The effect of abrasive cutting on the temperature of grinding wheel and its relative efficiency

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1 ARCHIVES OF CIVIL AND MECHANICAL ENGINEERING Vol. VIII 008 No. The effect of abrasive cutting on the temperature of grinding wheel and its relative efficiency JÓZEF KACZMAREK Technical University of Łódź Department of Production Engineering, Stefanowskiego 1/ Łódź, Poland The thermovision method was applied for determination of the temperature of grinding wheel during abrasive cutting. Abrasive cutting of steel workpieces by using grinding wheels purchased from 3 different manufacturers was conducted under different conditions. Temperature distribution along the grinding wheel circumference and along its radial cross-section was determined. The relative efficiency of grinding wheel was estimated. Keywords: abrasive cutting, cutting grinding wheels, temperature of grinding wheel, relative efficiency of grinding wheel, thermovision measurement 1. Introduction Grinding wheels have been applied for cutting in different branches of industry and by individual users. These cutting tools have been used in steel mills, steelworks, foundries, forges, mechanical and repair departments of factories, in building industry, for repair of roads and railway tracks as well as by individual users. Abrasive cutting is applicable to metal workpieces with different hardness and to very hard non-metal materials (e.g. concrete, silicon tiles). Studies on abrasive cutting have hitherto focused on mechanical durability of grinding wheels [6 13], which is of great importance for their operational safety and reliability [1 3]. Too high temperature of the grinding wheel during the process of cutting can have a negative impact on the bonding agent, which binds abrasive particles. This in turn affects the operational reliability of grinding wheel and can lead to a lower cutting efficiency and an increased emission of environmentally harmful gases. This work presents results of studies on distribution of temperature along the circumference of grinding wheel and along its radial cross section and on the relative efficiency of this process. The temperature of grinding wheel was measured by using the thermovision camera, which facilitated recording temperature in each site of the grinding wheel.. Heat emission during abrasive cutting The source of heat is work used during cutting for elastic and plastic deformations, removal of chips and friction interaction between abrasive particles and the workpiece. An approximate value of heat flux can be calculated from the Equation [17]:

2 8 J. KACZMAREK [ W ] Φ = F v, (1) c s where: F c cutting force [N], v s cutting speed [m/s]. The heat flux generated during abrasive cutting penetrates the workpiece (Φ w ), chips (Φ ch ), grinding wheel (Φ s ) and is dissipated to the atmosphere (Φ p ) in the form of irradiation (Figure 1). The shares of aforesaid components of the heat flux are as follows [18]: Φ ch 60%, Φ s 0%, Φ w 10%, Φ p 10%.Temperature of grinding wheel depends on the density of heat flux ϕ = dφ s /da, which passes through the grinding wheel-workpiece interface A, and on thermal characteristics of this grinding wheel. Cutting is usually carried out by using narrow grinding wheels. The surface of their interaction with the workpiece is relatively small. Also the thermal conductance of grinding wheels is low. Both these factors cause that an excessive increase in the temperature of the grinding wheel can negatively affect its cutting performance and, probably to a lesser extent, also its operational reliability. The distribution of the heat flux in the shearing zone can have a strand-like character for cutting with the radial feed [4, 15 16, 19], or a triangle-like character for cutting with the longitudinal feed [0 1]. The abrasive cutting is usually characterized by the radial feed ( f p ) which is conducive to the strand-like distribution of the heat flux. f p f p V s V Φ s F c Φ ch Φ Φ w Fig. 1. The heat flux and its distribution in the shearing zone The quantity of generated heat flux Φ depends mainly on the cutting speed v s and feed f p. Because the cutting speed is determined by standards and recommendations [ 6] so the feed value f p is the most important factor in this respect. Abrasive cutting is usually carried out by using manual grinders which are characterized by the lack of precise control of the feed. Too large feed superimposed by the small area of

3 The effect of abrasive cutting on the temperature of grinding wheel 83 the grinding wheel-workpiece interface and poor thermal conductance of the grinding wheel may result in intensive emission of heat. The high heat flux density may lead to grinding wheel melting and in consequence to its increased wear. Additionally, apparent changes of coloration in the shearing zone, structural alterations of the ground material (approaching the depth of 0. mm from top surface [3]), thermal flush, etc. can occur. 3. Relative efficiency of grinding wheels The wear of grinding wheels depends first of all on their technical characteristics, properties of the ground material and grinding conditions. The determination of this wear is usually based on determination of the coefficient of relative efficiency of grinding G [5]. This coefficient equals to the ratio of the volume of chips removed during grinding V M to the drop in the volume of grinding wheel during this process V S (). V G = V M S π d b = 4 π ( DS1 DS 4 ) T = ( D S1 d b, D ) T S () where: D S1 grinding wheel diameter before grinding [mm], D S grinding wheel diameter on completion of grinding [mm], d the diameter of the rod subjected to grinding [mm], b the width of slit on the rod [mm], T the width of grinding wheel [mm]. In case of abrasive cutting, the coefficient G equals to the quotient of the crosssection surface area of the workpiece subjected to cutting divided by the loss of the head surface of grinding wheel caused by cutting. In this process, an assumption that b = T leads to conversion of Equation () to (3). d G =. (3) D S 1 D S Under appropriate cutting conditions the values of coefficient G are large. Small values of this coefficient result from an excessive self-sharpening of the grinding wheel, which in turn increases the costs of machining. 4. Experimental studies The experiments were carried out by using the workstand schematically presented in Figure. The principal tool in this workstand was a grinder BS-300 (no. 1). It was

4 84 J. KACZMAREK used to cut-off cylindrical, reinforcing steel rods (no. 3) with two different diameters. The quantity of interaction force between the grinding wheel and the rod was determined by gravity, it means with the aid of the appropriately matched system of pull rod. An object glass of the thermovision camera PN 90 (no. 4) pointed toward the grinding wheel (no. ) and the rod subjected to cutting. The camera was coupled to the PC computer (no. 5) equipped with the program Termolab (The thermovision camera and the program Termolab, are the property of the Department of Electronic Systems and Thermography at the Institute of Electronics, Technical University of Lodz. They were made available during presented experiments). It ensured recording of measurements. The detailed description of the workstand was published elsewhere [14]. F Q PC P Fig.. Schematic diagram of the workstand used for investigation of grinding wheel efficiency during abrasive cutting: 1 grinder, grinding wheel, 3 workpiece, 4 thermovision camera, 5 computer, Q grinding wheel loading, F interaction force of grinder to cutting bar The grinding wheels A30 R6 BF, which were used in machining trials were purchased from three different manufacturers (further referred to as: no. 1, no., no. 3). This type of grinding wheels is most frequently applied for cutting. The experiments were carried out for three different interaction forces between the grinding wheel and the rod subjected to cutting, which were determined through preliminary trials [14]. Their values ensured smooth cutting. These friction forces were: F 1 = 6 N, F = 7 N, and F 3 = 8 N. The diameters of cylindrical rods subjected to cutting were: d 1 = 1 mm and d = 0 mm. Each cutting trial under these conditions was repeated twice.

5 The effect of abrasive cutting on the temperature of grinding wheel Results of measurements of temperature distribution The executed cutting trials were recorded by the camera and the soft-ware as films, which show the course of each trial. An example of a take (a frame) of such a film, also termed a thermogram, is shown in Figure 3. scala o C Fig. 3. An example of a grinding wheel thermogram, which was recorded during abrasive cutting trial under the following conditions: grinding wheel no. 3, F = 6 [N], d =1 [mm] The applied soft-ware ensures, among others: measurement of the temperature of randomly selected point on the grinding wheel (the temperature of one of selected points, equal to 138 C, is shown in Figure 3), recording changes in this temperature throughout the cutting trial and determination of temperature distribution along any circumference or segment, which were randomly selected against the recorded thermogram. These facilities enabled determination of temperature distribution: along the circumference of grinding wheel, as shown in Figure 4 (the plots reflect the true situation), along the radius of grinding wheel, as shown in Figure 5. Distribution of temperature may ensure determination of thermal load of grinding wheel during abrasive cutting. The maximum temperature at the circumference of the grinding wheel was determined on the basis of the recorded thermograms. The mean values of the maximum temperature of grinding wheel were estimated for the applied grinding conditions. They are shown in Table 1 and Figure 6.

6 86 J. KACZMAREK o C A B C Fig. 4. Distribution of temperature along grinding wheel circumference for 3 different values of the depth of cut, expressed as a ratio of the depth from the rod top surface g to the rod diameter d: A g/d = 1/3, B g/d = 1/, C g/d = 3/4 o C cutting zone 1 mm 5 points on X axis Fig. 5. Distribution of temperature along the radius of grinding wheel for the following cutting conditions: grinding wheel no 3, F = 6 [N], d =1 [mm] Table 1. Values of the maximum temperature of grinding wheel during the process of cutting No of wheel 1 3 Interaction force F [N] Maximum temperature t [ C] Diameter of bar d [mm]

7 The effect of abrasive cutting on the temperature of grinding wheel 87 t [ C] t [ C] nr o 1 nr o nr o a) F [N] b) F [N] Fig. 6. The effect of interaction force between the grinding wheel and the workpiece on the maximum temperature of grinding wheel during cutting for: a) the rod with diameter d 1 = 1 mm, b) the rod with the diameter d = 0 mm; no. 1, no., no. 3 numbers of grinding wheels purchased from three different manufacturers The analysis of temperature distribution along the grinding wheel circumference (Figure 4) leads to conclusion that the temperature decreases co-linearly to the direction of the grinding wheel rotation. It is caused by heat dissipation to the surrounding air despite the high circumferential speed (circa 50 m/s) and very short pauses between the contacts of each point on the grinding wheel circumference with the zone in which the heat is generated. The analysis of temperature distribution along the grinding wheel radius (Figure 5) indicates that the highest temperature occurs along the circumference of the grinding wheel, in the zone of a ring, which has the width of the depth of cut. A rapid decrease in temperature (in the direction from the grinding wheel circumference to its center), which is collinear to the radius (its gradient ranges from 60 C/mm to 80 C/mm) provides evidence of a low heat conductivity of this tool. The maximum temperatures of grinding wheels with the same technical characteristics but purchased from different manufacturers differ by approximately 30% if the cutting process was conducted under the same conditions. It means that it is very difficult to obtain the same functionability of grinding wheels produced by different manufacturers. The cutting trials showed that the temperature of cutting increases with the diameter of the ground workpiece (Figure 6). It is caused by the larger area of the interface between the grinding wheel and the workpiece, despite the lower unit pressures.

8 88 J. KACZMAREK A rise in the interaction force between the grinding wheel and the workpiece, within the range from 6 N to 7 N results in an increase in the maximum temperature of grinding wheel. The further increase in the interaction force up to 8 N brings about a drop in this temperature (Figure 6), which is most probably caused by self-sharpening of the grinding wheel, which results from high loading of abrasive particles. The self-sharpening is characterized by the occurrence of the larger number of sharp abrasive particles in the shearing zone. 4.. The relative efficiency of grinding wheel The relative efficiency of grinding wheels applied for the cutting trials was determined on the basis of Equation (3). The experimental results are shown in Table and Figure 6. Table. Results of measurements of the coefficient of relative efficiency G during abrasive cutting No of wheel 1 3 Interaction force F [N] G ratio Diameter of bar d [mm] G G nr o 1 nr o nr o 3 0 a) F [N] 0 b) F [N] Fig. 7. Values of the coefficient of relative efficiency of grinding wheels G for the cutting trial conducted for: a) the rod with a diameter of d 1 = 1 mm, b) the rod with the diameter of d = 0 mm; no. 1, no., no. 3 numbers of grinding wheels purchased from different manufacturers

9 The effect of abrasive cutting on the temperature of grinding wheel 89 Analysis of these results indicates that in case of cutting grinding wheels of cylindrical rods with lower diameter, the values of coefficient G decrease with a rise in the interaction force between these rods and the grinding wheel (Figure 7a). The lower values of coefficient G were also observed for cutting of cylindrical rods with larger diameters but only when the interaction force was increased (Figure 7b). The decreased relative efficiency of grinding wheels was most probably caused by intensification of the process of self-sharpening under the latter conditions. 5. Summary The thermovision camera coupled to the computer soft-ware, which were applied in the abrasive cutting trials ensured the accurate and easy recording of temperature distribution along the circumference and along the radius of each grinding wheel. The cutting trials proved that utilitarian properties of grinding wheels used for cutting can be accurately determined on the basis of temperature distribution and the coefficient of relative efficiency. The recorded, different maximum temperatures of grinding wheels purchased from different manufacturers and different coefficients of their relative efficiency can be used for improvement of technological processes of their manufacturing. Acknowledgments Research carried out under the Research Project no 4T07 D0109. References [1] Biegalski H., Feld M.: Prognozowanie właściwości uŝytkowych ściernic do przecinania, XXIV Naukowa Szkoła Obróbki Ściernej, Kraków-Łopuszna, 001, pp [] Biegalski H., Feld M.: MoŜliwość zmniejszenia zanieczyszczeń pyłami i gazami podczas przecinania ściernicowego, XXV Naukowa Szkoła Obróbki Ściernej, Wrocław-Duszniki Zdrój, 00, pp [3] Biegalski H.: Badania nieniszczące i koncepcja stanowiska do badań eksploatacyjnych ściernic do przecinania, XXVI Naukowa Szkoła Obróbki Ściernej, Łódź-Spała, 003, pp [4] Chanzin N.N. i inni: Razcziet temperaturnogo polia pri szlifowanii metałow, Stanki i Insrumient, No. 8, 1981, pp [5] Kaczmarek J.: Ocena skrawności ściernicy w wybranych odmianach szlifowania, Praca doktorska, Łódź, [6] Maszyna do badań wytrzymałości dynamicznej ściernic przy obciąŝeniach bocznych, XXII Naukowa Szkoła Obróbki Ściernej, Gdańsk-Jurata, 1999, pp [7] Kaczmarek J., Fiks W., Dębkowski R.: Urządzenia do badania wytrzymałości dynamicznej ściernic na rozrywanie, Magazyn Przemysłowy, Vol. 5, No., 1999, pp. 51. [8] Gołąbczak A., Kaczmarek J.: Elektroerozyjne obciąganie ściernic supertwardych elektrodą segmentową, XXV Naukowa Szkoła Obróbki Ściernej, Wrocław-Duszniki Zdrój, 00, pp

10 90 J. KACZMAREK [9] Kaczmarek J.: Odkształcenia ściernic do przecinania, XXVI Naukowa Szkoła Obróbki Ściernej, Łódź-Spała, 003, pp [10] Kaczmarek J., Kubiak T.: Metody określania napręŝeń w ściernicy do przecinania obcią- Ŝonej siłą boczną, XXVI Naukowa Szkoła Obróbki Ściernej, Łódź-Spała, 003, pp [11] Kaczmarek J.: Wpływ prędkości roboczej i jednopunktowego odciąŝenia bocznego na wytrzymałość ściernicy, XXVII Naukowa Szkoła Obróbki Ściernej, Koszalin, 004, pp [1] Kaczmarek J., Świniarski J.: Analiza wytrzymałości ściernicy uwzględniająca ich wymiary geometryczne i sposób zamocowania, XXVIII Naukowa Szkoła Obróbki Ściernej, Warszawa-Mszczonów, 005, [13] Kaczmarek J., Świniarski J.: Analiza wytrzymałości ściernicy do przecinania przy obciąŝeniu bocznym trójpunktowym, XIX Naukowa Szkoła Obróbki Ściernej, Gdańsk-Jurata, 006, pp [14] Kaczmarek J., Kuliński M., Ostrowski B., Więcek B.: Zastosowanie termowizyjnego pomiaru temperatury ściernicy podczas przecinania ściernicowego, Materiały VII Konferencji Krajowej Termografia i Termometria w Podczerwieni pod redakcją Bogusława Więcka, Ustroń-Jaszowiec, 006, pp [15] Konig W., Honscheid W., Lowin R.: Untersuchung der beim Schleifprozess endstchenden Temperaturen und ihre Auswirkungen auf das Arbeitsergebnis, Forschungsberichte des landes Nordhein-Westfalen Nr 68, Westdeutscher Verlag. [16] Marcler R., Malkin S., Molinsdorf J. C.: Thermal Stresses Form a Moving Band Source of Heat on the Surface of a Semi Infinite. [17] Marciniak M., Filipowski R.: Techniki obróbki mechanicznej i erozyjnej, Oficyna wydawnicza Politechniki Warszawskiej, Warszawa, 000. [18] Oczoś K., Porzycki J.: Szlifowanie. Podstawy i technika, WNT Warszawa [19] Snoyes R., Maris M., Peters J.: Thermaly Induced Damage in Grinding, Annals of the CIRP, Vol. 39, No., 1990, pp [0] Vensevenant E.: An Imroved Mathematical Model to Predict Residual Stresses in Sufrace Plunge Grinding, Annals of the CIRP, Vol. 36, No 1, 1987, pp [1] Vensevenant E.: A Subsurface Integrity Model in Grinding, Praca doktorska, KU Leuven, September [] DSA 10 Badanie narzędzi ściernych na stanowiskach badawczych. [3] PN-EN 1413, 001, Warunki bezpieczeństwa dla narzędzi ściernych spojonych. Wpływ warunków przecinania ściernicowego na temperaturę ściernicy i względną wydajność szlifowania W artykule przedstawiono zastosowanie termowizyjnego sposobu pomiaru temperatury ściernicy podczas przecinania ściernicowego. Opisano badania przecinania stali ściernicami trzech producentów w róŝnych warunkach tego procesu. Zaprezentowano wyniki badań dotyczące rozkładu temperatury na obwodzie ściernicy i wzdłuŝ jej przekroju promieniowego oraz ocenę względnej wydajności ściernicy. Analiza uzyskanych rozkładów temperatury wzdłuŝ obwodu ściernicy pokazuje, iŝ wartość temperatury ma tendencję malejącą w kierunku obrotu ściernicy określającą stopień jej wychłodzenia w powietrzu.

11 The effect of abrasive cutting on the temperature of grinding wheel 91 Analiza otrzymanych rozkładów temperatury wzdłuŝ promienia ściernicy wskazuje na to, iŝ największa temperatura ściernicy występująca przy jej obwodzie podlega spadkowi wyraŝonemu gradientem od 60 C/mm do 80 C/mm, co świadczy o niskiej przewodności cieplnej badanego narzędzia. Maksymalne temperatury uzyskane dla ściernic róŝnych producentów o tej samej charakterystyce technicznej w tych samych warunkach przecinania róŝnią się o około 30%. Ze wzrostem średnicy przecinanego przedmiotu temperatura przecinania rośnie. Jest to skutkiem zwiększonego kontaktu ściernicy z przedmiotem pomimo mniejszych nacisków jednostkowych. Wzrostowi siły docisku ściernicy w procesie przecinania od 6 N do 7 N towarzyszy wzrost maksymalnej temperatury ściernicy. Dalszy wzrost siły docisku do 8 N powoduje spadek tej temperatury, którego przyczyną moŝe być powstające przy duŝych obciąŝeniach ziaren ściernych zjawisko samoostrzenia ściernicy. Analiza wyników badań wydajności względnej ściernic wskazuje, iŝ ze wzrostem siły docisku ściernicy maleją wartości wskaźnika wydajności względnej przy przecinaniu prętów o mniejszej średnicy. Zmniejszone wartości tego wskaźnika zaobserwowano takŝe podczas przecinania prętów o większej średnicy tylko dla większych sił docisku. Wykazane tu pogorszenie wydajności względnej ściernic do przecinania naleŝy wiązać z intensyfikacją w tych warunkach przecinania zjawiska samoostrzenia ściernicy.