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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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38 gearsolutions.com
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.
40 gearsolutions.com
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.
42 gearsolutions.com
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.
44 gearsolutions.com
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.
46 gearsolutions.com
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).
48 gearsolutions.com
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