CNC Gear Grinding MethodsWhich technique is best for your .application?

m rinding in one form or anotherhas been used for mere than 50years to correct distortions ingears caused by the high temper-

atures and quenching technique associ-ated with hardening. Grinding improvesthe lead. involute and spacing character-istics. This makes Ihe gear capable ofcarrying the high leads ami running atthe high. p.itch line velocities required bytoday's most demanding applications.Gears that must meet or exceed theaccuracy requirements specified byAGMAQuality IO-IL ornINClass6-7must be ground or hard finished afterheattreatment.

Few manufacturers have been uc-cessful at developing a machine that iswithout some compromise. As a result ofthe enormous challenge of trying to

design one machine that fits aU needs, thevarious manufacturersapparentty havedecided that specializing their products totarget specific market niches is more effi-cient; therefore, over the past threedecades, machine tool manufacturershave engineered their gear grindersspecifically to be very efficient grindingcertain Iypes of parts. The remainder of!:his article is a.survey of the various typesof grinding processes available today.

Single Index Form GrindersAlthough most of today's modem

machine tools are quite good at grindingexternal spnrand helical gears, very fewmanufacturers have been successful atincorporating the ability to handle inter-nal gears without major compromises.Today, one of the only ways to efficient-ly grind intemalgears is the single-indexmethod with a dressable vitrified wheelor abonded CBN-plated wheel. A platedwheel maintains its form well; however.

its high purchase price and dedication toa single part result in a high perishableco t. A vitrified dressable wheel offers

Dennis Ri'chmondl

greater flexibility and is much lessexpensive, but is more susceptible toprocess variables that can lead to geargeometry errors.

Single indexform grinding i the easiestgear grinding process to understand, ]t is a."what you see is what you get" process.The form required en the workpiece isidentical to the form or shape put into thegrinding wheel. Gear teeth, or more accu-rately, gear space is ground one tooth at atime, The gear is indexed to correspond tothe number of teeth 0.11 the workpiece. Inmachines under 400 mm capacity. thepart to be ground is normally mountedbetween centers in a horizontal space.These machines have tables that recipro-cate under a stationary wheel spindle. Thewheel spindle is mounted to a verticalcolumn supported by a feed device,which raises and lowers the whee] spindleto realize the center distance and facilitatewheel dressing (see Fig. I),

Older machines use templates and acopy device totransfer the form onto avitrified wheel Newer machines useeither steel. body plated wheels thatinclude the desired form or dressablewheels that are profiled through themachine control.

This process derives its appeal. pri-marily from its simplicity and flexibility.Wheels can easily be made from alu-minum oxide for either internal forms orexternal shapes (Fig,. 2).

The operator has little to do with. puttingthe form onto the wheel; therefore, theprocess demands fewer operator skills. Forthe past decade, machine builders haveoffered grinders that use superabrasives,such as cubic boron nitride (CBN), whichare plated directly to a steel wheel body ormade as a vitrified bonded wheel that canbe dressed to the desired form. The user canexpect to achieve profile tolerances of.0002" or less. Nickel. is the bonding agent

that holds the CBN crystal IOlhe steelwheel (Fig. 3).

The crystal is sifted to the desired sizeprior to the plating process so that a finallapping operation is nOI. required. Thisallows the crystal to remain sharp andextends its useful life, permitting moreparts to be ground between rcplatingoperations. To further optimize wheellife. roughing wheels and additionalmachine axes are used, A machine 0·011-

figured to mount more than one wheelcan rough and finish the gear in the arnepass. resulting in 8. higher productivityand extended wheel lite. A high-volume!high-pressure coolant delivery y tempermits the removal of large amounts ofstock while reducing or minimizing therisk of burning or cracking from exees-sive heat during grinding ..

IFig.2

MAY/JUNE 1&87 43

_-------------1 GEAIRFUNiDAMENTAlS _

Fig~,3

ArmWssnowmotlnn for grindingboth lla nks at thagame lime.

Fig'. !5 IFig.6

Rolling Block

fig. 7

Generating;Motion

\\61

K Grinding Me.rIlad

/\xis I feed and additional'rotation for helical gearsCrossbeam set to basehelix angle

Fig. 8

44 GE.o,R TECHtoIOLOGY

I

{$-Modifications to the lead such as

crowning, taper and end relief are impleinputs to the control. Cams and templatesale a thing of the past for the newer breedof machine. J.lllllany cases, internal gearsor forms can be ground on the samemachine withlhe addition of a suitablegrinding ann.

Forms such as vane pump, male andfemale compressor rotors, gerotors,worm ball screw tracks, root-type rotarypistons, gear hobs and constant velocityjoints-any precise form that is evenlyindexable--can be ground on a singleindex. form grinder.

These machines are wen-suited forsmall to medium gears. and non-gearparts. They are capable of achieving theaccuracies required for applications sucha aircraft and aerospace gears, machinetools, screw compressor rotors, ballscrew tracks, gerotors, internal gears,vane pumps, worms, constant velocityjoints and automobile transmissions.

Single Wheel Generating GrindersSeveral different styles of gear grind-

ing machines use the generatingmethod. As with other types of machin-ing, the style sometimes takes on theDame of the original manufacturer whodeveloped the method. The Hofler orNiles method is sometimes referred to as"a single tooth rack process" method. Itcan best be described as one in which arack Or single ribbed wheel strokes par-allel to the axis of the workpiece whileat the same lime traversing tangentiallyto the workpiece (Fig. 4).

In addition to these two motions, theworkpiece rotates ro simulate the rolling act-ion required to generate the profile (Fig. 5).

____________ GEARIFUNDAiMENTALS _

The angle on the double-taperedwheel. corresponds to the pressure angleof the gear. Tile generating path, or dis-tance the wheel must. jravel, can bereduced by optimizing the wheel pres-sure ang1e, leading to shorter grindingtimes. Wheels call be made from eithervitrified CBN or aluminum oxide.Automatic wheel balancing on the spin-dle has been incorporated into the newermachines, allowing them to achievehigher quality and better surface finish.

The latest grinders are capable ofgrinding both flanks in the same motion,resulting in a productivity advantagewhen double-flank grinding (Fig. 6).

During the grinding cycle infeed pass,the left andright flank of one tooth spaceis machined before the gear is indexed tothe next tooth. The number of workpiecerevolutions, typically two to four, will bedetermined by thegrinding allowance onthe flank of the gear.

Modifications to the lead and profileare easily realized by imple menu inputsto the control. Different modifications onleft and right flanks are also possiblewhen using the double-flank technique.

These machines are ideally suited formedium to large gears. Tile single wheelgenerating grinder are capable ofachieving accuracies required for a vari-ety of demanding applications such aslarge rugh speed marinetransmissions,submarine drives, stationary turbines forpower generation, machine tools, windturbine generators, steel mills and largediesel engines ..

Doub'le Wheel 'GeneratingThis generating teci'mique use a pair of

saucer-shaped wheels. Vertical andhori-zontal machines are builr and dedicated togrindinggea:rs and pinions respectively.The horizontal machines. are equipped withdevices to' allow for lead modifications forpinions, while ·lhe vertical machinesgener-ally have no provisions. for modifications,as they areprimaJily used for grindinglarge gears (Fig. 7).

Aluminum oxide wheels dressed w:ithsingle point diamonds are the wheels ofchoice for this dry grinding process. Thewheels on these machines act as astraight-sided rack that rolls past the gearduring the grinding process. This motion

WHERE CANYOU FIND ...

generates the involute profile. The axesof the wheels call be set parallel to eachother or at a 10D angle (fig. 8).

The workpiece is reciprocated or"rocked" in the axial direction to providethe infeed motion as two flanks of differentteeth are ground :in one pass. At the end ofthe pass,the entire gear is indexed usingmechanical, index plates having the exactnumber of notches or a multiple of theteeth in the gear. Prime numbered gearsrequire dedicated index plates. The depth

of cut is determined by the infeed of thetwo grinding wheels toward each other.

When grinding spur gears, \:he axes ofthe grinding wheels are perpendicular tothe axis of the gear. A simple motion tosimulate the rolling of the gear on the rackrepresented by the wheels generate theinvolute. Steel bands attached to a station-ary support on one end and a rolling blockat the other produce the generatingmotion. The rolling block is generalJy thearne diameter asthe base circle of the

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_------------GEAR,FUNDAMENTALS------------gear, however, the: newer style machineincorporates the use of a variable diame-ter rolfulg block. Whe.1l grindi.ng a helicalgear, an additional motion must beimparted tollle workpiece to compensateIor the :helix angle. A helical guide mech-anism is used teacccmplish this motion.

The contact point between the wheelsand dte tooth flanks is confined to a verysmal] area at any given time, Thisexplains why the productivity of theprocess is described in hours per gearrather than gears per hour. "Tune COD-

suming" and "slow" also describe thisvery accurate process. H productivity isthe sbortcoming of this process, its abili-

ty to gri~d point by point lead and profilemodifications is a strength,

Tile Maag is one of tile onlymachines ever built to incorporate thepoint-by-point system for lead and invo-lute modifications (Fig. '9) .. It has beensaid tbat the primary purpose for thede. ign of Ibis machine was to support themanufacture of Maag's own high-speedgearbox and transmission business andnot necessarily as a commercial product.To urnmarize.jhe trade-off in produc-tivity must be weighed against theadvantage of flexibility.

These machines are best suited for smallto med:i;u~ lot size gears tbat require higl1Iymodified lootlil forms. They are I.ISed in thefollowing applications: gear rolling dies,having cutter sharpening, teel joolicg,

large marine transmissions. machine toolsand bigh speed gearboxes.

Form GeneratingFonn generating is a unique process

offered by a single manufacturer for highproduction grinding of helical automotive

traasmis ion gears. This process (see Fig.to} was first introduced to the market inthe mid 1980s. It i .ideal. for large volume.highly automated manufacturing environ-ments. Becan e the feed motion for stockremoval is rotary and Dotaxial, cycle limecan generally be stated as one second pertooth, regardless oflhe gear face width,Simply put, the wheeland the workpiecedonet traverse past ollie anomer during the

The aluminum oxide wheel has agloboid shape when viewed from theend (Fig .. U). Thi bape is dressed intothe wheel with a diamond-plated gearwith the de ired profile and lead char-acteristic required by the product gear.The wheel is po itioned above thedressing gear 0 that its axis is perpen-dicular to the lead or helix angle of thedressing gear. It is then plunged into thedressing gear and aUowed to rotateintwo direction , re ultil'lg in alhinningof the thread on tile wheel. The thinningof the wheel is necessary so that rotary

Fig. 10

I1g. 111

46 G~AR T,ECHNOLOG,Y

A

_------------'GEARlfUNIDAMENTALS------------_feed can take place during the actualgrinding. In this process, each sid ofthe gear is ground separately.

The grinding sequence can bedescribed as follows:

L The workpiece i transferred! intothe machine with an auto loader, and aprobe determines, Ithe optimum orienta-tion considering the runout and lockcond.itions (Fig. W2a),.

2. Rotary infeed begins,and the leftflank is roughed and finished andallowed to spark: out (Fig. 12b).

3. The workpiece is then ,electronical-ly advanced so that the riglu flank comeinto contact with the wheel (Fig. 12c).

4. Rotary feed begins. and the right

flank its roughed, finished and allowed tospark: out.

5. The workpiece is removed from themachine, andlh next cycle can begin,

The profiling sequence can bedescribed as follows (Fig. B):

L In the case of a shaft-type work-piece. the dressing gear is automaticallyloaded into the machine between centers.

2. The grinding wheel is plunged intothe dressing gear.

3. Rotary feed is applied to the dress-ing gear. and the left and right flanks aresequentially dressed, allowing for a cer-tainamount of dwell for born flanks.

4. The dressing gear is removed. and15-311 gears, are ground before the nextdressing operation is neee sary.

Whe.n. grinding bore-type work-pieces, the dressing gear is pan of thework spindle and (.ooling and is mount-ed on the machine at aU times. Forshaft gears, the dressing gear is resi-dent in the machine, but must be pre-sented to the wheel in 3. separatesequence. This allows for slightlyshorter dressing times.

Thi process i weD su:ited to themanufacture of large-volume. automo-tive helical gears with anneal quanti tie .of .approximately 20011)()()gears per yearof the same type.

Continuous GeneratingThis process is sometimes referred to

as threaded wheel grinding and is by farthe most wide-spread in II e today. Thistechnique has kinematics similar tothose in bobbing (Fig. [4).

A threaded wheel similar to a gearhob without gashes is used as a cuttingtool. The relationship of the workpieceand the wheel is described as follows:

Wheel [pm x no. of starts.no ..of teeth

When grinding heli.cal gears, the leadmust be compensated for as :it is in bob-bing. through a mechanical or electronicdifferentie]. This differential impart anadditional rotary motion to the work-

allowing the wheel to traverse

along the lead or helix angle of the part.The similarities do not Slap here, Thetool or grinding wheel is, ensitive to thesame constraints that affect a hob. As[ong as the diametral pitch or pre ureangle are not changed,the ' ame wheelcan be used to grind a variety of work-pieces, regardles . of Ithe hellix angle ornumber of teeth. This i a majol cliffer-ence from the single index teehnique,which is affected by both helix angle andvarying the number ofteeth,I ' .

I

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AiD MIi' iC Corp.AiMrican 1TrNlill Co.AmlriCIIII ,.._. LP.lIItcNaIIhd PrKisiDil Inc.lIotmIalocll M.clli .. Toal Co.Chicago Gllr· D.O.........CoIHial Tool G,..p Inc.DUnI-BarFellows Corpot'etionTh. Glel.on Wort.HolroydHydra Honing Laboratorie. lae,'""ai America Corp.Induetoh ... •nw H.. rttlnd •• ...,.., AlMric-. LP.LeColud 11Ie.•UebhlrrMlinllfac:tared Gllr' GageMechanical' StntctInI lIni,•• SoftwIn •Midwest GI.. & Tool. Inc.

lIilkrll E.,inuringlI_b_i Machinl ToollMooN Prodllcts Co.NnDIIII BrHCh & Mlchinl Co.N.,. GearCorp.PI"er hldllltri ... Inc.PII1'Y Technology,.. ... ,.1111, Cutting TooII, LP.Pro-GI.' CompanyProc ... G.n Co•• Inc.Profile Engineering. Inc.R.illla.er CorporationRoto- Tlchnology. Inc. •llulall. Holbrook' HlndenonSofIwlnl E.. inuring ServiciStareld hln, Inc.STO PrKilioll Glar • IIIIInIIIIIId •SytIC Corporation •Ty Miln, 11Ie.•Ullinral Technical SpI ...... Inc. •V•• GerpH • R.ac. EqillHrilll

iii A V/ J U N E I g g 7 47

_------------GiEARFUNDAMENTALS------------Method

- - -

CNC Gear Grlindling Techniques

Disadvllntag81

SingIe lndexform Grinders

Manufacturer

I(B·PPNational Broael!OkamotoKlingelnberg

Advllntages

• Ac curate control o~form.• Ability to grind finer pitches from tile solid.• No wheel dressing required.

I • Wheel can be stripped and replated., • Ability to grind roots and tips., • Ability to iJrtegrataon·machi ne,inspa clion.

• High stock removal rates,• Ability to grind noninvolute shapes .•' Grinds internal g;ears.

'. Dedicated steell wheel needed for each pitch.pressure angls' end teeth number c'ombination.

• form only correct once part is 'on size.o Steal wheels cannot be modified.• Expensive initial capital investment• Potentially large inventory of wheels.• Horizontal wo rkpiece ,axis.

Single WheelGenerating

Hofler, Niles

MeDg method

• Low perishable cost--wheelsand diamond dressing tools .•' Same wheel can be used for different pitches. -• High achievable accuracy.• Fast setup.• ~bility to integrate, on-machine inspection equipment.• Coarse and fine pitches possible.• Vertical workpiece axis.• High stock removal per pass.• Machines available for !Iarge gears up to 15,7".

• Fast process, but not considered nigh production.• High initial capital costs,.' large floor space required.• Specialloundations needed: for some,applications.• Cannot grind roots or ODs.

Double WheelGenerating:

form Generating IReishauer TYlle RZf

• Very accurate.• Long machine tool tife,• Can be inspected: prior to complete grinding of gear.'. Dry process makes washing unnecessary for inspection.'. POint-by-point grinding reduces the tendency to burn.• Ideal for low volume production.• 'Low tooling costs.

• Cycle time quite long.• Expensive capital ,investment• In g,ears.with a lorg.e number of teetil a jump

between the first and lasttooth ground maybe experienced .

'. Very short cvcle times.'. low tooling costs,'. Dedicated: tooling-no development time needed.'. Stable process with few variables.

i • Process suitable ,onlV'for helical gears., • Workpiec,e diameters up to 200 mm.I' Large ,capital invBstment

ContinuousGenerating

I !.iellherrI OkamotoI Gleason

Reishauer

, • High productivity..' Good precess stability..' Very low perisha bl'a costs..' lead and profile modifications made easily .•' Easily automated process.

Fig. 13

,48 Ge",R TECHNO~OGV

'. Unable rogrind close to should'ers.• No intemal Qear ,capability.• large initial investment,

AII··U

_------------GEARfUNDAMENTAL-SII. •Theeontinaousgeneratien technique

is aptly named. The threaded wheel has arack or traight-sided shape with the sideangle ecrresponding tojhe pre sureangle of the gear. This rack is in continu-ous contact with the gear during thegrinding operation while the involutegeneration i .occurring (Pig, 15).

In order to be able to accomplish thisprecess, the wheel, unlike the hob, mustrun at speeds of 45 mls or 8,800 sfm, Theindexing system must have mechanicaland electronic integrity many times thatof it roughing brother, the hobber, Tothink that one could simply modify amodem bobber and tum it into a grinderis. however, to misunderstand the kine-matics required.

In the pasl, complicated mechanicalsystems provided !he movements requiredfor index and differential motions oflhemachines. Today commercially availableelectronic gearboxes are used for thesemachine motions. Iln addition to the rela-tively JmpJe motions described here,thenewest machines incorporate "shift grind-ing," which is akin to creep feed grindingin the cylindrical grinding world. Theworkpiece is shifted tangentially as it is fedpast the grinding wheel (Fig. 16).

This jaagential movement is constantlyexposing the workpiece toa fresh 'cuttingedge of the wheel. which results in fewerpasses (typica1J'yone roughing and one fin-ishing) and significantly reduces Ilhe cycletime when compared to a conventionalcontinuous generating grinder using themulti-pa process (Fig. 17).

fig. IS

Wheel technology and balancingmaintenance have artempted to keeppace with machine tool advances. Formost applications, the vitrified alu-minum oxide wheel has been used withsuccess. The newer breeds of machineshave dictated that the wheels be "freecutting" to avoid burning. Wheels thathave induced porosity and eeded gelachieve this goal. Wheels that change

in size because of the con tam need forredressing must be balanced in twoplanes. Thi.s is auromatically donewhen the machine enses an out-of-bal-ance condition. One machine tool man-ufacturer has developed a machinearound a steel-bodied, threaded wheelplated with Borazcn, making the dress-ing and the balancing operation obso-lete, This makes modifications to the

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AIW Systems Co. announces that it is now amanufacturing source of spiral gear roughingland finishingl cutters and bodies.

We alsocan manufacture new spiralcutter bodies in diameters olS·through 12·at present.

A/IN can also supply roughing and finishingcutters, hardware and replacement :parts formast 5"-12" diameter bodies.

Whether it's service or manufacturing, con-sider us as an alternative source for replace-ment parts and hardware, as well as bodiesand cutters.You'll be in for a pleasant surprise.

NEW! Straight Bevel Cutters. Royal Oak. Michigan 48067T,el:(248) 544-3852' Fax: (248) 544-3922:

ICIRCLE 105

t.UVIJUNE 1U7 491

_------------GEAR.FUNDAMIEINTALS,------------

Infeed hIsreached'finished size

Fig. 16

Infeed hasreachedlfinished size

Roughing FirS!Workpiece ~-

" Finishing First Workpiece

Fig. 17

Grinding WormGuard in Position forProliJingO. D. Dressing HeadSetting Scraw m for UI I ,C. :ir----"==r

=~~~~~~~:A~=and RH[liamondlWhaelSetting Sc rew (2) for RH:Diamond W-heel Only ~~;;;;;;;;;;;;;~"

Profiling Slide

I-----Cenl r01Profile

Grinding WormProfile'

@~~~=-- DIsh Wheel.

DI.m"'d\''''~ Aligning the DishWheeland Profile

Fill·n,

!50 GEAR TECHNOL.OGY

wheel impo ible without stripping thewheel of its coating and regrinding andrepJating the body. This compromisemust be weighed against the produc-tivity benefits.

Vitrified bonded wheels, regerdle sof their compcsitioa, must be dre sedperiodically to remain sharp and cutefficiently, This can be done in a num-beT of ways. depending Oil the quantityof gears Ito be produced and Ute degreeof flexibililY required by the gear man-ufacturer (Fig. 18).

In the past it was difficult to auto-marethese machines because the workspindle was ina different position afl.erevery cycle due to the part shiftingaero s 'the face of the wheel. Micro-processors and modem machine controlsmakes this possible today,.

These machines are well-suited fororiginal eq,uipment manufacturers aswell as job shops. Productivity and flex-ibility standard's are set today by themachines capable of using the continu-ous generating process. These machinescan be found in the following industries:aircraft and aerospace. machine tools,printing. pre s manufacturers, automo-tive truck and tractor, marine transmi-sions, wind turbine generator andhigh- peed ind'uslrialgeaJ1boxes,O

Acknowledgement: This article isbased on. a paper presented at the 1995SME Advanced Gear Processing Clinic.Sept, 19-21, 1995. Chicago. n.

Dennis R!ichmondis vice presiden: of Reishauer Corporation. Elgin.ti; l1IlJlIu!aclurrr:r of precision g~ar grindingmachin~ry.

Tell III WIllI Yea TIIiIIII •..If you found 1hit article of inbJrest and/oruseful. pte... circle III.

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