grinding of tool steel - uddeholm global grinding of tool steel thermal durability hardness in air...

GRINDING OF TOOL STEEL

SS-EN ISO 9001SS-EN ISO 14001

This information is based on our present state of knowledge and is intended to provide generalnotes on our products and their uses. It should not therefore be construed as a warranty ofspecific properties of the products described or a warranty for fitness for a particular purpose.

Classified according to EU Directive 1999/45/ECFor further information see our “Material Safety Data Sheets”.

Edition 7, Revised 12.2012The latest revised edition of this brochure is the English version,which is always published on our web site www.uddeholm.com


3

CONTENTS

Introduction 4

Grinding wheel design 4

How the grinding wheel works 6

The grinding machine 9

Grinding fluid 9

The tool steel 10

Recommendations for grinding ofUddeholm tool steel 13

Cutting speed and feed 14

Grinding wheel dressing 15

Examples of suitable grinding wheels 15–17

4


THERMALDURABILITY

HARDNESS IN AIRABRASIVE KNOOP °C

Aluminiumoxide 2100 2000

Siliconcarbide 2500 1200

CBN 4700 1400

Diamond 7000 650

ABRASIVE

It is important that the abrasive fulfilsrequirements in respect of:

• hardness

• sharpness

• thermal resistance

• chemical stability

Today, the following four main groupsof abrasives (all synthetic) are used,fulfilling the above requirements togreater or lesser extents.

1. Aluminium oxide designation A (SG)

2. Silicon carbide designation C

3.Cubic boron nitride designation B

4.Diamond designation SD

Abrasives have different applicationareas, depending on their particularcharacteristics, as shown partially inthe table below.

IntroductionThe high alloy content of tool steelmeans that such steel are often moredifficult to grind than conventionalstructural steel.

In order to achieve successfulresults when grinding tool steel, it isnecessary to choose the grindingwheel with care. In turn, choosing theright grinding wheel and grinding datarequires an understanding of how agrinding wheel works.

This brochure provides a quitedetailed description of the make-upof the wheel, of how it works whengrinding and of the parameters thatdetermine the final result. It alsoincludes recommendations forgrinding wheels for use with Udde-holm tool steel.

Grindingwheel designIn principle, a grinding wheel consistsof the following components:• Abrasive• Binder• Air pores

Figure 1. The arrangement and proportionsof abrasives grains, air pores and bondbridges (made up of binder) determinegrinding wheel characteristics.

Certain special grinding wheels, suchas metallically bonded diamondwheels, contain no air pores.

It is the composition and variationof the above components that deter-mines the characteristic of a grindingwheel. An identification system, whichhas now been ratified as an interna-

Airpores

Abrasive

Binder

tional standard by ISO, indicates thecomposition of grinding wheels. Theidentification consists of numeralsand letters in a particular sequence,defining the abrasive, grain size, gradeand binder.

The table below shows how thecharacteristics of aluminium oxideabrasive can be varied by alloying it.

Unfortunately, the colour of a grind-ing wheel does not always necessarilyindicate the type of abrasive used init, due to the fact that some grindingwheel manufacturers colour theirabra-sives and binders.

There is also another type of alu-minium oxide named ceramic orsintered aluminium oxide. This abra-sive has a fine crystalline structure,which means that the grains retaintheir sharpness better. However, itsuse requires higher grinding pressure.A typical application for it is grindingtool steel in rigid grinding machines.Examples of this type of abrasive areSG (Seeded Gel) from Norton andCubitron from 3M.

2. Silicon carbide is an abrasive that isused primarily for grinding cast ironand austenitic stainless steel, althoughit can also be used for hardened toolsteel. It occurs in two main variants:the black silicon carbide and a some-what harder green variant, which ismore brittle than the black material.

3. Cubic Boron Nitride (CBN) is pro-duced in approximately the same wayas synthetic diamond, and is an abra-sive that is used primarily for grindinghardened high-carbide tool steel andhigh-speed steel. A drawback of CBNis its high price—almost twice that ofsynthetic diamond.

4. Diamond is seldom used, despiteits high hardness, for grinding toolsteel as a result of its low thermalresistance. Diamond is used primarilyfor grinding cemented carbide andceramic materials.

ABRASIVE COLOUR PROPERTIES

Normalcorundum Brown, grey

Mixedcorundum Yellowbrown

Red alumina Red

White alumina White

Example:

A 46 H V

Abrasive

Grain size

Grade

Binder

Har

der

Toug

her

1. Aluminium oxide, is the abrasivemost commonly used for grindingsteel, and is available in several vari-ants. It can be alloyed with otheroxides, of which the most common istitanium oxide.


5

The photo shows the difference between a CBN wheel and a conventional grinding wheel.As a result of the high price of CBN, wheels made from it consist of a thin layer of abrasiveapplied to a central hub, usually of aluminium.

ABRASIVE GRAIN SIZE

The grain size of the abrasive is animportant factor in selecting thecorrect grinding wheel.

Grain sizes are classified in accord-ance with an international mesh sizein mesh/inch, ranging from 8 (coarse)to 1200 (superfine). Grain sizes forgrinding tool steel are generally inthe range 24–100 mesh. Coarse grainsizes are used for rapid rate of remo-val, when grinding large workpieces,grinding softer materials or when thecontact surface of the grinding wheelis large. Fine grain sizes are used toproduce high surface finish, whengrinding hard materials or when thecontact surface of the grinding wheelis small.

The surface smoothness of theground part depends not only on thegrain size of the grinding wheel. Thesharpness of the wheel, the bondingmaterial used and the hardness of thewheel also play a considerable part indetermining the surface finish pro-duced.

In the case of diamond and CBNgrinding wheels, European grindingwheel manufacturers indicate grainsize by the diameter of the abrasivegrains in microns, while American andJapanese manufacturers indicate it inmesh size.

GRINDING WHEEL BINDERS

The following binders are used tobind the grains in a grinding wheel:

• Vitrified designation: V

• Resinoid ,, B

• Rubber ,, R

• Metal ,, M

GRINDING WHEEL GRADE

The grade of a grinding wheel refersto its hardness, i.e. how securely theabrasive grains are held by the binder.It does not, therefore, depend on thehardness of the abrasive used in thewheel.

The grade of a grinding wheel isdetermined primarily by the quantityof binder used in the wheel. A higherproportion of binder reduces theamount of air pores and produces aharder wheel.

The grade of a wheel is indicatedby a letter, indicating the hardness inalphabetical order:

E = very soft compositionZ = very hard composition.

For tool steel, the most commonlyencountered compositions are withinthe hardness range G–K. Indication ofthe grade is sometimes followed by anumeral, which indicates the spreadof the abrasive particles in the wheel.

Vitrif ied grinding wheels are thosemost commonly used for grindingtool steel.

Resinoid is used as a binder ingrinding wheels intended for highperipheral speeds, such as certainCBN wheels.

Rubber-bonded wheels are used forhigh specific grinding pressures, suchas for control wheels in centrelessgrinding.

Metallic binders are used fordiamond and certain CBN wheels.Such wheels can withstand very highperipheral speeds.

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Fine surface

Figure 3. A large chip size results in a rougher surface f inish onthe workpiece.

Rough surface

Small chip Large chip

High forces onthe abrasive grain

Low forces onthe abrasive grain

How the grindingwheel worksGrinding is a cutting process in whichthe cutting edges are formed by thegrains of abrasive. The same prin-ciples apply for grinding as for otherchip-cutting methods, although vari-ous factors mean that it is necessaryto consider the theory of grindingsomewhat differently.

Conditions that are special forgrinding.

• The cutting tool has an irregularcutting geometry and the abrasivegrains are irregularly placed, whichmeans that cutting, ploughing andsliding will occur, see figure 2.

• The cutting geometry can change.The method of working of an abra-sive tool includes a certain degreeof “self-sharpening”, which meansthat grains of abrasive break or arereplaced as they wear.

• Negative cutting angles. The irregu-lar “blunt” shapes of the grainsmean that the rake angles are oftennegative. GRINDING WHEEL WEAR

The grains of abrasive are initiallysharp, but in the same way as with allother cutting edges they wear downin use and become blunt. Finally, thegrains will have become so blunt thatthey have difficulty in penetrating intothe material of the workpiece. They

cease to remove material and gener-ate only heat. The grinding wheel isthen said to be burning the material,which can cause cracks in it.

For a grinding wheel to workcorrectly, the stresses in the binderand the strength of the binder mustbe so balanced that, as the grainsbecome as blunt as can be accepted,they are pulled out of the binder andare replaced by new, sharp grains.The grinding wheel, in other words,sharpens itself. Self-sharpening alsooccurs through grain breakage, whichcreates new cutting edges.

The degree of self-sharpening, i.e.whether the grinding wheel is hardor soft, is affected by the composi-tion of the wheel (its design hard-ness) and by the conditions underwhich it is working.

AVERAGE CHIP THICKNESS

Although the chips removed bygrinding are small and irregular, themean value of their thickness at anytime is relatively constant. This valuevaries, depending on the type ofgrinding operation and in response tothe changes in grinding data.

If a grinding wheel is cutting largerchips, this means two things:

1. Higher loading on each cuttingedge, i.e. higher specific forces. Thisincreases the self-sharpeningcharacteristic of the wheel and

GRINDING FORCES

The grinding forces that act on eachindividual grain of abrasive are re-ferred to as specif ic forces. A meanvalue of the specific forces can beobtained by dividing the total forceby the number of cutting edges,which depends on the size of thecontact area and the number of cut-ting edges in the grinding path. Thespecific forces determine variouseffects, including the degree of self-sharpening of the grinding wheel, i.e.its “working hardness”. The total forceis the force arising between thegrinding wheel and the workpiece.

Abrasive grain

Grindingdirection

Abrasive grain

Abrasive grain

Grinding

direction

Workpiece

Grinding

direction

Workpiece

Chip

Workpiece

Cutting

Ploughing

Sliding

Figure 2. Different conditions during grind-ing (highly schematic). Cutting angles aregenerally negative.

• A very large number of cuttingedges.

• Very high cutting speed. The mostcommon cutting speed for preci-sion grinding, 35 m/s = 2100 m/min.,is far above what is normal forother cutting processes.

• Very small chips, i.e. very smallcutting depth for each cutting edge.

Friction heat


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Internal grinding

Segmental surfacegrinding

Surface grinding

Cylindrical grinding

Figure 4. Differences in contact length fordifferent grinding operations.

STOCK REMOVAL RATE

When grinding, the amount of chipsremoved per unit of time can mosteasily be expressed as mm3/s. This isoften referred to as the stock re-moval rate, and depends on the ma-chine feed, the composition of thegrinding wheel, its cutting speed(peripheral speed) and (in certaincases) on the dimensions of theworkpiece.

It is often more meaningful to talkabout stock removal rate rather thanabout table feed speed, feed depthetc., and it is also quite easy to calcu-late. Cost considerations often dic-tate that the stock removal rateshould be as high as possible. If thestock removal rate is increased with-out increasing the number of grainsof abrasive performing the work, e.g.by greater infeed depth, the chip sizewill also naturally be increased.

CUTTING SPEED

The peripheral speed of a grindingwheel has a direct effect on thenumber of cutting edges that actuallyperform the machining work. If, forexample, the cutting speed is dou-bled, twice as many grains of abrasivewill pass the workpiece per unit oftime. If the workpiece speed is notincreased, the mean chip thicknesswill decrease, thus also reducing thecutting forces on each grain. Self-sharpening will be less effective, i.e.the grinding wheel will be effectivelyharder, producing a finer surfacefinish, but with greater risk of burningthe surface.

Conversely, reducing the speed ofthe wheel will increase the chip

GRINDING WHEELCONTACT SURFACE

It is at the contact surface betweenthe grinding wheel and the workpiecethat the actual cutting operationoccurs. A large contact surface meansthat a greater number of cuttingedges participate in the process, thusreducing the chip size and specificforces. Similarly, a reduced contactsurface area results in greater chipsize and higher specific forces.

In principle, the contact surface isin the shape of a rectangle. Its extentin the cutting direction is referred toas the contact length or contact arc,while its extent perpendicular to thecutting direction is referred to as thecontact width.

The contact length depends primarilyon the type of grinding operation. Inaddition, it depends on the diameterof the grinding wheel, the cuttingdepth and in all cases—except forsurface grinding—the dimensions ofthe workpiece. Differences in thecontact length are the main reasonfor having to select different grindingwheel compositions for differentgrinding operations.

If, when performing internal grind-ing, a grinding wheel is used that hasa diameter only a little less than thatof the ground hole, the contactlength will be very large, resulting inlow cutting force per grain.

If the wheel is to sharpen itself prop-erly, it must be of a softer composi-tion than one intended for externalcylindrical grinding of a similar part. Inthis latter case, the contact length isshorter, which means that there arehigher cutting forces on each grain.

The contact width may be equal to thewidth of the grinding wheel as, forexample, in plunge grinding. Howeverin operations such as surface grindingwith a moving table, only part of the

thickness, with the result that thegrinding wheel behaves as a softerwheel.

Generally, both peripheral velocityand workpiece speed are increased inorder to increase the total rate ofremoval.

THE G-RATIO OFA GRINDING WHEEL

The G-ratio of a grinding wheel refersto the relationship between theamount of material removed and theamount of grinding wheel consumed.The G-ratio is a measure of howeffectively a grinding wheel workswith the particular workpiece mate-rial.

gives it the characteristics of asofter wheel.

2. The surface of the part beingground is coarser, see Figure 3.

A reduction in the average chip thick-ness represents the opposite. It istherefore important to understandhow changes in grinding data andother conditions affect the averagechip thickness.

8


THE NUMBER OF CUTTINGEDGES IN THE CONTACT AREA

The number of cutting edges in thecontact area is a factor that has aconsiderable effect on the chipthickness and thus on the grindingprocess.

A large number of cutting edgesper unit area mean that the work ofremoving material is spread over alarger number of grains, reducing thechip thickness and the specific forces.

The grain size of the abrasive alsoaffects the number of cutting edges,which is the reason for the commonobservation that fine-grained cuttingwheels seem to be harder.

grinding wheel is actually cutting andthis part changes as the wheel wearsdown. It is sometimes possible toreduce the contact width, if this isrequired, by truing of the grindingwheel. This reduces contact surfacearea, resulting (as already described)in a greater chip thickness, higherloading on the abrasive grains and aneffectively softer grinding wheel.

Dressing is a conditioning of thewheel surface to give the desiredcutting action. Dressing the wheelexposes sharp cutting edges. One andthe same grinding wheel can be givencompletely different grinding charac-teristics through application of diffe-rent dressing tools or different dres-sing methods. Dressing is therefore aparticularly important parameter inachieving good grinding performance.

Dressing resulting in a smoothsurface on the wheel results in thecutting edges of the grains of abrasivebeing close together, while dressingresulting in a rough surface of thewheel gives the wheel a more openstructure. Dressing provides a meansof making the same grinding wheelgive completely different grindingresults.

The degree of self-sharpeningaffects the structure of the grindingwheel surface, i.e. the number ofcutting edges per unit of area.A grinding wheel that has a high self-sharpening performance has a differ-ent, more open structure than onehaving poorer self-sharpening per-formance.

creates space for chip formation. Inpractice this can be done by pushinga wet aluminium oxide stone into thewheel for a few seconds.

DRESSING AND TRUINGGRINDING WHEELS

Dressing and truing of a grindingwheel are often considered to be thesame thing because they are oftenperformed as one operation.Truing is made to produce any profilewhich may be required on the faceof the wheel and to ensure concen-tricity.

There are many different tools avail-able for dressing and truing grindingwheels, e.g. crushing rolls and dia-mond tools. CBN wheels are bestdressed using a diamond coatedroller.Certain types of grinding wheels, e.g.resinoid bonded CBN wheels, needto be “opened” after dressing. Thisreveals the abrasive particles and


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The grindingmachineThe type of grinding operation andthe machine available has a consider-able effect on the choice of appropri-ate grinding wheel composition.A grinding machine should be as rigidas possible, in order to allow it towork at high grinding pressures. Thisis because it is the rigidity of thegrinder and the method of clampingthe workpiece that determine thepermissible grinding pressure andtherefore restrict the choice ofwheels. If the machine is not suffi-ciently rigid, a softer grinding wheelcomposition or a smaller contactarea between the grinding wheel andthe workpiece should be chosen, inorder to achieve the required degreeof self-sharpening performance.

The speed of the grinder alsoaffects the choice of grinding wheel.CBN wheels often require peripheralspeeds of 45 m/s in order to providegood cutting performance.

Fine gridning of detailsin hardened UdddeholmMirrax ESR

Grinding fluidWhen grinding, as with all othercutting operations, a cutting fluid isused primarily to:

• cool the workpiece

• act as a lubricant and reducefriction between the chips, work-piece and grinding wheel

• remove chips from the contactarea

There are three main types of cuttingfluids that can be used when grinding.

• Water solutions. These are liquidsthat consist of water with syntheticadditives in order to increase itswetting performance and preventcorrosion. Such fluids contain no oiland provide good cooling perform-ance but poorer lubrication per-formance.

• Emulsions. These consist of waterwith an ad-mixture of 2–5% of oilin an extremely finely distributedform. Sulphur or chlorine additivesmay also be used as EP additives.

• Cutting oils. These are composed ofa mineral oil base with EP-typeadditives. Cutting oils provideeffective lubrication but poorercooling.

Water solutions are most suitablewhen grinding with diamond wheels.

Emulsions are used nowadays forthe majority of grinding operationsbecause they are ecologically bene-ficial and perform adequately.

Cutting oils give the best resultsfor profile and plunge grinding withfine grained wheels, e.g. when grind-ing threads. Cutting oil also providesthe longest life for resinoid bondedCBN wheels, although high-oilemulsions are often chosen in theinterests of pollution reduction.

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Ferr

ite

Aus

teni

te

Mar

tens

ite

Cem

enti

te

Mo

lybd

enum

car

bide

Chr

om

ium

car

bide

Nio

bium

car

bide

Tung

sten

car

bide

Van

adiu

m c

arbi

de

Tit

aniu

m c

arbi

de

Alu

min

ium

oxi

de

Silic

on

carb

ide

Cub

ic b

oro

n ni

trid

e

Dia

mo

nd

Hardness kp/mm2

7500

7000

6500

6000

5500

5000

4500

4000

3500

3000

2500

2000

1500

1000

500

0

Figure 5. Thehardness of grindingabrasives, basicphases found in a toolsteel and carbidesfound in tool steel.

The tool steelThe alloying constituents of a toolsteel have a considerable effect on itsease of grinding.

The Uddeholm range of tool steelextends from low-alloy steel, such asUddeholm UHB 11, to high-alloysteel, such as Uddeholm Vanadis 10.

There is seldom any problem ingrinding low-alloy tool steel. At theother end of the scale, however, thehigh-alloy carbide-rich steel can causeproblems when being ground, andrequire a careful choice of grindingwheel and operating parameters.

The higher the wear resistance of asteel, the more difficult it is to grind.The wear resistance of a steel, andthus also its ease of grinding, aredetermined by its basic hardness andby the size, hardness and quantity ofthe carbides in it.

In order to enhance the wearresistance of a tool steel, the steel isalloyed with carbide-forming alloyingelements, of which the most impor-tant are chromium and vanadium. Thesteel must also have a high carboncontent if carbides are to be formed.

The diagram, Figure 5, shows thehardness of the basic phases found ina tool steel, the hardness of the mostcommon carbides found in tool steeland the hardness of commonly usedgrinding abrasives.

As can be seen in the figure, it isonly diamond and CBN that areharder than all the carbides that arefound in a tool steel. However, asmentioned earlier, diamond is unsuit-able for grinding steel.

The quantity and the size of carbidesin a steel has a very considerableeffect on the ease of grinding of thematerial. The greater the number of,and the larger the carbides, the moredifficult the material is to grind.This is the reason why tool steelproduced by powder metallurgyprocesses, having smaller carbides, iseasier to grind than a conventionallyproduced steel having a similar com-position.

In practice, powder metallurgy isemployed to increase the quantity ofcarbide in a tool steel, i.e. such steelare more highly alloyed than conven-tional steel, which generally meansthat they are more difficult to grind.

The effect of hardness on ease ofgrinding is also dependent on thequantity of carbide-forming alloyingelements in the steel.

Figure 6. The effect of hardness ongrindability for:A – a low-alloy tool steel of Arne typeB – a material of Sverker typeC – material of Vanadis 10 type.

Grindability index100

10

1

0,1

As can be seen in Figure 6, hardnesshas a greater effect on grindability forhigh-carbide steel.

AB

C


11

Figure 7. Hardness prof ile through thesurface layer of an incorrectly ground tool.

Hardness, HRC

64

60

56

52

480,10 0,20 0,30 0,40 0,50

Depth below ground surface, mm

The diagram below shows thehardness profile through the surfaceof a tool steel, incorrectly ground insuch a way as to produce re-harden-ing.

GRINDING CRACKS ANDGRINDING STRESSES

The wrong choice of grinding wheelsand grinding parameters results in aconsiderable risk of causing cracks inthe workpiece.

Generally, grinding cracks are notas easy to see as in Photo 2. It isusually necessary to examine the partunder a microscope, or with mag-netic powder inspection, in order tosee the cracks.

Grinding cracks.

In order to obtain good grindingperformance with high-alloy carbide-rich tool steel, it is important toselect the correct grinding wheel.Materials in the Uddeholm Vanadisrange, for example, contain a largequantity of vanadium carbides. To cutthrough a vanadium carbide requiresan abrasive that is harder than alu-minium oxide or silicon carbide.CBN wheels are therefore recom-mended as first choice for grindingthis material. The fact that, despitethis, material can be removed fromUddeholm Vanadis steel by grindingwith aluminium oxide or silicon car-bide is due to the fact that it is thematerial enclosing the carbides that isground away, so that the carbides aretorn out of the basic material of thesteel. However, this occurs at theprice of high wear of the grindingwheel and a risk of poor grindingperformance.

The formation of grinding cracks,which tend to occur perpendicular tothe direction of grinding, usuallymeans the tool has to be scrapped.Hardened steel are more sensitive togrinding cracks than non-hardenedsteel. A material that has been onlyhardened, and not tempered, mustnever be ground: hardened materialsshould always be tempered beforegrinding.

Formation of grinding cracks can beexplained as follows:

Almost all the energy used ingrinding is converted into heat, partlythrough pure friction and partly as aresult of deformation of the material.If the correct grinding wheel hasbeen chosen, most of the heat will beremoved in the chips, with only asmaller part heating up the work-piece.

Re-hardened layer in anincorrectly ground tool.

Incorrect grinding of a hardened toolsteel can result in such a high tem-perature at the ground surface thatthe tempering temperature of thematerial is exceeded. This results in areduction in the hardness of the sur-face. If the temperature is allowed torise further, the hardening tempera-ture of the material can be reached,resulting in rehardening. This pro-duces a mixture of non-temperedand tempered martensite in the sur-face layer, together with retainedaustenite, as shown in Photo 3. Veryhigh stresses arise in the material,often resulting in the formation ofcracks.

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Figure 9. Three typical examples of stressdistribution in a ground surface.

Figure 8. Incorrect grinding can often resultin grinding cracks.

b

b

b

a

a

A

C

Best

Better

a

B

The surface exhibits a high hardnessdue to the untempered martensite.An overtempered zone occurs justbelow the surface, where the hard-ness is lower than the basic hardnessof the workpiece.

Incorrect grinding, resulting in amodified surface layer, often revealsitself through burn marks—discolora-tion of the ground surface. In orderto avoid burning and grinding cracks,it is necessary to keep down thetemperature of the ground part, e.g.by means of good cooling, and toemploy properly dressed grindingwheels that cut the material withsharp cutting edges instead of simplygenerating heat through friction.

A simple example of how incorrectgrinding can cause cracks is shown inFigure 8. A hardened punch with ahead is to be cylindrical-ground, withthe head (b) being ground flat in thesame operation.

Alternative A shows the use of agrinding wheel trued with a 90° edge.The grinding wheel, which is suitablefor cylindrical grinding of the surface(a), produces a good result on sur-face (a). Here the contact surface issmall so the self sharpening perform-ance is good. The head, on the otherhand, which is to be ground flat,presents a larger contact surface tothe grinding wheel. The specificforces on the abrasive grains are lowso that the wheel does not self-sharpen. Instead, surface (b) is sub-jected mainly to rubbing and the heatgenerated can cause grinding cracks.

Alternative B shows a better way togrind the punch. In this case, the sideof the grinding wheel has been truedas shown so that the contact surfaceat (b) is smaller. This results in im-proved self-sharpening and “cooler”grinding.

Case C shows the preferred way togrind this part. The grinding wheel isset at an angle, so that the two con-tact surfaces are of approximately thesame size.

The retained austenite content of ahardened material can also affect thegrinding result. High retained austen-

ite levels increase the risk of crackformation when grinding.

The majority of grinding operationsleave residual stresses in the groundsurface. These stresses are usually ata maximum close to the surface, andcan cause permanent deformation ofthe ground part when grinding thinmaterials.

Of the three examples shown inFigure 9, Example 1 is most at risk inrespect of crack formation. It exhibitstensile stresses in the surface whichcan, if they exceed the material’sultimate tensile strength, result in thematerial cracking.

Examples 2 and 3 are not as danger-ous—the surface stresses are com-pressive stresses, which result inimproved fatigue strength of theground parts.

It is, unfortunately, very difficult toproduce a simple check to determinethe stress pattern set up in theground part unless the stresses areso high that grinding cracks are vis-ible.Grinding stresses can be reduced bystress-relief tempering after grinding.The tempering temperature shouldbe about 25°C below the previoustempering temperature in order toavoid any risk of reducing the hard-ness of the workpiece.

Another way of reducing grindingstresses is to tumble or blast theground parts.

Example 3

Example 2

Example 1

Depth below the surface



+

–

+

–

+

–

Com-pression

Tension


13

Recommendationsfor grinding ofUddeholm tool steelGRINDING OFHIGH-CARBIDE TOOL STEEL

The high carbide content of high-carbide tool steel gives them excel-lent wear resistance, and requirespecial recommendations in respectof grinding operations and selectionof grinding wheels. For the majorityof grinding operations, CBN wheelsare the best choice for such steel.

There are two different types ofcarbide rich tool steel, conventionallymade steel and powder steel. Themain differences that affect the grind-ing properties are the hardness, sizeand distribution of carbides, seeFigure 10 below.• Powder steel, such as Uddeholm

Elmax, Uddeholm Vanadis andUddeholm Vancron, have in spite ofthe high alloying level relativelygood grinding properties due tothe small carbide/nitro carbide size.The small carbides will give thegrinding wheel good self-sharpeningproperties.

• Conventionally made steel, such asUddeholm Rigor, UddeholmSleipner and Uddeholm Sverker,

Figure 11. Surface grinding of UddeholmVanadis 10 with various grinding wheels.(Grinding wheel width: Al2O3

40 mm, CBN20 mm.)

Figure 10. Carbide size and distribution inhigh-carbide tool steel (highly schematic).

have not so good self-sharpeningproperties as powder steel due tothe bigger carbide size. However,the lower carbide hardness andcarbide content will compensatefor the grinding properties.

Figure 11 shows the results of sur-face grinding trials on UddeholmVanadis 10 with aluminium oxide, finecrystalline aluminium oxide and CBNgrinding wheels.

As can be seen in Figure 11, mate-rial is removed more quickly, and theG-ratio is higher, using CBN wheels.These wheels have a “colder” cut,with less risk of “burning” the sur-face.

If the material is to be profile-ground, bear in mind that a consider-able quantity of heat will be gener-ated. Experiments have shown thatvitrified CBN wheels are preferablefor this application. These wheels also

Conventionally madehigh-carbide steel

Powder steel

Carbides

Carbides

Work piece

Work piece

operate well for other grinding ope-rations, provided that a high periph-eral speed can be maintained.

Where boron nitride wheels cannotbe used, the type of grinding wheelmust be chosen with care. Whitealuminium oxide or green siliconcarbide wheels are recommended.Fine-crystalline aluminium oxidewheels, such as the Norton SG, givegood results if the grinding set-up isrigid.

When grinding high-carbide steel,the grinding wheel should always besomewhat softer in order to ensuregood self-sharpening performance.

In addition, the following pointsmust be borne in mind:

• the grinder must be vibration-free,rigid and in good condition

• the workpiece must be securelyclamped. Use a steady rest whengrinding long, thin workpieces

• use sharp conical diamonds whendressing Al

2O

3 and SiC wheels. The

dressed finish must be rough

• maintain a high peripheral speed ofgrinding wheels

• ensure an adequate supply ofcoolant to the grinding zone

• if grinding is carried out without acoolant, select a grinding wheelthat is one grade softer than wouldhave been used if grinding wasperformed with coolant

• never grind a hardened workpiecebefore it has been tempered

Stock removal rate mm3/s

11,811

13,3

14

12

10

8

6

4

2

270

240

210

180

150

120

90

60

30

Al2O3 Al2O3-SG CBN

G-ratio

0,68 2,4

253

Al2O3 Al2O3-SG CBN

14


CROSS-FEED

The cross-feed speed of a grindingwheel, i.e. its sideways motion, ishigher for rough grinding than forfine grinding.

In the case of cylindrical grinding,the cross-feed should be about1/3–1/2 of the width of the wheel foreach revolution of the workpiece. Forfine surface finish, this ratio should bereduced to 1/6–1/3 of the width ofthe grinding wheel per revolution ofthe workpiece.

If a very high standard of surfacefinish is required, cross feed can befurther reduced to 1/8–1/10 of thegrinding wheel width.

When surface grinding with astraight wheel, choose a transversefeed of 1/6–1/3 of the width of thegrinding wheel for each stroke. Again,reduce this feed for high surfacefinish requirements.

Note that when the cross-feed isincreased, the active contact surfacearea between the grinding wheel andthe workpiece becomes larger, result-ing in an apparent increase in hard-ness of the grinding wheel.

GRINDING OFCONVENTIONAL TOOL STEEL

This group covers all the otherconventionally produced tool steel.Providing that common grindingrecommendations are followed,problems are seldom encounteredwhen grinding these tool steel. Forthese steel, ordinary aluminium oxidegrinding wheels are perfectly suitable.CBN wheels can also be used if thesteel are to be ground in the hard-ened and tempered condition.

GRINDING OF PRECIPITATIONHARDENING STEEL

Precipitation hardening steel, such asUddeholm Corrax, behaves in a littledifferent way than other tool steelwhen grinding. It tends to clog thegrinding wheel, especially if the grind-ing wheel is hard and has a closestructure. The clogging can causeproblems like low material removalrate and rough surface finish. To prev-ent the clogging, observe followingrecommendations:

• the wheel should have an open andporous structure

• use a softer wheel grade (hardness)than for other types of tool steel

• the wheel dressing should be donefrequent and rough

• the coolant concentration shouldbe high (>5%) for efficient lubrica-tion

Conventional Al2O3 wheels arerecommended, but SiC wheels can bea better choice for high surface finishwhen a small amount of material is tobe ground. No particular difference ingrindability between solution treatedand aged condition. In the table withrecommended grinding wheels, page16–17, suitable standard type ofgrinding wheels are recommended.However, if a lot of grinding is to bedone in this type of steel, it is recom-mended to select a wheel with amore open structure than a standardwheel type.

WORKPIECE SPEED

For surface grinding, the speed of theworkpiece should be 10–20 m/min.For conventional cylindrical grinding,this speed should be 15–20 m/min.This speed should be reduced forsmaller diameter workpieces, forwhich 5–10 m/min is suitable.

Varying the workpiece speed alsoprovides a means of modifying thegrinding performance of the wheel.Increasing the speed of the work-piece makes the wheel seem softer,while reducing its speed produces aharder wheel.

Cutting speedand feedGRINDING WHEEL SPEED(CUTTING SPEED)

When using small grinding machines,the spindle speed often restrictschoice of cutting speed.

A common safety limit for vitrifiedgrinding wheels is 35 m/s. However,some grinding wheels are approvedfor peripheral speeds of 125 m/s.

A common cutting speed forsurface and cylindrical grinding is20–35 m/s. Varying the peripheralspeed of the wheel makes it possibleto modify its grinding performance.Increasing the peripheral speed ofthe wheel while retaining the sameworkpiece speed means that thewheel behaves as if it was harder.Reducing the peripheral speed makesthe wheel seem softer.

A suitable peripheral speed forresinoid CBN wheels is 30–40 m/s.For vitrified CBN wheels, a cuttingspeed ≥45 m/s is often necessary.

When grinding high-carbide toolsteel, the peripheral speed of thegrinding wheel should be high. Testson cylindrical grinding of UddeholmElmax have shown that the G-ratio ofthe grinding wheel dropped from 127to 28 when the peripheral speed wasdropped from 60 m/s to 30 m/s.Cutting speed, in other words, has aconsiderable effect on the economicsof grinding.


15

INFEED

The infeed of the grinding wheeldepends on the type of wheel andthe rigidity of the grinder and/orworkpiece clamping.

Guide values for cylindrical grindingusing conventional grinding wheelsare:

Rough finish ~0.05 mm/pass.Fine finish ~0.005–0.010 mm/pass.The above feeds should be halved forcylindrical grinding using CBNwheels.

For surface grinding using a straightgrinding wheel, the feed depths forconventional wheels are:

Rough finish ~0.025–0.075 mm/pass.Fine finish ~0.005–0.010 mm/pass.

The feed depths when using CBNwheels are:

Rough finish ~0,010–0,040 mm/pass.Fine finish ~0,005–0,010 mm/pass.

When using grinding wheels havingfine-crystalline aluminium oxideabrasive, such as the Norton SG type,feed depth should be increasedsomewhat over the above valuesin order to achieve higher grindingpressure and hence good self-sharpening performance.

Suitablegrinding wheelsThe examples of grinding wheels inthe tables, page 16–17, have beenmade in consultation with grindingwheel manufacturers, and are basedon our own and others experience.However, it must be emphasised thatthe choice of grinding wheel isstrongly dependent on the type ofgrinding machine, rigidity of clampingand the size of the workpiece, whichmeans that the recommendationsshould be seen as starting points,from which each particular processshould be optimized.

GRINDING PROBLEMS—REMEDIES

The table shows the most importantactions to solve different grindingproblems.

SYMPTOM REMEDY

Chatter marks Check the wheel balance.Ensure that the diamond is sharp.Ensure that the diamond is fixed.

Finish too coarse Use fine, slow traverse dress.Decrease work speed.Use finer grit wheel.Use harder grade wheel.

Burning, grinding cracks Ensure that the diamond is sharp.Use coarse dress.Ensure that the coolant reaches the contact point.Use softer grade wheel.

Short wheel life Ensure that the cutting speed is sufficient.Reduce depth of cut and feed.Use harder grade wheel.

Flecking on surface finish Check coolant filtration.Flush wheel guard.

Diamond is sensitive for high tem-peratures. Therefore, dressing withdiamonds should always be carriedout with plenty of coolant. Thecoolant should always be turned onbefore the diamond touches thewheel. Single point diamond dressingtool should be systematically rotatedto maintain the sharpness.

Grindingwheel dressingDuring dressing a helix along thewheel periphery is made. The lead ofhelix which the dressing tool is beingfed affects the structure of the grind-ing wheel. The lead of helix dependsboth of the r.p.m. of the grindingwheel and the speed of the dressingtool.

The following are rules of thumbsfor grinding wheel dressing withsingle point diamonds and similartools.

Rough Fine

dressing dressing

Diamond infeed(mm) 0,02–0,04 0,01–0,02

Diamondtransverse rate(mm/wheel rev.) 0,15–0,30 0,05–0,10

16


UDDEHOLM SURFACE GRINDING SURFACE GRINDING STEEL GRADE CONDITION CENTERLESS STRAIGHT WHEEL SEGMENT

Conventional steel:ALVAR Soft 1)33A 60 LVM 1)43A 46 HVZ 1)43A 24 FVZALVAR 14 annealed 2)89A 60 2 K5A V217 2)91A 46 I8A V217 2)88A 36 H8A V2ARNE 3)SGB 60 MVX 3)3SG 46 G10 VXPM 3)86A 30 G12 VXPMCALDIE 4)51A 601 L5V MRAA 4)WA 46 HV 4)WA 24 GVCALMAXDIEVARFORMAXHOTVAR Hardened 1)62A 60 LVZ 1)48A 46 HVZ 1)48A 46 FVZPMIRRAX ESR 2)89A 60 2 K5A V217 2)97A 46 2 H8A V217 2)97A 46 1 H10A V2ORVAR SUPREME 3)SGB 60 MVX 3)SGB 46 G10 VXPM 3)86A 36 F12 VXPCORVAR 2 MICRODIZED 4)48A 601 L8V LNAA 4)WA 46 GV 4)WA 36 GVPOLMAXQRO 90 SUPREMEREGIN 3STAVAX ESRTHG 2000UHB 11UNIMAXORVAR SUPERIORVIDAR SUPERIORVIDAR 1 VIDAR 1 ESR

HOLDAX Pre-hardened 1)33A 60 LVM 1)43A 46 HVZ 1)43A 24 FVZIMPAX SUPREME 2)97A 60 1 K5A V217 2)89A46 2 I7A V217 2)88A 36 H8A V2MIRRAX 40 3)SGB 60 MVX 3)SGB 46 G10 VXPM 3)86A 36 F12 VXPCNIMAX 4)51A 601 L5V MRAA 4)WA 46 HV 4)WA 24 GVRAMAX HH ROYALLOY

Precipitationhardening steel: Solution 1)33A 60 KVM 1)43A 46 GVZ 1)43A 36 FVZ

treated or 2)97A 60 2 K5A V227 1)15C 46 HVD 1)15C 36 GVDCORRAX aged 3)SGB 60 KVX 2)89A 46 1 H8A V217 2)89A 362 I 10A V237 P20

4)48A 601 J8V LNAA 3)3SG 46 G10 VXPM 3)1TGP 36 F12 VXPC4)WA 46 GV 4)WA 24 GV

High carbide steel:

ELMAX Soft annealed 1)33A 60 LVM 1)43A 46 HVZ 1)43A 36 FVZRIGOR 2)97A 60 2 J5A V227 2)455A 36 2 K15 V3 P22 2)454A 46 K13 V3SLEIPNER 3)SGB 60 LVX 3)3SG 46 G10 VXPM 3)53A 30 F12 VBEPSVERKER 3 4)51A 601 L5V MRAA 4)WA 46 HV 4)WA 24 GVSVERKER 21VANADIS 4 EXTRAVANADIS 6VANADIS 10VANADIS 23VANADIS 30VANADIS 60VANCRON 40

RIGOR Hardened 1)48A 60 LVZ 1)B151 R50 B3 1)420A 46 FVQPSLEIPNER 1)820A 60 LVQ 1)420A 46 G12VQP 2)89A 362 I8A V2SVERKER 21 2)97A 60 1 K5A V227 2)51B126 C50B Vib-Star 3)3SG 36 HVXVANADIS 23 3)SGB 60 LVX 2)455A 36 2 K15 V3 P22 4)WA 36 HVVANADIS 30 4)48A 601 L8V LNAA 3)SGB 46 HVXVANCRON 40 4)43A 601 L8V LNAA 3)3SG 46G10 VXPM

4)B126 V18 KR2374)27A 46 HV

ELMAX Hardened 1)48A 60 LVZ 1)B151 R50 B3 1)420A 46 FVQPSVERKER 3 1)820A 60 LVQ 1)420A 46 G12VQP 2)454A 46 K13 V3VANADIS 4 EXTRA 2)97A 60 K5A V217 2)51B126 C50B Vib-Star 3)3SG 46 FVSPFVANADIS 6 3)SGB 60 LVX 2)455A 36 2 K15 V3 P22 4)WA 46 FVVANADIS 10 4)48A 601 L8V LNAA 3)C150 QBAVANADIS 60 4)43A 601 L8V LNAA 3)SGB 46 HVX

4)B126 V18 KR2374)27A 46 HV

Example of suitable grinding wheelsThe grinding wheels are of SlipNaxos1) ,Tyrolit2), Norton3) and Unicorn4) type. The designations, however,essentially comply with international standards.


17

UDDEHOLM CYLINDRICALSTEEL GRADE CONDITION GRINDING INTERNAL GRINDING PROFILE GRINDING

Conventional steel:ALVAR Soft 1)33A 46 KVM 1)77A 60 K9VZ 1)42A 100 IVZALVAR 14 annealed 2)89A 60 2 K 5A V217 2)89A 60 2 K6 V112 2)89A 801 G11A V237 P25ARNE 3)19A 60 KVS 3)32A 46 L5 VBE 3)32A 100 KVSCALDIE 4)48A 46 LV 4)WA 46 JV 4)WA 100 LVCALMAXDIEVARFORMAXHOTVAR Hardened 1)48A 60 KVZ 1)77A 80 K9VZ 1)42A 1003 HVZMIRRAX ESR 2)92A 60 2 I6 V111 2)AH 120 K6 VCOL 2)89A 100 2 H11A V2ORVAR SUPREME 3)SGB 60 KVX 3)32A 60K5 VBE 3)32A 100 KVSORVAR 2 MICRODIZED 4)WA 60 JV 4)WA 60 IV 4)WA 120 JVPOLMAXQRO 90 SUPREMEREGIN 3STAVAX ESRTHG 2000UHB 11UNIMAXORVAR SUPERIORVIDAR SUPERIORVIDAR 1VIDAR 1 ESR

HOLDAX Pre-hardened 1)33A 46 KVM 1)77A 60 K9VZ 1)42A 100 IVZIMPAX SUPREME 2)89A 60 2 K 5A V217 2)97A 60 2 K6 V112 2)89A 80 1G11A V237 P25MIRRAX 40 3)19A 60 KVS 3)32A 46 L5 VBE 3)32A 100 KVSNIMAX 4)48A 46 LV 4)WA 46 JV 4)WA 100 LVRAMAX HH ROYALLOY

Precipitationhardening steel: Solution 1)42A 60 JVZ 1)42A 60 J9 VZ 1)42A 100 HVZ

treated or 1)15C 60 IVD 1)15C 60 IVD 2)89A 80 1G11A V237 P25

CORRAX aged 2)89A 60 2 J5A V217 2)64B91 K11 V333 VV 3)32A 100 JVS3)SGB 60 JVX 3)32A 46 K5 VBE 4)77A 100 J8V LNAA

4)77A 461 K7V LNAA 4)25A 601 J85VP MCNN

High carbide steel:

ELMAX Soft annealed 1)62A 60 KVZ 1)77A 60 K9 VZ 1)42A 100 IVZRIGOR 2)454A 80 J11 V3 2)AH 120 K6 VCOL 2)F13A 54 FF22V StratoSLEIPNER 3)SGB 60 KVX 3)32A 46 L5 VBE 3)32A 100 KVSSVERKER 3 4)48A 46 LV 4)WA 46 JV 4)WA 100 LVSVERKER 21VANADIS 4 EXTRAVANADIS 6VANADIS 10VANADIS 23VANADIS 30VANADIS 60VANCRON 40

RIGOR Hardened 1)B151 R50 B3 1)B151 R75 B3 1)B126 R100 B6SLEIPNER 1)48A 60 KVZ 1)430A 80 J VQA 1)820A 1003 GVQSVERKER 21 2)51B126 C50B Vib-Star 2)51B126 C100 B54 2)B126 C75 B53VANADIS 23 2)454A 80 J11 V3 2)C202 H5A V18 2)89A 80 1 G11A V237 P25VANADIS 30 3)SGB 60 KVX 3)CB150 TBA 3)CB150 TBEVANCRON 40 3)3SGP 70 JVX 3)3SG 60 JVX 3)5SG 80 KVX

4)B126 V18 KR191 4)B126 V24 KR237 4)B126K V24 KR2374)27A 60 JV 4)27A 60 HV 4)27A 100 JV

ELMAX Hardened 1)B151 R50 B3 1)B151 R75 B3 1)B126 R100 B6SVERKER 3 1)420A 54 JVQ 1)430A 80 J VQA 1)820A 1003 GVQVANADIS 4 Extra 2)51B126 C50B Vib-Star 2)51B126 C100 B54 2)B126 C75 B53VANADIS 6 2)454A 80 J11 V3 2)C202 H54 V18 2)F13A 54 FF22V Strato VANADIS 10 3)CB150 QBA 3)CB150 TBA 3)CB150 TBEVANADIS 60 3)SGB 60 KVX 3)3SG 60 JVX 3)5SG 80 JVX

3)3SGP 70 JVX 4)B126 V24 KR237 4)B126K V24 KR2374)B126 V18 KR191 4)27A 60 HV 4)27A 100 IV

4)27A 60 IV

18


www.assab.com www.uddeholm.com

Network of excellenceUDDEHOLM is present on every continent. This ensures you

high-quality Swedish tool steel and local support wherever you

are. ASSAB is our exclusive sales channel, representing Udde-

holm in the Asia Pacific area. Together we secure our position

as the world’s leading supplier of tooling materials.

UD

DEH

OLM

R-12.2012

UDDEHOLM is the world’s leading supplier of tooling materials. This

is a position we have reached by improving our customers’ everyday

business. Long tradition combined with research and product develop-

ment equips Uddeholm to solve any tooling problem that may arise.

It is a challenging process, but the goal is clear – to be your number one

partner and tool steel provider.

Our presence on every continent guarantees you the same high quality

wherever you are. ASSAB is our exclusive sales channel, representing

Uddeholm in the Asia Pacific area. Together we secure our position as

the world’s leading supplier of tooling materials. We act worldwide, so

there is always an Uddeholm or ASSAB representative close at hand to

give local advice and support. For us it is all a matter of trust – in long-

term partnerships as well as in developing new products. Trust is

something you earn, every day.

For more information, please visit www.uddeholm.com, www.assab.com

or your local website.

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