appropriate spark gap. Graphite is particulate. It is a structured material made of granules, all of which are approximately the same size.
Moreover, as the workpiece gets bigger, surface finish quality decreases even more so. This is due to the electrode’s grain particles ganging up, or sticking together. Grain particles gang up even more as the size of the electrode increases before they are able to exit the gap causing hard spots. In addition, its electrical resistivity elevates and the electrode becomes more difficult to control. Therefore, there is a direct correlation between the size of the electrode and the surface finish that can be achieved.
Additive or powder based EDM technologies have been around for about thirty years. By putting a powder additive acting as a semi-conductor in the dielectric fluid and sending amperage through the fluid in the gap itself, abnormal and secondary discharges can be eliminated. But this process has not become generally accepted because it uses toxic material, such as silicon, chromium and aluminum.
HQSF and uSC Additive
Makino’s HQSF process uses a proprietary powder additive, called SC. It is more cost effective than chrome and silicon particulate, costing approximately 90 percent less. Its specific gravity is almost zero, allowing it to float in oil at all times, resulting in even dispersion throughout the entire cavity. This provides the ability to machine deep cavities, while also achieving excellent sidewall finishes.
Since the powder has negligible specific gravity, both roughing and finishing can be done with a single dielectric system, thereby eliminating the need for two tanks and heavy cleanups between jobs. Flushing is unnecessary. Like standard EDM machining, orbital motion is enough to keep the additive moving and evenly dispersed. When using silicon or chrome, flushing becomes a critical issue due to the higher specific gravity of these metals.
The particulate size for SC is 5 microns. To get a 3 mRmax finish, the electrode has to be within 8 microns of the workpiece. But if you take the 5 micron SC particulate dispersed in oil and add the 10 micron granule from the graphite wear, together the two will not be able to exit the 8 micron spark gap, creating an unstable machining environment. This generates highly unproductive secondary discharging. However, the HQSF process allows energy to be fired through the SC semi-conductor, since it effectively dissipates the energy to the workpiece. Therefore, the electrode only has to get within 13 microns of the workpiece and 8 microns of the semi-conductor, meaning the 10 micron graphite granule can now exit the cavity gap, resulting in a smooth, continual burn.
Diminishing the Heat Affected Zone
The surface finish left by EDM can consist of a thermally affected layer of material, referred to as the HAZ. Sometimes it is termed as “recast”, but only a very small part of the HAZ has recast particles. These recast particles are present because they re-solidified on the workpiece surface. In other words, the material was in a molten state but was not removed by the flushing action of the dielectric. Ordinarily these particles can be removed by common but costly secondary operations.
HQSF minimizes HAZ by as much as 5:1 ratio, over standard EDM technology. HQSF reduces HAZ because less amperage needs to be sent through the electrode to burn the cavity in the workpiece, and the heat that does come through is dissipated by the SC semi-conductor. As a result, the heat does not penetrate very deeply into the workpiece. This diminishes the HAZ, and consequently, substantially reduces polishing. In fact, hand-polishing times can be reduced by up to 50% with HQSF. This also increases accuracy.
The HQSF process can be best utilized by the plastic injection molding industry, with its small to medium sized parts, which are very difficult to polish. By reducing hand polishing, accuracies are better maintained and cost reduced. Better surface finishes can also be achieved on larger parts. The other industry that is drastically impacted by HQSF is Die Casting. Here, HQSF provides greatly extended die life because HAZ is reduced. In die casting, when pouring hot metal into the finished workpiece, the workpiece will fatigue quickly exposing any blemishes in surface integrity.
Current EDM technology is limited, as compared to HQSF. EDMing small ribs with the prevailing EDM technology require hand polishing for better mold release. Smaller cavities, such as 50x50mm size, using copper or graphite electrodes need polishing, due to thick hardened layers in the HAZ. But with HQSF, when EDMing small ribs, side finish is much better. As a result, molded parts release without polishing. Also with smaller cavities, surface finish is easily polished, due to the uniform fine finish and reduced hardened layer.
HQSF solves the limitations of additive technology because it is cost effective to use, maintains accuracy, reduces HAZ, improves surface finish, consistency, roughing and finishing speeds, does not require additional machinery maintenance and is environmentally safe.