1、90 FTM 13AvFace Milling or Face Hobbingby: Theodore J. Krenzer, The Gleason WorksAmerican Gear Manufacturers AssociationIllllTECHNICAL PAPERFace Milling or Face HobbingTheodore J. Krenzer,The Gleason WorksThe Statements and opinions contained herein are those of the author and should not be construe
2、d as anofficial action or opinion of the American Gear Manufacturers Association.ABSTRACT:Face milling and face hobbing are the two principal processes used in the production of bevel and hypoidgears. A manufacturer must decide on one or the other. This paper defines the methods and theinherent char
3、acteristics they impose on the tooth design and manufacture. Geometric tooth designdifferences and the reasons for the differences are examined. TCA, finite element analysis and testresults for the two processes are included. The cutting processes and cutting tools are compared. Theadvantages and di
4、sadvantages of each process are enumerated. Based on this information and therequirements of the application, a criteria for the selection of one process over the over is proposed.Copyright 1990American Gear Manufacturers Association1500 King Street, Suite 201Alexandria, Virginia, 22314October, 1990
5、ISBN: 1-55589-565-4FACE MILLING OR FACE HOBBINGTheodore J. Krenzer, The Gleason Worksgenerating gear is rolled with the workpiece toMost spiral bevel and hypoid gears are produce tooth surfaces on one or both sides ofmanufactured by either the face millingor the a tooth slot. The part is indexed to
6、the nextface hobbing process. A great deal of tooth and the process is repeated.discussion has centered on which process isbest for a given application. In the past thediscussion was somewhat academic, in that itdepended on which machines were available,since older machines generated by only one ort
7、he other method. Newer machines arecapable of using either method. /Both face hobbing and face milling methods | _._0_are discussed. Blank design, tooth geometry, - i -contact pattern control, as well as the cutting /“ “ oprocesses and their respective tools, are ,/,examined. Comparisons are made an
8、d criteriafor selecting the most appropriate process arepresented based on geometric considerations.FACE MILLING ,/Basic Generating Member “, ,“/The tooth shape of a spiral bevel or hypoid .gear is more easily understood by considering .the basic generating gear. Face milling Fig. 1: Face Milling Me
9、thodemploys a circular face mill type cutter. Thecutter is designed and set into position relative Modifications of Generating Memberto “the work, so that it cuts the correctpressure angle and spiral angle at the This simple generation concept is complicatedcalculating point, and sweeps out the toot
10、h by two factors. First, gear blanks are designedform as it rotates about its axis. The with tapering depth teeth, and second, in orderlengthwise tooth form then is a circular arc to assemble the gear set in its finalwith a curvature equal to the curvature of the environment and for it to perform un
11、der load,cutter. Figure 1 shows a spread blade cutter regardless of the cutting method, mismatch isand the generating gear it emulates. The required between the mating tooth surfaces.Blanks are designed with tapering depth for profile directions of the tooth flank and isparabolic in form. These mism
12、atches are alsoseveralreasons: secondorder.SeeFigure4.1. Since the pitch surfaces are cones, it is _“-“natural that tooth depth should be afunction of the distance from the pitch _-“apex. SeeFigure 2. _E S2. The normal pitch is greater at theoutside of the blank than at the inside,which results in t
13、apered toplands andslotwidths. By designing the teeth tobe deeper at the outside, toplands andslotwidths are made optimum. Fig. 4: Design MismatchI Thegeneratinggearismodifiedtoaddressthefact that generating mismatch exists due to thetapered depth, and that this mismatch is notnecessarily the desire
14、d mismatch. When thepinion is generated with two setups thefollowingchanges are generally used to controlsecond order tooth contact. The cutter radius is adjusted to give the desired lengthwise mismatch. This is anobvious change which is shown in Figure 5.Fig. 2: Tapered Depth ToothIn most cases the
15、 gear cutter is set to followthe root line of the gear, and the pinion cutteris set to follow the root line of the pinion.For tapering depth teeth these lines are not Generating Gear Axisparallel. This resultsin a pressureangle (change along the pitch cone from inside tooutside of the blank. The cha
16、nge is opposite Aon the gear and pinion members producinggeneratedmismatch.Thechangeinpressure ed terangle in the lengthwise direction produces achange in the direction of the path of contact,a second order change, which is referred to asa bias change. A schematic of the condition is tershowninFigur
17、e3. -Pitch Line-“_ _ _D_endum Angle of Gear Fig. 5: Cutter Radius ChangeDedendum Angle of PinionThe generating cone distance is changed tocontrol the direction of the path of contact.Figure 6 shows the path of the cutter ischanged during generation, producing achange in spiral angle from the top to
18、theflank of the tooth, which is a bias change.Path of ContactThe generating pitch angle is adjusted to givethe desired profile mismatch. The generatingFig. 3: Bias Path of Contact Resulting from member becomes a tapered generating gearTapered Depth Teeth and tilt, or a simulation of tilt, is require
19、d inthe generating machine. The change in profileMismatch is deliberately designed into gear curvature can be envisioned by considering thesets to compensate for manufacturing and transverse plane layout shown in Figure 7. Ofassembly tolerances and for deflections under particular interest is the ge
20、nerating gearload. It is appliedin both the lengthwise and center, Ox. For crown gear generation the2A Modified Generating Gear Center _ O_ Generating Gear Center/“ ( Generating Gear CenterFig. 6: Change in Generating Cone Distance Work_/,- Ow Centercenter is at infinity in the transverse plane.Cons
21、ider the Euler-Savary equation, which Fig. 7: Transverse Plane Layoutsays the relative curvature of the mating toothprofdes at the pitch point is given by: Face Milling Completing1 = 1 1+1po sine In the completing process each member isgenerated in one cut from the solid with awhere: spread blade cu
22、tter. The root lines of theteeth are tilted so that the rate of change ofslotwidth at the mean section is zero, which isr is the pitch radius of the pinionR is the pitch radius of the gear called duplex taper.O is the pressure angle at the pitch The tilted root lines cause bias in on bothpoint sides
23、. The change in generating cone distanceFor the cases being considered, pois the radius cannot be used to remove this bias since itof curvature of the work, since the tool is works in the opposite direction on the twostraight sided. In the case of the crown gear sides of the tooth. Helical motion, a
24、 motion inR is infinite and the curvature on the work is: the direction of the generating gear axisduringgeneration, controls the bias by producing a1 = 1 1 change in pressure angle along the length ofthe tooth. Consider the generating gear inPw sin0 A tan Figure 8 with its axis pointing out of thew
25、here: paper. Helical motion is added by tilting theaxis in a plane parallel to the mean normalA is the cone distance of the work plane of the tooth. To hold the velocity in they is the pitch angle depthwise direction, the following relationshipmust exist:For the ease of the tapered generating gear R
26、is equal to A, tan rx : 1 L cosA = 1 sin A,A= cos ,=JR. 21r R.where:tan A = L 1A, is the generating cone distance 2rt A_cos _xr_ is the generating pitch angle where:The profile curvature on the work is:R. is the ratio of roll adjusted to1=1 1 + _2_11 maintain zero velocity in the normalPw sine A tan
27、 A_ tan rx direction,x is the generating spiral anglewhich is an increase in curvature. L is the helical lead3,.- Cutter AxessinA_ c./ R_ 2*tA_Generating Gear CenterIIFig. 9: Duplex Lengthwise MismatchFig.8: GenerationwithHelicalMotion _ Gear Cutter Axis/ -/J / , “Z7 tI iStandard Duplex “ Flared Cup
28、 DuplexFig. 13: TCA Comparisonsinai =n_b ACOS*FACE HOBBING No rBasic Generating Member It = 90 - _ + AFace hobbing employs a circular face hob type S = !/A_ + r2- 2A_ cos It“cutter. As the cutter turns through one bladepitch the generating gear and work index one cos n = A cos _ N, + nhpitch. The le
29、ngthwisetooth curveis an S Noextep.ded epicycloid that is formedEt tannkinematically. Figure 14 is a schematic that p = A cos _ an _ + l+tanA_(tan_+tan nshows the emulation of a face hobbinggenerating gear by a face hob cutter. Thegenerating gear and cutter roll togetheraccording to the following ra
30、tio, which giveszero velocity along the normal:N_= A cosn_ r sinACalculating Pointwhere: GeneratingGearCenter _ .)N. is the number of teeth inthe generating gearn_ is the number of bladegroupsr is the cutter radiusAt is the angle between the normaland the cutter radiusFig. 15: Face HobbingGeometryL
31、Blank Designwith uniform depth designs requires specialtopland and undercut attention during design.,“ _?, Tooth proportions are selected to avoid“ f“-?“/c_c_/ “ excessive undercut. Pointed or narrowtoplands at the inside are eliminated by“ “ . “ i introducing a secondary face angle as shown in, , F
32、igure 16./“, ,“4 I“._. Secondary Face AngleFig. 14: Face Hobbing Method “-_,x,_., iFace hobbing is a continuous indexing process.The generating motion which rolls out thetootti surface is superimposed on the indexingmotion. The process is also a conjugateqgeneration method which requires uniformdept
33、h teeth. See Figure 16.Lengthwise tooth curvature, at any point, iscalculated from the kinematics as shown in Fig. 16: Uniform Depth ToothFigure 15.6Since depth taper cannot be used to control Face bobbing cutters are designed with insidelengthwise tooth taper, cutter radius selection and outside cu
34、tting edges that areis more criticalthan with face milling. If the approximately on the same radius at theA lengthwise tooth curve was an involute, calculation height. See Figure 17. Toothslotwidth taper would not exist. Therefore, to thickness unbalance and non-equi-angularv avoid excessive slotwid
35、th taper, face hobbing blade spacing cause some radius difference.designs use effective tooth curvatures that are Two types of cutter design are generally used;usually not over 35% greater than involute one with special alternate cutting blades andcurvature at the calculation point. Continuous the o
36、ther with alternate cutting blades andindexing helps to minimize the effect of protective blades. The need for the specialslotwidth taper by dividingthe taper between blades or the protective blades is that thethe gear and pinion. Systems with non-equi- continuous indexing of the face bobbingangular
37、 blade spacing divide the taper process results in cutting on the clearance sideproportionately to the spacing, of the blades. The condition is most acute onpinions with low tooth numbers during theModifications of Generating Gear plunge portion of the cycle. The two-dimensional sketch in Figure 18
38、illustrates theBecause face bobbing is a conjugate system, problem. After the outside blade passesonly mismatch to compensate for through the work, the work has indexed half amanufacturing and build tolerances and pitch when the inside blade cuts through. Asdeflections under load is required. Normal
39、 tilt a result of the index the slot moves away fromis used to shorten the contact. Except for the cutter and the inside blade takes a smallhelical motion, the other freedoms described shallow chip, leaving stock to be removed byunder face milling apply as well to face the clearance side of the outs
40、ide blade.bobbing.Cutting tools with curved profiles arefrequently used as additional freedoms for Clearance Sidecontact control. Curved profiles affect both Blade Chipprofile and bias mismatch. The principledesignconstraintin tooth contactpattern _ ( _ ddevelopment is limited bias control.e ChipCUT
41、IING TOOLSFace milling cutters are relatively simple in Ill-/ _uethat the blades for completing from the solid Feed ,.or for roughing have al .-m._.te inside and .outside cutting edges with bottom lands thatoverlap. See Figcre 17. Single side fmishingcurets have all inside or outside cutting edges.1
42、I!1 _ Fig. 18: Clearance Side CuttingJ IVBlades used in the special alternate bladeI system have a double angle on the front faceII so that the clearance edge is sharp. Since thefront face is not ground during refurbishing,Face Milling Cutter TiN coating is used to improve blade life andsurface fini
43、sh. The resulting cutter isI extremely efficient, since it contains a large _ number of blade groups and uses both the) i_ i 1, : : cutting andclearance edges for stock removal.t 1_ The cycle is set up to plunge at the center of! _,/ roll, so as to distribute the cutting evenlyI betweeninsideandouts
44、ideblades.IIProtective blade cutters are designed witheither evenly spaced blades and a hook typeFace HobbingCutter blade precedingeach side cutting blade, orwith non-equi-angularly spaced blades and aFig.17 hooktypebladeprecedingonlythe burdenedblade. These latter designs are not as maximum tensile
45、 stress for the pinions andefficient, since the number of blade groups is gears. Maximum pinion tensile stress is 11%reduced. Further, with continuous indexing higher and gear stress is 14% lower on thethere is not a theoretically correct position for duplex design.the protective blade.STRESS TCAThe
46、 gear set defined in Table I was calculatedby the duplex face milling and the facebobbing methods. The finite element stress PSI (Thousands)TCA was used to analyze the contact pattern a0oand stresses of the two sets under load.Figure 19shows the contact patterns at bench, 2595%, 25% and full load fo
47、r duplex face milling 250in the upper grouping and for face hobbing in 224the lower grouping. Note the face milling i!iiiiiiiiiiii!icontact pattern does not cover the entire tooth 200 191 _i_:_liilil i!iliii!iiL“:!: !II:IEEIsurface at full load. The contact pattern could 172 iiii.:iiiii!i iii;ii!i!i
48、!be positioned more towards the inside of the irii_i._iiL-“ _:_=_=_,_:iiiliii ii!i!iiiblank and should have less lengthwise 150 ,_-_m_i_w.:_=:= iiliiiL.“!_mismatch. The face hobbed contact patterns ,i_i:i,ii!i!_ =_:m._,_:cover the tooth at full load. These patterns i!iiiiii ir,i!_iii!i!i_i_iiiiiI i
49、._iiimatch actual manufactured parts which were 100 iliiii iiiiiiiii_-.:_iEili- i!iiiiiitested. Figure 20 shows the calculated i!iiili !ii11iii“iiii_iiiiiiiNiiiii50 ii_:m:_ililii !iiii!iiiiiiiiiiiiiiiiii0Face Milling 17 ,_.aR. D_r._x 17 e,._rt. D_.EXPTNZON GEAR_ _. Duplex _. 20: FiniteElementMaximumTensileStreng-t:h._,_ u_,._-_,. _-_ 3.750“ Cutter Radius,A OU=I S,i=sSets manufactured to these designs werefatigue tested at 60,000 pound inches of geartorque. The three duplex sets had an averag
