GREEN PRODUCTION SUPPLEMENT DRY MACHINING/MQL
Vertical milling is one of the most challenging operations for MQL, says Seco
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.
OPERATIONAL ANALYSIS CNC routers are also commonly equipped with MQL systems, due to their open-bed design. Those that are not equipped with MQL by the manufacturer are typically sold without any cutter lubrication system and intended to run dry. These machines can easily be retro-fitted 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%, while the surface finish is simultaneously improved, owing to more consistent lubrication during the cut. Because cutters last two to four times longer using MQL, a significant amount of downtime for tool changes is eliminated. MQL has been widely accepted and embraced by the aerospace industry on large gantry mills used to machine spars out of solid aluminium. As a result, MQL has become the preferred lubrication method, with one or two external nozzles applying small amounts of lubricant, with vacuum systems typically being integrated to evacuate the chips. MQL has also proven its effectiveness in the operation of portable positive feed drills.
www.machinery.co.uk May 2013
These drills are commonly mounted in large fixtures to drill and ream holes during the assembly of aircraft spars and fuselages. The drills are typically equipped with through- coolant tools to direct fluid to the cutting edge. In the past, water-soluble coolants were used for this application, until the effectiveness of MQL was discovered. Minimum-quantity lubrication can also be effective on CNC lathes/turning centres, but the ease of application depends on the design of the machine – and, in particular, its turret.
The best way to supply MQL on
lathes/turning centres is through the same fluid path to the turret that is typically employed for flood coolant.
A single-outlet MQL system is typical, but the oil is not mixed with the air until it is as close to the turret as physically possible. At this point, the oil is mixed with the air to create a mist that is carried through the turret passages. This mist can be directed to the tools (externally) through small stainless steel nozzles or, internally, through boring bars or modified tool holders. During turning operations, the tool is submerged in the workpiece material for significant periods of time. As a result, it is often more effective to apply the lubricant to the backside of an insert where a proportion of it will migrate to the cutting edge and some of it will adhere to the workpiece until it meets the cutter on the next revolution. Because chips fall away from the cutter/workpiece interface on turning centres, there is no need for coolant to help evacuate chips. Vertical machining centres pose one of the bigger challenges when converting to MQL, because of the horizontal position of the workpiece, where chips tend to accumulate. There are two ways to apply the lubricant to the cutting tool on VMCs: internally, through the spindle when using through-coolant tools; or externally, with a nozzle when the tools are too small for oil holes, or where the machine is not equipped for through-spindle fluid application. The preferred method is through the spindle, because it directs the fluid to the cutting edges, while the air carrying the fluid effectively blows the chips from the cutting area, preventing them from being recut. ■
The Benefits of MQL over flood coolant
● Increased cutting tool life (owing to reduced friction) that can range from 25% – 200%, depending on the application, material, type of cutter etc
● Faster production times, due to the higher feed rates that MQL enables. Depending on materials and processes employed, feeds can be 25% – 100% faster
● Clean and nearly-dry chips are produced that require no post-processing and generate higher recycling prices
● Reduced costs and time required to treat and replace old or rancid coolant ● Elimination/reduction of costs associated with the disposal of old coolant ● Elimination of the need for coolant skimmers ● No coolant testing required ● Cleaner floors and the creation of a safer work area ● The creation of clean finished parts with a light film of protective oil ● The entire MQL process is environmentally friendly, with the fluid itself 100% bio-degradable and consumed in the process
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