Fluid Selection Can Make or Break You
They are important for cutting, cooling, and cleaning
By Robert B. Aronson
The main tasks performed by machine tool fluid are lubrication, cooling, and waste removal. Lubrication involves the cutting action of the tool or wheel against the workpieces, or the lubrication of the machine tool's moving parts. Cooling controls the temperature of the cutting actions and machine tool. In waste removal, chips and swarf are flushed away from the work areas. Some fluids do one job, some several, depending on the manufacturing operation.
Several trends have emerged:
Natural oil, such as soybean oil, is becoming more common. Due to growing interest in renewable resources, and pressure from farm groups, lubricants based on soybean, canola, and other "vegetable" sources are becoming more popular.
These oils can have features unavailable with more conventional fluids. The downside is higher cost to manufacture. Some claim that the amount of petroleum needed to process these oils is greater than the volume of vegetable oil produced.
Minimum quantity lubrication (MQL) seems to be getting greater use. This involves getting a small amount of fluid to the cutting area instead of traditional flood cooling, eliminating coolant disposal problems. This MQL method of cooling has been strong in Europe and Japan and is experiencing renewed interest in the US.
According to Robert Myers of ITW Accu-Lube (Glenview, IL), "In the past, near-dry machining was limited to spraying fluid externally on the work area. Now the move is to more internal (through the tool) applications. We like to see air/fluid because it's a simple system without waste disposal problems. The fluid mist handles the lubrication and air blows away the chips. Design changes in machine tools have been necessary to accommodate through-the-tool cooling and air/fluid mists.
"Another trend in traditional flood coolants is to develop flood coolants that naturally resist organic growth. Fluids have to be inherently 'bug proof' or biocide has to be added.
"Generic versus specialized fluids is a common problem. It's the user's call as to whether to go with generic fluids for all the machines, or use specialized fluids. If you will take a small drop in tool life with a generic system, that will probably be acceptable. But with some metals, you have to use specialized fluids and incur the costs of separate sumps and filtering systems. Disposal now plays a big part in the type of system installed," Myers concludes.
Machine tool design plays an important role in fluid performance. Sumps tend to become smaller and more difficult to get to. Toolholders have also been modified to handle the higher fluid pressures. "The toolholders have to withstand the increased forces, particularly the sealing," explains Billy Grobe, process development manager Makino (Mason, OH). "In addition, for systems using mist cooling and MQL there are issues with maintaining the mist in its most effective form.
"On Makino machines we have modified the toolholder retention stud some time ago to give a better seal between the machine tool and toolholder. These joints have to be able to withstand fluid pressures up to 1450 psi (10 MPa) plus the strains of high-speed rotation.
"This is a particular problem with mist cooling/lubrication. In that operation you have a column of air containing droplets of lubricant. You want to keep the droplets as fine as possible so they adequately lubricate the cutting area. But centrifugal force tends to cause the mist droplets to form larger drops of lubricant.
"We avoid this problem by placing small fluid reservoirs that feed fluid through a venture nozzle at the spindle's tip. This short flow path eliminates the drop formation problem, plus it allows the operator to easily change the type of lubricant to match the material being cut.
Highly specific applications is what our customers are looking for," according to Joseph Gentile, product manager and manufacturing engineer for Hangsterfer's Laboratories Inc. (Mantua, NJ). "They want to be the very best in a very tight field and limit part-size range as opposed to being generalists after any job they can find. To achieve this aim, they are now more willing to buy specialized oils."
Another trend, Gentile notes, is that environmental laws are becoming more strict, particularly involving shop mist and fluid disposal. "We are well positioned for any changes in US laws," he says. "This is because we have designed our product for the European market, which has had far more stringent laws in force for more than 10 years.
"Europeans are more concerned with reusing fluids while in the US the issue is usually fluid disposal. In Europe, with less land available for disposal sites, it's more economical to invest in filtration and other recovery equipment.
"As a result, European countries are leading the pack as far as environmental rules. For example, we look at Sweden as kind of a test bed. They often have or are proposing very stringent laws. So we watch them carefully to see what may be coming up. Although ISO has standardized the way of reporting test procedures, the actual requirements vary from country-to-country.
"Some larger manufacturers invest in costly recycling or disposal operations, rather than fluid reuse. We feel it's more economical to maintain the fluid life and our products are designed to be reused to achieve this goal. However, it is important that the recycling technology be appropriate for this use and not strip out or destroy key fluid elements.
"We find that vegetable oils, particularly the canola are getting more popular despite their higher price. They usually pay for themselves in certain specific applications. For example, in one case the diamond inserts went from 7 to 21 days because the company switched to Hangsterfer's polar technology in the form of base canola oil. The main benefits they offer are superior lubricity and good performance under extreme pressure. In some cases they mist less because of their molecular structure.
"However, water-soluble fluids are still the most commonly used and are the answer to more common manufacturing problems. The two most common issues to resolve are to identify the specific process needs and confirm you have the best chemistry for issues such as bio-stability and long life.
"A large variety of materials can be a problem with some of the new technology. You have to be careful about the chemical technologies as you mix metals such as titanium, copper, and aluminum with certain fluid. Sometimes filtering takes care of the problem, but not always.
"And finally, we have noticed that customers want lighter-viscosity fluids because they are now working with smaller parts. For example, if you are working with a drill only a few thousands of an inch in diameter, you can't do the job with fluid that flows like honey. The fluid has to get into the hole," Gentile concludes.
Fluid maintenance plays a major role in fluid performance. Keeping the fluid reservoir topped off is one of those annoying maintenance jobs that when ignored or done poorly can be a problem.
Keller Products (Lexington, MA) offers a system to simplify that operation. It consists of a portable drum, a filler unit, and a dispensing system. The operator first fills the drum with a blend of water and coolant. Any coolant/water ratio can be easily obtained by drawing off the desired quantity of coolant concentrate into a calibrated container, and then mixing the coolant with water using the mixing and dispensing pump. Then the unit is wheeled to the machines and the dispenser pump tops off the fluid reservoir. This eliminates the need for hand-carried fluids or hoses on the shop floor.
Keller also offers a number of fluid-tending systems to either filter solids from the coolant or totally drain a sump. The unit can either remove settled or suspended solids from the sump, or vacuum the sump contents to a disposal container.
"Mist reduction is a problem is we are working on," comments Randy Bates, grinding fluids manager, Benz Oil (Milwaukee, WI). "Fluid particles in the air have been the source of worker complaints for some time. Much of the pressure for anti-misting is coming from the United Auto Workers. Limits on the amount of undesirable products in the air have caused a major rethinking of safety issues. Undesirable coolant and lubricant particles in the air can be reduced by going to minimal techniques that use minimal amounts of fluid, machine enclosures, and particulate filtering systems. We formulate fluids that are less likely to mist. Reducing the volatility and viscosity of the fluids also helps.
"We are working to reduce fluid-process variables because it is important that the properties of the fluid do not change with time. We are also concerned about fluid oxidation. If it oxidizes, you are shot.
"One aspect of lubrication and cooling that the user often ignores is fluid delivery. It's important that the user understands the type of nozzle used, its position, the type of flow (it should be laminar), and the pressure. All these factors combine to ensure that the fluid reaches the work zone.
"An area that can use more research is the benefit of monitoring power consumption," says Bates. "We know that when power draw increases for a motor it may mean the wheel needs dressing in a grinding operation or a tool has become dull in machining work. But there may be other process variables that might be deciphered from this occurrence that could be looked into."
Filtration is an often-ignored factor in fluid handling. As Bates reports, "It pays for itself in several ways:
- Machined and ground parts are of higher quality and the finishes are better because the fluids carry no solids that can act as abrasives.
- Machine wear is less so equipment lasts longer and retains its accuracy. Plus, an often-forgotten feature, a less-worn machine has higher resale value
- Recovering valuable materials from the swarf is one way some manufacturers have found to mitigate the cost of fluids. Some materials are worth retrieving, such as carbide grinding swarf.
- Fluids last longer.
Machine design plays an important role in fluid performance. Sumps tend to become smaller and more difficult to get to. "And when sumps are out of the way and difficult to reach, it is more likely that they will not be cleaned properly.
"Unfortunately, one of the negative results of some 'lean' programs has been elimination or reduction of maintenance and safety personnel. Fluid performance can suffer as a result," Bates concludes.
Fluid delivery systems that carry oil or grease from a reservoir to a machine tool are an important element in any fluid-management program. Bijur Lubricating Corp. (Research Triangle Park, NC) offers a variety of systems that transfer lubricant to machine tool spindles, bearings, and slides. The FluidFlex system is an air/oil system used for cutting tool lubrication or to dispense a variety of fluids in many applications. There is also a Fluid Transfer system available to collect oil from a hydraulic leak and recycle it to a central collection point.
Bijur systems are sold to both the OEM and aftermarket. In addition to machine-specific systems, there are also systems that are flexible enough to serve a wide variety of machinery and applications. Control of lubricant application can be manual, by timer, by number of parts, or by command from the machine's operating program.
What Do Customers Want?
As a way to guide their research and customer service efforts Benz Oil surveyed existing and potential customers to find out what the wanted in an "Ideal" lubricant. Here are the results:
- Reduced grind burn
- Surface finish
- Oxidation stability
- Thermal stability
- Minimal Odor
- Minimal Foam
- Hydraulic oil compatibility
- Misting properties
- Operating cleanliness
- Seal and wiring compatibility
This article was first published in the November 2005 edition of Manufacturing Engineering magazine.