1、91 FTM3High Efficiency Gear Hobbingby: G. Ashcroft, Pfauter-Maag Cutting Tools andB. Cluff, American PfauterIIIAmerican Gear Manufacturers AssociationiI| ITECHNICAL PAPERHighEfficiencyGearHobbingG. Ashcroft, Pfauter-Maag Cutting Tools and B. Cluff, American PfauterTheStatementsand opinionscontained
2、hereinare thoseof the authorand shouldnotbe construed asan officialaction oropinion of the American Gear ManufacturersAssociation.ABSTRACT:This paper discusses the design advances of disposable gear cutting tools, specificallythose which have produced thenon-resharpenable WaferTMhob, the application
3、of the tools, andthe benefits derivedfrom applying these tools in gearmanufacturing. The concurrent development of hobbingmachines capable of efficientlyapplying these tool designs isalso detailed.Copyright 1991American Gear Manufacturers Association1500 King Street, Suite 201Alexandria, Virginia_22
4、314October, 1991ISBN: 1-55589-600-6HIGH EFFICIENCY GEAR HOBBING- A New Approach To Hob Design i.e., profile and There is an obvious limit to such ansize, and cutting geometry; i.e., clearance and approach. In fact, in some high productionrake angles, applications, particularly automotive mann-The sm
5、all amount of material in the facture, this limit was reached or exceeded inWafer TM itself allows for the use of premium recent years.tool steels and the absence of the need toresharpen allows for optimizing of coating type In some extreme cases the forces appliedand thickness, to the workpiece wer
6、e greater than can beWhen the convenience factor is taken effectively resisted by the available clampinginto account through the elimination of changes area in the workpiece and the tooling. In otherin size and stroke settings on the shaping ma- cases the feedmarks are too deep for economicalchines
7、and the elimination ofresharpening and finishing by normal methods and the effect inrecoating, the concept becomes attractive, the trochoid and tip areas is unacceptable (seeExperience has proven that such tools Figure 6). ! ! “ , k,m t i ii I m # i i _mFigure 3. Disc Type Shaper Cutter.allow machin
8、es to be run at higher strokingrates, cutting many more pieces pertool change An Approach to Higher Efficiencywith significant reduction in tool change costs Obviously, given the limitations of ap-and downtime (See Table 1 for hob example), plied feed, every possible means to increase thespeed must
9、be considered if appreciable gainsHobbing Limitations in productivity are to be achieved.Until now, improvements in gear hobbing Given that the best available tool mate-productivity have resulted solely from the ap- rials and coatings for a specific application areplication of“ brute force“. Increas
10、ingly heavier, to be used there are two opportunities for themore rigid machines with greater horsepower tool designer to increase productivity. Theseand the application of multiple thread hobs areas are the overall dimensions of the hobwith teeth or segments of heavy cross section (diameter and len
11、gth) and the cutting tooth(inserted blade hobs) to withstand the heavy geometry (particularly in respect to clearanceloads applied from ever-increasing feed rates angles).has been the general direction. It is necessary to understand the limi-In fact, apart from cutting speed in- tations of the tradi
12、tional design approach andcreases resulting from improved tool steels and the compromises required to achieve traditionalcoatings, all machine and tool design develop- concepts of value in this tool.ments havebeeninthe area ofincreased effective Because the hob continuously advances4Figure 4a. Two t
13、hread hob with 12 gashes cutting a Figure 4c. Single thread hob with 24 gashes cutting19 tooth gear. 19 teeth.Figure 4b. Trochoid area of 19 tooth gear cut with a Figure 4d. Trocho/d area of 19 tooth gear cut with atwo thread hob with 12 gashes, single thread hob with 24 gashes.5ACustomerSpindle0 Ad
14、apterClampIIIi_ KeyscrewCustomerBoltFigure 5. Exploded View of a Wafer TM Shaper CutterAssembly.6diameter and reduce productivity._ lilt Rethinking the Basics- _“ liilliii Jillv _ l illllli J Ilil Experience with shaper cutters of the_h IIIIIIIi IIII I throwaway type (Wafer TM) showed clearly that-_
15、 III1ti It = Illl an improvement in cutting geometry could sig-i II I111 nificantly increase the allowable cutting speedit:_lil lill_:_ I_1111 - l lll for a given combination of cutting tool andi_llll _ _ IIII workpiece materials. Further, this change inI:_l,llll _.- tlllF I,IIt“lqII I “ I I I geome
16、try could be readily achieved if the needIlllllll IIIIIIIIIIII IIII to resharpen was eliminated or greatly reduced.I I I t I II I I I I I When this single change in thinking wasIfililii iliiIIIIIIII t111 applied to hob design the results were dramatic.So dramatic in fact that new hobbing machinesFig
17、ure 6. Helical gear tooth topography as “seen“ by a - .had to be designed to take full advantage of theone lead and one involute by the inspection machine.“High“ feed rate hobbing.through the workpiece at a given feed rate once _i i_-,per revolution of the workpiece the time based i Ifeed rate and t
18、he cutting time are determined BDIA. 1.6by the speed of the workpiece rotation, sos DIAThe workpiece rotational speed is the _ .result of the speed (Figure 7) of the hob (and Teeh in Work - constant, e.g. 50.the number of threads in the hob). As the hob SFM - constant, K1RPM is determined by the di_
19、meter of the hob Feed per work revolution - contstant, K2v at a given surface footage speed it follows thatthe desirable, higher hob rpm can only be _:M Hob =-SFMx 12 RPM Hob - SFM x 12achieved via a smaller diameter hob and/or a 3.5 x _ 1.8 xhigher surface footage limitation. RPM Work =RPM Hobx No.
20、 of Hob ThreadsNo. of Teeth in WorkTraditional Approach to Hob DesignFigure 8 shows a typical, traditional, _PMWork- K1 x 12 RPM Work- K1 x 12well balanced, hob design. In this design the 3.5 x _ x 50 1.8 x _ x 5_nll_,h_ ,_ _o _ is held to a mirdmum neces-saryto generate an acceptable profile. The g
21、ash Feed on time base =feed/rev, x RPM workis the minimum required to allow the grinding =K2 x K1 x 12wheel used for the hob profiling operation to 1.8x = x 5oclear the next tooth in the hob and the backoffor relief is the lowest which will give an ac- FigureT. As hob diameter is the only variable i
22、t followsceptable resultant side clearance for cutting, that hobbing time is in inverse prportion to diameter ofThe tooth length is that which will give hob for a given hob travel. In thiscase by a factorofl.8/an acceptable number of resharpenings to the 3.5. A further increase in productivity is ga
23、ined from thebuyer and the “end of life“ tooth thickness is reduction in approach distance for the smaller hob.that which will give an adequate beam strengthto withstand a heavy feed. possibilities.The diameter of the hob is the minimum A New Approach to Hob Designwhich will accommodate all the abov
24、e factors. Through the elimination of the “sharp-Any improvement in the cutting geom- ening amount“ the hob tooth thickness is thatetry or “value“ of the tool will increase the required only to give sufficient beam strength7to withstand the cutting forces at their maxi- required to manufacture the n
25、ew hobs.m!lm,Obviously this reduction in tooth thick- The Throwaway Hobhess gives the designer the opportunity for The final design for the new hob was amore teeth (or gashes) in a given hob diameter, hob with approximately 2“ diameter (50 ram) byThis increase in the m_n_imum number of teeth 7.99 ov
26、erall length (203 ram). This diameter toin the cutter reduces the chip load resulting in length ratio proved adequate to support thea further opportunity to reduce hob tooth cutting forces from veryhigh speed hobbingupthickness and the smaller chips require a to 10 DP (2.5 Module).smaller gash for c
27、learance. The long length was used to give maxi-harpenable/ I / IIit _ llfeFigure 8. Sharpenable Life of a Hob. Figure 9. Non-resharpenable Wafer TM Hob.This reduction in hob tooth thickness mum “shiftability, or pieces per tool, and aand shorter gash to gash width allows the simple shank design eli
28、minated the costly anddesigner to reduce the hob dismeter while rigidity reducing bore.maintaining an adequate number of teeth to A special high speed hobhead was de-generate an acceptable profile. The reduced signed with precision coUet arrangements athob diameter is, of course, the most attractive
29、 drive and outboard ends to greatly simplify toolattribute in regards to increased productivity, changes. Location of the tool in the hobhead isThe result of this rethinking of the basics identical to that used in manufacturing the hob.was a new hob design incorporating a high Working with grinding
30、machine toolnumber of gashes, small diameter and free- manufacturers the engineers csme up with acutting geometry (Figure 9). manufacturing concept which allowed a veryThis design was able to take full advan- high number of teeth to be ground in a singletage of the high speed capabilities of present
31、 pass with tolerances and quality levels whichday tool steels and coatings, far exceed previous capabilities.The departure from traditional concepts The tool is a true throwaway design. Thewas such that not only were new hobbing ma- clearance angles and geometry, while optimizedchines required to fu
32、lly accommodate the new for productivity, are such that a true profile ishobs, but new machine tools and methods were generated only by the original manufacturedcutting edge.8A_! L 5.91 . _ I. 18-_.71 _.49 - , ,750 T.P.F. .*-.98-,Ill ._-Sl- _ -I_1 I-Sl_LL5,IS-laUXC-2B. !._3 DEEP.343 CBORE, .13 DEEP.
33、03 x 60“ CHAMFERFigure 10. Design of a helically gashed, shank type, solid WaferTM Hob for 10 DP workpieces and finer.As the design principles were the result Limltatlons in Applicationof the original work on the Wafer TM shaper The disposableWafer TM hobis, ofcourse,cutter the tool was named the Wa
34、fer TM Hob an attractive solution to new production appli-despite the fact that it does not have disposable cations in relatively fine pitches (DP 10 andblades, finer), but there are obviouslya majorityofgearSee layout and photo in Figure 10. hobbing applications outside these criteriawhich would be
35、nefit from these new approaches.The productivity and quality produced A true throwaway design necessarilybythis hob are outstanding and examples are to requires a relatively low cost. The small di-be foundlater in this paper, ameter and simpleshank designofthe WaferTM9hob achieved their goal.Existin
36、g hobbing machines require, gen-erally, the use of a shell type hob with a preci- _ision ground bore and keyway to fit on an arbor. CONVENr_ONAL“OBCoarserpitchapplicationswouldrequireheavier 16GAS_S.330UFE22SHARPENINGS2 _G. PROT.A._LEshanks and higher torque capacity coUets than / .130CAM_.00“00thos
37、e of the “standard“ Wafer TM hob. /Wafer TM type hobs with larger diameters /and standard bores are available to meet thisrequirement (Figure 111Although there is no theoretical limita-tion on size or pitch of such a hob the higherinitial cost and the speed limitations of themachines on which they m
38、ay be applied placean effective limit on the economy of the true Figure 12a. Conventional hob with 16 gashes, 4“ OD,throwaway hob. 22 resharpenings.At some point the tool cost per piece willincrease to apoint of elimlnation of the economicsgained from productivity.A Small Step Back (/ t _ OPT_.-_ASH
39、HOBThe CAD techniquesand programs de- j / 23 GASHES.150LIFEveloped for the design ofthe Wafer TM hob, when _ j /_/ 10 SHARPENINGS/ “h4:;-_-_ ie BEG. PROT. ANGLEproperly applied, can be used to optimize hob / j- _o0 c_. 4.00oD/ k. TFigure lZb. Opti-GashTM Hob with 23 gashes, 4“ OD,10 resharpenings.Fi
40、gure 11. Bore type non-resharpenable WaferTM Hob.design to achieve the best possible compromisebetween cost of tool, cost of production, qualityof hobbed part and machine capability.Such a hob is generally smaller in diam-eter than a standard hob and has a much highermlmber of gashes. Typical is red
41、uction of 25%in diameter and an increase of 50% in numberof gashes when compared to a traditional solidhob and even more favorable when compared toan inserted segment hob (See Figures 12a-c).A small mlmber of resharpenings isprovided, typically 3 to 8 and it is generallyexpected that such hobs will
42、be recoated after Figure 12c. Opti-Gash TM Hob.10each resharpening, including finishing, the hobs are designed withThe actual decrease in diameter and in- multiple threads. Depending on the steel or- crease in number of gashes depends on the alloy to be machined this means hob spindleapplication. In
43、 extreme cases the number of rpmsbetween500and3000. Whenmultithreadgashes may be increased to the maximum to hobbing workpieces, such as automotive pin-improve the involute with no increase in pro- ions, or power drill armature shafts, with lowductivity to be gained from a decreased diarn- n, lmbers
44、 of teeth worktable rpms can easilyeter. In other cases the reverse logic may be exceed 400.applied. To allow efficient application of Wafer TM ,As this design approach optimizes all the Wafer TM type and Opti-Gash hob designs in afactors of the end use these tools have been modern hobbing system Am
45、erican Pfauter andnamed “Opti-Gash“. Pfauter engineers concurrently designed a highspeed hobbing machine with a hob spindleDelimiting The Application drive range of 500 to 3000 rpm; a worktable rpmAll older, conventional mechanical hob- range up to 500; a 2 second load-unload inte-bing machine desig
46、ns, and some more recent grated automation system (Figure 13); and aCNC designs, limit worktable rotational speed hob spindle drive which would accept solid,to a maximum somewhere between 25 and 50, shank driven 7.677“ (195 ram) long hobs.depending on the model. This severely con- The resulting mach
47、ine design, thestrains the efficient use of Wafer TM, Wafer TM PfauterPE80,wasfurtherenhancedbyasupertype and Opti-Gash hob designs, compact design (Figure 14a cases, eliminate shaving.consequently, tool resharpening costs are The accuracy capability of Wafer TM hobseliminated. For new hobbing cells
48、 the capital also offers the opportunity to optimize hardcost of a hob sharpener is eliminated, shaving by producing extremely good gears in3. Because tools are no longer the green in efficient times with minimal stockresharpenable all faces are Titanium Nitride envelopes for hard shaving.coated wit
49、hout additional recoating charges Some double helical marine gear pinionafter resharpenings, and gear applications can be dromatically im-4. Tools in float are reduced since corn- proved in terms of productivity and efficientpensation in the float volume for sharpening is tool life by the use of Wafer TM and Wafer TM typeno longerrequired, hobs.5. The accuracy of the tool is completelyin the manufacturing ability of the supplier and Exemples of Wafer TM Hobbing Applicationnot dependent on the users ability to resharpen Tables 1-4c
