Important informationPageImportant information 4Services 5An introduction to FETTE 6
4Important informationArticle numbersTo speed up order supply and toavoid confusion, orders should al-ways specify the article numberslisted in this catalogue.PricesThis catalogue does not containprices. Prices can be found in thelatest price list for standard arti-cles. Please consult us for a quotewith regard to semi-standard orspecial items.Minimum order valueOrders with a total value of lessthan DM 200.00 are subject to aprocessing surcharge of DM50.00. We trust that you will appre-ciate the need for this measure.Tool groupsOur wide range of hobbing tools isdivided into tool groups, which aremarked in the index at the side ofthe page and are thus easily locat-ed.Product rangeThe entire FETTE catalogue prod-uct range with some 15,000 stan-dard items, 1,100 in the hobbingarea alone, is subject to continu-ous improvement. As part of thisprocess, we not only introducenew and therefore technologicallysuperior products into our range,but also take care to remove out-dated products from it.In some cases it could happen thatwe do not carry in stock the itemwhich you have ordered. In thiscase you will in general receiveproducts from us technologicallybetter product, but at least anequivalent alternative. In case ofdoubt, our sales team is availableto determine a design that will pro-duce best possible results for you.By following this procedure, youcan be sure that you are always besupplied with tools, which aretechnologically to the newest stan-dard. For that reason, we do notfeel not obliged to supply tools,which are still shown in the cata-logue, or which have been clearedfrom the programme already inter-nally.Catalogue number indexAll catalogue numbers, arranged innumerical order and with the pagenumber, are listed on page 193.DIN Standard indexAn index on all DIN Standard num-bers covered is listed on page194.Technical detailsTechnical application details of ageneral nature commence on page125, whereas the specific techni-cal details concerning individualproduct groups are directly as-signed to the section concerned.Special formsShould you be unable to find a so-lution to your machining tasksamong the 1,100 items which westock, special forms are availableupon request, including formsmanufactured specifically to yourdrawings.
6Application advice and serviceProduction on modern machine toolscombined with up-to-date CNC techniqueDesign and developmentFETTE – a brief introductionQuality assuranceTrainingHeat treatmentEcology and environment protection are part of the companyphilosophy, recognizable on the factory grounds
Hobsfor spur gears, straight- or helical tooth, with involute flanksCat.-No. PageHobs for the manufacture of straight spur gears,straight or helical tooth, with involute flanks 10Explanatory notes on the descriptions and size tablesfor hobs for straight spur gears 11Solid-type hobsrelief ground, DIN 58411 2002 13relief ground, in solid carbide 2008 14relief ground, DIN 8002 A 2022 15relief turned, DIN 8002 B 2031 16relief ground, DIN 8002 B 2032 16relief ground 2033 17relief ground, for spur gears to DP 2042 18for straight spur gears 2026 47Multiple-gash hobs 19Solid carbide hobs 26Roughing hobs 32relief turned, 20 gashes, with drive slot 2051 34relief ground, 20 gashes, with drive slot 2053 34relief turned, 16 gashes, with drive slot 2055 34relief ground, 16 gashes, with drive slot 2057 34relief turned, 20 gashes, with keyway 2061 35relief ground, 20 gashes, with keyway 2063 35relief turned, 16 gashes, with keyway 2065 35relief ground, 16 gashes, with keyway 2067 35Roughing hobs with indexable carbide inserts 36with 19 blade rows 2163 39Carbide skiving hobs 40Solid carbide 2028 44with 12 or 15 brazed-on blade rows 2129 45with indexable carbide inserts 2153 46
FfsfoFfsfuHobs for producing straight- and helical-tooth spurgears with involute flanksThe fundamental geometrical con-cepts of a spur gear hob for gener-ating gears with involute flanks arelaid down and explained in detailin DIN 8000. According to this, thebasic body of a hob is always aworm. If this worm is now providedwith flutes, cutting teeth result.These become capable of cuttingby being backed off or relieved.This relieving operation is carriedout on machine tools specially de-veloped for this process; it is verytime consuming and therefore alsoexpensive. For hobs to moderateaccuracy specifications, reliefturning is sufficient; for stricterquality requirements the hob is re-lief ground.Generally, relief turned hobsachieve quality class B approxi-mately to DIN 3968. Relief groundhobs achieve quality classes A, AAand higher. The highest qualityclass in DIN 3968 is AA. For ex-ceptionally high quality require-ments it is usual to restrict the tol-erances of quality class AA stillfurther. Quality class correspond-ing to AAA to DIN 3868, withoutcomment, means the restriction to75 % of the AA tolerances for allmeasurable variables.If special tolerance restrictions ofthe AA tolerance are required, thisis also done with the AAA refer-ence, but the individual measur-able variables and the tolerancerestriction are now given in % ordirectly in µm. E.g. quality classAAA to DIN 3968, item nos. 16 and17 restricted to 50 % of the toler-ance of AA.The purpose of hob tolerances isto assign the tools to a qualityclass according to their accuracy.On the basis of the hob qualityclasses, the expected gear qualitycan then be forecast.Not all requirements aimed at a”good gear quality“ in the widersense, e.g. very quiet running or aspecific addendum- and deden-dum relief are achieved solelythrough a high cutter quality. Forsuch needs, hobs with a definedcrowning depth have proved suc-cessful. Depending on the loadand the required gear perfor-mance, the suitable crowningdepth can be selected from thevarious tables N102S, N102S/3 orN102S/5. It must be noted that thetool depth crowning is not trans-mitted completely to the gear. Thelower the number of teeth of thegear, the less the effective convex-ity portion.10Tolerances for hobs with special class tolerance values in 1/1000 millimetres0,63–1 1–1,6 1,6–2,5 2,5–4 4–6,3 6,3–10 10–16 16–25 25–40FfSfo 25 28 32 36 40 50 63 80 100FfSfu 12 14 16 18 20 25 32 40 50N 102 SFfSo 4 4 4 5 6 8 10 12 16FfSu 0 0 0 0 0 0 0 0 0FfSao 16 16 16 20 24 32 40 50 64FfSau 8 8 8 10 12 16 20 25 32FfSfo 12 14 16 18 20 25 32 40 50FfSfu 8 8 8 10 12 16 20 25 32N 102 S/3FfSo 4 4 4 5 6 8 10 12 16FfSu 0 0 0 0 0 0 0 0 0FfSao 12 14 16 18 20 25 32 40 50FfSau 8 8 8 10 12 16 20 25 32FfSfo 8 8 8 10 12 16 20 25 32FfSfu 4 4 4 5 6 8 10 12 16N 102 S/5FfSo 0 0 0 0 0 0 0 0 0FfSu 0 0 0 0 0 0 0 0 0FfSao 8 8 8 10 12 16 20 25 32FfSau 0 0 0 0 0 0 0 0 0ModuleTolerance rangeinvolute withtop convexityFfSfoFfSfuFfSaoFfSauTolerance rangeFfSfoFfSfuTool root section Tool tooth tipForm deviation ofthe cutting edge
Notes to the descriptions and size tablesfor spur gear hobsOwing to the many different hobversions available, their presenta-tion in a product catalogue mustbe restricted to a range which isintended as a representative se-lection. Standardized referenceprofiles to DIN 3972 or DIN 58412and size series to DIN 8002 or DIN58411 were selected for inclusionin the catalogue.For cutter designs such as broach-tooth type roughing hobs or skiv-ing hobs, the size tables werebased upon works standardswhich maximize usefulness withinthe constraints of the design crite-ria.These standard tools can, how-ever, only cover part of therequired hob range, and possiblevariants are therefore brieflylisted below.DimensionsThe four main dimensions of thehobs are stated in the followingsequence: cutter diameter, cuttingedge length, total length and borediameter; e.g. for module 8, cat.no. 2032; dia. 125 x 130/138 x dia. 40.Diverse measurements may be-come necessary due to the work-piece shape, because of the limi-tation of the cutter dimensions dueto the measurements and perfor-mance of the hobbing machine,through the dimensions of theavailable cutter arbors or toachieve specific cutting par-ameters or machining times.Cutter materialsThe standard material is the high-speed EMo5Co5 (material no.1.3242).Gear materials whose tensilestrength values exceed 1200N/mm or which are intended forvery high cutting speeds andfeeds are manufactured from pow-der metallurgical high-speed steel.Carbides are increasingly beingemployed for high-performancehobbing and for skive hobbing.CoatingA hard coating with a thickness of2 to 3 µm increases the life of thehobs, or permits higher cuttingrates. Further information on thecoatings can be found on Pages151 and 152 in the technical sec-tion of the catalogue.Basic tooth profilesThe definition and description ofthe various reference tooth profilesare found in the technical part ofthe catalogue on pp. 137 to 148.Pressure angleThe pressure angle, as also themodule, is determined by the gearcutting data of the workpiece andmust be taken into account whendeciding on the basic hob profile.Tip edge chamferTo protect the tip edges againstdamage, they are chamfered. Thistip edge chamfer can be producedduring manufacture with a suitablydimensioned hob. To determinethe hob reference or basic profilecorrectly, the complete gearcutting data are needed. The sizeof the tip edge chamfer dependson the number of teeth, i.e. whenusing the same hob for differentnumbers of gear teeth, the cham-fer will decrease with a smallernumber of teeth. For a large toothnumber range, several differentcutters are needed.Information about these relation-ships and recommended chamfersizes can be made available on re-quest.Profile modificationThe purpose of the profile modifi-cation is to reduce or avoid theinterference when the teeth rollinto mesh while a gear pair is run-ning under load. To decide on thebasic profile of the hob, the com-plete tooth cutting data or theworkpiece drawing are necessary.The size of the profile modificationproduced depends, similarly aswith the tip edge chamfer, on thenumber of teeth.ProtuberanceThe protuberance creates a clear-ance cut in the root of the tooth, sothat during the next operation thegrinding wheel or the rotary shav-ing cutter does not machine thetooth root. This prevents stresspeaks through grinding- or shav-ing stages.The protuberance basic profilesare not standardized and are sup-plied on request to your require-ments. If you do not have relevantexperience, we can submit sug-gestions and if necessary prepareprofile plots for your gear cuttingdata.Multi-start hobsMulti-start hobs are used to in-crease hobbing output. This ap-plies particularly in the case ofgears with small modules (ͨ mod-ule 2.5) and relatively large num-bers of teeth. In the case of hobswith axially parallel flutes, thenumber of starts should be select-ed so that a lead angle of 7.5° isnot exceeded. The approachingtooth flanks of the hob can other-wise be expected to produce aninferior surface quality on the gearflanks.Lead directionWith the usual uni-directional hob-bing of helical spur gears, the leaddirection of the hob and the helixdirection of the gear are the same;with contra-directional hobbingthey are opposite. In the case ofstraight spur gears both right-hand- and left-hand cutters can beused. Normally, one uses right-hand cutters.11
12Topping cuttersThe outside diameter of the gearis topped by the tooth root of thehob. Changes in the tooth thickn-ess also result in changes of theoutside diameter.ChamferWhen hobbing helical spur gearswith large diameters, the hobscannot always be chosen longenough to cover the entire workingarea. To prevent excessive wear ofthe hob teeth in the approacharea, the hob is provided with a ta-pered chamfer. For gears withdouble-helical teeth, two hobswith chamfer may be necessary, ifthe distance between the twotooth rows is relatively small.Depending on whether hobbing isby the climb or conventional meth-od, the chamfer — generally 5 to 6x module long and 5° to 10° angleof inclination — is situated on theentering- or leaving end of thecutter.RakeUnless otherwise agreed, hobshave a rake of 0°. This does notapply to broaching tooth typeroughing hobs, which have a rakeof +8°, and indexable insert andskive hobs, which have a rake of-10° to -30°.GashesA high number of gashes increas-es the cutting capacity of the hobsand the density of the envelopenetwork; they do however also re-duce the useful tooth length,unless the cutter diameter isincreased accordingly. For solidtype hobs the gashes are up to ahelix angle of 6° made axially par-allel and over 6° with helix.DP and CPIn English-speaking countries, dia-metral pitch and circular pitch areused instead of the module. lt isbest to convert the above valuesinto module and to proceed withthe calculated module in the usualway.The equations for the conversioninto module are:m = 25.4 / DPm = 25.4 · CP / 3.1416
20Coated solid-type hobs with a highnumber of gashes are ideally suit-ed to high-performance hobbingof straight spur gears. Solid-typehobs are more stable than anytype of composite hob. The highnumber of gashes permits a highrate of chip removal, and the toollife is increased substantially bythe coating and, where applicable,re-coating.Compared to conventional hobs,high-performance hobs are re-quired to have:■ A higher tool life quality;■ Shorter machining times;■ At least equal if not superiorgear quality.These requirements are interrelat-ed, such that measures which forexample reduce the machiningtime may have a detrimental effectupon the tool life or the gearquality.Hobs can be optimized only inconsideration of the machining en-vironment. Based upon the ge-ometry and the material and qual-ity characteristics of the gear inquestion, the hob design andcutting parameters must bematched such that the require-ments are broadly fulfilled.Tip chip thicknessThe tip chip thickness is an impor-tant criterion for hob design andoptimization.The tip chip thickness is the theo-retical maximum chip thicknesswhich can be removed by the hobsteeth.The following hob characteristicsand cutting parameters are takeninto account during calculation ofthe tip chip thickness:■ Module■ Number of teeth■ Helix angle■ Profile displacement■ Cutter diameter■ Number of gashes■ Number of starts■ Axial feed■ Cutting depth.Increased tool life qualityAn increase in the number ofgashes is a design measure with adecisive, positive effect upon thetool life quality. The increase in thenumber of gashes results in thevolume to be machined being dis-tributed over a greater number ofcutter teeth, and the tip chip thick-nesses being reduced.Smaller tip chip thicknesses re-quire smaller cutting forces, whichreduce the stresses placed uponthe cutting edges of the hob andlead to lower wear. Lower tip chipthicknesses enable higher tool lifequalities to be achieved.Assuming that the hob diameterremains unchanged, however, anincrease in the number of gashesreduces the number of regrindswhich are possible. If the numberof gashes is selected so that onlyone to three regrinds are possible,the hob is described as an super-fine-tooth cutter.Hobs with 20 to 30 gashes and auseful tooth length for approxi-mately 10 regrinds are describedas multi-tooth cutters.Whether multi-tooth or superfinetooth hobs are the ideal tools for aspecific gear hobbing task mustbe determined by means of a costanalysis. The cost structure andcapacity exploitation of the usersinstallation are also decisive fac-tors.Developments over recent yearshave shown that in the majority ofcases, the multi-tooth cutter is themost suitable tool.A cutter with a high number ofgashes also generates a denserenvelope network, i.e. the profileform of the gear is improved. Thisis particularly significant for work-pieces with a small number ofteeth.faδxdδx [mm] =facos β02·sin αn4 · da0δx [mm] = depth of the feed markingfa [mm/WU] = axial feedβ0 = helix angleαn = pressure angleda0 [mm] = tip circle diameter of the hobDepth of the feed markings
21In order to achieve a high tool lifequality, high-performance hobsmust be coated. Titanium nitride(TiN) is generally employed as acoating at present. The high de-gree of hardness of the TiN coat-ing and the reduction in frictionbetween the chips and the cuttingfaces and flanks of the cutter teethpermit higher cutting speeds andfeeds together with considerablylonger tool life.When the hob is sharpened, theTiN coating is removed from thecutting faces. Pitting increases onthe now uncoated cutting faces,and the tool life quality is reduced.In order to exploit the high perfor-mance potential of these hobs tothe full, it follows that hobs forhigh-performance machining mustbe re-coated.The tool life quality is obviously al-so increased if the cutter length isextended, since the shift distanceis extended by the same quantitywith which the cutter length is in-creased.The shift strategy has a consider-able influence upon the tool lifequality. The strategy for high-per-formance hobbing is described ascoarse shifting.The shift increment is calculated inthe familiar way by dividing theavailable shift distance by thenumber of workpieces or work-piece packs which can be ma-chined between two regrinds. Onconventional hobbing machines,the standard procedure was toshift the hob through once by theshift increment calculated in thisway, and then to regrind it. Practi-cal experience has shown how-ever that the tool life quality israised considerably if the hob isshifted through several times withan increasing shift increment. It isimportant that the starting point forthe subsequent shift pass is dis-placed with each shift by a smalldistance in the direction of shifting.Coarse shifting also enables thewear development to be observedclosely and the specified wearmark width to be adhered to with-out difficulty.Shift distanceShift increment withconventional shiftingshift passshift passshift passshift passShift directionSGSKSK =Shift increment withcoarse shiftingSG =Starting point offsetCoarse shiftingConventional shiftingStarting pointth3rd2nd1stShift strategy: coarse shifting
22Shorter machining timesThe machining time (productiontime) for the hobbing process isdetermined on the one hand bythe gear width and number ofteeth and on the other by thecutting speed, hob diameter,number of starts, and axial feed.The gear width and the numberof teeth are fixed geometricvalues. The cutting speed islargely dependent upon the gearmaterial, and its tensile strengthand machineability.The machining time changes as afunction of the hob diameter,however. With a small hob diam-eter and with the cutting speedunchanged, the hob spindle andtable speeds increase, and themachining time is reduced. At thesame time, a reduction in hob di-ameter results in a reduction inthe machining distance for axialmachining.When selecting the hob diameter,note that the number of gashes islimited by this dimension, andthat a high number of gashes isrequired for good tool life qual-ities and lower cutting forces.The cutter diameter should there-fore only be sufficiently small toenable a specified cycle time tobe achieved. An unnecessarilysmall cutter diameter impairs thetool life and gear quality.High axial feeds and multi-starthobs reduce the machining timeconsiderably. However, they alsolead to higher tip chip thick-nesses, the increase in which isinfluenced more strongly by thenumber of starts than by the in-creased axial feed.A relatively high feed should beselected, and the number ofstarts kept as low as possible.This combination produces thelowest tip chip thickness. Thetwo variables are of equal import-ance for calculation of the ma-chining time, i.e. the machiningtime is determined by the prod-uct of the feed and the number ofstarts.The number of starts should al-ways be increased when the feedis limited by the depth of the feedmarkings before the maximum tipchip thickness is reached.The depth of the feed markings isdependent upon whether the gearis to be finish-hobbed or subse-quently shaved or ground.δ y [mm]z0mnαnz2i=====envelop cutdeviationnumber of startsof the hobnormal moduleprofile anglenumber of teethon the gearnumber of gashesof the hobδydδ y [mm] =π2 · z02 · mn · sinαn4 · z2 · i2=Envelop cut deviationsth =z2 · da0 · π · (E+b+A)z0 · fa · vc · 1000thz2da0EbAz0favc=========machining timenumber of teeth of thegear to be machinedtip circle diameterof the hobapproach lengthof the hobtooth width of the gearto be machinedidle travel distance of the hobnumber of starts of the hobaxial feedcutting speed[min][mm][mm][mm][mm][mm/WU][m/min]Machining time (production time) for hobbing
23Gear qualityThe gear quality is determined pri-marily by the accuracy of the hob-bing machine, the quality of thehob, stable clamping of the work-piece, and zero radial and axialrunout of the workpiece and hob.The axial feed and the diameter ofthe hob are decisive for the depthof the feed markings. In considera-tion of the gear quality producedduring finish-hobbing or subse-quent processes such as shavingor grinding, the depth of the feedmarkings and therefore the feedmust be limited.The number of starts and the num-ber of gashes have a bearing uponthe magnitude of the envelopingcut deviations. The hob diameter,number of gashes, number ofstarts, axial feed, and cuttingdepth are included in the calcula-tion of the tip chip thicknesses,and therefore influence the cuttingforces and thereby also the qualityof the gear.With regard to the quality aspects,not only must the correct hobquality to be specified to DIN 3968or comparable hob standards foreach hobbing arrangement; the tipchip thickness, feed markings andenveloping cut deviations must al-so be checked to ensure that theylie within the specified limits.SummaryOptimization of the hobbing pro-cess must entail consideration ofthe entire system, comprising thehobbing machine, workpiece, hob,and cutting parameters.Should one variable in this systemchange, the effects upon the vari-ous targets must be examined,with regard to both economicaland quality aspects.An ideal high-performance hob isalways geared to the individualgear generating task. The sizetable shown on Page 25 shouldtherefore only be regarded as aguide by means of which the hugerange of possible hob diameterscan be limited and a contributionconsequently made towards re-duction of the costs.1234681012141618202224262830ModuleCutting Speed Vm/min60504030201010 20 30 40 50 60 70Machineability in %
24Description of theworkpiece:■ Module■ Pressure angle■ Helix angle■ Number of teeth■ Tip circle diameter■ Depth of tooth or root circlediameter■ Profile displacement factor orstandards for setting the tooththickness■ Width of the gear■ Material and tensile strength■ Number of workpieces to bemachined; lot size, if applicableDescription of thehob employed:■ Hob diameter■ Cutting edge length■ Number of gashes■ Number of starts■ Cutting material■ Coated/uncoated■ Coating with hob in new condi-tion, reground with or withoutre-coatingDescription of theprocess parameters:■ Cutting speed■ Feed■ Shift increment■ Number of workpiecesclamped in the pack■ Single-cut/multiple-cut process■ Climb or conventional hobbingDescription of theresults:■ Tool life quality per regrind■ Length of the wear mark on thehob■ Machining time per workpieceor workpiece packIn the event ofquality problems:■ Quality attained on theworkpieceFormulation of theoptimization objectives:Possible targets may include:■ Shorter machining times■ Superior tool life quality■ Superior gear qualityNote when formulating the objec-tives that measures which are suit-able for attainment of, for exam-ple, the objective "improvement ofthe gear quality" influence the ma-chining time and gear generationcosts.The objective must therefore al-ways be supplemented by a qual-itative and quantitative specifica-tion of the remaining processresults.Limit values imposed bythe machine must bespecified, such as:■ Max. cutter diameter■ Max. cutter length■ Max. cutter spindle and tablespeed■ Max. shift distanceWe can alsooptimizeyour hobbing processFor this purpose we requirea complete description of theworkpiece, the hob previouslyused, the process parameters,and the results. A clear targetmust be specified foroptimization.
25Multiple-gash hobsRecommended structural dimensionsKHSS-E EMo5Co5 – TiN-coatedDimensions in mm Number ofgashesm d1 I3 I1 d21 to 4 80 120 130 3290 13, 15, 17, 19140 150 or 20170 1801 to 6 100 140 150170 180110 140 150 40 13, 15, 17, 19200 210 20, 21120 160 180 321) or 24190 210 40125 200l1l3d1 d21) Or bore diameter 40 mm
26Solid carbide hobsModern solid carbide hobs boast the followingcharacteristics:■ High cutting speeds■ Short machining times■ Long tool life■ High suitability for dry machining■ Re-coating not required for P carbides■ Lower gear generation costs (according to themachining task)IntroductionCarbide hobs permit cuttingspeeds into the high-speed cutting(HSC) range, and significantlyhigher than those possible withhigh-speed steel hobs.The development of suitably ratedhobbing machines enables the ad-vantages of solid carbide cuttersto be exploited in practical use.The combination of high-speedcutting (HSC) and dry machiningpresents substantial potential forrationalization.
27mm onwards is considerably low-er. The substrate reacts more fa-vourably.By contrast, fine-grain carbideshave as yet only been developedfor the K types. Fine-grain car-bides permit very high hardnessvalues and consequently a high re-sistance to wear, combined withexcellent toughness.Consequently, fully coated K sub-strates generally permit higher toollife qualities when compared withhobs manufactured from P car-bides, which lose their cutting facecoatings at the first regrind at thelatest. P carbide hobs must there-fore be changed more frequently.TiN, manufactured by means ofPVD, continues to be the mainsubstance employed for the hardmaterial layer of hobs. TiN pos-sesses excellent chemical resis-tance to the hot steel chips. In ad-dition to its hardness of 2200 HV,its relatively high toughness makesit particularly attractive for hobs.The logistics aspect represents adecisive advantage. TiN is thecoating which, owing to its lowpressure characteristics, can bere-coated more easily. This is es-sential following grinding of thecutting face of hobs with a K typesubstrate.Newly developed coatings such asTiCN and (TiAIN)can attain longer tool life travel fora given application, but have yet tobe accepted by the market, partic-ularly with regard to the re-coatingaspect.Carbide types andcoatingsThe carbide types generally usedare those of the main machininggroups K and P. The types presentadvantages and disadvantagesaccording to their material compo-sition (alloying elements and com-ponents) and their grain size.Whereas K carbides, owing to thetendency of chips to bond to theuncoated substrate, can only beemployed fully coated, P carbidescan also be employed in uncoatedform. There is therefore no needfor the cutting face to be re-coatedfollowing regrinding. This reducesthe maintenance costs for P car-bide hobs considerably.In addition, P carbides are lesssensitive to temperature, and thestrong progressive increase inwear which takes effect from aflank wear of approximately 0.2Advantages: Disadvantages:Advantages: Disadvantages:● Re-coating not necessary following regrinding● Low maintenance costs (regrinding only)● Shorter maintenance times, consequently:● Fewer tools in circulation (lower capital investment)● Lower progressive rise in wear when the coating is penetrated,consequently:● Lower risk of built-up edges● Shorter tool life in the reground condition, therefore:● More frequent tool changes required● Generally longer tool life, therefore:● Less frequent tool changing● Fine-grain grades possible, therefore:● Greater toughness and greater hardness● Cannot be employed uncoated, i.e. removal of the coating andre-coating is required, therefore:● Higher maintenance costs● Longer maintenance times, therefore:● More tools in circulation (greater capital investment)● Strongly progressive increase in wear following penetration ofthe coating, consequently:● Greater risk of built-up edgesUse of coated solid carbide hobs with P type substrateMaintenance process: regrinding (flank coated, cutting face uncoated)Use of coated solid carbide hobs with K type substrateMaintenance process: removal of coating - regrinding - re-coating (flank and cutting face coated)
28Cooling lubricants are not eco-nomically justifiable, because theyincrease the production costs ow-ing to the very high costs of theirsupply and disposal. Up to 16% ofthe total gear production costs canbe saved by dry machining.Furthermore, cooling lubricantsmay pose disadvantages for tech-nological reasons. The use of cool-ing lubricants in many hobbing op-erations involving carbide cuttingedges, for example, may lead topremature failure of the tool owingto stress cracking (temperatureshock). For this reason, cuttingspeeds are limited to 250 m/minfor wet machining (in comparisonwith 350 to 450 m/min for dry ma-chining). The table shows the ad-vantages and disadvantages ofcooling lubricant with regard tocarbide hobbing.The main problem with dry ma-chining lies in the increase incutting temperature. Up to 80% ofthe heat which is generated is dis-sipated with the chips, providedattention has been paid to correcttool design and suitable cuttingparameters are employed.The configuration of the tool is de-pendent upon the data of the gearto be manufactured. A significantinfluencing factor is the tip chipthickness, which is derived fromthe cutter design (number ofstarts, number of gashes, diame-ter), the workpiece geometry(module, number of teeth, cuttingdepth, helix angle) and the select-ed feed. An important considera-tion is that dry machining requiresobservance not only of an upperlimit to the tip chip thickness, butalso of a minimum thickness value.The greater the chip volume, thegreater the quantity of heat whichan individual chip can absorb. Thismust be taken into account in or-der to ensure that during dry ma-chining, the greater part of the ma-chining heat is dissipated by thechips.Machining with andwithout coolantThe machining of steel materialsgenerates considerable quantitiesof heat at the point of chip re-moval. If the temperatures reachexcessive levels, the cutting edgesof the tool are rapidly destroyed.In order to cool the tool and at thesame time to lubricate the cuttingedge, cooling lubricants have inthe past been applied to the con-tact point between the cuttingedge and the material to be ma-chined. Cooling lubricants alsohave the function of flushing awaythe chips which are produced.Cooling lubricants, however, haveconsiderable ecological, econom-ic, and in many cases also techno-logical disadvantages.Cooling lubricants present an eco-logical hazard since they impactthe environment in the form of oilvapour and oil mist, and canpresent a health hazard to hu-mans.Advantages DisadvantagesMachine ● Supports chip removal ● Aggregates (filters, pumps, etc.), therefore:● Lower heating up of the machine ● Greater space requirements● Additional operating expenditure (maintenance,power, etc.)Tool ● Cooling of the tool ● Lower tool life owing to the formation of cracksperpendicular to the cutting edge (thermal shock)● Lubrication of the friction zonesWorkpiece ● Lower heating ● Cleaning necessary● Lower dimensional deviations● Protection against corrosionEnvironment ● Binding of graphite dust ● Health riskduring cast iron machiningFurther costs ● Tempering of the workpiece, ● Purchasing coststhus faster measurement● Inventory costs● Contaminated chips, therefore:● Expensive recycling processes and● Higher disposal costsAdvantages and disadvantages of the use of cooling lubricant during hobbing
29At the point of chip generation,however, far higher temperaturesoccur which under certain circum-stances may rise to approximately900 °C, as indicated by incandes-cent individual chips. Based uponthese observations, a transversemicrosection from a workpiecesubjected to the dry machiningprocess under optimum machiningconditions for the HSC hobbingprocess was examined for pos-sible changes to the microstruc-ture.The tooth flanks machined by theHSC process and the referencesamples of a turned blank ana-lysed for the purpose of compari-son revealed no changes to themicrostructure attributable to themachining process.As already mentioned, HSC ma-chining must be considered inconjunction with dry machining.The first studies were performedon HSC hobbing machines in theearly 1990s. This process nowpermits dry machining of gears ina secure process at cuttingspeeds of up to 350 m/min.320300280260240220200180160140120600 700 800 900 1000 1100Cuttingspeedvc[m/min]Tensile strength [N/mm2]Dry machiningWet machiningCutting speeds for a range of material tensile strengths, carbide hobbing, dry and wet, module 2High-speedcutting (HSC)The advantages of high-speedcutting are:■ High surface quality and shortmachining times(depending upon the machiningapplication)■ Low cutting forces, withresulting benefits for thedimensional accuracy of theworkpiece and the tool lifeOwing to the low contact timebetween the chip and the cuttingedge, the heat which is generateddoes not have time to flow into thetool or the workpiece. The tool andthe workpiece thus remain rela-tively cold. By contrast, the chipsare heated very strongly and mustbe removed very quickly in orderto prevent the machine from heat-ing up.In an example application, HSCmachining without cooling lubri-cant led to the workpieces beingheated to approximately 50-60 °C.Applications andcutting dataThe proven applications for solidcarbide tools for gear and pinionmanufacture lie in a module rangefrom m = 0.5 to m = 4. The toolsare generally manufactured asstable monoblocs with bore- orshank-type mounting arrange-ment. The shank type is recom-mended for smaller tools. Thecutting speeds are in the rangefrom 150 to 350 m/min, accordingto the module size and process(dry or wet machining).The diagram shows the differencein cutting speeds for dry and wethobbing of materials with a rangeof tensile strengths. The values inthe diagram apply to a solid car-bide hob, m = 2.Substantially higher cutting speedscan be achieved with dry hobbingthan with wet hobbing.
30MaintenanceWhen regrinding solid carbidehobs, ensure that the thermalstress on the tooth tip is kept to aminimum. A defined edge treat-ment is also recommended. De-pending upon the hob design (e.g.positive or negative rake angle,width of the tooth lands), approxi-mately 10 to 20 regrinds are pos-sible.The "de-coating" and "re-coating"processes are required in additionfor hobs manufactured from Ktype carbide.Further information on the mainte-nance of solid carbide hobs canbe found on Page 168.Wear behaviorFlank wear is the chief form ofwear occurring on carbide hobs.Pitting, which occurs on HSShobs, is not normally significant oncarbide hobs. Chipping at thecutting edge following penetrationof the carbide coating may occa-sionally be observed. The chipsmay adhere to the uncoatedcutting edge of K types followingpenetration of the coating. Thepoint of first penetration of thecoating must therefore be delayedas long as possible.The increase in wear is progressivefrom a wear mark width of approx.0.1 mm upwards, and has a con-siderable influence upon the eco-nomic viability of the process. Wetherefore recommend that a wearmark width of 0.15 mm not be ex-ceeded, and that the cutter be re-coated following each regrind.Chip adhesion to the worn andtherefore uncoated cutting edgesis much less common with the Ptypes. Re-coating is not thereforenecessary with these types.t3H12dH5AA0,2b3H11f2f2r3r3Drive slot dimensions of a carbide hobStructural dimensionsThe size table indicates the hob di-mensions for which FETTE stockscarbide blanks. The blanks do nothave drive slots. A drive slot cantherefore be provided on either theleft-hand or the right-hand indica-tor hub, as desired by the cus-tomer.FETTE recommends drive slotswith reduced gash depth for car-bide hobs. The gash dimensionscan be found in the table below.Bore diameter b3 t3 r3 f2Permissible deviation Permissible deviation8 5,4 2,00 0,6 –0,2 0,4 0,110 6,4 2,25 0,8 0,513 8,4 2,50 1,016 2,80 –0,3 0,6 0,222 10,4 3,15 1,227 12,4 3,50 0,832 14,4 4,00 1,6 –0,440 16,4 4,50 2,0 –0,5 1,0 0,350 18,4 5,0060 20,5 5,6070 22,5 6,25 2,5 1,280 24,5 7,00100 8,00 3,0 1,6 0,5t3 = 1/2 depth to DIN 138
Roughing hobsHigh cutting capacities areachieved with the heavy dutyrouching hob when roughing gearsfrom module 6 onwards with hightooth numbers and large gearwidths.These high cutting capacities aremade possible by a favourablecutting edge geometry and thedistribution of the metal removalcapacity over a relatively largenumber of tool cutting faces.Because of its even cutting edgeload, this tool is particularly quietin operation, even with maximumfeeds and high chip thickness.The design of the heavy dutyroughing hob is based on the fol-lowing considerations:■ The volume of metal to be re-moved when cutting gears in-creases quadratically with themodule, whereas the number ofgashes, because of the greaterprofile height, becomes smallerin the usual cutter sizes. Thisresults in a greater load on theindividual cutter teeth.■ Approximately 75 % of the me-tal removal work takes place inthe tip area of the cutter teeth.This results, particularly whenroughing, in an extremely un-even load and wear distributionon the cutter teeth. The greatertip corner wear determines theduration of the service life,whereas the cutting edges inthe tooth centre- and root areashow only very little wear.■ An efficient and economicalhob must therefore have a verylarge number of gashes, with-out making the outside diam-eter of the cutter too large. Thenumber of tip cutting facesshould exceed that of the flankand root cutting edges.32
33These requirements are met per-fectly by the FETTE heavy dutyroughing hob with its verticallystaggered teeth. The cutter teethonly have the full profile height inevery second tooth row. The inter-mediate teeth are limited to about1/3 of the profile height.This design principle makes it pos-sible to acommodate 16 or 20flutes on a still practicable cutterdiameter.The 8 or 10 complete teeth on thecutter circumference are generallysufficient for producing the profileshape within the required toler-ances. The heavy duty roughinghob can therefore also be used asa finishing cutter.Depending on the quality required,the heavy duty roughing hob isavailable either relief turned or re-lief ground.For roughing, the cutter teeth canbe provided with offset chipgrooves, which divide the chipsand reduce cutting forces andwear.Roughing hobs can be regroundon any standard hob grinder. Onceset, the gash lead can be retained,independent of the gash depth.Roughing hobs are manufacturedwith axially parallel gashes up tolead angle of 6°, which is a condi-tion for sharpening by the deepgrinding method.The design principle of the rough-ing hob is of course not limited tothe basic profiles for involute toothsystems to module or diametralpitch, but can also be used for allother common profiles and forspecial profiles.00BASchnitt A–0 Schnitt B–02,25 · m1,5 · m0,75 · mF2 F1 F2Metal removal areas on the cutter tooth:tooth tip corresponds to area F 1 ≈ 75 %tooth root corresponds to area F 2 ≈ 25 %tooth gash volume = 100 %Face plane of a roughing hobSection A-0 Section B-0
34Dimensions in mmm d1 I3 I0 d26 150 108 140 507 126 1588 160 144 1769 162 19410 170 180 214 6011 180 198 232 6012 190 216 25013 200 234 26814 210 252 28615 230 270 310 8016 240 288 33018 260 318 36020 290 360 406 10022 300 396 442 10024 310 432 47827 330 486 53230 340 540 586Heavy duty roughing hobsl0l3d1 d2(roughing type hobs)for spur and helical gears tomodule pitch20° pressure anglebasic profile III to DIN 3972with positive rake (undercut)optionally with chip breakergroovessingle start right-handedwith drive slotKHSS-E EMo5Co52051 relief turned ■ Quality grade B/C to DIN 3968 ■ with 20 gashes2053 relief ground ■ Quality grade A to DIN 3968 ■ with 20 gashes2055 relief turned ■ Quality grade B/C to DIN 3968 ■ with 16 gashes2057 relief ground ■ Quality grade A to DIN 3968 ■ with 16 gashesCat.-No.
35Ident No.2067Ident No.2065Ident No.2063Ident No.2061Dimensions in mmm d1 I3 I0 d26 150 108 118 50 1208017 1208053 1209205 12090237 126 136 1208019 1208055 1209214 12090258 160 144 154 1208021 1208057 1209223 12090289 162 172 1208023 1208059 1209232 120903010 170 180 190 60 1208025 1208061 1209241 120903211 180 198 208 60 1208027 1208063 1209250 120903412 190 216 226 1208029 1208065 1209269 120903713 200 234 244 1208031 1208067 1209278 120903914 210 252 262 1208033 1208069 1209287 120904115 230 270 280 80 1208035 1208071 1209296 120904316 240 288 300 1208037 1208073 1209303 120904618 260 318 330 1208039 1208075 1209312 120904820* 287 1208041 1208077 1209321 120905020 290 360 372 100 1208043 1208079 1209011 120905222 300 396 408 100 1208045 1208081 1209013 120905524 310 432 444 1208047 1208083 1209015 120905727 330 486 498 1208049 1208085 1209017 120905930 340 540 552 1208051 1208087 1209019 1209061Heavy duty roughing hobsl1l3d1 d2(roughing type hobs)for spur and helical gears tomodule pitch20° pressure anglebasic profile III to DIN 3972with positive rake (undercut)optionally with chip groovessingle start right-handedwith keywayKHSS-E EMo5Co52061 relief turned ■ Quality grade B/C to DIN 3968 ■ with 20 gashes2063 relief ground ■ Quality grade A to DIN 3968 ■ with 20 gashes2065 relief turned ■ Quality grade B/C to DIN 3968 ■ with 16 gashes2067 relief ground ■ Quality grade A to DIN 3968 ■ with 16 gashesCat.-No.* For hobbing machines with max. capacity = 290 mm dia. and for max.cutter lenght = 330 mm.
36Roughing hobs with indexable carbide insertsRoughing hob with indexable carbide inserts in operation
37The rough hobbing of gears formmodule 5 onwards can be carriedout extremely economically withthis modern tool.The design concept is the combi-nation of the known advantages ofthe hobbing process with the per-formance of carbide and the econ-omy of indexable inserts. Using in-dexable carbide inserts, largevolumes of metal can be removedwithin a given time at high cuttingspeeds.Regrinding, which is necessarywith conventional hobs, is elimin-ated. This saves the cost of sharp-ening and of tool changes. Thewear marks on the individualcutter teeth vary according to theprocess. In the large-gear sector,these can only be partly equalizedby shifting. Hobs therefore alwayscontain teeth with different wearmark widths. When using index-able inserts, only those insertsneed be turned or replaced whichhave reached the maximum wearmark width.To change the indexable inserts orthe segments, it is not necessaryto remove the cutter form the ma-chine. This results in short hobbingmachine downtimes.Changing the indexable inserts al-so makes it possible to match thecarbide grade optimally to the gearmaterial.To use these carbide tipped toolssuccessfully, it is necessary tohave hobbing machines which of-fer sufficient rigidity as well as therequired speed and drive power.ConstructionFETTE indexable insert hobs con-sist of a cutter body, onto whichthe tooth segments are screwedand indexable carbide inserts.The latter are held by clampingscrews in the insert seats of thesegments.A helical groove has been re-cessed into the cylindrical cutterbody. The flanks of the grooveground according to the cutterlead.The parts of the ground cylindricalshell which remain between thegroove windings act as supportsurfaces for the tooth segments.Two cylindrical pins arranged inthe tooth segments are guided inthe groove and determine the po-sition of the segments.The segments are fixed to thecutter body by inhex screws.The seats for the indexable car-bide inserts are arranged tangen-tially on the tooth segments.Within a segment, the seats are ar-ranged alternately if possible.The purpose of this arrangement isto keep the axial reaction forces onthe cutter and the tangentialcutting force components on thegear as low as possible.Cutter bodyTooth segment
38The indexable carbide insertsmust completely cover the cuttingedges of the cutter tooth. The ne-cessary number of indexable in-serts and their arrangement de-pend on the dimensions of theinserts and on the size of the gear.To render the pre-cutting of thegear optimal for skive hobbing orgrinding, the carbide hobs with in-dexable inserts can be made sothat they produce both a rootclearance cut (protuberance) and achamfer on the gear (see fig. be-low).In the range from module 5 tomodule 10 each cutter tooth onlyholds one insert, which covers theentire flank length.From module 11 onwards, eachflank is fitted with an indexable in-sert offset to the opposite flank. Inspecial cases, versions with asingle insert covering each flankare also possible with these mod-ule sizes.Cutting edge construction module 5–10Cutting edge construction module 11–20Profile design with protuberance and chamfer
39Roughing hobsl1l3d1 d2with indexable carbide insertsfor spur and helical gearsto module pitch20° pressure anglebasic profile by arrangementsingle start right-handedwith keywayCarbide - TiN-coatedDimensions in mm Ident No.Number ofindexableinserts1)Number ofsegmentsNumber oftooth rowsm d1 I3 I1 d22163Cat.-No.5 190 95 144 60 19 24 96 –6 114 165 12242067 210 133 185 –8 152 206 12242159 171 227 –10 190 248 122422411 280 209 269 80 19 24 192 –12 229 289 122423313 248 310 –14 267 331 122424215 280 286 352 80 19 24 192 –16 300 305 373 122425117 324 394 –18 343 415 122426019 362 436 –20 300 382 457 80 19 24 192 1224279Spare parts and indexable inserts: design on request.1) The number of indexable inserts may change according to the basic profile.
40Carbide skiving hobsDesignsDepending on the module size andthe accuracy requirements, 3 skiv-ing hob designs can be basicallydistinguished:■ Solid carbideup to and including module 4FETTE Cat. no. 2028■ Brazed-on carbide tipsfor modules above 4FETTE Cat. no. 2129■ Indexable carbide insertsfor modules from 5 upwardsFETTE Cat. no. 2153Process andrange of applicationsSkive hobbing is a machining pro-cess in which skiving hobs areused for cutting rough-milled andhardened gears.The main area of application is thehobbing of straight and helicalspur gears. In addition, externalsplines, roll profiles and a largenumber of special profiles whichcan be generated by the hobbingmethod can be machined with theskiving hob. There are various rea-sons for using this process:Finish-hobbing of gearsSkive hobbing eliminates harden-ing distortion and improves thequality of the gear.The metal removal capacity is con-siderably higher with skive hob-bing than with the usual grindingprocesses. It is therefore economi-cal to replace grinding by skivehobbing in the range of coarse andmedium gear tolerances.Gear quality grade 6 to DIN 3962can be quoted as an approximatevalue for the attainable accuracy.Profile- and flank modifications,too, such as depth crowning, toothface setback or width crowning,can be produced by suitable hobprofiles and corresponding ma-chine motions.Preparation for grindingFor high gear quality requirements,the gears are ground. The gearcutting costs can be markedly re-duced if the hardening distortion isbefore grinding removed by skivehobbing, at the same time remov-ing material down to the necessarygrinding allowance. Grinding timesand costs are reduced while gain-ing additional grinding capacity.The toolDesignThe characteristic design featureof skiving hobs is the negative tiprake angle. The tip rake angle isdescribed as negative when thecutting faces of the teeth lie, in thedirection of the cutting motion, infront of the tool reference plane.The tool reference plane is theplane in which lie the tip cuttingedges of the axially parallel cutterand cutter axis.Due to the negative tip rake angle,the flank cutting edges are inclinedin relation to the effective refer-ence plane (plane perpendicular tothe cutting motion) and in this wayproduce a peeling cut.The negative rake angle is greaterin the root area of the hob teeththan in the tip area. The tip cuttingedges have no effective back rakeand cannot therefore generate acurling cut. It therefore follows thatthe skiving hobs should only pro-duce flank chips and that protu-berance cutters are used forroughing the gears.Tool materialLow chip thickness and hardenedgear materials make severe de-mands on the edge strength of thetool material. As the tool materialfor skiving hobs, carbides of ISOapplication groups K 05 to K 15are used.Fig. 1– γλsvc–γ = Kopfspanwinkelλs = Neigungswinkel der Flankenschneidevc = SchnittgeschwindigkeitFig. 2- γ = tip rake angleλs = back rake of the flank cutting edgevc = cutting speed
41A special position among theabove designs is occupied by theskiving hob with indexable carbideinserts. This cutter type does notrequire regrinding. Only those in-serts which have reached themaximum wear mark width areturned or changed.It is understandable that a cutterassembled from cutter body, toothsegments and indexable insertscannot offer the same accuracy asa cutter in solid carbide. This iswhy the cutter with indexable in-serts is particularly suitable forpreparing the workpiece for grind-ing.By far the most common skivinghob is the bore type. Solid carbideskiving hobs have a drive slot onone or both ends, for manufactur-ing reasons. For hobs with a highquality grade, preference shouldwhere possible be given to boreswith drive slot over those with key-way. A precise bore can be manu-factured more easily without akeyway, and the run-out of the hobon the hobbing machine is also re-duced. For extreme accuracy re-quirements, a shank-type tool alsopermits compensation of the run-out between cutter arbor andcutter.Quality gradesSkiving hobs are generally manu-factured in quality grade AA to DIN3968. If required, the solid carbideand brazed-on carbide tip typescan also be manufactured in qual-ity grade AAA (75% of the toler-ances of AA).A concave flank shape is usual forthe skiving hob, to achieve a slighttip relief on the workpiece.Preparation forskive hobbingThe machining allowance dependson the module size and the hard-ening distortion. Experience hasshown that for the module range 2to 10 it lies between 0.15 and 0.30mm/flank.The tooth root must be pre-machined deeply enough to pre-vent the tooth tip of the skivinghob from cutting into it.We recommend hobs protuber-ance, e.g. FETTE Cat. no. 2026.The hardness of the gear must forthe skive hobbing process be lim-ited to HRC 62 +2.Solid-carbide skiving hobSkiving hob with indexablecarbide insertsSkiving hob with brazed-oncarbide stripsSkiving hob with brazed-oncarbide tips
42For high quality requirements,hobbing must always be done inseveral cuts. For the last cut, a re-moval of 0.1 mm/flank should beaimed at, to affect the structure ofthe gear material as little as pos-sible.CoolingIntensive cooling of the tool, work-piece, holding fixture and machinewith the cutting oils usual for hob-bing, the temperature-dependenterror values are reduced and theservice life of the skiving hobs isextended.Wear and tool lifevaluesWear mark widthThe wear mark width on the skiv-ing hobs should not exceed 0.15mm.Cutting forces increase with great-er wear mark width and with verythin chips deflection of the hobcutting edges will occur.This may have the following con-sequences:quality losses, chipping of carbidecutting edges and excessivestructural changes through tem-pering and re-hardening process-es on the gears.Uniform wear throughshiftingWear only occurs on the toothflanks of the skiving hobs. Thewear marks are relatively shortand follow the contour of the en-gagement lines.By shifting the hob in the axial di-rection after hobbing a gear or setof gears, the wear is distributedevenly over the flank cutting edgesand over the entire cutting edgelength of the hob. This process isfurther facilitated if the hobbingmachine is equipped with a syn-chronous shifting arrangement.This arrangement ensures that themachine table makes an additionalturn when the tangential slide ismoved. The relative position of thehob motion then remains as setduring centering.Tool life between regrindsThe life between regrinds of a hobequals the sum of the lengths of allhobbed workpiece teeth betweentwo regrinds of the hob.The calculation of the life betweenregrinds, the tool requirement, theproportional tool costs etc. isbased on the life between regrindsper cutter tooth. This depends onthe module value and on the hard-ness of the material being cut. Ex-perience has shown the tool lifebetween regrinds to lie between 2and 4 m per cutter tooth for skivehobbing.Gear cutting qualityThe gear quality when skive hob-bing depends on the interaction ofa large number of components andparameters, such as:■ skiving hob (cutting material,correctly sharpened, sufficentaccuracy)■ rigid hobbing machine■ accurate and stable clampingof hob and workspiece■ hob aligned with an absoluteminimum of runout■ accurate centering■ correct selection of cuttingspeed, feed and metal removalper flank■ adherence to the maximumwear mark width■ material, preparation and heattreatment of the workpiecesCutting conditionsCutting speedThe cutting speed depends on themodule size and on the hardnessof the gear. As an approximate val-ue, a cutting speed of 36 m/mincan be quoted for module 30 andof 110 m/min for module 2.For the lower modules, higher val-ues between 140 and 160 m/minare also possible. These highcutting speeds do however reducethe service life of the skiving hoband the workpiece structure is in-creasingly affected.For workpiece hardness valuesfrom HRC 62 upwards, the cuttingspeed should be limited initially to70 m/min and then optimized inconsideration of the cutting resultand the service life of the tool.FeedThe structure of surfaces ma-chined with hobs is affected by thedepth of the feed markings. Thedepth of the feed mark increasesquadratically with the value of thefeed. It is therefore logical to dis-tinguish between feeds for finish-ing and for roughing.Approximate value for the feed:For the finishing cut1.5 to 2 mm/workpiece rotationfor the roughing cutup to 4 mm/workpiece rotationClimp hobbing methodClimb hobbing for skive hobbing ispreferred since this yields the bestservice life of the skiving hobs.Removal per flankTo maintain a reasonable servicelife of the hobs, not more than0.15 ÷ 0.20 mm/flank should beremoved in one cut.
43angle, the grinding wheel must beset off-centre. The measurementfor the setting of the grindingwheel depends on the cutter diam-eter in question and is shown inthe regrinding diagram, which isenclosed with every cutter.Cutting faces must be ground withlow roughness depth in order toprevent flaws and micro-chippingon the cutting edges. The toler-ances of DIN 3968, insofar as theyconcern the gashes, must bemaintained.= cutting face offset= cutter diameterudaudaCutterdiameter(mm)189188187186185184183–45,4 –45,8 –46,2 –46,6 –47,0 –47,4Cutting face offset u (mm)Cutting face regrinding diagram for carbide skiving hobsPitch- and tooth trace deviationsare caused by the hobbing ma-chine.The profile shape depends basi-cally on the quality of the hobs.The cutting parameters, the hard-ness of the workpieces and thewear condition of the cutters affectmainly the cutting forces, whichreact on tool and machine andthus contribute to the tooth quality.Under good conditions and withcareful working the gear qualitygrade 6 to DIN 3962 can beachieved with a surface roughnessof 1to2 µm.Hobbing machineIn principle, conventional hobbingmachines are also suitable forskive hobbing. The decisive factoris the condition of the machine.It is vital to keep the play in thehob spindle thrust bearing and inthe table- and feed drive as low aspossible.Obviously, modern hobbing ma-chines with dual-worm table driveor hydraulic table pre-loading, withcirculating ball spindle for the axialfeed and prestressed thrust bear-ing of the hob spindle offer betterpreconditions for good gear qual-ity. Arrangements for automaticcentering and for synchronousshifting are also desirable.Maintenance of theskiving hobThe skiving hob should be sharp-ened when the wear mark hasreached a width of 0.15 mm. Dia-mond wheels are used for grindingwith the traverse grinding or thedeep grinding process.Because of the negative tip rake
44Skiving hobsl1l3d1 d2Solid carbidefor finishing hardened (highly tempered)spur and helical gears to module pitch20° pressure anglebasic profile: ha0 = 1.15 · m, öa0 = 0.1 · mquality grade AA nach DIN 3968single start right-handedwith keywayCarbide – TiN coatedDimensions in mm Number ofgashesIdent No.m d1 I3 I1 d22028 relief groundCat.-No.2 80 100 120 32 15 23528902,5 23528913 90 40 23528923,5 100 120 140 23528934 4021516
49Hobsfor internal gears, with straight or helical teeth, involute flanksCat.-No. PageExplanatory notes 50Solid-type hobs 2082 51
50Explanatory notesHobs for internal gears are de-signed for a specific gear. Themeasurements for the maximumand the minimum cutter diameterand the maximum cutter widthmust then be taken into account,for which the internal hobbinghead is dimensioned.ln the case of internal gears withlarge profile displacement, themaximum permissible cutter widthmay be insufficient for cutting thecomplete teeth, if the hob is di-mensioned in the usual way. It isthen necessary to fix the moduleand the pressure angle of the hobdifferently from those of the inter-nal gear.On the hob, one tooth is definedas a "setting tooth" and markedaccordingly. The cutter must bepositioned on the hobbing ma-chine so that the setting tooth iswhen new placed in the "machinecentre". Although the setting toothwill shift in the axial direction whenthe hob is reground, it is not nec-essary to correct the position ofthe hob determined in the newcondition and fixed by spacers.The hobs offered for finishinginternal gears are only to a limitedextent suitable for roughing. Bear-ing in mind the tool costs, reliefturned hobs with a lead matchedto the workpiece should be usedfor roughing.
51Solid-type hobsl1l3d1 d2for internal gears to module pitchstraight or helical teeth20° pressure anglebasic profile II to DIN 3972quality grade AA to DIN 3968single start right-handedwith keywayKHSS-E EMo5Co5Dimensions in mm Number ofgashesm d1 I3 I1 d22082 relief groundCat.-No.5 360 45 65 100 306 52 728 66 86 2410 80 90 2212 94 10414 108 11816 122 132 2018 136 146 1820 150 160 16The structural dimensions listed are approximate values, which can be changed according to thesize of the internal hobbing head and the tooth data of the gear.For internal hobs greater than module 20, workpiece drawings and dimensions of the internalhobbing head must be submitted, so that the structural dimensions of the hob can be deter-mined accordingly.
Hobsfor compressor rotors and pump spindlesCat.-No. PageHobs for compressor rotors 54Rotor hobsRoughing cutters, as roughing hobs (broach-tooth type) 2091 55Finishing cutters, as solid-type hobs 2092 56Hobs for pump spindlesFinishing cutters, as solid-type hobs 2094 57
Hobs for compressor rotorsRotors are the multi-thread feedscrews of a screw compressor,which are arranged in pairs insidea housing. The meshing screwthreads have a symmetrical or anasymmetrical profile.Quiet running and good efficiencyof the rotors are determined by theaccuracy of the rotor profiles.The advantages of hobbing pro-duce favourable results in rotormanufacture:■ High pitch accuracy■ Low distortion owing to even,constant chip removal in allgaps■ Trouble-free maintenance ofthe hob, which is reground onlyon the cutting faces.The use of this technology for rotormanufacture requires the develop-ment of the required analysis pro-grams for rotor and hob profilesand high standards of manufactur-ing in the area of precision hobs.High demands are placed on therigidity, output, thermal stabilityand feed accuracy of the hobbingmachines.The successful use of hobs alsodepends on the degree to whichthe tool manufacturer on the onehand and the rotor producer or-designer on the other hand com-municate with each other aboutthe production constraints im-posed on profile shape, amount ofplay and play distribution. Thisprocess then does allow modernand economical production, whenquality and output depend primari-ly on the tool and the machine.54Female rotorMale rotorRotors: face plane view
55Rotor hobs, for roughingl1l3d1 d2for screw compressorsfor male and female rotorsas heavy-duty roughing hobswith 16 flutesaxially parallel gashessingle startwith keywayKHSS-E EMo5Co5Dimensions in mmCutter dimensionsRotor dia. m Profile height d1 I3 I1 d22091 relief turnedCat.-No.47/44,5 ≈ 5,2 ≈ 10,2 112 90 106 4081,6 ≈ 9,1 ≈ 17,5 140 154 170 50102 ≈ 11,4 ≈ 22 170 184 200 60127,5 ≈ 14,2 ≈ 27,5 212 234 250163,2 ≈ 18,2 ≈ 35,5 265 299 315 80204 ≈ 22,7 ≈ 44 305 319 335 100204 ≈ 22,7 ≈ 44 335The structural dimensions are approximate values for rotor measurements L/D = 1.65.When ordering, workpiece drawings of the rotors and data abaut the profileat the face plane (list of coordinates) must be made available.Owing to their size, not all rotorscan be generated by hobbing. Fur-thermore, the choice of tools is al-so influenced by the process al-ready in place and the machineswhich are available.FETTE played a leading part in theintroduction of the hobbing pro-cess for the manufacture of rotors.FETTE can therefore call uponconsiderable experience in advis-ing its customers.The advantages of the hobbing method are undisputed and can be summar-ized as follows:■ Quick and trouble-free production of rotors with good surfaces and accu-rate profiles and pitch.■ The sealing strips on the tooth tip and the sealing grooves in the tooth rootof the rotors can be generated in one operation with the flanks.■ Hobbed rotors can be exchanged at any time, thanks to their uniform ac-curacy.■ Simple and economical maintenance of the tools, since the hobs are onlyreground on the cutting face.for male rotorsfor female rotors
56Rotor hobs, for finishingl1l3d1 d2for screw compressorsfor male and female rotorsquality grade AA restricted to DIN 3968axially parallel flutessingle startwith keywayKHSS-E EMo5Co5Dimensions in mmCutter dimensionsRotor dia. m Profile height d1 I3 I1 d22092 relief groundCat.-No.47/44,5 ≈ 5,2 ≈ 10,2 140 74 90 6081,6 ≈ 9,1 ≈ 17,5 190 124 140 80102 ≈ 11,4 ≈ 22 236 154 170127,5 ≈ 14,2 ≈ 27,5 265 196 212 100163,2 ≈ 18,2 ≈ 35,5 300 249 265204 ≈ 22,7 ≈ 44 305 299 315204 ≈ 22,7 ≈ 44 335for male rotorsfor female rotorsThe structural dimensions are approximate values for rotor measurements L/D = 1.65.The entire profile, including the sealing strip and slot, is machined one on operation. The out-side diameter of the rotors is ground to finish size.When ordering, workpiece drawings of the rotors and data about the profile at the faceplane (list of coordinates) must be made available.
57Hobsl1l3d1 d2for screw pumpsfor drive or trailing spindlequality grade AA restricted to DIN 3968single startwith keywayKHSS-E EMo5Co5Dimensions in mmHob dimensionsd1 I3 I1 d22094 relief groundCat.-No.18 x 10,8 10,8 x 3,6 100 52 60 32 1620 x 12 12 x 4 55 6330 x 18 18 x 6 112 72 8035 x 21 21 x 7 118 82 9038 x 22,8 22,8 x 7,6 125 87 95 4045 x 27 27 x 9 140 98 106 1852 x 31,2 31,2 x 10,4 150 104 112 5060 x 36 36 x 12 160 110 11870 x 42 42 x 14 180 122 132Hob for drive spindleHob for trailing spindleNumber ofgashesDrive spindleD x d1)Trailing spindleD x d1)1) D = Outside diameter, d = inside diameterThe overall dimensions shown are recommended values and may be adapted to the working space ofthe hobbing machine both in length and in diameter.When ordering, the following workpiece data must be quoted: measurements about the profile atface plane, outside diameter, inside diameter, lead and direction of lead – normally drive spindleright-hand, trailing spindle left-hand.Drive and trailing spindles
Hobsfor sprockets, timing belt pulleys and splinesCat.-No. PageHobs for sprocketsrelief turned 2301 60relief turned 2311 61relief turned 2331 62relief ground 2341 63Hobs for timing belt pulleysrelief ground 2342 64relief ground 2352 65Hobs for spline shaftsrelief ground 2402 66relief ground 2412 66relief ground 2422 67relief ground 2432 67relief ground 2442 68Hobs for p.t.o. shaftsrelief ground 2444 69relief ground 2472 70Hobs for involute spline shaftsrelief ground 2452 71Hobs for serrated shaftsrelief ground 2462 72
63Hobsl1l3d1 d2for sprockets with involute flankswith tip relief30° pressure anglequality grade A to DIN 3968single start right-handedwith keywayKHSS-E EMo5Co5Dimensions in mm Number ofgashesIdent No.PitchInch d1 I3 I1 d22341 relief groundCat.-No.5/16 70 63 69 27 16 12275063/8 80 32 12275151/2 90 70 78 12275245/8 80 88 14 12275333/4 100 92 100 12275421 110 120 130 12 122755111/2 150 160 170 50 12275602 190 215 225 1227579
64Hobsl1l3d1 d2for synchroflex timing belt pulleystopping cutterquality grade A to DIN 3968single start right-handedwith keywayKHSS-E EMo5Co5Dimensions in mmPitchTooth numberrangeIdent No.Number ofgashesd1 I3 I1 d22342 relief groundCat.-No.T 2,5 se 12– 20 50 25 31 22 14 1228006T 2,5 21– 45 1228015T 2,5 46– 80 1228024T 5 se 10– 14 56 32 38 22 14 1228033T 5 se 15– 20 1228042T 5 21– 50 1228051T 5 51–114 1228060T 10 se 12– 15 70 50 56 27 14 1228079T 10 se 16– 20 1228088T 10 21– 45 1228097T 10 46–114 1228104T 20 se 15– 20 90 80 88 32 14 1228113T 20 21– 45 1228122T 20 46–119 1228131The "se" tooth gap form is applied up to 20 teeth incl., over 20 teeth = normal profile
65Hobsl1l3d1 d2for timing belt pulleyswith involute flanks to DIN/ISO 5294topping cutterquality grade A to DIN 3968single start right-handedwith keywayKHSS-E EMo5Co5Dimensions in mmPitchTooth numberrangeIdent No.Number ofgashesd1 I3 I1 d22352 relief groundCat.-No.0,08 MXL10 up to 23 50 25 31 22 14 1203010from 24 22573981/8 XXL from 10 12030121/5 XL from 10 56 32 38 12283003/8 L from 10 70 50 56 27 12283191/2 H 14–19 63 69 12283281/2 H from 20 12283377/8 XH from 18 100 80 88 40 122834611/4 XXH from 18 115 100 108 1228355Hobs for timing belt pulleys with straight flanks to DIN/ISO 5294 on request.Our range also includes hobs for timing belt pulleys with special profiles.
66Hobsl1l3d1 d2for spline shaftsquality grade A to DIN 3968single start right-handedwith keywayKHSS-E EMo5Co5Dimensions in mm Dimensions in mmIdent No.NumberofgashesNumberofsplinesForshldr.dia.Spline shaft nominal dimesions Hob dimensions2402 relief ground2412 relief groundCat.-No.d1 I3 I1 d2I.d. O.d. Wdth ofg 6 a 11 spl. h 91), 2), 3), 4) This hob is absolutely identical with the hob marked with the same index number under Cat.-No. 2442.23 26 6 6 29 56 30 36 22 12 122946126 30 33 63 34 40 27 122947028 32 7 35 122948932 36 6 8 39 39 45 122949836 40 7 43 122950442 46 8 50 122951346 50 9 54 70 44 50 122952252 58 10 62 50 56 122953156 62 66 122954062 68 12 73 122955972 78 10 83 57 63 122956882 88 93 80 32 122957792 98 14 103 90 65 71 1229586102 108 16 113 100 1229595112 120 18 126 72 80 122960211 14 3 6 16 56 26 32 22 12 123021713 16 3,5 18 123022616 20 4 22 30 36 123023518 22 5 25 34 40 1230244211) 25 28 63 27 1230253232) 28 6 31 39 45 1230262263) 32 35 1230271284) 34 7 37 44 50 123028032 38 6 8 41 123029936 42 7 45 123030642 48 8 52 70 50 56 123031546 54 9 58 123032452 60 10 64 57 63 123033356 65 69 80 32 123034262 72 12 77 65 71 123035172 82 10 87 90 123036082 92 97 72 80 123037992 102 14 107 100 1230388102 112 16 117 112 40 1230397112 125 18 131 82 90 12304042412 For spline shafts to DIN ISO 14 – medium series ■ Cutting to shoulder, with 2 lugs and chamfer2402 For spline shafts to DIN ISO 14 – light series ■ Cutting to shoulder, with 2 lugs and chamfer
682442 relief groundHobsl1l3d1 d2for spline shaftsquality grade A to DIN 3968single start right-handedwith keywayKHSS-E EMo5Co5Dimensions in mm Dimensions in mmIdent No.NumberofgashesNumberofsplinesForshldr.dia.Spline shaft nominal dimesions Hob dimensionsCat.-No.d1 I3 I1 d2I.d. O.d. Wdth ofg 6 a 11 spl. h 91), 2), 3), 4) This hob is absolutely identical with the hob marked with the same index number under Cat.-No. 2412.211) 25 5 6 28 63 34 40 27 12 1232420232) 28 6 31 39 45 1232439263) 32 35 1232448284) 34 7 37 44 50 123245732 38 8 42 70 50 56 123246636 42 46 123247542 48 10 52 90 57 63 32 14 123248446 52 12 57 123249352 60 14 65 100 65 71 123250958 65 70 123251862 70 16 75 123252768 78 83 112 72 80 40 123253672 82 87 123254578 90 95 140 82 90 16 123255482 95 100 123256388 100 105 123257292 105 20 111 92 100 123258198 110 116 1232590105 120 126 160 102 110 50 1232607115 130 136 1232616130 145 24 151 12326252442 For spline shafts to DIN 5472 ■ Cutting to shoulder, with 2 lugs and chamfer
692444 relief groundHobsl1l3d1 d2for p.t.o. shafts to DIN 9611quality grade A to DIN 3968single start right-handedwith keywayKHSS-E EMo5Co5Cat.-No.63 50 56 27 12 123268956 32 38 22 14 123266163 40 46 27 14 1232670Form 128.91 ± 0.05 x 34.79 ± 0.06 x 8.69 –0.09–0.166 splines with 2 lugs and chamferForm 2DP 16, EW 30°, da = 34.6721 teeth with chamferForm 3DP 12, EW 30°, da = 44.3320 teeth with chamferP.t.o. shaftsDimensions in mmHob dimensionsd1 I3 I1 d2Number ofgashesIdent No.
70Hobsl1l3d1 d2for spline shafts with involute flanksto DIN 548030° pressure anglequality grade A to DIN 3968single start right-handedwith keywayKHSS-E EMo5Co5Dimensions in mm Number ofgashesIdent No.m d1 I3 I1 d22472 relief groundCat.-No.0,6 50 25 31 22 14 12339190,8 12339281,0 12339371,25 12339461,5 56 32 38 12339552,0 63 40 46 27 12339642,5 70 50 56 12339733 12339824 80 63 69 32 12339915 90 70 78 12340086 100 80 88 12340178 115 100 108 40 123402610 125 130 138 1234035
71Hobsl1l3d1 d2for spline shafts with involute flankspreviously DIN 548230° pressure anglequality grade A to DIN 3968single start right-handedwith keywayKHSS-E EMo5Co5Dimensions in mm Ident No.Number ofgashesSpl. shaft nom. size m d1 I3 I1 d22452 relief groundCat.-No.15 x 12 1,6 56 32 38 22 1217 x 1418 x 1520 x 17123301822 x 1925 x 2228 x 25 1,7530 x 2732 x 28123302735 x 3138 x 34 1,9 63 40 46 2740 x 36 123303642 x 3845 x 41 248 x 4450 x 4552 x 47 123304555 x 5058 x 5360 x 5562 x 57 2,165 x 6068 x 6270 x 6472 x 66123305475 x 6978 x 7280 x 7482 x 76 2,25 70 50 5685 x 7988 x 8290 x 8492 x 86123306395 x 8998 x 92100 x 94
72Hobsl1l3d1 d2for serrated shafts to DIN 5481with staight flanks for involute flankform on the componentquality grade A to DIN 3968single start right-handedwith keywayKHSS-E EMo5Co5Dimensions in mm Ident No.Number ofgashes*Serr. sh. nom. size Pitch d1 I3 I1 d22462 relief groundCat.-No.7 x 8 0,842 50 25 31 22 16 12334108 x 10 1,010 123342910 x 12 1,152 123343812 x 14 1,317 123344715 x 17 1,517 123345617 x 20 1,761 56 32 38 123346521 x 24 2,033 123347426 x 30 2,513 123348330 x 34 2,792 123349236 x 40 3,226 123350840 x 44 3,472 63 40 46 27 123351745 x 50 3,826 123352650 x 55 4,123 123353555 x 60 4,301 123354460 x 65 4,712 70 50 5665 x 7070 x 7575 x 8080 x 8585 x 9090 x 95 123355395 x 100100 x 105105 x 110110 x 115115 x 120120 x 125* Hobs will be supplied with 12 gashes whilst stocks last.
76Hobs for worm gearsFlank formsThe flank form of a worm gear hobis determined by the flank form ofthe driving worm. The various flankforms are standardized in DIN3975, which distinguishes be-tween ZA, ZN, ZI and ZK worms,according to the generating meth-od.■ The ZA worm has a straight-line flank profile in its axialplane. This flank form is op-tained when a trapezoidal turn-ing tool is applied so that itscutting edges are in the axialplane.■ The ZN worm has a straight-line flank profile in its normalplane. This flank form isachieved when a trapezoidalturning tool set at axis height isapplied so that its cutting edgeslie in the plane inclined by themean lead angle and the wormprofile is generated in this set-ting.■ The ZI worm has involuteflanks in its face plane. Thisflank form is produced, for ex-ample, when the worm profile isgenerated by a straight-linedcutting or grinding elementwhose axis is inclined to theworm axis by the mean lead an-gle and to the normal plane onthe worm axis by the pressureangle "α0".■ The ZK worm has a convexflank form in the axial plane.This worm form is generatedwhen a double taper wheeltrued under the pressure angleα0 is inclined into the mean leadangle, where the line of symme-try of the wheel profile passesthrough the intersection of theaxes and generates the wormprofile in this position.Apart from the standardized flankforms, there are special forms, ofwhich the follow flank form is themost used.The above worm profile forms canalso be used in DUPLEX wormdrives. DUPLEX worms have dif-ferent leads on the left- and right-hand flanks. As a result, the tooththicknesses on the worms changecontinuously in the course of thelead, and an axial displacement ofthe worm in relation to the wormgear makes it possible to adjustthe backlash.The specification factors of wormgear hobs are determined essen-tially by the worm gear data.In order to prevent edge bearing ofthe driving worm in the worm gear,the hobs used for producing theworm gears must under no cir-cumstances have a pitch cylinderdiameter that is smaller than thecentre circle diameter of the worm.Owing to the relief machining, thediameter of the hob is reduced byregrinding. The pitch cylinder di-ameters of the worm gear hob inthe new condition must thereforebe greater than those of theworms. This dimension is deter-mined as a function of the module,the centre circle diameter, and thenumber of threads.The outside diameter of a newworm gear hob is thus calculatedas follows:Centre circle diameterof the worm+Pitch circle increase+2 x addendum of the worm+2 x tip clearance
77Processes and designsWorm gear hobs are available in arange of designs. A distinction isdrawn between the followingtypes:Radial methodCylindrical hobs are employed forthis method. The hob enters theworm radially to full tooth depth,and can be displaced tangentiallyby a small distance in order to im-prove the enveloping cut on theflanks. This hobbing method hasthe shortest machining time and isgenerally employed for worm gearhobs with helix angles up to ap-proximately 8°. The cutting edgelength must be at least as long asthe penetration length for theworm gear to be machined. Long-er hobs can of course also beshifted.γmαoαoabαoαodo = ∞dmγmαododmγmBore-type hob with drive slot for radial hobbingturning tool shaper-type cutter turning tool milling cuttergeneratinglineZN-WormZA-WormZI-Worm ZK-Wormgrinding wheelgrinding wheel