November 7, 2011
For fabricators looking to reduce the amount of lubricant they use for sawing applications, lubricants designed for near- dry sawing may be the answer. These minimum quantity lubricants, as they are called, are designed to provide complete coverage of the blade with as small an amount as possible.
Is your sawing operation underwater because of a flood of cleanup costs?
The shop uses traditional flood coolants because they do the job: keep production up and running and extend the saw blade life. However, it’s not the cleanest scenario (see Figure 1). Lubricant is sprayed all over the saw and workpiece. A combination of rags and buckets are used to keep the mess in check, but it’s an uphill battle.
Perhaps it doesn’t have to be. Research and development efforts have led to the emergence of lubricants that can be applied minimally and still dissipate the heat that is associated with sawing. This type of minimum-quantity-lubrication (MQL) technology is environmentally responsible, because less of it is needed to do the job, and also can reduce manufacturing costs by increasing saw blade life and production rates.
Although it is not quite as simple as simply exchanging your coolant for an oil, using these lubricants for near-dry sawing can make a tremendous difference in your operation when properly implemented.
MQL, more specifically, is the process of applying a minute amount of a lubricant directly into the cutting tool-workpiece interface. This approach is often associated with machining applications, such as drilling, milling, tapping, and turning, but also is used in sawing.
Manufacturers using MQL have reported benefits related to better cutting tool life, reduced coolant usage and handling, decreased maintenance, and increased production time. Traditionally, these advantages were limited only to light-duty applications because heat generated during more aggressive applications could not be eliminated effectively with small amounts of oil mist. However, the development of new additives for inclusion in MQL holds the potential to extend similar cost benefits to manufacturers that do aggressive sawing, as well as machining.
Of course, choosing the right lubricant for these near-dry sawing applications is important. Recent studies have demonstrated that oil-based lubricants cover metal surfaces better than water-based fluids. It should be noted, however, that not all oils are created equal.
In the previously mentioned study, organic oils performed better than mineral-based oils because of their polar molecules. Polar molecules have opposite electrical charges at each end that create a thin, consistent bond between the lubricant and the metallic surface. This gives an even, strong, and durable layer of lubrication.
These types of MQL fluids are also stable against oxidative and thermal breakdown. Being plant-based, they are completely biodegradable.
MQL can be successful only with precise fluid application. To do its job effectively, the lubricant needs to get between the cutting edge and the workpiece. With too little lubricant, friction is not reduced and heat builds up in the blade. Using too much lubricant is wasteful and can lead to premature wear. The goal of this near-dry sawing application is to maintain a consistent thin film of oil that lubricates the cutting interface.
The application system requires two key components:
The application system must be able to deliver fluid to the nozzles in a precise and repeatable manner. A positive-displacement pump system has proven to be an effective method. These systems typically incorporate one or more small, air-operated, positive-displacement pumps that can dispense as little as one-tenth of a drop per stroke. These pumps are normally cycled five to 20 times per minute with a pneumatic pulse generator to provide a near-continuous yet minute output of oil.
A metered amount of compressed air mixed with this oil at or near the nozzle propels the small droplets of lubricant to the blade. Often this air is kept separate from the oil with a coaxial hose (see Figure 2) in which the fluid line is inside the air line from the pump to the nozzle.
Airflow rate can play an important role in overall system performance. Too much airflow may cause misting, which can fog up the air. The goal is to use just enough air to carry the oil to the blade.
The second key component in effective MQL application is the nozzle. Sawing jobs using flood coolants typically require multiple nozzles or nozzles with multiple outlets to ensure the lubricant is applied uniformly throughout the cut. It is important to lubricate both the cutting edges of the saw blade as well as the blade guides on band saws. Ensuring the lubrication is present at the cut is important.
A number of specialty nozzles and fittings are available to perform this operation.
Figure 3 shows a nozzle with three outlets that apply the oil directly into the gullet as well as the sides of the teeth to ensure an even and adequate coating. Similar specialty nozzles also are available for band saws. It is important to note that, even with a nozzle specifically designed for the application, it is crucial to position the nozzle so the oil is applied to the blade before it enters the cut (see Figure 4).
For circular saws, nozzles often need to be mounted into the blade cover by putting a hole in the cover, which allows the nozzle to protrude through it. Care should be taken to position the nozzles so they spray onto the blade and are not too far away from it. If mounted incorrectly, the nozzles will reduce the performance of the system.
In the case of band saws, the MQL fluid can serve as both a guide and saw blade lubricant. Where guide lube points already are present on the saw, a separate lubricant output can be used to add lube through the points. Look at your saw to determine the optimal location of the nozzle and the guide lube points. If the blade is lubricated before the guide, it possibly could get wiped off before the cut. If the lubricant is applied to the blade before the cut, the cutting operation may consume so much of the lubricant that an insufficient amount is left for the guide. Because of this, some systems have separate guide and blade lubrication nozzles that can be controlled independently (see Figure 5).
Although MQL is effective in most sawing applications, it is more common on larger saws because the cost to retrofit is small compared to the cost of purchasing a new saw and the potential production benefits are greater. In some instances, return on investment can be as little as two months, depending on production volumes.
Does this type of near-dry sawing work for every shop? Of course it doesn’t, and it’s important to remember that sawing applications can greatly vary. Variables such as blade speed, chip clearance, and pitch may need to be examined before some of the benefits of MQL can be fully realized. However, it has been proven in labs and in the field to work well on many materials, including most nonferrous metals, steels, titanium, and specialty alloys.
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