New Innovations inHobbing Part II

IRoben Phillipspt. te -,M'- -- C -Hing 1001!s LP'au er _ aa,g u___ _ I __ •

Loves Park, IL

IntroductionThe first part of this article, which ran in

the September/October 1,994 i ue, explainedthe fundamentals of gear bobbing and omeof the latest techniques, including methods ofhob performance analysis and new tool con-figuration . being u ed to olve pecifie appli-cation problems. In thi ,i ue, the author con-tinues his exploration of hobbing by de crib-ing the effect of progres Olll requirements inaccuracy, as well as the latest in materials,coatings and dry hobbing,

by Machining Method

Is ~ 4 1 141i ~ 4..... 4 5 3...:l 6 5

~' 7 sI 6 4~ 6

=QI 8 7 5

9 II 6

10 9 6

11 9 10 7 9!O 10 12

I~10 II R 10 12 7II II

I

Fr = Runout

fp = Pitch variationFf ;;;Protile

P~ = Lead

Ii = Tooth-to-tooth composite

Pi ;;;Total composite

hming Method

Possible only under IIspecial condlrions

SOUR EO~lPJ\RISO, S: DIN 3962 (1974) and AGMA 390.03NOTE- SEE UIE ACTUAL ST II DARDS FOR EXACT COMPAR1SO S

F,ig. l

26 GEAR TECHNOLOGY

A.ccur.acy]mprmtemell1.tsWhile considerable 'progress has been

made toward solving application problems,part requirements are becoming more andmore demanding. to a point where the qualitylevel expected from the tool has to be raised.II ha long been under toad that there is avery direct relationship between the accuracyof the hob and tile quality of the part beingproduced. It has been common 10 see thequa.ity requirement of the tool. raised fromClass B to Class A or even Class AA ..Thereare even applications for whichthe require-ment actually exceeds the industry standard10 a point of developing tolerances thai areClass AAA.

For an idea of the "normal" quality levelthat are achievable with different manufactur-ing processes, refer to' Fig. t.

The twoelements In the hob that directlyaffect the quality of thegear being producedare the lead in one pitch and Ihe profile. In aingle-thread hob. if the error. in these ele-

ment are added together, the re ult will beequivalent 10 the involute error possible inthe past.

Historically. the method to measure thiserror is a convennonal lead check and profilecheck (Fig. 2). With this method, it is possi-ble to find the worst : pot in the lead and addil to the profile error and be relatively confi-dentthat thi. will represent the worst-caseinvolute error.

The lise of eN inspection equipmentmakes it po ible to rake this mea urementdirectly. The line-of-action check (Fig. 3)enables the evaluator '10 review the combina-lion 0:1'the lead and profile in one tep, The

method involves making the same movementax.ially as checking the lead, but in addition,the probe is moved radially to simulate thecontactpattern realized when hobbing thegear. One drawback: to the present method ofline-of-action check is that it represents onlyone generating zone ef the hob.

With multiple-thread hobs, a similarapproach to evaluating the accuracy can beused with the addition of one more element,referred to as thread-to-thread error. In thesecases, the urn of the lead error in one pitch,the profile error and the thread-to-threaderrorwill give the involute error that can beexpected. This additional element limited theuse of rnultiple-thread hobs to roughing andpre-finishing operations, but today hob manu-facturers can produce very accurate multiple-thread hobs.

With the introduction of the latest CNCtechnology to the grinding and inspection ofhobs, tolerances that were virtually impossi-ble to hold now are maintained routinely. Insome cases.the thread-to-thread error hasbeen held within.OOOI-.0002". As mentionedearlier, the requirements of today's tools havein some cases even surpassed the industrystandards, Here Class AAA tolerances thatare equal to 60% of Class AA have beendeveloped. Of course, it must be understoodthat in these ca es, the sharpening tolerances,hub faces, hub diameter, bore, etc" all mustbe modified to support the tight tolerances onlead and profile.

Ma.terialsWhen the idea of improving the productiv-

ity of an application i discussed. one areathat normally receives considerable attentionis the substrate material. of the tool. The intentof this article is not to give a completedescription of the different grades of steel, butinstead to rai e the level of awareness aboutthe multiple pos ibilities for a elution to aspecific problem ..The successful introductionof particle metallurgy some years ago hasgiven the application engineer materials withcharacteristics of wear resistance. toughnessand red-hardness levels (Fig. 4) considerablybetter than the original high-speed steels.

The general direction of the industry inrecent years has been to upgrade the steel fora given applicarion, normally by increasing

SpecifiedPlotrlng Path Radial Plane (-) (+)

.........L-

SpecifiedPloaTrue

Plot

I1

Profile~Error

Hob Profile InspectionOne Axial Pitch

(12 Gashes)

r--L-1 i;=r.--t---- ......Lead Error!I

Fig .. 2:

Lead Error .--,---+t--r----<:

Om: Axial Pitch(12 Gashes)

Hob Lead Chart

Fig. 3 Line-or-Aclion Check

Hligh-SpeedSteel Comparagraph200 I

I I v-II' V /

V \ ,;f ,I; r-Wenr Resistance / \1 ~ - I

~ V ~ '" / I-Red Hardness I II I

Toughness (CPM]I

I III

L,----- II1 I 'I

rT!Ugh~e" r - ---- ./ .......l ... ~..... _J I-,(CoovenlioruUl

I I I II

180

160

141)

100

811

611

20

oMI \12 MIO TI M7 \1.1 \i4

(PM (PM (PMT4 MJS MB TIS RJ'.X M4J M~~ REXM-16 REX

zs 20 76\Ifil M62 M48

L-J ICohal' Super

High - Speed Steeb. Fig. 4Genen ..dPurpose

Vanadium

its alloy content. The advantages of thisapproach can be realized in a number of dif-ferent areas. The first one is the improved lifefactor, which results in increased lineal inchescut per sharpening when compared to the ini-tial base grade. A table showing relative life

Roben Phlill!ipsis Vice-Presidelll ofEngineering withPfauter-Maag CUllingTools. He is the author ofmarly articles and papers011 gear curling subjects.

NOVEMBERIDECEMBER 1994 21

M3100

REX45

110

Fig.S

(Percent. Re]!ati.\'e Life)

Mil

us

REX76

120

M35

110

T1S

120

M1I2.110

ASP23

tOO

REX20

.110

ASP30

HO

M3100

REX4S

115

Fig. 6

(Percent Relative Feed)

M4105

REX76

115

M35100

Tl5115

M42lIS

ASP23

100

REX20

115

ASP30100

M3100

RlEX45

120

Fig. 7

(Percent Relative Speed)

M4100

REX76

200

M3S

120

TIS200

M41125

ASP23

100

REX20

125

ASP30120

M3100

Mil

100

REX 76

175

(Percent Relative Prlee)

M35115

TlS165

M42125

ASPB100

REX20

125

ASP30125

REX45

125

Fig. 8

28: GEAR TECHNOLOGY

factors as compared with M3 steel for differ-ent high-speed steels is shown in Fig. 5. Thefigures in this table and the two that followdepend on how aggressively the original toolis being applied. Actual results may vary, butgenerally this win give the manufacturer agood place to start,

In addition to the increase in tool life, thegain achieved in productivity improvementsi another area that deserves attention.

In a typical hob bing application, approxi-mately 85% of the total manufacturing cost ismachining cost. Thi machining cost is direct-ly related to feeds and speeds in any givenapplication. Some typical figures for relativefeeds and speeds are shown in Figs ..6 and 7.

When compared with the total manufac-turing cost per metal-cutting operation, theprice of the tool is minor. It is not uncommonfor the purchase cost of a tool to amount toonlyabout 5% of the tala 1cost per part. Withthis in mind, it follows that simply buying thecheapest tool is not an effective way ofreducing cost. ]f the purchase price of thetool is combined with the cost of resharpen-ing and recoating, we find that. the tool cost isapproximately 15% of the total, Relativeprice increase compared wirh M3 steel areshown in Fig. 8. This table takes into accountboth tile increase due to material cost and theadditional machining cost encountered by thetool manufacturer.

One area that has shown significantadvances in the past few years is the ability totailor the hardening of high-speed steel to theperformance of a pecific application. Thisallows the tool manufacturer yet anotheropportunity to addre s wear or failure con-cerns. The approach that has been taken con-sists of hardening the tool to a higher or lowerhardness than what might be considered "nor-mal." In fact, we have seen occasions whereeven one point higher or lower in hardnesscan make the difference between catastrophicfailure and succes .

Next in the series of material improve-ments is the field of carbide tools. There hasbeen a great deal of effort to apply carbide linsituations where high-speed steel hobs nor-mally would be used. The main reason forthis is to take advantage of the highproduc-tion rates that are possible with carbide .. The

gear hobbing industry has realized that inmany cases, the relatively high tool cost of acarbide hob can more than be offset by thereduction of machining co t.

The availability of some newer grades ofcarbide have solved some of the earlier prob-lems of applying carbide tools, The cuttingprocess of hobbing is a very severe interrupt-eel cut that demands certain characteristics toas ure success. Intensive research in the past.few years has Ied to the developmern ofmicrogram carbide, which addressesthe prob-lems associated wit.h this process.

One factor that must be taken into consid-eration when applying carbide is the speed atwhich the hob is being run. In many cases, themachinery needs to be capable of speeds twoto three times faster than those used withhigh-speed steel in order to take full advan-tage of what carbide has to offer, There iseven a possibility of failure if these speeds arenot possible. Running these speeds, of course,reduces the machining cost significantlywhile at the same time yielding extended lifeas compared with high-speed steel,

A comparison of the quality of parts thatwere cut with a high-speed steel hob and partscut with a carbide hob, each at the optimalconditions, shows a great advantage in favorof Lhe carbide hob (Fig. 9). The reason is thelower force components resulting from theconventional process, the lower number ofthreads, a lower axial feed and the higher cut-ting speeds.

CoatingsSince the early 19808. titanium nitride

coatings have been very succe sful at improv-ing the process of hobbing. This coatingproved to be extremely adaptable to mostapplications and gained acceptance relativelyquickly within the gear cutting industry. It isassumed the reader is familiar with the advan-tages of coatings in general, soa derailedexplanation of economic justification is notoffered with this article.

In recent years, there has been consider-able work to improve on what titaniumnitride had done within this industry. Thenumber of hard coatings that are available hasincreased dramatically. Today, there are asmany as a dozen different coatings and multi-layer coatings available to choose from.

HSS C6, P20/25I I I

I i

I .-17 ( ,~

~'-II ,

IJ J I

i\ 1\ I ) If,

A jE

.........L... 'I TT, II1)- 1'" P If" r',\ J I ! f !I

~~I \1 ! I

I fT I I III

I

I , II

,

,I I I II

II,

I' j III I

'II 11 \

IA Root 'I E

Z, =2RHVO = 280 m/minfa = 3 rnrn/rev

Opposite Hand

m = 1.75 mma = 150dal= 110mmnj = 12Workz2 =36~. =290 LHb = 15.2 mm

Rm= 600 -700 N/mm2

Core Hardened Steel

Some of these include• TiCN- Titanium Carbonitride• TiAIN-Til.anium Aluminum Nitride• CrN-Chromiurn Nitride• CrC-Chromium CarbideAU of these coatings have certain advan-

tages ..Titanium nitride is very versatile, whilesome of the newer coatings are more applica-tion-specific. For example, TiCN has shownpromise in areas that are very abrasive (castiron), where TiN may not be performing at anacceptable level. TiCN, on the other hand. issomewhat sensitive to temperature, so incases where high speeds are attempted, thiscoating may not show the same improve-ments, 11seems that in cases where tempera-ture is an issue, TiAlN may be a better selec-

NOVEt.!BER/OECEMBER 199<. ,29

ZI",3 LH'Ie = 80 m/minfa = 4.5 mm/rev

Same HandProcessCoolant: Aral f' 3105 NClimb

Fig. 9

30 GEAR TECH'NOLOGY

tion, There has been some work recentlyapplying TiAIN to high-speed steel in dryhobbing attempts for just that reason.

It now 'becomes even more imponant forthe end user to work very closely with the1001 manufacturer to coordinate the design,material, application and coating to obtain thebest possible solution 1.0 a specific process. Itis still generally accepted thar a starting pointfor the addition of coatings is TiN. It may notbe the final solution, but the user can 'beginwith a relatively high level of comfort as tothe success of the coating.

Dry MobbingOne last area to explore is the recent work

being done in dry bobbing of gears. This sub-ject has a number of areas being developed,but the common goal is to successfully hobgears without using coolants. The advantagesof hobbing without coolant are numerous,including cost reduction in

• Cleaning/washing parts.•Piltration• Chip di posal (dean)• Coolant requirement• Coolant additivesThese factors, as well as the environmental

issue of disposal, have driven the efforts to

progress with this new technology. There area number of different approaches to how thiscan be accomplished. The actual solution, aswith the new coatings, may prove to bedependent on the application. The basic meth-ods to develop this technology can begrouped as

• High-speed steel hobs

• Carbide hobs•.Cermet hobsHigh-speed reel hobs have been applied

with TiALN coatings with initial success. Tbematerial removal rate of this application iscomparable to carbide hobbing at high speedand low feed .. The tool life was even betterthan a TiN-coated tool with o.il coolant

Carbide tools have been applied as bothcoaled (TiN) and uncoated, Considerable caremust be used to select the best grade of carbidefor each application. The ability of carbide towithstand high temperatures while providingvery high wear resi lance has significantlyaffected the success of the approach.

Cermet materials are multi-component cut-

ling materials generally based on titaniumcarbide (TiC), with nickel. (Ni) a a bindermatrix ..They were primarily developed toprovide thermal stability.edge-lrolding abilityand toughness. In many cases, coating thistype of too] can also improve its performance.There are certain advantages as well as risksinvolved with applying this material in thehobbing applications.

The advantages include:• Three to six limes the life of carbide.•Higher cutting speedThe possible risks include:• Process fine luning-slight deviations

from the optimal setup can result in catastroph-ic failure.

• Limited toughness characteristics com-pared to carbide. This currently limits the max-imum feed rates.

• Grinding difficulty and cost. These factsresultin tool prices in the range of two to threetimes the price of a carbide tool ThL material

more susceptible 10 grinding cracks.Conclusion

It should be apparent to the readerthatthere have been significant improvements ingear cutting technology ..The goal, as stated inthe opening comments, should always be toimprove the process in all respects. Theadvances in new technology will continue inthe years to come, and it must be the respon-sibility of everyone to exploit the advance-ments. Simply to continue with the process asit was developed years ago is not goodenough in today's market. To fully utilize thelatest in these developments, it becomes evenmore important to get the tool manufacturerinvolved in the early stages of this optimiza-tion cycle. It should also be evident that thefinal solution for success can be considerablydifferent depending on the re ults expected,even within the same application. One pointto keep in mind, however: all processes haveroom for improvement .•

Acknowledgement; This article was first presented at theSME Gear Processing and MWlIIj(u'rurit!g Clinic. April 1994.Indianapolis, IN.

r'ellilUs What You Ihink ...If you found this article ofinterest ,and/or useful, please' circle Reader ServiceINo.A-1J.