- Email: [email protected]
Experimental study on grinding of a sintered friction material
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 6 ( 2 0 0 8 ) 184–189
journal homepage: www.elsevier.com/locate/jmatprotec
Experimental study on grinding of a sintered friction material E. Atzeni ∗ , L. Iuliano Polytechnic of Turin, Department of Manufacturing Systems and Economics, C.so Duca degli Abruzzi 24, 10129 Turin, Italy
a r t i c l e
i n f o
a b s t r a c t
Article history:
The grinding of friction material brake pad, necessary to ensure a flat surface, generates
Received 11 September 2006
the surface roughness which highly influences the friction coefficient. This paper deals
Received in revised form
with an experimental activity focused on the evaluation of the surface roughness after a
14 May 2007
grinding operation of an innovative sintered friction material. A regression analysis has
Accepted 16 May 2007
been used to model the relationship among surface roughness and kinematic parameters. An instrumented tangential grinding machine controlled by a CNC system has been used in this activity. During the process the normal and tangential grinding forces have been
Keywords:
measured by a force measurement device. © 2007 Elsevier B.V. All rights reserved.
Grinding Friction material Surface roughness Grinding force
1.
Introduction
Friction materials are used in applications when slow or decreasing movement is desired, such as a disc brake pad in braking system. During the braking process the kinetic energy is transformed into heat and then emitted to the surroundings, so the thermal properties of the pad material are very important. Sintered friction materials made by powder metallurgy process give the highest performance, having thermal stability and permanent friction coefficient at temperatures up to 800 ◦ C. These materials are environmental friendly and have a long life. Typical applications are motorcycle, aircraft and race cars brake pads. The manufacturing process of brake pads includes grinding as the final finishing because of its ability to satisfy the stringent flatness tolerance required. On the other hand, the surface roughness mainly influences the friction coefficient. Surface roughness and tolerance are closely interrelated and in this application it is the tolerance requirement which lim-
∗
Corresponding author. Tel.: +39 011 567 7267; fax: +39 011 564 7299. E-mail address: [email protected] (E. Atzeni). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.05.037
its the maximum achievable roughness. In the last decades many models have been proposed to predict the ideal surface roughness in terms of grinding operation and wheel topography. However, these models predict roughness values smaller than real ones because of the number of the uncontrolled variables affecting the process (Malkin, 1989; Xun and Rowe, 1996; Hecker and Liang, 2003). Only experimental investigation can give a realistic indication restricted at specific application case. In this paper an experimental activity focused on evaluation of friction material surface roughness after a grinding operation is presented. The friction material studied is a metallic matrix made of Cu, Zn and Sn and graphite and silicon zirconium oxide friction modifiers.
2.
Experimental set-up
The CNC machine used in this experimental study was an ABA tangential grinding machine (Fig. 1) instrumented with a quartz 3-component dynamometer. The force data were col-
185
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 6 ( 2 0 0 8 ) 184–189
Table 1 – Test parameters Parameter
Values
Cutting speed (m/s) Feed per grain (mm/rev)
15–18.5–22.7–27.9–34 0.01–0.011–0.013–0.014–0.016
Fig. 3 – Grinding pass.
Fig. 1 – Photograph of the grinding system.
lected by a sampling device installed on a PC. The acquisition board was a Microsystem CIO-DAS 1402/16 and the signals were sampled at the frequency of 2000 Hz. The grinding wheel used for experiments was 37C 36 HVP (silicon carbide for non-steel application, vetrified, medium grit size), 225 mm diameter, 20 mm thickness. The topography of the wheel surface influences the surface roughness of the workpiece. To describe the topography the grain density was measured with the following procedure: • active surface of grinding wheel has been marked on a paper by rolling over a piece of carbon paper; • three areas 20 mm × 15 mm (A, B and C in Fig. 2) have been chosen; • in each area the points (markers of the grains) have been counted. The mean value is 260 points (Fig. 2). Under assumption of uniform distribution of the grains, the density d can be determined as number of points/area ratio d=
grains = 0.87 (grains/mm2 ) area
(1)
Fig. 4 – Roughness Ra vs. cutting speed vt and feed per 2 = 0.995; null hypothesis grain ag . Statistical parameter Radj p = 0.01 (*experimental data).
The mean distance between grains is
=
2.1.
Fig. 2 – Markers of the grain of the grinding wheel.
1 = 1.07 (mm) d
(2)
Experiment design
A plan of experiments was prepared in order to test the influence of the kinematic parameters (cutting speed and feed) on the surface roughness. The test conditions are detailed in Table 1. The cutting speed is varying in geometric progression from 15 m/s to 34 m/s (maximum allowed on the grinding machine); assuming an analogy with the milling process, the feed per grain is varying in geometric progression from 0.010 mm/rev to 0.016 mm/rev, corresponding to a 8.30 m/min feed speed, within the operating range of the machine. For each parameter combination, five repeated tests were conducted. The grinding cycle consists on four stages each having
186
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 6 ( 2 0 0 8 ) 184–189
Fig. 5 – Output forces signal [N] after filtering, 2000 Hz sampling test 9 rough pass (cutting speed vt = 18.5 (m/s), feed per grain = 0.014). 50 m in feed (roughing) and 10 stages each having 10 m in feed (finishing). The total removed material thickness is 0.3 mm. During the grinding pass, the workpiece motion is first in the opposite direction of the wheel velocity (up grinding) and then in the same direction (down grinding) (Fig. 3).
Fig. 6 – Cutting, ploughing and rubbing modes of deformation in grinding process.
The surface roughness has been measured after the grinding cycle has finished. Surface roughness measurements were taken on a 4.8 mm length using a portable roughness measuring station. Measurements were made on each test piece along the grinding direction, which is the sliding direction of the disc pad. The surface parameters were roughness average
Fig. 7 – Forces vs. cutting speed vt and feed per grain ag in rough grinding (*experimental data).
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 6 ( 2 0 0 8 ) 184–189
187
Fig. 8 – Forces vs. cutting speed vt and feed per grain ag in finish grinding (*experimental data).
(Ra ), mean peak to valley roughness (Rz ) and maximum peak to valley roughness (Rm ). The flatness was measured on each test piece using a coordinate measuring machine. During the roughing and the finishing passes in the tests the normal and the tangential forces were measured by the force measurement device. The force signals were recorded by the PC based acquisition system.
2.2.
Experimental results
The surface finish value Ra is in the range 1.97–2.68 m; it increases with decreasing wheel speed and increasing table speed. Rz is 7–10 times Ra . The maximum flatness is 0.02 mm. The data were elaborated in order to identify the relationships among roughness and kinematic conditions: the roughness values obtained for the different combinations of process parameters (cutting speed and feed per grain) were statistically analysed using multiple linear regression. The diagram obtained from the elaboration of the results is described in Fig. 4, where the curves were plotted in the
domain of validity. The validity of the model was checked using variance analysis. The obtained model is valid in the range of data. The results show that the roughness is mainly influenced by the feed per grain and to a lesser degree by the cutting speed. The feed per cutting point, analogous to plain milling, corresponds to the spacing between successive peaks along the workpiece. Decreasing the feed per grain, this spacing and consequently the depth of engagement decrease, producing a smoother surface. The reason of contribution of cutting speed is that increasing the wheel speed the number of active cutting point increases, decreasing the effective spacing between successive cutting points (Malkin, 1989; Shaji and Radhakrishnan, 2003; Brinksmeier and Giwerzew, 2003; Mayer and Fang, 1995; Rowe, 1993; Zhou and Van Luttervelt, 1992; Qi et al., 1997). The force signals, recorded by a PC based acquisition system, were processed to obtain an accurate value of the grinding forces. In order to remove noise included dynamometer’s original output signal, the characteristics of existing noise were identified. First, the amplitude spectrum of the original
188
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 6 ( 2 0 0 8 ) 184–189
signal was obtained by FFT method. The results show that noise before grinding and after grinding were almost the same, have a similar large component at 50 Hz, which reflects the electric power used. Another characteristic noise frequency is the same as the wheel’s rotation. In order to remove above mentioned components, a digital filter was employed. Fig. 5 shows signal after noise removal, relative to the test 9, rough pass. Three stages of material deformation take place when a grain interacts with a workpiece: rubbing, ploughing, and cutting. These are illustrated schematically in Fig. 6. In the rubbing process material removal is negligible and only elastic deformation and some plastic deformation occurs: the force on each grain is too small to cause large penetration of the workpiece. Ploughing occurs when penetration of grains is increased. In the ploughing process, grain deforms the workpiece and material is dislodged ahead of the grain forming side ridge. Rate of material removal remains negligible. In the cutting process the chip removal takes place. Active grains are distributed on the external surface of grinding wheel and also radially, below the surface. So when active grains on the surface are cutting, other ones below the surface are ploughing or rubbing at once. It can be seen in Fig. 5 that in down-grinding forces are slightly smaller than in up-grinding. The reason is that the initial ploughing is reduced or eliminated since each grain engages the workpiece at its maximum depth of cut (Malkin, 1989; Wager and Gu, 1991). The average value of tangential and normal forces in rough and finish grinding are shown in Figs. 7 and 8, limited at the up-grinding. The normal force ranges from 27.8 N to 80.9 N in rough grinding and from 7.5 N to 45.8 N in finish grinding, while the tangential grinding force ranges from 21.1 N to 61.6 N in rough grinding and from 5.5 N to 36.9 N in finish grinding. The forces are directly dependent upon the undeformed chip thickness, so the influence of depth of cut on forces is justified. The tangential/normal force ratio depends on the workpiece material and is constant (Malkin, 1989; Li et al., 2002) the force ratio value for this material is 0.6. These data have been statistically analysed in order to identify the relationships among forces and kinematic conditions. The diagrams obtained from results’ elaboration are described in Figs. 7 and 8, where the curves were plotted in the domain of validity. The validity of the models was checked using variance analysis. The obtained models are valid in the range of the data. It can be seen that forces increase almost proportionately with cutting speed and feed per grain. The reason is that the normal force is proportional to the contact length, which is a function of the feed per cutting point. Increasing the cutting speed, the number of active grains increases, contributing to an increase of the force value. In the case of finish grinding cutting speed is found to have more contribution in forces: at fine depth of cut, forces increases less than proportionately with the cutting speed–feed per grain product, and this effect is bigger at higher cutting speed. This is the evidence of the size effect (Malkin, 1989; Li et al., 2002; Liu et al., 2005; Midha et al., 1991).
3.
Conclusions
In this paper an experimental activity performed to evaluate the surface roughness of friction material pad after a grinding operation have been presented. An instrumented tangential grinding machine has been used to finish some pads, allowing to monitor the forces during the process. A plan of experiments has been prepared in order to test the influence of cutting speed and feed per grain on the surface roughness after grinding cycle. The obtained data have been statistically processed using multiple linear regression to identify the relationship among roughness and kinematic parameters. The obtained model shows that the roughness is mainly influenced by the feed per grain and to a lesser degree by the cutting speed. Decreasing the feed per grain, the spacing between successive peaks along the workpiece and consequently the depth of engagement decrease, producing a smoother surface; while an increase of cutting speed results in a decrease of roughness because the number of active cutting point is higher and consequently the effective spacing between successive cutting points is reduced. The force signals, recorded by a PC based acquisition system, were processed to obtain accurate values. The grinding forces in down grinding are slightly smaller then in upgrinding due to the reduction of the plowing. The force data in rough and finish grinding have been statistically processed using multiple linear regression to identify the relationship with kinematic parameters. The obtained models show that forces increase almost proportionately with cutting speed and feed per grain. The reason is that the normal force is proportional to contact length, which is function of the feed per cutting point. Increasing cutting speed, the number of active grains increases, contributing to increase force value. In finish grinding the size effect is evident and forces increases less than proportionately with cutting speed and feed per grain.
references
Brinksmeier, E., Giwerzew, A., 2003. Chip formation mechanisms in grinding at low speeds. CIRP Ann., 253. Hecker, R.L., Liang, S.Y., 2003. Predictive modeling of surface roughness in grinding. Int. J. Mach. Tools Manufacture 43 (8), 755–761. Li, Z.C., Lin, B., Xu, Y.S., Hu, J., 2002. Experimental studies on grinding forces and force ratio of the unsteady-state grinding technique. J. Mater. Process. Technol. 129 (1–3), 76–80. Liu, C.H., Chen, A., Chen, C.-C.A., Wang, Y.-T., 2005. Grinding force control in an automatic surface finishing system. J. Mater. Process. Technol. 170 (1–2), 367–373. Malkin, S., 1989. Grinding Technology: Theory and Applications of Machining with Abrasives. Ellis Horwood, Chichester. Mayer, J.E., Fang, G.P., 1995. Effect of grinding parameters on surface finish of ground ceramics. CIRP Ann., 279. Midha, P.S., Zhu, C.B., Trmal, G.J., 1991. Optimum selection of grinding parameters using process modeling and knowledge based system approach. J. Mater. Process. Technol. 28 (1–2), 189–198. Qi, H.S., Rowe, W.B., Mills, B., 1997. Experimental investigation of contact behaviour in grinding. Tribol. Int. 30 (4), 283–294.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 6 ( 2 0 0 8 ) 184–189
Rowe, W.B., 1993. The effect of deformation on the contact area in grinding. CIRP Ann., 409. Shaji, S., Radhakrishnan, V., 2003. Analysis of process parameters in surface grinding with graphite as lubricant based on the Taguchi method. J. Mater. Process. Technol. 141 (1), 51–59. Wager, J.G., Gu, D.Y., 1991. Influence of up-grinding and down-grinding on the contact zone. CIRP Ann., 323.
189
Xun, C., Rowe, W.B., 1996. Analysis and simulation of the grinding process. Part II. Mechanics of grinding. Int. J. Mach. Tools Manufacture 36 (8), 883–896. Zhou, Z.X., Van Luttervelt, C.A., 1992. The real contact length between grinding wheel and workpiece: a new concept and a new measuring method. CIRP Ann., 387.
journal homepage: www.elsevier.com/locate/jmatprotec
Experimental study on grinding of a sintered friction material E. Atzeni ∗ , L. Iuliano Polytechnic of Turin, Department of Manufacturing Systems and Economics, C.so Duca degli Abruzzi 24, 10129 Turin, Italy
a r t i c l e
i n f o
a b s t r a c t
Article history:
The grinding of friction material brake pad, necessary to ensure a flat surface, generates
Received 11 September 2006
the surface roughness which highly influences the friction coefficient. This paper deals
Received in revised form
with an experimental activity focused on the evaluation of the surface roughness after a
14 May 2007
grinding operation of an innovative sintered friction material. A regression analysis has
Accepted 16 May 2007
been used to model the relationship among surface roughness and kinematic parameters. An instrumented tangential grinding machine controlled by a CNC system has been used in this activity. During the process the normal and tangential grinding forces have been
Keywords:
measured by a force measurement device. © 2007 Elsevier B.V. All rights reserved.
Grinding Friction material Surface roughness Grinding force
1.
Introduction
Friction materials are used in applications when slow or decreasing movement is desired, such as a disc brake pad in braking system. During the braking process the kinetic energy is transformed into heat and then emitted to the surroundings, so the thermal properties of the pad material are very important. Sintered friction materials made by powder metallurgy process give the highest performance, having thermal stability and permanent friction coefficient at temperatures up to 800 ◦ C. These materials are environmental friendly and have a long life. Typical applications are motorcycle, aircraft and race cars brake pads. The manufacturing process of brake pads includes grinding as the final finishing because of its ability to satisfy the stringent flatness tolerance required. On the other hand, the surface roughness mainly influences the friction coefficient. Surface roughness and tolerance are closely interrelated and in this application it is the tolerance requirement which lim-
∗
Corresponding author. Tel.: +39 011 567 7267; fax: +39 011 564 7299. E-mail address: [email protected] (E. Atzeni). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.05.037
its the maximum achievable roughness. In the last decades many models have been proposed to predict the ideal surface roughness in terms of grinding operation and wheel topography. However, these models predict roughness values smaller than real ones because of the number of the uncontrolled variables affecting the process (Malkin, 1989; Xun and Rowe, 1996; Hecker and Liang, 2003). Only experimental investigation can give a realistic indication restricted at specific application case. In this paper an experimental activity focused on evaluation of friction material surface roughness after a grinding operation is presented. The friction material studied is a metallic matrix made of Cu, Zn and Sn and graphite and silicon zirconium oxide friction modifiers.
2.
Experimental set-up
The CNC machine used in this experimental study was an ABA tangential grinding machine (Fig. 1) instrumented with a quartz 3-component dynamometer. The force data were col-
185
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 6 ( 2 0 0 8 ) 184–189
Table 1 – Test parameters Parameter
Values
Cutting speed (m/s) Feed per grain (mm/rev)
15–18.5–22.7–27.9–34 0.01–0.011–0.013–0.014–0.016
Fig. 3 – Grinding pass.
Fig. 1 – Photograph of the grinding system.
lected by a sampling device installed on a PC. The acquisition board was a Microsystem CIO-DAS 1402/16 and the signals were sampled at the frequency of 2000 Hz. The grinding wheel used for experiments was 37C 36 HVP (silicon carbide for non-steel application, vetrified, medium grit size), 225 mm diameter, 20 mm thickness. The topography of the wheel surface influences the surface roughness of the workpiece. To describe the topography the grain density was measured with the following procedure: • active surface of grinding wheel has been marked on a paper by rolling over a piece of carbon paper; • three areas 20 mm × 15 mm (A, B and C in Fig. 2) have been chosen; • in each area the points (markers of the grains) have been counted. The mean value is 260 points (Fig. 2). Under assumption of uniform distribution of the grains, the density d can be determined as number of points/area ratio d=
grains = 0.87 (grains/mm2 ) area
(1)
Fig. 4 – Roughness Ra vs. cutting speed vt and feed per 2 = 0.995; null hypothesis grain ag . Statistical parameter Radj p = 0.01 (*experimental data).
The mean distance between grains is
=
2.1.
Fig. 2 – Markers of the grain of the grinding wheel.
1 = 1.07 (mm) d
(2)
Experiment design
A plan of experiments was prepared in order to test the influence of the kinematic parameters (cutting speed and feed) on the surface roughness. The test conditions are detailed in Table 1. The cutting speed is varying in geometric progression from 15 m/s to 34 m/s (maximum allowed on the grinding machine); assuming an analogy with the milling process, the feed per grain is varying in geometric progression from 0.010 mm/rev to 0.016 mm/rev, corresponding to a 8.30 m/min feed speed, within the operating range of the machine. For each parameter combination, five repeated tests were conducted. The grinding cycle consists on four stages each having
186
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 6 ( 2 0 0 8 ) 184–189
Fig. 5 – Output forces signal [N] after filtering, 2000 Hz sampling test 9 rough pass (cutting speed vt = 18.5 (m/s), feed per grain = 0.014). 50 m in feed (roughing) and 10 stages each having 10 m in feed (finishing). The total removed material thickness is 0.3 mm. During the grinding pass, the workpiece motion is first in the opposite direction of the wheel velocity (up grinding) and then in the same direction (down grinding) (Fig. 3).
Fig. 6 – Cutting, ploughing and rubbing modes of deformation in grinding process.
The surface roughness has been measured after the grinding cycle has finished. Surface roughness measurements were taken on a 4.8 mm length using a portable roughness measuring station. Measurements were made on each test piece along the grinding direction, which is the sliding direction of the disc pad. The surface parameters were roughness average
Fig. 7 – Forces vs. cutting speed vt and feed per grain ag in rough grinding (*experimental data).
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 6 ( 2 0 0 8 ) 184–189
187
Fig. 8 – Forces vs. cutting speed vt and feed per grain ag in finish grinding (*experimental data).
(Ra ), mean peak to valley roughness (Rz ) and maximum peak to valley roughness (Rm ). The flatness was measured on each test piece using a coordinate measuring machine. During the roughing and the finishing passes in the tests the normal and the tangential forces were measured by the force measurement device. The force signals were recorded by the PC based acquisition system.
2.2.
Experimental results
The surface finish value Ra is in the range 1.97–2.68 m; it increases with decreasing wheel speed and increasing table speed. Rz is 7–10 times Ra . The maximum flatness is 0.02 mm. The data were elaborated in order to identify the relationships among roughness and kinematic conditions: the roughness values obtained for the different combinations of process parameters (cutting speed and feed per grain) were statistically analysed using multiple linear regression. The diagram obtained from the elaboration of the results is described in Fig. 4, where the curves were plotted in the
domain of validity. The validity of the model was checked using variance analysis. The obtained model is valid in the range of data. The results show that the roughness is mainly influenced by the feed per grain and to a lesser degree by the cutting speed. The feed per cutting point, analogous to plain milling, corresponds to the spacing between successive peaks along the workpiece. Decreasing the feed per grain, this spacing and consequently the depth of engagement decrease, producing a smoother surface. The reason of contribution of cutting speed is that increasing the wheel speed the number of active cutting point increases, decreasing the effective spacing between successive cutting points (Malkin, 1989; Shaji and Radhakrishnan, 2003; Brinksmeier and Giwerzew, 2003; Mayer and Fang, 1995; Rowe, 1993; Zhou and Van Luttervelt, 1992; Qi et al., 1997). The force signals, recorded by a PC based acquisition system, were processed to obtain an accurate value of the grinding forces. In order to remove noise included dynamometer’s original output signal, the characteristics of existing noise were identified. First, the amplitude spectrum of the original
188
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 6 ( 2 0 0 8 ) 184–189
signal was obtained by FFT method. The results show that noise before grinding and after grinding were almost the same, have a similar large component at 50 Hz, which reflects the electric power used. Another characteristic noise frequency is the same as the wheel’s rotation. In order to remove above mentioned components, a digital filter was employed. Fig. 5 shows signal after noise removal, relative to the test 9, rough pass. Three stages of material deformation take place when a grain interacts with a workpiece: rubbing, ploughing, and cutting. These are illustrated schematically in Fig. 6. In the rubbing process material removal is negligible and only elastic deformation and some plastic deformation occurs: the force on each grain is too small to cause large penetration of the workpiece. Ploughing occurs when penetration of grains is increased. In the ploughing process, grain deforms the workpiece and material is dislodged ahead of the grain forming side ridge. Rate of material removal remains negligible. In the cutting process the chip removal takes place. Active grains are distributed on the external surface of grinding wheel and also radially, below the surface. So when active grains on the surface are cutting, other ones below the surface are ploughing or rubbing at once. It can be seen in Fig. 5 that in down-grinding forces are slightly smaller than in up-grinding. The reason is that the initial ploughing is reduced or eliminated since each grain engages the workpiece at its maximum depth of cut (Malkin, 1989; Wager and Gu, 1991). The average value of tangential and normal forces in rough and finish grinding are shown in Figs. 7 and 8, limited at the up-grinding. The normal force ranges from 27.8 N to 80.9 N in rough grinding and from 7.5 N to 45.8 N in finish grinding, while the tangential grinding force ranges from 21.1 N to 61.6 N in rough grinding and from 5.5 N to 36.9 N in finish grinding. The forces are directly dependent upon the undeformed chip thickness, so the influence of depth of cut on forces is justified. The tangential/normal force ratio depends on the workpiece material and is constant (Malkin, 1989; Li et al., 2002) the force ratio value for this material is 0.6. These data have been statistically analysed in order to identify the relationships among forces and kinematic conditions. The diagrams obtained from results’ elaboration are described in Figs. 7 and 8, where the curves were plotted in the domain of validity. The validity of the models was checked using variance analysis. The obtained models are valid in the range of the data. It can be seen that forces increase almost proportionately with cutting speed and feed per grain. The reason is that the normal force is proportional to the contact length, which is a function of the feed per cutting point. Increasing the cutting speed, the number of active grains increases, contributing to an increase of the force value. In the case of finish grinding cutting speed is found to have more contribution in forces: at fine depth of cut, forces increases less than proportionately with the cutting speed–feed per grain product, and this effect is bigger at higher cutting speed. This is the evidence of the size effect (Malkin, 1989; Li et al., 2002; Liu et al., 2005; Midha et al., 1991).
3.
Conclusions
In this paper an experimental activity performed to evaluate the surface roughness of friction material pad after a grinding operation have been presented. An instrumented tangential grinding machine has been used to finish some pads, allowing to monitor the forces during the process. A plan of experiments has been prepared in order to test the influence of cutting speed and feed per grain on the surface roughness after grinding cycle. The obtained data have been statistically processed using multiple linear regression to identify the relationship among roughness and kinematic parameters. The obtained model shows that the roughness is mainly influenced by the feed per grain and to a lesser degree by the cutting speed. Decreasing the feed per grain, the spacing between successive peaks along the workpiece and consequently the depth of engagement decrease, producing a smoother surface; while an increase of cutting speed results in a decrease of roughness because the number of active cutting point is higher and consequently the effective spacing between successive cutting points is reduced. The force signals, recorded by a PC based acquisition system, were processed to obtain accurate values. The grinding forces in down grinding are slightly smaller then in upgrinding due to the reduction of the plowing. The force data in rough and finish grinding have been statistically processed using multiple linear regression to identify the relationship with kinematic parameters. The obtained models show that forces increase almost proportionately with cutting speed and feed per grain. The reason is that the normal force is proportional to contact length, which is function of the feed per cutting point. Increasing cutting speed, the number of active grains increases, contributing to increase force value. In finish grinding the size effect is evident and forces increases less than proportionately with cutting speed and feed per grain.
references
Brinksmeier, E., Giwerzew, A., 2003. Chip formation mechanisms in grinding at low speeds. CIRP Ann., 253. Hecker, R.L., Liang, S.Y., 2003. Predictive modeling of surface roughness in grinding. Int. J. Mach. Tools Manufacture 43 (8), 755–761. Li, Z.C., Lin, B., Xu, Y.S., Hu, J., 2002. Experimental studies on grinding forces and force ratio of the unsteady-state grinding technique. J. Mater. Process. Technol. 129 (1–3), 76–80. Liu, C.H., Chen, A., Chen, C.-C.A., Wang, Y.-T., 2005. Grinding force control in an automatic surface finishing system. J. Mater. Process. Technol. 170 (1–2), 367–373. Malkin, S., 1989. Grinding Technology: Theory and Applications of Machining with Abrasives. Ellis Horwood, Chichester. Mayer, J.E., Fang, G.P., 1995. Effect of grinding parameters on surface finish of ground ceramics. CIRP Ann., 279. Midha, P.S., Zhu, C.B., Trmal, G.J., 1991. Optimum selection of grinding parameters using process modeling and knowledge based system approach. J. Mater. Process. Technol. 28 (1–2), 189–198. Qi, H.S., Rowe, W.B., Mills, B., 1997. Experimental investigation of contact behaviour in grinding. Tribol. Int. 30 (4), 283–294.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 6 ( 2 0 0 8 ) 184–189
Rowe, W.B., 1993. The effect of deformation on the contact area in grinding. CIRP Ann., 409. Shaji, S., Radhakrishnan, V., 2003. Analysis of process parameters in surface grinding with graphite as lubricant based on the Taguchi method. J. Mater. Process. Technol. 141 (1), 51–59. Wager, J.G., Gu, D.Y., 1991. Influence of up-grinding and down-grinding on the contact zone. CIRP Ann., 323.
189
Xun, C., Rowe, W.B., 1996. Analysis and simulation of the grinding process. Part II. Mechanics of grinding. Int. J. Mach. Tools Manufacture 36 (8), 883–896. Zhou, Z.X., Van Luttervelt, C.A., 1992. The real contact length between grinding wheel and workpiece: a new concept and a new measuring method. CIRP Ann., 387.