In the past, cutting-fluid care was not an issue on which many manufacturers spent a lot of time. But today's more astute managers are learning that clean, properly maintained fluids have a big payback.
Coolants are important to the manufacturing process because they can:
- Carry away heat from the process,
- Reduce friction between the tool and the freshly cut surface,
- Reduce friction between the chip and the face of the tool,
- Control edge buildup, and
- Flush chips from the cutting zone.
Why filter? While coolants perform all of these functions, they pick up fines and other undesirable substances that make a filtering system necessary. These fines, chips, and other debris provide surfaces on which biofilms and biomasses can grow and prosper. So, the cleaner the sump, the fewer bacterial problems. Another contributing factor is the buildup of tramp oil.
In addition, freshly cut chips are chemically reactive, and the longer they are exposed to the fluid, the greater the need for corrosion-inhibiting additives to protect the workpiece and the machine tool. This is particularly true of systems that run at low coolant concentrations or with very low carry-off.
Fluids from machining and grinding operations will have somewhat different contaminants. Generally, the swarf produced by grinding is a combination of small pieces of the workpiece plus wheel grit. Machining produces fluid contaminates that are almost entirely workpiece chips. Comparing the two operations, grinding requires more attention to filtering fine grit. Recovery of material is also different. Normally, there is an effort to recover and recycle chips from a machining operation, which is not as common with grinding. In the typical grinding system, the process is very coolant-efficient without a lot of carry-off or splash-out. As a result, little fresh coolant is added to the system.
Heat generated by the machining process can actually heat-treat the chip as it forms. Tempered by the coolant, the chips and swarf are then much harder than the materials that are not work-hardened. A good example of this is bending a wire coat hanger back and forth. Working the wire causes it to get hard and brittle, until it finally breaks. It becomes very hot at that point as well.
Recirculating chips or swarf can be very hard on tool or wheel life, as well as cause surface-finish problems. "Fish tails" are almost always associated with dirty coolant loading up the grinding wheel, or the fluid not properly cleaning the grinding wheel. Grinding grit and chips then get caught in the grinding wheel, fill the voids between the abrasive grain, and soon the pressure of the wheel squeezing the chips against the workpiece causes them to "weld to" the workpiece.
The benefits of a good filtration program are:
- More consistent machine tool performance and better tool life, because the chips are not eroding the tool or wheel, and very hard chips are not pushed back into the "guts of the machine."
- Increased coolant cost-savings because clean coolant lasts longer.
- Better machining, longer tool life, improved surface-finish and more consistent performance, because chips are not affecting the process.
Filter selection. Two factors to consider are the size and number of the particles. Deciding which type of filter works best is situation-specific. There are two basic filtration methods: those that rely on gravity and those that use some type of filter element.
Gravity systems include settling tanks, cyclones, and centrifuges. A gravity filtration system uses time to separate the various phases in the fluid. Problems arise when holding time is reduced. Also, changing from a synthetic fluid with a very low surface tension to a soluble oil or oil with a higher surface tension will require additional time for the same-size chip to separate. If the chips get smaller or lighter or sometimes even change shape, settling efficiency will be affected.
In media-type filters, fluid passes through a chamber that has a cartridge, roll, or disk of media using gravity or pressure. The pressure-assisted systems are designed chiefly to remove fine particles and can be used in conjunction with a primary filtering system, such as a settling tank. Some filter elements are preloaded to assist in cake buildup; most are disposable, some can be cleaned. Those that are reusable can be cleaned by back flushing, either automatically or manually. The cleaning cycle may be determined by time (set interval) or by pressure drop across the filter. Filter elements use filter paper, wedge wire, and microscreens. Frequently, a "filter aid," a material that precoats or cakes the barrier media for better filtration, is used to improve efficiency. In most cases, the "fine" filtration that occurs in these situations is the result of cake building on the filter media and creating a deeper filter bed.
What to watch for. If the quality of the filter media changes, then the size of what gets past changes also. When these barriers are "blinded off" with tramp oil, bacterial films, fungus or antifoam, the media must be replaced or indexed, which can significantly drive up media costs. If it happens often enough, it can cause the clean side of the filter to be starved for clean coolant.
You do not want to pay for too fine a filter, but make sure that once you have settled on a filter media, you get a material that is the same, time after time. Paper or other filter media are often sold based on the weight of the paper. It is an indirect measure of how thick and how porous the paper is. If the weight (density) of the filter media is not evenly distributed across the width and length of the fluid flow, filter performance will be very inconsistent. Filter media often collect tramp oil, and if you don't get rid of the oil quickly or prevent its occurrence, you can use a lot of media (paper) to take out a very small amount of oil.
Don't forget the fluid. Five factors to consider in fluid selection are:
- Foam is not desirable since it neither cools nor lubricates. Fluid type, as well as nozzle placement, can have a major affect on foam generation.
- Wetting ability is a critical factor, and therefore the surface tension of the working solution is critical. In addition, the speed with which the fluid wets (dynamic surface tension) is important in high-velocity operations.
- Viscosity is an issue in cutting and grinding oils, because it influences the type of pump and plumbing sizes needed.
- Flashpoint is a concern since some oils may more easily catch fire.
- Mist formation can be a major issue in some of the newer machines that move a lot of fluid around very fast.
The importance of these factors varies with each application:
- Hydrodynamic lubrication
- Boundary lubrication
- Extreme pressure lubrication
Be sure to select a fluid based on the application and what needs to be accomplished.
Following are several examples of commercially available filtering systems.
One of the problems with coolant can be its aroma. Often, before the fluid goes bad, the smell caused by the action of the organic content can be a serious issue. Vanson HaloSource (Redmond, WA) offers an answer with their Coolant Xtender odor-control system. The system consists of a portable pump and filter assembly. In operation, coolant is pumped from a machine's sump to a treatment chamber filled with chlorinated beads. Odors are removed as the coolant passes through the chamber. The process does not add any free chlorine to the coolant. The Xtender can be used with any water-miscible coolant and sumps up to 400 gallons (1514 L). In some cases, use of the system has reduced the number of annual fluid changes by half.
An often overlooked advantage of properly cleaned fluids is prolonged pump life. Pumps that have to move fluid with a heavy contaminate load are going to wear faster and have a tendency to fail at just the wrong moment.
Cartridge filter systems from Pall Corp. (East Hills, NY) are used in operations ranging from intermediate machining to finishing and super-finishing applications, where the level of filtration is critical. The use of this type of filtering system is based primarily on the level of filtration required. And this, in turn, depends on the machining or grinding process, tool material, finish quality, and material being worked.
According to Darren Nowicki, Vice President, "The major advantages of this design include filtering efficiency up to 99.98%, the removal of target particle sizes, more consistent performance than products requiring cake-building, and long service life through the incorporation of a graded-pore media structure."
In operation, the filter elements fit into a housing through which the coolant flows. Contaminants are captured by the filters based on their micron removal rating. After an interval, determined by increased pressure drop across the filters (preferred) or a specific time, the filters are discarded and new elements are installed. In a machining operation these filters are used to control particles in the 1 - 100 µm range, depending on the application requirement. With grinding, the size range is typically 20 - 70 µm.
Filters can be used as either the primary filtration or as polishing (secondary) filtration. According to Nowicki, "This choice is usually determined by the size and amount of ingression [the quantity of particulate generated by the machining/grinding operation]. High-ingression applications will often have a 'bulk-removal' device [mechanical or disposable] employed to remove large quantities of coarser debris. Our products are used in these systems for control of critical contaminant sizes to ensure product quality or finish."
"Most of our filters are used in grinding applications," says Tom Oberlin, President, Oberlin Filters (Waukesha, WI). "The waste accumulates as a cake on the filter medium, and is periodically removed. One of our design advantages is that the cake is essentially dry, which means it can be disposed of more easily. Any wet residue is considered a toxic product, and has to be disposed of in a more costly way."
These filters can remove particles from 40 - 0.5 µm. Currently it is not economically practical to recycle grinding swarf unless a very costly metal, such as nickel, is involved. The need for high-pressure coolant flow is growing because wheel speeds are getting high. When wheels are moving faster, you need higher pressure and more precise and costly pumps. These pumps will wear faster when the coolant is dirty.
"We offer a pressure filter that has either disposable or cleanable woven media. It works like a fancy coffee filter. The fluid is forced through the filter media and builds up a cake. This cake becomes part of the filtering media.
"Our design uses less filtering media. Normally, we get a 30 - 35 psi [207 - 242 kPa] pressure drop before we shift the media. It's usually a pressure differential that triggers the media change. Gravity and vacuum- filter systems have to shift at much lower pressures. Because fluid is forced out of the cake, it is much dryer and therefore easier to handle."
This article was first published in the June 2004 edition of Manufacturing Engineering magazine.