1、07FTM16Straight Bevel Gear Cutting and Grindingon CNC Free Form Machinesby: Dr. H.J. Stadtfeld, The Gleason WorksTECHNICAL PAPERAmerican Gear Manufacturers AssociationStraight Bevel Gear Cutting and Grinding on CNCFree Form MachinesDr. Hermann J. Stadtfeld, The Gleason WorksThe statements and opinio
2、ns contained herein are those of the author and should not be construed as anofficial action or opinion of the American Gear Manufacturers Association.AbstractManufacturingofstraightbevelgearswasinthepastonlypossibleonspeciallydedicatedmachines.Onetypeof straight bevel gears are the gears cut with a
3、 circular cutter with a circumferential blade arrangement. TheMachinesandthegearstheymanufacturehavetheGleasontradenameConiflex.Thecuttersarearrangedinthemachineunderanangleinaninterlockingarrangementwhichallowsacompletingcuttingprocess.Thetwo interlocking cutters have to be adjusted independently d
4、uring setup which is complicated and timeconsuming.The outdated mechanical machines have never been replaced by full CNC machines but there is still aconsiderabledemandinanhighvarietyoflowquantitiesofstraightbevelears.Justrecentlyitwasdiscoveredthat it is possible to connect one of the interlocking
5、straight bevel gear cutter disks to a free form bevel geargenerator and cut straight bevel gears of identical geometry compared to the dedicated mechanical straightbevel gear generator. A conversion based on a vector approach delivers basic settings as they are used inmodern free form machines. The
6、cutter is mounted to a shaft which is connected to the cutter spindle.Additional features like reverse cutter mounting, vector feed and root limited roll finally enabled the straightbevel cutting process on modern free form machines.Theadvantagesarequicksetup,highaccuracy,easycorrectionsandhighrepea
7、tability.Becauseoftheuseofasinglecutter disk,itwillbepossibletogrindConiflex bevelgears onstandardfreeformgrindingmachines.The grinding technology of straight bevel gears whose geometry bases on an interlocking cutter system isbeing developed in the next development step.Copyright 2007American Gear
8、Manufacturers Association500 Montgomery Street, Suite 350Alexandria, Virginia, 22314October, 2007ISBN: 978-1-55589-920-21Straight Bevel Gear Cutting and Grinding on CNC Free Form MachinesDr. Hermann J. Stadtfeld, The Gleason WorksIntroductionManufacturing of straight bevel gears was in thepast only
9、possible on specially dedicated mechani-calmachines. Suchmachines areby naturedifficultto set up with limited ability to precisely repeat thesetup. Mechanical machine setup has beenhistori-cally slowand timeconsuming. Newdevelopmentsallow the use of the straight bevel gear cutting sys-tem known as C
10、oniflex on the Phoenix free formmachine. This patent pending methodology takesadvantageofthePhoenixfreeformflexibilityandre-duces setup time to a minimum while applying theConiflex cutting system used previously onmechanical machines.Figure 1. Interlocking Coniflex cuttersConiflex straight bevel gea
11、rs are cut with a circularcutter with a circumferential blade arrangement.The Coniflex cutters are arranged in the machineunder an angle of e.g. 24 to a plane which is per-pendicular to the generating plane or cradle plane.The upper cutter disc is inclined to point down withthecuttingzoneoftheblades
12、andthelowercutterisinclined to point up (Figure 1). The blades of thelower cutter move through the gaps between theblades of the upper cutter while rotating (and viceversa). This pair of cutter disks and the arrange-ment in the cutting machine is called interlockingcutter system.The upper cutter cut
13、s the upper flank only andtherefore is equipped with blades that have theircuttingedgestowardsthetopofthefigure.Thelow-er cutter inturncuts thelower flank andisequippedwith blades that have their cutting edges towardsthebottom of thefigure. Thetwocutters generateacombination of profile and length cr
14、owning in theflank surfaces. A side effect of the cutter arrange-ment is a curved root line, depending on the cutterdiameter.Thetwocutters represent onetoothof ageneratinggear which subsequently requires the setup possi-bilities for a variety of job designs. The cutters canbe moved away from the cen
15、ter of the cradle toachieve a certain mean cone distance. Since theConiflexflanklinetangentspointtothepitchapex,itisnecessarytoadjusttheangleof theflank linetan-gent with the so called space angle. To control theslotwidthusingexistingcuttersthecutterscaninde-pendently be shifted apart or towards eac
16、h other.Some Coniflex machines allow an additional free-dom to adjust both cutters in order to modify thelength crowning (swing angle). Coniflex is a com-pleting process which, compared to spiral bevelgearwetcuttingisconsideredaratherfastprocess.Figure2shows aGleasonNo.2A Coniflexgenera-torwiththein
17、terlockingcuttersengagedinaslotofaring gear.Figure 2. View into the work chamber of aGleason No. 2A Coniflex Machine2In case of a 20 pressure angle system and a tiltangle of 20 for each cutter the planes of the cutterdisks would generate flank surfaces which consistof precise spherical involutes. Ho
18、wever, also instraight bevel gears a located flank contact is de-sired which lead to a modified cutter and cutter tiltdesign.Ifinthecaseofa20pressureanglesystemthetiltangleofthecutterdisksis24andthecuttingedge,ratherthenbeingpartof thecutter diskplane,form an internal cone of 86 cone angle (90 - 4),
19、then the internal cone generates a certain lengthcrowning. This effect in connectionwith curvedcut-ter blades is used to produce the crowning instraight bevel gears. As mentioned before, it is al-ways a pair of cutters, one left hand and one righthand cutter, required to realize the interlocking ar-
20、rangement in the cutting machine in order to cutboth flanks of a slot at the same time.There is still a considerable demand for straightbevel gears and machines for their manufacturing.However, there was never a full CNC machine de-velopedtosatisfythepresentdemand.Manufactur-ers of straight bevel ge
21、ars were relying on reman-ufacturedmechanicalmachines,someof whichareequipped with partial CNC functions like cradle roll,sliding base, ratio of roll and indexing. The reasonwhy no Coniflex CNC free form machine was de-signedbecomesevident afterstudyingthegeomet-ricalconceptinFigure2.It isnot possib
22、letoutilizea6-axis machine because of the double tilted cutterspindles and the interlocking cutter arrangement.To configure a regular 6-axis free form machinewith the Coniflex double spindle head including anautomated head setup would have introduced 6additional axes with complicated setup actuators
23、.The cost to build such a machine is between 150and200%ofaregulartodaysPhoenixIIfreeformgenerator.Motivation of Coniflex cutting on freeform machinesManufacturers of straight bevel gears are process-ingmostly lowquantities andahighvariety ofdiffer-ent designs. The applications are differential gears
24、for special equipment, electrodes for forging diesand powder metal dies, machine tool applicationsetc. The demand for higher quality straight bevelgears has been raised together with the desire toapply a defined hard finishing method such asgrinding.Manufacturers with a high job variety and manychan
25、geovers between jobs would invest in a mod-ern machine tool for their straight bevel gear needsif it was alsopossibletoput themachinetodifferentuses e.g. to cut spiral bevel gears. A six axis freeform machine can present a tool in every desiredpositionandangularinclinationtoawork.Thisisthebasis for
26、the ideas that realized Coniflex cutting onPhoenix free form machines. It is only possible touseonecutterwhichmakesthecompletingprocess“semicompleting”. Thecycletimeisstillacceptablesince the free form machine with direct spindlemo-tors indexes extremely fast and since the higherstiffness of a Phoen
27、ix machine allows a cycle ofrapid plunge and limited roll.There are additional features like independentcorrection of upper and lower flank (even using dif-ferent ratio of roll, root angle, tilt angle and more),summarystorageandfastsetupsandthepossibilityto implement cutter disks with carbide tips,
28、thatwouldallowahighspeeddrycuttingofConiflexgearsets.Conversion of Coniflex summaries toPhoenixII machinesConiflexsummarieswhichhavebeencalculatedforthe Gleason machine Nos. 2A, 102, 104 and 114can be converted to general basic settings in orderto perform Coniflex cutting on Phoenix II free formmach
29、ines. Theprocedure that has beendevelopedfollows the list of steps below: Conversion of Coniflex summary into generalbasic settings Conversion of upper and lower Coniflex cutterseparately Transformation of basic settings into free formmotions Reverse mount Coniflex cutter on the free formmachine to
30、avoid too negative machine rootangles Rolling of root only to the bottom of the involute(avoidsundercutandreducesnegativemachineroot angle), rollingfacelikeonConiflexmachine Calculate work phase angle set over betweenupper and lower cutting in order to manufacturecorrect slot width Use vector feed t
31、o allow cutting slot from solidwith outside blades only (no alternating blades)3Coniflex straight bevel gear cutting summaries thatuse interlocking cutters contain the following geargeometry relevant settings. The settings of group1are initially identical for the upper and lower cutterbut may be cha
32、nged in the course of contact opti-mizations onthemechanicalmachine. Thesettingsof group 2 are always identical for the upper andlower cutter in the mechanical machine. In a freeform machine all settings of group 1 and 2 may bechanged in order to optimize a pinion or gear:Summary settings Group 1Spa
33、ce angleCutter offsetCutter cone distanceCutter swing angleSummary settings Group 2Cradle test rollWork test rollStart roll positionEnd roll positionMachine root angleSliding baseFor the correct positioning of the cutter in the ma-chine the tool related dimensions of group 3 arerequired:Summary sett
34、ings Group 3Actual cutter diameterCutter reference heightIn order to convert asummary of a mechanicalma-chine into basic settings the machine constants ofgroup 4 are additionally required:Summary settings Group 4Cutter tilt angleSwing axis constantCutter gage reference radiusWithallinformationof gro
35、ups 1through4available,the basic machine setup can be calculated accord-ingtothevector diagram inFigure3.Thefirststep,after the cutter is positioned with the tip of the RWvector in the origin of the coordinate system (cutteraxisparalleltoY-axis) isacuttertiltrotationaroundtheZ-axis, followedby amove
36、ment ofthecuttertipRw to the cutter cone distance (vector Rm1). Thenthe cutter tip is moved in X-axis direction about thecutteroffsetETandrotatedaroundtheY-axisaboutQstoachievethespaceangleinclination.Thelowerdiagram in Figure 3 shows the movement XBin di-rectionoftheY-axistotheproperslidingbaseposi
37、-tion. In some cases there is a swing angle setting(not showninFigure3) whichrequires anadditionalrotationofthecutteraroundtheX-axis.Alsothead-justment of the work root angle is not shown in thediagrams, it requires a rotation of the work aroundtheX-axis. Asimilar conversionis donefor thelow-er cutt
38、er setup. Althoughinitially thelower cutterar-rangement is amirror imageof theupper cutterset-up (mirror in a horizontal plane that contains thecradleaxis),theconversionsaredoneseparateandindependent. If corrections have been made to thetheoretical summary, the upper and lower cuttersetup may differ
39、 from each other and theindependentconversionwillcorrectlytransferthosecorrections into the free form machine setup.Figure 3. Diagram of Coniflex setup (uppercutter)Reverse Coniflex cutter mountingFigure4demonstrates thattheslotwidthtaperandadditionally the work root angle leads in case of aregular
40、mounted cutter to highly negative machineroot angles in a free form machine (center rollposi-tionisshown).Byreversingthecutterdisksuchthatthecuttingedges pointaway fromthecutterspindle4the cutter finishes the right flank in the left slot inFigure4,whichswingsthecutteraxisinthepositivedirection.Theco
41、mmonlimitationof=-3onfreeform machines would present a problem with regu-lar cutter mounting. This problem is completelyeliminatedwithoutanynegativesideeffectifthecut-ter is used in the reverse mounting arrangement.Figure 4. Machine root angle in case ofreverse cutter mountingCutting cycle and gener
42、ating roll strategyAbasiccyclecomparisonisshowninFigure5.Theupper diagram in Figure 5 represents the cycle of aNo. 102 Coniflex machine. Roll angle changes areplotted along the abscissa, plunge advances of thecutter are plotted along the ordinate. The cyclestarts with a first plunge, followed by a r
43、ough roll tothestartrollposition.Thenextstepisasetinplungetofulldepth, followedbyafinishrollfromstart rolltotheendrollposition. Thenthecutteris withdrawntothe index position, the cradle rolls back to center ofroll and the indexing to the next slot occurs.Figure 5. Cycle diagram mechanical machineand
44、 free form machineThe lower diagram shows the cycle of cutting thesame bevel gear on a free form machine. The redvectors represent the cutting of the first cut lowerslots.Itbeginswithaplungetofulldepth(at therootroll position), followed by the finish roll to the top ofthe profile. Then the cutter wi
45、thdraws, rolls to thestart roll position and indexes to the next slot. Afterallflanks inthelower positionarefinishedasetoverofthecutterintheupperpositionandasetoverrota-tion of the work occurs. Now the cutter approachesthe full depth with rapid plunge feed, since the slot-ting operation had already
46、been conducted in thelower positionandimmediately thefinishrollbeginsand rolls the second flank from root roll position tothe top of the flank.The lower diagram in Figure 5 leaves some ques-tionunansweredsuchashowacutterwithonly“out-side” blades can perform a slotting operation with-out damage and w
47、hy the roll angle required for one5flankisonlyabout65%oftherolltravelofamechan-ical machine with interlocking cutters.Asthelowerslotiscutfirstinthepresentedexampleitisimportanttoapproachtheworkwiththecutterinamanner toprevent theclearancesideof thecutterfrom any cutting action. Figure6 shows schemat
48、icthecutterinthestart rollposition. Thefeedvectorisderived in the final plunge position at the start rollpositionattachedtotheclearancecornerof thecut-teratthecenterfacewidthsuchthatenoughstockisleftonthenotyetgeneratedflank2.Thefeedvectoris preferably perpendicular to the root at the meanface width
49、. If the cutter follows with the clearancecuttingedgesidethevectorshowninFigure6,thenonly the cutting edges and the blade tips are sub-jected to any chip load, which prevents damage tothe clearance side blade edges and leads to anefficient and smooth cutting action.Figure 6. Vector feed approachThe plunge position in Figure 6 forms already theroot of flank 1. The cutting edge generates a zerodegree pressure angle area which generally is notpart of the operating portion of the flank anymore.Zero degree is sufficient to form the trochoidal partof the
