© Strojni{ki vestnik 50(2004)4,206-218 © Journal of Mechanical Engineering 50(2004)4,206-218 ISSN 0039-2480 ISSN 0039-2480 UDK 621.921.3:621.924 UDC 621.921.3:621.924 Pregledni znanstveni ~lanek (1.02) Review scientific paper (1.02) Pregled bru{enja z velikimi hitrostmi in u~inkovitih abrazivnih orodij A Review of High-Speed Grinding and High-Performance Abrasive Tools Peter Krajnik - Janez Kopa~ Prispevek se nanaša na pregled sodobnih vidikov brušenja glede na zahteve po povečani produktivnosti in kakovosti obdelave. Predstavljen je osnovni mehanizem brušenja in uporabe, ki se nanašajo na najnovejšo tehnologijo brušenja z velikimi hitrostmi (BVH) z zelo učinkovitimi orodji. Z napredkom tehnologije BVH je treba zahtevam obdelave z velikimi hitrostmi prilagoditi tudi obdelovalni stroj, sistem za hlajenje in nadzor postopka. Nadalje je v manjšem obsegu obravnavana še celovitost brušene površine in proizvodna ekonomika BVH. © 2004 Strojniški vestnik. Vse pravice pridržane. (Ključne besede: brušenje zelo hitro, abrazivi, integriteta površin) This paper reviews the modern aspects of grinding with regards to enhanced productivity and manufacturing quality demands. The basic mechanism of grinding and the applications for the state-of-the-art technology of high-speed grinding (HSG) with high-performance grinding wheels are presented. In addition to the improvements in the technology associated with HSG, the grinding machine-tool, the coolant system and the process monitoring also need to adapt to high-speed machining. In addition, the ground-surface integrity and the economic efficiency of HSG are also briefly discussed. © 2004 Journal of Mechanical Engineering. All rights reserved. (Keywords: high speed grinding, abrasives, surface integrity) 0 UVOD Povečevanje zahtev po zelo kakovostnih in robustnih proizvodih, predvsem v avtomobilski industriji, proizvodnji turbin in ležajev, terjajo nove in izboljšane postopke brušenja, ki zagotavljajo najboljše učinke glede na produktivnost in natančnost ob sprejemljivih stroških. Povečanje postopkovne učinkovitosti ob zmanjšanju stroškov je bilo v zadnjih letih doseženo z izrabo prednosti brušenja z velikimi hitrostmi, ki uporablja najnovej še abrazive, veziva z izboljšano obrabno odpornostjo in ustrezne stroje. Izbira ustreznega postopka je odvisna od koncepta obdelovalnega stroja, uporabe različnih abrazivnih orodij in okvira krmiljenja parametrov postopka. Uporabe BVH so razširile področje brušenja od tradicionalnih končnih obdelovalnih postopkov do zelo učinkovitih natančnih obdelav. Postopkovni razvoj je pripeljal do novega primera brušenja, ki se nanaša na konfiguracijo izboljšanega postopka z zelo učinkovitimi zmožnostmi [1]. Dandanes, ko je kakovost obdelovalnih postopkov z velikimi hitrostmi prav tako pomembna 0 INTRODUCTION Increasing demands for high-quality and robust products, particularly in the automotive, turbine and bearing industries, require new and improved grinding processes that provide the best performance with respect to productivity and precision at a reason-able cost. Increased manufacturing performance and reduced costs have both been achieved in recent years through the exploitation of advances in high-speed grinding (HSG), which utilizes the latest abrasives, bonding systems with improved wear resistance, and appropriate machine-tools. The selection of an effec-tive procedure is dependent on the machine-tool con-cept, the employment of different abrasive tools and a frame of process parameters’ manipulation. Applications of HSG have expanded the field of grinding from the traditional finish machining proc-ess to high-performance precision machining. Proc-ess development has led to a new grinding paradigm, which refers to the configuration of an improved proc-ess with high-performance capabilities [1]. Nowadays, when the quality in high-speed machining processes is just as important as the effi- VH^tTPsDDIK stran 206 Krajnik P., Kopa~ J.: Pregled bru{enja z velikimi hitrostmi - A Review of High-Speed Grinding kakor njihova učinkovitost, morajo biti postopki brušenja predrugačeni. Ekonomična in učinkovita izraba BVH je omejena na zožen postopkovni okvir, ki omogoča optimalno gospodarnost. Primerno BVH lahko dosežemo le z učinkovito integracijo upravljanja kakovosti ([2] in [3]), ki se nanaša na neposreden sistem nadzora postopka, večnivojsko nadzorovanje napak, vizualizacijo, neposredne/posredne meritve, prilagodljivo krmiljenje in diagnozo kakovosti na daljavo. Široki vidiki integracije upravljanja kakovosti presegajo okvir tega prispevka. Cilj tega prispevka se nanaša na pregled osnovnih vidikov brušenja s poudarkom na primerih sodobnega BVH težko obdelovalnih kovinskih materialov z orodji iz zelo učinkovitih abrazivov. 1 OSNOVE BRUŠENJA IN ODREZOVALNO NAČELO Brušenje je postopek odrezovanja, ki se nanaša na obdelavo z geometrijsko nedefiniranimi rezalnimi robovi. Mehanizem brušenja predstavlja zapleteni postopek odvzema materiala, ki vključuje brus s popolnoma naključno strukturirano topografijo in zrno z običajno negativnim kotom cepilne ploskve (sl. 1a). Brušenje se nanaša na odvzem materiala posameznih zrn, katerih rezalni rob je omejen s silo in tirnico. Začetek odrezovanja karakterizira elastična deformacija, kateri sledi plastično tečenje materiala obdelovanca (sl. 1b). Nastanek odrezka se začne v točki, kjer se debelina odrezka ujema z globino rezanja. Na nastajanje odrezka bistveno vplivajo pogoji trenja, lastnosti tečenja materiala obdelovanca in rezalna hitrost. Dosleden opis mehanizma odrezovanja torej vključuje zapletene zveze prodiranja med dvema trdima materialoma, elasto-plastno mehaniko in tribološke vidike, ki vplivajo na odrezovalno kinematiko in dotikalne pogoje. ciency, grinding processes have to be redesigned. The economic and effective employment of HSG is limited to a narrow machining frame, which leads to optimum cost efficiency. Adequate HSG is only achieved by effective quality-management integration ([2] and [3]), which refers to an online process monitoring system, multilevel error control, visuali-sation, in/post process measurements, adaptive con-trol and remote quality diagnosis. The broad aspects of integrated quality management are beyond the scope of this paper. The objectives of this paper are therefore to review the basic aspects of grinding and to point out the paradigms of the modern HSG of hard-to-machine ferrous materials with high-performance abrasives. 1 FUNDAMENTALS OF THE GRINDING AND CUTTING PRINCIPLE Grinding is a machining cutting operation that refers to machining with geometrically undefined cutting edges. The mechanism of the grinding proc-ess is a complex material-removal process, including a grinding wheel with a perfectly randomly struc-tured topography and a grain with a generally negative rake angle (Figure 1a). Grinding refers to material removal by individual grains whose cutting edge is bounded by force and path. The initial cutting interface is characterized by elastic deformation, which is followed by plastic flow of the workpiece material (Figure 1b). The chip formation initi-ates at a point where the chip thickness corresponds to the cutting depth. The interface friction conditions, the flow characteristics of the material and the cutting speed have a significant influence on chip formation. A consist-ent cutting-mechanism description therefore comprises complex penetration relationships between two hard materials, elasto-plastics mechanics and aspects of tribology, which all influence the kinematics and contact condition. *.K\ i 'i ,¦'A-, Chip a) b) Sl. 1. Makro in mikro načelo brušenja ([5] in [6]) Fig. 1. Macro and micro grinding principle ([5] and [6]) I ^TJT^3Ce)cJ][RI]D^[jd] 04 stran 207 I^IMl^lflJ^QDCC Krajnik P., Kopa~ J.: Pregled bru{enja z velikimi hitrostmi - A Review of High-Speed Grinding Porabljeno delo za nastanek odrezka je zmnožek mehanske moči in časa brušenja. Pri brušenju mehanska moč pomeni moč za brušenje, ki je zmnožek obodne rezalne sile in rezalne hitrosti. Večina dela za nastanek odrezka se spreminja v termično energijo, tj. toploto, ki se porazdeli v obdelovanec, brus, odrezke, hladilno-mazalno sredstvo in okolico. 2 ZELO UČINKOVITI ABRAZIVI Bistvene prednosti povečane produktivnosti pri brušenju z velikimi hitrostmi se nanašajo na povečano obstojnost zelo učinkovitih brusov. Zahteve za ta orodja se nanašajo na njihovo dopustno vrtilno hitrost in odpornost proti obrabi in lomu. Poleg tega so zaželene še dobre značilnosti dušenja, velika togost in ustrezna toplotna prevodnost [1]. To poglavje se nanaša na BVH jekel z uporabo brusov iz: - kubičnega borovega nitrida (KBN) - mikrokristalinskega aluminijevega oksida (Al2O3) 2.1 Abrazivi CBN Zaradi visoke trdote, termične in kemične odpornosti je KBN idealen zelo učinkovit abraziv V primerjavi z običajnimi abrazivi ima KBN najmanjšo obrabo in zaradi tega stabilno rezalno zmožnost v daljšem časovnem obdobju. Prednosti se zlasti nanašajo na brušenje težko obdelovalnih legiranih jekel s trdoto nad 55 HRC, npr. hitrorezna in kromova jekla. Posamezno zrno KBN (sl. 2) je trdo in ima veliko toplotno prevodnost ter v primerjavi z običajnim zrnom stokrat večjo obstojnost. Zaradi inherentne ostrine omogočajo hladnejšo obdelavo in zagotavljajo veliko celovitost obdelane površine z manjšo hrapavostjo [4]. Brusi KBN se običajno sestojijo iz aluminijastega, jeklenega, keramičnega, smolnatega nosilnega telesa in tankega brusilnega sloja. Brusne značilnosti brusov KBN določajo karakteristike zrna, njihov tip, koncentracija in vezivo. Omogočene so rezalne hitrosti do 280 m/s. The work performed during chip formation is the product of the mechanical power input and the grind-ing time. In grinding, mechanical power refers to the grinding power, which results from the product of the tangential cutting force and the cutting speed. The work for chip formation is mainly transformed into thermal energy, i.e., heat, which is distributed to the workpiece, grinding wheel, chips, coolant and the surroundings. 2 HIGH-PERFORMANCE ABRASIVES The important advantage of increased pro-ductivity during grinding at high cutting speeds is the increased tool life of high-performance grinding wheels. The grinding tools for HSG are subjected to special requirements in terms of their allowed rotational speed and resistance to wear and fracture. Good damping char-acteristics, high rigidity, and adequate thermal conduc-tivity are also desirable [1]. This section refers to the HSG of steels with grinding wheels of: - Cubic boron nitride (CBN) - Microcrystalline aluminium oxide (Al2O3) 2.1 CBN abrasives Because of its high level of hardness, its ther-mal and chemical resistance, CBN is a perfectly suited high-performance abrasive. Compared with conven-tional abrasives, CBN has minimal wear and therefore its cutting ability is stable over a longer period of time. In addition, it offers advantages, especially in the grind-ing of hard-to-machine steels with large proportions of alloy and hardnesses of 55 HRC and above, e.g., highspeed steels and chrome steels. An individual CBN grain (figure 2) is hard and has a high thermal conductivity and will therefore have a life that is a hundred times that of a conventional grain. Because of its inherent sharp-ness, CBN tends to machine cooler, providing a high surface integrity and a superior surface finish [4]. CBN grinding wheels generally consist of an aluminium, steel, ceramic or artificial resin base and a thin grinding layer. The grinding characteristics of a CBN grinding wheel are determined by the grain char-acteristics, the type of grain, its concentration and the bonding system. Cutting speeds of 280 m/s are possi- Sl. 2. Mikrostruktura KBN [1] Fig. 2. CBN microstructure [1] VH^tTPsDDIK stran 208 Krajnik P., Kopa~ J.: Pregled bru{enja z velikimi hitrostmi - A Review of High-Speed Grinding Sodobna večplastna veziva za bruse KBN vključujejo kovinsko sintrana in keramična veziva, velike trdote in odpornosti proti obrabi. Poravnava takšnih orodij je težavna glede na njihovo veliko trdnost. Prednosti brusov KBN je povezana z visoko ceno. 2.2 Mikrokristalinski aluminijev oksid (sintrani korund Al2O3) V nasprotju z dragim brušenjem s KBN so različni proizvajalci orodij nedavno ponudili orodja, ki integrirajo običajne abrazive, ki omogočajo večjo stopnjo odvzema materiala pri velikih hitrostih. Keramično vezani brusi omogočajo ekonomske prednosti kakor tudi odlično učinkovitost orodja velike poroznosti, čigar odprta struktura omogoča samoostrenje. Orodja iz sintranega korunda omogočajo BVH do rezalnih hitrosti 180 m/s. Zaradi njegove zmogljivosti in osnovnih značilnosti zapira vrzel med zelo trdimi superabrazivi (diamant in KBN) ter običajnimi brusnimi abrazivi. Sintrani korund združuje lastnosti keramike (K) in zlitega aluminijevega oksida (S) v keramično vezanem brusu za BVH [4]. Ta kombinacija (sl. 3a) omogoča agresivno odrezovanje, dobro stabilnost oblike in veliko obstojnost. Običajni korund določajo prednostne makrolomne ploskve (sl. 3b), iz katerih se krhajo razmeroma veliki delci. Zaradi mikrokristalinske strukture (0,5 mm) sintranega korunda ta nima prednostnih lomnih površin (slika 3c), zato so odlomljeni delci razmeroma majhni, s čimer zrna obdržijo svojo ostrino [6]. Sintrani korund je sorazmerno poceni abraziv, ki se je cenovno uvrstil med običajne in zelo drage abrazive KBN z merljivimi izboljšavami brušenja za faktor tri do pet [4]. Nova generacija brusov s skrajno velikimi hitrostmi (rezalne hitrosti do 500 m/s) se nanaša na keramično vezan brus KBN s plastičnim nosilnim telesom, ki je ojačan z ogljikovimi vlakni [9]. Zahteve ble. Modern multi-layer bonding systems for CBN grind-ing wheels include sintered metal bonds and vitrified bonds, which possess a high bond hardness and wear resistance. The dressing of these tools is difficult be-cause of their high mechanical strength. However, the performance of CBN wheels comes at a price. 2.2 Microcrystalline Aluminium Oxide (sintered corundum Al2O3) To compete with to CBN grinding, various grinding-tools manufacturers have recently launched grinding tools that integrate conventional abrasives for a higher MRR at high cutting speeds. Vitrified bonded wheels offer not only economic advantages but also the excellent performance of a highly porous tool, whose open structure enables self-sharpening. Sintered corundum tools offer HSG up to cutting speeds of 180 m/s. Because of their characteristics and efficiency they are closing the gap between the very hard superabrasives (diamond and CBN) and the conventional grinding abrasives. Sintered corun-dum combines the properties of the ceramic (K) and fused aluminium oxide (S) into vitrified grinding wheels [7]. This combination (Figure 3a) exhibits a high level of sharpness, aggressive cutting, good form holding and a long life. Conventional corundum has preferential macro fracture planes (Figure 3b), from which relatively large particles chip away. Because of its microcrystal-line structure (0.5 mm), sintered corundum does not have preferential fracture planes (Figure 3c), hence the parti-cles that chip away are relatively small and the grains retain their sharpness [6]. Aluminium oxide is a rela-tively low-cost abrasive, which has positioned itself in economic terms between the conventional and the very expensive CBN abrasives, with measurable improve-ments in grinding by a factor of three to five times [4]. The new generation of ultra-high-speed grind-ing wheels (cutting speeds up to 500 m/s) consist of a vitrified CBN wheel with a CFRP (carbon-fibre-rein-forced plastics) core [9]. The requirements for such Sl. 3. Mikrostruktura sintranega korunda ([7] in [6]) Fig. 3. Sintered corundum microstructure ([7] and [6]) stran 209 Krajnik P., Kopa~ J.: Pregled bru{enja z velikimi hitrostmi - A Review of High-Speed Grinding teh brusov se nanašajo na zelo veliko trdnost nosilnega telesa z majhnimi radialnimi raztezki, kvaziizotropične značilnosti materiala, majhno težo in dobre lastnosti dušenja. 3 BRUŠENJE Z VELIKIMI HITROSTMI BVH opredeljuje težnja po izboljšanju ekonomike postopka s povečanjem stopnje odvzema materiala ob hkratnem izboljšanju stabilnosti in zmožnosti postopka ter kakovosti obdelave. V tem pomenu je povečanje rezalne hitrosti prvi pogoj za povečanje produktivnosti. V tehnološkem načelu se BVH nanaša bodisi na: - zelo učinkovito brušenje v smislu zmanjšanja časa obdelave, pri čemer je kakovost obdelave nespremenjena; - zelo kakovostno brušenje za doseganje velike kakovosti obdelave pri stalni ravni obdelovalne zmogljivosti. Postopek brušenja je zaradi kompleksnosti zelo težaven za analizo. Običajno se tehnološke študije postopka brušenja nanašajo na model, ki je veljaven le v omejenem področju z danimi robnimi pogoji. Model na osnovi vstopnih veličin omogoča simuliranje izstopnih veličin in je tako lahko namenjen za ocenitev postopkovnih učinkov [8]. Poenostavljena primerjava med različnimi postopki BVH lahko temelji na primerjavi različnih parametrov, ki se nanašajo na vstopne, postopkovne in izstopne veličine. Postopkovna razmerja so lahko povzeta grafično (sl. 4). wheels include a very high core-material strength with a small radial expansion, quasi-isotropic material char-acteristics, low weight and good dumping proper-ties. 3 HIGH-SPEED GRINDING PROCESS HSG is characterised by efforts to improve the economics of the process by increasing material removal rates (MRR) and simultaneous improvements of process stability, capability and machining qual-ity. In this way, an increased cutting speed is a pre-requisite for increased productivity. In technological terms HSG refers to either: - High-performance grinding in order to reduce the machining time while maintaining the same level of quality. - High-quality grinding in order to enhance machin-ing quality while maintaining a constant machin-ing capacity. The grinding process is very difficult to analyse due to its inherent complexity. Usually, a tech-nological study of a grinding process refers to a model that is valid in only a limited field within given bound-ary conditions. A model can be used to predict proc-ess output and thus establish a relation between in-put and output quantities [8]. However, a simplified comparison between various HSG procedures can be established by comparing different parameters, which refer to input, process and output characteris-tics. The process relations can be summarized graphi-cally (Figure 4). VHOD / INPUT Specifična stopnja odvzema materiala Specific material removal rate vfr -----------— Podajama hitrost Feedrate *W Hitrost obdelovanca Workpiece speed POSTOPEK / PROCESS IZHOD / OUTPUT Rezalne sile H 1 Cutting forces ^L^ *- v, Rezalna hitrost Cutting speed Vw Prostorninski odvzem ma eriala Machining volume 4 Geometrijska netočnost Geometrical inaccuracy Hrapavost površine Surface roughness #t Temperatura Temperature -»- V„ Čas brušenja Grinding time — V„ -*- v, Toplotno prizadeto območje Heat affected zone Z* — v, Sl. 4. Postopkovna razmerja BVH [11] Fig. 4. HSG process coherence [11] VBgfFMK stran 210 Krajnik P., Kopa~ J.: Pregled bru{enja z velikimi hitrostmi - A Review of High-Speed Grinding Glede na naključno porazdelitev in obliko rezalnih robov so za analizo nastanka odrezka uporabljene izkustvene metode. Odvzem materiala in nastanek odrezka je odvisen od strukture brusa ter kinematičnih in geometrijskih parametrov. Debelina nedeformiranega odrezka h , enačba (1), je torej odvisna od statične gostote rezalnih robov C , kinematičnih in geometrijskih veličin; kjer se st v nanaša na hitrost obdelovanca, v na rezalno hitrost, a na globino rezanja, d na ekvivalenten premer brusa in a, b, g na pozitivne eksponente. In view of the random distribution and shapes of cutting edges, empirical methods are applied to analyse chip formation during grinding. Material removal and chip formation depend on the structure of the grind-ing wheel as well as on kinematical and geometrical parameters. The undeformed chip thickness, hcu (Eq.1), is therefore dependent on the static density of the cut-ting edges, Cstat, and on the kinematical and geometrical variables; where vw is the workpiece speed, vc the cut-ting speed, a the depth of cut, deq the equivalent grind-ing-wheel diameter, and a, b, g are positive exponents. Na podlagi tega razmerja lahko sklepamo, da se povečanje rezalne hitrosti ob nespremenljivosti vseh preostalih parametrov izraža v zmanjšanju nedeformirane debeline odrezka. Tipična ekvivalentna debelina odrezkov pri BVH znaša med 0,5 in 10 mm. Specifična stopnja odvzema materiala Q ’ (2) označuje količino odvzetega materiala obdelovanca v časovni enoti na širino brusa in lahko rabi kot cenilka učinkovitosti in ekonomičnosti BVH. dV a d (1). On the basis of this relationship it can be established that an increase in the cutting speed, assuming all other conditions are constant, will re-sult in a reduction in the undeformed chip thickness. Equivalent chip thicknesses of between 0.5 and 10 mm are a characteristic feature of HSG. The specific material removal rate Qw’ (Eq.2) indicates the amount of workpiece volume removed per unit of time and the grinding contact width, which can be used as an assessment criteria for HSG performance and economics. Qw Qw dt bDbD mm3 /mmxs (2). Sposobnost brušenja določa brusno razmerje G, enačba (3), ki se nanaša na razmerje odvzete količine materiala obdelovanca in za to potrebne količine abraziva. Za doseganje ekonomičnega BVH mora biti velikost brusnega razmerja G vsaj 100. V primeru optimiranega postopka lahko dosežemo razmerja G 1000 in več. S povečanjem rezalne hitrosti se razmerje G eksponentno povečuje; odrezki postajajo krajši in tanjši, obremenitev abrazivnih zrn in veziva se zmanjša, s tem pa se poveča obstojnost orodja in razmerje G. Nadalje je pomembna določitev toplotnih lastnosti BVH, ki so odvisne od več medsebojno odvisnih spremenljivk, to so toplotne lastnosti obdelovanca in abraziva, variacije toplotne porazdelitve itn. Natančno razmerje med parametri brušenja in temperaturo je odvisno od celotnega toplotnega toka in porazdelitve toplotne energije. Slednji cenilki sta potrebni za napoved toplotnih značilnic postopka. Toplotni tok lahko ocenimo s specifično energijo za brušenje e en. (4). ec=A-[Q The grinding capacity is stated in terms of the grinding ratio G (Eq.3), which corresponds to the ratio of material removed from the workpiece and the necessary volume of abrasive required. In order to make a HSG process economic, a minimum G ratio of 100 must be aimed for. In the case of optimised proc-esses, G ratios of 1000 and higher are not unusual. mm3/mm3 (3). With an increase of the cutting speed the G ratio begins to grow exponentionally; the chips be-come shorter and thinner, the load on the abrasive and the bonding system is reduced, and therefore, the wheel life and the G ratio increase. Furthermore, it is important to determine the thermal characteristics of HSG process, which are dependent on several interacting variables, such as the thermal properties of the workpiece and the abra-sive, the variations in heat partitioning, etc. The ex-act relationship between the grinding parameters and the temperature depends on the total heat flux and the partitioning of the thermal energy. The latter two estimates are required for a prediction of the thermal characteristics of the process. The heat flux can be estimated from the specific grinding energy ec (Eq.4). J/mm3 (4). gfin^OtJJlMlSCSD 04-4 stran 211 |^BSSITIMIGC Krajnik P., Kopa~ J.: Pregled bru{enja z velikimi hitrostmi - A Review of High-Speed Grinding Sl. 5. Specifična energija [12] Fig. 5. Specific energy [12] Specifična energija za brušenje variira s specifično stopnjo odvzema materiala, kjer sta stalni A in t odvisni od materiala obdelovanca in abraziva ter razmer pri brušenju [12]. Pri majhni specifični stopnji odvzema materiala se specifična energija za brušenje povečuje in eksponentno zmanjšuje z večanjem specifične stopnje odvzema materiala (sl. 5), še posebej pri velikih rezalnih hitrostih [13]. Zanimivo dejstvo pri BVH je, da se pri zelo velikih hitrostih brusa temperatura obdelovanca značilno zniža. S povečanjem hitrosti brusa se najprej zviša, potem pa se začne pri določenih okoliščinah in pri dovolj veliki hitrosti brusa nižati (sl. 6). To si lahko razlagamo s tem, da je čas stika med abrazivnimi zrni in obdelovancem izredno kratek. Površina obdelovanca ni v toplotnem ravnotežju in tako se toplotni impulz v začetni fazi porazdeli po površini še preden prodre v obdelovanec. Morebitno prodiranje toplote v obdelovanec pa v naslednji fazi prepreči oblikovanje naslednjega odrezka, ki jo odnese. Ti učinki so očitni pri mejni hitrosti brusa okoli 100 m/s [20]. 4 CELOVITOST BRUŠENE POVRŠINE IN HLADILNO MAZALNA SREDSTVA Toplotne lastnosti brušenja se nanašajo na skoraj celotno spremembo energije za brušenje v toploto, vendar se manjši del te energije porabi za generiranje površine in kopičenje v odrezkih in obdelovancu v obliki zaostalih napetosti. Tako so vse komponente v brusni coni izpostavljene toplotnim obremenitvam [8]. Raztros toplote je odvisen od toplotne prevodnosti komponent, strategije hlajenja in značilnic postopka. Lastnosti brušene površine bistveno vplivajo na funkcionalne lastnosti obdelane komponente. Toplotni vnos lahko povzroči strukturne spremembe, kakor so lokalne zakalitve ter formiranje trde in krhke martenzitne strukture. V povezavi z zaostalimi napetostmi lahko tako pride do nastanka inširjenja razpok (sl. 7). ^BlgFUTMHlC | stran 212 *» i--------------------------------------------------------------- rrirrrf Imilnl hmi j'*—~ . C«i^Mwnal» h«l j t —^.^ X J| am / \ i,\ JT \ KBN brus i X_ g i------------------------------------------------------------------- GO bo mn i?n Kti inn inn K.v.'iiii liurUMiilm 3| Clil|iiiJ*]W