Ignoring Fluids Can Be Costly
They're finally getting some respect
By Robert B. Aronson
The time has passed when coolants and lubricants were valued somewhere around wiping rags when ordering expendables. Companies that have learned high quality is the key to survival know differently.
Picking the right fluid is not the only consideration. A fluid can be useless, or worse, if not delivered efficiently at the proper temperature and pressure, through well-positioned and appropriately shaped nozzles, and using a filtering system matched to the job.
Technology is not the only challenge faced by coolant and lubricant suppliers. "Our company's focus, as well as that of most other suppliers, is meeting the challenge of rising base-stock cost," says Gene Venso, business manager, ITW Rocol North America (Glenview, IL). It's a dual problem of less petroleum available, along with massive price increases.
"We have to work more closely with customers to ensure we provide the optimum solution to their needs," says Venso. "Products that before were not considered practical because of their high cost are now 'in the ballpark' of many other fluid prices."
ITW Rocol North America is following the dual path of providing fluids for traditional flood cooling while also supplying fluids for the growing minimal quantity lubrication (MQL) system.
For the MQL market, they have developed the Accu-Lube line of organic fluid/vegetable oil for near-dry machining. "The MQL market is growing as manufacturers see the benefits of eliminating water-based coolants with their inherent issues of sump maintenance and fluid disposal," explains Venso. For traditional flood cooling, the latest focus is on biostable semisynthetic coolants like Rustlick Ultracut 370R.
Which to choose—flood or MQL—is determined by the specific job. The key question is: Can you get the fluid to the work area?
Another factor in the background is how serious is the move to limit mist in the work environment. "It's been under consideration for some time. If they finally get serious, there will be a lot of changes for both customers and suppliers of metalworking fluids," predicts Venso.
Basic research has been an emphasis for Benz Oil Inc. (Milwaukee, WI) for some time, in particular the surface chemistry of lubricants under high-pressure situations, for example, what occurs in the contact between a tool and workpiece? "We want to know how chlorine, phosphorus, and sulfur in the fluids can be efficiently used to control surface finish," explains Peter V. Kotvis, Research Director. "But we want to quantify the process of how films are formed and what makes them up. Then, what is perhaps most important, how these chemicals can facilitate the removal of material by a complex process that breaks bonds between surface atoms of, for example ferrous, workpiece materials, and opens up the surface by controlled microcracking. They function as kind of high-speed hydrogen embrittlement agents which also control friction by forming films on the surfaces that remain in the sliding contact region."
This research is relevant to any process in which the contact pressures are in gigapascals (GPa) or the point at which one has plastic deformation at any level.
Filtration is a key issue in grinding, and with petroleum prices constantly rising should be an important part of any grinding operation. One of the reasons behind the move to more solvent-refined base oils is that they are less sensitive to thermal shock. And thermal stability is essential to any precision-grinding operation. Researchers discovered that water-based fluids had problems under severe grinding conditions. There was workpiece damage due to heat. The water would boil off at the cutting face and the system was essentially grinding dry. But users who move to oil have to be aware of the problems of misting, plus the need for a higher-pressure flow system to move the oil.
"Vegetable oils are a big market for us," says John Wiley, VP, Blaser Swisslube (Goshen, NY). "Some think that it's solely because of environmental issues, but productivity gains are the primary benefit." In one application, Blaser engineers were able to help reduce cycle time by 30% by switching to vegetable oil. They have found that the biggest productivity gains are in the more difficult applications. For example, they found more benefits from using vegetable lubricants on titanium alloys than on aluminum.
Blaser's hottest market is currently medical. The medical manufacturers not only enjoy the productivity benefits on titanium, but on stainless and cobalt chrome as well. Vegetable oils are polaric in nature. This allows vegetable oils to effectively deliver additives in a very fine, evenly dispersed layer, not unlike the coating on carbide tools. According to Wiley, "Blaser customers have repeatedly commented on their ability to make gains in productivity from running faster and more frequently, after switching to a vegetable-based product."
In addition, vegetable oils drain off the machine, chips, and parts better. "You use 20-30% less fluid," says Wiley. This helps make up for the cost difference between petroleum and vegetable-based fluids. It's not uncommon to see a $5 to $9 per gallon jump in price when looking at some of the premium vegetablebased cutting fluids. Another plus is that most vegetable oils have a higher flashpoint than mineral oils, which can be great safety advantage for those using straight oil products.
Another hot market for Blaser has been the spread of high-pressure coolant and cutting oil units. "We have been particularly effective at developing fluids that control foam and aeration, which allows our customers to take full advantage of the productivity benefits of high pressure," says Wiley. Water quality, sump size, cycle times, pump configuration, pressure, and through-tooling coolant delivery are all important to consider when selecting the appropriate metalworking fluid for a high-pressure application.
Fluid filters are important to fluid performance. "Your filter should be chosen so that there are no particles circulating in the fluid bigger than 10% of your tolerance band," says Andy Nelson, manager product applications, Master Chemical Corp. (Miamisburg, OH). "For example, if your tolerance is plus or minus 0.0005" [0.13 mm] then pick a filter that will reliably remove particles larger than 0.0001" [0.0025 mm] to be sure that scratches or buildup left by the chips don't push you out of tolerance. This is particularly true when looking for superior surface finishes where a single scratch can spoil both the appearance and the measured surface finish. Even with very fine and effective filtration, particle buildup will occur over time. It is critical to periodically get these particles out of the fluid, as they provide reactive surfaces that can chemically stress the fluid unnecessarily."
For coolant to do its jobs of heat and friction reduction, it is critical that the fluid quickly wet the grinding wheel (the wheel serves as a rapidly moving sponge that carries fluid into the arc of contact where it cools and lubricates). The more open the wheel is to receive the fluid, the better the coolant can reduce parasitic drag. So along with all the standard concerns about keeping the wheel properly dressed, clean, and free of built-up swarf, it is also critical to consider the affect of residues being deposited on the wheel. One of the sources of these residues is the hard water minerals contained in the mixing water, and another is tramp oil. These cations, predominately magnesium and calcium, can react with the wetting agents in the fluid to produce a soft, greasy residue that can "coat" the wheel, reducing its ability to pick up the fluid. Additionally, these reactions reduce the amount of wetting agent present in the fluid, thus reducing its ability to "efficiently" wet the wheel. Similar things can occur with the "over addition" of conventional "topical" antifoams where the typical active ingredient is a water-insoluble "oil-like" material (typically an emulsion of one or more siloxane compounds). These problems are most often seen when "small abrasive grain sizes are used in hard closed wheels" but can occur any time.
The amount of mineral oil allowed in a metalworking plant's mist may be about to change. This move has been in the works for some time and, according to Greg Foltz, engineering and development manager, Cimcool Industrial Fluid Div., of Milacron (Cincinnati, OH). The American Council of Governmental and Industrial Hygienists has proposed a limit of 0.2 mg/m3. And although the ACGIH is not a regulatory agency, OSHA accepts their recommendations.
Should the new limit be put in place, it would affect both equipment manufacturers and end users, and require a number of equipment modifications. "This could mean more use of synthetics, requirements for systems that do a better job of mist control, or force a lot more moves to Mexico," explains Foltz. Theoretically, vegetable oil would not be included, which might increase its use.
Also working in favor of vegetable oils is the constant increase in the price of petroleum oils and the greater use of higher lubricant pressures and through-the-tool lubrication. Vegetable oils are more resistant to foaming, and are more bioresisteant and more biostable.
The use of both high and low-alloy magnesium is growing, particularly in the auto industry, because of this material's good strength-to-weight ratio. "However, this trend introduces other metalworking fluid concerns," explains Foltz. "It's a very reactive metal, and on contact with water can liberate hydrogen. This can cause a fire or explosion hazard. It is essential, therefore, that the fluids used in these operations inhibit these types of reactions," he says.
Medical market buyers are also concerned about reactions between metalworking fluids and metals, particularly chlorinated substances. "We have found that many products used in aircraft applications are also good for medical parts."
Fluids and Grinding
It's incredible that a company will buy a $1 million grinding machine, put $10,000 CBN wheels on it, then run it on $5/gallon cutting fluid through $1 crushed tubes. But that too often is the case with many in the abrasive-cutting industry.
Coolants have a special role to play in grinding operations, and are important not only in determining product quality, but operating costs.
The power consumed by the grinding wheel spindle motor is partitioned into the wheel, work, chip, and coolant. The amount that enters the workpiece must be cooled quickly, to prevent high local temperatures and the resulting phase transformations from developing.
Phase transformations are often responsible for tensile residual stresses, white layer formation, reduced fatigue life, and surface and subsurface cracking.
Cooling of the process is achieved by the application of a cooling and lubricating fluid, as well as selecting process parameters that minimize the heat being transferred into the ground surface, and cause more heat to be carried away by the wheel, chips, and coolant.
Pressure, flow rate, temperature, and direction of flow all influence the quality and economics of the process. The pressure controls the velocity of the fluid, the flow rate and temperature controls the rate of heat transfer into the fluid, and the direction allows the fluid to remove the air-barrier that travels with the wheel. The jet must be aimed at the wheel just before the grinding zone to give the air an escape path.
Flow rate is dependent on the type of grinding wheel used, and the spindle power consumed during grinding that needs to be cooled. In many cases, the effort put into the design of the coolant nozzles and pumping/filtration is often done close to the time for machine run-off. In other cases, the machine tool manufacturer will supply a simple nozzle system based on plastic bendable nozzles or small-diameter, bent-metal tubes. This is especially true with tool and aerospace product grinding machines. While the pressure for the coolant jet to match wheel-speed should be between 60-30 psi (0.4-0.2 kPa), plastic nozzles will move or separate at much lower pressures, and open small-diameter tubes kick-back because of the reaction force of the jet.
Other issues associated with these nozzles is high jet dispersion, high turbulence inside the tube before exit, very close proximity to the grinding wheel, and trial-and-error targeting.
The combination of coherent-jet round nozzles with a high degree of reconfigurability has been extensively tested. In a recent case study, grinding turbine blade root-forms in mineral oil, the customer has experienced the following advantages:
- Doubled plated wheel life and a 25% increase in vitrified CBN parts-per-dress.
- Lower push-off forces, resulting in more accuracy.
- Elimination of grinding burn and cracks.
- Reduction in flow rate to 33% of previous setup.
- Faster setup using laser aiming.
- Reduced pumping energy and chiller power consumption.
- Adjustability of nozzles for different component geometry.
Coolant application should be strategic, that is, put the fluid only where it is needed. It should also be energy efficient, economical, and an engineered product with all degrees of adjustability required to reduce setup time.
Choose a fluid that gives you the benefits you need. The choices are numerous and include: mineral oils, synthetic oils, synthetic water-soluble, semisynthetic water-soluble, and oil/water emulsions. The straight-oil products typically have higher lubricity and the water-soluble type the best cooling.
Dr. John A. Webster
Dr. Webster is chairman of the SME Abrasive Removal Community.
This article was first published in the July 2006 edition of Manufacturing Engineering magazine.