Lubricants and Lubrication Part 9 pot

Thevery low viscosities are used for spark erosion applications EDM and for honingand super-finishing while the higher viscosities are mostly used for difficult machin-ing work at low cu

438 14 Metalworking Fluids

oils The use of zinc- and other heavy metal-based additives in cutting oils is ing due to environmental and waste water considerations Chlorinated paraffins arestill used as universal EP agents throughout the world but combinations of sulfuradditives and ester oils are being increasingly used in Germany and Western Europe

declin-as substitutes The redeclin-asons for these developments are the considerably higher posal costs for products containing chlorine in Germany (Fig 14.16) [14.140].14.4.2.2 Significance of Viscosity on the Selection of Neat Products

dis-The viscosity of most metalworking oils is between 2 and 46 mm2s–1at 40 C Thevery low viscosities are used for spark erosion applications (EDM) and for honingand super-finishing while the higher viscosities are mostly used for difficult machin-ing work at low cutting speeds such as broaching and gear cutting

In general, high cutting forces, continuous chips and large chip cross-sectionsrequire higher viscosities These higher viscosities also have a positive effect on theevaporation and misting behavior of the oils However, the disadvantages of higherviscosity oils are higher drag-out losses on chips and components and the resultingincreased pollution of washing lines For this reason, the current trend is towardslow viscosity products The disadvantages of low viscosities, i.e lower flashpoints aswell as greater evaporation and misting can be compensated for by new hydro-cracked base oils or ester oils

Small components also normally require low viscosities because thicker oils cancause parts to stick together and thus cause transport and positioning problems.Thick oil films can also interfere with monitoring of routine manufacturing toler-ance and here again, low viscosity oils are the only solution if component de-oiling

is to be avoided Low viscosity oils offer better heat dissipation and this is a clearbenefit in processes which create high heat such as high-performance grinding.Also the better flushing properties of low viscosity oils make then ideal for honing,grinding and lapping operations which require the efficient removal of abradedmaterial from the cutting zone for perfect results Deep hole drilling also benefits

Cutting oil

AdditivesBase fluids

Synthetic oil

Anti wear additivesExtreme pressure additives(Corrosion protection additives)Additives to improve wettingand flushing effectAnti mist additives

Solvent raffinates

Hydrocracked stocks

White oils

Synthetic ester(natural ester)PolyalphaolefinsFig 14.16 Composition of neat metalworking oils.

439 14.4 Neat Cutting Fluidsfrom low-viscosity oils because chips are more rapidly flushed out of the hole Theother advantages of low viscosity oils relate to machinery peripherals For example,the necessary size of belt filters falls exponentially with viscosity so that both costsand space can be saved.

14.4.3

Oil Mist and Oil Evaporation Behavior

Hygiene and dermatological problems occur in the metalworking industry whenmachine personnel are subjected to repeated skin contact with metalworking oils.Chip-forming metalworking is particularly affected because of the regular and closecontact between personnel and the machines An overall reduction in the number

of dermatological problems and a general reduction in the oil pollution of workshopatmospheres was achieved by mechanical barriers on machines as well as changes

to manufacturing technologies, in particular, the automation of machining cesses in transfer lines The pollution of workshop air with oil mists and vapors isstill a problem in many companies Measures such as machine encapsulation,improved machining procedures, automation and above all, extraction at themachine or at a central point in the workshop have certainly improved the situation.However, extraction without de-oiling simply shifts the pollution outdoors and this

pro-is in conflict with general environmental protection measures

Many attempts have been made to reduce not only the misting properties ofstraight oils but also the misting of water-mixed coolants [14.141, 14.159] Initialreduction of misting has been achieved but the effect could not be maintained over

an 8-h working shift Two effects must be mentioned – the mist-suppression effectgradually decreases as the molecular weight of the polymer is reduced and the poly-mer will be filtered out and is, therefore, no longer available for mist suppression.This means, in practice, that permanent addition of a suitable polymer is necessary

to reduce the misting of a water-mixed coolant in workshops

Further studies show that total inhalable particulates can be used to estimatemineral oil mist exposure but cannot be used for water–mix MWF concentrate mistexposure [14.160]

14.4.3.1 Evaporation Behavior

The evaporation of cutting fluids on hot components, tool and chips and from thelarger surface area of oil mist droplets leads to vapor pollution of workshops andeven to condensed mists As low-viscosity oils become more popular, the evaporationbehavior of base oils is becoming increasingly important This subject is covered indetail in Chapter 4

While a number of countries specify an oil mist threshold value in mg m–3culates), Germany has in recent years moved over to evaluating the total hydrocar-bon concentration in the air In most metalworking factories, this is 5 to 20 timeshigher than the oil mist concentration

(parti-440 14 Metalworking Fluids

14.4.3.2 Low-Misting Oils

The development and application of low-misting neat cutting oils have significantlyimproved the oil mist pollution situation Development work as well as the generalacceptance of these oils in the metalworking industry was significantly influenced

by new methods of measuring oil mists and of determining the misting tics of oils [14.56–14.58]

characteris-14.4.3.3 The Creation of Oil Mist

If an oil mist is analyzed, a variety of different factors are involved lt is soon ent that the whole process is highly complex in which mechanical, physical and phy-sico-chemical factors are intermixed

appar-The causes of neat cutting oil mists can be roughly listed as follows:

. As the fluid exits the nozzle, air friction acts on the jet in proportion to itsexit velocity Important factors are the geometry of the nozzle and the exitspeed The result is that droplets of oil are dispersed into the surrounding air.. If the oil jet rebounds off the machine bed, the component or tool, oil mistcan be formed This is greatly influenced by the oil pressure but also thequantity The amount of oil mist with droplet sizes < 5 lm increases drama-tically with increasing pressure [14.59]

. Mechanical stress on the oil during the machining process also creates oilmist Critical factors here include the machining speed, the geometry of therotating machine parts and component, chip formation, the quantity of oiland the oil pressure Oil misting is a major problem in machining processeswhich use geometrically non-defined cutting edges, i.e particularly grinding.The porosity of the grinding face allows oil to be thrown-off at high periph-eral speeds and form a dispersion

. When the oil is returned to the tank, air can be trapped in the oil As the air

is released, it carries out oil in the form of a finely-dispersed mist Importantfactors here are the geometry of the jet which impacts the surface of the fluid

in the tank and the velocity of the jet

. Particularly if high oil pressures are used and depending on the related solubility equilibrium, larger volumes of air can be dissolved in theoil When the pressure falls, the air is released from the oil This escaping aircan transport oil droplets to the workshop air The decrease in the solubility

pressure-of air in oil as the temperature pressure-of the oil increases causes air to be releasedfrom the oil as the temperature of the oil increases in the cutting zone.. Apart from the above-mentioned aerosols, metalworking and other processescan cause condensation aerosols to form A large part of the energy consumed bythe machining process is converted into heat and this can lead to very high tooland component temperatures This, in turn, can cause a partial evaporation ofthe oil This evaporation process can continue beyond the cutting zone insofar asoil can still evaporate on the hot machining chips Shortly after this evaporation,the vapor cools somewhat and condenses This sequence can create condensa-tion aerosols with very fine droplet sizes Apart from the surface temperature of

441 14.4 Neat Cutting Fluidsthe wetted components, the surface itself, the thickness of the oil film and lastbut not least, oil-specific factors such as its vapor pressure also effect the creation

of oil mist Outside influences such as the amount of dust and moisture in theair influence condensation In a simple laboratory misting test, an oil mist with apronounced maximum droplet distribution at 1.2 lm was examined [14.60]

14.4.3.4 Sedimentation and Separation of Oil Mists

All the above-mentioned possible causes of oil mist can create mists with a very largespectrum of droplet size distributions Immediately after creation, oil mists begin tocollapse While droplets which are much larger than those in the mist itself normallyprecipitate in the immediate vicinity of the machine, smaller droplets can spread muchfarther if they have enough kinetic energy A major factor in the precipitation of an oilmist is the droplet size Suspended oil droplets agglomerate until they reach a maxi-mum diameter of 3 lm when they begin to sink slowly (0.5 m h–1at 2 lm diameter)[14.61] Sedimentation can also be accelerated by coagulation Air flow conditions andBrownian movement of smaller droplets can cause droplets to collide and thus grow bycoagulation And finally, small and large oil mist droplets can undergo an interchangecaused by surface-activated evaporation and condensation

In plants in which chip-forming machining is performed with neat cutting oils,the oil mist in the workshop atmosphere can have droplet sizes up to a maximum of

3 lm [14.61] The maximum numeric distribution is about 1 lm The maximummass distribution is naturally near to the upper droplet size limit if one assumesthat the mass of a droplet increases by the third power of its radius The averagedroplet size measured at various points on a centerless external grinding machinewas about 95 % < 3 lm [14.61]

14.4.3.5 Toxicity of Oil Mist

An oil mist is a dispersed system with droplet sizes between 0.01 and 10 lm Thisvery wide range of sizes justifies the term, polydispersed system The toxicologicalevaluation of oil in the air in the form of gas should be viewed totally differently tothe evaluation of oil mists

Research on aerosols and dusts has shown that only particles or droplets smallerthan 5 lm can reach the lung’s alveoles Larger droplets are filtered out by the nose

or are trapped in the bronchial tubes and made relatively harmless for the body’sorganism [14.62] Medical research on silicosis has shown that (as far as dust is con-cerned), particle sizes of 0.5 to 1.5 lm are efficiently retained, i.e these particlesizes are effectively trapped by the alveoles in the respiratory system Hydrocarbonoil mists tests on animals [14.63] have shown that this droplet size range is particu-larly important Most oil mist tests have examined mineral oil hydrocarbons withoutadditives These are viewed chemically as relatively inert substances and this ismostly the case if one disregards the above-mentioned discussion about aromatichydrocarbons or the tests performed on highly condensed aromatics Nevertheless,toxicological evaluation of pure hydrocarbon mists is still extremely complicated Iflubricant additives are included in the evaluation, which are a much more reactivegroup of substances than hydrocarbons alone, generalized statements on the toxicity

442 14 Metalworking Fluids

of neat cutting oils would be almost impossible This proviso must however formthe focus of the health hazards of oil mist

According to Reiter’s retention charts [14.64], the lung’s alveoles only retain about

10 % of oil droplets with a diameter of 0.1 lm but 70 % with a diameter of 1 lm.This should be noted when oil mists are evaluated even if the toxicological detailsare not available

As was the case with dust contamination, these medical considerations led to thecreation of oil mist thresholds The last time oil mists were mentioned in the Ger-man MAK (Maximum Workplace Concentration) list [14.65] was 1966 However,uncertainties regarding the toxicological evaluation of oil mists was the reason whyMAK values were not established for oil mist

Some countries set the limit at 3 mg m–3(and 5 mg m–3for longer exposure) TheAmerican TLV list (Threshold Limit Values) contains a threshold of 5 mg m–3oil mist(particulates) suggested by the 1973 Conference of Government Industrial Hygienists.This threshold was based on animal tests with a naphthenic white oil (molecular weight:

350 to 410) with no additives or aromatics [14.66] lt is therefore particularly significantthat the animal tests were oriented to oil mist droplet sizes found in practice The max-imum droplet size distribution was about 1.3 lm (90 % of the droplets were < 1.6 lm).This droplet size distribution roughly corresponds to that of mists found in metalwork-ing shops and also to the type of mists which are toxicologically important Withoutdoubt, aromatic-free white oils are much less problematic than the mineral oil cuts nor-mally used in cutting and grinding oils Above all, the effect of additives was excluded.Two OSHA air contaminant permissible exposure limits currently apply to metal-working fluids These are 5 mg m–3 for an 8-h time-weighted average (TWA) formineral oil mist and 15 mg m–3(8-h TWA) for particulates not otherwise classified(PNOC) (applicable to all other metalworking fluids) [14.142] There are no otherrequirements

There are also other recommended exposure limits In 1998 the National Institute forOccupational Safety and Health (NIOSH) published a criteria document which recom-mended an exposure limit (REL) for metalworking fluid aerosols of 0.4 mg m–3for thor-acic particulate mass as a time-weighted average (TWA) concentration for up to 10 h perday during a 40-h working week Because of the limited availability of thoracic samplers,measurement of total particulate mass is an acceptable substitute The 0.4 mg m–3con-centration of thoracic particulate mass approximately corresponds to 0.5 mg m–3totalparticulate mass The NIOSH REL is intended to prevent, or greatly reduce, respira-tory disorders causally associated with exposure to metalworking fluid It isNIOSH’s belief that in most metal-removal operations it is technologically feasible

to limit metalworking fluid aerosol exposure to 0.4 mg m–3or less [14.143]

The American Conference of Governmental Hygienists (ACGIH) threshold limitvalue (TLV) for mineral oils is 5 mg m–3for an 8-h TWA and 10 mg m–3for a 15-min short-term exposure limit (STEL) In 1999 the OSHA Metalworking FluidsStandards Advisory Committee also recommended a new 8-h time-weighted averagepermissible exposure limit (PEL) of 0.4 mg m–3 for thoracic particulates(0.5 mg m–3 total particulates) The committee based the recommended PEL onstudies of asthma and reduced lung function

443 14.4 Neat Cutting Fluids

In Germany, the toxicological evaluation has been based in recent years on thetotal hydrocarbon concentration in the atmosphere (which means the total of oilmists and vapor) The threshold value currently used (regulation officially not valid,February 2006) is set at 10 mg m–3, which applies both to water-miscible and neatcutting fluids Because of the diversity in measurement methods, many TLV(threshold limit values; MAK values) have been established; all are based on thesame dimension of mg m–3, but the results can never be compared with each other

As an example, if the method used in the USA gives a result of 0.5 mg m–3themethod applied in Germany under identical conditions leads to a result which issometimes more than 30 mg m–3 This is because the method used in the USAmeasures particulates in a specific size range only whereas the German methoddetects aerosols and hydrocarbon vapor of different droplet size [14.144]

14.4.3.6 Oil Mist Measurement

Determining the total hydrocarbon content of air is possible with good accuracy usingcarbon tetrachloride solvent washing with subsequent infrared spectroscopic examina-tion of the CH-valence shifts [14.67] These measurements do not analyze oil mist con-centration because the total hydrocarbon content in workshop atmospheres in themetalworking industry can be many times greater Most suitable of all is a scatteredlight spectrometer which can be adjusted to measure the droplet sizes which are rele-vant to the retention characteristics of the lung’s alveoles This method ensures that thetoxicologically relevant oil mist is measured [14.56, 14.161] This measuring method issimple and fast and can be used in all areas of a workshop For comparative measure-ments of an oil’s misting behavior, a scattered light spectrometer is installed in a unitwhich is schematically described in Fig 14.17 (Fuchs procedure) [14.68]

Air, at a given pressure, volume and temperature, is blown into the oil This ates an oil mist which is measured by the scattered light spectrometer over time.Figure 14.18 illustrates the influence of viscosity on the oil mist characteristics of aseries of neat cutting oils

cre-Fig 14.17 Measuring apparatus for determining the misting

characteristics of cutting and grinding oils.

444 14 Metalworking Fluids

14.4.3.7 Oil Mist Index

To give the oil misting characteristics of metalworking oils a figure, the oil mist centration in mg m–3is determined in line with defined machining conditions andthis oil mist concentration set against that of a reference fluid This ratio has nodimension but is multiplied by 100 to provide the Oil Mist Index If the referencefluid is di-n-octyl phthalate or di-iso-octyl phthalate, whose misting behavior is simi-lar to that of a standard cutting fluid at a given viscosity, an Oil Mist Index as inFig 14.19 results Figure 14.19 shows values for standard and low-misting oils andthe viscosity of the oils The oil mist index of medium viscosity standard oils(40 mm2s–1at 40 C) is between 80 and 120 but between about 4 and 6 for low-mist-ing versions This provides a possible definition of low-misting oils For low-mistingoils with a viscosity > 30 mm2s–1at 40 C, the index is < 10 According to Fig 14.19,greater differentiation must be made for oils with lower viscosities [14.57]

con-14.4.3.8 Oil Mist Concentration in Practice

Back in 1978, Fuchs [14.69] performed a far-reaching study in Germany to mine the oil mist concentrations near to machine tools using neat cutting oils.Table 14.15 shows values from 350 measuring points in 65 companies for standardand low-misting oils The large deviation in values was mainly due to varyingmachine-specific counter-measures (encapsulation, extraction etc.) Figure 14.20shows the oil-mist concentration in a machining shop over a period of 12 h (onlyneat cutting oils via a central system) Apart from the positive effect of low-mistingoils, the extraction system’s effect on reducing the oil mist concentration can beseen Fig 14.21 shows the high sensitivity of the scattered light spectrometer andthe effect of low-misting oils on a Gleason gear cutting machine Each tooth cut bythe machine can be identified by the pattern of oil mist concentration and the timescale Such measurements also allow the cutting oil feed to be optimized in terms ofoil mist pollution Such measurements and mist concentration profiles are also ofgreat assistance when extraction equipment is installed and/or monitored

445 14.4 Neat Cutting Fluids

Tab 14.15 Values from 350 measuring points in 65 companies for standard and low-misting oils.

Type of oil

Measuring point

Standard oils, mg m–3 Low-misting oils, mg m–3

Workshop locations, various machining

ops; points at some distance from

machines; walkways between machines

Automatic lathe; Type I, head-hight,

< 1.5 m from the machining point

95 to 2 13.3 24 to 0.8 5.2

Automatic lathe: Type II 72 to 1.5 6.3 7 to 1.2 3.1

Automatic lathe: Type III: head-hight,

< 1.5 m from the machining point

(a) standard oils (not low-misting);

(b) low-misting oils; (c) value for dioctyl phthalate (DOP, according to definition).

NI = k p / r  100; k p = mist concentration

of the oil under test, mg m –3 ; k r = mist concentration of the reference oil (DOP), mg m–3.

Fig 14.21 Gear cutting on Gleason machines 1, conventional

neat cutting oil; 2, equiviscous anti-mist neat cutting oil.

Fig 14.20 Oil mist concentration in a machining shop over a

period of 12 h (a) conventional neat cutting oil without

extraction; (b) conventional neat cutting oil with extraction; (c)

anti-mist cutting oil with extraction; (d) basic extractor switched

on; (e) complete extraction system switched on.

447 14.5 Machining with Geometrically Defined Cutting Edges14.5

Machining with Geometrically Defined Cutting Edges

14.5.1

Turning

With a turning operation it is possible to produce parts which are round in shape.Cylindrical and conical surfaces are generated from a rough cylindrical blank whichrotates about its longitudinal axis and against a cutting tool

Most turning operations use single-point tools Normally, a tool holder contains

an indexable insert with multiple cutting edges

The speed of the workpiece, the feed rate of the tool, and the depth of cut are mainspecifications for turning operations Roughing cuts, which remove metal at the maxi-mum rate, are followed by finishing cuts at a higher cutting speed, a lower feed rate, and

a smaller depth of cut [14.70] These machining conditions depend on the workpieceand tool materials, the surface finish, the dimensional accuracy, and the machine toolcapacity Each combination of workpiece and tool materials has an optimal set of toolangles Tool geometry will affect the direction of chip flow The objective is to avoid longcontinuous chips which can interfere with the machine tool operation or damage thepart surface Chips will break when they hit the workpiece or tool holder

. The major turning processes performed on a lathe are straight or cylindricalturning, taper turning, facing, and boring

. Turning operations occur on the external surface of a part whereas facing isused to produce a flat surface

. Boring is an internal turning process and may be done to enlarge a holemade by a previous process, to enlarge the inside diameter of a hollow tube,

or to machine internal grooves

. For turning operations, water-mixed metalworking fluids are used only whenrunning automatic lathes; neat oils are often preferred

14.5.2

Drilling

The most common shape found in a manufactured part is a circular hole, and many ofthese holes are produced by drilling Since the chips are formed within the part, theflutes or grooves normally serve two purposes In addition to providing a conduit for theremoval of chips, they also allow the cutting fluid to reach the tool–workpiece interface.Most drills are made from HSS which demands the application of a cutting fluid [14.71].The coolant pressure recommended in drilling depends on several factors Themost important factors include the following: workpiece hardness, feed rate, holediameter, depth, tolerance, and finish As the coolant pressure is increased, recircu-lating the coolant could become a problem [14.72]

Drilling times can be reduced by using internally cooled drills with inserts sten carbide inserts and inserts with PVD or CVD coatings are commonly used inthese applications An efficient coolant system supplies that coolant at the proper

Tung-448 14 Metalworking Fluids

pressure and flow rate An inefficient coolant system can lead to poor surface finishinside the drilled holes The drill diameter is the most important factor in determin-ing the coolant pressure and flow rate [14.73] Chips can block the channels thatdeliver the coolant to the inserts if the coolant system is inefficient This conditioncan cause tool breakage

Water-mixed metalworking fluids are used where high cooling properties arerequired Recent developments show that this operation may be run also with MQL(Minimum Quantity Lubrication) or by using dry cutting technology Both techniquesdepend on the material machined and the machine tool designed for this application.14.5.3

Milling

A variety of milling processes is available where end milling, slab milling, and facemilling are included High metal removal rates are possible since the tools havemultiple teeth and each tooth produces a chip In most applications, the workpiece

is fed into a rotating tool The feed motion usually is perpendicular to the tool axis,and cutting occurs on the circumference of the tool

In end milling, the axis of cutter rotation is perpendicular to the workpiece face to be milled End mills commonly have two, three, or four flutes

sur-Flat surfaces, recess cuts for making dies, grooves and profiles around thin partsare common operations for end mills

Hollow end mills are used on automatic screw machines The internal cutting teethwill machine a cylindrical surface to an accurate diameter from round bar stock.Face milling is similar to end milling However, a face-milling cutter is relativelylarge in diameter compared to its length Face-milling cutters are designed tomachine flat surfaces The geometry of the cutter and the major application areappropriate for carbide inserts which can be indexed Due to the relative motionbetween the cutting tool and the workpiece, tool marks, such as those produced inturning, are found on face-milled surfaces also

The direction of cutter rotation in milling can produce different effects In thetraditional method, called up milling or conventional milling, the tool rotatesagainst the direction in which the workpiece is fed In down milling or climbmilling, the cutter rotates with the feed direction Up milling produces a chip thatgets thicker, but the chip gets thinner in down milling

The major advantage of up milling is that tool wear is not affected by the surface dition of the workpiece A smoother surface is possible if the cutting edges are sharp.When down milling materials with a hard surface, such as castings and hot-worked metals, the teeth will wear faster and can be damaged Advantages of downmilling include a reduced tendency to have tool marks on the part and a surfacefinish that is not affected by a built-up edge if one is created on the cutting edge.When machining cast iron, dry milling has been state-of-the-art for many years.High tensile steels and aluminum alloys are lubricated with water-mixed metalwork-ing fluids but recent developments show that MQL can in some cases providefurther advantages from a cost perspective

con-449 14.5 Machining with Geometrically Defined Cutting Edges14.5.4

Gear Cutting

The most important machining operations are gear hobbing, gear shaping and gearshaving The different forms of teeth and various types of machines with differenttool kinematics very frequently lead to different oil recommendations by themachine tool maker Nevertheless, in this case the users very clearly want to have aunique product for some gear cutting applications Today in large serial automobilegearbox production this has already been generally achieved and all hobbing andgenerating shaping operations are carried out with just one oil Broaching is a com-petitive application to run with hobbing cutters and shapers in the production ofexternal toothing and should be mentioned here; uniform metalworking fluid gen-erally presents no problems

As far as tool cooling is concerned, the tool cutting edges only cut for a tively short time and sufficient time is available for cooling As a consequence suffi-cient cooling is provided by neat cutting oils However, the use of neat oils is calledfor because of the high standards set for a high quality surface finish; the high qual-ity surface finish becomes a prime consideration particularly when the gears are not

compara-to be precision shaped afterwards by shaving Although compara-today water miscible ants are available and can achieve good results, the majority of machine tools forhobbing and shaping are not designed for water miscible products The latest trendsare towards dry machining and the basis for this is coated tools and chip transporta-tion with compressed air

cool-Should the quality of surface finish not be adequate when hobbing and shaping,then precision machining may be used to ensure finished gears attain correct loadcarrying capacity and quiet running In this context gearwheel shaving and morerecently even gearwheel rolling have become established operations Oils withapproximate 40 mm2s–1 at 40 C are particularly suitable for gear shaving with ahigher module (m > 2.5); recommended for smaller modules are less viscous oilswith viscosities between 15 and 25 mm2s–1at 40 C

Gearwheel rolling is not a cutting operation but the area of application is so lar to that of shaving that the requirements for the cutting oil performance in thiscase have to be considered the same The surface is smooth and minimum material

simi-is dsimi-isplaced by the rolling and sliding motion of the rolling wheel under high face pressure on the gear flank This, in turn, can lead to burring When using oilsfor this forming operation they should contain polar additives As a general rule oilsfor shaping and rolling can be the same at least for gearwheels with a larger module

sur-if the additives are balanced

In the case of very sensitive surfaces, when precision machining of gearwheelssometimes corrosion problems can occur as a result of additives containing chlor-ine However, this can be remedied by adding suitable corrosion protection concen-trate but in this case it must be ensured that the metal removing capacity of the oils

is not reduced

450 14 Metalworking Fluids

14.5.5

Deep Hole Drilling

Current deep hole drilling techniques, previously employed for the most part only

in the armaments industry (hence the term
gun drilling’ which is often tered), have been steadily modified and are now concentrated to a greater extent incivil applications

encoun-These techniques are now used in the automotive, shipbuilding, aircraft, machineand tool industries, in industrial plant construction and for hydraulic and pneu-matic equipment

The borderline between conventional drilling using HSS twist-drills and the ious special-purpose drills such as deep hole drilling tools or short hole drillsequipped with indexable tungsten carbide inserts can best be defined as follows:. if the holes are shallower than 2 to 3 times the diameter (d), and on theassumption that powerful machining centers of great rigidity are availablefor the purpose, drilling with indexable tungsten carbide inserts is likely tobecome the most popular method in the future

var-. this method gives lower feed and cutting forces than twist drills On account

of the high cutting speeds (5 to 10 times higher than with twist drills), ever, correspondingly more powerful drives are needed

how-. for holes deeper than 10  d, and in cases where a better quality surface ish, reduced surface irregularity and closer dimensional tolerances are calledfor, deep hole drilling tools are used

fin-. in between these two areas, special designs of conventional HSS twist drillare employed

14.5.5.1 Deep Hole Drilling Methods

A distinction is made between three drilling methods, which are known as solid,trepanning and counter-boring methods Drilling from solid means cutting out theentire volume of material representing the hole to be drilled Core drilling (trepan-ning) cuts away only an annular area, leaving the solid core untouched Counter-boring-diameter a second operation, at the same time improving the surface finishand accuracy of the hole Counter-boring processes include sciving and finish-roll-ing or burnishing The three best-known deep drilling methods, BTA (the initials ofthe Boring and Trepanning Association), ejector and gun drilling, all have differentforms of special cutting fluid supply

Gundrilling System (Fig 14.22)

The gun drill has internal cutting fluid supply and external chip flow The cuttingfluid is pumped at high pressure through the oil duct to the tip to lubricate the cut-ting edge and support pads Because of its high velocity it carries the chips backthrough the V-shaped groove

451 14.5 Machining with Geometrically Defined Cutting Edges

Ejector system (Fig 14.23)

The Ejector system has an external cutting fluid supply and an internal chip flow.The fluid is pumped between the drill tube and inner tube to the drill head Most ofthe fluid is forced through holes in the drill head and cools and lubricates the sup-port pads and cutting tips The remainder is forced through the nozzle in the innertube and diverted back to the outlet This creates a partial vacuum in the inner tube

so that the fluid which has performed the lubrication and cooling is sucked into theinner tube together with the chips, and passed to the outlet

BTA system (Fig 14.24)

The BTA (or STS) drill has external cutting fluid supply and internal chip flow Byforcing the cutting oil between the drill tube and the hole being drilled, the velocity

of the fluid becomes so high that it is able to provide effective transportation of thechips through the drill tube and the drilling spindle back to the outlet

14.5.5.2 Tasks to be Fulfilled by the Cutting Fluid

As one of the process parameters, the cutting fluid can make a considerable bution to increased economic viability, assuming that all other influencing factorshave been optimized Its principal task is to cool and lubricate the cutting edges.Chemically active extreme-pressure (EP) additives incorporated into the cuttingfluid and polar effective substances help to reduce wear and friction

452 14 Metalworking Fluids

In addition to the actual cooling and lubricating functions, the cutting fluid,which is fed to the tool at high pressure, must also carry away the swarf from thecutting point The high-pressure supply system requires not only a carefully de-signed fluid circuit (with a recirculation rate no higher than 6 h–1),but also for prod-ucts with low foam and in particular low oil-mist characteristics

Unusually critical conditions occur when deep hole drilling with gun drill tools,which, have an off-center cutter tip This offset cutting action means that resultantcutting forces must be absorbed at the bore wall, usually by way of support pads.This in turn increases the proportion of friction, as a result of friction between thesupport pads and the bore wall (Fig 14.25)

Accordingly, lubrication of these support pads is one of the important tasks whichhave to be performed by the deep hole drilling oil

14.5.6

Threading and Tapping

Frequently water miscible products are used in the production of external threads.The focus is clearly on neat cutting oils for tapping where the materials to be cut aredifficult or most difficult (material groups 2 and 3, section 1); typical tapping andthreading oils of this type are sulfur active oils with viscosities between 15 and

40 mm2s–1at 40 C; the proportion of polar substances is also important Tight drillholes and the low pitch of the internal threads call for less viscous oils because of thefavorable flushing properties Here again the risk of cutting edge build-up is greatbecause of the generally low cutting speed, and damage to the cutting edge can result.Lubrication is also important during reverse movement of the tap If chips becomestuck between the flank and the workpiece in the reverse movement phase this can lead

to edge build-up on the flank Breakaway is more likely when this edge build-up occursclose to the cutting edge Frequently recommended where tapping is carried out and ondifficult and most difficult-to-cut materials are thread cutting pastes (sometimes diluta-ble with water up to 1:5) or oils containing solvent In particular, products with chlori-nated hydrocarbons, more especially 1,1,1-trichloroethane as solvent, are easy to apply

by brush or in drops Many problems can be solved with such oils when tapping blind

453 14.5 Machining with Geometrically Defined Cutting Edges

hole threads Such oils and paste containing solvent are not suitable for coolant tion systems Today for environmental reasons such products have been replaced by lowviscosity hydrocarbon solvents and/or other synthetic fluids

circula-14.5.7

Broaching

Broaching is a type of machining operation which is carried out with a clear focus onneat cutting oils Far more than is the case of many other cutting operations the mainconsideration is tool wear or tool life The reason for this is the fact that the broachingtool is a very complicated part and, the tool is manufactured from one single part Thematerial is predominantly high speed steel; carbide metal as a tool material is only usedfor gray iron machining In the case of broaching several teeth engage at the same timeand very frequently the chip width is large Chip removal can be very problematic whichthen generally also calls for relatively low viscosity oils In view of the cutting fluid sup-ply internal broaching is more problematic than external broaching and horizontalbroaching is more difficult than vertical broaching

Temperature sensitivity of the broaching tool combined with cutting from the solidworkpiece which follows each tooth without any soft initial cut, call for relatively lowcutting speeds As a result this leads us into the build-up cutting edge zone whichdemands special and even particularly high amounts of additives in broaching oils Aproportion of active sulfur is necessary to reduce the build-up on the cutting edge Fre-quently active sulfur additives are used in conjunction with substances containing chlor-

Fig 14.25 Sectional view of a gundriller with schematic

presentation of friction forces at the guide pads F 1 , F 2 , normal

forces transmitted from the guide pads to the bore wall; R 1 , R 2 ,

friction forces at the guide pads; S, cutting edge Drilling force =

cutting force + friction force Friction force = l(F 1 + F 1 )  d/2

454 14 Metalworking Fluids

ine [14.74] Increasing the cutting speed, in so far as machine technology permits, canavoid the critical cutting edge build-up zone; in this case lubrication conditions forbroaching oils are generally more difficult If one is successful in reducing cutting edgebuild-up, this generally improves the workpiece surface which is an interesting aspectespecially in serial production; in mass production, where broaching is very often not aprecision machining operation but can be classified as a roughing operation with highstock removal rates, the question of quality of the surface of the workpiece is of second-ary importance and under these circumstances protection of the tool by the metalwork-ing fluid has priority At very high broaching speeds and high stock removal rates goodresults are achieved on free machining steels by using water miscible coolants , moreespecially EP emulsions and synthetic products free of mineral oil with relatively highconcentrations (10 to 35 %); in this case, compared with neat oils, the improvements ofthe cooling effect and of chip removal are important factors Also important whenbroaching, as far as the actual lubricating operation is concerned, unlike almost all othercutting operations with geometrically defined cutting edge, this operation employs anextremely low clearance angle As a result of this the front clearance is under consider-able stress and flank wear and the wear on the rake can be greater and this should also

be noted for the coolant supply

During linked broaching in simple cutting operations, an attempt should at least

be made to use EP emulsions on transfer lines running water miscible coolants inlow concentration

External cylindrical grinding is used with symmetrical rotating workpiece tours We differentiate between centerless grinding and grinding between centers,depending on how the workpiece is mounted Industrial applications are, for exam-

con-454 14 Metalworking Fluids

ine [14.74] Increasing the cutting speed, in so far as machine technology permits, canavoid the critical cutting edge build-up zone; in this case lubrication conditions forbroaching oils are generally more difficult If one is successful in reducing cutting edgebuild-up, this generally improves the workpiece surface which is an interesting aspectespecially in serial production; in mass production, where broaching is very often not aprecision machining operation but can be classified as a roughing operation with highstock removal rates, the question of quality of the surface of the workpiece is of second-ary importance and under these circumstances protection of the tool by the metalwork-ing fluid has priority At very high broaching speeds and high stock removal rates goodresults are achieved on free machining steels by using water miscible coolants , moreespecially EP emulsions and synthetic products free of mineral oil with relatively highconcentrations (10 to 35 %); in this case, compared with neat oils, the improvements ofthe cooling effect and of chip removal are important factors Also important whenbroaching, as far as the actual lubricating operation is concerned, unlike almost all othercutting operations with geometrically defined cutting edge, this operation employs anextremely low clearance angle As a result of this the front clearance is under consider-able stress and flank wear and the wear on the rake can be greater and this should also

be noted for the coolant supply

During linked broaching in simple cutting operations, an attempt should at least

be made to use EP emulsions on transfer lines running water miscible coolants inlow concentration

External cylindrical grinding is used with symmetrical rotating workpiece tours We differentiate between centerless grinding and grinding between centers,depending on how the workpiece is mounted Industrial applications are, for exam-

con-Lubricants and Lubrication 2nd Ed Edited by Th Mang and W Dresel

Copyright
2007 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim

455 14.6 Machining with Geometric Non-defined Cutting Edgesple, shafts and roller bearing elements, cam shaft bearing seats, valve push rods,rotor shafts and injector needles In the case of external cylindrical grinding, watermixed, generally transparent synthetic cutting fluids, which are used primarilybecause of the better cooling and flushing properties.

Internal cylindrical grinding is comparable to external cylindrical grinding ever, since the contact zone between workpiece and grinding wheels is considerablylarger, the cutting fluid and its supply is of greater importance As is the case withdeep grinding, very coarse porous grinding wheels with lower hardness and openstructure are to be used so that the temperature in the contact zone is kept low

How-Gear grinding is a special grinding method In principle, this method can be divided into discontinuous and continuous hob and profile grinding The methodselected also depends on both the specific size of the component and the quantityinvolved, as well as the quality of the toothing required For example, as far as con-tinuous profile grinding is concerned only very short machining times can beachieved, with average quality machining results, as against partial hob grindingwhich ensures maximum quality toothing with longer grinding times

sub-Also important as special grinding methods are thread grinding, form and cut-offgrinding and sharp grinding These grinding methods frequently make special cuttingfluids necessary

14.6.1.1 High-speed Grinding

High speed grinding atVc > 80 m s–1has been marked by the endeavor to makethe grinding process more economical through the higher time-related stockremoval rates (Fig 14.26) [14.76, 14.77] The principle of high speed grinding is tomachine at higher feed control rates and at the same time, at a high cutting speedand at an over proportional rate of feed As a result, the demands put on grinding

3027,52522,52017,51512,5107,552,50

c c c

456 14 Metalworking Fluids

machine stiffness, grinding wheel driving capacity and workpiece drive are ably higher than those put on conventional grinding The use of grinding oils isspecial to high speed grinding with CBN grinding wheels (cubic boron nitride) Thehigh lubricating effect reduces the friction between workpiece and the grain pointsand, as a result can prevent the generation of heat In this way it is possible to pre-vent thermal problems such as grinding crack, soft skin formation and re-harden-ing This is also reflected in the reduced tangential force when suitable oils are used

consider-At the same time better quality surfaces are produced with neat oils

14.6.1.2 Grinding Wheel Abrasive Materials and Bondings

Today, abrasive materials are mainly produced synthetically and have replaced ural materials The most important materials are classified according to increasinghardness: Corundum (Al2O3), silicon carbide (SiC), cubic boron nitride (CBN) andsynthetic diamond The choice of abrasive materials depends both on the material

nat-as well nat-as on the grinding method Although corundum is used especially for ventional grinding of cast and steel materials, CBN is suitable for high performancegrinding of ferrous materials because it is considerably harder and dissipates heatbetter [14.78] With regards to their reaction to chemicals CBN and diamond are to

con-be viewed more critically than corundum and silicon carbide There is a tendencyfor diamonds to graphitize at 900 C CBN is subject to hydrolytic decompositionabove 1000 C Hydrolysis depends greatly on the machining conditions and thecoolant used This hydrolysis is one of the reasons for using oils

In all abrasives to DIN 69111 the abrasive grains are bonded with each other tothe base material The purpose of the bond is to secure the abrasive grains until theybecome dull and then release them We differentiate between resinoid ceramic andmetallic bonds because of the difference in their elasticity, temperature resistance,heat dissipation and dressing capacity Their resistance to chemicals is particularlyimportant as far as the coolants used are concerned

Resin bonded grinding wheels are not stable to hydrolysis and can lose their sile strength when water-mixed coolants are used [14.79]

ten-Ceramic materials are highly resistant to chemicals and temperature but are tle and glass-like in character Metallic bonds are becoming more and more impor-tant as far as superabrasives are concerned because of their high resistance to wear.14.6.1.3 Requirements for Grinding Fluids

brit-The grinding process, as a precision machining process, is used frequently for thefinal stage machining of workpieces As a result, high demands are set on compo-nent accuracy Particularly significant in this respect is the coolant

Its first task is to reduce the friction between the abrasive material and surface ofthe workpiece in the contact zone and, as a result, lower the temperature in thegrinding area It is necessary to reduce the contact zone temperature to increase thetool life of the grinding wheel and prevent the peripheral zones of the workpiecefrom being affected in any way

A further task of the coolant is, of course, to prevent thermal damage to either theworkpiece or grinding wheel by dissipating the heat generated Flushing the grind-

457 14.6 Machining with Geometric Non-defined Cutting Edgesing wheel clean is an important criterion as far as tool life is concerned Here theviscosity and quality of the base oil as well as the coolant additives play a decisiverole.

We must differentiate between the following criteria to be able to select the mum lubricant for the respective grinding process

opti-. workpiece material

. grinding wheel

. grinding method

. cutting parameters (cutting speed, feed, stock removal rate over time, etc.)

Whether a water-miscible coolant or a straight oil is suitable for a process alsodepends, apart from the above-mentioned criteria, on the peripheral system Herethe machine tool equipment plays a particularly decisive role with regards to extrac-tion, sealing, wiring and type of filter system

14.6.1.4 Special Workpiece Material Considerations

Cemented carbide grinding sets particularly high demands for cooling of the used bide grinding wheels during use As a result, synthetic water-miscible coolants are pri-marily used for this purpose In this case the coolants should certainly contain inhibitorswhich prevent cobalt complexes Water soluble cobalt compounds lead to waste-waterdisposal problems and are to be considered particularly critical for occupational healthreasons The formation of water soluble cobalt complexes [14.80] can be reduced to aminimum by using suitable inhibitors whereby full stream fine filtration with pre-coat-

car-ed filters and return flushing filters are an advantage Latest trends in Europe also reflectthe use of very low viscosity synthetic hydrocarbon fluids for cemented carbide grinding.Straight cuts with low boiling points and with flash points just above 100 C are used toensure adequate heat dissipation

14.6.1.5 CBN High-speed Grinding

In recent years the trend towards high speed grinding has promoted CBN grindingwheels to an ever increasing extent Cutting speeds from 80 to 250 m s–1are beingachieved today in CBN high speed grinding Galvanic bonded CBN grinding wheels,which can be dressed, have up to 100 times longer tool lives than conventional corun-dum grinding wheels when a suitable neat grinding oil is used In the case of CBNgrinding wheels, one special feature is that the coolant reduces the friction in theengagement zone of the grinding wheel, which leads to the temperature being reducedconsiderably As a result, despite a poorer cooling effect, neat grinding oils ensure con-siderably lower temperatures in the contact zone (Fig 14.27)

Consequently, the use of suitable oils for CBN grinding brings the followingadvantages for the user: longer grinding wheel tool life (factor 10 higher than in thecase of corundum wheel grinding with water-miscible coolants) and considerablyless thermal damage to the workpiece

The use of straight oils in CBN grinding wheel also enables the user to filter to avery fine degree using pre-coated filters, which has a favorable effect on both the sur-face finish of the component and the tool life of the grinding

458 14 Metalworking Fluids

CBN grinding operations are also carried out to some extent with water mixed ants for system capacity or process compatibility reasons Highly additived products areused in this case which, according to their lubricating properties, are almost equivalent

cool-to neat cutting oils Component quality is achieved in this case at a slightly reducedstock removal rate However, the user has to accept as a disadvantage the clearly reducedgrinding wheel tool life because of the higher grain breakaway

Ester oils have proved particularly favorable for flute and twist drill grinding, aswell as for tooth cutting

Unlike mineral oils, ester oils offer numerous technical cutting advantages as aresult of their chemical properties Their excellent air release behavior proves to be aconsiderable plus point [14.81], especially when machining with non-defined cuttingedges In the meantime, pressures of over 100 bar are more and more becomingstate-of-the art technology for high speed grinding operations On the other hand,conventional mineral oils take up a great amount of air and tend to foam The airtrapped in the oil while machining leads to considerably poorer heat dissipation.Some ester oils with a specific chemical structure immediately release the air and

do not foam The more favorable heat dissipation frequently permits cutting speeds

to be increased [14.145, 14.146] Apart from their better air release capacity whencompared with mineral oils, ester oils also have a more favorable friction coefficient[14.82] As a result, lower viscosity ester oils can very often be used for the samemachining process Drag-out losses through chippings and machined parts are, as aresult, considerably less

14.6.1.6 Honing

As with grinding, the honing tool has grit or grain bonded multiple cutting edges.The most important grain materials are corundum, silicon carbide, CBN (cubicboron nitride) and diamonds

Honing is applied to improve the form and accuracy of a workpiece by continuoussurface contact with the tool

459 14.6 Machining with Geometric Non-defined Cutting EdgesThere are four main categories of honing to be considered:

move-is generally < 10 mm, and the honing stone oscillates at a higher frequency Gearhoning is a hard machining process with which a inner tooth honing stone made ofhigh-grade corundum or silicon carbide is used for honing external straight or heli-cal pinion wheels and shafts Gearwheels with inner toothing are produced in thesame way using external toothed tools Laser honing is a combination of honing andlaser processing Specific amounts of oil are used within a precisely defined area tocreate precisely defined surfaces [14.147, 14.148]

The advantage of all honing methods is that, with high measurement accuracy,surface roughness values in the region of Rz = 1lm can be achieved, which liewithin the surface class 3 in accordance to DIN 3969, Part 1 [14.83]

Since honing tools have area contact, the hydrodynamic proportion of the stockremoval can be controlled by the hold-down pressure, machining speed and honingfluid viscosity The hydrodynamic effects increase as the honing operation proceedsand this reduces the surface roughness; there is no stock removal where fully hydro-dynamic tool floating is concerned This is made clear in Fig 14.28 in which the

460 14 Metalworking Fluids

stock removal at various hold down pressures is plotted against honing time In thecase of the lowest contact pressure no further stock removal takes place after a hon-ing time of 80 s because the hydrodynamic lubricating film leads to complete sepa-ration of the honing stone and workpiece When the hold-down pressure is not onlytransmitted via the honing stone but also over the wear guides, this must be main-tained as far as possible without wear This is possible by selecting a honing oil withsuitable viscosity and additives, but is frequently contrary to the demands for stockremoval, where a fluid with a lower viscosity is required to ensure sufficient stockremoval The same applies for rinsing the honing stones, where low viscosity oilsare also an advantage

Figure 14.29 shows the influence of honing oil viscosity on the workpiece stockremoval of cast iron in relation to contact pressure (according to Haasis)

14.6.1.7 Honing Oils

During honing, due to this type of operation, there is a very large contact zonebetween tool and workpiece with relatively low pressures This means that the heatformation is relatively low compared with grinding so that the coolant does not tohave to dissipate any great amount of heat

An important criterion when honing is flushing the honing stone to remove thestock removed material to ensure that the working surface retains its grip and is selfsharpening

As a consequence, very low viscosity oils between 2 and 10 mm2s–1at 40 C areused when honing These are applied, as a general rule, at low pressure but at ahigh flow rate through ring nozzles To achieve the component accuracy required,the honing oil is kept at a constant temperature by chillers If an attempt is made toassociate certain honing oil qualities with the workpiece materials to be machined,short chipping hard materials are associated more with very low viscosity oils, andlong-chip tough materials are better machined using higher viscosities [14.84]

Oil 2 = honing oil approx.

Fig 14.29 Effect of honing oil viscosity on the workpiece stock

removal of cast iron in relation to hold down pressure

(according to Haasis).

461 14.6 Machining with Geometric Non-defined Cutting EdgesThere is a special situation, for example, with sulfur bonded honing stones If thesulfur dissolves in the honing oil this can lead to reduced stock removal Specialinhibitors in the honing oil can prevent the sulfur from dissolving As is the casewhen grinding it is essential when selecting a coolant to also consider both the hon-ing-stone bonding and the material to be machined.

Apart from straight oils, water mixed coolants are also used, especially when ing cylinder liners with diamond honing stones in the automobile industry Reasonsfor this are to be found frequently in the process line, because previous machining

hon-is generally carried out with water-mixed fluids Thhon-is leads to reduced process costswhen uniform media are used

14.6.1.8 Lapping

Unlike grinding and honing, lapping is a precision finishing process with bonded abrasive In lapping, the workpiece surfaces are machined by the frictionbetween the workpiece surfaces and the appropriate counter surface in the form of aworking disc The stock removal is by means of a lap (grit and liquid) which isapplied between the workpiece and lapping wheel surface The stock removal in thiscase is effected by the grit movement in the contact zone [14.85] The grit pointspress into the material to indent depths of between 5 and 10 % of the grain diameter

non-In accordance with DIN 8589, lapping is subdivided into surface, round and helicallapping, tooth lapping, form lapping and copy lapping Further classifications arepossible based on the effective surface of the lapping tool When the tool axis andthe workpiece surface are parallel to each other the process is called peripheral lap-ping; side lapping when the tool axis and the workpiece surface are vertical to eachother A distinction can also be drawn between broad and fine lapping, depending

on the fineness of the surface finish

14.6.1.9 Lapping Powder and Carrier Media

Mainly silicon carbide (SiC), corundum (Al2O3), boron carbide (B4C) and diamondare used as abrasive grain Preference is given to corundum for soft steels, brass andbronze and castings, while SiC is preferred for machining annealed steels and glass.Boron carbide and diamond are used for very hard materials such as cemented car-bides and ceramic materials

The main characteristics for the quality of the lapping powder are the distribution

of the grain by size, its hardness and the type and number of grit cutting edges Thegrain size spectrum ranges from 5–40lm, but the bulk lie between 12 and 18 lm.The purpose of lapping oil is to induce homogeneous mixing of the grain How-ever, lapping powder agglomeration must certainly be avoided The lapping oil has

an important task to perform in the operation, in that it lubricates properly to vent cold welding between workpiece and working disc The lubricating film mustnot be applied too thickly as otherwise hydrodynamics will prevent stock removal

pre-In the case of lapping oils we differentiate between the very low viscous oils,which can only maintain the lapping powder in suspension for a short period oftime, and the thixotropic, high viscous products, which suspend the lapping powderfor days on end The advantage of low viscous oils is the very good washing behavior,

462 14 Metalworking Fluids

which is reflected in clean surfaces Due to the lack of carrier capacity for the ping powders these products are only used on machines where the lapping mediumtank is equipped with an agitator

In general steel is an alloy of iron and carbon The basic differentiation between castiron and steel is the carbon content, ranging up to 1.5 % in steel and up to 4 % incast iron

The alloying elements used in steel have specific functions and their tion ranges have been developed for the various grades to produce specific proper-ties

concentra-Carbon determines the ultimate hardness a steel can achieve Steels with lowranges up to 0.20 % carbon achieve maximum hardness up to about 35 HRC (Rock-well C hardness) When used for applications requiring high hardness, these steelscan be carburized, a case-hardening heat treatment Steels with medium carbon, up

to about 0.50 %, may be fully hardened to as high as 60+ HRC

Manganese, vanadium, chromium, and molybdenum are very effective at ing hardenability, but unlike manganese, they form also strong carbides These ele-ments offer increased wear resistance, but this effect leads to increasing processseverity when machined

increas-Nickel generally improves hardenability, promotes toughness and is frequentlyused for applications requiring high impact resistance

Sulfur and phosphorus are added to promote machinability while forming metallic inclusions with low melting points During machining operations theseinclusions lubricate the cutting tool and also act as chip breakers Both sulfur andphosphorus have a negative effect on strength and toughness Lead is another alloythat falls into the category of a free machining additive but due to its toxicity isbeing used less and less [14.87] The alloy steels can contain a maximum of about

non-5 % alloying elements Steels for more demanding applications such as the toolsteels require much higher alloy additions

14.7.1.2 Tool Steels

These steels are difficult to categorize They are used are for molds, bearings, wearparts, and a wide variety of structural components

463 14.7 Specific Material Requirements for Machining Operations14.7.1.3 High-speed Steels (HSS)

All metal cutting was once performed with high-carbon steels This grade is capable

of developing high hardness but tends to soften rapidly when it heats up The tion of tungsten and chromium to cutting steel made it much more resistant to soft-ening when heated and, therefore, made it possible to increase the cutting speed to

addi-a remaddi-arkaddi-able degree These steels caddi-ame to be known addi-as high-speed steel

Compared with this group the other classes of tool steels are less important forthe cutting application

14.7.1.4 Stainless Steels

Stainless steels can be categorized by their crystallographic structures: ferritic, tensitic, and austenitic The most corrosion-resistant grades are to be foundamongst the austenitic series followed by the ferritic The martensitic grades, gener-ally have the poorest corrosion resistance

mar-The primary mechanism by which stainless steels gain their corrosion resistance

is through the development of a stable, protective surface oxide film It is generallyaccepted that a minimum chromium content of 12 % is necessary to form the pro-tective oxide film Stainless steels form this film naturally by reaction with oxygen inthe atmosphere

Stainless steel is subject to a serious reduction in corrosion resistance through amechanism called sensitization Chromium has a very strong affinity for carbonand tends to form a very stable carbide One common fabrication process that caninduce sensitization is welding Sensitization can be reversed by a specific heat-treat-ing process [14.88, 14.89]

14.7.1.5 Cast Iron

Cast iron has good mechanical properties and is easily machinable due to its uniquemicrostructure In its broadest description, cast irons are alloys of iron, carbon, andsilicon The carbon is usually present in the range of 2.0 to 4.0 % and the silicon inthe range of 1.0 to 3.0 % The form taken by the excess carbon is the basis of thethree major subdivisions of cast irons: gray iron, white iron, and malleable iron

The most common variety of cast iron has the excess carbon present in the form

of graphite flakes and is called gray iron

In irons with the carbon and silicon content minimized and where a very rapidsolidification rate was attained, the excess carbon is present as a carbide and there is

no free graphite This type is called white iron, is very hard and frequently used inapplications where extreme wear resistance is required

White iron can be converted to so-called malleable iron by a heat treatment process.Another method of producing a ductile form of cast iron is by a nodulizing inocu-lation If magnesium or rare earth metals are added to the molten iron, the excesscarbon forms spheroidal nodules of graphite rather than the flake form found ingray iron The nodular graphite structure results in a substantial increase instrength and ductility Ductile iron castings can compete with steel castings or forg-ings in some near net shape applications

464 14 Metalworking Fluids

14.7.2

Aluminum

14.7.2.1 Influence of the Type of Aluminum Alloy

In addition to the machining conditions, the type of workpiece material plays animportant role

Malleable Aluminum alloys

As these alloys contain few coarse abrasives, the fastest cutting speed possibleshould be chosen (minimum 100 m min–1) to avoid the formation of cutting edgebuild-up The most important machining parameters for malleable Aluminumalloys are chip type and surface finish The tendency of the material to smear’ can

be reduced by using sharp tools, high cutting speeds, good cooling and large rakeangles

Free-cutting Materials

Free-cutting materials are characterized by their heavy metal additives which assistchip breaking Similar criteria apply to the selection of tool materials and cuttingspeeds as do for malleable aluminum alloys

Cast Aluminum

As long as the cast Aluminum contains no silicon and is structurally uniform, it can

be treated similarly to free-cutting Aluminum and malleable aluminum alloys.Greater difficulties can be expected however when machining cast aluminum–sil-icon materials

Aluminum–Silicon Alloys

Aluminum–silicon alloys tend to lead to cutting edge build-up When not forged, ting edge build-up occurs when the material contains more than 5 % silicon The cuttingedge build-up problem reaches its peak when the silicon content is between 8 % and

cut-12 % (silicon alloys), i.e when the structure is almost eutectic Above 13 % cutting edgebuild-up steadily decreases and at about 22 % disappears altogether If these figures arecompared to the usage of Aluminum alloys in Germany, one can clearly see that 80 % ofAluminum alloys contain a percentage of silicon which makes machining difficult andwhich causes cutting edge build-up (Table 14.16) It should also be remembered thatalthough a silicon content of over 12.7 % reduces the cutting edge build-up problem andassists chip breaking, it also causes much more wear This means that the water-misci-ble cutting fluid must not only provide good cooling but also offer adequate lubrication

in order that the demands made on Aluminum machining can be met

465 14.7 Specific Material Requirements for Machining Operations Tab 14.16 Aluminum–silicon alloys.

31 % AlSi 8 Cu 3 Thermally stable, multipurpose alloy

16 % AlSi 12 Cu As AlSi 12 but harder

14 % AlSi 11 Automobile wheels

8 % AlSi 12CuMgNi Alloys for thin-walled pistons

5 % AlSi 12 For thin-walled, cast components

3 % AlSi 10 mg As AlSi 12 but hardenable and stronger

< 3 % Other alloys

Hypoeutectic (< 12.7 % Silicon)

Surface finish and chip formation are not problems with hypoeutectic aluminums.The abrasiveness which increases in proportion to the increase in silicon content,requires the use of a cutting fluid

Eutectic (approx 12.7 % Silicon)

The characteristic feature of these aluminum–silicon alloys is their softness compared

to other aluminum–silicon alloys The abrasive structural components are pressed intothe material during machining and therefore cannot exercise their abrasive effect Thetool wear they cause is similar to that of cast materials With regard to surface finish,these alloys tend to smear’ just like malleable Aluminum alloys

Hypereutectic (> 12.7 % Silicon)

The very high tool wear caused by such materials makes the use of a high lubricitycutting fluid a necessity Although cutting edge build-up is rare, false chip’ forma-tion is very common

14.7.2.2 The Behavior of Aluminum During Machining

The specific cutting energy required for Aluminum is approximately 25 % less thanthat of steel However because of the much higher cutting speeds used, greatermachine performance is necessary The good thermal conductivity of Aluminumserves to transfer heat away from the cutting face as well as reducing heat build-up

at the tool’s cutting edge The modulus of elasticity (70 000 N mm–2) of aluminumwhich is about one third less than that of steel as well as its relatively low tensilestrength means that far less energy is needed by the tool to penetrate the material.Cutting Speeds and their Effect on the Surface Finish

The two special problems associated with the machining of Aluminum are cuttingedge build-up and the formation of false chips’ The effect of cutting speed on sur-face roughness can be seen in Fig 14.30

The curve pattern (the extremities and the optimum zone) depends on the rial being machined

mate-466 14 Metalworking Fluids

Cutting Edge Build-up

The lower cutting speed limit for Aluminum machining is determined by the point atwhich cutting edge build-up occurs This usually means that cutting speeds of less than

90 m min–1should be avoided If lower cutting speeds are unavoidable, for examplewhen drilling, broaching, thread cutting or boring, countermeasures must be taken.The formation of cutting edge build-up is determined by the following parameters:. cutting speed

. cutting angle

. lubrication at the cutting zone

. feed

. type of Aluminum

Measures to eliminate cutting edge build-up can include:

. increasing the cutting speed, insofar as this is technically possible

. increasing the cutting angle Naturally this is only possible to a certain extent

as thin tools used on brittle materials can break

. polishing the cutting surfaces

. the use of suitable lubricants which assist chip transportation and avoid ting edge build-up by reducing friction

cut-False Chip’ Formation

An additional form of wear called false chip’ formation only usually effects cast minum–silicon materials which display high wear factors Higher cutting speedscause higher temperatures to be generated at the cutting zone If the temperaturerises to such an extent that the workpiece material becomes pasty, the material issqueezed out of the contact zone This material then solidifies and forms falsechip’ This usually builds up on the clearance faces of the tool As the false chip’and the workpiece come into contact with each other, the surface of the workpiece isdamaged Apart from high cutting speeds and high fluctuating temperatures, themain cause of false chip formation is tool bluntness To prevent the formation offalse chips, the cutting speed must be reduced All measures which lead to a reduc-

Cutting speed

Fig 14.30 Effect of cutting speed on surface roughness.

1, Cutting edge build-up; 2, No cutting edge build-up;

3,
False chip’ formation zone.