Manufacturing Technology I. Exercise 9. General principles of machining with a geometrically undefined

Transcription

1 Lehrstuhl für Technologie der Fertigungsverfahren Laboratorium für Werkzeugmaschinen und Betriebslehre Manufacturing Technology I Exercise 9 General principles of machining ith a geometrically undefined cutting edge Werkzeugmaschinenlabor Lehrstuhl für Technologie der Fertigungsverfahren Prof. Dr. - Ing. F. Klocke RWTH - Aachen Steinbachstraße Aachen

2 Inhaltsverzeichnis Table of Contents 1 INTRODUCTION PRINCIPLES OF CUTTING EDGE CONTACT DESIGNATION OF GRINDING TOOLS CORUNDUM AND SILICON CARBIDE GRINDING TOOLS DIAMOND AND BORON NITRIDE GRINDING WHEELS GRIT MATERIALS CORUNDUM SILICON CARBIDE DIAMOND CUBIC BORON NITRIDE BONDS ARTIFICIAL RESIN BONDS CERAMIC BONDS METALLIC BONDS OTHER CHARACTERISTICS OF GRINDING TOOLS GRIT STRUCTURE HARDNESS CONCENTRATION PROCESS VARIANTS AND PROCESS CHARACTERISTICS IN GRINDING OPERATIONS SURFACE GRINDING CYLINDRICAL SURFACE GRINDING Cylindrical surface grinding beteen peaks Centreless grinding INTERNAL CIRCULAR GRINDING FT I Allgemeine Grundlagen zum Zerspanen mit geometrisch unbestimmter Schneide 2

3 Einleitung 1 Introduction Grinding is defined in DIN Standard 8589 as Cutting by the mechanical effect of cutting edges on the orkpiece material. Material is removed in this manufacturing process by ensuring that more or less randomly formed grains of hard material are brought into contact ith the material to be cut. The requirements, hich the various grinding techniques and processes must satisfy, are many and varied. The extend from ultra-precision machining, ith its exacting requirements in terms of the roughness, form and dimensional accuracy of the surface to be produced to high performance and abrasive cutting processes, in hich minimum manufacturing times are the decisive factor. In addition to machine characteristics and the selection of suitable machine variables and cooling lubricant conditions, the use of a suitable tool is vital. It is essential to have knoledge of the characteristics of the components of any given grinding heel and of the resultant overall characteristics, if the most appropriate grinding heel is to be selected for the task in hand. The design of a grinding process is a very complex task, due to the large number of influencing factors and their interaction ith one another. The suitability, or the process behaviour of a nely specified grinding heel for a specific grinding task, cannot be clearly predicted. Hoever, there are some aspects explained in the folloing, hich relate to the selection of the type of grit, grit size, bond, hardness and structure and hich form a useful basis on hich to specify the grinding heel characteristics. 2 Principles of cutting edge contact Investigations aimed at identifying the underlying correlations in machining operations conducted using a geometrically undefined cutting edge, are fraught ith problems. The extremely complex micro-structure of grinding heels makes it very difficult to determine ho many cutting edges are in contact at any given moment. Because of this large number of cutting edges, material removal is the outcome of the sum of a large number of different cutting edge contacts, each of hich cuts individual chips from the orkpiece surface. It is very expensive and time-consuming to measure precisely the geometry of each individual cutting edge on one grinding heel, due to the large number of FT I Allgemeine Grundlagen zum Zerspanen mit geometrisch unbestimmter Schneide 3

4 Grundlagen zum Schneideneingriff cutting edges concerned. In addition to this, there are problems caused by the fact that the shape of the grains changes as a result of ear during the machining operation. In general, the grit cutting edges have a strongly negative rake angle in grinding operations. Grinding is based on the active principle of path-bound grit contact. During a machining process involving path-bound cutting edge contact, the cutting edge of the grit penetrates the orkpiece on a very flat path and triggers plastic flo in the orkpiece material after a phase of elastic deformation Fig As a result of the cutting edge shape, the angle beteen the cutting edge contour and the orkpiece surface is initially very small. Consequently, no chips form at the beginning. The orkpiece material is simply pushed to the side and forms mounds and/or flos underneath the cutting edge till it reaches its flank. FnS FtS grain Ve h chip bulage Tm I elastic deformation friction grain/ material II elastic and non-plastic deformation friction grain/ material internal material friction hcu eff h cu III elastic and plastic deformation + chip removal friction grain/ material internal material friction Fig. 2.1: Chip removal on grit contact The chip itself does not begin to form until the cutting edge has penetrated so deep into the orkpiece, that the chip thickness hcu, corresponds to the grain cutting depth Tµ. Since the displacement actions and chip formation occur simultaneft I Allgemeine Grundlagen zum Zerspanen mit geometrisch unbestimmter Schneide 4

5 Grundlagen zum Schneideneingriff ously in the further course of the operation, it is decisive for the efficiency of the material removal, ho much of the chip thickness h cu is actually removed as a chip and ho large the effective chip thickness h cu,eff therefore is. The friction conditions at the cutting edge are very important in this context. 3 Designation of grinding tools 3.1 Corundum and silicon carbide grinding tools The grit materials corundum and silicon carbide belong to the group of conventional grinding materials. Grinding heels made of these materials, consist entirely of the grinding material and do not have a base as an abrasive carrier. The total volume of the grinding abrasive heel V ges, is thus composed of proportions of grit volume V K, bond volume V B and pore volume V P, and is described by the equation: V ges = V K +V B +V P. Before a tool can be described, it is necessary to determine the geometry and composition. Whereas the geometry is determined by the orkpiece and by the machine available, i.e. by the machining process, the composition can initially be freely selected ithin certain boundaries. The grit sizes used to describe the grinding heel consisting of bonded abrasives, are listed in DIN and DIN , Fig FT I Allgemeine Grundlagen zum Zerspanen mit geometrisch unbestimmter Schneide 5

6 Bezeichnung von Schleiferkzeugen grinding heel DIN A x 250 x 304,8 - P 390 x F 20 A 60 L - 5 V - 50 name DIN - number form number edge profile dimension (list depending on each form) material abrasive grain size hardness texture bonding maximum velocity Fig. 3.1: Description of conventional grinding heels in accordance ith DIN (König/ Klocke Volume 2, P. 47, Fig. 3-11) A more detailed explanation of the nomenclature is given in Fig Within the group of conventional abrasives, a distinction is dran beteen: Corundum (i.e. aluminium oxide, designation»a«) and Silicon carbide (designation»c«). The grit size is given an index number ranging from the roughest don to the finest grain size. This number is identical to the number of meshes per inch in the grit classification sieve. For example, the number»6«stands for a very rough grain and the number»1200«, for a very fine grain. The grain size designation if folloed by information relating to the level of hardness, the structure and the bond used in the grinding heel. The grinding heel designation is completed in accordance ith DIN by a number shoing the maximum operating speed. In the case of grinding heels for general maximum speeds (< 40 m/s), this information need not be included. Hoever, here the intention is to use grinding heels ith higher circumferential speeds, this must be indicated by the use of a coloured block. The coloured marks specified by 4 of the regulations for the prevention of accidents, published by the German Employers Liability Insurance Association, are as follos (extract): Blue strip up to v s = 50 m/s, FT I Allgemeine Grundlagen zum Zerspanen mit geometrisch unbestimmter Schneide 6

7 Bezeichnung von Schleiferkzeugen Yello strip up to v s = 63 m/s, Red strip up to v s = 80 m/s, Green strip up to v s = 100 m/s. abrasive type abrasive grain size hardness grade texture bonding bonding type maximum velocity example:... A 60 L 5 B abrasive A elektro corundum C silicon carbide grain size macrograin size micrograin size DIN part 1 DIN part 1 coarse medium fine very fine hardness grade A B C D extremely soft E F G very soft H I J K soft L M N O medium P Q R S hard T U V W very hard X Y Z extremely hard V R RF B BF E Mg maximum velocity [ m/s ] DSA part ) ) 1) ithout practical application bonding vitrified bonding rubber bonding rubber bonding ith fibre resin bonding resin bonding ith fibre shellac bonding magnesit bonding texture etc. closed texture opened texture Fig. 3.2: Composition of conventional grinding heels as set out in DIN (König/Klocke Volume 2, P. 49, Fig. 3-13) 3.2 Diamond and boron nitride grinding heels There is no compulsory standard relating to the further sub-division of diamond or boron nitride grinding heels and to the characteristic quantities. Hoever, the characteristics quantities can be compiled from the information provided by the manufactur and from the FEPA-Standard. According to these, the designation of a diamond or CBN grinding heel, like that of the conventional grinding tools, is composed partly of the tool geometry and partly of the grinding surface itself, Fig This contains information about the type of grit, grit size, bond, bond hardness, base and concentration. FT I Allgemeine Grundlagen zum Zerspanen mit geometrisch unbestimmter Schneide 7

8 Bezeichnung von Schleiferkzeugen GA D 126 K-plus 888 J A C50 form diameter D grinding volume W coating level drilled hole grain type and grain size bonding bonding hardness eehl body concentration X Fig. 3.3: Structure and designation of diamond grinding heels (König/Klocke Volume 2, P. 59, Fig. 3-18) The diamond and CBN grinding heels consist mainly of a base and a relatively thin layer of abrasive (i.e. grinding surface) on top of the base. The reasons for this, include the very high cost of the abrasive (diamond, CBN) lo tool ear and the fact that they are suitable for use at very high grinding heel speeds. The total volume of the grinding abrasive heel V ges is made up of the proportions of grit volume V K, bond volume V B, pore volume V P and base volume V G. It is described by the equation: V ges = V K +V B +V P +V G. Fig. 3.4 shos a further sub-division of the designation for the grinding surface. According to this, the letters»d«and»b«are used to indicate the to available type of grit, diamond and CBN. The figure hich follos the abrasive designation, stands for grit size and rises in this case ith increasing grain size. This figure reflects the mesh idth of the appropriate test sieve (FEPA-Standard) and is thus more or less the specification of the actual average grit diameter. NOTE: In the case of conventional grinding tools, the grit is classified in completely the opposite ay, since there, it is the number of meshes per inch hich is used. FT I Allgemeine Grundlagen zum Zerspanen mit geometrisch unbestimmter Schneide 8

9 Bezeichnung von Schleiferkzeugen abrasive grain size bonding bonding hardness heel body concentration example : D 126 K J A C50 diamond D Kt/cm3 index cubic cristalline 1,1 C 25 B bornitrid 1,65 C 38 2,2 C ,3 C ,4 C ,5 C C ,6 C vol % index K resin bonding V V Bz M 24 V metall bonding G S soft A KSS resin bonding J N mean B GSS sinter-metall bonding MSS galv. metall bonding R hard VSS ceramic bonding T very hard diamond cubic cristalline bornitrid diamond CBN alu-resin aluminium phenol resin Fig. 3.4: Composition of the grinding heels ith abrasives consisting of diamond and CBN (König/Klocke Volume 2, P. 60, Fig. 3-19) The selection of the most suitable and most economic concentration depends on the grinding task, heel shape, heel dimensions and grit size required in order to achieve the grinding outcome specified. 4 Grit materials The grit materials most commonly used for grinding tools, are corundum, silicon carbide, CBN and diamond. The grit material must have the folloing characteristics in order to ensure that material can be removed over a substantial period of time: High levels of hardness and toughness, so that material is removed from the orkpiece and the sharpness once achieved, is maintained; (Alternating) temperature stability, so that the grit can ithstand high machining temperatures and rapid temperature changes; Chemical stability, so that no chemical reactions take place hich might eaken the grit at high pressures and temperatures. FT I Allgemeine Grundlagen zum Zerspanen mit geometrisch unbestimmter Schneide 9

10 Kornerkstoffe These criteria can certainly not be met by one hard material alone. A number of grit materials, hich ill be presented in the folloing, are therefore used. 4.1 Corundum Corundum is a crystalline aluminium oxide (Al 2 O 3 ). In principle, a distinction can be dran, depending on the manufacturing technique used, beteen aluminium oxide abrasive or fused corundum and sintered corundum. aluminium oxide abrasives or fused corundums are technical products, hich are melted in an electric furnace made of ra bauxite. They are then broken up, pulverised and sieved. Distinctions are dran beteen normal, semi-precious and precious corundum, depending on the proportion of the oxides TiO 2 and SiO 2 they contain. Sintered bauxite and solgel corundum belong to the group of sintered corundums. The sol-gel corundums developed recently have led to considerable improvements in grinding operations. These corundums have a micro-crystalline structure ith individual crystals smaller than 0.5 µm, hich results in greater hardness and toughness. Corundum has a very ide range of applications. It is used mainly to machine the folloing materials: Unalloyed and alloyed as ell as unhardened and hardened steel; cast iron; Wood; Plastic and Non-ferrous metals. The folloing general statements can be made in relation to the material characteristics of corundum: Toughest of all grit materials; Toughness increases and hardness diminishes as the oxide content rises Toughness increases as the grit size falls lo level of thermal conductivity approx. 6 W/m C; High level of thermal stability up to approx C; Lo Knoop hardness - approx HK. 4.2 Silicon carbide In addition to corundum, silicon carbide (SiC) is used as a grit material for a number of grinding tools. It is melted out of glass sand in a resistance furnace. Silicon FT I Allgemeine Grundlagen zum Zerspanen mit geometrisch unbestimmter Schneide 10

11 Kornerkstoffe carbides of various levels of purity and quality can be produced by adapting the management of the melting process accordingly. A distinction is dran beteen the less pure, black silicon carbide and the purer, green silicon carbide (containing the smallest proportion of nitrogen). Silicon carbide is the hardest of the conventional grinding materials. It splinters easily, forming sharp cutting edges. It is the preferred material for machining short chipping materials, such as: Ceramic, glass, stones; Wood; Plastic; Hard metal; Aluminium; GG and Titanium alloys hich are unsuitable for the use of the ultra-hard abrasives CBN and diamond (due to price, quantity, economic efficiency). The folloing general statements can be made in relation to SiC: Less tough than corundum Toughness and hardness diminish ith increasing impurity Thermal conductivity approx. 55 W/m C better than corundum; Thermal expansion coefficient loer than that of corundum; Thermal stability up to approx C Knoop hardness up to 3000 HK. 4.3 Diamond Diamond grinding grit is predominantly synthetically manufactured. Synthetic diamond, like cubic boron nitride, is one of the ultra-hard grit materials and its manufacture requires high levels of energy and technical expertise. One of the most striking characteristics of diamonds used as grit material, is their extreme hardness, hich is unmatched by any other material. In comparison ith conventional grit materials, diamonds have very high levels of thermal conductivity. The diamond grit therefore conducts the heat generated in the machining zone quickly to the bond. Nickel, cobalt or composite metal coatings around the dia- FT I Allgemeine Grundlagen zum Zerspanen mit geometrisch unbestimmter Schneide 11

12 Kornerkstoffe mond grit, therefore act as a heat brake and ensure improved adhesion beteen the grit and the bond. Under the influence of lo pressures, diamonds are affected by graphitisation at temperatures beteen 700 C and 1400 C, provided sufficient oxygen is available. The areas of application of the diamond (100 % carbon) are severely restricted by the chemical affinity ith materials hich can absorb carbon (e.g. steel). Only the folloing materials can, therefore, be machined: Carbon-saturated steel Glass, natural stone, ceramics Hard metals, cermets GFK and Non-ferrous heavy metals The folloing characteristics apply to diamonds: Nigh thermal conductivity beteen W/m C; Lo level of thermal stability up to approx. 900 C and High Knoop hardness up to 8000 HK. 4.4 Cubic boron nitride Cubic boron nitride (CBN) as produced industrially and used as a grinding material for the first time in Cubic boron nitride is very much harder and has a considerably higher level of thermal conductivity than the conventional grinding materials, but rates belo the values recorded by diamonds. The high temperature stability of CBN compared ith that of diamonds, is a special feature of CBN. Under atmospheric pressure, CBN remains stable at temperatures belo 2000 C. These characteristics ensure that CBN has a high level of ear resistance in machining operations. This is used to advantage particularly in high-poer grinding operations conducted on ferrous materials. Grinding heels ith boron nitride surfaces are used to machine the folloing materials: Hardened steel Lo-allo steel and HSS. The folloing characteristics apply to CBN: FT I Allgemeine Grundlagen zum Zerspanen mit geometrisch unbestimmter Schneide 12

13 Kornerkstoffe Thermal conductivity beteen W/m C; Highest thermal stability up to approx C and High Knoop hardness up to 4700 HK. 5 Bonds The purpose of the bond is to retain the grinding grit in place until it is blunted by the cutting operation. The bond should then release the grain so that the next sharp grain can be used. The retention function is met only hen the bond meets the folloing three requirements: The Material must be firm enough, Cross-sectional areas of the bond bridges must be sufficiently large and A durable joint must form beteen the bond and the grit. Whereas the first and the third characteristic are determined by the selection of the bond material and by the manufacturing process, the second one is controlled almost solely by the volumetric proportions of the grinding heel components (c.f. König/Klocke Volume 2, P. 50, Fig. 3-14). 5.1 Artificial resin bond The artificial resin bond is made of artificial resin or on a combination of artificial resins, ith or ithout filling materials. Despite the range of artificial resins no available, the phenolic resins or phenoplasts are still the market leaders and the indications are, that they ill remain unrivalled in the foreseeable future. The characteristics of grinding heels manufactured using artificial resin bonds, can be summarised as follos: Insensitive to impact and shock or to lateral pressure; High peripheral speeds and material removal capacities in cutting and roughgrinding operations; High levels of elasticity of polishing and precision-grinding heels permit high surface quality; Lo manufacturing cost and Lo temperature stability. FT I Allgemeine Grundlagen zum Zerspanen mit geometrisch unbestimmter Schneide 13

14 Bindungen When these tools are conditioned, the artificial resin bond must be removed back in a sharpening operation folloing the profiling operation in order to crease sufficient pore space, i.e. chip space, thus regulating the required cutting capacity of the grinding heel. 5.2 Ceramic bonds Compounds containing natural silicates, red and hite clay, kaolin and felspar, quartz and fritts as additives, are referred to as ceramic bonds. compounds. The fritts are glass-type compounds of an organic and anorganic nature, hich serve as fluxing agents and hich give the ceramic bond certain characteristics. The characteristics of the ceramic bond can be characterised as follos: Brittle and therefore very sensitive to shocks Good capacity for carrying cooling lubricant into the grinding gap as a result of ell-positioned pores High module of elasticity Temperature-stability, but sensitive to temperature change Good profile-holding and dressing characteristics and Chemical resistance to oils and ater. Some ceramic bonds can be provided ith sufficient pore space in the production process, thus eliminating any need in these cases to sharpen the grinding heels after profiling (c.f. self-sharpening effect). 5.3 Metallic bonds There are a number of variants of these bonds, including both multi-layer and single-layer metal bonds. One characteristic shared by all metallic bonds, is that they have higher thermal conductivity than artificial resin and ceramic-bond grinding heels. Metal compounds are more resistant to abrasive ear than the artificial resin bonds. The characteristics of metallic bonds can be summarised as follos: High resistance to ear Difficult to dress Single-layer metallic bonds (galvanically bonded) cannot be dressed High edge strength FT I Allgemeine Grundlagen zum Zerspanen mit geometrisch unbestimmter Schneide 14

15 Bindungen High thermal conductivity Increased generation of friction heat. 6 Other characteristics of grinding tools 6.1 Grit The grain size required, is specified according to the folloing criteria: Surface quality required and Planned material removal rate. When the grain concentration in the grinding surface is constant, the number of cutting edges or grain tips involved in the machining operation, diminishes ith increasing grain size. Since the material removal is the same under the same settings the individual grain tips inevitably penetrate further into material, i.e. the chip thickness increases. The level of surface quality hich can be achieved deteriorates hilst the volume of material hich can be removed, increases. Rough grit is therefore used for pre-grinding and fine grit for finish-grinding operations. This allocation shos that any comparison of the grit sizes must take account of the rate of material removal. The correlation beteen process parameters and tool specifications ill be examined in greater detail on the basis of grit and cutting speed. It is knon that generally speaking, there is a reduction in surface roughness ith increasing cutting speed. This is initially attributable to the fact that more grain tips per unit of the path to be covered, become active. The penetration depth of the individual grain tips is smaller, thus reducing automatically reducing surface roughness. This explanation of the improvement in surface quality is one aspect; a further aspect is the machining mechanism involved. 6.2 Structure The purpose of the pores, or pore space, is to guide the chips aay from the area in hich they ere produced (i.e. the contact zone) and then to release then. Any chips hich become stuck to the pore space, are usually flushed out by a highpressure flo of cooling lubricant. FT I Allgemeine Grundlagen zum Zerspanen mit geometrisch unbestimmter Schneide 15

16 Sonstige Charakteristika von Schleiferkzeugen One of the other functions of the pores is to supply the contact zone ith cooling lubricant. The process can be adjusted so as to ensure that the pore spaces fill up ith the cooling lubricant and transport it to the contact zone. When the pore space is reduced and the grit size is maintained at a constant level, the bond volume increases, resulting in a harder heel (c.f. König/Klocke Vol. 2, P. 50, Fig. 3-14). 6.3 Hardness The term hardness is used to describe the resistance to the tendency for abrasive grains to become detached from the abrasive material. The level of hardness depends on the adhesion of the bond to the grit and on the strength of the bond bridges. It is therefore determined directly by the structure of the abrasive material, i.e. by the volumetric composition of grain, bond, pore space and their distribution (c.f. formulae in Sections 2.1 & 2.2). Generally speaking, hard grinding heels improve the grinding ratio and surface quality. The grinding ratio G, is the quotient of the amount of material hich has been removed from the orkpiece to the volume of orn grinding heel. G = V W /V S Hoever, the cutting force and thermal load to hich the orkpiece is exposed increase ith grinding heel hardness. 6.4 Concentration The concentration of abrasive material in the grinding surface is one of the most important features of a CBN grinding heel. This has a very considerable impact on the volume of material removed, the ear characteristics of the grinding heel and on the form and dimensional accuracy of the orkpiece to be machined. The concentration must be co-ordinated ith the other conditions hich affect the grinding process. It influences the number of active cutting edges as a function of grit size, via the number of grinding grains per carat, (1 carat = 0.2 g). Since the mean grit size per carat increases ith diminishing grain size, loer concentrations are usually selected for smaller grit sizes. FT I Allgemeine Grundlagen zum Zerspanen mit geometrisch unbestimmter Schneide 16

17 Verfahrensvarianten und Prozesskenngrößen beim Schleifen 7 Grinding process variants and parameters Depending on the shape of the parts, distinctions are dran beteen cylindrical surface, internal circular and surface grinding operations. The term used to describe the technique, depends largely on hether the orkpiece surface is generated by the peripheral area or the lateral face of the grinding heel and hether the main direction of feed is parallel (transverse grinding) or vertical (longitudinal grinding) to the reference plane of action. By definition, this is vertical to the grinding heel axle. The specific material removal rate Q, hich is used as a measure for the material removal rate in the process, is an important quantity in grinding operations. It specifies, the volume of orkpiece per unit of time and per mm of grinding heel idth in contact ith the orkpiece. Q Q = b s,eff V = b t s,eff c The material removal rate Q, hich is also an important parameter in machining operations conducted using a geometrically defined cutting edge, is calculated from the cross-sectional area of cut and the and the velocity perpendicular to the area of cut. In grinding operations, the rate of material removal is generally scaled to the idth of the grinding heel in use. In contrast to machining operations conducted using a geometrically defined cutting edge, the cutting speed has no influence on the rate of material removal in grinding operations. 7.1 Surface grinding The surface grinding technique is used to generate surfaces hich are either completely even or hich are linear in the main feed direction of the grinding heel. The most common variations on surface grinding are: Peripheral plunge grinding Peripheral traverse grinding Grooves or profiles are produced using the first of these techniques, hilst the second variation is preferred hen large, even surfaces are required. A further FT I Allgemeine Grundlagen zum Zerspanen mit geometrisch unbestimmter Schneide 17

18 Verfahrensvarianten und Prozesskenngrößen beim Schleifen distinction is dran beteen the deep grinding and sing grinding variations of each of these techniques. In deep grinding operations (full-idth grinding, craling feed grinding), the orkpiece shape is ground from solid material at a large infeed, frequently in one travel and ith reduced table feed speed from solid material. In contrast, the material is removed in reciprocating grinding operations, over several travels ith small individual feed motions and relatively high feed speeds. The larger infeed and the associated increase in contact length beteen the orkpiece and the grinding heel in creep feed grinding operations, hinder the transport of coolant to the grinding contact zone and the removal of grinding chips. Open-pored grinding heels ith lo levels of hardness and an effective coolant supply (high volume flo at high pressures, favourable nozzle shape) are therefore used in creep feed grinding operations. The advantages of creep feed grinding over reciprocating grinding, are better levels of orkpiece surface quality, loer heel ear and shorter grinding times as a result of the elimination of any need for reversal times and empty runs. FT I Allgemeine Grundlagen zum Zerspanen mit geometrisch unbestimmter Schneide 18

19 Verfahrensvarianten und Prozesskenngrößen beim Schleifen Fig. 7.1 shos and example of a surface-peripheral longitudinal grinding operation: n s grinding heel orkpiece a e V a p = b s removal cross section material removal rate A = a a p e Q = A v specific material removal Q = a a v p e Q = Q / b seff = Q /a = a v p e Fig. 7.1: Surface-peripheral-longitudinal grinding 7.2 External cylindrical grinding External cylindrical grinding beteen centres The to techniques most commonly used in practice are, as shon in Fig. 7.2 and Fig. 7.3, peripheral external cylindrical grinding (plunge grinding) and peripheral longitudinal external cylindrical grinding ( peal grinding). Whereas in plunge grinding operations, the grinding heel is moved normal to the orkpiece axis, in longitudinal grinding operations, is moves equidistant to this axis. FT I Allgemeine Grundlagen zum Zerspanen mit geometrisch unbestimmter Schneide 19

20 Verfahrensvarianten und Prozesskenngrößen beim Schleifen n orkpiece d a e V a p = b s grinding heel V fr n s Fig. 7.2: Peripheral external cylindrical grinding n orkpiece d V a p a e grinding heel V fa n s removal cross section material removal rate A = a a p Q = A v e specific material removal Q = a a v p e Q = Q / b seff = Q /a = a v p e Fig. 7.3: Peripheral longitudinal external cylindrical grinding In principle, a distinction can be dran beteen single-stage and multi-stage processes. In single-stage grinding process, there is a continuous grinding heel feed speed v f perpendicular to or at an angle to the orkpiece axis and therefor a continuous reduction in the diameter of the orkpiece until the finished dimension is reached. Multi-stage grinding operations are virtually alays carried out in order to FT I Allgemeine Grundlagen zum Zerspanen mit geometrisch unbestimmter Schneide 20

21 Verfahrensvarianten und Prozesskenngrößen beim Schleifen reduce the machining time and, at the same time, maintain the high quality of the orkpiece. The peal grinding operation (v f high, high level of material removal per unit of time) is folloed by a finish or precision finish-grinding operation (v f lo, small amount of material removal per unit of time). A spark-out operation can be carried out at the end (feed speed v f = 0) if required by the part quality specifications. Hoever the folloing problems arise in connection ith the design of external cylindrical grinding processes: On one hand, the material removal rate and the alloance should be as high as possible in order to reduce the grinding time. Hoever, this results in high levels of grinding heel ear and roughs up the grinding heel surface. Consequently, the machining outcome required, cannot be achieved in the folloing finish-grinding or spark-out operation. On the other hand, hen the material removal rate selected for the rough-grinding operation is too lo, or the point at hich the operation sitches from rough to finish machining is too soon, the orkpiece quality exceeds the requirements but the grinding process is uneconomical Centreless grinding The main feature hich distinguishes centreless grinding from cylindrical grinding ith a orkpiece clamped in, is that the orkpiece is not guided in its rotational axis by peaks, but rests along to surface lines on its circumference on a bearing rail and a regulating heel. The geometry and kinematics involved in centreless cylindrical surface-peripheral grinding operations, are illustrated clearly in Fig Here too, a distinction is dran beteen the various forms of this technique, depending on the orientation of the main feed direction to the surface to be produced in the peripheral external cylindrical grinding (plunge grinding) operation, in hich the rotating orkpiece is moved normal to the grinding heel and external cylindrical longitudinal grinding (through-feed grinding), in hich the orkpiece is guided along a surface line on the circumference of the grinding heel. The kinematics in centreless plunge or through-feed grinding, are the same as in external cylindrical grinding. Hoever, it is important to note that at identical infeed rated, the values FT I Allgemeine Grundlagen zum Zerspanen mit geometrisch unbestimmter Schneide 21

22 Verfahrensvarianten und Prozesskenngrößen beim Schleifen of the material removal rate are halved due to the diameter-related infeed to stationary regulating or grinding heels. Workpieces ith steps, profiles, or ith several diameters to be treated, are machined in centreless plunge grinding operations. Centreless through feed grinding is suitable for the manufacture of cylindrical parts, hich are only to be machined to the largest diameter. This technique is used mainly in industrial scale manufacture and in mass production. Fig. 7.4: Geometry and kinematics in centreless cylindrical surface-peripheral grinding. Advantages of centreless grinding: Due to the linear support of the orkpiece, even bendable and brittle parts ith large material removal rates, can be machined, i.e. ith high grinding forces and lo levels of deformation. Since the orkpiece needs no preparation for clamping or in order to transfer the rotational movement, both the ork this ould involve and the possible sources of error in centring and re-clamping, are eliminated. FT I Allgemeine Grundlagen zum Zerspanen mit geometrisch unbestimmter Schneide 22

23 Verfahrensvarianten und Prozesskenngrößen beim Schleifen The orkpiece changing operation is uncomplicated and easy to automate. The requirement for this procedure is eliminated completely in longitudinal grinding operations conducted in continuous machining mode. Extremely long parts can be machined even by small machines, in longitudinal grinding operations. Drabacks of centreless grinding: It is vital to ensure that the process is geometrically stable. Any reduction in the Regenerative Effect, i.e. the displacement of an existing circularity fault to another point on the circumference, must be minimised by careful selection of the grinding gap geometry. The inflexibility of centreless grinding renders it unsuitable for single part or small part manufacture. 7.3 Internal circular grinding As in external cylindrical surface grinding, a distinction is made in internal circular grinding operations beteen peripheral cross grinding (plunge grinding) and peripheral longitudinal grinding (longitudinal grinding). Since the kinematics in this operation are the same as those in cylindrical surface grinding beteen peaks, the formula derived for that technique also apply to the relative rate of material removal here. In internal circular grinding operations, the contact arc beteen the grinding heel and the orkpiece is, as shon in Fig. 7.5, considerably larger than in comparable cylindrical surface, and surface grinding operations. This is an obstacle to chip removal and to sufficient supply of the contact zone ith cooling lubricant. Grinding heels ith rough grit, lo levels of hardness and open structure, hich permit a free cut, i.e. ith lo contact force and lo contact zone temperatures, are therefore used for internal circular grinding operations. The lo level of hardness and the small dimensions (smaller grains are engaged more frequently in contact) go hand in hand ith particularly high levels of ear. Additionally, the cutting speeds hich can be applied in the case of small grinding tools, are frequently FT I Allgemeine Grundlagen zum Zerspanen mit geometrisch unbestimmter Schneide 23

24 Verfahrensvarianten und Prozesskenngrößen beim Schleifen lo, since the spindle speeds required are frequently very difficult to achieve, ith the result that the benefits of high cutting speeds such as reduced ear and improved surface quality, cannot be exploited. Usually only lo material removal rates are therefore possible in internal circular grinding operations. d d d Internal cylindrical Face grinding External cylindrical Fig. 7.5: Contact conditions under various grinding conditions FT I Allgemeine Grundlagen zum Zerspanen mit geometrisch unbestimmter Schneide 24