Wire EDM Filtration Comes of Age

A look at the evolution of wire EDMs over the past 10 to 15 years has shown dramatic improvements in machine tool construction, power supplies, CNC controls, and operator interfaces. There have been improvements in almost every area except one: EDM filtration. Wire-EDM filtration, admittedly not a glamorous subject, seems to be one of those items that goes by the rule, "If it ain’t broke, don’t fix it."

However, there are many EDM owners and operators who feel that wire-EDM filtration needs to be improved, if not fixed altogether. The traditional approach to filtration using paper cartridges is both economically and environmentally costly. Shops with multiple EDM machines know this better than anyone; their costs to purchase and dispose of these filters, as well as the cost of downtime on their machines, add up to a significant portion of their overhead. Improvements in EDM filtration are slowly starting to appear and offer a viable solution for real-world production machining.

Traditional Filtration Media

Filtering is necessary to remove the contaminants that the wire-EDM process leaves in the dielectric fluid. If left in the fluid, these contaminants can interfere with the electrical discharge between the wire and the workpiece, reducing the quality and the speed of the cut. Unfortunately, these contaminants pose a threat to humans and the environment in the form of groundwater contamination; therefore, there are few safe places to put them once they have been removed from the fluid.

Table 1: An analysis of the particulate in the dielectric water after EDMing a 7"-high, A-2 tool-steel workpiece with 0.010"-dia. wire at 10.0 amps. 

Table 2: The federal Environmental Protection Agency’s allowable limits for hazardous metals. State regulations may be stricter.

The EDM wire itself is the source of much of this hazardous contamination. One analysis of the contaminants produced when an A-2 tool-steel workpiece was EDMed found that about 48.5% of the waste was copper and zinc eroded from the EDM wire (Table 1). The disposal of materials such as copper and zinc is regulated by local and state laws, because these materials can leach out and poison an area’s groundwater if they are disposed of in a landfill improperly. Table 2 shows the federal Environmental Protection Agency’s allowable limits for metals considered hazardous to ground-water.

Because dielectric-fluid contaminants pose a threat to both the EDM operation and the environment, EDM shops must consider how much it will cost to both remove and dispose of the contaminants when they evaluate their filtration needs. EDM shops use a variety of different media types and particulate-retention capabilities in search of the holy grail that is EDM filtration. Every shop is searching for an acceptable balance between filtration efficiency and cost.

Most knowledgeable EDM users would agree that clean water is essential for fast, high-quality cutting. Generally, shops have considered a filter that can remove pArticles down to 5µm or 10µm in size sufficient to get their water clean. However, many applications produce smaller pArticles and could benefit from the use of a higher quality filter with a rating of 3µm or better. The EDMing of D-2 tool steel is an example of such an application. One analysis found that 81% of the pArticles produced by such an operation were less than 5µm. Generally, finer filtration will produce the most dramatic improvements when used with machines capable of finer finishes. These machines typically produce smaller particulate. Also, the cleaner water produces fewer false voltage readings that cause the machine to back up in applications where a smooth, steady servo action is required to produce a good surface finish.

Additionally, the use of dirtier than normal dielectric fluid or dielectric fluid filtered through media larger than 5µm will reduce resin life, because the resin (which is designed to remove soluble particulate) also is an excellent filter for the finer solid particulate that passes through many of the conventional filter cartridges on today’s market. By using a system that filters particulate down to 3µm, a shop can extend resin life by 200% or more. Because the finer filtration produces cleaner EDM water to start with, less particulate is trapped in the resin filter columns, which are really designed to deionize the water, not filter out contamination.

But despite the advantages that can be gained through finer filtration, such a quality level is neither practical nor cost effective when traditional pleated cartridge elements are used. The finer the retention rating of the filter, the more expensive it is to purchase. Filter cartridges capable of fine retention also drive up the operating cost of the equipment, because these filters get clogged easily and need to be replaced more often. In addition to the cost of the media itself, the user must consider the cost of the nonproductive time that accumulates as the machine sits idle during filter changeout and the labor cost required for this routine maintenance.

Shops find it a daunting task to assess the total financial impact their choice of filter-media type and filter rating will have on their operation. As a result, many shops accept their filtration expense as a necessary evil, which they pay for on a filter element-by-element basis; they never track what they are paying for the elements or for any associated costs. Some shops can tell you what they are paying for filter media, but they haven’t analyzed any other costs.

It is usually the more experienced shops with multiple machines that have determined what their total filtration expenses are, including their costs for such elements as media, machine downtime, and labor. Shops with multiple machines are more concerned with costs because they usually operate these machines closer to their full capacity, running two or three shifts six to seven days a week. Not only are their filter costs higher, but the cost of machine downtime and the labor required to change filters and perform the extra maintenance to keep the machine dielectric tanks clean has a greater impact on their profitability. With machines that can earn $50 to $80 per hour, shops with multiple machines place value on a "hassle-free" solution.

If you are running a wire-EDM operation and you haven’t analyzed your filtration needs or explored alternatives, your filtration costs may be unnecessarily high. You might be able to reduce your maintenance costs, increase machine uptime, and work with cleaner dielectric fluid (which would help you produce parts with better finishes in less time) by replacing your paper-cartridge system with a different solution.

To know for sure, you will have to conduct a thorough investigation to answer these questions: Do you know what your current filter costs are? Do you know what it will cost you to stick with the OEM filtration system that came with the machine? Are you paying for 10µm capability when you really need 5µm results (or better).

Common Choices

As you explore your options, you will find that the two most common wire-EDM filtration systems on the market today are cartridge-element systems and diatomaceous earth (DE) systems. Cartridge-element systems are the most popular choice among wire-EDM users today. Users like these systems because of their low to moderate element costs, almost universal acceptance, and good performance for almost all applications. Cartridge-style filter elements usually are nominally rated and come in a variety of micron ratings. The filter elements are manufactured from a variety of media, including pleated and stacked paper, polyester, fiberglass, resin-impregnated cellulose, silicone-treated cellulose, micro-glass fiber, synthetic fiber, and string-wound types, as well as others.

Although cartridge-element systems are still universally accepted, some users are beginning to question their use. Increasingly, users who have analyzed their filtration costs are finding that these systems represent an unjustifiable manufacturing expense. They are also discovering that, as local regulations become more stringent, there is more demand on shops to dispose of the elements in an ecologically responsible manner.

These environmental regulations vary from state to state. Some states are clamping down on the EDM industry, having recognized that both wire- and die-sinker-EDM shops are contributing to the waste stream at the landfills and that improper disposal of filter cartridges has promoted the pollution of the locality’s groundwater. In California, EDM shops are now required to dispose of their filter cartridges in 55-gal. drums, which must be taken to an approved site by EPA-registered hazardous-waste haulers. This is an extreme case, however. In almost all the other states, EDM shops can continue to dispose of their filters in the industrial-waste container in the parking lot, despite the groundwater-contamination risks.

As an alternative to cartridge-element filtration, shops have typically chosen DE filtration. Although not as popular today as it was in the late 1970s and the early 1980s, DE is still effective. The filtration media is an inexpensive powder made from ground-up diatoms or other materials such as lava. Diatoms are a class of plankton-like algae with skeletons of silica. Ground into a powder, this silica becomes a filter media when it is mixed with dielectric oil or water.

As the fluid circulates, the suspended DE forms a filter cake on the outside of a flexible tube (actually a series of tubes or "fingers") that serves as a filter support. This cake traps the particulate. At some point, the fingers are flexed (a process also known as bumping) to release the DE powder, which is then redeposited onto the fingers. The fresh filtration media is thus re-exposed to the incoming dirty water.

Releasing and redepositing the DE helps extend the filtering life of the media. This procedure can be used in both pressure- and vacuum-filter applications, and it can be repeated several times before the DE powder needs to be replaced. Operators know it is time for fresh DE powder when the contamination of the DE reduces the flow rate of the water to the clean tank. Another signal is an increase in the back pressure of the water going through the filter as registered on a pressure gage.

The primary drawback to DE filtering is the labor required to clean the fingers. The operator must perform this task periodically when the system is taken apart for maintenance. To remove the DE and contaminants, the maintenance worker must soak the fingers in diluted muriatic acid, a process that poses some risk to the worker. The process also poses some environmental risks as well, because the acid-soaked fingers typically are rinsed in the shop or at a car wash, and the runoff is allowed to wash into the nearest drain.

It is not just the contaminants in the DE-filtered water that can damage the environment. There also are environmental concerns with the disposal of the DE powder. DE filtration generates a larger volume of waste than filter-cartridge systems, because users must throw out the powder as well as the captured particulate. The diatoms that make up the powder are approximately 70% silica. For companies that EDM carbide, this powder poses a hazard, because it can release chlorides into the water that can chemically attack the cobalt binder that holds the carbide pArticles together. DE powder has been outlawed in Germany as a filter aid because of these environmental concerns.

Filtration Advances

Over the years, engineers have been challenged to develop an efficient, safe, and economical alternative to traditional wire-EDM filtration. Several designers have thought they had the "next greatest thing" for wire-EDM users only to discover that their solution lacked some key feature or capability. In the meantime, the need for a better filtration system has grown more urgent as automation, thicker workpiece capacity, and automatic-threading capabilities have increased wire-EDM’s output, putting a strain on the operating life of conventional OEM and aftermarket pleated-paper cartridge systems.

After false starts and dashed hopes, several new players are finally achieving commercial success in this marketplace. Typically, it is the European manufacturers who are leading the way with these innovations. When one considers Europe’s stringent environmental regulations for filter-element disposal, it is easy to understand why Europe has taken the lead. In Italy, all companies must pay to have their cartridges disposed of by certified waste haulers. The companies also are responsible for keeping records of the generated waste, and they can be fined if these records are not properly maintained. In Germany, the seller of the goods is responsible for the disposal and recycling of the original product packaging (in other words, the box that the cartridge comes in) if the consumer chooses to leave it at the store or return it after the product has been removed. Europe’s answer to these and other regulations (such as the newly implemented ISO 14000 regulations) has been to develop deep-bed mineral filtration for wire EDMs.

Mineral-filtration technology is now available in the United States. Although the principle of deep-bed mineral filtration is not new—it has been used in swimming-pool and municipal-water filtration for years—manufacturers have made it practical for wire-EDM use by careful selection of the mineral-filtration media and the development of microprocessor controls and a comprehensive method of capturing the backwashed waste byproducts from the wire-EDM process. Mineral-filtration systems have been in use since 1989 in more than 200 wire-EDM operations worldwide. Although there are differences between the systems available on the market (due to operating philosophies and international patents), they are all based on the same, clearly effective, principle.

Figure 1: A centralized mineral-filtration system.

Figure 1 shows a modular mineral-filtration system. As is true of any well-designed centralized filtration system, this system is designed and built not only to meet today’s needs but also to be expandable and upgradeable as the user’s business grows and new equipment is added. The system includes a filtration module with a programmable logic controller (center), a backwash module (left), and a centralized dielectric module (right). These modules work together to process the dirty water from multiple EDMs and return filtered water back to the OEM clean-dielectric tanks as required, automatically and with minimal operator attention.

How Mineral Filtration Works

Figure 2: A schematic of a centralized mineral-filtration system.

While mineral-filtration equipment is available for single-machine installations, it is more cost effective from a capital-purchase point of view to install a centralized system that can service multiple machines. Figure 2 offers a schematic to show how such a centralized system works.

The filtration system consists of two distinct operating cycles. The first is a continuous filtration cycle that cleans the dirty water routed to it from the EDMs and returns it clean for use. The filtering is performed through two or more filter columns in the filtration module (#1). The required capacity of this module depends on the flow-rate requirements of the shop’s wire EDMs.

Users planning the installation of a centralized system can usually find the required flow rates in the technical specifications listed in their machines’ operation manuals. The flow rate also can be measured by capturing the overflow of water pouring from the machine’s clean tank to the dirty tank. Should a shop need a higher flow rate at a later date, the system’s modular design allows its capacity to be increased simply by adding a filtration module to the existing equipment.

Each column contains a graduated layering of quartz and other proprietary inert materials that capture wire-EDM byproducts down to 3µm (#3). All mineral-filtration systems filter to this level, and some systems come with options to "polish" the water to approximately 1µm. The user never has to change the filtration media; since it is made from minerals that have been around for millions of years, it will not wear out.

Before it travels to the filtration module, the dirty water from the wire-EDMs’ dirty tanks flows into a central dirty tank (#8). It is then pumped by a filter pump (#5) to the filter columns. EDM particulate measuring approximately 1µm to 20µm in diameter is trapped by the graduated layers of quartz as the water flows through the column. The quartz stores this EDM waste until the automatic backwash cycle removes it. The cleaned fluid is returned to a central clean tank (#7), where it is then pumped back to the machines’ clean reservoirs.

The second cycle is the programmed backwash cycle that cleans the filter-column media of the debris and contaminants that it has collected and restores the media to optimal working conditions. The operation of the backwash cycle is automatic and is controlled by the programmable logic controller (#2). Operational status is easily monitored on a liquid-crystal display.

The timing of the backwash cycle can be automatically controlled by an inline pressure sensor that measures the back pressure of the water going through the filter columns and then sends this information to the controller. As the media in the column becomes contaminated, the pressure rises. At a predetermined pressure, the controller sends a signal to begin the backwash process.

The operator can also control the process by entering into the controller the number of minutes the controller should wait between backwash cycles, using the keypad/display panel. The operator must rely on experience with the type and amount of work being done to choose an appropriate interval. If the interval is too long and the contamination level of the filter media rises past a predetermined set-point, the system will intervene and begin a backwash cycle automatically.

The backwash operation takes only a few minutes and does not interrupt normal wire-EDM operations. Since only one column is backwashed at a time, the flow of dielectric fluid can continue through the other column. The filtration operation and flow of water to the dirty tank from the EDM and the flow from the clean tank back to the EDM is not interrupted. The EDMs can continue cutting, so productivity is maintained. When using conventional paper-cartridge or DE systems, the operator must shut the system down for filter changeover.

The closed-loop backwash cycle begins when the backwash pump (#6) pumps water from a clean-water reservoir (#11) through the filter columns. This water flows in the opposite direction from the dielectric fluid being filtered. After picking up the contaminants collected by the filter media, the backflow is routed into a settling tank (#9).

A liquid flocculent (not shown) is metered in small amounts into the settling tank, where it binds the small pArticles of wire-EDM contaminants together. The use of a flocculent dramatically reduces the time it takes for the debris to settle out of the water. Without the flocculent, it would take three hours. With the flocculent, the process takes 10 minutes. This reduction in time increases the capacity of the filtration system and makes it possible to connect more machines to it.

At a signal from the controller, the flocculated dirty water in the settling tank is released onto an inexpensive coolant-filtering fabric as used on grinding machines (#10). This filter fabric traps the debris and recycles the cleaned backwash water back into the clean-water reservoir, where it is used again during subsequent backwash cycles.

After a number of backwash cycles, the filter fabric becomes contaminated. This contamination is monitored by a sensor. When the contamination reaches a certain level, the sensor activates a motor to expose fresh material by unwinding the roll of fabric a few inches. Each 100-yard roll of filter fabric lasts about 600 cutting hours and is easier to change than a conventional cartridge filter. The used filter fabric and the EDM debris it has trapped are collected in a basket to be disposed of later.

There are several factors that make mineral filtration a more ecologically sound process than cartridge-element filtration. First, fabric is much lower in volume than cartridges, so the user can get much more waste into a drum. Also, there is no heavy metal structure to throw away. With a cartridge, the user must dispose of end caps, an outer support grid, and an inner metal support tube in addition to the filter media. As an alternative to disposal, the filter fabric and the waste it holds can be sent to a metal recycler that can burn off the fabric and reclaim the melted metal.

Centralized mineral filtration is a viable and economical solution to one of the last remaining pieces of the puzzle in the wire-EDM process. Now, with the realization of round-the-clock wire-EDM filtration, and advances in machine construction, power supplies, and controls, today’s wire-EDM job shops are achieving the productivity and the profitability potential that has always been visible, but often tantalizingly just out of reach.

About the Author
Christopher Wilkins is president of EDM Mechatronics Inc., Hauppauge, NY.