0903 lubrication

8/6/2019 0903 Lubrication

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By Chuck Boelkins, Unist

W hile flood coolants have tra-ditionally been used almostexclusively in machining oper-

ations, the concept of minimum-quan-tity lubrication has been steadily gaining

momentum during the last several years.MQL, also known as near-dry machin-ing, replaces flood coolant with a minuteamount of high-efficiency lubricant ap-plied precisely to the cutting tool/work-piece interface. The process has proveneffective for machining all nonferrousmetals and most grades of steel.

The most common lubricants forthis process are biodegradable vegetableoil derivatives, which, because of theirpolarity, are known for their extensivefriction-reducing properties. Unlike min-eral-based and synthetic fluids, vegetable-based fluids are developed in the presenceof oxygen, which causes them to organizeand align better as they bond with thesurface of metals, providing superior lu-bricity. These fluids, when properly ap-plied, significantly reduce the frictionand corresponding heat created whencut metal slides over the cutting tool be-fore breaking off and becoming a chip.

Reducing that friction not only extendstool life, but also eliminates the need forcooling the tool with flood coolant. Ad-

ditionally, the amount of fluid used is

often less than 8 oz. per shift, which iseither consumed in the cut or remains asa light film on the part and chips.

The Benefits of MQLThe benefits of MQL are almost all a

direct result of eliminating flood cool-ant, including:

n Significantly longer cutting tool lifefrom reduced friction, ranging from25 to 500 percent.

n Reduced production times due tothe higher feed rates MQL allows.Depending on materials and pro-cesses, feeds can be 25 to 100 per-cent faster.

n Clean, nearly dry chips that require

no post processing and generatehigh prices for recycling.n No treating and replacing old or

MQL: LeanandGreenThe benefits of minimum-quantity lubrication—

which is suitable for manytypes of machine tools—are numerous

All images: Unist

MQL uses precise fluid application coupled with a high-quality lubricant, such as in thisstep-drilling operation.

Clean, near-dry chips from MQLapplications can yield higher prices forrecycling than flood-coolant applications.

MARCH 2009 / VOLUME 61 / ISSUE 3



rancid coolantn No disposal costs for old coolantn No coolant skimmers requiredn No coolant testing requiredn Cleaner floors and a safer work arean Clean finished parts with a light film

of protective oiln The entire process is environmen-

tally friendly, i.e., there is no fluid totreat, recycle or dispose of. The fluiditself is 100% biodegradable, and isconsumed in the process.

There are many ancillary costs asso-ciated with the use of flood coolants—typically 15 to 18 percent of the totalmanufacturing cost of machined parts.

Switching to MQL can typically reducethat number to around 5 percent, whileincreasing tool life and feed rates (seeitems one and two above).

Because it is different than tradi-tional cooling and lubricating methods,MQL has been slow to gain acceptance.Machine builders as well as machinistsare accustomed to the way things havealways been done. As a result, the initialmarkets to readily accept MQL were ap-plications where flood coolant simply wasn’t feasible. Although this is chang-ing somewhat, there are still industry reservations.

Circular Saws and BandsawsMQL has become commonplace in

most sawing operations and has replaced

flood coolant in many applications. Thistransition was prompted by the saws’open design and their application in

MQL: Lean and Green(continued)

Te most commonlubricants or this process arebiodegradable vegetable

oil derivatives, which,because o their polarity,are known or their extensive riction-reducing properties.



cutting tubes, structural shapes and ex-trusions with hollow profiles. Flood cool-ant, flowing over the saw blade, naturally migrates down the part being cut andonto the floor.

A simple MQL system can be easily retrofitted on almost any circular saw or

bandsaw. These systems typically use oneor two external nozzles, which spray intothe gullet of the saw teeth prior to thecut. In many cases, special nozzles withmultiple outlets made specifically for saw blades make the installation clean andeffective. In the case of band saws, theMQL fluid serves double duty as a guidelubricant on the blade sides.

Although MQL is effective in mostsawing applications, it is more com-mon on larger saws because the cost to

retrofit is small compared to the cost of the saws, and the production benefitsare great. The average cost to retrofit isless than $2,000, and when blade cost,fluid cost and maintenance benefits areconsidered, the ROI can be as little as 2months, depending on production vol-umes. Smaller saws, while still being ableto benefit from MQL, are less commonly converted because of the comparatively higher cost of conversion.

Routers, Mills and DrillsCNC routers are also commonly

equipped with MQL systems from thefactory due to their open-bed design.Those that aren’t equipped with MQLby the builder are typically sold withoutany cutter lubrication system and are in-tended to run dry. These machines caneasily be retrofitted in the field with a sin-gle-nozzle MQL system where the noz-zle 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 fin-

ish is simultaneously improved becauseof more consistent lubricity during thecut. Because cutters last two to four timeslonger using MQL, a significant amountof downtime for tool changes is elimi-nated as well.

MQL has been widely accepted and

embraced by the aerospace industry onlarge gantry mills used to machine sparsout of solid aluminum or titanium. Be-cause the machines are so large, it isimpossible to contain them for floodlubricant application. As a result, MQLhas become the preferred lubricationmethod, with one or two external nozzles

applying small amounts of lubricant. Vacuum systemstypically evacuate the chips.

MQL has also provenits effectiveness in the op-eration of portable posi-tive feed drills. These drillsare commonly mounted inlarge fixtures to drill andream holes during the as-sembly of aircraft sparsand fuselages. The drillsare typically equipped with

through-coolant tools to direct fluid tothe cutting edge. In the past, water-sol-uble coolants were used for this applica-tion until the effectiveness of MQL wasdiscovered.

In these applications, a sensor deter-mines when the air motor on the drill is

started. When the motor starts, a meteredmixture of air and oil is automatically de-livered to the cutting tool through a coax-ial hose with a fluid tube routed inside anair line. The air and fluid are mixed justbefore they enter the cutting tool. Con-sistent air pressure should be maintainedthroughout the cut so the fluid flow and

Sawing is an ideal application for MQL.

Tere are many ancillary costsassociated with the use o food coolants—typically 15 to 18percent o the total manu acturing cost o machined parts. Switching

to MQL can typically reduce thatnumber to around 5 percent.



chip evacuation remain constant. Sys-tems like these have been used success-fully not only in the aerospace industry but also in the shipbuilding industry formore than 20 years.

Tapping MachinesMQL is also effective in tapping.

Because taps are buried in the work-piece when they’re cutting threads,they require a lubricant—not acoolant. For dedicated tapping ma-chines, such as lead screw tappers, a spe-cial MQL system provides a single shotof fluid mixed with a small amount of compressed air to the tap just before themixture enters the hole. These systems

provide the same quantity of fluid to thetap during each cycle using a small noz-zle aimed at the side of the tap near thetip. Multiple taps can be lubricated si-multaneously or sequentially by apply-ing a multiple-nozzle system and limitswitches or a PLC.

CNC Turning CentersMinimum-quantity lubrication can

also be effective on CNC turning cen-ters, but the ease of application dependson the design of the machine turret. Thebest way to supply MQL to turning cen-ters is through the same fluid path to the

turret typically used for flood coolant. A single-outlet MQL system is typi-cal but the oil is not mixed with the airuntil it is as close to the turret as phys-ically possible. At this point, the oil ismixed with the air to create a mist that iscarried through the turret passages. This

mist can be directed to the tools exter-nally through small stainless steel noz-zles or internally through boring bars ormodified toolholders.

When turning, the tool is submergedin the workpiece material for significantperiods of time. As a result, it is oftenmore effective to apply the lubricant tothe backside of an insert where some of it will migrate to the cutting edge andsome of it will adhere to the workpieceuntil it meets the cutter on the next rev-olution. Because chips fall away fromthe cutter/workpiece interface on turn-ing centers, there is no need for coolantto help evacuate chips.

Vertical Machining CentersVertical machining centers pose one

of the bigger challenges when convert-

ing to MQL because of the horizontalposition of the workpiece, where chipstend to accumulate. There are two waysto apply the lubricant to the cutting toolon VMCs: internally through the spin-dle when using through-coolant tools, orexternally with a nozzle when the tools

MQL: Lean and Green(continued)

When externalapplication is the only option, the e ectivenesso MQL can be greatly improved by adding aseparate blow-o nozzleto evacuate the chips.



are too small for oil holes or the machineis not equipped for through-the-spindlefluid application. The preferred methodis through the spindle because it directsthe fluid to the cutting edges while theair carrying the fluid effectively blowsthe chips from the cutting area so they are not recut.

When external application is the only option, the effectiveness of MQL canbe greatly improved by adding a sep-arate blow-off nozzle to evacuate thechips. The blow-off nozzle is typically required when milling pockets wherechips tend to accumulate and wouldotherwise be re-cut. Other operations,such as drilling and tapping, can usu-ally be accomplished without using achip blow-off nozzle. In the case of deep-hole drilling without a through-

coolant drill, the effectiveness of theoperation can be greatly improved by pecking the drill, which reapplies lubri-cant as it drills deeper. A programmedcycle to blow the chips off the entireworkpiece can easily be integrated so theentire part is chip-free during unloading

and subsequent reloading. As environmental pressures grow,

the benefits of minimum-quantity lubrication will become more appar-ent. The future of U.S. manufacturingdepends on individuals with open

minds who can prove to the world that American manufacturers are still the bestand most innovative in the world. Thechange from flood coolant to MQL willnot only make manufacturing opera-tions leaner, but will also keep the worldgreener. CTE

About the Author: Chuck Boelkins ispresident o Unist Inc., Grand Rapids,Mich. He has a BS degree in MechanicalEngineering rom Calvin College and hasworked extensively in the feld o lubricationtechnology or metal cutting and metalorming since 1993.

MQL being appliedthrough the turret in aturning operation.

For more information, visit“Articles Archive Index” and selectthe “Lubricants” category.

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