Advances in Wire EDM Filtration

A look at the evolution of wire EDM machines during the past 10 to 15 years has shown dramatic improvements in machine tool construction, power supplies, CNC controls and operator interfaces. Improvements have been made in almost every area except one - EDM filtration. 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, as multiple machine users will attest. Improvements in EDM filtration are slowly starting to appear and as we will see, offer a viable solution for real-world production machining.

 

Traditional Filtration Media

Manufacturers use a variety of different media types and particulate retention capabilities in search of the Holy Grail that is EDM filtration. Each one is searching for that balance between filtration efficiency and cost. Most knowledgeable EDM users would agree that clean water is essential for quality cutting. Generally, this means a filter with a five- or 10-micron rating. Many applications could benefit from the use of a higher quality filter with a rating of three microns (or better), however such a quality level is neither practical nor cost-effective when we are talking about traditional, pleated-cartridge elements.

Speaking of cost, the finer the retention rating of the filter, the more expensive it is to purchase and the more often the filter media has to be replaced. We not only have to consider the cost of the media itself, but also the cost of the non-productive, idle machine time during filter change-out as well as the labor costs required for this routine maintenance.

So clearly, choosing filter media type and filter rating with the associated nonproductive costs of each system is a daunting task. Many shops accept filtration expense as a necessary evil that is paid for on a filter-by-filter element basis without analyzing where the costs really are. Some shops can tell what the filter media costs are, but usually it's the more experienced shops with multiple machines that have an idea what the total filtration expense is-media, machine downtime and labor, etc.

Perhaps a different solution other than paper cartridges would offer cost savings through reduced maintenance, increased machine up-time, parts with better finishes and parts produced in less time-due to improvements in filtration quality.

• How well have you analyzed where your filter costs are going?
• Do you know the costs associated with using that same OEM filtration system that came with the machine?
• Are you paying for 10 mm capability, but really need 5 mm results (or better)?

The only way to know for sure is to do your own investigation.

The following is a summary of the different types of filtration solutions on the market today for wire EDM machines:

1. Cartridge Element
This is the most popular type today. Its advantages are low-to-moderate element cost, almost universal acceptance and good performance for almost all applications.

Cartridges are being increasingly seen as an unjustifiable manufacturing expense and ecologically risky to dispose of as EPA regulations become more effective.

Filter elements come in a wide variety of micron ratings (usually nominally rated) and in different filter media that include, 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.

2. Diatomaceous Earth
Although not as popular today as it was in the late '70s and early '80s, diatomaceous earth (DE) filtration is still an effective solution. 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.)

This powder is mixed with the dielectric-oil or water-where it forms a coating on a flexible tube that serves as a filter support. The contaminated particles are trapped as a filter cake on the outside. At some point, this tube-actually a series of tubes or "fingers"-is flexed to release the DE powder where it is re-deposited, exposing fresh filtration media for additional filter life. This process-also known as bumping-can be repeated several times before the DE needs to be replaced. It can be used in both pressure and vacuum filter applications.

 

Wire Filtration Comes of Age

Wire EDM filtration brings its own unique set of problems to the prospective designer of such systems. Just ask any number of engineers who were sure that they had the "next greatest thing" for wire EDM'ers. Well, after false starts and dashed hopes, several new players are achieving commercial success in this marketplace.

The impact of such trends as automation, thicker workpiece capacity and auto- threading capabilities put a large strain on the operating life-actually lack of it-of conventional OEM and aftermarket pleated-paper filter cartridge systems. Factor in increasingly stringent regulations for filter element disposal in an ecologically responsible manner, and you'll understand why Europe is the leader in mineral filtration for wire EDM machines. This 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-it has been made practical for wire EDM use by the careful selection of mineral filtration media, microprocessor controls and a comprehensive method of capturing the backwash, waste by-products from the wire EDM process.

Mineral filtration systems have been in use since 1989 on more than 200 wire EDM machines worldwide. Although there are differences among the systems available on the market-due to operating philosophies and international patents-clearly the principle works well.

A well-designed, centralized filtration system should be designed and built to not only meet today's needs, but also to be expandable and upgradeable as the business grows and adds new equipment.

One such modular approach could include a:

• Filtration module with programmable logic controller in the center;
• Backwash module placed on the left; and,
• Centralized dielectric module on the 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 absolute minimal operator attention. Figure 2 shows how such a centralized system works for a multiple machine installation.

 

Understanding Centralized Mineral Filtration Systems

Refer to Figure 2 as you follow along with the system operation description and you'll see why this solution makes sense for busy, wire EDM shops with multiple machines.

The filtration system consists of two distinct operating cycles:
1. A continuous filtration cycle to clean the dirty water from the EDM machines and return it clean for use.

The filtering is performed through two or more filter columns (Figure 2 #1), depending on the flow rate requirements of the wire EDM machines. Each column contains a graduated layering of quartz and other proprietary inert materials, which capture wire EDM by-products down to three microns.

The dirty water from the wire EDM dirty tank flows into a central dirty tank (#8) and is pumped by the filter pump (#5) to the filter columns (#1). EDM particulate in the dirty water is trapped by the graduated layers of quartz, which temporarily store this EDM waste prior to an automatic backwashing of the filter column media. The mineral filtration media cleans and returns the wire EDM process water back to a central clean tank (#7) where it is then pumped back to the machine's clean reservoir.

2. A programmed backwash cycle to clean the filter column media and restore it to optimal working conditions.

The debris that is temporarily collected in the filter column beds is periodically removed by a backwashing cycle. The backwashing cycle is independent of the filtration cycle and does not interfere with normal filtration. The backwash cycle takes only a few minutes and works as follows: This closed-loop cycle, pumps water with the backwash pump (#6) from a clean water reservoir (#11) in a reverse-flow direction through the bottle(s) (#1) and into a settling tank (#9). A liquid flocculent (not shown) is metered in small amounts into the settling tank (#9) where it binds the small wire EDM particles-approximately one to 20 microns in diameter-together. This dramatically reduces the settling time of the debris-laden dirty water from about three hours to 10 minutes. This allows for more machines to be connected to the filtration system-since waiting long periods of time for the dirt to settle from the process water is not necessary. The programmable logic controller (PLC) (#2) releases the flocculated, dirty water in the settling tank 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 tank (#11) where it is used again during subsequent backwash cycles. As this filter fabric (#10) becomes contaminated, a sensor unwinds the roll of fabric ahead a few inches to expose fresh material to trap subsequent discharges of dirty water from the settling tank (#9). The thin, filter fabric and the EDM debris is collected in a basket. The amount of waste is minimal compared to bulky, pleated-paper filter cartridges and can be incinerated or disposed of in a much more ecologically responsible manner.

The backwashing cycle, which restores the filter media to its original clean condition, is automatically controlled by an in-line pressure sensor or by a process cycle time input by the operator into the keypad/display panel (#2). The backwashing operation takes only a few minutes and does not interrupt normal wire EDM operations unlike conventional paper cartridge or diatomaceous filter systems, which may require that the system be shut down for filter changeover. Today's mineral filtration system maintains the productivity of your machine by allowing cutting to continue. The operation of the filter is automatic and is controlled by an on-board programmable logic controller (PLC). Operational status is easily monitored on a liquid crystal display (LCD).

 

'Real World' Experience

Bill Mader, vice president of MicroCut (York, PA) knows first-hand the benefits of a centralized mineral filtration system. MicroCut has long been known for its "Give Us A Challenge" philosophy when it comes to tackling unusual or large wire EDM jobs. This production facility operates six wire EDM machines on a seven day, 24-hour schedule. Obviously machine up-time is one of several important requirements for this busy shop.

"We have found that this ecological, mineral filtration system has been a capable performer with more than 5,000 cutting hours-handling everything from 14-inch tall, alloy steel workpieces to stacked aluminum prototype parts," according to Mader.

He also cites several other advantages including the elimination of paper filter cartridges, reduced operator maintenance (MicroCut's OEM dielectric tanks remain clean even after six months of cutting), increased up-time and a 500 percent increase in the life of its DI resin.

Summary

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 profitability potential that has always been visible, but often just out of reach.