In today's metalworking environment, lowering operating and maintenance costs is a major goal. New permanent media filters, designed for modern CNC machine tool operations, are a key asset to maximizing the productive use of these costly machines.
Disposable media designs, which include gravity bed, hydraulic vacuum, flatbed pressure, and tubular precoat filters, have long histories of successful application to both abrasive and chip-removal machines. For many operations, disposable media filters will continue to be the design of choice, whether due to proven application efficiency, capital cost, or other factors.
Where practical, new permanent media filter designs can play an important role in reducing the overall cost of coolant filtration. Successfully applied to high production operations, coolant filtration and chip removal systems that use long life recleanable belts or elements can increase tooling life, reduce nozzle clogging, and extend coolant life. Precision woven of nylon, polyester blends, nylon-backed felt, or a combination of fibers for specific applications, permanent media can protect the environment and reduce direct costs associated with media replacement and direct operator involvement.
Permanent media filters, while representing only a small part of the overall investment, are playing an important role in today's advanced manufacturing systems. In many cases, these new designs are modeled along the same platform as proven disposable media designs.
Permanent Media Vacuum Filters
Vacuum filters with permanent recleanable belts are a recent innovation that can offer the potential, in many instances, for operating without disposable media.
These filters have been successfully applied to production CNC operations in which chip removal and efficient filtration of smaller particles provide multiple benefits. Permanent media vacuum filters use recleanable belts that allow free passage of the coolant, while retaining a high percentage of particles as small as 10 to 40 microns.
Dirty coolant flow is induced through the media, which quickly builds a blanket of particulate on the media surface. This blanket effect enhances removal of smaller particles. Pressure produced by the restriction is sensed by a vacuum switch that initiates the "index" cycle.
Thorough cleaning of the permanent media belt is critical to the continued successful operation of the filter. Belt cleaning is accomplished by a high-pressure reciprocating header that sprays clean liquid directly onto the underside of the filter belt. As the belt indexes, this action continues and completely dislodges any solids embedded in the belt surface. The ability to rapidly reclean the media provides an important benefit in instances where tramp oil, graphite, or small particle distribution might cause shortened index cycles.
Permanent media vacuum filters are currently used for a variety of operations ranging from CNC machining to tight-tolerance grinding operations. The following examples illustrate how these filters can be used in production machining operations.
Cast Iron And Aluminum Machining
A large automotive parts manufacturer selected permanent media vacuum filtration to service a high-production dial machine producing cast iron workpieces. An additional requirement was that the system accommodate future conversion of the workpiece to aluminum.
The system design included a chip drag conveyor, permanent media vacuum filter, and separate clean tank. Bulk chips were removed by the chip conveyor and placed into a common hopper. The coolant was then filtered through a 30-micron retention permanent media belt at a rate of 400 gpm.
In addition to flush coolant, the machine also required high-pressure coolant. The high-pressure pump was protected by 10-micron canister elements that require periodic replacement. Illustrative of the overall efficiency of the permanent media filter is the low 6-to-8-week changeover frequency of these elements based on a three-shift, seven-day-per-week cast iron machining operation.
Conversion of the part to aluminum required no changes to the filtration system. Current belt life is nine months to one year, and conversion to aluminum will extend belt life to more than two years.
"Convertible" Vacuum Filtration
A major aerospace manufacturer recently required retrofit of an existing central filtration system that served a CNC machining area. The filter design selected was a "convertible" vacuum filter capable of running straight-oil coolant with a recleanable permanent media belt or with a 2-to-3-oz. disposable paper media. Dirty oil is pumped from the existing chip removal system, filtered through the permanent media belt, and pumped to the machining line as clean oil.
Small floating magnesium chips and fines had been passing through a screen, causing excessive nozzle plugging and frequent machine downtime. Subsidiary problems included broken tools, reject parts, fire hazards, and reduced oil life. The company had installed automatic-flushing "Y" strainers at various locations throughout the system. These strainers contained No. 40 mesh screens (around 375 microns) and were flushed at each tool change cycle. Because of the high maintenance involved, this solution proved only temporary.
Consultation with the manufacturing team indicated a required oil flow of 650 gpm at 55 psi outlet pressure and a cleanliness target of 30 microns. Possible future tooling requirements, however, produced an additional request for capability to provide 10-micron clarity without adding additional filtration equipment.
The convertible vacuum filter system is currently operating successfully in the permanent media mode, and nozzle plugging has been completely eliminated. Should future conditions warrant, the permanent media belt can be replaced in under one hour with a carrier belt transporting disposable paper media. This design flexibility means the micron cleanliness can be further raised if required.
Honing Transmission Parts
A recent case study at an automotive transmission plant revealed filter media costs of $13,000 annually, oil loss of 7,000 gallons at $2.00 per gallon, and potential for significant tooling costs savings.
Problems with maintenance staff availability were leading to occasional "media out" conditions, causing system contamination, part rejects, and excessive tooling consumption. After a careful cost evaluation, the existing central filter was replaced with a new permanent media vacuum system including a chiller unit.
The floor-mounted filter is currently in operation, with dirty oil pumped in from two honing machines. Consistent oil clarity in the 7-to-10-micron range is maintained with the use of a thick, recleanable media belt. Frequency of filter maintenance has been reduced, and machine productivity has been increased. Large savings in honing stone and increased mandrel life, coupled with additional savings in media, oil loss, and plant labor produced a rapid return on investment.
This type of permanent media vacuum filter can be successfully applied to various abrasive machining applications in which medium-to-high rates of coolant flow and consistent 10-to-20-micron clarity must be maintained.
Filtering Below 10 Microns
Tubular precoat filtration has generally been recognized as the surest approach to removal of particles down to 1 micron in operations such as electrical discharge machining, rolling mills, bearing honing, super finishing, and other tight filtration applications. Disadvantages of tubular precoat filters have included frequent operator attention, high cost of filter powder, oil loss, and increased disposal volume when the spent filter powder is included.
Precoat filters work by depositing a thin layer of diatomaceous earth on the surface of tubes that are mounted inside the filter vessel. This coating aids the filtering process. When the filter aid coating is completely formed, dirty liquid is pumped through the vessel and particles 1 micron or smaller are captured by the diatomaceous earth. Cleaning of the filter vessel involves reversing the coolant flow in order to backwash or flush both filter aid and collected solids from the vessel to a sludge dryer.
New permanent media tubular filter designs are now available that provide consistent 1-micron particle removal efficiency while eliminating the need for precoat powders. Tubular filter operation can require backwash and precoat with the new filter powder once, and sometimes twice per shift. Frequency is usually determined by various load and sizing factors, which may change over time.
Recent engineering improvements to tubular filter design have changed this picture. Reverse-flow tubular filters use individual filter discs tightly packed onto a central guide mandrel. This design eliminates the need for filter powder and thus the need for constant operator attention. Unattended operation for up to 120 hours is now possible.
In a rolling mill application, oil loss was reduced by 85 percent and solid-waste disposal was reduced by more than 90 percent in the first year of operation. Operator attention was limited to 30 minutes every four weeks versus the daily attention required previously. Elimination of the filter powder reduces direct media costs and the indirect cost of subsequent disposal. Reduced solids volume means less oil carried off in the waste.
Gravity Screen Filters
Generally applied to chip-making machines, gravity screen filters are available in several designs, including continuously revolving belts and drums. A combination chip removal and filtration design normally includes a chip drag conveyor coupled with a permanent media screen for removal of suspended chips and fines. A low-pressure fixed spray header continuously cleans the revolving screen or belt and dislodges solids, which are removed by the chip drag conveyor.
These units have mainly found use on individual machines, particularly where floating aluminum chips are a problem. Cast iron applications are sometimes less satisfactory, with small particle distribution, graphite, and coolant life as sources of concern under some conditions.
Because dirty coolant passes through the filter screen by gravity, the micron retention rating of the screen determines the level of cleanliness possible. Because screen opening and flow restrictions quickly become a factor with gravity designs, filtration performance in the 60-to-100-micron range is the norm. Long, stringy or nested chips can sometimes pose a problem if a drag flight conveyor is part of the design. In some cases, a steel belt or auger conveyor may be required to remove this material.
Although not strictly a filtration process, magnetic separators continue to be a "no media" workhorse with wide application in the metalworking industry. Operations such as grinding, honing, broaching, gundrilling, and gear shaving all lend themselves to efficient magnetic removal of ferrous chips or fines from machining coolants. Properly designed and sized magnetic separators can be expected to remove more than 98 percent by volume of ferrous solids larger than 20 to 40 microns.
Magnetic separators offer several advantages, including small size relative to flow rates, efficient removal of ferrous particles and chips, and low operating and maintenance costs. Magnetic separators with indexing drum controls form a "filter cake" with the larger solids reducing velocity past the magnetic drum and aiding in the capture of additional solids. Indexing the magnetic drum can also serve to slow ejection of the solids, reducing coolant loss, and producing drier sludge for disposal.
Used as the primary coolant cleaner or as a prefilter, magnetic separators are effective with either water-based or straight-oil coolants. When used as a prefilter, magnetic separators reduce the dirt load and extend the life of expensive disposable paper media and cartridges. To assure proper coolant cleaning, larger separators should be used for higher viscosity straight-oil coolants.
Available in a range of sizes and capable of coolant flows as high as 500 gpm, magnetic separators continue to offer reliable "no media" coolant cleaning performance and low operating costs.
The permanent media designs mentioned in this article are being aggressively used by an increasing number of manufacturers to lower variable and fixed costs associated with production machine tool operations. The bottom line is intelligent solutions that both protect the environment and at the same time reduce expenses. Continuing improvements in permanent media filtration will play a growing role in making this goal possible.
About The Author. Larry Sweeney has served as the president of CMP Company, a manufacturer's rep organization based in Glen Ellyn, Illinois, since 1980. His company has been involved with specification and design of coolant filtration systems in both large and small metalworking plants.