Minimum Quantity Lubrication (MQL) replaces the flood coolant commonly used in machining operations with a minute amount of high-quality lubrication precisely applied to the interface of the cutting tool and work piece.
While flood coolants have traditionally been used almost exclusively in machining operations, the concept of minimum-quantity lubrication has been steadily gaining momentum during the last several years.
MQL, also known as near-dry machining, replaces flood coolant with a minute amount of high-efficiency lubricant applied precisely to the cutting tool/workpiece interface. The process has proven effective for machining all nonferrous metals and most grades of steel.
The most common lubricants for this process are biodegradable vegetable oil derivatives, which, because of their polarity, are known for their extensive friction-reducing properties.
Unlike mineral-based and synthetic fluids, vegetablebased fluids are developed in the presence of oxygen, which causes them to organize and align better as they bond with the surface of metals, providing superior lubricity.
These fluids, when properly applied, significantly reduce the friction and corresponding heat created when cut metal slides over the cutting tool before breaking off and becoming a chip. Reducing that friction not only extends tool life, but also eliminates the need for cooling the tool with flood coolant.
Additionally, the amount of fluid used is often less than 10ml. per hour, which is either consumed in the cut or remains as a light film on the part and chips.
The Benefits of MQL
The benefits of MQL are almost all a direct result of eliminating flood coolant, including:
- Significantly longer cutting tool life from reduced friction, ranging from 25 to 500 percent.
- Reduced production times due to the higher feed rates MQL allows. Depending on materials and processes, feeds can be 25 to 100 percent faster.
- Clean, nearly dry chips that require no post processing and generate high prices for recycling.
- No treating and replacing old or rancid coolant
- No disposal costs for old coolant
- No coolant skimmers required
- No coolant testing required
- Cleaner floors and a safer work area
- Clean finished parts with a light film of protective oil
- The entire process is environmentally friendly, i.e., there is no fluid to treat, recycle or dispose of. The fluid itself is 100% biodegradable, and is consumed in the process.
There are many ancillary costs associated with the use of flood coolants - typically 15 to 18 percent of the total
manufacturing cost of machined parts. Switching to MQL can typically reduce that number to around 5 percent, while increasing tool life and feed rates (see items one and two above).
Because it is different than traditional cooling and lubricating methods, MQL has been slow to gain acceptance. Machine builders as well as machinists are accustomed to the way things have always been done. As a result, the initial markets to readily accept MQL were applications where flood coolant simply wasn’t feasible. Although this is changing somewhat, there are still industry reservations.
Circular Saws and Bandsaws
MQL has become commonplace in most sawing operations and has replaced flood coolant in many applications. This transition was prompted by the saws'
open design and their application in cutting tubes, structural shapes and extrusions
with hollow profiles. Flood coolant, flowing over the saw blade, naturally migrates down the part being cut and onto the floor.
A simple MQL system can be easily retrofitted on almost any circular saw or bandsaw. These systems typically use one or two external nozzles, which spray into the gullet of the saw teeth prior to the cut. In many cases, special nozzles with multiple outlets made specifically for saw blades make the installation clean and effective. In the case of band saws, the MQL fluid serves double duty as a guide lubricant on the blade sides.
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 the saws, and the production benefits are great.
Routers, Mills and Drills
CNC routers are also commonly equipped with MQL systems from the factory due to their open-bed design. Those that aren't equipped with MQL by the builder are typically sold without any cutter lubrication system and are intended to run dry.
These machines can easily be retrofitted in the field with a single- nozzle MQL system where the nozzle is mounted to the gantry and aimed down at the cutting edge periphery of the tool. Feed rates can be increased by up to 50 percent while the surface finish is simultaneously improved because of more consistent lubricity during the cut. Because cutters last two to four times longer using MQL, a significant amount of downtime for tool changes is eliminated as well.
MQL has been widely accepted and embraced by the aerospace industry on large gantry mills used to machine spars out of solid aluminum or titanium. Because the machines are so large, it is impossible to contain them for flood lubricant application. As a result, MQL has become the preferred lubrication method, with one or two external nozzles applying small amounts of lubricant. Vacuum systems typically evacuate the chips.
MQL is also effective in tapping. Because taps are buried in the workpiece when they're cutting threads, they require a lubricant - not a coolant. For dedicated tapping machines, such as lead screw tappers, a special MQL system provides a single shot of fluid mixed with a small amount of compressed air to the tap just before the mixture enters the hole. These systems provide the same quantity of fluid to the tap during each cycle using a small nozzle aimed at the side of the tap near the tip.
CNC Turning Centers
Minimum-quantity lubrication can also be effective on CNC turning centers, but the ease of application depends on the design of the machine turret. The best way to supply MQL to turning centers is through the same fluid path to theturret typically used for flood coolant.