:38 GEAR TECHNO~OGY

The Gear HobbingProcess

Dennis ,GimpertK'oepfer America Limited Partnership"

South Elgin.IL

ear hobbing is a generating proces .The term generating refers to the factthat the gear tooth form cut is lIotlllcconjugate form of the cutting tool,lhe

hob. During hobbing both the hob and theworkpiece rotate ill a continuous rotational.relationship. [)uring this rotation, the hob itypically fed axiaUy withall the teeth beinggradually formed as the tool traverses the workface (see Fig. Ia).

FOIl" a spur gear being cut with a single starthob, the workpiece will advance one tooth foreach revolution of the cutter. Wilen bobbing atwenty-tooth gear" the hob will rotate twentytimes, while the workpiece will rotate once.The profile is formed by the equally spacedcutting edgesaroundthe hob, each taking sue-

cessive cuts on the workpiece, with the work-piece ,in a slightly different po ition for eachcut (see Fig. Ib). Several cutting edge of thetool will be cutting at the arne time.

The hob is basically a wonn with gashes.CUi axially across it to produce these cuttingedges. Each cutting tooth is also relievedradially to provide chip clearance behind thecutting edge. This also allowsthe hob face tobe sharpened and still maintain the originaltooth shape. The final profile of the tooth j

created by a number of flats blending togeth-er. The number of flats corresponds to thenumber of cutting gashes which pass theworkpiece tooth during a single rotation.Thus, the greater the number of gashes in thehob, thegreater the number of flats along the

18fig.l a & b

1b:

~.15J"DP

"Fine Pitch is

Length

Active Length

- ---

Filg. 2

profile which improves the "smoothness" ofthe tooth profile.

The CutterBobbing is a generating process, and the

hob win not cut the same shape as the cuttingtool form, An unmodified involute gear toothis produced by .<11 hob with straight-sided cut-ting edges. Involete gear cutting is the largestapplication of bobbing (Fig. 2). In contrast, astraight-sided spline tooth is produced by a hobwith curved cutting edges (see Fig. 3).

Cutter ModificationsIt is possible to design the shape of a cutting

tool to produce modified tooth forms. Theseare done for various reasons. The hob toothroot can be designed to cut the outside diame-ter of the gear tooth. With this "topping" hob,the tooth involute and the outside diameter ofthe blank will be hobbed in one operation (Fig.4). This may eliminate finish turning of thegear blank, reducing machine operations.

The outside diameter of the gear will beconcentric with the operating pitch diameter ofthe gear. This win provide a locating surfacefor subsequent operations and a method ofmea uring size.

Sharp corners between the tooth flank: andoutside diameter can be eliminated with a"semi-topping" or "tip chamfering" hob (seeFig. 5). With proper design such a hob mayalso correct the problem of gear tooth bendingunder load,

Gears which will be finished by a subse-quent operation, such as skiving, shaving orgrinding, may require clearance in the geartooth fillet area for the finishing tool. This canbe cut using a "protuberance" hob. which will

-

Fig. 3

Whole Depthof Cut

tTopping

Fig.4

Gear Toolh

Fig. 5

Dennis G,imp'eirtis rile pre idem ofKoepfer Amuletl LimitedPartnership ill SouthElgin, IL. He is theauthor of several articleson gearing subjects.

JANUARVIFEBRUARY1994 39

F. IIg.6 I

Shaved'

Paint ofIntersection of

Pre-ShavedProlilawlth

Undercut

Lowest Point01 Contact

./" with MatingGear

~ Greater ~\--_.....,than-

Shaving Stock(Depends

upon DiarnetralPitch)

Root Fillet AsGenerated byPre-ShavingHob

fig.7- -

Index Worm Gear (R.H.)

DifferentialChange Gears

Speed Change Gears- --

Fig. 8

Mechanical Vertical Hobblng Machine

Fig.9

40 GEAF\ TECHNOLOGY

produce undercut (see Fig. 6).. The protuber-ance hob is designed to provide a uniformstock for the finishing tool and to provide ablend between the bobbed root area and thefinished flank. Caution: On any modified cut-ter, we are changing the correct rack shape.Thus, the tool may cut only a certain range ofgear teeth numbers correctly (Fig. 7).

The Gear Hobbingl MachineA gear bobbillg machine consists of five

common elements:• A work spindle to rotate the work.• A hob spindle to rotate the hob.o A means of rotating the work spindle and

hob spindle with a constant of ratio, dependingon the number of teeth in the workpiece amithe number of threads ill the hob.

'. A means of traversing the curnag too!across the face of the work in the direction ofthe work axis for spur and helical gears.

• A means of adjusting the center distanceof the work and the hobs for different sizeworkpieces, Figs. 8- W show schematics forthree typical bobbing machines.

Hobbing FeedDuring bobbing, the cutting tool can be fed

in a manner similar to' a milling machine; bothconventional and climb bobbing are used (seeFig. 11). A general rule of thumb is that climbhobbing yields better tool life, and!convention-al hobbing yields a better finish, In all. cases,the cutting force should be directed against the

work spindle, never against the tailstock.The directions of feed on a hobbing

machine correspond to the work axis .. Thus,three feeds are possible - axial, radial andtangential (Figs. 12a, b, c).

It is also possible to combine more than oneaxis of feed sequentially or simultaneouslyduring the machine cycle, A radial feedapproach followed by axial feed across theface is very typi:calin fine pitch gear work oron a workpiece where an open axial approachis not possible (Fig. 13).

Axial and tangenti,ai feed are used imulta-neously for several purposes on very large,coarse pitch, wide-faced gears. The tangentialfeed presents a. sharp portion of the tool as theaxial feed cuts across the gear face (Fig .. 14).

Taper root splines are cut with simultaneousaxial and tangenti .al feed (Fig. I5).

A "jump" or "skip" cycle is used. to cut rnul-

Radial, Feed

DriveMotor

Axial FeedConventional

Climb Feed Conventional Feedl

~- :

t

Fig. 11

Tang.enlial Feed Used for Worm Wheel Generation

1994 41Fig.12c

t Radial Feed

Radial Feed Approach with Axiall Feed

JANUARY/FEBRUARY

Axial and Tangential Feed--

Fig. 14

Taper Root Hob Taper Root Spline

Fig. 15

Jump Cycle

Tapered Gear Crowned Gear

42Fig. 11

GEAR TECHNOLOGY

tip le gear elements o.n a single part. This maybe required for gear tooth alignment or simplecutting efficiency (See Fig. 16).

Tapered gears or crowned gears are pro-duced with simultaneous radial and axial feed(Fig. 17).

At the beginning of the hobbing machinecycle, the cutter will not. be generating the fulldepth of the gear tooth form. Only a small cutis made by each tooth in the hob graduallyfeeding into the part. This is known as the"approach" portion of the hobbi ng cycle. It ispossible to utilize a different feed rate duringthis approaeh Iength with a reduction in cycletime (Fig. 18).

During the hob bing of some gears orsplines, the cutter win not feed completelythrough the workpiece face ..This is known as a"blind" cut. To complete all of the teeth evenlyaround the circumference of the gear, "dwell"is utilized. During dwell the hob and work-piece continue to rotate ill a timed relationshipfor one or two more work revolutions, hutwithout feed (Fig. 19).

Multilple, Start HollsThe hob is a series of racks positioned

around the circumference of a cylindrical tool.Each successive rack is shifted axially to createa worm, typically a single thread. Thus, foreach revolution of a single start hob, the gearmust advance one tooth space (see Fig. 20).This is accomplished by the bobbing machinekinematic indexing system.

At this point, it is important to understandwhat causes the cutting marks on a hob bedgear. The marks axially across the face of tinegear wiH carre pond directly with the axialfeed per work: revolution of the tool. Themarks positioned across the profile of the gea.rwill correspond directly with the number ofgashes or flutes on the hob (Fig. 2 O. Normallyit is not possible to see the generating flatsalongthe profile of the material.

It is possible to increase the speed of thehobbing operation by utilizing a hob with morethan one thread. or example, if the hob hastwo threads, the gear must advance two toothspaces for each revolution of that hob. Thiswill double the speed of the work and doublethe production. an other factors being equal(Fig. 22). With four threads in the hob, fourteeth on the part will index with every revolu-

lion of the cutter. However, there are factorthat limit. or prevent the use of multiple startshob for all ca e .

The HobAs more and more threads are designed into

the tool, the lead of the thread will increase.Normally. a thread lead angle of 2-60 will beacceptable. Beyondix degrees, the :Ieft andright side of the cutting tooth will be loadedunequally, which will cause poor tool life. Tocompensate fo£, !:his pmb.lem, ,!he diameter of'the [001 can be increa ed slightly. but. with areduction in RPM to maintain the arne SFM.Alternanvely, the gash of the hob can be madehelical to position the cutting tooth perpendicu-lar to the cutting action.

To calculate the thread angle of a hob. 1.1 ethe following formula:

Tan a = Thread in HobDP x Hob Diameter

Example:

Tan a=_]_20x.2

Tan ,a = .025

a= 1° 2S' 56"

Another problem with multiple thread hobsis the number of effective gashe generatinglbe profile, Again. all factor being equal. atwo-start hob with twelve gashe will generatethe gear profile with six of the gashes ver usasingle-start hob with twelve gashe .

The Ho'bbing Macbine,As the number of threads in the tool

increa e , the work will index. faster. Thismeans that. the work pindle of the bobbingmachine must be able to rotate at higherspeed . On workpieces with high numbers ofteeth. the machine peed is not aproblem, butfor gears with a low number of teeth. the hob-bing machine mu t be designed correctly.

Several machine design solutions are used.For traditional worms and worm wheel workspmdle drives. a multiple- uart worm andworm wheel can be u ed. Four, eight and morethread worms are common. Another approachis to utilize a helical gear index system. Bothsy terns work effectively in providing a mod-

Fig. 118

fig. 19

Height ofFeed Mar1<s

1i

Feed Marks froml Hobbing

---

Fig. 21

JANUARYJFEIIRU"RY laa. 43:

on. e HO.b Re..v.oru.tion. W. A.S .M.any w.ork TeethAdvances the Work As There Areby . . . Threads on the .

• . Hob ('In This ., . Instance, .2).

CuI in Same time

Double-Start Hob Single-Start Hob

Mark of Enveloping Cut

Fig. 22

Pitch

1'-:;f1

Tooth Spacing Inspection Chart Showing Hob Errorfrom Even Ratio -

Fig. 23

Cut with .Double-StartHobEven ,Ratio

C1Jtwith Double-StartHobHunting Ratio

Fig. 24

I· , . II•• I,

Lead Inspection Chart Showing: Hob Error fromHunting Ratio

Fig. 25

GEAR TECHfoiOLOGY

em high-speed bobbing machine.Another consideration for the bobbing

machine is the cutting capacity. As multiple-start hobs are used, the metal removal rateincreases. If a subsequent finishing operation isused, such as shaving. rolling or grinding, it ispractical to use much higher feed rates than usedin finish hobbing. These feed rates can approachthe maximum cutting capacity of a machine.Although the actual horsepower used in the hob-bing operation is a small percentage of that usedto drive the machine itself, the "effective" powerof the machine must be considered. This effec-tive power includes horsepower, rigidity, fixmr-ing, maintenance. condition, etc.

l1hread Spacing IErrorsThe multi-start hob will have manufactur-

ing errors between the threads: in other words,the threads will not be in the correct position.This thread spacing error mayor may not havean influence on the gear cut

There are three possible conditions betweenthe number of teeth in the part and the numberof threads in the hob. First, an even ratio, suchas a 2-starl hob cutter and a 22-tooth gear (seeFig 20). to this case, one thread will cut theeven teeth, and one thread will cut the oddteeth. Thus, all of the thread-to-thread errorwill be seen in the tooth-to-tooth spacing. butnot in the lead inspection (see Fig. 23).

The second condition occurs when certainthreads of the hob will cut certain teeth, suchas a 4-start bob cutting the same 22-tooth gear(Fig. 24a).

The final condition occurs when all threadscut all the teeth. This is the most ideal condi-tion, as the thread-to-thread errors become dis-tributed among all the gear teeth and "cancel"each other out. This is known as a huntingratio (See Fig. 24b). This occurs when a 4-starthob cuts a 21-ttooth gea.r. Thethread-to-threadhob spacing errors are not seen in the tooth-to-tooth spacing, but will be seen in the leadinspection (Fig. 25).

It is possible to purchase cutnng 'tools withimproved thread-to-thread spacing .• 1

Acknowledgement; Reprinted with permission of IheAmerican Gear Manufacturers Association. The opinions.statements and conclusions presented in this article arethose of me Author and ill no way represent the position oropinion of the AMERiCAN GEAR MANUFACTURERSASSOCIATION.