Cyclocut Bevel Gear Production

Cyclocut Bevel Gear ProductionThe following technical paper from Gleason describes the Cyclocut process for advanced cutting, skiving, and semi-completing of bevel gears in low quantities.

By Dr. Hermann J. Stadtfeld

The finishing blades are replaced by skiving blades,

which commonly have brazed on CBN (cubical boron

nitride) inserts on the cutting edges. The roughing blade

slots are not used during the skiving operation.

Cyclo-Palloid is a continuous indexing face hobbing

method with parallel depth teeth that is based on

conjugacy. This means without any flank corrections

the pinion and gear flanks would contact along contact

lines in every roll position. While rolling through an

entire tooth mesh, the transmission ratio in case of

conjugacy is perfectly constant and equal to the ratio

of the pinion and gear tooth count. The motivation to

use two-part interlocking cutters is based on the idea

of applying a mathematically simple method to gener-

ate length crowning by combining nominal inside blade

diameters with outside blade diameters that are larger

than nominal. The enlarged outside blade radii generate

outside flanks that, in interaction with the nominal inside

outside flanks, will lead to length crowning controlled by

the amount of outside blade point diameter increase.

The graphic in fig. 2 shows how the two different pitch

point diameters are arranged to be approximately tan-

gential in the center roll position at the tooth mid-face. In

order to accomplish the correct position of the two inter-

locking cutter parts, a main (outer) spindle carries the

outer cutter part, while the inner cutter part is connected

to a secondary (inner) spindle. The secondary spindle is

positioned at an eccentric position relative to the main

spindle, such that the two pitch point circles contact each

other at the calculation point in mid-face position and the

offset lines of both cutter parts lie on top of each other

and are perpendicular to the flank line tangent (fig. 2).

LLoW-quaNTITy BeveL gearSeTS CaN Be maNufaCTureD WITH THe CyCLo-PaLLoID®

meTHoD. THe CyCLo-PaLLoID uSeS INTerLoCkINg faCe HoBBINg CuTTerS WITH fIve

STarTS IN moST CaSeS. for THe SofT CuTTINg, eaCH CuTTer STarT INCLuDeS SePa-

raTe ouTSIDe aND INSIDe rougHINg aND fINISHINg BLaDeS. fIgure 1 SHoWS aN

INTerLoCkINg CyCLo-PaLLoID CuTTer HeaD WHere THe CeNTer ParT CarrIeS aN

INSIDe rougHINg BLaDe aND aN ouTSIDe fINISHINg BLaDe. THe INTerLoCkINg SeC-

oND ParT of THe CuTTer CarrIeS THe ouTSIDe BLaDeS. THe CuTTer aS SHoWN IN

fIg. 1 IS SeT uP for THe SofT CuTTINg oPeraTIoN. THe Same CuTTer HeaD CaN Be

uSeD for HarD fINISHINg By SkIvINg.

Fig. 1: Interlocking Cyclo-Palloid cutter head.

Fig. 2: Principle of length crowning by dif-ferent cutter radii.

DECEMBER 2011 37

Profile crowning in Cyclo-Palloid is generated with curved blade

cutting edges. Both pinion and gear are strictly generated bevel

gears; there is no non-generated version of Cyclo-Palloid avail-

able [1]. a hypoid offset of the pinion versus the gear is basically

possible, but very seldom used in practical applications.

IntroductIon to cyclocutInterlocking face cutters are time consuming to build and have

lower stiffness than single part cutters. also, the provisions on

the machine of a main spindle with an adjustable secondary

spindle inside requires a complex design and results in reduced

stiffness.

Cyclocut™ successfully replaces the interlocking cutter with

a single part cutter, which can be used on freeform Phoenix®

II bevel gear generators in order to produce gears with flank

surfaces that match Cyclo-Palloid flanks. Length crowning is

generated with single part cutters (in a completing process) by

utilizing a cutter head tilt as shown in fig. 3. The untilted cutter

to the left requires two cutter parts, rotating about different cut-

ter axes (∆ρ), like the case in Cyclo-Palloid. It is also possible to

tilt a single part cutter about the mid-face flank line tangent and

adjust the blade angle by the same amount to achieve identical

curvature radii as in the case of different cutter axes for inside

and outside blades. The mentioned blade angle adjustment also

Fig. 3: Conversion of an interlocking cutter into a single part cutter.

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assures the correct pressure angles on both flanks.

figure 4 shows a comparison of a simulated tooth contact analy-

sis based on the Cyclo-Palloid cutters and machine kinematics to

the left and based on the single part Cyclocut cutter and machine

kinematics to the right. ease-off, tooth contact, and motion error

between Cyclo-Palloid and Cyclocut show the identical character-

istics. Practical trials proved that Cyclo-Palloid pinions can roll

with Cyclocut gears and vice-versa without significant differences

in contact patterns and rolling characteristics [2].

tools and cuttIng ProcessThe first generation Cyclocut cutter head (shown in fig. 5 to the

left) used rectangular blade sticks and was mostly applied to wet

cutting with HSS (high speed steel) blades. The second generation

Cyclocut cutters (fig. 5, right) utilizes five-sided Pentac® slots that

provide a positive seating which is beneficial for all applications

of carbide blades [1]. Pentac blades eliminate basically all micro-

movement of the blades in the cutter head slot, which is important

in order to reduce or eliminate cutting edge chipping.

The machining process for soft cutting is a standard face hob-

bing cycle with a center plunge and double roll cycle. In order to

maximize tool life, the center plunge utilizes a vector feed [2] in

order to balance cutting edge wear between inside and outside

blades. The preferred hard finishing after heat treatment is skiv-

ing. In Cyclocut, the soft cutting prepares the root geometry for

the following skiving process with a root skiving distance, a flank

stock allowance, and a protuberance between flank and root fillet

radius. figure 6 shows those three geometric elements that are

standard features of Cyclocut [3].

cyclocut skIvIngCyclocut skiving is a hard cutting process that utilizes coated

carbide blades with a chip-forming facet that has a 20° negative

side rake angle (T-Land in fig. 7). In the skiving process the blade

cutting edge is required to develop a high normal force contact

with the flank surface in order to penetrate into the hardened steel

and form a chip. The same Pentac cutter as shown in fig. 5 for the

previous soft cutting is used for the skiving only the soft cutting

blades are exchanged with skiving blades. It is very advantageous

to use single-part Pentac cutters instead of interlocking cutters

mounted to a double spindle because of the high stiffness require-

ment between work and tool in the case of skiving. Cyclocut skiving

is performed at surface speeds of 120 m/min and removes chip

thicknesses of 0.1mm, which is equivalent to an end chip value of

0.34mm.

a dry skiving as shown in fig. 8 is possible and might be the

process of choice if the Cyclocut soft cutting was already done

as high speed dry PowerCutting®. However, wet skiving is still the

DECEMBER 2011 39

standard process that delivers better tool

lives than dry skiving.

Skiving chips for wet cutting have a differ-

ent formation and color than dry cutting. a

comparison of dry and wet chips is shown in

the photos of fig. 9. In any case, wet or dry,

an all-around coating of the blade cutting

edges is recommended in order to achieve

optimum tool life results. future develop-

ments will concentrate on a strictly dry skiv-

ing process without any tool life penalties

compared to wet skiving.

a typical Cyclocut gearset with a face

width of 112mm and a whole depth of

25mm is shown after hard finishing by

skiving in fig. 10. The tool life for the parts

shown was 20 ring gears and 24 pinions,

amounting to 640 gear slots and 312 pinion

slots, which is a remarkable result com-

pared to generally smaller numbers for skiv-

ing with CBN cutting edges. The reasons

can be found in the carbide blade design

and the higher stiffness between blades,

cutter head body, supporting machine com-

ponents, and work piece.

Tool spacing measurement results, sur-

face roughness, and waviness of the ring

gear are shown in fig. 11. The excel-

lent spacing quality together with the low

surface roughness and waviness values

also confirm the optimal conditions of the

Cyclocut skiving process.

tooth contact develoPment and correctIonsalso for Cyclocut pinions and gears, 3D

measurement of flank surfaces and flank

form corrections are available standard fea-

tures of the gleason metrology and correc-

tion software “g-age.” Smaller manufactur-

ers often have no coordinate measurement

Fig. 4: Tooth contact analysis (TCA) of Cyclo-Palloid (left) and Cyclocut (right).

Fig. 5: First (left) and sec-ond (right) generation of Cyclocut cutter heads.


capability available and like to rely on the tooth contact pattern

appearance from roll testing as shown in fig. 12.

If a correction of the contact positions is desired, the gleason

“Tooth Contact Corrections” can be utilized for a profile and

face width contact movement on coast and drive side. figure 13

shows the input screen to the left. The input values show the

desire to move the contact bearing on the drive side 5mm toward

the toe and 2mm toward the top. as correction target, pinion or

gear for soft cutting and skiving is possible. The output of the

“Tooth Contact Corrections” is similar to corrections known from

g-age, which do address the major basic settings in the Phoenix

summary (fig. 13, right side). Cyclocut tooth contact corrections

do not require blade modifications, which makes their execution

very easy and fast.

semI-comPletIngIn some cases grinding is the desired or required hard finishing

process. The Cyclocut flank geometry has three specific geom-

etry features similar to other face hobbing processes. Those

features are the parallel depth teeth, the constant slot width

taper (which reduces the tooth thickness and slot width propor-

tional to the distance from the pitch apex) and the epicyclical

surface function of the flanks in face width direction. figure 14

shows the face hobbed Cyclocut slot and the difference between

a circular and an epicyclical lead function. The tooth depth (par-

allel or tapered) has no influence on the success of a grinding

process. The proportional change in slot width will require grinding

Fig. 6: Semi-finish strategy for skiving.

DECEMBER 2011 41

the convex flank with different machine settings than the concave

flank (setting change in spiral angle direction). In case of grinding

with a cup-shaped grinding wheel the epicyclical lead function will be

replaced by a circle, which requires an uneven stock removal. The

questions are, to what extent is the uneven stock removal accept-

able, and how will the changed surface lead function influence the

properties of a Cyclocut gearset?

Large bevel gear sets, which are not quenched in a die, have rath-

er large distortions in the vicinity of 0.15 to 0.3mm. In most of those

cases the stock has to be removed in multiple steps by repeating the

hard finishing cycle several times. The additional non-uniformity due

to a circular flank lead as final surface function seems acceptable

in practical cases as long as the magnitude of the non-uniformity is

below the maximum permissible stock allowance.

The influence of the surface function change on the Cyclocut

Fig. 9: Wet skiving chips (left) and dry skiving chips (right).

Fig. 10: Typical Cyclocut gearset.

Fig. 11: Pitch Variation, sur-face roughness and waviness of a skived part.

Fig. 7: Cutting edge facet (T-Land) with negative side rake.

Fig. 8: Photo of skiving process with Cyclocutcutter on Phoenix II 600HC.


gearset performance is minimal because of

the low number of five cutter starts. With a

decreasing number of cutter starts, the face

hobbed lead function migrates to a circular

function. The v-H characteristic in case of a

17-start cutter system would in fact change

dramatically if the epicycloid was replaced

by a circle. In case of five starts, the effect

is negligible. Since all other properties of

the Cyclocut geometry such as parallel

tooth depth and proportional slot with taper

can be preserved, the remaining question is

only how such a “Semi-Completing” grinding

process can be realized, regarding machine

settings and motions as well as grinding

wheel geometry?

From cyclocut to semI-comPletIngThe conversion task from Cyclocut to Semi-

Completing is basically twofold.

1) a cutter head and blade geometry has to

be found which first will assure a larger

outside blade point radius than inside

blade point radius, and secondly form

flank lead curvature which are identical

to the Cyclocut lead curvature at a cer-

tain point at the face width.

2) Separate basic settings for convex and

concave flank grinding have to be calcu-

lated which match the slot width taper

from face hobbing and minimize the

surface deviation between epicycloid and

circle.

The changes to the cutter head and blade

geometry between Cyclocut and “Semi-

Completing” are shown in fig. 15 in two

separate steps. Step one, from “Cyclocut”

to “Step 1 Completing cutter,” requires to

the outside blade (arrow tip of roB) to a

reduced radius and the inside blade (arrow

tip of rIB) to a larger diameter such that the

negative cutter point width from face hob-

bing becomes positive (see fig. 15, change

from top to middle graphic). Step 1 causes a

curvature radius change to roB (larger than

rIB), which would produce flank lead curva-

ture that causes negative crowning between

pinion and gear. Besides, those curvatures

Fig. 12: Tooth

contact roll test results.

Drive Side Contact Coast Side Contact

DECEMBER 2011 43

Fig. 13: Tooth contact correction software.


would also result in rather high deviations in flank form compared

to the originally manufactured Cyclocut geometry. In order to re-

establish the curvature radii from Cyclocut, the radii roB and rIB

are drawn under the same angle (using the vector length from the

top graphic in fig. 15) with their tip pointing at the same profile point

as in the middle graphic of fig. 15, resulting in two new vector origin

locations (bottom of fig. 15). If the vector origins are connected, the

new “actual cutter axis” and the tilt angle for semi-completing will

be found as shown in the bottom graphic in fig. 15.

It has to be mentioned that the graphics in fig. 15 are a 2D sim-

plification of a 3D problem. The correct solution is computed with a

3D vector approach, based on the explained principle.

dIFFerence Between ePIcycloId and cIrcleThe deviation between epicycloid and circle will be compensated

for in the interaction between pinion and gear flank surfaces. They

have congruent deviations, which cancel out the differences in

the tooth contact and ease-off. regarding the non-uniform stock

removal between soft machining with face hobbing and hard finish-

ing with face milling, the knowledge about the deviations is still

an important basis for the minimization of the difference between

epicycloid and circle or as a guideline for the required case depth in

the heat treatment process.

The pinion flank form deviations (soft manufacturing to hard fin-

ishing) have been calculated and graphically shown in fig. 16 for the

same sample gearset, which has been used throughout this paper

to demonstrate the Cyclo-Palloid conversion to Cyclocut with tooth

contact analysis (TCas).

The optimal cutter radius is calculated from the epicyclical cur-

vature in mid-face. The two graphics on top of fig. 16 show the

pinion convex and the pinion concave deviations if the spiral angle

is matched in the center of the face width. a considerably better

surface match was found in the lower two graphics of fig. 16. Since

the epicycloid has a constantly increasing radius of curvature from

heel to toe, it is possible to find a spiral angle match about 25 per-

cent of the face width away from the heel, and at the same time a

spiral angle match 25 percent of the face width away from the toe.

If the radial setting, which dictates the spiral angle, is calculated

from the epicyclical kinematic from those two positions, then the

average value of the two slightly different results will always lead

to the optimal match conditions. The gleason Semicom software

automatically calculates the optimal settings for minimum devia-

tions, and therefore minimized stock variations during the hard

finishing process.

FInal conversIon to semI-comPletIngfor the final conversion of the Cyclocut basic settings, as well as

cutter head and blade specifications, pinion and gear are developed

with mirror image generating gear specifications (basic settings).

This reduces the influence of different kinds of side effects, like

diamond contact bearing in case of highly asymmetric outside and

inside blade angles. The result of the Semi-Completing conver-

sion of the example gearset used in this paper is shown in the

tooth contact analysis (TCa) results in fig. 17. Bias direction of

the tooth contact and motion graph compare well to the original

Cyclo-Palloid analysis. The curved path of contact and the twisted

ease-off topography (on top in fig. 17) are the result of cutter tilt

Fig. 14: Difference between epicycloid and circle.

Fig. 15: Conversion of Cyclocut into a Semi-Completing setup.


and machine setting adjustment in order to accomplish the conver-

sion task.

The following steps are performed by the gleason Cyclocut

and Semicom Software to come from Cyclo-Palloid to Semi-

Completing:

• Conversion of Cyclo-Palloid to Cyclocut;

• Switch face hobbing to face milling;

• Calculation of optimal average cutter radius and radial distances

to achieve minimal deviations like shown in fig. 16, bottom

part;

• Establish positive cutter point width with clearance to the inner

slot width (average cutter radius is kept constant), shown in fig.

15 middle part;

• Calculation of tilt angle from bottom graphic in fig. 15;

• Contact optimization as required.

The restrictions in tooth contact development may seem low

because of the single side freedoms a Semi-Completing process

offers, but it has to be considered that in contrast to a fixed set-

ting process, a single grinding wheel is used to grind both convex

and concave flank. The grinding wheel point width is defined by the

inner slot width and the mean radius was derived from the midpoint

of the epicyclical kinematic in order to minimize the deviations

between epicycloid and circle. also the radial distances for the two

setups for convex and concave flank grinding are predetermined

to minimize the deviation in fig. 16. The final freedoms, cutter tilt

combined with tool profile angle changes as well as tool profile cur-

vatures, are used to accommodate the correct effective length and

profile curvature. Since the setups of pinion and gear are basically

mirror images, the tooth contact will always be central as desired.

The exception of the mirror image setups is the case of previously

optimized Cyclo-Palloid or Cyclocut designs.

The method to use blade angle modification in connection with

cutter tilt is a well-known technology that has been used for many

years in fixed setting, as well as completing designs, which was

originally developed by gleason in the 1940s [4]. In 1988 the

blade angle modification in connection with cutter tilt was expand-

ed by Dr. gerhard Brandner [5] in order to hard finish previously

face hobbed parts with a circular tool in face milling. Brandner

describes in his teachings that the same tool can be used in con-

nection with different basic settings in order to finish both the

convex and the concave flanks of a bevel gear. This method was

later named “Semi-Completing.”

summaryThe flow chart in fig. 18 (see pg. 49) shows the different pos-

sibilities of data input. The soft machining process could be per-

DECEMBER 2011 47

formed using klingelnberg Cyclo-Palloid,

oerlikon Spiroflex/Spirac, or gleason TrI-

aC® designs.

Software modules like Cyclocut and

Spiroform convert the input files into gleason

basic machine settings and tool definitions.

The processed Cyclocut, Spiroform, and

TrI-aC can either be used for soft cutting

and hard skiving or for soft cutting only. In

the second case, grinding is possible as

the hard finishing process after Semicom

converts the face hobbing-completing data

into face milling Semi-Completing data.

It is an important task of the conversion

to minimize the non-uniformity of stock

removal due to the difference between epi-

cycloid and circle. one grinding wheel with

outside and inside profile definition and two

sets of basic settings are generated by the

Semicom software. The grinding stock is

removed on the convex flanks and the con-

cave flanks in two separate passes, which

can be accomplished in a “up-roll -> convex,

down-roll -> concave” grinding. In cases of

large bevel gear grinding with high amounts

of stock removal, the single side grinding

can be viewed as an advantage that con-

tributed to high accuracy, good surface fin-

ish, and the avoidance of thermal material

damages.

reFerences:1) krumme, W. klingelnberg Spiralkegelräder.

Springer verlag, Berlin, Heidelberg, New

york, 1976

2) Stadtfeld, H.J. Cyclocut™-a Jobbing

System for Bevel and Hypoid gears.

gleason Publication, rochester, New

york, January 1998

3) Stadtfeld, H.J. advanced Bevel gear

Technology, manufacturing, Inspection

and optimization. gleason Publication,

rochester, New york, may 2000

4) NN. Calculating methods for spiral

bevel, zerol bevel and hypoid gears on

gleason machines. gleason Publication,

rochester, New york

5) Brandner, g. verfahren zum

fer tigbearbeiten vorverzahnter

kegelräder. Patentschrift Deutsche

Demokratische republik DD 257 781

a1 1988

* Cyclo-Palloid® is a registered trademark

of the klingelnberg Corporation. Phoenix®,

PowerCutting® and TrI-aC® are registered

trademarks of The gleason Works.

Fig. 16: Actual deviation between epicycloid and circle.

about the authoR:

Dr. Hermann J. Stadtfeld is vice president of bevel gear technol-ogy at The gleason Works. go online to www.gleason.com.

Fig. 17: Semi-Completing tooth contact analysis (TCA).


Fig. 18: Data input and flow for different Semi-Completing gear types.

Cyclo PalloidOld Style

Calculation Output

INT-FileASCII File with Blankand Machine Settings

CYCLOCUTGleason CYCLOCUT

Cutter System & Basic Settings

Cyclo PalloidNeutral Data

Output

Spiroflex/SpiracOerlikon CDS

Files

SPIROFORMGleason SPIROFORM

Cutter System & BasicSettings

SEMICONCalculation of Semi-Completing

Machine Settings and Grinding Wheel

TCADownload File CMMGrinding SummarySubset of UNICAL

Spiroflex/SpiracNeutral Data

Files

Gleason NeutralData Converter

to SPA-File

ConversionSPA-AAA

File

Gleason TRI-ACSPA-File

ConversionSPA-AAA

File

AAA-FileAAA-FileAAA-File

AAC-File

SPA-FileSPA-FileCDS-File

DECEMBER 2011 49

Cyclocut Bevel Gear Production

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