30 GE",R TECHNOLOGY Carbide Hobs Robert IP..PhiUi'ps Pfauter-:Maaig Cutting Tools •.IUd.• Loves P'ark, IL Introduction The following article is a. collection of data intended to give the reader a general overview of information related to a relatively new subject within the gear cutting industry. Although carbide hobbing itself is not necessarrly new, some of the methods and types otapplication are. While the subject content of this article may be quite broad, lt should not be considered all-inclusive. The actual results obtained and the speeds. feeds, and tool life used in carbide hobbing applications can vary significantly. History of ear bide eutting Tools The use of carbide has been accepted in the metal cutting tool industry for many years. Generally, when we talk about carbide cutting tools. we direct our attention to carbide inserts used mainly for machining centers and lathes. Carbide lends itself particularly to these types of tools for several reasons. One is that the insert itselfhas a relatively simple geometry (compared to a hob, for example). This geometry makes maintaining dimensional. control of the tool. / somewhat easier during manufacturing process, These types of tools take advantage of the benefits of carbide and at the same time tolerate '-:':-, --t+--it--- some of its disadvantages. In most applications, the tool was used in high-speed, uninterrupted cuts that required the high heat qualities of the carbide material, yet was not hindered by the brittleness and lack of abiHty to withstand shock loads encountered during interrupted cuts. The stability of carbide at elevated temperatures also, allowed insert manufacturers to take advantage of the properties of CVD titanium carbide and other CVD coatings to enhance the tool performance. Early Carbide Hobbing The benefits realized in the successful appli- cation of carbide in the insert industry made the ability to apply the same technology to the hob- bing industry look. quite attractive. Initially. the use of carbide here was somewhat limited to those applications with special needs that. could not be met by the use of more conventional. materials, such as high-speed steel, Ex amples of these types of applications include bobbing gear materials, such as plastics, pheno- lics, or cast iron, These materials have a tendency to be very abrasive and can be very difficultto hob using high-speed steels. The high abrasion resis- tanceof carbide in these cases offset the early manufacturing problems to be disc ussed in greater detail later. Another example of the early use of carbide in the !'tobbing process is in hard fini shing or ski ving I NE~G.RA Body hobs. (See Fig. 1.) Here. the tool was used to finish hob gears after hardening. In many such cases, th.epart to be hobbed reached hardnesses in the range of 60Rc. High-speed steel simply would not hold up because of high part hardness .. Robbing St.eel From Solid More recently, a great deal of effort has been applied to using carbide in cases where normally high-speed steel bobs would be used. The main reason for this is the desire to take advantage of the high production rates that are possible with
Transcript
30 GE",R TECHNOLOGY
Carbide HobsRobert IP..PhiUi'ps
Pfauter-:Maaig Cutting Tools •.IUd.•Loves P'ark, IL
IntroductionThe following article is a. collection of data
intended to give the reader a general overview ofinformation related to a relatively new subjectwithin the gear cutting industry. Although carbidehobbing itself is not necessarrly new, some of themethods and types otapplication are. While thesubject content of this article may be quite broad,lt should not be considered all-inclusive. Theactual results obtained and the speeds. feeds, andtool life used in carbide hobbing applications canvary significantly.
History of ear bide eutting ToolsThe use of carbide has been accepted in the
metal cutting tool industry for many years.Generally, when we talk about carbide cuttingtools. we direct our attention to carbide insertsused mainly for machining centers and lathes.Carbide lends itself particularly to these types oftools for several reasons. One is that the insertitselfhas a relatively simple geometry (comparedto a hob, for example). This geometry makesmaintaining dimensional. control of the tool. /somewhat easier during manufacturing process,
These types of tools take advantage of thebenefits of carbide and at the same time tolerate '-:':-,--t+--it---
some of its disadvantages. In most applications,the tool was used in high-speed, uninterruptedcuts that required the high heat qualities of thecarbide material, yet was not hindered by thebrittleness and lack of abiHty to withstand shockloads encountered during interrupted cuts. Thestability of carbide at elevated temperatures also,allowed insert manufacturers to take advantage ofthe properties of CVD titanium carbide and otherCVD coatings to enhance the tool performance.
Early Carbide HobbingThe benefits realized in the successful appli-
cation of carbide in the insert industry made theability to apply the same technology to the hob-bing industry look. quite attractive. Initially. the
use of carbide here was somewhat limited to thoseapplications with special needs that. could not bemet by the use of more conventional. materials,such as high-speed steel,
Examples of these types of applications includebobbing gear materials, such as plastics, pheno-lics, or cast iron, These materials have a tendencyto be very abrasive and can be very difficultto hobusing high-speed steels. The high abrasion resis-tanceof carbide in these cases offset the earlymanufacturing problems to be disc ussed in greaterdetail later.
Another example of the early use of carbide inthe !'tobbingprocess is in hard fini shing or ski ving
I
NE~G.RA
Body
hobs. (See Fig. 1.) Here. the tool was used tofinish hob gears after hardening. In many suchcases, th.epart to be hobbed reached hardnesses inthe range of 60Rc. High-speed steel simplywould not hold up because of high part hardness ..
Robbing St.eel From SolidMore recently, a great deal of effort has been
applied to using carbide in cases where normallyhigh-speed steel bobs would be used. The mainreason for this is the desire to take advantage ofthe high production rates that are possible with
carbide. The gear bobbing industry has realizedthat in many cases the relatively high tool cost ofa carbide hob can be more than offset by thereduction of machining costs. We are now usingcarbide in applications such as soft bobbing gearsfrom the solid.
Transfering the technology learned from manysuccessful years of carbide insert applicationstothe hobbing industry was very desirable. How-ever, a number of problems needed to be over-come before carbide bobbing could be considereda viableaUemative to high-speed steel hobbing,
One of the first obstacles was the inability ofthe older hobbing machines to provide the rightconditions totake advantage of all that carbidehobs had to offer, Two key factors had to beaddressed. The first was the rigidity of the ma-chine being used. Because of its extreme hard-ness, carbide has an inherent tendency to chip, soevery attempt had to be made to minimizelooseness, vibr.ation, and chatter. In many casesthe older machines were not capable of with-standing the eonditiens created when attempt-ing to use carbide hobs.
The second item that was critical to the successof carbide hobbing was the hob head speed ca-pabilities. To take fun advantage of carbidematerials, hob speeds that had never before beenrealistically possible (because of the high-speedsteel. hobs being used) would have to now bemade available, In many cases, hob peeds in thearea of 900 sfm would have to be accommodated.
The bobbing machines were not the only earlyobstacles to applying carbide to the hobbing in-dustry. One of the major problems that needed tobe addressed was the ability of carbide to with-stand the severe cutting conditions that are presentduring the hobbing process. The carbide would
have to be tough enough to aUow the cutting'edges (individual hob teeth) to continuously enterand exit the cut duringthe generating process.The impact and shock resistance of tile carbidewould have to be increased if it were to be usedsuccessfully. Fortunately, over the last few years,a great deal of progress has been made wi.th.newgr:adesof carbide, as wellasgrain size control, sothat now numerous selections of grades for agiven application areavailable,
Carbide Hob ManufacturingBesides the machine and the material Itself,
the problem of manufacturing the carbide tool.still remained. In most cases, the hob manufac-
turers werenottooled for or experienced in manu-facturing tools made out of carbide. One attemptto overcome this was the "bladed' or "tipped"design. (See Fig ...2.) Withthis design, IIsteel hobwas manufactured. the teeth were removed bygashing, and carbide blades were inserted andfinished by grinding the teeth in the carbide fromsolid. This allowed the manufacturer to use many
of the same processes used on standard high-speed steel. hobs.
Thi.smethod. however. did present sorneprob-
lems, By using a composite design, the manufac-turer had to develop an effective means of holdingthe carbide blade in the steel body. Early on themost widely used method was brazing. Thismethod was successful, but. did have drawbacksbecause of the heat required to braze. The highheat made it too difficult to accurately locate thecarbide tip in its proper position. In some cases,the heat also caused the carbide to crack. becausenf'the difference in thermal expansion propertiesbetween the steel body and tbe carbide blade. Theintroduction of new adhesives in recent yearsallows the blade to be held effectively without theneed for high heat Positioning of the blades cannow be done very accurately, and cracking hasbeen virtu any eliminated ..Using this method doesrequire a more accurate interface between theblade and body and. in most cases, requires groundsurfaces to assure proper adherence.
To help minimize the amount of form grindingrequired to produce the teeth in the blade, one oftwo different approaches can be used. The firstis to use preformed tips supplied with the teetoformed ineach blade by the carbide manufacturer.The second is to use wire EDM to cut the teeth ineach blade. (See Fig. 3.) The use of blades withteeth make accurate location of the blades in
Robert :P. Phillipsis Vice President ofEngineering and QualityAssurance al· P!au!er.Moog Curling Tools, LId.
His background is in /00/
desig!! engineering. andhe is responsible for thedepartments of ToolDesign cngi.fluring .Research and Develop-mellI, Manu/ac/uringEngineering, and SPC
lmplementaiion.
M ... VIJUf.lE 199' 31
the body even more critical, so here, normally the comer will result in a stress riser thatcould causeadhesive bond technique is used. cracks to propagate. An external sharp comer
The ability of carbide manufacturers to accu- creates a structurally less sound edge thatcouldrarely produce preformed blanks has solved an- lead to premature failure due to chipping.other carbide bob manufacturing problem. With high-speed steel. hobs, it is possible toThrough different methods, ranging from pre- grind with an assortment of different types ofform pressing to CNC machining before sinter- grinding wheels (aluminum oxide, silicon car-ing, manufacturers are now able to deliver much bide, Borazon,® etc.), When grinding with car-more complex blanks than. in the past. It is nowpossible to recei ve preformed hob blanks that
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have the bore, hubs, and gashes roughed intothem. (See Fig. 4.) The hob manufacturer is thenrequired to complete the grinding operations to
the hole. hubs, sharpening, and tooth form tofinish the hob.
Solid carbide blanks have an advantage overthe composite design in that it is possible toenhance theperfonnance of the tool with coatingswithout the threat of contaminating the furnacewith the braze or adhesi ve bond material.
DesignCertain common practices should be followed
to assure an acceptable design for carbide hobs,An example of one of these considerations is theavoidance of sharp comers. An internal sharp
bide, the choice is limited to diamond. Althoughwork is currently being done to develop adressable, vitrified diamond wheel. the majorityof carbide form grinding is done with eitherplated diamond or resin bond diamond grindingwheels. The inability to readily form the diamondwheels has, in the past, Limited the profile modi-fications thatarepermissible on the moth flanks.
MaintenanceWhen discussing the proper maintenance of
carbide hobs, a number of areas must. be ad-dressed. The first one - often overlooked - is thespecial care that must be taken when handlingcarbide hobs. Although precautions must betaken when handling any cutting tool, carbide issomewhat more brittle than high-speed steel andmore susceptible tochipping and breakage. Whendealing with carbide hobs, special cases fortrans-porting and storage should be invesngated,
Tooling is another area that should be exam-ined. Some toelingthat may be considered stan-dard when u ing high-speed steel hebs, simplymust be avoided when using carbide materials.One example is any kind of "press type" arbor,Obviously •.given some ofthe material character-istics of carbide, any excessive force used withthis type of arbor could lead to cracking soonerthan with. steel hobs.
The only maintenance with which the finaluser of a. carbide hob must be concerned is theproper sharpening of the cutting face of the hob
32 GEA" TECHNOLOG.Y
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when the tool is worn, In practice, the same carbide andcebalthas the widesaindu trialuse. 11'1theprocess is used to sharpena carbide hob asa steel case of machining steel, the chips are much different.hob. The differences are the type 'of grinding from those produced in the early carbide hob applica-wheel used, the feeds and speeds, and possibly the tions discussed previously. The chip in this case cancoolant used Diamond gJiinding wheels should be relatively long and stringy. The properties of thebe used when sharpening carbide hobs, We have newer grades 'of carbide for these applications havebad success using hath plated diamond and re in been tailored to meet ,the needs present here.bonded diamond grinding wheels. The actual The strength and hardness ofa cobalt-bonded tung-wheel specification can vary according to the sten carbide section depends primarily on the unifor-machine being used, and the surface finish and miry and the thickness achieved in the cobalt filmamount of stock removal required, The wheelswe have found successful are:
• Plated - Universal Super Abrasive
(Elgin !Diamond) 180 Grir
• Resin Bond - Universal Super
Abrasive (Elgin Diamond)
[80 Grit 100 concentration
Both wheels are capable of producing surfacefinishes of ]6J,! or better (resin as low as as 6-Bmm) if applied correctly.
Grinding wheel speeds will vary according tothe type of wheel being used" but, typically, toprovide the proper cutting conditions, the speedwill be in the range of 6500/6700 sfm. The tablefeed rate should stay in the area of four inchesperminute. The stock removal rate is very criticalwhen sharpening carbide hobs. The rate of re-movalis much less than with steel. hobs. Nor-mally .001 stock removal perpass of the grindingwheel should not be exceeded. Stock removalrates much higher than this can lead to heatgeneration" causing sharpening cracks. Note thatusing magnaflux to detect cracks in carbide tools(as, might be done with steel hobs) is not recom-mended. A more readily accepted method forerack detection in carbide is a. chemical means,such as Zyglo.
One final. topic to cover in the proper mainte-nance of carbide hobs is the coolants being used,not only in the sharpening operation, but also inthe bobbing application. Cutting oils should beevaluated to assure that no elements in the chemi-cal make-up of the oil may be detrimental to thecarbide materia] itself. Certain sulfur and chlo-rine additives can tend to leach the carbide bind-ers, leaving an.extremely brittle cuttingedge thatwill lead to premature failure of the tool.
Carbide PropertiesOf the many different types of carbide materi-
als available today" the one based on tungsten
surrouadingfhe carbide particles, This is controllableby adjusting the proportion of cobalt to tungsten car-bide and, to a lesser degree, by varying the particle sizeof'the tungsten carbide. Smaller amounts of cobalt willresult in the structure assuming properties more liketungsten carbide itself; namely, higher hardness andincreasing tendency toward brittleness and lowerstrength. The effect of cobalt on hardness and strengthis represented in figs. 5 and 6.
Aside from its tremendous compressive strengthand hardaess.the best known characteri stic of carbide
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is its abrasionresistance, (See Fig, 7.) However, in Fig. 9. In many cases, within a specific grade,a direct relationship exists between these two there may be variations as far as grain structurecharacteristics and the transverse rupture strength. and size to help tailor the performance to anThis is a measure of shock resistance. (See Fig. 8.) application. The carbide vendor should be in-
Carbide Grades eluded in discussions regarding grade selection ..
Each carbide vendor has a wide selection of Application Resultscarbide grades available for a given application, As stated in the introduction, tile actual resultsThe primary bases for selection of a specific obtained in specific applications of carbide hob-grade of carbide are the material being cut and the bing can vary great! y. The following examplestype of cutting application being performed. A are included for reference only, but can help givechart showing a number of different carbide veri- an indication as to the possible benefits obtain-docs and their specific grade designations is shown able with the proper application of carbide hobs.
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