___________ GEA!R FUNIDI.AMENTAtS, _

Gear Shaving Basics - Part IJohn P. Dugas

gear teeth. Its purpose is to correct errors

in index. helix angle, tooth profile andeccentricity (Fig. 1). The process also

improves tooth surface finish andelimi-nates by means of crowned tooth formsthe danger of tooth end Load concentra-tions in service. Shaving provides for

profile modifications that reduce gearnoise, increase a gear's load carryingcapacity, its factor of safety and its ser-vice life. Gear finishing (shaving) is not

to be confused with gear cutting (rough-ing). They are essentially different. Anymachine designed primarily For one can-

not be expected to do borh witl! equaleffectiveness or with equal economy,

Gear shaving is the logical remedy forthe inaccuracies inherent in gear cutting,

It is equally effective as a control for

those troublesome distortions caused byheat treatment.

The form of the shaving cutter canbe reground to make profile allowancefor different heat treat movements

caused by varying heats of steel. Theshaving machine can also be reset tomake allowance for lead change inheat treatment.

Rotary gear shaving is a productionprocess that utilizes a high speed, hard-ened and ground, ultra-precision steelshaving cutter. The cutter is made inthe form of a helical gear. It has gashes

in the flanks of the teeth which act asthe cutting edges.

The cutter is meshed with the workgear m a crossed-axes relationship' (Fig.2) and rotated in both directions during

rhe work cycle while the center distanceis reduced in small increments. Sirnulta-

neously the work is traversed back and

diagonal to ihe work gear axis. depend-ing on the type of work gear. the produc-

tion rate and finish requirements. The

gear shaving process can be performed athigh production rates. It removes materi-

al in the form of fine hair-like chips.

Machines are available (0 shave exter-nal spur and helical gears up to 200" indiameter. Other machines are also avail-

able for shavinginternal spur and helicalgears, For best results with shaving, the

hardness of the gear teeth should notexceed 30 Rockwell C scale. If stockremoval is kept to recommended limits.and the gears are 'properly qualified, the

shaving process win finish gear teeth inthe 7 to 2D pitch range to the followingaccuracies: involute profile, 0.0002":toorh-eo-tootn pacing, 0.0003" and leador parallelism, 0.0002".

In any event, it should be rememberedthat gear shaving can remove 65-80% of

the errors in a hobbed or shaped gear. II

will make a good gear better. The finalquality of the shaved gear is dependent to

a large degree upon having good hobbedor shaped gear teeth,

Excellent surface finish is achievedwith gear shaving. A value of appro xi-mately 25° is the normal finish obtainedwith production gear shaving, although

much finer finishes are possible byslowing the process, In some cases,shaving cutters will finish up to 80.000

gears before they need sharpening.They may generally be sharpened fromfour to ten times,

To a gear designer. tile shaving processoffers attractive advantages in the abilityto modify the tooth form, Ifa crownedtooth form ora tapered. tooth form aredesired to avoid end bearing conditions.these can be easily provided by shaving,

If modifications are desired in theinvolute profile. these can be made by

Fig'.3 _ Contact between cvlinders change 8S forth across the width of the cutter. The suitable modifications in the ground cut-crossed 8.1es aru, variedl• traverse path can either be parallel. or ter tooth form .. If heat treatment distor-2.6 GEAR TECHNOLOGY

_ •• GEA'R, FUNDAIMENTAtS,_tions can be kept to a minimum. 'the mostinexpensive way to produce an accurate"quiet. high-performance gear i to speci-fy hobbing followed by gear shaving ..Th having proees u e variety ofrandardized production equipment rang-

ing from hand loading t fully automaticloading and unloading.

Basic PrinciplThe rotary gear having proce is

based on fundamental principle. This'process u e a gashed rotary euuer in thefonn of a helical gear having a helixangle different from 'thal of ttl gear to behayed. The axe of the cutter and the

gear are ceo sed at a predetennined angleduring the ' having operation. When thecutter and the work gear are rotated indo e mesh, the edge of each cutter gashshaves a fine hair-like chip as it movesover the surface of a work ,gear tooth.The finer the cut. the le s the pre urerequired between tooll and work, elimi-nating the tendency to cold-work 'the sur-face metal of the work: gear teeth.

This proees is performed ma shav-ing machine, which has ,3 motor-driveneujter head and a reciprocating worktable. The cutter head is, adjlllSlable [0

obtain !'he desired era sed axis relation-hip with the work. The work: carried

between live centers is driven by lh cut-ter. During the having cycle, 'the work ireciprocated parallel to its axis aero theface of lbe cutter and upfcd an incrementinto 'the cutler with each sl1'oke of thetable, This conventional. shaving, cycleione of everal method .

The Crossed-Axi .Princip eTo visualize the ere sed-axis princi-

ple, con ider two parallel cylinders ofthe arne diameter (Fig. 3). Whenbrought (,ogeiber ullder pressure, theircommoa comact urface is a rectangleha.Ying the length of a cylilnder andwidth which varie with contact pres-sure and cylinder diameter.

When one of these cylinders is swungaround. 0 that the angle between its axisand that. of the other cylinder is increased'Up '10 9O°,their common plane remains aparallelogram, but its area steadilydecreases as the 8xi31 angle increases.The arne conditions prevail whenin tead of the two plain cylinders. a hav-

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_-----------."GEA;R FUNDAMENTAlS _iagcutter and ill work gear are meshedtogether. When the angle between theiraxes is from 10<> to 15", toothsurface con-tact is reduced, and the pressure requiredfor cutting is small. As the work gear ismoved away axially from the point ofintersection of the axes, backlash devel-ops .. Conversely, as it is returned to thepoint of axial in~ersectjon, backlashdecreases until. the two members engagein tight mesh w:ith the teeth of the cutterwedging between those of the work gear.Thus, each succeeding cutting edge sinksdeeper into the work: gear tooth until thepoint of axial intersection is reached.

For shaving, the cutter and work gearaxes are crossed at an'angJ'e usually ill the

CIRCLE 124Z8 GEAR TECHNOLOGY

range of 10" to ]5" or approximatelyequal to the difference [II their angles.

Crossing the axes produces reasonablyuniform diagona] sliding action from thetip of the teeth to the root. This not onlycompensates for the nonuniform involuteaction typical of gears in mesh on parallelaxes, but provides the necessary shearingaction for stock removal.

Relationship' Between Cutting& Guiding Action

Increasing the angle between the cut-ter and work axes increases cuttingaction, but, as this reduces the width ofthe contact zone, guiding action is sacri-ficed. Conversely, guiding action can beincreased by reducing the angle of

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crossed axes, but at the expense of cut-ting action.

Prepar.ation Prior to ShavingThe first consideration in manufactur-

ing a gear is to select the locating sur-faces and use theml.ltroughout theprocess sequence. Close relationshipbetween the locating surface and the faceof the gear itself mest be held. Otherwise,when the teeth are cut and finished withtooling that necessarily contacts the gearfa"ces,!.he teeth will be in an improperrelationship with 'the locating or relatedsurface on which the gear operates. Gearsthat locate on round diameters or splineteeth must fit the work arbors closely or,these critical hole-to-face relationsmpswiII be destroyed.

Typical manufacturing tolerances forgear blanks prior to cutting of the teethare shown in Table l.

Once the gear blank has been manu-factured, it. is necessary to cut the gearteeth. The most common methods todayfor rough-cutting gear reeth are hob bingand shaper cutting. Of primary concernto the having cutter manufacturer is thefillet produced by the roughing opera-tion. The tips of the shaving cutter teethmust not contact the gear root fillet dur-ing the shaving operation. If such con-tact does occur, excessive wear of thecutter results, and the accuracy of theinvolute profile is affected.

The shaving cutter just finishes thegear tooth below its active profile. Thus,the height of the fillet should not exceedthe lowest point of contact between theshaving cutter teeth and the teeth. on thework gear..

Protuberance type hobs and shapercutters are often used prior to shavingto produce a slight undercut or reliefnear the base of the gear tooth. Thismethod assures a smooth blending ofthe shaved tooth profile and. theunshaved tooth fillet, as well as reduc-ing shaving cutter tooth tip wear (Fig.4), The amount of undercut producedby the protuberance type tool shouldbe made for the thin end of the tooth.The position of the undercut should besuch that its upper margin meets theinvolute profile at a point below itscontact diameter.

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Fig'. 4: - Undercut produced bV protuberance hob and basic hob tooth form.

TABLE 1 - TYPICAL GEAR BLANK TOLERANCE

Blank Face Hole Hole Hole O.'D.Dia .. Runout Size Tape Roundness O.D. RunoutIn. In. In. In./In. I".-Max. In.-Max. iln.

Up to 1 0.0003- 0.0003- 0.0002- 0.0002- 0.003 0.003l-in 0.0005 0,0006 0.'0003 0,0003Thick

1-4, upto 1 in. 0.0004- 0.0005- 0.0002- 0.0003- 0.005 0.005Thick 0.'0008 0.001 0.0003 0.0005

4 to a, 0.0006- 0.0008- 0.0002- 0.0004- 0.005 0.0070.0012 0.0012 0.0003 0.0006

8 to 12 0.001- 0.001- 0.0002- 0.0005- 0.005 0.0080.002 0.001'5 0.0003 0.0007

TABLE 2 - RECOMMENDED SHAVING STOCK AND UNDERCUT FOR PRESHAVED GEARS.

Normal Diametral Shaving, Stock Total UndercutPitch (In. per Side of loath 1 Un. per Side of Tooth)

2-4 0.0015-0.0020 0.00.25rlJ.0030

5-6 0.0012-0.0018 0.0023-0.0028

7-10 0.0010-0.0015 1),0015-0.0020

lH4 0.0008--0.0013 0.0012-0.00 17

16-18 0.0005-0.0010

20-48 0.0003-0.0008

52-12 0.0001-0.0003

30 GEAR TECHNOLOGY

Shaving StockThe amount of stock removed during

the shaving process is a key to its SUlC-

cessful application. Sufficient stockshould be removed to permit correctionof errors in the preshaved teeth.However, if too much stock is removed,cutter life and part accuracy are effective-ly reduced,

Table 2 shows the recommendedamounts of stock to be removed during theshaving operations and the correspondiagamount of' undercut required. 0

Acknowledgement: Presented byNational Broach &. Machine at the Societyof Manufacturing Engineers 1st Inter-national Advanced Gear Processing &.Manufacturing Conference, June, 1996.

The second half of this article, coveringshaving methods and design, will appearin our Jan/Feb, 1998, issue.

John P.Dugasfor mOllY years was the Chief Tool Engineer atNational Broach & Machine ..He is tile author ofnUl/lemus papers and presentations 011gear sub-jects. He is /lOW retired.

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