cs

IntroductionSome years back, most spiral bevel

gear sets were produced as cut, casehardened, and lapped. The case harden-ing process most frequently used wasand is case carburizing, Many largegears were flame hardened, nitrided, orthrough hardened (hardness around 300BHN) using medium carbon alloysteels, such as 4140, to avoid higherdistortions related to the carhurizingand hardening process.

The use of a quench press can con-trol, but not eliminate distortions .. Alapping operation cannot remove run-out, pitch error, profile error, and othererrors caused by heat treatment dis-tortions. It can only improve activetooth profile finish and tooth contactlocation, provided that gears did nothave excessive errors during soft cuttingor high distortions in heat treatment.As a matter of fact, overlapping some-times does more harm than good on abevel set,

Bevel tooth grinding was very limiteddue to cost and size.

Advancement in bevel generators,carbide technology, and many otherfactors allowed introduction of hard36 Gear Tecnnology

Yogi Sharma,Philadelphia Gear Corporation

King of Prussia, PA

cutting inbevel gears. Initially, the pro-cess was limited to special requirementsbecause oflowcarbide tool life, the needfor frequent sharpening, and limited ex-perience. But the picture changed dra-matically after some time. The ex-perience gained and the development ofthe CBN (Cubic Boron Nitride) toolmade bevel gear hard cutting very efiec-tive from cost and quality viewpoints.

As with any PIlOCe5S, hard cutting hasits limitations and problems. A properlycontrolled process, starting from thedesign concept, good bevel generators,hard cutting tools, including sharpeningfixtures, special machines, and trainedwork force, are a must for successfulbevel hard cutting.

This article describes the process andsteps required for spiral bevel hard cut-ting on small batches or in a jobbingatmosphere.

Process DescriptionBevel hard cutting can be defined as

an operation in which gear teeth flanksare finished by removing the stockallowance left during the soft or roughteeth cutting. The process is similar to

gear grinding, with almost all the opera-tions remaining the same, with the ex-ception of tooth grinding, which is re-placed by hard cutting.

A simplified manufacturing processsheet for a hard cut bevel gear will con-tain the following:• Complete machining of gear blanks

for teeth cutting - including variousoperations, such as turning, milling,drilling and tapping, etc.

• Bevel teeth cutting - consisting ofteeth cutting, testing, and any toothcontact development with master ormate, teeth deburring, etc.

• Heat treatment- mostly case car-burizing and hardening (nitriding,flame hardening, and inductionhardening are rarely used for hard cutbevel gears).

• Finish machining - all machiningoperations required before teeth hardcutting, such as turning O.D.ll.o..grinding, special machining, etc.

• Hard cutting - bevel gear arrives athard cutting with most or all opera-tions done. It is important that themating part or master is available fortesting purposes. Normally, the gear(member with higher number of

teeth) is finished first and pinion teethare modified to get correct tooth con-tact along profile and length of tooth.The modification for length of contactis nonnally made by change of radiusof curvature of the cutting blades. Thelocation of tooth contact along thelength is usually controlled bymachine settings. The profile correc-tion or modification can be made bydifferent means depending on thetype ·ofmachine or system used to cutbevel gears, size of gears, and pitch.On fine pitch gears, high pitch linehearing can be obtained by using cut-ter blades wi th modified profiles ..Oncoarse pitch gears, profile modifiea-tion 'can be made using taper shims.Once the tooth contact requirementsare met, the gear teeth are finallychecked for spacing and mounted ona gear checker.

• Final inspection - includes dimen-sional checks, magnaflux, and a-"yother specialrequirements.

Ad.v.antages ,ofHard. Cutting• Higher power transmission by

spiral bevel gears, as both members arecase carburized and hardened andfinished by hard cutting.

• Higher and predictable qualitylevels in gear teeth.

• A surface finish of 16 Rl\1S orbetter.

'. Lower noise level lower internaldynamic forces due to higher geometricaccuracy, and better load distribution.A hard cut bevel set signifkantlyreduces the gear box vibration problemcaused by higher intemal dynamicforces due to poor quality gear teeth,which can cause premature gear boxfailure,

On the other hand the gear grindingoperation is always very sensitive tomany factors, such as rate of material.removal, grinding wheel, coolant, 'etc.Any compromise or loose control cancause surface t.emperingor cracks orboth ..Surface temperingor cracks are

practically unknown problems in hardcutting .

•' Hard cutting is performed on thesame 'type of machines as soft-cutting,making the process much moreeconomical ..Of course, special toolingis required.

• The same person or group ofper-sons are involved in soft and hard cut-t:ing. It has been found that control inroughing operation (soft cutting inbevelgears) is quite important fora successfulfinishing operation. Proper stock allow-ance, tooth depth, tooth contactretc.are necessary for bevel hard cutting.Too much stock. allowance can causeloss of case depth and longer cuttingtimes, while too Iittle stock allowancealso can cause a variety of problems.Hard cutting time cycles can be reducedby making some adiustrnents at softcutting for heat treatment distortions,and it can be done very simply, as bothoperations are performed by the sameperson or group of persons.

• Consistency in bevel hard cuttingcan eliminate the need for matchedbevel sets by careful. planning. Some ofthe requirements for elimination ofmatched sets are as follows:- Manufacturing and storing of casehardened and hard cut master gearandpinion for checking the gears and pi-nions in all future setups.- Optimization of design and cuttingdata so that it does not have to changefor the period unmatched sets arerequired.- Tighter control of critical dimen-sions on gear Ipinion blanks. Hard cutunmatched sets are not only useful. inassemblies, but they can eliminatemany production problems, as eachmember can be processed independentof others. The unmatched set approachmust be used very selectively, as it needscareful planning, customized fixtures,optimized design, cutting summaries,and long term commitment. The un-matched set approach combined withstandardized bevel sets can cut down

delivery times and cost very effectivelyin spiral bevel gear boxes,

PreparationFollowing are some items which

should be considered in detail for suc-cessful and economical bevel hardcutting.

Practical Tooth Design. A balancedtooth geometry is a must fora goodhard cut set. All new tooth geometrymust be reviewed carefully from amanufacturing point of view. In a job-bing or low batch environment, the newtooth geometry should try to use ex-isting tools, as new tool requirementscan cause cost and delivery problems.Even in high batch production wheretools can be designed around gears,poor tooth geometry can cause multipleproblems, such as low too] life, smallerfillet radius, etc.

Gear Blank Design. Fig. 1 to Fig. 4show a bored pinion, a stem spiral bevelpinion, iii solid gear, and a ring typegear. As shown in F.ig. 2, the bevel pin-ion can be indicated in both planes bymeans of an extra extension in front ofthe teeth. Both gears show prool bandsfor indicating the gear blank at hard cut-ting. Special attention must be paid inblank design so that revalidated proofsurfaces at final machining can be usedfor indication purposes at hard cutting.In high batch production, blanks, tru-ing, and use of proof surfaces are not re-quiredbecause of special customizedfixtures. Still, it is good practice to createproof bands at final machi,ning for in-spection or assembly purposes. In the

AUfHOR:

YOCI SHARMA is employed in gearmanufacturing .and design at PhiladelphiaGetlr Corp. He holds an M.S. degr:ee ,inmechanical engineering from ViIlanovaUniversity and one in jndustrial.engineeringfrom Perm State. Mr. 5hatmais a .licensedmechanical engineer .in tne state .of Penn-syivaniJ:! and' a-senior member of SME.

November/December 1990 37

-

Fig. 2

38 'GeoT Technology

, r CC 1. H

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November/December 1,990 39'

case of a bored pinion/gear, thesubassembly with shaft should be donewherever possible to avoid concentric-ity and squareness problems (Fig. 5).Fig. 6 shows the subassembly of a ringgear and spider. In pinions, the one-piece design does offer some manufac-turing advantages over the two-piecedesign. Also, small to medium size gearsshould be investigated for one-piecedesign, as there are some manufacturingadvantages in the one-piececonfiguration.

Previously, gears were mostly de-signed as bored rings so that they couldbe die quenched, but hard cutting haschanged the picture somewhat. Distor-tions in free quenching of small tomedium size solid gears can. be kept lowwith proper control. Hard cutting willremove the distortion from teeth. Cau-tion must be used for any switch ove.rfrom two-piece design to one-piece.There are many factors which affectdistortions in heat treatment, such asmaterial blank configuration, rixturingin heat treatment, etc. Vl/herever possi-ble, some experimental pieces should bemanufactured to evaluate the situationbefore making the final decision, as ex-cessive distortion is highly undesirableand can cause various problems at hardcutting.40 Gear lechnoloov

Soft and Hard Cutting Summaries. Atooth cutting summary for a beveJgenerator provides machine set up data,cutter data, tooth measurements, andother tooth geometry informationrelated to a specific spiral bevel set.Bevel hard cutting blades are very sen-sitive to proper feed and speed. Selec-tion of correct amount of crowningstock allowance affects the overallqual.ity and cutting times. Normally, inhigh batch production, a sample set ordummy set is always useful to fine tunethe summary ..In a jobbing or low batchatmosphere, a dummy set or experi-mental set mayor may not be possiblefor various reasons; therefore, a jobbingatmosphere requires initial selection ofall variables affecting tooth contact.The length and location of tooth contactshould be based on factors, such asassembly conditions (overhung orstraddle mounted members), tooth stif-feners, and loading, application,andpast experience. Wherever possible, finetuning of tooth contact can be achievedthrough load testing of bevel gears.

Heat Treatment Process ControL Ex-cessive heattreatment distortions arehighly undesirable for hard cutting ofbevel gears for a variety of reasons, suchas loss of case thickness, longer cuttingtimes, tooth thickness problems, and

mounting distance problems. Ringgears must be die quenched' utilizingproper quenching dies. Fig. 7 shows a.quench press. A pre-quench and temperoperation of rough-turned blanks forlarge gears and pinions can assist instabilizing the pieces during heat treat-ment. In the jobbing atmosphere, gearsand pinions should be checked. fordistortion belore releasing for finalmachining.

Finish Machining Operation. Thefinish machining of gears and pinionsfor hard cut bevel. gears is very im.por-tant. The runout and squal'eness and aliother geometric tolerances of beari:ngdiameters, proof diameters, etc. must bekept to less than 40 % of the valuesallowed on the gear teeth. Any loosecontrol in finish machining operationscan add to distortion, leading 'to longercutting cycles and various problems. Itis vita] that proof surfaces in both p1anesare indicated within specified valuesbefore proceedingwi.th final machining.The big difference in the fina1 machin-ing of a lapped and a hard cut gear isthat, in the event of a lapped set, theback face or mounting face is usuallyleft as it is, while in hard cutting, it isground or turned square to bore. In ad-dition, it is critical 'that som extramaterial is left in the back of a hard cut

.fbI' Y01ur tougllestg ~.._~rcutt.--ng Jobs,

,tile lIardest ,ste,el JS,the easl ~'.-t clJQ -'ce

For hobs, shaper cutters and other gear cuttingtools. that are more than a cut above the rest, specifyGrucible CPM REX~ 76. With 33% total alloy con-tent and an attainable hardness ot HiRC68-70, thishliglh speed steel provides. the highest availableco-mbination of red hardiness, wear resistance andtoughness, either coated or uncoated,

A 9ig- Three US. auto maker mcenUy realized a300% -improvement in gear cutting tooll life byswitchingl from 1M3 HSS to CPM REX 716. Withgr,ealer too~lllifeand excellent grindability, CPM REX76 means less downtime because resharpening iseasier and less frequent -

Ill/iii?!_____ 11

• I•• CPMREX'76CPM REX 76 is just one of 10 high speer! steels

produced by the Crucible Particle Metallurgy pro-cess. With the industry's widest material selection,Crucible can meet your specific needs at any pro-ductivity level. You can selectively upgrad:e to thebest CPM material for the right eppllcaton.

On your next order, specify a high speed steelthat's hard to beat ... CPM REX. 76 or another rnem-ber of the CPM REX family. To learn mare, contactyour nearest Grucible service center or call to'll free:

11..800~PAR;"XC;EL(1~800-7.27-9235)

A Division of Crucible Matenals Corporation

CiReLli A-19 ON REAIDERIREPlY CARD

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42 Gearlechnology

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gear at blank turning, which will thenbe removed at final machining .. Thiswill reduce the variations in mountingdistances .. In multiple pieces, consis-tency of the crown to back dimension isimportant for hard cutting cycles,Variation will cause adjustments forevery piece on 'the machine, resulting inlonger cutting cycles. Proper proofbands and surfaces must be created atfinal machining, with concern given totheir location as well, so that they can beeasitly used for indication purposes athard cutting ..

Soft Cutting Blades. Fig. 8 depicts ablade with protuberance.Where everpossible, protuberance blades should beused ..They will cut down hard cuttingtime, extend too] life, and reduce thepossibility of steps in fillet. The use ofcustomized protuberance blades is noproblem in high batch production, butin jobbing or low batch production thepicture is quite opposite. Amount andheight of protuberance selection be-comes difficult as blades are stretched tocover maximum range. Blades can stillbe obtained with protuberance selectedvery carefully, which 'Will help hard cut-ting. Setting middle blades slightlydeeper than side blades also helps inhard cutting and is a common practicein bevel cutting. Fig. 9 shows a filletcreated witha deeper middle blade. Fig.10 shows a blade for soft cutting.

Hard Cutting Blades. Bevel hard cut-ting began with the use of carbideinserts. Carbide inserts are damped tosteel bodies for coarse pitches andbrazed for medium to fine pitch gears.In the beginning, low tool life with car-bide inserts, frequent tool sharpening,insufficient experience, high cost. andmany other Factorskept hard cutting'suse quite low. As hard cut bevel gearsreached into the field, their performancein all aspects was the single most reasonto increase the use ofbevell hard cutting.

In the meantime, CBN inserts wereintroduced for hard cutting, The BZN(BZN is a trade mark of General ElectricCompany) compact blank isa combin-ation of a layer of Borozon CBN (Cubic

'Gear Tooll S;peela:lIsls3601 WEST T'OUHY AVENUE

UNC'OLNWOOD" 11'lILlNOIS 606451'08-6,75-2100

1-·800k628-2220 In IlL '·8'00'-628·2221

GEAR TRAINING PROGRAM1. BASIC FUNDAMENTALS

1. a..r HImIryA. Cyefotdal TeethB. Involute TeethC. Gear Cutting MachinesD. Gear Cutting Tools

2. a..rTypeeA. Par811e1AxisB. Intersecting AxisC. Skew Axis

3. OHr Aattoa4. Involut. Gear o.om.try

A. NomenclaturaB. Involutomelry - Contract Ratio, etc,C. Helical Gearw - Lead - Helical Overlap

5. Gear Tooth BystemeA. Full DepthB. Full AlletC. Stub Depth

I. a.n.r.I Formu_7. M8them8tIca - (I.T.W. Trig Book)

2. HI SPEED STEELSA. Common TypesB. Special TypesC. Heat Treatment - M8IaIlurgyD. ControlsE. Sur1lK:e TreatmentsF.Special C_

3. CUTTING THE GEAR1. FonnIng

A. MillingB. BroachingC. Shear Cutting

2. o....,.ungA.She

~lCircu'!'~ypec) Machine Types and Manufacturers

d) Schematic - Principlesa) Speeds - FeedsII Machine Cutting Conditions

B. Hobbinga! The Mobbing Machinab Types and Manufacturersc Schematic - Differential and

Non-Differentialdl Sl*KIs - Feedsa Climb CuI - Conventional CUiII Shifting - Types

3. The Hob ... Cutting ToolA. How It CutsB. Tolerances and ClassesC. Multiple ThreadsD. Hob Sharpening and ControlE. The Effect of Hob and Mounting Errors

on lhe Gear4. The Shaper CUtt..... a Cutting Tool

A. Know Your Shape! CuttersB. Design LlmltallonsC. SharpeningD. The Effect of Cutter Mounting and

Errors on the GearE. Manufacturing Methods

5. Tool Tolet'ance Va. GHr Toter.nc:eA. Machining TolerancesB. Gear Blank Accuracy and Design

Umitations

... FINISHING THE GEAR1, Gear F1nl8tMng - BatON Hardening

A. ShaVInga) The Shaving Cutterb) Types of Shaving - Conventional.

Underpass. Diagonalc! Crown ShaVingd Shaving Cutler Modificationse Co-ordinatlng Tool Design-

The Shaver and Pre-Shave ToolII Re-Sharpeningg) Machines

B. Rolling2. a..r F1rHehlngaft ... HMt: TfNl

A. HoningB. LappingC. Grinding

a) Methods - Formed Wheel·Generating - Threaded Wheel

b) Machine Typas

5. GEAR INSPECTION1. Functional

A. Gear RollersB. Gear Charters

a) Reading the Chartb) Tooth-to-Tooth Composite Errorc) Total Composite Error

C. Master Gears~!=~~i"c Special Types2.~1

A. Size - Tooth ThlcknenB. RunoulC. SpacIngD. LeadE. Involute

3. Automatic: and SamI-AutCMMtk:A. How They WorkB. What Can Be CheckedC. How Fast

4. ChIIr1I~ - An8IytIcIII and FunctionalA. Reading the ChartaB. Which Errors Affect Other ElementsC. How to COI'rectthe Error the Chart Shows

6. INDIVIDUAL INSTRUCTIONAND SPECIFIC PROBLEMSOR PLANT TOURThis Program is IleKlble and chenged 10 meet !heneeds of each group.

The 4-day program consIIts of a coordinated aeries of lectures given by the Engineering,Production and lnapection ata1'l8 of Illinois Tool Works Inc. They represent ovar sixty years ofexperience In gear and tOOlspecialization.

The .... ion. are conducted with a technique and approach that leads to Interningdiscussion and participation. Staning with the basic fundamentale, the program I. directedinto those areas of g.... test interest and value to the people attending the particular session.The groups are kept .maIllO that this object is readily accomplished.

All mentioned, the planned program IU18 tour days. One-half of the fourth day I. for In-dividual dlscunion of apeclflc problems In a detailed manner with members of the illinoisTOOlWorks' ataff.

More than 4,000 Individual. from hundreds of companies repreeenting manufacturing,engineering, Inspection and management, have come to Chicago for these programs. Theyhaw been conducted on a monthly basis .Inea 1958. CI.... have also been conducted InEurope. We are c.rtaln that this well rounded program has helped all of them to a better Joband a110 given them a better underltandlng of engineering, manufacturing and Inspection.

All thole attending are aaaIgned to the same hotel. This promote. friendly contact anddiacuaalon of mutual problems and Interesta. Tuition for the course Includes transportationfrom the hot.' to ITW and back, one group dlnn.r, ali continental breakfasts and all lunche •.

We hope we may Include your company in one of our Training Program •.CIRCLE A-20 ON READER REPlY CARD

ITW ILLINOIS TOOLSA Division of IIIInoI8 Toot8 Wofka Inc.

1991 GEAR TRAINING PROGRAMFour Day Seminara

JanuaryFebrualyMarch .Aprj ""

''''7"."11·"15-1e

MayJuneJulyAugua1

13-1e10-1315-1812·15

,e-li'417"·:4

2·5

TUITION FEE: II1II8.00

NOTEI Additional siudenla, aame CCIInPMy, __ ct.. teeo.OO

Includes ,he ,r8l18pO!1abon from !he hotei to ITW and bKk. one group dIM«.hoepI!aI1\y meeting. conllnantai br~. and aIIl~

Ths 5ocI8Iy 01 Manufaclurlflg Englneera ISM E.) has IIPP"\MId m,. School lorPr01888iona1 Cred<18 10Ward !he S.M E. Recer\IIIca1oo Program Creda are ......m.d

on !he baN 01 ont creel' per c:ontenl hOurFor addII#oMI ~ aonIICt;

ROBERT H. MOOiAOWManager. TI'1Ii'*'II708-675-2100

l.aoo.e21-2Z!OIn IL CIlI: l.aoo.e21-2221

Fig. 10 - Courtesy of Klingelnberg Corp.

Boron Nitride) and a cemented tungstencarbide substrate produced as anin-tegral blank using an advanced highpressure, high temperature process. Theuse of CBN inserts increased the tool lifemany times. Cutting times decrease aJFpreciably as less sharpening isrequ.ired.From a performance standpoint, 'theCBN mserts excelled considerably overtheecarbide inserts in every asped , eventhough the price of the CBN insert is. ':1;.. k;~k . hh . f th .bid .mUUI rugner t...<ill that 0 iecar 1 te1Il~sert, tool price per gear did not risebecause of improved tool life.

The CBNtools need. much morecareful sharpening, including the use ofspeciaJ machines, special fixtures, anddiamond grinding wheels. The bladesare sharpened on the cutting face. Extracare should be taken in storing, hand-ling, and the use of CBN inserts. Thecorrect feed, speed, and depth of cut isalso very critical in the usage of CBNinserts.

LimJtation of Bevel Hard. Cutting• Machine Accuracy. Bevel hard cut-

Hog and soft or rough teeth ,cutting areperfonned on the same or similar typesof machines. The accuracy of the bevelgenerator .reflects directly on rhe qual-ity of gear teeth. Where more than onesimilar machine is available, it may bebeneficial in the long run to use the bet-44 Gear Technology

ter machine for hard cutting all the time.• Single Indexing Versus Continuous

Indexing System ..Bevelgenerators withsingle indexing directly affect toothspacing; whereas, continuous indexingoffers significant accuracy:in tooth spac-ingdue to the natural hunting actionbetween gear teeth and cutter blades.

• Multi-Start Cutter System. In rut-ting systems which utilize multi-startcutter heads, attention must be paid, sothat the number of teeth is not divisibleby the number of starts in the cutterhead . Otherwise, the cutter head bladespacing can aHect tooth spacing in hardcutting. Inhigh batch production, thisis never aconstrainr, as special cutterheads can. be obtained for a set, but itcan become a limiting factor in. jobbing,where the exact ratio is a necessity. Amulti-start cutter head can also be usedas a single start cutter head, which willeliminate this problem; however, cut-ting times will increase.

• Fillet Finishing. in bevel grinding,the tooth fillets are completely ground,and the desired fiUet radius is obtainedby dressingthe grinding wheel. In. thebevel hard cutting process, the filletsarenormally finished at soft cutting. Some-times step problems may appear at hardcutting due to low or no protuberance,higher distortions, etc ..The hard cuttingblades are manufactured with a certain

blades are manufactnred with a certainfillet radius, which should closely matchwith the roughing blade radius. Duringhard cuttiflg, the fillet comers can beslighHy skimmed with hard cuttingblades, but any excessive fillet materialremoval compromises the tool life.

SummaryThe hard cut bevel gears have per-

formed very well in all applications.Hard cutting has improved load carry-ing capacity and has made quality bevel.gears available Ito the industry atjustifiable costs.

The improvement in large gears iseven more noticeable as tooth grindingwas not possible because of size. The in-troduction of CBN tooling has broughtthe toolingcest and cutting times toreasonable and economical values. Fur-thermore, consistency of hard cuttinghas made unmatched sets a realityunder a controlled atmosphere and forcertain applications. Hard cutting alsoprovides. gear teeth with the quality andsurface finish of grinding without anypossibility of any metallurgical damage .FinaJ]y, the utilization of CBN tools forhard cutting is being applied more fre-quently to parallel axis gears usingshaper type machines.

Acknowledge:me:nl: Reprinted with permissionof the American Gear Milnufllc!u rers AssociR-tion. The opinions, statementsand conclusionspresentedin this pt:lper t:lre those of the A utharand in no wily represent the position or opinionof the AMERICAN GEAR MANUFAC-TURERS ASSOClA TrON,

References:

1. TRAPP. 'Quality Achievements 111Spiral Bevel Gears By The HPG-Method", AGMA Semi-AnnualMeeting, OctO~I, 1981.

2, lOFFELMANN, E. "Hard FinishingOf Spiral Blroel and' Hypoid GearsUsing BZN Cutting Toots", AGMAMeeting Fall, 1985.

3. WIEGAND. R. "Finish MachiningC~e-Hardened Spiral Bevel Gears ToThe Klingelflberg HPG-S Method,"