Shaper Cutters -Design & Application-Part ,2

Editor's Note: This is part two of afeature begun in our last issue, In theMar / Apr issue, Mr. Ianninck discussedthe shaping method, spur and helicalcutters, herringbone cutters,cutterblank sizes, tooth shape change, toothforms, and cutting internal gears.

Cuner SharpeningCutter sharpening is very important

both during manufacturing and subse-quently in resharpening after dulling.Not only does this process affect cutter"over cutting edge" quality and thequality of the part cut, but it can also af-fect the manner in which chip flow takesplace on the cutter face if the surfacefinish is too rough or rippled.

The sharpening of spur cutters andthose helicals under 8° helix angle thatare made with a conical sharpening, areusually sharpened on a rotary table sur-face grinder using a centering plug, andwith the table set at the sharpeningangle. See Fig. 2-1, The sharpeningangle has been standardized at 5°, sonormally the table is set at 5°. Smalldownfeeds are used along with a goodcoolant flow to prevent material burn.No wavy pattern should appear.

The cutter manufacturer would rec-ommend using two separate grindingoperations, one a roughing pass with acoarser gri], and the second with a finegrit to get a fine, almost polished finishon the tooth face. Usually for expedi-ency only, one operation is used.

38 Gear Technology

William L JanninckITW - Illinois Tools, Lincolnwood. IL

The sharpening of helical cutters withthe step-sharpening, as seen in Fig. 2-2,which is also called normal sharpening,requires more elaborate equipment. Asurface grinder fixture which containsthe geometric requirements is shown inFig. 2-3. These fixtures have been madein several styles and are usually used insmall shops. The fixture is usually madewith a 5° angle ..Fig. 2-4 shows a view ofthe sharpening plane on the cutter toothface and the face sharpening angle.

For quantity sharpening of steppedface cutters, say on a high productiongear line, specially constructed ma-chines with automatic operation areused.

Vl/hile the standard practice is toplace the cutting face normal to the cut-ter helix, experience has proven abenefit in cutter utility by consideringthe off-normal sharpening. This is calleda chip cent .rol sharpening, and the hel-ical sharpening angle is usually 80 lessthan the cutter helix angle. See Fig. 2-5.

Cutter Lead GuidesFor a spur cutter the reciprocation

takes place in a straight line; thus, for a11spur cutters a single spur guide is used inmoving the cutter along a straight path.

For a helical cutter a helical guide ofthe same hand and lead as the cutter be-ing used must be available, As a helicalcutter is reciprocated it must pass thecutter teeth through a helical lead path,See Fig. 2-6 for a schematic view of thefunctioning of typical spur and helicallead guides.

For a vertical axis machine, the guideand shoe are usually located on the topend of the cutter ram, where they are ac-cessable for changing. Lead guides are arequired setup item.

Frequently the gear manufacturerwants to utilize lead guides that areavailable in-house, avoiding extra costand possible delay in procurement.First, confirmation is made of the handof guide and cutter. Then a comparisonis made of the lead of guide and gearand number of teeth of cutter and gearaccording to this formula:

LEAD OF GEAR LEAD OF GUIDE

GEAR TEETH CUTTER TEETH

H the data meets equality in this equa-tion, the guide can be used. Varying thecutter teeth or altering the gear datamay find a. solution.

Gear Clearance GroovesWhen cutting a gear located adjacent

to.a shoulder, it is necessary to providea groove for the cutter to pass into at thebottom of the stroke, Fig. 2-7 showssuch a case for a spur cutter. Dependingon the helix angle and the type of shar-pening used, the groove width for heli-cal gears will usually be wider. SeeFig. 2-8 ..

Indicating DiametersAn accurately ground ring located on

the shoulder diameter just behind theteeth of a disk or deep counterbore

WHEEllRAVE'L

rig. 2-1-Circular sharpening of a cutter.

Fig. 2-2 - Helical cutter with step sharpening and its cut gear.

. .HELlXo J+. .. IHELI~ ~ .- . ~"IItGL!~· ~IS Of TOOTH "'NGL~ ~J(IS 'Of TOOTH

Fig. 2.3 - Helical cutter sharpening fixture.

Fig. 2-4-Normal sharpening plane.

FACE SHARPENING .ANGLEHELLCI.'L. IIHAI'E'RCUTTER

.--.~

IDOWNHIIL!.SIDE

,Fig. 2-S-Nonnal and chlpcontrol sharpening.

Fig. l-6 - Diagramstic: view of spur and helical lead guides.

May/.1Jne1990 39

Fl-l.2-7- Clearance gTilove Ior a spur cutter.

CLEARANCE ABSORBEDBY SHARPENING STEP

RECESS

Fig. 2-8- Clearance groove for a helical cutter,

40 'Gear Technology

r------INDICATING:

BAND II

INDICATING BAND

Fig..2-9- Cutler indicating rings.

• DP 20 PA Spur4-PD CUTTEII

S"PD GIAII

10

CUTTEIt TOL.ONLY

CUT TEll II ..ACHINEALLOWANCE

Fig. 2-1'0 - Shaper cutlet accuracy capabUi ty,

shaper cutter ,or on the diameter behindthe teeth of a hub type or shank typecutter, is used as a truing ring for check-ing cutter runout while it is mounted inthe gear shaping machine. See Fig. 2-9 ..

Cutter TolerancesThe first SOUIc.e for information on

the tolerances for shapercutters is thepublication ANSI 894.21, "Gear ShaperCutters." Only one level of accuracy isspecified, which is called "CommercialTolerance", MCTI, Meta] Cutting ToolInstitute, has similar standards whichdo include tolerances for "CommercialGround" and, for certain cutter sizesonly, tolerances for "Precision Ground".

Fig. 2-10 shows a comparison chartof what could be expected by cutting agear using commercial tolerances, first,where only cutter tolerances are consi-dered,and second, where some allow-ance is made for machine and machin-ing influences. The best cutting ability isabout A:GMA Q-10 and, with allow-ances, about AGMA Q-8.

Tolerances are published for profile,spacing, and PO runout for the various,types of cutter - disk, deep counter-bore, shank, and herringbone. Toler-ances are also given for bore, tooththickness, flatness of cutter back, 00angle, sharpening angle, side reliefangle, shank runout, shank diameter,indicating band runout, 00 runout,and 00 size. For herringbone cutters,matching tolerances on 00 and widthare given,

Cutter Base Cir,clesWith 'the repA!:sentat:ion of the shaper

AUIHOR:

W.L JANNINCK is III consultant for lTW- J1Iinois Tools, III division of Illinois ToolWorks, Inc. He has nearly 40 years' ex-pen"ence in an engineering and manufactur-ing environment in' III cutting toot plantworking: in the ,design, development, and ap-plicatio.I1' of gearshaper cutters and gear hobsused in the generating: process of gear cui-nng. He has sensed 011 various committees ofAGMA andMCTI and is past chairman ofthe AGMA Cutting Tools Committee" Hehas also served on .the SAE - ANSI.1nvoluJeSpline Committee find ASME - Committcuon Power Transmission Chains ,andSprockets. He has written several articles on,tool applications, gaging. gear design. andgear inspection.

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42 GeorTechnology

cutter asa gear, it is also inspected on aninvolute checking instrument. The basecircle diameters are not the theoreticalones expected, but are corrected for thecutting clearances on the cutter. For aspur cutter the base diameters are thesame on both Flanks, right and left.,With the additional geometry of thehelix and normal sharpening on ahelical cutter, base circles differ ft~mside to side,

The base diameters ef spur or helicalcutters are eonstant throughout 'the lifeof the cutter.

Because of the need to maintain a cut-ting edge in the transverse plane, her-ringbone cutters are not corrected andthe base diameter is equal to the 'theoret~ical value and is the same on bothHanks.

Cutter GenerationThe traditional process used in.mak-

ing shaper cutters is by abrasive wheelgrinding,. which can provide both at

good finish and accuracy. All processesuse some form of involute gen ration,whether it be a single Hank grind usingthe radial side of the grinding wheelsuch as seen in fig. 2-11, or using a dualHank grind with a reciprocating vee-shaped wheel, as is shown in the dia-gram in Fig. 2~12.. Another methodusing a ribbed or threaded abras:ivewheel. running in timed relation to thecutter has been used for some unmodi-fied fine diametral pitch tools.

The single flank method requires a1'argewheell relat.ive zo the rutterwidthand is positioned tangent to the cuttertooth flank. The wheel position is sta-tionary, and the cutter is rotated andtranslated below the wheel to generatethe involute. Each flank is ground 5ep"

arately,and the number of teeth is es-tablished by an indexing mechanism.

The dual Hank grinder uses a vee-shaped grinding wheel dressed to gener-ate the fonn required on the cutter . Thecutter is rolled tangent to a generatingcirde and passed or 'translated throU8-hthe reciprocating grinding wheel. Index-ing takes place at the end ofa generationcycle, The entire profile of the cutter,tip, root, and both.Hanks can be groundatone time with one space being com-pleted between indexing cydes.

Fig. 2-12- Dual flank generating grinder.

HA SET

ROJ .

.Fig ..2"13 - Madune mounting of various culler types.

TAPER S.MUKCUnER

DIS nPE CUnE.R HU 8 TYPE curt n

tUnERREDuCERHUH I

TUE ISUNKIDHHil

Cutter Clearance AnglesThe side relief angle of a shaper cut-

ter generally ranges between 1.5" and2.50

, with the norm being 2.00••This is

a relatively low amount .of clearance fora metal cutting tool, and any substantialincrease in this angle can cause drasticchanges in cutter geometry and designand would reduce useful cutter life.

The cutter outside angle \s a functionof cutter-gear geometry, amount of siderelief, and pressure angle ..It ranges ap-proximately from 3.50 for 30PA to 5.50

for20PAand8.5° for14.5PA. The geo-metrically true cutter outside angle isnot usually a straight angle, but is reallya curve that is compromised to astraight line, The cutter designer mustbe fully aware of the deviation amountin order to make proper allowances.

Cutter PDThe cutter PD is defined In the same

way as an equivalent gear using thesame formulae. With shaper cutters thePO' is frequently referred. to in a generalway, such as a 3" or 4" PO cutter, mean-ing a nominal or approximate size.

Cutter Oversize and UndersizeIn optimizing cutter life and utility it

is normally expected that a cutter will bemade somewhat oversize or larger ondiameter than standard. Some geargeometries may require that a gear bemade undersize or less than. standard ondiameter.

DesWt OptimizationIn the process of designing a gear

shaper cutter, several possible extremesare considered. Going toward a maxi-mum. oversize design will reduce cuttertip land toward a sharp. Setting a prac-tical limit on the tip flat and maintain-ing proper part geometry will yield themaximum oversize cutter.

Inconsideration of the smallest possi-ble diameter undersize for the cutterwithout damaging effects on the cutteror part, the targeted end life point on thecutter is determined. With allowancefor other factors in the total. tool layout,the final design will fall in between theselimits .. Such designs are usually com-puter aided and are planned solely forone part.44 Gear TecihnoloQV

Fig ..2-]4 - Involute profile inspection of a cutter.

SHARPENING,ANGLE

~=:l======iIi!I"i-I--=fTHICKNESS

L

SHOULDERFig ..2-15 -Cutter blank definitions.

OUTSIDEANGL.E

Fig. 2~1O- Helical cutter dehnitions.

Fig. 2-17 - Roller chain sprocket euner.

Fig. 2-18- Tandem cutter,

Fig. 2-19 - Cutter with teeth removed,

KeywaysKeyways are used on some shaper

cutters, especially on helical cuU:ers,where there is some chance that the cut-ter may slip rotationally under load.Herringbone cutters, always hell cal, usea keyway in the bore. Most othercut-ters use a slot in the mounting face.Another purpose of the keyway is fortiming or alignment. Bcamples of key-ways can be seen in some of the figures,

CuUer MountingsShaper cutters are made in various

types to suit the special needs for ,applic-ation to the part or cutting machine. fig.2-13 miustrates various ways that thesecutters can be mounted. \Nhile disk anddeep counterbore cutters are fairlysecure in their mountings, any longshank type cutter, including those usedwith taper or straight shank adapters,should be checked for possible excessiverunout,

Cutter InspectionShaper cutters are inspected ina

similar fashion to gears using some ofthe same involute, lead. and spacingiil-struments, Fig. 2-14 shows a view of acutter during the involute profile check.

NomenclatureCutter nomenclature has been listed

in several publications indll.ding theANSI standard. Fig. 2-15 shows some ofthe terminology for a hole Itype cuUer,and Fig. 2-16 illustrates some of thehelical element definitions. Figs.2-17-2-20 show cutters of varioustypes and conditions.

Tool MatedalsMost cutters are made of AISI M-2,

M-.3, and M--4 high-speed steels, withsome special applications of part ma~erc..ial alloy content, part hardness. or ma-terial' abrasiveness requ.lring the supermaterials, sumas M-42, T-15, or Rex,}6. Most of these materials are availablein the particle metal. process.

The titanium nitride coatings are alsobeing used successfully on gear shapercutters.

Acknowledgement: Presented ,at the' SME GearProcessing and Ivb:rufw:furil'lg ainic. NovemlNr1-3, 1988. .Reprinted with permission of the Societyof Mcmufacturing Engineers.

Mav/JUne~990 45