G H bb'._ear _ 0- '-lngWithout Coolant

10,r.Lothar 'Op'heyLiebhen Verzahnt'echnik 'GmbH

K'empten. Germany

IntroductionFor environmental and economic reasons,

the u e of coolant in machining processes isincreasingly being questioned. Rising coolantprices and di posol costs. as wen as strains onworkers and the environment. have fueled thedebete. The use of coolant has given rise to ahighly technical system for handling coolantin the machine (cooling, filtering) and protect-ing the environment (filler, oil-mist collector),In this area the latest cutting materials-usedwith or without coolant=heve great potentialfor making the metal-removal process moreeconomical. The natural progression to com-pletely dry machining has decisive advantagesfmhobbing.

\1)\ \'\'1' \(;ES DIS \1)\.\ '\'1' \(;ES

nNISHING I Reduced machinetime/i ncreasedtool life/improvedquality

MACH1NE Chip llu hing Filtration unitRefrigeratorFire Extinguisher

IIPempsand hoses

I

CUTTlNG OIL Purchase costDisposal costMonitoring co t

ENVIRONMENT Heakh riskEnvironmental hazardContaminated chips

,,

PROCESS I Additional part,washing

Fig. I - Con equenee ,orusing CUlling oil.

20 GEAR TECHNOLOGY

The CoolantCoolams serve many purposes in metal

removal. With lheir help, the workpiece and1001are cooled. wear is reduced and chips areflushed out. of the machine. But there are essen-ti I.di advantage : the sub l:antiaUy increasedcost of the machine because of the need (0process and circulate the coolant, the 'co t ofprotecting the environment from oil :mi t, thecosts of purcha ing and di sposing of thecoolant, the col of disposing of the contami-nated chips and the strain on workers and theenvironment caused by coolant in all its forms.

In Europe, the purchase price of coolanttoday is about $2,060 per cubic meter of oiland $185 per cubic meter of emulsion. To1'.110 e price add disposal costs in the range of$76 per cubic meier of used! oil and $185 percubic meter of ,emulsion, as wen as disposalcosts of about 6/lon for chips. The e figuresare valid at the mom.ent, bul. we have to assumethat they will skyrocket in the near future. Athorough examination of the snoject can befound in [he 1993 addles es to the AachenMachine Tool Kolloquium (Ref. 2).

Since bobbing usually precedes heat treat-ing, the use of coolant during metal removalabo necessitates an additional wash operation.Fig. I dearly shows thatthe disadvantages ofuaing coolant outweigh the advantages.Nonetheless, economic and technical consider-ation will not allow coolant. to be abandonedin the immediate future.

The T.o.o1Elimination of coolant ha far-reaching

effects on the machine, tool andproce .. The1001 mu t withstand the thermal and mechani-cal stresse of me process and still be econorni-

The MachineThe machine design must be adapted to the

new requirements of the coolant-free process ..First the thermal load of the entire machine Fig. 1- Development of Cutting Speed for Steets (See Ref. U.

changes. According to Koenig, Severt andBerthold ("Cutting Without Coolant," Ref. 4),75% of process heat is removed by the chips.Since there i no coolant to wa h them away,the chips must be removed from the process inLite most direct way possible so that the heatthey contain is transferred out of the machinequickly. Furthermore, the use of modern cut-ting materials requires a. different performanceprofile worn the machine. Increased speeds andpower axe both necessary.

Fig. 3 shows the design ofa modem hob-bing machine that meets these requirements.Integrating the chip conveyor in the machinebed satisfies the requirement that the chips becentrifuged away from the process directly tothe chip conveyor below. In addition .. puttingthe chip conveyor at the center of the machineallows the side walls to be substantially steeperthan they could be if the chip conveyer werebuill onto the side. In this way the chips takethe shortest possible route out of the machine.

The table is driven by a compound, pre-loaded gear train that allows maximum speedsof up to 450 rpm. The hob head is equippedwith a heavy duty drive that offers maximumspeeds of 3000 rpm with a drive power of18kW. With a maximum hob diameter of 90

cal; that is, it must allow large batch sizes withconsistent workpiece surface quality. In thisregard, modem cutting materials, such as car-bide or cermet with suitable carbide coatings,open completely new per pectives,

Fig. 2 shows the progress that has beenachieved in steel cutting speeds over the pastfew years because of improvements in cuttingmaterials, Today, coated carbides and cuttingceramics allow speeds that are many timesg:reater than the cutting speeds possible withclassic tools or high-speed steels. For example.catting ceramics allow cutting speeds of about2,000 m/min. At the same time, the improvedcutting material's have increased tool life, sothat in the final analysi these new tools offer asubstantial economic advantage in spite oftheir high cost.

mm, these result in a cutting speed of 850m/min, These performance data=-in compari-

6000 II

,

II

I ..-.12000 I,

I II LV -,

,~uttin~~]000 I :1 / I Ceramic, I I ,.

I I I i'I

600i

~ 1/I Coated

400II J !~ I"""Carbide/

I /11/ ___-~-- Uncoated200 /]1 CarbideI ~-100 I

!J I~ 1/ I

60 I 1/ ! II I.I ri High-40 /f- I--"

I

I SpeedJ I JT:::./ , I20 I! II ,V I I»: ,,

II V -Io I I / II' Steel

II

1850 1870 1890 1910 1930 1950 1.970 1990YEAR

Fig. 3 - Concept. Hobbing Machine for Higb-Speed Machining.son with the technology data used in currentpractice-leave sufficient reserves for furtherincreases beyond the performance potential oftoday's enning marerials,

Since these enormously high cutting speedsreduce machining times to less than 10 seconds(depending on the workpiece), part change hasto be considered an integral aspect of the basicmachine design. For that reason, a conveyorsystem for part load/unload is integrated withthe machine. Part change is so closely com-bined with workpiece clamping that the entireprocess is complete in about 2 seconds.

Fig. 4 shows the prototype of the newly

Dr. tothar Oph1eyis Managing Directorof LiebherrV(!I'=ahlrl~chllik GmbH.

NOVEMBER/DECEIliBER leu 21

Power: 24 hpMax. Spindle Speed: 3000 rpmMax. Table Speed: 450 rpmFloor Space Including Automation: 4.8 m2

Work. Example. Speed Gear

Part Data:o.ofTeeth

Module (DP)Helix AngleFace WidthCutting Depth

Hob Data:MaterialNo. of StartNo. of GasheHob Diameter in mm (")Hob Length in mm (")

Teehnology:CoolantSpeed in rn/min, (sfm)

I HobrpmAxial feed in mmltr. (ipr)Cutting time lin sec.Load/unload in sec.Time per pad in sec.

Casts: (Guide Values)Hob Co I inNo. of ResharpeningCost/Resharpening in $Parts.Resharpen ingTeol Cor/Part hI 'Machine Co Is/Hour inMC-Co 1 per Part in $

3]2 mm 02.7)32"RH18 mm (,71")5.8 mm (.23")

HSS33090 (3.5)70 (2.75)

HSSOil120 (393)4243 (.12)25328

HSS2200335054000.1£;:750.58

Totall Costs per Part in $ 0.73*Assumed

CARBIDE]

3090 (3.5)70 (2.75)

CARnIDENone450 (1475)1.5904.5 (.18)14317

CARD.rDE350033567000.]7750.35

0.52

CERMET]

3090 (3.5)70 (2.75)

CERMETNone600 (1970)2J204.5 (.I8)10313

CERMET525033506700*0.24750.27

0.51

Fig. 5 - Habbln "withlll!d. coolant (Co tlPerformance Cl.lm,parison).

,22 GEAR TECHNOLOGY

developed heavy-duty bobbing machine andlists it . fundamental technical performancecharacteristics.

In addition to the associated technologicalrequirements, the current push toward "lean"manufacturing was taken inro account. Theinternal. function of l1le machine were simpli-fled as much as po sible, andthe electrical andmechanical designs were closely integrated. Inthis way, a production-ready machine wasdeveloped. Installation at the eu lamer's planti limited to setting it up and plugging it in.And the entire machine-s-including integratedauromation=has a footprint of les than 5 m2•

Machining E"'~mple ,For one of the test cuts. a gearthat is typi-

cally u ed a a drive gear in passenger cartransmissions was selected. The workpiecedata are shown in Fig. 5.

The table shows the time that can be savedby u ing dry cutting with carbide or cermettools rather than conventmnal wei enning withH5S hobs.

The use of Ti -coated HSS hobs withcoolant i considered the current state of the art.Al cutting peeds of 120 m/min. with a feed of 3mm/rev .• the machining lime is 25 second,TiN-coated carbide tools, used with a cuttingspeed of 350 m/rnin., make i.1 possible to reducethe machining lime to 17 seconds and eliminatethe u e of coolant. A funherreduction to 10 ec-ond machining time i achieved withthe u e ofcermet hobs. This process is also carried outwithout coolant, In. all, there i. an improvementof the overall machining time from 28 to 13 sec-onds: thai is,today's standard value i reducedby more than half. orin other words the capaci-Iy of the machine is more than doubled ..Wilhrelation to hob bing. thi increase in performancedirectly reduced piece co 'Is by 15-25% beforethe saving from the elimination of coolant. istaken into account. To be complete, our roughanalysis of productivity increases would alsohave to include a recalculation of the steps ingear manufacturing such as extrusion molding(See "Cold Impact Forming of GearedWorkpieces." Ref. 6). Simply switching hobha the additional advantage of increasing theflexibility of the bobbing process.

Along with purely time-related issues, thetargeted workpiece quality plays an essentialrole in assessment. of the process. Fig. 6 shows

the profile and lead measurements of a drivegear. The workpiece was cut with a TiN-coatedcermet hob using the machining data given inthe right-hand column of Fig. 5. The profiles ofthe left and right flanks are identical, showingthat. the process is stable. The same conclusioncan. be drawn from the leads, on which the feedmarks show up as ripples about 16 J..Lmin sizebecause of the high feed rate. Influences on theleads and face width resulting from theincreased workpiece temperatures associatedwith dry cutting can be corrected through suit-able compensation strategies. The tooth-spacemeasurement (Fig. 7) shows outstanding valuesfor tooth-spacing error (DIN quality 4-:5). Thecumulative tooth-spacing error is also good(DIN quality 6-7), though it was affected by theexcessive runout of the gear.

In summary, excellent machining resultswere achieved simultaneously with the enor-mous reduction in time from 28 to I 3 seconds.This dry-cutting process is compatible withsubsequent heat treatment and hard gear finish-ing (honing) or with an alternative finishingprocess, such as shaving and hardening ..

For another test, a steering pinion washobbed. Fig. 8 shows the steering pinion alongwith workpiece and technology data. The mainhurdle with this workpiece is the need forsmooth operation, which is hindered by largefeed marks. For economic reasons, of course,the feed rate should be as high as possible.Derived from practical experimentation, themetal-removal rates in Fig. 8 represent a com-promise between these opposing requirements.The steering pinion was machined with a TiN-coated, single-start carbide hob with. 11 gashesand a diameter of 45 mm. Once again, nocoolant was used.

Using the technological possibilities provid-ed by dry cutting with carbide hobs reducesmachining time by 50% compared to conven-tional machining with HSS bobs,

Fig. 9' documents the machining results inthe form of profile and lead measurement. Thesmall deviations on the profiles are due to thequality of the hob. The lead tracing shows rip-ples of about 2 11mcaused by feed marks. Thejump that can be seen on the lower part of theright flank was caused by a change in cuttingforce that resulted from a change in the work-piece diameter in this area.

Pretu- urcd FI!IonkenlinienmeS-lSun-g

Fig..6- Profile and lead measurements of drive gear.

Fig. 7 - Tooth spacing error measurement.

NQVEM6E,RIOEGEMBER 1994 ,23

Work Example: Steering Pinion

Part Data:No. of Teeth 8Module (DP) 1.95 mm (20)

Helix Angle 13.67° RHFace Width 26 mm (1.02")

Cutting Depth 3.2 mm (.13")

Hob Data:Material HSS CARBIDENo. of Starts 1No. of Gashes 11 11Hob Diameter in nun (") 45 (1.75) 45 (1.75)

Hob Length in mm (") 100 (4) 100 (4)

Technology: HS8 CARBIDECoolant Oil NoneSpeed in m/min. (sfm) 107 (360) 250 (820)

Hob rpm 760 1800

Axial feed in mm/tr, (ipr) 0.9 (0.35) 0.9 (0.35)

Cutting time in sec. 38 16Load/unload in sec. 6 6Time per part in sec. 44 22

Costs: (Guide Values) HSS CARBIDEHob Cost in $ 600 2200No. of Resharpenings 8 20Cost/Resharpening in $ 100 200Parts/Resharpening 2700 2400

Tool Costs per Part in $ 0.06 0.12Machine Costs/Hour in $ 75 75Me·Costs per Part in $ 0.82 0.46

Total Costs per Partin $ 0.88 0.58

The steering pinion has all the propertiesrequired for trouble- free actuation. Once again,using carbide hobs without coolant has directeconomic advantages through shortenedmachining times as well as indirect advantagesthrough the elimination of coolant.

SummaryFor ecological and economic reasons, the

use of coolants in production has been at thecenter of debate for some time. The advantagesand disadvantages are well known, and therehave already been numerous attempts to devel-op economical dry cutting methods. Improve-ments in tools and the incorporation of dry-cut-ting requirements in a suitable machine designhave established the prerequisites for eliminat-ing coolant from the hobbing process. Thiselimination leads to other process advantages,which in direct comparisons have already yield-ed considerable economic advantages in favorof dry cutting. The positive ecological balanceresults in still more pluses, which also show upas cost reductions .•

Fig. 8 - Hobbiog without coolant (Cost/Performance Comparison).

References:I. "Design of Spmdle-Bearirig Systems for High-SpeedMachining." Expert- Verlag. 1990.2. "Production Technology as a Factor in Competitiveness-Aachen Perspective." Addresses to the Aachen Machine Too]Kolloquiurn 1993, VDI·Yerlag.3. "Hobbing with Cermet: A New Technology with a Future,"Information from Liebherr- Verzahntechnik GmbH and WilhelmFette GmbH ..released for EMO '93.4. Koenig, W., W. Severt and A. Berthold. "Cutting WithoutCoolant." Automating Production. May 1993. p.3l.5. Koenig, W. "Low. or No-coolant Gear Manufacturing?" and"Transmission Experiments in Laboratory for Machine Tools andManagement Instruction." RWTH Aachen, May 28·29,1991.6. Kol, W. "Cold Impact Forming of Geared Workpieces."Dissertation, R WTH Aachen. 1990.

Aeknowledgement: Presented at tile SME Advanced GearProcessing and MallujaClrIrillg Clinic. April 1994. Reprintedwitl: permission.

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Fig. 9 - Profile and lead measurements ..

,24 G EA R TECHNO LOG Y