Lubricants and Lubrication Part 10 pptx

Although disadvantages include expensive technicalequipment and high energy costs, the most important advantage is its suitability foralmost all types of water-miscible cutting fluids..

497 14.8 Metalworking Fluid Circulation SystemPetroleum Ether-extractable Components

The presence of petroleum hydrocarbons are most often determined by extractionwith petroleum ether However, it should be remembered that other products mayalso be extracted Exact determination of hydrocarbon content requires another ana-lytical step (e.g chromatography) Petroleum ether extractables have been divided

up into saponifiable and non-saponifiable components

Thresholds and Legislation

There are no national guidelines on the quality of separated-water in Germany and

in Britain which can assist the operator of cutting fluid separation plants olds are set by local water authorities which can also reflect the local situationregarding waste water treatment plants and the condition of rivers and streams.This leads to greatly differing evaluations between areas Apart from BOD5 andCOD and petroleum ether extractables, pH value, insoluble solids, a series of metalsand ions such as sulfate, cyanide and nitrite have thresholds As regards the latter,the importance is either their toxicity or, as in the case of sulfate, the damage itcauses to concrete pipes and drains

Thresh-In Germany, national legislation only applies indirectly to water separated-out ofemulsions Among other things, the German Law on Water regulates the disposal ofwastes into public sewers and rivers, the notification obligation if petroleum prod-ucts are stored or the location of protected water zones

German Waste Disposal Law deals with the treatment of sludges which remainafter emulsion separation processes This includes incineration, land-fill and specialland-fill sites for hazardous wastes

The German Law on Emissions is focussed on keeping the air clean As regardsdisposal of cutting fluids, this is limited to incineration of separated oils and usedemulsions Extreme Pressure (EP) agents with high levels of sulfur and chlorine can

of the cutting fluids [14.108–14.110, 14.155]

14.8.7.3 Electrolyte Separation

Salt Splitting

Splitting water-miscible cutting fluids with salts is still widely practiced for tional emulsions The addition of salts such as sodium chloride, magnesium chlo-

conven-498 14 Metalworking Fluids

ride or calcium chloride affects the efficiency of the emulsifiers This breaks downthe emulsion and the lighter oil phase floats to the top This floating oil can be col-lected by an overflow pipe or by pumping the water phase out from below Largeseparation plants use centrifuges to accelerate the process Overall, the capitalinvestment costs of salt separation are comparatively low Figure 14.43 illustratesthe simple principle of salt splitting

The splitting process can also be accelerated by heating the emulsion up to 90 C.The emulsion and the splitting salts should be stirred vigorously to assist the pro-cess

Price is a major factor in the selection of the salt Sodium chloride is the cheapestbut also the slowest Ferrous and aluminum salts have good separation propertiesbut require subsequent hydroxide precipitation with adsorption

Separated water normally contains more than 150 mg l–1petroleum ether table components As most thresholds range from 10 to 20 mg l–1, this means thatsubsequent treatment is necessary The considerable salt contamination in separat-

extrac-ed water which can be over 1500 mg l–1, is a further limitation to the use of saltsplitting because such salt concentrations are unacceptable for waster water treat-ment plants and, of course, rivers The best salt splitting results are obtained withconventional emulsions using high levels of anionic emulsifiers As the generaltrend is towards greater electrolytic stability of emulsions, electrolytic splitting pro-cesses are gradually disappearing due to their less satisfactory results with moremodern coolants

Acid Splitting

Similarly to salt splitting, emulsions can be split with the acids which many plantshave used for etching processes (sulfuric and hydrochloric acid) Separating emul-sions with acids is faster than with salts and, in the case of stable emulsions, alsomore effective

Particularly effective acid separation processes are often combined with physical

or physico-chemical splitting methods Successful processes are those which employsplitting temperatures of over 90 C and a downstream separation column filledwith inorganic solids to accelerate the splitting process Such acid splitting combina-

Stirrer

Salt solution feed

Split oil overflow

Treated waste water Heating

499 14.8 Metalworking Fluid Circulation Systemtions can generate petroleum ether extractable values of under 20 mg l–1 Neutrali-zation of the separated water is necessary The electrolyte content in the separatedwater, as in salt splitting, is also a problem.

14.8.7.4 Emulsion Separation by Flotation

In this procedure, fluid droplets are dispersed by air bubbles Emulsion oil dropletsand solid impurities are carried to the surface by the air However, as only hydropho-bic oil droplets float, hydrophilic oil droplets must be treated by breaking-down theemulsion The air bubbles necessary for flotation can be created in a number ofways [14.111]:

. in release flotation, the air bubbles are formed as water which is saturatedwith air is de-pressurized

. in stirring flotation, air is distributed throughout the fluid by rapid agitation.Electro-flotation has become an accepted form of emulsion separation The elec-trolytic breakdown of water releases hydrogen and oxygen If total-loss electrodes areused, the metal ions (iron, magnesium and aluminum) which end-up in solutionsplit the emulsion The precipitating hydroxide floc absorb the oil and then floatback to the surface through a layer of oily sludge This method is a combination ofsalt splitting, adsorption splitting and flotation A major hazard of electro-flotation

is the formation of mixed oxygen and hydrogen gases

14.8.7.5 Splitting of Emulsions with Adsorbents

Adsorption with Amorphous Silica

Adsorption with hygroscopic, fine-grain (ca 10lm) amorphous silica has been ing popularity The process involves the powdery adsorption medium being placed

gain-in the emulsion The oily and watery sludges formed can be de-watered with type filters or filter presses

belt-To keep the cost of the expensive adsorption medium under control, adsorption aration is often preceded by salt splitting because the adsorbent consumptiondepends on the amount of oil in the emulsion A rough guide to the consumption

sep-of adsorbent is 30 % weight sep-of the oil in the emulsion

It must be remembered that non-ionic emulsifiers which are often unaffected byelectrolyte splitting are easily adsorbed by amorphous silica On the other hand,anionic emulsifiers are poorly adsorbed [14.112] Hydrophilic amorphous silica,which is of major importance to adsorptive emulsion separation, is most effective

on anionic-active emulsifiers

Adsorption with Metal Hydroxides

In the case of salt splitting with ferrous or aluminum salts, subsequent alkalizationcan precipitate the metals as hydroxides The hydroxide flakes thus formed readilyadsorb oil droplets and emulsifiers Anionic emulsifiers are much easier to processthan non-ionic products

500 14 Metalworking Fluids

In general, such combinations of salt and adsorption separation processes usingaluminum and ferrous salts can provide good splitting results with low oil and saltlevels in the separated water

However, the disposal of oily hydroxide sludges is a big problem The problem issomewhat eased if filter presses are used to de-water the sludge

Figure 14.44 shows a schematic diagram of the emulsion splitting process withthe two stages: salt splitting and adsorptive hydroxide precipitation

14.8.7.6 Separating Water-miscible Cutting Fluids by Thermal Methods

Thermal separation processes use heat to evaporate the water in cutting fluid sions which is then re-condensed Such processes require the implementation ofsome sort of energy recovery Although disadvantages include expensive technicalequipment and high energy costs, the most important advantage is its suitability foralmost all types of water-miscible cutting fluids Contrary to the previously-men-tioned methods, thermal processes do not require a dispersed organic phase but canalso separate real organic solutions, popularly known as fully synthetic cuttingfluids

emul-Immersion Heaters

Immersion heater separation involves a gas- or oil-fired immersion heater beinglowered into the emulsion The water which evaporates and then re-condenses mustnormally be treated because organic compounds can be released during the evapora-tion process This method has never found great acceptance because the exhaust gasproblems can only be eliminated at great expense

Thin-film Evaporator

This method has a promising future The method involves the water in the sion being evaporated on indirectly-heated evaporation plates The quality of theevaporated water normally requires some subsequent treatment but generally lessthan in the immersion heater method Active charcoal filters have proved successfulfor treating the water

emul-Incineration

The incineration or thermal treatment of water-miscible cutting fluids works withsome highly-concentrated cutting fluids Burning cutting fluids in heating systemsrequires the cutting fluid to be clean and also some modification to the nozzles Inthe case of conventional emulsions, heat output is reduced if the emulsion concen-tration is less than 6 % The incineration of cutting fluids containing chlorine com-pounds can cause damage to the furnaces And finally, cutting fluids with large pro-portions of EP additives can cause exhaust gas emission problems

14.8.7.7 Ultrafiltration

This method of separating water-miscible cutting fluids has gained the most ance over the past few years Ultrafiltration involves the cutting fluid being passedthrough a semi-permeable membrane under pressure Water and low-molecular-

accept-501 14.8 Metalworking Fluid Circulation System

Stirrer

Coagulatingagent feed

Treated waste waterProcess tank

Acid water

Acid splittingprocess tank

Hydroxide

precipi-tation tank

Hydroxide tation waste water

precipi-Treated water

Oily hydroxide sludge

Dehydrated hydroxide

sludge outlet

Fig 14.44 Splitting of emulsions with adsorbents and

schematic diagram of the emulsion splitting process with the

two stages; salt splitting and adsorptive hydroxide precipitation.

As separation performance falls with increasing concentration in the retainedmaterial, 30–50 % has proved to be an acceptable figure The enriched, retainedmaterial could be treated by evaporators to produce material with less than 10 %water content.

Ultrafiltration plants consume little energy and they can mostly be run ously and automatically The capacity can be varied to meet needs by selecting thenumber of modules in the circuit There are ultrafiltration plants with capacitiesranging from a few hundred liters per day through to 1000 l h–1

continu-The retained filtrate has a low hydrocarbon content but often a relatively highCOD This could become an important factor if ultrafiltration is evaluated according

to the German waste water levy system In some circumstances, ultrafiltered fluidscan be reused similarly to the recycling of degreasing baths With the necessarytechnical complexity through to reverse osmosis which can generate drinking waterquality, this type of cutting fluid separation offers a wealth of further developmentopportunities

14.8.7.8 Evaluation of Disposal Methods

The selection of the disposal method must take into consideration the applicablethresholds, the cutting fluid volumes, the disposal of filter residues, investmentcosts, operating costs and personnel costs If a metalworking factory only generatessmall quantities of used cutting fluid emulsions (e.g less than 3 m3per week), in-house splitting is not recommended and it is cheaper to hire a disposal company.The most reliable method of meeting all waste water thresholds for all types of cut-

Module

Concentrated oil

Pressure feed to membrane module

Feed of cutting fluid free of solid contaminants

Floating oil outlet

Fig 14.45 The continuous separation of water-miscible cutting fluids by ultrafiltration.

503 14.9 Coolant Coststing fluids including fully synthetic solutions are the thermal processes However,these normally require high capital expenditures For lower investment costs, goodwaste water values, low personnel requirements and very low operating costs, ultra-filtration is a good alternative Salt and acid splitting methods require the leastinvestment but high operating costs, high personnel requirements and above all,residue disposal problems limit their practicability For special applications, electro-flotation and thermal acid splitting in columns are possible solutions.

Optimized combination of evaporation with ultrafiltration as methods of ment could eliminate production of waste water in workshops This complete water-recycling concept has been realized in some European manufacturing plants

treat-14.9

Coolant Costs

This issue has had a considerable influence on the development of coolants in thenineties This was not so much a matter of the costs of the volume of coolantsbought by the consumer but rather the costs for the use of the coolants which areconsiderable, at approx 12 % of production costs (serial production, central circula-tion systems) In this case the actual coolant costs themselves are only about 1 %.Pioneer work in system cost analysis has been done especially by Fuchs Petrolub

AG [14.113], Daimler Chrysler [14.114], K Weinert [14.115]

14.9.1

Coolant Application Costs

Coolant application costs include the costs for the operation of all equipment, theinvestment costs as well as the coolant costs, costs for their preparation and dispo-sal, energy costs, costs for coolant care as well as the costs for all auxiliaries

However, to assess coolant systems the influence on the entire productionsequence also has to be considered, for example, on tool costs, setting up times fortool changing, degree of machine utilization, workplace safety, workpiece quality

14.9.1.1 Investment Costs (Depreciation, Financing Costs, Maintenance Costs)

This is the most significant proportion of the costs and is some 60 % of the coolantapplication costs in serial production with central systems This includes the costs

of the plant tanks, pumps, pipelines, filter equipment, delivery of the coolant to themachine tool, de-oiling the chips, tramp oil separators and extraction systems It isalso possible to consider depreciation and maintenance for coolant splitting and dis-posal systems Since preparation and disposal are frequently carried out by thirdparties it is more expedient to total these costs in a cost analysis (for example, ascosts m–3)

504 14 Metalworking Fluids

14.9.1.2 Energy Costs

These include all the energy costs to operate the above-mentioned equipment cluding lighting

in-14.9.1.3 Coolant and Coolant Additives

Summarized under this are the costs for buying the coolant, chemical additives aswell as the mix water

14.9.1.4 Coolant Monitoring

These are the costs for the analysis and labor costs for coolant care and monitoring

14.9.1.5 Other Auxiliaries

These are filter materials for coolant filtration and extraction

14.9.1.6 Coolant Separation and Disposal

Summarized here are the costs for the splitting of water-miscible products and thedisposal of the oil and water phase

14.9.2

Coolant Application Costs with Constant System

Shown in the following is an analysis for the use of water-miscible coolants with astandard continuous system It is assumed that the coolant circulation system is pre-determined and cost minimization can only be achieved by selecting the right water-miscible coolants [14.113]

14.9.2.1 Specific Coolant Costs

To make the following optimization considerations more understandable it is firstnecessary to define some terms

Circulation Factor,f

This shows how many times the total volume of a coolant system – whether it is anindividually filled machine or a central system – is circulated by pumping in 1 h Forexample, the same number of machines can be supplied with a small volume in acentral system and higher circulation number as with a high volume and a smallcirculation number Here it is also obvious that the coolant in the system with ahigh circulation number is more stressed and the specific drag out losses – related tothe volume – are greater

The coolant requirement for machine tools is normally given in l min–1 If thecirculation factor, f, is only defined for the volume flow which runs through themachine tools and possible hydraulic chip transport by special coolant nozzles isignored, then the following is valid:

505 14.9 Coolant Costs

f = 0.06B/V (h–1

)whereV is the coolant system volume and B coolant machine tool requirement in

l min–1

Drag-out Coefficient,a

This is understood to be the number of times the volume of a plant is changed permonth A drag-out coefficienta = 1 means that the total volume is taken out of thesystem and has to be replaced every month; a drag-out coefficient a = 4 meanschange every week anda = 0.25 change of the volume after 4 months

Figure 14.46 shows the most important working area of coolant circulation tems concerning the drag-out coefficient and circulation factor

sys-The coolant losses take place on the one hand by chips, especially in the serialproduction sector with close interlinking of different working processes and throughdrag-out losses through geometrically complex parts such as, for example, enginecrank or transmission housings

High costs are incurred for the coolant itself and for the disposal of the coolantfrom systems with high drag-out coefficienta

In the case of a lower drag-out coefficient, these costs are low by comparison;however, the maintenance costs are higher through the rapid increase in the concen-

Fig 14.46 Most important working area for central circulation

systems in respect of drag-out coefficient and circulation factor.

506 14 Metalworking Fluids

tration of harmful substances Since, in systems with high drag-out, fresh coolanthas to be filled over and over again, the concentration of harmful substancesremains at a lower level In this case the most significant harmful substances aretramp oils from leakage, other dragged-in coolants, corrosion protection agentsfrom machined parts, dragged-in cleaners, heat treatment salts, non-filterable con-tamination from outside, water salt enrichment, decomposition products of coolantand microorganisms

The following consequences arise due to over-concentration of harmful substancesdue to drag-out: either high care costs are obtained when using coolants with low drag-out coefficients, or more stable and thus also more expensive products must be used.Figure 14.47 shows how the maintenance costs per kilo of water-miscible coolantused can depend on the drag-out coefficient; the curve shows the average valuesgiven in serial production over many years in studies

The easiest way is to consider costs for coolant systems when both the differenttypes of costs and the full costs are related to 1 m3volume of the system; 1 year isselected as the calculation period The various costs can then be expressed as costsunits (KE) per m3

and year The following breakdown has been split into three costgroups for the specific total costsK:

k1= Costs for coolant change

k2= Costs for drag-out losses

k3= Costs for coolant maintenance

This is then applicable:

507 14.9 Coolant Costsk1are the costs of changing the coolant Included here are firstly the costs for thenew mix, that is to say, coolant concentrate + water, and secondly the disposal costsfor the used coolant Also considered is the work necessary to empty the coolantsystem and refill it; in the main these are labor costs, possibly machine and energycosts If cleaning work has to be carried out which is only necessary in the case ofcoolant related changing, these costs are likewise to be included under k1; coolantchanging is often carried out in the usual factory standstill periods to avoid interrup-tion to the working process However, if the coolant change has to be carried outwhile the factory is in operation and machine downtime costs are incurred, thesehave to be also considered under the costs for changing the coolant.

k2are the losses for coolant drag-out In this case it can be assumed that the totalvolume of coolant carried off has to be disposed off through a separate drain, fromthe chip centrifuges or even through the washing fluid In this case the disposalcosts have to be considered

Thek3cost is the expenditure for coolant maintenance and monitoring The costsfor microbiocide and alkaline tankside additions must be considered here However,also the laboratory monitoring costs to determine fluid condition, and also themicrobiological tests are significant

These costs can be calculated easily by determining the following variables

Specific costs for the coolant changek1in KE m–3year–1:

k1=w(10cP + E + N + WA)

Costs for the coolant dischargek2in KE m–3year–1:

k2= 12a(10cP + E + WA)

Maintenance and monitoring costsk3in KE m–3year–1:

The maintenance and monitoring expenditure costs must be determined dually The total costs for one circulation system over one year must be determinedand the result must be divided by the circulating volume of coolant

indivi-The following equation is obtained for the specific total costs:

K = (12a + w)  (10cP + E + WA) + wN + k3

In these equationsKE is the cost unit, P is the Coolant price in KE m–3

,WA isthe price of water in KE m–3,E are the disposal costs in KE m–3,N is the workinghours per change, i.e possible machine standstill costs in KE m–3,a is the drag-outcoefficient in 1 month,c is the coolant concentration in %, and w is the number ofchanges per year

By means of these equations the specific coolant costs can be determined

relative-ly easirelative-ly for the different system situations

The costs for changing the coolant and drag-out are easy to determine from tory records At least the labor costs can be estimated to a large extent Specific costs

fac-K, k1, k2 andk3 can be presented by graphics, with their dependency on the quency of the coolant change w according to the example shown in Fig 14.48.There is no mathematical dependency for the care costsk3; normally the mainte-nance costs increase with the extension of the coolant service life (reduction of the

fre-508 14 Metalworking Fluids

change numberw) The example in Fig 14.48 shows observations for a large centralsystem and also reflects many years of experience gained in calculating maintenancecosts with different service lives The addition of specific partial costs shows mini-mum costs (service life optimization) in the course of specific total costs,K

14.9.2.2 Optimization of Coolant use by Computer

The cost equation developed for specific total costsK offers a favorable computerizedroute to optimize costs for coolant systems If a number of different working condi-tions are programmed by drawing up a cost matrix, a document can be producedwhich simplifies both selection of coolant as well as measures for the coolant circu-lation system Integration of care costs, coolant price, coolant concentration andcoolant service life as variables is recommended; these are included in the cost equa-tion by use of the termsU, V, X and Z:

Fig 14.48 Dependency of the specific coolant costs on the

number of coolant changes every year (from observations of a

large central system).

509 14.9 Coolant Costs

L = coolant changing: labor cost, Euro change–1

;M = coolant changing: chemicals,Euro change–1; O = coolant changing: machine downtime costs, Euro change–1

;

Q = disposal costs: Euro m–3

; R = water costs: Euro m–3

; S = coolant price:Euro kg–1(these definitions were used to adapt the usual records made in practicefor the specific total costs in the cost equation [14.116])

The overall cost equation is highly suitable for computer evaluation The number

of different variations ofU, V, X, and Z can be selected as required from the values

10, 20 or 30 Calculating about 20 different values has proved adequate in practice.Because, in the first instance, the actual status of a coolant system is determinedwith a view to cost, the following questions can be answered:

. How are the specific total costs changed by the price of the water-misciblecoolant (for example, by being 30 % higher, 30 % lower or half the price; inthis case 1.3, 0.7 and 0.5 are programmed for Z in the cost matrix

. How are the specific total costs changed when the coolant concentration isaltered, the service life is changed or the coolant care costs are amended?

Results can also be obtained with the following problem when selecting coolant:

By changing over to a new coolant which is 50 % higher in price (Z = 1.5), only halfthe maintenance expenditure (U = 0.5) should be necessary in the opinion of thecoolant manufacturer or an own estimate; at the same time coolant service life isdoubled (X = 2.0) and a 30 % lower concentration (V = 0.7) should be possible with-out negative effects to corrosion protection and cutting performance The computercalculation of these variants in the cost matrix now shows whether the specific totalcosts for the studied system are lower or higher than the actual costs achieved

When studying these costs it is assumed that the total coolant volume draggedout must be disposed of at full costs If this is not the case this can also be consid-ered in the cost formula [14.116]

Cost analysis for a large number of coolant systems show that the total specificcoolant costs for individually filled machines and small central systems (up toapprox 3 m3) are 5 to 20 times higher than those for large central systems(> 50 m3) It can also be shown that coolants in the upper price bracket (three timesthe price of the bottom price bracket) with high stability and low requirements onmaintenance in small systems, especially individually filled machines and even sys-tems with particularly low drag-out, can result in lower overall costs than the lowpriced, less stable products On the other hand it is also revealed that the lowest totalcosts are given in medium size and large central systems with particularly highcarry-off of low priced products

510 14 Metalworking Fluids

14.10

New Trends in Coolant Technology

14.10.1

Oil Instead of Emulsion

Back in the early nineties, the discussion about replacing conventional emulsionswith neat oils was based on the concept of a total process cost analysis [14.117] Thebackground was the high cost of the machining fluids application (5–17 %) com-pared to overall process costs caused by the expense of maintaining and disposing

of water-miscible cutting fluids

These days, the oil instead of emulsion’ trend is seen as an answer to a number

of problems Not only are cost benefits realizable, but environmental, safety-at-workand technical performance are superior From a safety-at-work point of view, neatoils are more skin compatible than emulsions They do not contain biocides andfungicides Another major aspect is the almost unlimited life of oils compared tothe fluid change cycles of water-miscible fluids (6 weeks for individually-filledmachines and 2–3 years max for central systems) Oils are also better in terms ofenvironmental protection and protecting resources As regards pure machining per-formance, oils can satisfy more than 90 % of all machining operations

Replacing emulsions with oils offers better lubricity, improved surface finishesand significantly longer tool life A cost analysis performed in a gearbox plant gen-erated the factor 2 as an average of all machining operations [14.118]

Ten to twenty times longer tool life was recorded in CBN grinding and deep hole ling trials In addition, in-house corrosion protection for corrosion-prone materials such

dril-as cdril-ast iron and mild steel is normally not required The same applies to machinetools which are protected against corrosion even if the paint finish is damaged.The only disadvantages of machining with oils are in processes which generate alot of heat With a four-fold reduction in cooling, tool and machining problems canoccur Critical operations include the manufacture of carbide drills from solid stock

To perform these processes with oil, the viscosity has to be reduced to a mum This generates the first conflict between technical performance and safety-at-work The evaporation of conventional oils based on paraffinic solvent raffinatesrises almost exponentially to falling viscosity At the same time, the flashpoint falls.This problem can only be solved by the use of unconventional base oils which com-bine the advantages of a high flashpoint with low evaporation at very low viscosities

mini-In response to these requirements, the first cutting oils based on hydrocrackedoils with esters appeared at the end of the eighties with pure ester oils established

on the market in the early nineties

The evaporation characteristics of different organic base oils are shown in ter 4 The very low values for ester oils are particularly interesting Ester oils includeproducts with different chemical structures, originating, above all, from oleochem-ical products, i.e from animal and vegetable raw materials

Chap-Apart from low evaporation, ester oils also display tribological advantages due totheir significantly better friction behavior [14.119]

511 14.10 New Trends in Coolant TechnologyEven without any additives, they also offer improved friction and wear character-istics due to their polarity This advantage as well as a high viscosity index, low waterpollution hazards and good biodegradability predestines this group of base oils foruse in machine tools as cutting and machine lubricants.

There are some special oils in use which, as alternatives to pure ester oils, aremixtures of hydrocracked oils and ester oils and thus combine excellent tribologicalproperties with the somewhat lower price of hydrocracked products

14.10.1.1 Fluid Families and Multifunctional Fluids for Machine Tools

The decisive step towards cost optimization by harmonizing machine lubricants wasfirst made possible by the trend towards metalworking with oils

A much underestimated aspect of total cost analyzes is the influence of trampoils on cutting fluids Investigations in Europe and the USA have shown that 3 to 10times the entire hydraulic fluid volume gets mixed with total cutting fluid volumeevery year [14.120]

Figure 14.49 illustrates this effect over a period of 10 years at a European bile manufacturer In the case of water-miscible cutting fluids, the significant quan-tities of dragged-in oils caused some pronounced changes to the emulsion whichled to machining problems, corrective action and considerable costs In the case ofneat oils, contamination with tramp oils cannot be seen and can only be assumedwhen problems such as poor dimensional accuracy and increased tool wear arenoticed

automo-The trend towards machining with neat oils presented a range of cost-reductionpossibilities One analysis by a leading German machine tool manufacturer showedthat an average of seven different lubricants are used in every machine tool Apartform the leakage problem and the incompatibility of some lubricants, costs are alsogenerated by the stocking of all necessary lubricants Incorrect application of lubri-cants or grade confusion can cause considerable damage to machines or evenproduction stoppages

Year0

Fig 14.49 Hydraulic oil losses in machine tools (example taken

from mass production in the European automotive industry).

512 14 Metalworking Fluids

One possible solution is the introduction of multifunctional’ products whichsatisfy a wide range of machining operations at the same time as fulfilling thedemands of advanced machine tool lubrication A solution to the problems arisingfrom the various lubricants necessary could take the form of a universal fluid for allmachine lubrication points

A factor acting against the introduction of such a universal fluid solution is thefact that conventional machine tools require ISO VG 32 or 46 hydraulic fluidsbecause controls, valves and pumps are all designed around these viscosity grades

On the other hand, the trend in metalworking fluids is towards lower viscositieswhich offer lower drag-out losses on chips and grinding sludge Another reason forlow viscosity oils is the trend towards high speed cutting which requires good heatdissipation for good results This divergence between the viscosity of metalworkingoils and machine tool lubricants still allows process cost optimization by harmoniz-ing the additives in all machine tool lubricants A machine tool fluid family containsthe same or compatible additives in lubricants of different viscosities

The advantages are:

. the unavoidable leaks of hydraulic fluids and slideway oils have no negativeeffect on the cutting fluid

. manufacturing quality remains constant without complex analytical sures

mea-. tramp oils function as cutting fluids and thus do not create additional costs.. higher process reliability, good machining results and reduced tool wear allserve to lower manufacturing costs

. universal application

Users can benefit from enormous rationalization potential regarding lubricantgrades because a fluid family not only satisfies all lubrication requirements ofmachine tools but also those of various machining processes and materials A typi-cal example is engine manufacturing One oil can be used for the rough machining

of the block through to the honing of the cylinders This technology offers tional savings potential

sensa-14.10.1.2 Washing Lines

In process lines, water-based cleaning operations can be eliminated because able mixtures are avoided as a result of compatible lubricants Chips and solid con-taminants are removed from the cutting fluid by ultra-fine filtration Apart from thehigh investment costs of washing lines, detergent, energy, water and monitoringcosts are also eliminated

undesir-14.10.1.3 De-oiling of Chips and Machined Components

The harmonized additives in all process oils allows the economical recycling of dual oil from chips, grinding sludge and components Up to 50 % of the oil can bereturned to the cutting fluid circuit

resi-513 14.10 New Trends in Coolant Technology14.10.1.4 Future Perspectives – Unifluid

In the near future there will be one low viscosity oil which can be used as hydraulicoil and for different cutting and grinding operations This universal fluid’ (Uni-fluid) has been developed and tested in a German research project sponsored by theMinistry for Agriculture [14.121] The “Unifluid” with 10 mm2s–1 at 40 C showsexcellent results in a german automotive engine plant for machining and lubricating

a complete transfer line (including the hydraulic system)

14.10.2

Minimum Quantity Lubrication

The ever-changing legislation and an ever increasing awareness of the environmenthave led to a change in the production processes used up to now and especially inthe production auxiliaries such as coolants The increasing pressure on costs in con-sideration of international competitiveness for the German industry likewise callsfor reduced prices within the given possibilities

The analysis of an automobile manufacturer [14.122] published early in the ties revealed that a considerable proportion of the process costs are caused by theapplication of metal working fluids In this respect the costs for coolant concentratesthemselves played an insignificant role The actual costs are caused by system costs,personnel costs for the care and monitoring of materials, the high personnel andinvestment costs associated with splitting plants associated with water purificationsystems and last but not least, the costs for disposal (Fig 14.50)

nine-This has led to much more attention being paid to the difference between drymachining and minimum quantity lubrication [14.123] The drastic reduction in theamount of coolants used as a result of these new technologies only offers a consider-able potential for savings in process costs However, dispensing with coolants alsomeans that the primary functions of the coolants such as cooling, lubrication andchip transportation have to be replaced by other suitable technical solutions

16,9%

5,5%

Fluid costsonlyManufacturing costs

94,5%

Maintenancedisposalplant investmentetc

Cutting fluid related costs

Fig 14.50 Coolant process costs [14.22]

514 14 Metalworking Fluids

14.10.2.1 Considerations When Dispensing with Coolants

If there is no coolant available in the process then the heat generated when ing by the workpiece, tool and chips cannot be dissipated Inadequate heat dissipa-tion can have considerable effects on the peripheral zones such as structural trans-formation, re-hardening and cracking and from deviations in measurement, whichcan make the component unusable

machin-Apart from the component properties, the influence on the tools should be cially noted [14.124] A decisive role in this is firstly the heat dissipation and sec-ondly the lubrication of the chips when the lubricant flows over the cutting edge Inthe case of carbide metals the cooling effect has a negative influence on the servicelife because cooling can lead to cracks in the material As a general rule, however,cooling the tool and especially the favorable friction conditions when the chips arecarried off lead to the service life of the tool being increased However, it has beenshown that, particularly in the case of processes with a geometrically non-definedcutting edge, both cooling as well as lubrication are necessary to increase the servicelife of a tool In the case of grinding and honing the cooling and rinsing effect of thelubricant is particularly important to the process The coolant system plays animportant part in the functionality of a machine tool The chip transport plays aimportant role apart from controlling the temperature of the machine Since thechip take up 80 % of the cutting heat the coolant has a double function in that it firstdissipates this heat and secondly, the process heat and the chips can be removedfrom the machine tool to prevent the system becoming overheated, which can causeconsiderable dimensional fluctuations

espe-Figure 14.51 shows the coolant requirement dependent on the machining tion Although dry machining is already possible today with interrupted cutting

opera-Grinding

Honing

Lapping

ReamingDrillingTurningMilling

Geometricallyundefined cutting

Geometricallydefined cutting

Coolant

Fig 14.51 Dependence of coolant requirement on the machining operation.

515 14.10 New Trends in Coolant Technologysuch as milling, operations such as turning or drilling are only possible to a limitedextent without coolant Dry machining with geometrically non-defined cuttingedges cannot be carried out because the cooling and rinsing effect of the coolant has

a decisive influence on the quality of the workpiece and the tool service life

The dry milling of cast iron and steel is quite achievable today with the help oftool geometries However, the chips have to be cleared from the machine tool byother methods such as extraction or compressed air As a result of this a number ofnew problems have arisen such as, for example, the noise nuisance, additional costsfor compressed air as well as finest filtration of dust Cr–Ni and Co dusts need to beassessed critically from the toxicological point of view, for which there are alreadythreshold values on the workplace Not to be ignored is the risk of dust explosionparticularly when machining aluminum and magnesium

14.10.2.2 Minimum Quantity Lubrication Systems

By definition one talks of minimum quantity lubrication when the coolant volumeflow does not exceed 50 ml h–1 A minimum quantity lubrication system design isshown in the following Fig 14.52 which reflects the general principle

By means of a proportioning device a small amount of coolant spray (max

50 ml h–1 is applied to the machining point Of all the dosing equipment on themarket only two systems have proved successful [14.125] The pressure systemshave found widespread use, whereby differences can be made in this case betweentwo strategies for preparing the mix Firstly, there are the spraying systems wherethe medium (oil) and compressed air are mixed in the tank and the aerosol/air mix

is transported by means of a supply hose directly to the machining point Secondly,there are units with separate medium and compressed air supplies, where the medi-

um and compressed air are delivered separately to the nozzle by the positive sure in the storage tank The volume per piston stroke and the piston frequency can

pres-be freely adjusted over a wide range The amount of compressed air is dosed rately from this Of advantage in the case of dosing pump systems is the possibilityfor computer control during a complex working process, apart from the many possi-

Flow rate control/

of the machining point through the tool In this way it is possible to achieve verydeep bore holes (l/d > 3) and positioning errors by the operator are avoided Apartfrom this chip removal from blind drill holes is greatly assisted.

The minimum tool diameter is limited to approx 4 mm because of the coolantchannels Since the medium has to be supplied through the machine spindle sys-tem conversion is generally associated with high costs

All minimum quantity coolant systems have one thing in common The coolant

is dispersed into more or less fine drops which reach the machining point as anaerosol However this causes the first problems with regards to toxicological andworkplace hygiene Although for some years now, in the case of conventional cool-ant flooding at the workplace, efforts have been made to minimize spray develop-ment by new technologies and low emission coolants, sprays are now being inten-tionally developed for minimum quantity lubrication which are released into theworkshop atmosphere

The large number of minimum quantity coolant systems on the market [14.126]leads to wide choice of droplet size combination with greatly varying coolants whichstill have not been studied sufficiently concerning concentration and particle size.14.10.2.3 Coolants for Minimum Quantity Lubrication

Today greases and oils as well as esters and fatty alcohols are in use, apart from ventional mineral oils and water-miscible coolants Since minimum quantity lubri-cation is a matter of pure total loss lubrication and the coolant is frequently com-pletely dispersed in the working area in the form of vapors and mist, particularattention should be paid to safety aspects Ester oils and fatty alcohols with additiveswith low toxicological values have proved particularly successful in this respect.Natural oils and fats have the disadvantage of poor oxidative stability Ester oilsand fatty alcohols should be given preference in the case of processes where highheat is developed to avoid residues on workpieces and machines Table 14.22 showsthe differences between esters and fatty alcohols [14.127]

con-For low-emission metalworking with minimum quantity lubrication, correctselection of the lubricant is of decisive importance To minimize emissions, thelubricants used should be toxicologically and dermatologically safe, with favorablelubrication properties and high thermal stress capacity

517 14.10 New Trends in Coolant TechnologySynthetic ester oils and fatty alcohols, in particular, which have low evaporationcharacteristics, are toxicologically harmless, and have a high flashpoint, have proventhemselves in practice [14.157].

Flashpoint (DIN EN ISO 2592) and Noack evaporation loss (DIN 51581 T 01), inparticular, have been shown to be suitable reference values for selection of a low-emitting lubricant The lubricant should have a flashpoint of at least 150 C, an eva-poration loss at 250 C of 65 % maximum, and a viscosity at 40 C of > 10 mm2s–1[14.156, 14.158]

. Viscosity at 40 C (mm2s–1) > 10 DIN 51562

Tab 14.22 Differences between ester/fatty alcohols [14.123].

Equiviscous fatty alcohols have a lower flash point than ester oils They evaporaterelatively easily and as a result have a low cooling effect Compared with ester oilstheir lubricating effect is relatively low

This is why fatty alcohols are used with preference for cutting operations wherethe lubricating effect takes second place to the cooling effect Examples of this arethe machining of gray cast iron where the graphite in the material provides thelubricating effect, or also when sawing cast iron, steel and aluminum Fatty alcoholsoffer the advantage that the machined parts are dry as a result of the rapid evapora-tion However this evaporation must be considered critically because of the thresh-old values for oil mist and oil vapor in the workplace (10 mg m–3)

Ester oils are used with preference for all machining operations in which thelubrication effect between tool and workpiece and the passing chips is paramount.Examples of this are thread cutting, drilling and turning

Ester oils have the advantage that they have a high boiling range and flash pointdespite lower viscosity As a result of this considerably less vapor will be emitted intothe workroom At the same time the thin film remaining on the workpiece has acorrosion protecting effect Beside these properties ester oils are rapidly biodegrad-able and, due to their biodegradable properties, are classified as non-water hazar-dous or as water hazard class 1 [14.128] Table 14.23 shows a few examples of appli-cations where ester and fatty alcohols are specifically used

518 14 Metalworking Fluids

Tab 14.23 Examples of applications for the minimum quantity lubrication technique.

MQL-Lubricants

(base oil)

No residues up to 210 C Fatty alcohol CK 45 Drilling, reaming, milling Housing components Ester 42 CrMo4 Thread rolling High surface quality Fatty alcohol St37 Pipe bending Exhaust systems Ester 17MnCr5 Drilling, rolling, shaping Splining of drive shafts

The main points to be considered when developing a coolant for minimum tity processing are shown below Apart from the performance aspect it is particularlyimportant to make available low emission, dermatologically and toxicologicallyacceptable media with a high flash point New research results are being presentedwhich have been achieved in the fields of emission and of optimization of a coolantmedium for minimum quantity lubrication areas

quan-14.10.2.4 Oil Mist Tests with Minimum Quantity Lubrication

When metalworking with minimum quantity lubrication systems, aerosols are erated which need to be delivered to the machining point, and high concentrations

gen-of the aerosol gets into the working atmosphere, particularly when using externalspraying systems Under less favorable conditions a major proportion of the sprayedaerosols are given off as an oil mist which, with a particle size between 1lm and

5lm, are defined as in a range accessible to the lung and should be viewed verycritically

The factors which influence the development of oil mist have been studied[14.129] (Fig 14.53) It was of particular interest to assess the influence of viscosity

of the fluid on development of oil mist The aim was to discover whether and howgreatly the oil mist (measured variable: oil mist index) can be reduced by increasingthe viscosity Also determined was whether the development of oil mist in the lungaccessible range could be reduced by adding anti-mist additives In a further series

of test the delivery pressure of the spraying equipment used was varied to seewhether significant differences were apparent in respect of the tendency to emit A

Oil mist index

519 14.10 New Trends in Coolant Technology

testing facility was built for this purpose with which it was possible to measure thegiven oil mist when spraying, by means of a Tyndallometer (particle size up to

5lm) (Fig 14.54)

The Tyndallometer used to measure the oil mist was positioned at a certain tance from the spray nozzle The data determined by the measuring instrument wasdown-loaded to a computer which enabled the oil mist content and the characteristiccurve of the spraying unit to be shown

dis-From the results it can be seen that the oil mist increases enormously as thespraying pressure increases Especially in the case of low viscous esters, increasingthe pressure has a very strong influence on the development of oil mist Doublingthe spraying pressure also led to the aerosol volume being doubled However, wherethe spraying pressure was very low the starting behavior of the equipment was poor,i.e the period before which the equipment is able to provide a constant volume oflubricant is extended At the same time the oil mist index strongly increased withthe reduction of viscosity of the medium On the other hand, the starting behavior

of the spraying equipment where low viscous media was concerned was ably better than that with high viscosity media

consider-As a general rule it is possible, by adding special anti mist additives, to reduce theoil mist behavior in all the media of varying viscosity used (Fig 14.55) Primarilywith low viscosity media use of anti-mist additive enables the development of oilmist to be reduced by more than 80 % The measuring methods presented alsomake detailed statements possible on the spraying properties of a system in respect

of start-up , spraying consistency and oil mist behavior

It can be seen from the findings presented here that the emission can be erably reduced by selecting the right spraying pressure and media viscosity Addingspecial anti-mist additives also reduced the emission in all cases

consid-Serial port Calibration screw Nozzle holder

Table

To the sprayapparatus

Manometer

NozzleExtraction unit

Tyndallometer

Control switches

Fig 14.54 Structure of oil-mist measuring facility.

520 14 Metalworking Fluids

14.10.2.5 Product Optimization of a Minimum Quantity Coolant Medium for DrillingThe machining of materials with minimum quantity lubrication (deep hole drilling(l/d > 3) with external spraying) was examined in a further study [14.130] The dril-ling tests were carried out on a DMG milling machine (Table 14.24) The workpiecewas a high alloyed steel (X90MoCrV18); this had a tensile strength of 1000 N mm–2for drilling blind holes A full carbide metal drill with SE shaft with high edge andbending fracture resistance with a PVD-TIN coating By selecting a high alloyedsteel (X90MoCrV18) for machining as well as external spraying the lubricant usedwas tested and optimized under very demanding conditions The aim of the test was

to determine on tone hand, how the viscosity of the used ester and, on the otherhand, the addition of specific combinations of additive, effected service life whendrilling

Tab 14.24 Drilling of steel with minimum quantity lubrication.

Tool Tungsten carbide with a TIN-layer

Type: SE Drill

D = 8.5 mm

L > 26 mm Material X90MoCrV18, high alloyed steel

Chuck Hydraulic-expansion chuck

The test bed allows the cutting forces to be measured in z direction with a Kistlermeasuring platform The performance of the working spindle of the machine dur-ing the entire drilling operation was measured continuously at the same time Thetwo measuring methods allowed a statement to be made on both the forces used in

a single drilling, as well as the continuous forces over the entire drilling test ring faults such as, for example, chips sticking as well as the tilting of drills could be

Ester 8 cst (with anti misting additives

Fig 14.55 Aerosol behavior: influence of anti misting additives.

Ester (viscosity 40 C = 8 mm 2 s –1 , pressure: 2.5 bar).

521 14.10 New Trends in Coolant Technology

easily registered and as a result the series of measurements could be assessed betterwith regards to quality

Figure 14.56 shows the performance of two esters, each with the same additives

As a general rule the drilling capacity of both media can be increased by selectingsuitable additives However, it could be clearly seen that special combinations ofadditive to both base oils in precisely the same dose had extremely different effects.Finally it only remains to establish that the occurring oil mist emission can bedecisively reduced by selecting suitable base oils, correct equipment setting and byincorporating anti mist additives This leads to improvement in working safety andthe improvement of toxicological, dermatological and safety implications in theworkplace Through the correct combination of additives the efficiency of a coolantfor minimum quantity lubrication can also be considerably improved which, lastbut not least, leads to longer service lives and better workpiece finish

Additive type 0

According to DIN 8582 one differentiates between six main groups of ing processes: primary shaping, forming, separating, assembling; changing proper-ties of materials and coating The processes listed under forming (DIN 8583 form-ing under compressive conditions, DIN 8584 forming under combination of tensileand compressive conditions, DIN 8585 forming under tensile conditions DIN 8586forming under bending conditions, DIN 8587 forming under shearing conditions),all refer to non-cutting production Over and above this, forming technology alsoincludes some sub-divisions under severing operations (DIN 8588 severing opera-tions, shear cutting) and jointing (DIN 8589 jointing by forming)

manufactur-Apart from the differentiation according to DIN 8582, in practical applicationsone normally divides the area of forming technology into sheet-metal forming andextrusion forming This conventional distinction is based on the fact that unlikeextrusion, sheet metal forming workpieces are produced with nearly uniform metalthickness over the complete product Additionally, extrusion forming uses muchhigher forces than are required when forming sheet metal

15.1

Sheet Metal Working Lubricants

Theo Mang, Franz Kubicki, Achim Losch and Wolfgang Buss

The tribological system consists of three components: The workpiece, the tool andthe lubricant The workpiece and the tool are characterized by their metallurgicalfeatures Their lattice structure is determined by their composition, heat treatment

as well as by subsequent cold processes such as temper rolling or deep drawing.These determine the physical stability, the flow characteristics, the anisotropy andfinally, the formability of the material As friction and wear take place at the surface,the material’s micro structure near the surface and any coatings have a fundamentaleffect on any operations on the material Finally, the lubricant is described by itschemical additives and physical properties such as viscosity

15

Forming Lubricants

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

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

523 15.1 Sheet Metal Working Lubricants15.1.1

Processes

Sheet metal forming processes have not only gained considerable importance in thethin sheet metal processing sector but significant progress has also been made inmaterials which in earlier times could not be processed by forming Especially note-worthy in this respect is the forming of stainless, austenitic and ferritic, sheet metalwhich initiated great activity in both the development of lubricants and materials forthe tools While the main objective in sheet metalworking is to produce the maxi-mum possible forming in as few working steps as possible without the production

of scrap parts, whether as a result of cracks or surface imperfections, the main focus

of tribological development in high tensile steels, titanium and stainless materialslies in reducing tool wear

During large scale production, for example in the production of parts for vehiclebodies, the greatest expense in development over recent years as far as lubricantsare concerned was not for the actual lubrication but for fulfilling new secondaryrequirements of the lubricant Advancements in production technologies made itnecessary for lubricants to fulfill specific requirements for removability, new appli-cation methods, corrosion protection, behavior during sheet metal adhesive bond-ing, welding, temperature resistance, compatibility with special coatings along withevolving workplace health and safety concerns

There are so many processes in sheet metal working that these cannot all be ered here Besides deep- and stress-drawing stamping and fineblanking will be cov-ered in this chapter Process combinations are frequently used and secondarydemands placed on lubricants in forming operations including stamping and fine-blanking are often similar

cov-15.1.2

Basic Terms in Forming Processes

To properly understand the friction and wear in forming, it is essential to becomefamiliar with some of the basic terms used It is especially necessary to accuratelyestimate the degree of difficulty of the forming process in order to be thus able toselect the correct lubricant

15.1.2.1 Lattice Structure of Metals

If the shear stress applied to a metallic body achieves a specific level then the ecule layers are displaced in the glide planes determined by the lattice structure ofthe material Forming of metals is based on being possible to make this displace-ment without losing material cohesion, i.e., without the occurrence of material frac-ture The higher the pressure normal to the lattice plane the more the material isable to displace without fracture in the glide plane

mol-The shear stress which causes this displacement with a permanent change inform is called yield point

524 15 Forming Lubricants

15.1.2.2 Yield Strength

If plasticity conditions are achieved in a material after applying yield shear stressthen the now present material strength is designated yield strength k1(flow stress).The yield strength can be determined in the laboratory, through compression andtension tests (cone compression test) as well as, with certain restrictions, tensiletests Knowledge of the yield strength, and particularly its dependence on the level

of the forming, temperature and the forming speed are of great importance forbeing able to estimate the degree of difficulty of a forming operation As a result ofhigh yield strength values, high forming forces and high surface pressures with dif-ficult friction conditions occur during forming These same conditions may alsofavor the prevention of cracks in the workpiece

The yield strength increases in carbon steels in that the carbon content increasesand in steels in general as much as the alloy concentration increase The latter isalso true for copper and aluminum alloys

15.1.2.3 Strain

To define strain, reference is made to the law of constant volume Here the tionships can be illustrated by using the exemple of a compressed rectangular bodywith initial dimensions h0, b0, and l0 Through compression the height of the bodyh0changes to h1 The following is valid:

interrela-h0 b0 l0= h1 b1 l1

h1/h0 b1/b0 l1/l0= 1

or

ln h1/h0+ ln b1/b0+ ln l1/l0= 0

The logarithmic addends in this last equation are termed logarithmic strain or u1,

u2, and u3 Therefore the following is applicable:

u1+ u2+ u3= 0

or

u1= –(u2+ u3)

15.1.2.4 Flow Curve

In a forming operation, the material hardness increases in that the strain increases

If a material is formed so that different degrees of strain are achieved and then thevalues for the flow stress, which are to be determined for every degree of strain, arespread over the whole strain u, then one gets the flow curve of a material (flowstress versus strain) In tensile testing, the flow stress is similar to the strainstrength values determined in other tests Figure 15.1 shows the flow curves ofsome materials up to strain u = 1.0 This also shows that less than half of the form-ing work has to be performed to form the copper material listed here than is thecase with steel C35

525 15.1 Sheet Metal Working Lubricants

For exact understanding and evaluation of flow curves for forming it is essential

to know the annealing conditions, grain structure, strength properties and the testconditions

The most important mathematical interpretation of the flow curve is the tial equation, which, in its simplest form, is approximately correct for a variety ofapplications:

exponen-k1= a  un

where a is a material-specific factor and n a hardening exponent Using logarithmsthe equation changes to:

log k1= log a + n log u

If one assumes that the forming area material falls in line with this equationthen, using a double logarithmic calculation, a is the ordinate value when u = 1 and

n the increase in the straight line

The hardness exponent n has considerable significance in the description of ing operation

form-High n values lead to high surface pressures during the forming operation.The achievable degree of forming normally declines with the n value during deepdrawing (b0max, Section 15.1.3) and increases during stretch forming (Erichsentest)

Fig 15.1 Flow curves for some metals up to strain u = 1.0.

526 15 Forming Lubricants

15.1.2.5 Efficiency of Deformation, Resistance to Forming, Surface Pressure

The forming force to be applied for a forming operation is given from the product ofstressed surface A and the yield strength k1; in this case one also talks about theideal forming force:

where gFis the efficiency of deformation

This relationship is particularly important for friction and lubrication because inmany forming processes the plastic deformation surface pressure between the mate-rial and the tool roughly equals the resistance to forming and can therefore be muchlarger than the yield strength This means, for example, that for thinner sheets and

a corresponding reduction of the forming efficiency during a drawing operation,difficult friction conditions can arise (see Fig 15.6)

15.1.2.6 Strain Rate

Strain rate is defined as du/dt or u, is measured in s–1, and can be calculated from theforming velocity Its influence on the yield strength in sheet metal working and manyother processes is relatively low In fast turning roller frames and during wet wire draw-ing, with yield speeds of 103to 104s–1, the yield strength can be 50 % higher than thevalue calculated from the flow curve In warm forming processes the influence of thestrain rate on the yield strength can be much larger than in cold forming processes.More important than the influence on the yield strength, though, is the materialheating which takes place because of the forming and heat from friction generated

at high forming speeds This can have a considerable influence on the reaction ofpolar lubricant additives and EP additives under boundary lubrication conditions

On top of this, even small temperature changes at low temperatures, e.g well below

100  C, bringing about the melting of lubricant compounds (e.g waxes) can have aconsiderable effect on the lubrication process

15.1.2.7 Anisotropy, Texture, Rvalue

Above all in the area of sheet metal forming, especially in deep drawing, differentmaterial properties working in different directions (anisotropy) can have a signifi-cant effect on the degree of difficulty of the forming operation Textures and orienta-tion of the crystal structure, e.g to the rolling direction of the sheet metal, lead tothis anisotropic directional behavior In deep drawing the result can be wrinkle for-mation and earing The directional variations in thickness are of particular impor-tance for the surface pressures and the lubrication