GRINDING of TOOL STEEL This Information Is Based on Our Present State of Knowledge and Is Intended to Provide General Notes on Our Products and Their Uses

GRINDING OF TOOL STEEL This information is based on our present state of knowledge and is intended to provide general notes on our products and their uses. It should not therefore be construed as a warranty of specific properties of the products described or a warranty for fitness for a particular purpose. Classified according to EU Directive 1999/45/EC For further information see our “Material Safety Data Sheets”.

Edition 7, Revised 12.2012 The latest revised edition of this brochure is the English version, which is always published on our web site www.uddeholm.com SS-EN ISO 9001 SS-EN ISO 14001 GRINDING OF TOOL STEEL

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 of Uddeholm tool steel 13 Cutting speed and feed 14 Grinding wheel dressing 15 Examples of suitable grinding wheels 15–17

3 GRINDING OF TOOL STEEL

tional standard by ISO, indicates the The table below shows how the Introduction composition of grinding wheels. The characteristics of aluminium oxide The high alloy content of tool steel identification consists of numerals abrasive can be varied by alloying it. means that such steel are often more and letters in a particular sequence, difficult to grind than conventional defining the abrasive, grain size, grade ABRASIVE COLOUR PROPERTIES structural steel. and binder. In order to achieve successful Normal Example: corundum Brown, grey results when grinding tool steel, it is Mixed Abrasive necessary to choose the grinding corundum Yellowbrown

ougher

T wheel with care. In turn, choosing the Grain size Red alumina Red Harder right grinding wheel and grinding data A 46 H V Grade White alumina White requires an understanding of how a Binder grinding wheel works. This brochure provides a quite Unfortunately, the colour of a grind- ABRASIVE detailed description of the make-up ing wheel does not always necessarily of the wheel, of how it works when It is important that the abrasive fulfils indicate the type of abrasive used in grinding and of the parameters that requirements in respect of: it, due to the fact that some grinding determine the final result. It also • hardness wheel manufacturers colour their includes recommendations for • sharpness abra-sives and binders. grinding wheels for use with Udde- • thermal resistance There is also another type of alu- holm tool steel. • chemical stability minium oxide named ceramic or sintered aluminium oxide. This abra- To day, the following four main groups sive has a fine crystalline structure, of abrasives (all synthetic) are used, which means that the grains retain fulfilling the above requirements to Grinding their sharpness better. However, its greater or lesser extents. use requires higher grinding pressure. wheel design 1. Aluminium oxide designation A (SG) A typical application for it is grinding In principle, a grinding wheel consists 2. Silicon carbide designation C tool steel in rigid grinding machines. of the following components: 3. Cubic boron nitride designation B Examples of this type of abrasive are • Abrasive 4. Diamond designation SD SG (Seeded Gel) from Norton and • Binder Cubitron from 3M. • Air pores Abrasives have different application 2. Silicon carbide is an abrasive that is areas, depending on their particular Binder used primarily for grinding cast iron characteristics, as shown partially in and austenitic stainless steel, although the table below. it can also be used for hardened tool Air steel. It occurs in two main variants: pores THERMAL the black silicon carbide and a some- DURABILITY Abrasive HARDNESS IN AIR what harder green variant, which is ABRASIVE KNOOP °C more brittle than the black material. Aluminium 3. Cubic Boron Nitride (CBN) is pro- oxide 2100 2000 Figure 1. The arrangement and proportions Silicon duced in approximately the same way of abrasives grains, air pores and bond carbide 2500 1200 as synthetic diamond, and is an abra- bridges (made up of binder) determine CBN 4700 1400 sive that is used primarily for grinding grinding wheel characteristics. Diamond 7000 650 hardened high-carbide tool steel and high-speed steel. A drawback of CBN Certain special grinding wheels, such is its high price—almost twice that of synthetic diamond. as metallically bonded diamond 1. Aluminium oxide, is the abrasive wheels, contain no air pores. most commonly used for grinding 4. Diamond is seldom used, despite It is the composition and variation steel, and is available in several vari- its high hardness, for grinding tool of the above components that deter- ants. It can be alloyed with other steel as a result of its low thermal mines the characteristic of a grinding oxides, of which the most common is resistance. Diamond is used primarily wheel. An identification system, which titanium oxide. for grinding cemented carbide and has now been ratified as an interna- ceramic materials.

4 GRINDING OF TOOL STEEL

ABRASIVE GRAIN SIZE GRINDING WHEEL GRADE Vitrified grinding wheels are those The grain size of the abrasive is an The grade of a grinding wheel refers most commonly used for grinding important factor in selecting the to its hardness, i.e. how securely the tool steel. correct grinding wheel. abrasive grains are held by the binder. Resinoid is used as a binder in Grain sizes are classified in accord- It does not, therefore, depend on the grinding wheels intended for high ance with an international mesh size hardness of the abrasive used in the peripheral speeds, such as certain in mesh/inch, ranging from 8 (coarse) wheel. CBN wheels. to 1200 (superfine). Grain sizes for The grade of a grinding wheel is Rubber-bonded wheels are used for grinding tool steel are generally in determined primarily by the quantity high specific grinding pressures, such the range 24–100 mesh. Coarse grain of binder used in the wheel. A higher as for control wheels in centreless sizes are used for rapid rate of remo- proportion of binder reduces the grinding. val, when grinding large workpieces, amount of air pores and produces a Metallic binders are used for grinding softer materials or when the harder wheel. diamond and certain CBN wheels. contact surface of the grinding wheel The grade of a wheel is indicated Such wheels can withstand very high is large. Fine grain sizes are used to by a letter, indicating the hardness in peripheral speeds. produce high surface finish, when alphabetical order: grinding hard materials or when the E = very soft composition contact surface of the grinding wheel Z = very hard composition. is small. For tool steel, the most commonly The surface smoothness of the encountered compositions are within ground part depends not only on the the hardness range G–K. Indication of grain size of the grinding wheel. The the grade is sometimes followed by a sharpness of the wheel, the bonding numeral, which indicates the spread material used and the hardness of the of the abrasive particles in the wheel. wheel also play a considerable part in determining the surface finish produced. GRINDING WHEEL BINDERS In the case of diamond and CBN The following binders are used to grinding wheels, European grinding bind the grains in a grinding wheel: wheel manufacturers indicate grain • Vitrified designation: V size by the diameter of the abrasive •Resinoid ,, B grains in microns, while American and •Rubber ,, R Japanese manufacturers indicate it in • Metal ,, M mesh size.

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 abrasive applied to a central hub, usually of aluminium.

5 GRINDING OF TOOL STEEL

•A very large number of cutting cease to remove material and gener- How the grinding edges. ate only heat. The grinding wheel is wheel works •Very high cutting speed. The most then said to be burning the material, which can cause cracks in it. Grinding is a cutting process in which common cutting speed for preci- For a grinding wheel to work the cutting edges are formed by the sion grinding, 35 m/s = 2100 m/min., correctly, the stresses in the binder grains of abrasive. The same prin- is far above what is normal for and the strength of the binder must ciples apply for grinding as for other other cutting processes. be so balanced that, as the grains chip-cutting methods, although vari- •Very small chips, i.e. very small become as blunt as can be accepted, ous factors mean that it is necessary cutting depth for each cutting edge. they are pulled out of the binder and to consider the theory of grinding are replaced by new, sharp grains. somewhat differently. GRINDING FORCES The grinding wheel, in other words, Conditions that are special for sharpens itself. Self-sharpening also grinding. The grinding forces that act on each individual grain of abrasive are re- occurs through grain breakage, which • The cutting tool has an irregular ferred to as specific forces. A mean creates new cutting edges. cutting geometry and the abrasive value of the specific forces can be The degree of self-sharpening, i.e. grains are irregularly placed, which obtained by dividing the total force whether the grinding wheel is hard means that cutting, ploughing and by the number of cutting edges, or soft, is affected by the composi- sliding will occur, see figure 2. which depends on the size of the tion of the wheel (its design hard- • The cutting geometry can change. contact area and the number of cut- ness) and by the conditions under The method of working of an abra- ting edges in the grinding path. The which it is working. sive tool includes a certain degree specific forces determine various of “self-sharpening”, which means effects, including the degree of self- AVERAGE CHIP THICKNESS that grains of abrasive break or are sharpening of the grinding wheel, i.e. Although the chips removed by replaced as they wear. its “working hardness”. The total force grinding are small and irregular, the • Negative cutting angles. The irregu- is the force arising between the mean value of their thickness at any lar “blunt” shapes of the grains grinding wheel and the workpiece. time is relatively constant. This value mean that the rake angles are often varies, depending on the type of negative. GRINDING WHEEL WEAR grinding operation and in response to The grains of abrasive are initially the changes in grinding data. If a grinding wheel is cutting larger Cutting Grinding sharp, but in the same way as with all chips, this means two things: direction other cutting edges they wear down Chip Abrasive grain in use and become blunt. Finally, the 1. Higher loading on each cutting grains will have become so blunt that edge, i.e. higher specific forces. This they have difficulty in penetrating into increases the self-sharpening Workpiece the material of the workpiece. They characteristic of the wheel and

Grinding Ploughing direction Small chip Large chip Abrasive grain

Workpiece

Grinding High forces on direction Low forces on Sliding the abrasive grain the abrasive grain Abrasive grain

Friction heat Workpiece Fine surface Rough surface Figure 2. Different conditions during grinding (highly schematic). Cutting angles are Figure 3. A large chip size results in a rougher surface finish on generally negative. the workpiece. 6 GRINDING OF TOOL STEEL

gives it the characteristics of a thickness, with the result that the If the wheel is to sharpen itself prop- softer wheel. grinding wheel behaves as a softer erly, it must be of a softer composi- 2. The surface of the part being wheel. tion than one intended for external ground is coarser, see Figure 3. Generally, both peripheral velocity cylindrical grinding of a similar part. In and workpiece speed are increased in this latter case, the contact length is A reduction in the average chip thick- order to increase the total rate of shorter, which means that there are ness represents the opposite. It is removal. higher cutting forces on each grain. therefore important to understand how changes in grinding data and The contact width may be equal to the THE G-RATIO OF other conditions affect the average width of the grinding wheel as, for A GRINDING WHEEL chip thickness. example, in plunge grinding. However The G-ratio of a grinding wheel refers in operations such as surface grinding to the relationship between the with a moving table, only part of the STOCK REMOVAL RATE amount of material removed and the When grinding, the amount of chips amount of grinding wheel consumed. removed per unit of time can most The G-ratio is a measure of how Cylindrical grinding easily be expressed as mm3/s. This is effectively a grinding wheel works often referred to as the stock re- with the particular workpiece mate- moval rate, and depends on the ma- rial. chine feed, the composition of the grinding wheel, its cutting speed GRINDING WHEEL (peripheral speed) and (in certain CONTACT SURFACE cases) on the dimensions of the It is at the contact surface between workpiece. the grinding wheel and the workpiece It is often more meaningful to talk that the actual cutting operation about stock removal rate rather than Surface grinding occurs. A large contact surface means about table feed speed, feed depth that a greater number of cutting etc., and it is also quite easy to calcu- edges participate in the process, thus late. Cost considerations often dic- reducing the chip size and specific tate that the stock removal rate forces. Similarly, a reduced contact should be as high as possible. If the surface area results in greater chip stock removal rate is increased with- size and higher specific forces. out increasing the number of grains In principle, the contact surface is of abrasive performing the work, e.g. in the shape of a rectangle. Its extent by greater infeed depth, the chip size in the cutting direction is referred to will also naturally be increased. as the contact length or contact arc, Internal grinding while its extent perpendicular to the CUTTING SPEED cutting direction is referred to as the The peripheral speed of a grinding contact width. wheel has a direct effect on the number of cutting edges that actually The contact length depends primarily perform the machining work. If, for on the type of grinding operation. In example, the cutting speed is dou- addition, it depends on the diameter bled, twice as many grains of abrasive of the grinding wheel, the cutting will pass the workpiece per unit of depth and in all cases—except for time. If the workpiece speed is not surface grinding—the dimensions of Segmental surface increased, the mean chip thickness the workpiece. Differences in the grinding will decrease, thus also reducing the contact length are the main reason cutting forces on each grain. Self- for having to select different grinding sharpening will be less effective, i.e. wheel compositions for different the grinding wheel will be effectively grinding operations. harder, producing a finer surface If, when performing internal grind- finish, but with greater risk of burning ing, a grinding wheel is used that has the surface. a diameter only a little less than that Conversely, reducing the speed of of the ground hole, the contact the wheel will increase the chip length will be very large, resulting in Figure 4. Differences in contact length for low cutting force per grain. different grinding operations.

7 GRINDING OF TOOL STEEL grinding wheel is actually cutting and Dressing is a conditioning of the creates space for chip formation. In this part changes as the wheel wears wheel surface to give the desired practice this can be done by pushing down. It is sometimes possible to cutting action. Dressing the wheel a wet aluminium oxide stone into the reduce the contact width, if this is exposes sharp cutting edges. One and wheel for a few seconds. required, by truing of the grinding the same grinding wheel can be given wheel. This reduces contact surface completely different grinding charac- area, resulting (as already described) teristics through application of diffe- in a greater chip thickness, higher rent dressing tools or different dres- loading on the abrasive grains and an sing methods. Dressing is therefore a effectively softer grinding wheel. particularly important parameter in achieving good grinding performance. Dressing resulting in a smooth THE NUMBER OF CUTTING surface on the wheel results in the EDGES IN THE CONTACT AREA cutting edges of the grains of abrasive The number of cutting edges in the being close together, while dressing contact area is a factor that has a resulting in a rough surface of the considerable effect on the chip wheel gives the wheel a more open thickness and thus on the grinding structure. Dressing provides a means process. of making the same grinding wheel A large number of cutting edges give completely different grinding per unit area mean that the work of results. removing material is spread over a The degree of self-sharpening larger number of grains, reducing the affects the structure of the grinding chip thickness and the specific forces. wheel surface, i.e. the number of The grain size of the abrasive also cutting edges per unit of area. affects the number of cutting edges, A grinding wheel that has a high self- which is the reason for the common sharpening performance has a differ- observation that fine-grained cutting ent, more open structure than one wheels seem to be harder. having poorer self-sharpening performance.

DRESSING AND TRUING There are many different tools avail- GRINDING WHEELS able for dressing and truing grinding Dressing and truing of a grinding wheels, e.g. crushing rolls and dia- wheel are often considered to be the mond tools. CBN wheels are best same thing because they are often dressed using a diamond coated performed as one operation. roller. Truing is made to produce any profile Certain types of grinding wheels, e.g. which may be required on the face resinoid bonded CBN wheels, need of the wheel and to ensure concen- to be “opened” after dressing. This tricity. reveals the abrasive particles and

8 GRINDING OF TOOL STEEL

The grinding • Emulsions. These consist of water with an ad-mixture of 2–5% of oil machine in an extremely finely distributed The type of grinding operation and form. Sulphur or chlorine additives the machine available has a consider- may also be used as EP additives. able effect on the choice of appropri- • Cutting oils. These are composed of ate grinding wheel composition. a mineral oil base with EP-type A grinding machine should be as rigid additives. Cutting oils provide as possible, in order to allow it to effective lubrication but poorer work at high grinding pressures. This cooling. is because it is the rigidity of the Water solutions are most suitable grinder and the method of clamping when grinding with diamond wheels. the workpiece that determine the Emulsions are used nowadays for permissible grinding pressure and the majority of grinding operations therefore restrict the choice of because they are ecologically bene- wheels. If the machine is not suffi- ficial and perform adequately. ciently rigid, a softer grinding wheel Cutting oils give the best results composition or a smaller contact for profile and plunge grinding with area between the grinding wheel and fine grained wheels, e.g. when grind- the workpiece should be chosen, in ing threads. Cutting oil also provides order to achieve the required degree the longest life for resinoid bonded of self-sharpening performance. CBN wheels, although high-oil The speed of the grinder also emulsions are often chosen in the affects the choice of grinding wheel. interests of pollution reduction. CBN wheels often require peripheral speeds of 45 m/s in order to provide good cutting performance.

Grinding fluid When grinding, as with all other cutting operations, a cutting fluid is used primarily to: •cool the workpiece •act as a lubricant and reduce friction between the chips, work- Fine gridning of details piece and grinding wheel in hardened Udddeholm •remove chips from the contact Mirrax ESR area There are three main types of cutting fluids that can be used when grinding. • Water solutions. These are liquids that consist of water with synthetic additives in order to increase its wetting performance and prevent corrosion. Such fluids contain no oil and provide good cooling performance but poorer lubrication performance.

9 GRINDING OF TOOL STEEL

In practice, powder metallurgy is The tool steel employed to increase the quantity of The alloying constituents of a tool carbide in a tool steel, i.e. such steel steel have a considerable effect on its are more highly alloyed than conven- ease of grinding. tional steel, which generally means The Uddeholm range of tool steel that they are more difficult to grind. extends from low-alloy steel, such as The effect of hardness on ease of Uddeholm UHB 11, to high-alloy grinding is also dependent on the steel, such as Uddeholm Vanadis 10. quantity of carbide-forming alloying There is seldom any problem in elements in the steel. grinding low-alloy tool steel. At the other end of the scale, however, the high-alloy carbide-rich steel can cause Hardness kp/mm2 problems when being ground, and 7500 require a careful choice of grinding wheel and operating parameters. 7000 The higher the wear resistance of a 6500 steel, the more difficult it is to grind. 6000 The wear resistance of a steel, and thus also its ease of grinding, are 5500 determined by its basic hardness and 5000 by the size, hardness and quantity of the carbides in it. 4500

In order to enhance the wear 4000 resistance of a tool steel, the steel is alloyed with carbide-forming alloying 3500 elements, of which the most impor- 3000 tant are chromium and vanadium. The steel must also have a high carbon 2500 content if carbides are to be formed. 2000 The diagram, Figure 5, shows the Figure 5. The 1500 hardness of the basic phases found in hardness of grinding a tool steel, the hardness of the most 1000 abrasives, basic common carbides found in tool steel phases found in a tool 500 steel and carbides and the hardness of commonly used found in tool steel. grinding abrasives. 0

rite

As can be seen in the figure, it is r

only diamond and CBN that are Diamond

Austenite

Cementite harder than all the carbides that are Martensite

Silicon carbide

ungsten carbide

anadium carbide anadium

Niobium carbide

Titanium carbide found in a tool steel. However, as T

Aluminium oxide

Chromium carbide Chromium mentioned earlier, diamond is unsuit- nitride Cubic boron

Molybdenum carbide able for grinding steel. The quantity and the size of carbides As can be seen in Figure 6, hardness in a steel has a very considerable has a greater effect on grindability for effect on the ease of grinding of the high-carbide steel. material. The greater the number of, Grindability index and the larger the carbides, the more 100 difficult the material is to grind. A This is the reason why tool steel 10 B produced by powder metallurgy C Figure 6. The effect of hardness on processes, having smaller carbides, is 1 grindability for: easier to grind than a conventionally A – a low-alloy tool steel of Arne type produced steel having a similar com- B – a material of Sverker type position. 0,1 C – material of Vanadis 10 type.

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In order to obtain good grinding The formation of grinding cracks, The diagram below shows the performance with high-alloy carbide- which tend to occur perpendicular to hardness profile through the surface rich tool steel, it is important to the direction of grinding, usually of a tool steel, incorrectly ground in select the correct grinding wheel. means the tool has to be scrapped. such a way as to produce re-harden- Materials in the Uddeholm Vanadis Hardened steel are more sensitive to ing. range, for example, contain a large grinding cracks than non-hardened quantity of vanadium carbides. To cut steel. A material that has been only Hardness, HRC through a vanadium carbide requires hardened, and not tempered, must an abrasive that is harder than alu- never be ground: hardened materials 64 minium oxide or silicon carbide. should always be tempered before CBN wheels are therefore recom- grinding. 60 mended as first choice for grinding Formation of grinding cracks can be this material. The fact that, despite explained as follows: 56 this, material can be removed from Almost all the energy used in Uddeholm Vanadis steel by grinding grinding is converted into heat, partly 52 with aluminium oxide or silicon car- through pure friction and partly as a 48 bide is due to the fact that it is the result of deformation of the material. material enclosing the carbides that is 0,10 0,20 0,30 0,40 0,50 If the correct grinding wheel has Depth below ground surface, mm ground away, so that the carbides are been chosen, most of the heat will be torn out of the basic material of the removed in the chips, with only a Figure 7. Hardness profile through the steel. However, this occurs at the smaller part heating up the work- surface layer of an incorrectly ground tool. price of high wear of the grinding piece. wheel and a risk of poor grinding performance.

GRINDING CRACKS AND GRINDING STRESSES The wrong choice of grinding wheels and grinding parameters results in a considerable risk of causing cracks in the workpiece. Generally, grinding cracks are not as easy to see as in Photo 2. It is usually necessary to examine the part under a microscope, or with mag- Re-hardened layer in an netic powder inspection, in order to incorrectly ground tool. see the cracks.

Incorrect grinding of a hardened tool steel can result in such a high temperature at the ground surface that the tempering temperature of the material is exceeded. This results in a reduction in the hardness of the surface. If the temperature is allowed to rise further, the hardening temperature of the material can be reached, resulting in rehardening. This produces a mixture of non-tempered and tempered martensite in the surface layer, together with retained austenite, as shown in Photo 3. Very high stresses arise in the material, often resulting in the formation of Grinding cracks. cracks.

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The surface exhibits a high hardness ite levels increase the risk of crack It is, unfortunately, very difficult to due to the untempered martensite. formation when grinding. produce a simple check to determine An overtempered zone occurs just The majority of grinding operations the stress pattern set up in the below the surface, where the hard- leave residual stresses in the ground ground part unless the stresses are ness is lower than the basic hardness surface. These stresses are usually at so high that grinding cracks are vis- of the workpiece. a maximum close to the surface, and ible. Incorrect grinding, resulting in a can cause permanent deformation of Grinding stresses can be reduced by modified surface layer, often reveals the ground part when grinding thin stress-relief tempering after grinding. itself through burn marks—discolora- materials. The tempering temperature should tion of the ground surface. In order Of the three examples shown in be about 25°C below the previous to avoid burning and grinding cracks, Figure 9, Example 1 is most at risk in tempering temperature in order to it is necessary to keep down the respect of crack formation. It exhibits avoid any risk of reducing the hard- temperature of the ground part, e.g. tensile stresses in the surface which ness of the workpiece. by means of good cooling, and to can, if they exceed the material’s Another way of reducing grinding employ properly dressed grinding ultimate tensile strength, result in the stresses is to tumble or blast the wheels that cut the material with material cracking. ground parts. sharp cutting edges instead of simply Examples 2 and 3 are not as danger- generating heat through friction. ous—the surface stresses are com- A simple example of how incorrect pressive stresses, which result in grinding can cause cracks is shown in improved fatigue strength of the Figure 8. A hardened punch with a ground parts. head is to be cylindrical-ground, with the head (b) being ground flat in the same operation. Alternative A shows the use of a grinding wheel trued with a 90° edge. The grinding wheel, which is suitable A for cylindrical grinding of the surface b (a), produces a good result on sur- a face (a). Here the contact surface is small so the self sharpening perform- + ance is good. The head, on the other hand, which is to be ground flat, Example 1 presents a larger contact surface to the grinding wheel. The specific forces on the abrasive grains are low – B Better Depth below the surface so that the wheel does not self- sharpen. Instead, surface (b) is sub- b jected mainly to rubbing and the heat + a generated can cause grinding cracks. Example 2 Tension Alternative B shows a better way to grind the punch. In this case, the side Depth below the surface of the grinding wheel has been trued Com- as shown so that the contact surface pression at (b) is smaller. This results in im- – proved self-sharpening and “cooler” C Example 3 grinding. Best + Case C shows the preferred way to b grind this part. The grinding wheel is Depth below the surface a set at an angle, so that the two contact surfaces are of approximately the same size. – The retained austenite content of a hardened material can also affect the Figure 9. Three typical examples of stress grinding result. High retained austen- Figure 8. Incorrect grinding can often result in grinding cracks. distribution in a ground surface.

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Recommendations have not so good self-sharpening operate well for other grinding ope- properties as powder steel due to rations, provided that a high periph- for grinding of the bigger carbide size. However, eral speed can be maintained. the lower carbide hardness and Where boron nitride wheels cannot Uddeholm tool steel carbide content will compensate be used, the type of grinding wheel for the grinding properties. GRINDING OF must be chosen with care. White HIGH-CARBIDE TOOL STEEL Figure 11 shows the results of sur- aluminium oxide or green silicon The high carbide content of high- face grinding trials on Uddeholm carbide wheels are recommended. carbide tool steel gives them excel- Vanadis 10 with aluminium oxide, fine Fine-crystalline aluminium oxide lent wear resistance, and require crystalline aluminium oxide and CBN wheels, such as the Norton SG, give special recommendations in respect grinding wheels. good results if the grinding set-up is of grinding operations and selection As can be seen in Figure 11, mate- rigid. of grinding wheels. For the majority rial is removed more quickly, and the When grinding high-carbide steel, of grinding operations, CBN wheels G-ratio is higher, using CBN wheels. the grinding wheel should always be are the best choice for such steel. These wheels have a “colder” cut, somewhat softer in order to ensure with less risk of “burning” the sur- good self-sharpening performance. There are two different types of face. In addition, the following points carbide rich tool steel, conventionally If the material is to be profile- must be borne in mind: made steel and powder steel. The ground, bear in mind that a consider- • the grinder must be vibration-free, main differences that affect the grind- able quantity of heat will be gener- rigid and in good condition ing properties are the hardness, size ated. Experiments have shown that • the workpiece must be securely and distribution of carbides, see vitrified CBN wheels are preferable clamped. Use a steady rest when Figure 10 below. for this application. These wheels also • Powder steel, such as Uddeholm grinding long, thin workpieces Elmax, Uddeholm Vanadis and • use sharp conical diamonds when Stock removal rate mm3/s Uddeholm Vancron, have in spite of dressing Al2O3 and SiC wheels. The dressed finish must be rough the high alloying level relatively 14 good grinding properties due to 11,8 • maintain a high peripheral speed of 12 13,3 the small carbide/nitro carbide size. 11 grinding wheels The small carbides will give the 10 • ensure an adequate supply of grinding wheel good self-sharpening coolant to the grinding zone properties. 8 • if grinding is carried out without a • Conventionally made steel, such as 6 coolant, select a grinding wheel Uddeholm Rigor, Uddeholm that is one grade softer than would Sleipner and Uddeholm Sverker, 4 have been used if grinding was 2 performed with coolant •never grind a hardened workpiece

Conventionally made Al2O3 Al2O3-SG CBN before it has been tempered high-carbide steel

Carbides G-ratio 270 240 253 Work piece 210 180

Powder steel 150

Carbides 120

90 60 Work piece Figure 11. Surface grinding of Uddeholm 30 Vanadis 10 with various grinding wheels. 0,68 2,4 (Grinding wheel width: Al2O3 40 mm, CBN 20 mm.) Figure 10. Carbide size and distribution in Al2O3 Al2O3-SG CBN high-carbide tool steel (highly schematic).

13 GRINDING OF TOOL STEEL

GRINDING OF Cutting speed WORKPIECE SPEED CONVENTIONAL TOOL STEEL For surface grinding, the speed of the This group covers all the other and feed workpiece should be 10–20 m/min. conventionally produced tool steel. GRINDING WHEEL SPEED For conventional cylindrical grinding, Providing that common grinding (CUTTING SPEED) this speed should be 15–20 m/min. recommendations are followed, This speed should be reduced for When using small grinding machines, problems are seldom encountered smaller diameter workpieces, for the spindle speed often restricts when grinding these tool steel. For which 5–10 m/min is suitable. choice of cutting speed. these steel, ordinary aluminium oxide Varying the workpiece speed also A common safety limit for vitrified grinding wheels are perfectly suitable. provides a means of modifying the grinding wheels is 35 m/s. However, CBN wheels can also be used if the grinding performance of the wheel. some grinding wheels are approved steel are to be ground in the hard- Increasing the speed of the work- for peripheral speeds of 125 m/s. ened and tempered condition. piece makes the wheel seem softer, A common cutting speed for while reducing its speed produces a surface and cylindrical grinding is GRINDING OF PRECIPITATION harder wheel. 20–35 m/s. Varying the peripheral HARDENING STEEL speed of the wheel makes it possible Precipitation hardening steel, such as to modify its grinding performance. Uddeholm Corrax, behaves in a little Increasing the peripheral speed of CROSS-FEED different way than other tool steel the wheel while retaining the same The cross-feed speed of a grinding when grinding. It tends to clog the workpiece speed means that the wheel, i.e. its sideways motion, is grinding wheel, especially if the grind- wheel behaves as if it was harder. higher for rough grinding than for ing wheel is hard and has a close Reducing the peripheral speed makes fine grinding. structure. The clogging can cause the wheel seem softer. In the case of cylindrical grinding, problems like low material removal A suitable peripheral speed for the cross-feed should be about rate and rough surface finish. To prev- resinoid CBN wheels is 30–40 m/s. 1/3–1/2 of the width of the wheel for ent the clogging, observe following For vitrified CBN wheels, a cutting each revolution of the workpiece. For recommendations: speed ≥45 m/s is often necessary. fine surface finish, this ratio should be • the wheel should have an open and When grinding high-carbide tool reduced to 1/6–1/3 of the width of porous structure steel, the peripheral speed of the the grinding wheel per revolution of • use a softer wheel grade (hardness) grinding wheel should be high. Tests the workpiece. than for other types of tool steel on cylindrical grinding of Uddeholm If a very high standard of surface • the wheel dressing should be done Elmax have shown that the G-ratio of finish is required, cross feed can be frequent and rough the grinding wheel dropped from 127 further reduced to 1/8–1/10 of the • the coolant concentration should to 28 when the peripheral speed was grinding wheel width. be high (>5%) for efficient lubrica- dropped from 60 m/s to 30 m/s. When surface grinding with a tion Cutting speed, in other words, has a straight wheel, choose a transverse considerable effect on the economics feed of 1/6–1/3 of the width of the Conventional Al2O3 wheels are of grinding. grinding wheel for each stroke. Again, recommended, but SiC wheels can be reduce this feed for high surface a better choice for high surface finish finish requirements. when a small amount of material is to Note that when the cross-feed is be ground. No particular difference in increased, the active contact surface grindability between solution treated area between the grinding wheel and and aged condition. In the table with the workpiece becomes larger, result- recommended grinding wheels, page ing in an apparent increase in hard- 16–17, suitable standard type of ness of the grinding wheel. grinding wheels are recommended. However, if a lot of grinding is to be done in this type of steel, it is recommended to select a wheel with a more open structure than a standard wheel type.

14 GRINDING OF TOOL STEEL

INFEED Grinding Suitable The infeed of the grinding wheel depends on the type of wheel and wheel dressing grinding wheels the rigidity of the grinder and/or During dressing a helix along the The examples of grinding wheels in workpiece clamping. wheel periphery is made. The lead of the tables, page 16–17, have been Guide values for cylindrical grinding helix which the dressing tool is being made in consultation with grinding using conventional grinding wheels fed affects the structure of the grind- wheel manufacturers, and are based are: ing wheel. The lead of helix depends on our own and others experience. Rough finish ~0.05 mm/pass. both of the r.p.m. of the grinding However, it must be emphasised that Fine finish ~0.005–0.010 mm/pass. wheel and the speed of the dressing the choice of grinding wheel is strongly dependent on the type of The above feeds should be halved for tool. grinding machine, rigidity of clamping cylindrical grinding using CBN The following are rules of thumbs and the size of the workpiece, which wheels. for grinding wheel dressing with means that the recommendations For surface grinding using a straight single point diamonds and similar should be seen as starting points, grinding wheel, the feed depths for tools. from which each particular process conventional wheels are: should be optimized. Rough finish ~0.025–0.075 mm/pass. Rough Fine Fine finish ~0.005–0.010 mm/pass. dressing dressing Diamond infeed The feed depths when using CBN (mm) 0,02–0,04 0,01–0,02 wheels are: Diamond Rough finish ~0,010–0,040 mm/pass. transverse rate Fine finish ~0,005–0,010 mm/pass. (mm/wheel rev.) 0,15–0,30 0,05–0,10 When using grinding wheels having fine-crystalline aluminium oxide Diamond is sensitive for high tem- abrasive, such as the Norton SG type, peratures. Therefore, dressing with feed depth should be increased diamonds should always be carried somewhat over the above values out with plenty of coolant. The in order to achieve higher grinding coolant should always be turned on pressure and hence good self- before the diamond touches the sharpening performance. wheel. Single point diamond dressing tool should be systematically rotated to maintain the sharpness.

GRINDING PROBLEMS—REMEDIES The table shows the most important actions to solve different grinding problems.

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.

15 GRINDING OF TOOL STEEL Example of suitable grinding wheels The grinding wheels are of SlipNaxos1) ,Tyrolit2), Norton3) and Unicorn4) type. The designations, however, essentially comply with international standards.

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 FVZ ALVAR 14 annealed 2)89A 60 2 K5A V217 2)91A 46 I8A V217 2)88A 36 H8A V2 ARNE 3)SGB 60 MVX 3)3SG 46 G10 VXPM 3)86A 30 G12 VXPM CALDIE 4)51A 601 L5V MRAA 4)WA 46 HV 4)WA 24 GV CALMAX DIEVAR FORMAX HOTVAR Hardened 1)62A 60 LVZ 1)48A 46 HVZ 1)48A 46 FVZP MIRRAX ESR 2)89A 60 2 K5A V217 2)97A 46 2 H8A V217 2)97A 46 1 H10A V2 ORVAR SUPREME 3)SGB 60 MVX 3)SGB 46 G10 VXPM 3)86A 36 F12 VXPC ORVAR 2 MICRODIZED 4)48A 601 L8V LNAA 4)WA 46 GV 4)WA 36 GV POLMAX QRO 90 SUPREME REGIN 3 STAVAX ESR THG 2000 UHB 11 UNIMAX ORVAR SUPERIOR VIDAR SUPERIOR VIDAR 1 VIDAR 1 ESR HOLDAX Pre-hardened 1)33A 60 LVM 1)43A 46 HVZ 1)43A 24 FVZ IMPAX SUPREME 2)97A 60 1 K5A V217 2)89A46 2 I7A V217 2)88A 36 H8A V2 MIRRAX 40 3)SGB 60 MVX 3)SGB 46 G10 VXPM 3)86A 36 F12 VXPC NIMAX 4)51A 601 L5V MRAA 4)WA 46 HV 4)WA 24 GV RAMAX HH ROYALLOY

Precipitation hardening 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 GVD CORRAX 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 VXPC 4)WA 46 GV 4)WA 24 GV

High carbide steel: ELMAX Soft annealed 1)33A 60 LVM 1)43A 46 HVZ 1)43A 36 FVZ RIGOR 2)97A 60 2 J5A V227 2)455A 36 2 K15 V3 P22 2)454A 46 K13 V3 SLEIPNER 3)SGB 60 LVX 3)3SG 46 G10 VXPM 3)53A 30 F12 VBEP SVERKER 3 4)51A 601 L5V MRAA 4)WA 46 HV 4)WA 24 GV SVERKER 21 VANADIS 4 EXTRA VANADIS 6 VANADIS 10 VANADIS 23 VANADIS 30 VANADIS 60 VANCRON 40 RIGOR Hardened 1)48A 60 LVZ 1)B151 R50 B3 1)420A 46 FVQP SLEIPNER 1)820A 60 LVQ 1)420A 46 G12VQP 2)89A 362 I8A V2 SVERKER 21 2)97A 60 1 K5A V227 2)51B126 C50B Vib-Star 3)3SG 36 HVX VANADIS 23 3)SGB 60 LVX 2)455A 36 2 K15 V3 P22 4)WA 36 HV VANADIS 30 4)48A 601 L8V LNAA 3)SGB 46 HVX VANCRON 40 4)43A 601 L8V LNAA 3)3SG 46G10 VXPM 4)B126 V18 KR237 4)27A 46 HV ELMAX Hardened 1)48A 60 LVZ 1)B151 R50 B3 1)420A 46 FVQP SVERKER 3 1)820A 60 LVQ 1)420A 46 G12VQP 2)454A 46 K13 V3 VANADIS 4 EXTRA 2)97A 60 K5A V217 2)51B126 C50B Vib-Star 3)3SG 46 FVSPF VANADIS 6 3)SGB 60 LVX 2)455A 36 2 K15 V3 P22 4)WA 46 FV VANADIS 10 4)48A 601 L8V LNAA 3)C150 QBA VANADIS 60 4)43A 601 L8V LNAA 3)SGB 46 HVX 4)B126 V18 KR237 4)27A 46 HV

16 GRINDING OF TOOL STEEL

UDDEHOLM CYLINDRICAL STEEL GRADE CONDITION GRINDING INTERNAL GRINDING PROFILE GRINDING Conventional steel: ALVAR Soft 1)33A 46 KVM 1)77A 60 K9VZ 1)42A 100 IVZ ALVAR 14 annealed 2)89A 60 2 K 5A V217 2)89A 60 2 K6 V112 2)89A 801 G11A V237 P25 ARNE 3)19A 60 KVS 3)32A 46 L5 VBE 3)32A 100 KVS CALDIE 4)48A 46 LV 4)WA 46 JV 4)WA 100 LV CALMAX DIEVAR FORMAX HOTVAR Hardened 1)48A 60 KVZ 1)77A 80 K9VZ 1)42A 1003 HVZ MIRRAX ESR 2)92A 60 2 I6 V111 2)AH 120 K6 VCOL 2)89A 100 2 H11A V2 ORVAR SUPREME 3)SGB 60 KVX 3)32A 60K5 VBE 3)32A 100 KVS ORVAR 2 MICRODIZED 4)WA 60 JV 4)WA 60 IV 4)WA 120 JV POLMAX QRO 90 SUPREME REGIN 3 STAVAX ESR THG 2000 UHB 11 UNIMAX ORVAR SUPERIOR VIDAR SUPERIOR VIDAR 1 VIDAR 1 ESR HOLDAX Pre-hardened 1)33A 46 KVM 1)77A 60 K9VZ 1)42A 100 IVZ IMPAX SUPREME 2)89A 60 2 K 5A V217 2)97A 60 2 K6 V112 2)89A 80 1G11A V237 P25 MIRRAX 40 3)19A 60 KVS 3)32A 46 L5 VBE 3)32A 100 KVS NIMAX 4)48A 46 LV 4)WA 46 JV 4)WA 100 LV RAMAX HH ROYALLOY

Precipitation hardening 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 JVS 3)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 IVZ RIGOR 2)454A 80 J11 V3 2)AH 120 K6 VCOL 2)F13A 54 FF22V Strato SLEIPNER 3)SGB 60 KVX 3)32A 46 L5 VBE 3)32A 100 KVS SVERKER 3 4)48A 46 LV 4)WA 46 JV 4)WA 100 LV SVERKER 21 VANADIS 4 EXTRA VANADIS 6 VANADIS 10 VANADIS 23 VANADIS 30 VANADIS 60 VANCRON 40 RIGOR Hardened 1)B151 R50 B3 1)B151 R75 B3 1)B126 R100 B6 SLEIPNER 1)48A 60 KVZ 1)430A 80 J VQA 1)820A 1003 GVQ SVERKER 21 2)51B126 C50B Vib-Star 2)51B126 C100 B54 2)B126 C75 B53 VANADIS 23 2)454A 80 J11 V3 2)C202 H5A V18 2)89A 80 1 G11A V237 P25 VANADIS 30 3)SGB 60 KVX 3)CB150 TBA 3)CB150 TBE VANCRON 40 3)3SGP 70 JVX 3)3SG 60 JVX 3)5SG 80 KVX 4)B126 V18 KR191 4)B126 V24 KR237 4)B126K V24 KR237 4)27A 60 JV 4)27A 60 HV 4)27A 100 JV ELMAX Hardened 1)B151 R50 B3 1)B151 R75 B3 1)B126 R100 B6 SVERKER 3 1)420A 54 JVQ 1)430A 80 J VQA 1)820A 1003 GVQ VANADIS 4 Extra 2)51B126 C50B Vib-Star 2)51B126 C100 B54 2)B126 C75 B53 VANADIS 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 TBE VANADIS 60 3)SGB 60 KVX 3)3SG 60 JVX 3)5SG 80 JVX 3)3SGP 70 JVX 4)B126 V24 KR237 4)B126K V24 KR237 4)B126 V18 KR191 4)27A 60 HV 4)27A 100 IV 4)27A 60 IV

17 GRINDING OF TOOL STEEL

18 Network of excellence

UDDEHOLM 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.

www.assab.com www.uddeholm.com UDDEHOLM 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.