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Dimensional Characterization of Grinding Wheel Surface through Acoustic Emission
Dimensional Characterization of Grinding Wheel Surface through Acoustic Emission Jo80 F. Gomes d e Oliveira, David A. Dornfeld (2), Bernhard Winter, University of California, College of Engineering, Berkeley, CA, USA Received on January 4,1994
Abstract
An approach is proposed for measuring the grinding wheel geometric characteristics by using the dressing tool as a touch probe and the acoustic emission level generated in the contact as a trigger for the dimensional measurement. The interaction between the dressing tool and the grinding wheel is experimentally studied. Three interaction levels are observed: turbulence, elastic and brittle. In the elastic contact the grinding wheel surface is not damaged by the dressing tool. This condition is investigated based in the topographic characteristics of the interacting surfaces. The analysis shows that it is possible to measure grinding wheel characteristics using this system.
Keywords: Grinding, Grinding Wheel Measurement, Acoustic Emission.
Introduction Recent increasing demands for small lot production and gradual shortage of skilled machining operators require fully automated and flexible grinding systems. In order to develop such systems, in-process diagnostic functions, which identify grinding process conditions and perform decision making to recover from undesired conditions, are to be installed. Since the basic grinding relationships for modeling and simulation are well established [13] most of the grinding automation needs are now the development of monitoring systems for grinding control. The use of acoustic emission (AE) as a source of information for grinding control has been investigated extensively in the last 5 years. The AE generation is evaluated regarding other phenomena besides the wheel and workpiece first contact. AE has been used in grinding research for detecting malfunctions and dressing cycle quantities such as: grinding wheel loading [3], chatter [4],grinding bum [9], grinding wheel sharpness [6,7] and dressing monitoring [8,9]. In that research work, AE features such as mean value, root mean square value (AERMs) or frequency demodulation were investigated during the cutting process or dressing operation [ 5 ] . The use of information from the cutting phenomena combined with information measured directly on the workpiece surface (e.g. diameter or surface finish) is often enough for a grinding feedback control system. However in multi-diameter. interrupted cutting or non circular grinding operations the in-process measurement of the workpiece can be not feasable or too expensive. Also, the measurement of the workpiece diameter doesn't take in account the machine deformation by the cutting force, making it necessary to use longer grinding cycles with several spark-out phases or intermediate plunge speeds to relieve the deformation. The thermal deformation in grinding machines has been studied regarding temperature analysis and the use of thermal solutions (such as high efficient heat transfer elements or thermocouple feedback) to compensate for the thermal deflections [ 11. In industrial applications the correction of thermal deformation is normally achieved through an additional dressing operation or based on the in-process workpiece measurement. The additional dressing operation increases the tool costs, which can be high for superabrasive wheels. The changes in the dressing depth of cut caused by the thermal deformation can also result in undesired sharpness [ 101. To avoid the described problems related to workpiece measurement the compensation could be based in the measured position of the grinding wheel surface.
Annals of the ClRP Vol. 43/1/1994
The measurement of the AE from the dressing operation shows whether the last dressing stroke was successful or not, but doesn't give information before dressing about the wheel dimension or profile. Therefore in-process characterization of the grinding wheel surface can provide important information for increasing the efficiency of a control system. The objective of this research is to develop a system for grinding wheel characterization able to provide information about the grinding wheel surface position and profile. This information can then be used for thermal compensation and for deciding when and how to dress the wheel by a control system.
Monitoring for Dressing Decision Two main factors necessitate grinding wheel dressing: the sharpness loss and the profile loss. Therefore the in-process grinding wheel surface characterization has to be done with respect to dimensional and sharpness characteristics. In the case of monitoring sharpness, the grinding wheel topography can be measured through the reflection characteristics of its surface. This measurement can be done by using a CCD [ 111 or a PSD [2,14] sensor as a source of information to achieve the average grain wear flat area or the topographic profile. The grinding wheel profile is modified during the grinding process by irregularities in the wheel wear. These irregularities are caused by differences in the grinding specific volumes to be removed along the workpiece axis or by variations in the G ratio caused by wheel and workpiece non-homogenities. In order to measure the grinding wheel profile a system for detecting the position of the wheel surface is desirable. The determination of the position of the grinding wheel surface (or simply grinding wheel position) regarding the workpiece is important information to compensate for the grinding wheel wear and thermal deviations. Information about the superficial wear profile of the grinding wheel is an important feature for dressing decision and dressing strategy. The position of the grinding wheel surface can be achieved by measuring the air flow that surrounds the grinding wheel using an anemometer [121. However this method doesn't give the necessary accuracy required by precision grinding machines. In the following sections a method for measuring the grinding wheel is proposed by using a diamond probe and AE signal processing technology.
Proposed System In this system a dressing tool is used as a probe for measuring the grinding wheel position (Figure 1). A displacement measuring system provides information about the dresser tip position when the dresser approaches the grinding wheel surface. When the dresser touches the grinding wheel surface the elastic energy of contact can be measured as AE RMS by 291
an AE transducer. This AE signal can be amplified and. using a threshold voltage comparator. used to trigger the acquisition of the grinding wheel position and retract the dresser. The displacement transducer, in the case of a CNC grinding machine. can be a linear scale or an encoder on the machine crossfeed axis. This measurement can be repeated along the wheel surface to obtain the grinding wheel profile.
grinding wheel profile is carefully copied into a small workpiece by plunge grinding and the profile of the workpiece is measured by a profilometer. The obtained profile provides the value of the maximum depth of cut. From the maximum depth of cut and the displacement values measured by the laser interferometer is possible to find the position and the A € RMS level in the transition between elastic and brittle interaction. This position, which represents the limit at which the wheel damage starts, is called here the virtual position of the grinding wheel surface.
Contact and Acoustic Emission Figure 2 gives an illustration of the typical experimental results observed. As soon as the tip of the dressing tool enters AE RMS
level (V)
I
n "0
Plunge speed: A -7.8 pm/s o -4.1 um/s Figure 1 - System for compensation of Grinding Wheel Wear and thermal Deviation.
The performance of this system depends on the AE RMS level that can be obtained in the contact without damaging the grinding wheel surface. To evaluate the feasibility of this system, it is necessary to investigate the grinding wheel and dressing tool interaction during contact. This is needed to determine the maximum AE RMS energy obtained without grinding wheel damage, i.e. due to elastic contact. Previous experiments, proved that the generation of acoustic emission during the approach between workpiece and grinding wheel starts before the contact. This happens due to coolant/grinding wheel interaction [3]. A detailed analysis of this interaction is needed to evaluate the accuracy of this method and the behavior of the interaction in different grinding wheels, diamond profiles and approach speeds.
Experimental Setup The experimental setup is installed on a cylindrical grinding machine. The acoustic emission signal generated by interaction between the dresser and the grinding wheel is detected by a broad band AE transducer, with a frequency range of 50 - 1000 kHz. The AE signal passes through an amplifier cascade and the RMS value is calculated. The AE sensor is installed on the dresser shaft using a spring loaded magnetic support. The displacement of the grinding wheel relative to the dresser is measured by a laser interferometer system. The interferometer is attached to the dressing tool holder and the retro reflector is fixed to the grinding wheel head, so the measurement takes into account any machine deflection caused by force or by thermal deviations. The interferometer resolution is set to 0.1 pm.
In order to analyze the transitions between non-contact to elastic contact to brittle contact, the interaction between the dressing tool and the grinding wheel is done in a plunge cycle. Eight series of tests are done to evaluate the influence of different factors in the elastic range: wheel hardness grade (L,M), dressing tool sharpness (0.3, 0.1 mm tip lengh) and plunge speed. Each dresser engagement is done until it produces a groove in the grinding wheel so it is known that the brittle interaction was achieved. To measure the depths of engagement the
292
o -1.5 Gm/s
0.24-1
0
-1.5 ym/s(dry)
Turbulence
I
I Gr. wheel: I A1203- Vitr.
ISoeed: '28 m/
'-
1
Elastic
!
, ,
A
Brittle Contact (dressing)
Contact
Contict Position
Virtial Position (limit for damage)
Figure -3 - AERMS in the dressing tool and grinding wheel interaction. the boundary layer of air and coolant that surrounds the grinding wheel, the AERMS signal increases, because of shock waves caused by the interaction between the turbulent flow and the dresser tip (area A). When the dresser approaches the grinding wheel surface, interactions with the highest (here largest diameter) parts of the abrasive grains take place. First only elastic deformations (area B), like sliding or rubbing, occur. If the dresser is retracted from this point, there would be no measurable damage on the grinding wheel surface. The point at which grain fracture (area C) begins, and therefore volumetric wheel wear, is just beyond the "virtual position".. This is the maximum depth of interaction that can be used for the wheel measurement.
Elastic Interaction The elastic behavior is influenced mostly by the characteristics of the grinding wheel bond, since the elasticity of dressing tool and the abrasive grain materials is relatively low and can be neglected. The rigidity of the machine does not affect the behavior because here the displacement is measured between the grinding wheel head and the dresser. The "elastic length" can be defined as the distance between the first contact position and the virtual position. This length can be influenced by the wheel hardness. Figure 3 shows the AERMS level in the elastic contact area for interaction among the 60 M grinding wheel for two different dressing tools (dull and sharp). The dresser tip length is 0,3 mm and 0,lmm for the dull and sharp dressers respectively. In these experiments a higher gain was used in the AE amplifiers for better resolution in the analog to digital conversion. It should be noticed that the AERMS signal rises faster in the case of engagement with the dull dresser than with the sharp dresser. In each case the AERMS performance is the same for each
dresser. This means the infeed speed has no influence on the AERMS signal in the elastic contact range. The same results were obtained for the 60L grinding wheel. This behavior is not 6
Grind. Wheel 38A 60M V
5
Dry dressing plunge
cn I
a E
.-: : 4
Dresser, plunge speed: Dull, 10.5 mm/s Dull 3.8 mmls
a-u)
5% 3 *-
.-0 CI
a
=I
0
2
5
* 1
0 2 4 Displacement [pm]
0
6
8
Figure 3 - AE Signal in the Elastic Contact Area
the same in the brittle contact range, where the AERMS level is higher for higher infeed speeds, as can be observed in figure 2.
Topography of Interacting Surfaces The interaction between a dresser and a grinding wheel can be characterized as the interaction of two surfaces. Different quantities can be used to describing the topography of a surface. For the present investigations, the surfaces of the dresser and the grinding wheel are described with roughness quantities. particularly the bearing ratio. The idea is to evaluate the bearing ratios of two interacting surfaces through the intersection of the two bearing ratio curves. In addition to the uni-dimensional view of the interaction of surfaces a bidimensional consideration can be made. The equivalent to the bearing ratio is the "surface bearing ratio" which is defined as the ratio between the bearing area in a specified depth to the total area of the analyzed surface. As with the bearing ratio. the depth is measured from the top peak. Figure 5 shows the results of the calculations of the surface bearing ratios for the 60 L grinding wheel, the dull and the
Figure 4 shows the average elastic length for interaction between dresser and grinding wheel for the hardness grades M and L as well as for the dull and the sharp dresser obtained in 32 experiments. The average is calculated by considering different infeed speeds for each dresser - wheel combination. It was found that there is no dependency between the elastic length and infeed speed and that the maximum deviation from the mean value is up to 1.5 pm. The tendency is that, in contrast to the softer wheel, the average elastic length is shorter for the harder wheel considering both dressers. The elastic length is influenced by the bonding since the grain hardness is the
5 CR
Dull Dresser
E
Q)
60 M 60 M 60 L Grinding Wheel Figure 4 - Elastic Length and Gradient for Different Wheel Hardness Grades and Different Dressing Tools
60 L
same. When the dresser contacts the grinding wheel in the elastic area the grains deflect due to the applied forces. The deflection is easier in a soft grinding wheel and the dresser can interact over a longer distance before the grain is cut. In a hard wheel the grain is fixed more tightly and will be cut or removed since it cannot deflect. Hence the elastic length is higher for a soft wheel. The elastic gradient of the AE signal shown in figure 4 is calculated as the slope of the acoustic emission increase in the elastic length. This value is important because it is related to the sensitivity of the proposed measuring system. Since the grains deflect more easily in the soft wheel, the rise of contact forces is smaller than in the harder wheel. Smaller contact forces mean that the energy consumption in the contact area due to friction is lower as well. Consequently, the increase of AE. related to energy consumption, is smaller for the softer wheel. The higher gradient for the dull dresser can be explained by a topographic analysis of the grinding wheel and the dressing tool. This will be presented in the next section.
-6
-2
-4
Depth of Interaction [pm]
Figirre 5 - Surface Bearing Ratios of Dressers and Grinding Wheel . -
sharp dresser. The depth of interaction can be read on the xaxis. In this case data for 4 pm are displayed. The corresponding "interaction surface bearing ratio" can be read as the y- axis value of the intersection. A new picture can be built by shifting the surface bearing ratio for the two dressers to a new position, meaning a new depth of interaction. The intersection of the grinding wheel and the shifted dresser curves produce a new interaction surface bearing ratio. The results of this simulation of the relation between the interaction surface bearing ratio and depth of interaction are displayed in figure 6. The curve for the dull dresser rises faster since the y- value of the intersection for the dull dresser is always located above the intersection point for the sharp dresser, (figure 5).
P
1 $
0.9 0.7 0.6
rn Dull Dresser
0.2 0.1
0
0
2
Sharp Dresser
4
6
Depth of Interaction [pm]
Figure 6 - Interaction Surface Bearing Ratio as Function of Depth of Engagement
Figure 7 shows the relation between the measured AERMS level and the calculated interaction surface bearing ratio. All series show a linear behavior, meaning the generation of AE is
293
directly proportional to the interaction surface bearing ratio of the two surfaces and the AERMS level rises with increasing interaction surface bearing ratio. This also means that a relation between generation of AE and contact area exists. It is also seen that all series show the same gradient. The behavior is related only to the surface bearing ratio and is independent of the dresser profile. The dispersion in figure 7 is due to differences in the grinding wheel topography as well as to profile and bearing ratio measurements. For example, one higher peak in the profile can shift the whole beating ratio curve. Since the AER,MS signal rises with increasing interaction surface bearing ratio. the AERMS signal is higher for the same depth of cut in the case of the dull dresser. Therefore the gradient of the AERMS vs displacement curve is higher for the dull dresser.
A E R M S signal rises faster for a hard grinding wheel and for a worn dressing tool.
The profiles of grinding wheel and dresser are described with the "surface bearing ratio", which is a bi-dimensional consideration of the bearing ratio. The generation of AE is proportional to the "interaction surface bearing ratio" of two interacting surfaces. AERMS rises faster during interaction for a dresser with a higher rise of the surface bearing ratio curve. This dresser is more suitable for monitoring purposes. Future efforts should be focussed on building a complete system for measuring the position of the grinding wheel surface and compensating grinding wheel wear and thermal deviations of the grinding machine based on the present results.
Acknowledgments rn 0
This work is supported in part by the affiliates of the Laboratory for Manufacturing Automation at the University of California. The Norton Company supplied the grinding wheels for the tests.
Dull Dresser, 12 I m / s Sharp Dresser, 7.6 pm/s Sharp Dresser, 2.7 Hm/s
References [ IIBolin, Z. et al., "Improving the Thermal Behaviour of
0.0001
0.0003 0.0005 Interaction Surface Bearing Ratio
0.0007
Figure 7 - Relation between AERMS and Interaction Surface Bearing Ratio The monitoring system must guarantee that damage of the grinding wheel surface does not occur. Even light dresser grooves on the wheel can deteriorate the grinding results. In addition, the repeatability of the measurements is required since grinding is often the final operation in a series of machining processes and has to provide the workpiece with high accuracy. In order to check this grinding wheel measurement system a threshold value was established and several plunge cycles were done with the maximum depth of interaction limited by the threshold. After these measurements the grinding wheel profile was copied into a workpiece. No damage was observed in the measured profiles. The repeatability in the measurement of the grinding wheel position was determined for the setup as 1.2pm (+/- 30).
Conclusions Measurement of the dressing tool and grinding wheel interaction and the determination of the position of the grinding wheel surface is possible by using AE sensing technology and a single point diamond dresser as a measuring probe. An AE sensor attached to the dresser provides the information about the interaction between both surfaces. This information can be used to trigger the acquisition of the grinding wheel position. The contact conditions between grinding wheel and dresser can be distinguished as non contact, elastic contact, and brittle contact. The AERMS signal in the elastic contact range is used for the monitoring system. The generation of AE in the elastic contact area depends on the hardness grade of the grinding wheel and the profile of dresser and wheel. It is not influenced by the relative approach speed between dresser and grinding wheel. The
294
Precision Grinding Machine", Annals of the CIRP, ~O1.34/1,1985, pp. 305-308. ['IBrinksrneier, E., Werner, F., "Monitoring of Grinding Wheel Wear", Annals of the CIRP, vo1.41/1, 1992, pp. 373376. [3]Dornfeld, D.A., Cai, H.G., "An Investigation of Grinding and Wheel Loading using Acoustic Emission", Trans. of ASME, J . Eng. Ind.. vol. 106, Feb. 1984, pp. 28-33. [4]Dornfeld, D.A., Chang, Y.P., "Chatter and Surface Pattern Detection for Grinding Process using a Fluid Coupled Acoustic Emission Sensor", Proc. Int. Conf. Machining of Advanced Materials. NIST, July, 1993, pp 159-167. [SIDornfeld, D. A.. Blum, T. "Grinding Process Feedback using Acoustic Emission"; Proc.4th Int. Grinding Conference, Dearborn, Michigan, USA, Oct. 1990 [6]Inasaki,I., Wakuda,M., "Detection of Malfunctions in Grinding Process", Procdth Word Meeting on Acoustic Emission and 1st International Conference on Acoustic Emission in Manufacturing, Sept. 1991, ASNT, Boston, Mass. p. 494-501 [7lInasaki, I., "Monitoring and Optimization of Internal Grinding Process". Annals of the CIRP, vo1.40/1, 1991, pp. 359-362. [8]Inasaki, I. "Monitoring of Dressing and Grinding Processes with Acoustic Emission Signals"; Annals of the CIRP, ~01.34/1,1985, pp.277-280. [9]Konig, W., Klumpen, T., "Monitoring and Sensor Concepts for Higher Process Reliability", 5th International Grinding-SME,Oct. 1993, Cincinnati, Ohio. [ 10]01iveira, J.F.G., et al., "Grinding Process Dominance By Means Of The Dressing Operation"; Proc. 29th International Matador Conference, Manchester UK, 1992, pp 547-560. [ I I]Oliveira, J. F. G., "Sharpness Control Of Grinding Wheels In Precision Grinding", Associate Professor Thesis (in Portuguese), S2o Carlos, SP, Brazil, 1992, 165p. [ 121 Shibata, J., Goto, T., Yamamoto, M., "Characteristics of Air Flow Around a Grinding Wheel and their Availability for Assessing the Wheel Wear". Annals of the CIRP, vol 31/1, 1982, pp.233-238. [ 131Tonshoff, H. K., Peters, J., Inasaki, I., Paul, T., "Modeling and Simulation of Grinding Process, Annals of the CIRP, ~O1.41/2. 1992. pp. 677-688. [ 14]Tonshoff, H. K., Wobker. H.G., Werner, F., "Analysis of Wheel Waviness and Grain Wear in Grinding", Transaction of NAMRIJSME , vol XXI, 1993, pp. 145-149.
Abstract
An approach is proposed for measuring the grinding wheel geometric characteristics by using the dressing tool as a touch probe and the acoustic emission level generated in the contact as a trigger for the dimensional measurement. The interaction between the dressing tool and the grinding wheel is experimentally studied. Three interaction levels are observed: turbulence, elastic and brittle. In the elastic contact the grinding wheel surface is not damaged by the dressing tool. This condition is investigated based in the topographic characteristics of the interacting surfaces. The analysis shows that it is possible to measure grinding wheel characteristics using this system.
Keywords: Grinding, Grinding Wheel Measurement, Acoustic Emission.
Introduction Recent increasing demands for small lot production and gradual shortage of skilled machining operators require fully automated and flexible grinding systems. In order to develop such systems, in-process diagnostic functions, which identify grinding process conditions and perform decision making to recover from undesired conditions, are to be installed. Since the basic grinding relationships for modeling and simulation are well established [13] most of the grinding automation needs are now the development of monitoring systems for grinding control. The use of acoustic emission (AE) as a source of information for grinding control has been investigated extensively in the last 5 years. The AE generation is evaluated regarding other phenomena besides the wheel and workpiece first contact. AE has been used in grinding research for detecting malfunctions and dressing cycle quantities such as: grinding wheel loading [3], chatter [4],grinding bum [9], grinding wheel sharpness [6,7] and dressing monitoring [8,9]. In that research work, AE features such as mean value, root mean square value (AERMs) or frequency demodulation were investigated during the cutting process or dressing operation [ 5 ] . The use of information from the cutting phenomena combined with information measured directly on the workpiece surface (e.g. diameter or surface finish) is often enough for a grinding feedback control system. However in multi-diameter. interrupted cutting or non circular grinding operations the in-process measurement of the workpiece can be not feasable or too expensive. Also, the measurement of the workpiece diameter doesn't take in account the machine deformation by the cutting force, making it necessary to use longer grinding cycles with several spark-out phases or intermediate plunge speeds to relieve the deformation. The thermal deformation in grinding machines has been studied regarding temperature analysis and the use of thermal solutions (such as high efficient heat transfer elements or thermocouple feedback) to compensate for the thermal deflections [ 11. In industrial applications the correction of thermal deformation is normally achieved through an additional dressing operation or based on the in-process workpiece measurement. The additional dressing operation increases the tool costs, which can be high for superabrasive wheels. The changes in the dressing depth of cut caused by the thermal deformation can also result in undesired sharpness [ 101. To avoid the described problems related to workpiece measurement the compensation could be based in the measured position of the grinding wheel surface.
Annals of the ClRP Vol. 43/1/1994
The measurement of the AE from the dressing operation shows whether the last dressing stroke was successful or not, but doesn't give information before dressing about the wheel dimension or profile. Therefore in-process characterization of the grinding wheel surface can provide important information for increasing the efficiency of a control system. The objective of this research is to develop a system for grinding wheel characterization able to provide information about the grinding wheel surface position and profile. This information can then be used for thermal compensation and for deciding when and how to dress the wheel by a control system.
Monitoring for Dressing Decision Two main factors necessitate grinding wheel dressing: the sharpness loss and the profile loss. Therefore the in-process grinding wheel surface characterization has to be done with respect to dimensional and sharpness characteristics. In the case of monitoring sharpness, the grinding wheel topography can be measured through the reflection characteristics of its surface. This measurement can be done by using a CCD [ 111 or a PSD [2,14] sensor as a source of information to achieve the average grain wear flat area or the topographic profile. The grinding wheel profile is modified during the grinding process by irregularities in the wheel wear. These irregularities are caused by differences in the grinding specific volumes to be removed along the workpiece axis or by variations in the G ratio caused by wheel and workpiece non-homogenities. In order to measure the grinding wheel profile a system for detecting the position of the wheel surface is desirable. The determination of the position of the grinding wheel surface (or simply grinding wheel position) regarding the workpiece is important information to compensate for the grinding wheel wear and thermal deviations. Information about the superficial wear profile of the grinding wheel is an important feature for dressing decision and dressing strategy. The position of the grinding wheel surface can be achieved by measuring the air flow that surrounds the grinding wheel using an anemometer [121. However this method doesn't give the necessary accuracy required by precision grinding machines. In the following sections a method for measuring the grinding wheel is proposed by using a diamond probe and AE signal processing technology.
Proposed System In this system a dressing tool is used as a probe for measuring the grinding wheel position (Figure 1). A displacement measuring system provides information about the dresser tip position when the dresser approaches the grinding wheel surface. When the dresser touches the grinding wheel surface the elastic energy of contact can be measured as AE RMS by 291
an AE transducer. This AE signal can be amplified and. using a threshold voltage comparator. used to trigger the acquisition of the grinding wheel position and retract the dresser. The displacement transducer, in the case of a CNC grinding machine. can be a linear scale or an encoder on the machine crossfeed axis. This measurement can be repeated along the wheel surface to obtain the grinding wheel profile.
grinding wheel profile is carefully copied into a small workpiece by plunge grinding and the profile of the workpiece is measured by a profilometer. The obtained profile provides the value of the maximum depth of cut. From the maximum depth of cut and the displacement values measured by the laser interferometer is possible to find the position and the A € RMS level in the transition between elastic and brittle interaction. This position, which represents the limit at which the wheel damage starts, is called here the virtual position of the grinding wheel surface.
Contact and Acoustic Emission Figure 2 gives an illustration of the typical experimental results observed. As soon as the tip of the dressing tool enters AE RMS
level (V)
I
n "0
Plunge speed: A -7.8 pm/s o -4.1 um/s Figure 1 - System for compensation of Grinding Wheel Wear and thermal Deviation.
The performance of this system depends on the AE RMS level that can be obtained in the contact without damaging the grinding wheel surface. To evaluate the feasibility of this system, it is necessary to investigate the grinding wheel and dressing tool interaction during contact. This is needed to determine the maximum AE RMS energy obtained without grinding wheel damage, i.e. due to elastic contact. Previous experiments, proved that the generation of acoustic emission during the approach between workpiece and grinding wheel starts before the contact. This happens due to coolant/grinding wheel interaction [3]. A detailed analysis of this interaction is needed to evaluate the accuracy of this method and the behavior of the interaction in different grinding wheels, diamond profiles and approach speeds.
Experimental Setup The experimental setup is installed on a cylindrical grinding machine. The acoustic emission signal generated by interaction between the dresser and the grinding wheel is detected by a broad band AE transducer, with a frequency range of 50 - 1000 kHz. The AE signal passes through an amplifier cascade and the RMS value is calculated. The AE sensor is installed on the dresser shaft using a spring loaded magnetic support. The displacement of the grinding wheel relative to the dresser is measured by a laser interferometer system. The interferometer is attached to the dressing tool holder and the retro reflector is fixed to the grinding wheel head, so the measurement takes into account any machine deflection caused by force or by thermal deviations. The interferometer resolution is set to 0.1 pm.
In order to analyze the transitions between non-contact to elastic contact to brittle contact, the interaction between the dressing tool and the grinding wheel is done in a plunge cycle. Eight series of tests are done to evaluate the influence of different factors in the elastic range: wheel hardness grade (L,M), dressing tool sharpness (0.3, 0.1 mm tip lengh) and plunge speed. Each dresser engagement is done until it produces a groove in the grinding wheel so it is known that the brittle interaction was achieved. To measure the depths of engagement the
292
o -1.5 Gm/s
0.24-1
0
-1.5 ym/s(dry)
Turbulence
I
I Gr. wheel: I A1203- Vitr.
ISoeed: '28 m/
'-
1
Elastic
!
, ,
A
Brittle Contact (dressing)
Contact
Contict Position
Virtial Position (limit for damage)
Figure -3 - AERMS in the dressing tool and grinding wheel interaction. the boundary layer of air and coolant that surrounds the grinding wheel, the AERMS signal increases, because of shock waves caused by the interaction between the turbulent flow and the dresser tip (area A). When the dresser approaches the grinding wheel surface, interactions with the highest (here largest diameter) parts of the abrasive grains take place. First only elastic deformations (area B), like sliding or rubbing, occur. If the dresser is retracted from this point, there would be no measurable damage on the grinding wheel surface. The point at which grain fracture (area C) begins, and therefore volumetric wheel wear, is just beyond the "virtual position".. This is the maximum depth of interaction that can be used for the wheel measurement.
Elastic Interaction The elastic behavior is influenced mostly by the characteristics of the grinding wheel bond, since the elasticity of dressing tool and the abrasive grain materials is relatively low and can be neglected. The rigidity of the machine does not affect the behavior because here the displacement is measured between the grinding wheel head and the dresser. The "elastic length" can be defined as the distance between the first contact position and the virtual position. This length can be influenced by the wheel hardness. Figure 3 shows the AERMS level in the elastic contact area for interaction among the 60 M grinding wheel for two different dressing tools (dull and sharp). The dresser tip length is 0,3 mm and 0,lmm for the dull and sharp dressers respectively. In these experiments a higher gain was used in the AE amplifiers for better resolution in the analog to digital conversion. It should be noticed that the AERMS signal rises faster in the case of engagement with the dull dresser than with the sharp dresser. In each case the AERMS performance is the same for each
dresser. This means the infeed speed has no influence on the AERMS signal in the elastic contact range. The same results were obtained for the 60L grinding wheel. This behavior is not 6
Grind. Wheel 38A 60M V
5
Dry dressing plunge
cn I
a E
.-: : 4
Dresser, plunge speed: Dull, 10.5 mm/s Dull 3.8 mmls
a-u)
5% 3 *-
.-0 CI
a
=I
0
2
5
* 1
0 2 4 Displacement [pm]
0
6
8
Figure 3 - AE Signal in the Elastic Contact Area
the same in the brittle contact range, where the AERMS level is higher for higher infeed speeds, as can be observed in figure 2.
Topography of Interacting Surfaces The interaction between a dresser and a grinding wheel can be characterized as the interaction of two surfaces. Different quantities can be used to describing the topography of a surface. For the present investigations, the surfaces of the dresser and the grinding wheel are described with roughness quantities. particularly the bearing ratio. The idea is to evaluate the bearing ratios of two interacting surfaces through the intersection of the two bearing ratio curves. In addition to the uni-dimensional view of the interaction of surfaces a bidimensional consideration can be made. The equivalent to the bearing ratio is the "surface bearing ratio" which is defined as the ratio between the bearing area in a specified depth to the total area of the analyzed surface. As with the bearing ratio. the depth is measured from the top peak. Figure 5 shows the results of the calculations of the surface bearing ratios for the 60 L grinding wheel, the dull and the
Figure 4 shows the average elastic length for interaction between dresser and grinding wheel for the hardness grades M and L as well as for the dull and the sharp dresser obtained in 32 experiments. The average is calculated by considering different infeed speeds for each dresser - wheel combination. It was found that there is no dependency between the elastic length and infeed speed and that the maximum deviation from the mean value is up to 1.5 pm. The tendency is that, in contrast to the softer wheel, the average elastic length is shorter for the harder wheel considering both dressers. The elastic length is influenced by the bonding since the grain hardness is the
5 CR
Dull Dresser
E
Q)
60 M 60 M 60 L Grinding Wheel Figure 4 - Elastic Length and Gradient for Different Wheel Hardness Grades and Different Dressing Tools
60 L
same. When the dresser contacts the grinding wheel in the elastic area the grains deflect due to the applied forces. The deflection is easier in a soft grinding wheel and the dresser can interact over a longer distance before the grain is cut. In a hard wheel the grain is fixed more tightly and will be cut or removed since it cannot deflect. Hence the elastic length is higher for a soft wheel. The elastic gradient of the AE signal shown in figure 4 is calculated as the slope of the acoustic emission increase in the elastic length. This value is important because it is related to the sensitivity of the proposed measuring system. Since the grains deflect more easily in the soft wheel, the rise of contact forces is smaller than in the harder wheel. Smaller contact forces mean that the energy consumption in the contact area due to friction is lower as well. Consequently, the increase of AE. related to energy consumption, is smaller for the softer wheel. The higher gradient for the dull dresser can be explained by a topographic analysis of the grinding wheel and the dressing tool. This will be presented in the next section.
-6
-2
-4
Depth of Interaction [pm]
Figirre 5 - Surface Bearing Ratios of Dressers and Grinding Wheel . -
sharp dresser. The depth of interaction can be read on the xaxis. In this case data for 4 pm are displayed. The corresponding "interaction surface bearing ratio" can be read as the y- axis value of the intersection. A new picture can be built by shifting the surface bearing ratio for the two dressers to a new position, meaning a new depth of interaction. The intersection of the grinding wheel and the shifted dresser curves produce a new interaction surface bearing ratio. The results of this simulation of the relation between the interaction surface bearing ratio and depth of interaction are displayed in figure 6. The curve for the dull dresser rises faster since the y- value of the intersection for the dull dresser is always located above the intersection point for the sharp dresser, (figure 5).
P
1 $
0.9 0.7 0.6
rn Dull Dresser
0.2 0.1
0
0
2
Sharp Dresser
4
6
Depth of Interaction [pm]
Figure 6 - Interaction Surface Bearing Ratio as Function of Depth of Engagement
Figure 7 shows the relation between the measured AERMS level and the calculated interaction surface bearing ratio. All series show a linear behavior, meaning the generation of AE is
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directly proportional to the interaction surface bearing ratio of the two surfaces and the AERMS level rises with increasing interaction surface bearing ratio. This also means that a relation between generation of AE and contact area exists. It is also seen that all series show the same gradient. The behavior is related only to the surface bearing ratio and is independent of the dresser profile. The dispersion in figure 7 is due to differences in the grinding wheel topography as well as to profile and bearing ratio measurements. For example, one higher peak in the profile can shift the whole beating ratio curve. Since the AER,MS signal rises with increasing interaction surface bearing ratio. the AERMS signal is higher for the same depth of cut in the case of the dull dresser. Therefore the gradient of the AERMS vs displacement curve is higher for the dull dresser.
A E R M S signal rises faster for a hard grinding wheel and for a worn dressing tool.
The profiles of grinding wheel and dresser are described with the "surface bearing ratio", which is a bi-dimensional consideration of the bearing ratio. The generation of AE is proportional to the "interaction surface bearing ratio" of two interacting surfaces. AERMS rises faster during interaction for a dresser with a higher rise of the surface bearing ratio curve. This dresser is more suitable for monitoring purposes. Future efforts should be focussed on building a complete system for measuring the position of the grinding wheel surface and compensating grinding wheel wear and thermal deviations of the grinding machine based on the present results.
Acknowledgments rn 0
This work is supported in part by the affiliates of the Laboratory for Manufacturing Automation at the University of California. The Norton Company supplied the grinding wheels for the tests.
Dull Dresser, 12 I m / s Sharp Dresser, 7.6 pm/s Sharp Dresser, 2.7 Hm/s
References [ IIBolin, Z. et al., "Improving the Thermal Behaviour of
0.0001
0.0003 0.0005 Interaction Surface Bearing Ratio
0.0007
Figure 7 - Relation between AERMS and Interaction Surface Bearing Ratio The monitoring system must guarantee that damage of the grinding wheel surface does not occur. Even light dresser grooves on the wheel can deteriorate the grinding results. In addition, the repeatability of the measurements is required since grinding is often the final operation in a series of machining processes and has to provide the workpiece with high accuracy. In order to check this grinding wheel measurement system a threshold value was established and several plunge cycles were done with the maximum depth of interaction limited by the threshold. After these measurements the grinding wheel profile was copied into a workpiece. No damage was observed in the measured profiles. The repeatability in the measurement of the grinding wheel position was determined for the setup as 1.2pm (+/- 30).
Conclusions Measurement of the dressing tool and grinding wheel interaction and the determination of the position of the grinding wheel surface is possible by using AE sensing technology and a single point diamond dresser as a measuring probe. An AE sensor attached to the dresser provides the information about the interaction between both surfaces. This information can be used to trigger the acquisition of the grinding wheel position. The contact conditions between grinding wheel and dresser can be distinguished as non contact, elastic contact, and brittle contact. The AERMS signal in the elastic contact range is used for the monitoring system. The generation of AE in the elastic contact area depends on the hardness grade of the grinding wheel and the profile of dresser and wheel. It is not influenced by the relative approach speed between dresser and grinding wheel. The
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Precision Grinding Machine", Annals of the CIRP, ~O1.34/1,1985, pp. 305-308. ['IBrinksrneier, E., Werner, F., "Monitoring of Grinding Wheel Wear", Annals of the CIRP, vo1.41/1, 1992, pp. 373376. [3]Dornfeld, D.A., Cai, H.G., "An Investigation of Grinding and Wheel Loading using Acoustic Emission", Trans. of ASME, J . Eng. Ind.. vol. 106, Feb. 1984, pp. 28-33. [4]Dornfeld, D.A., Chang, Y.P., "Chatter and Surface Pattern Detection for Grinding Process using a Fluid Coupled Acoustic Emission Sensor", Proc. Int. Conf. Machining of Advanced Materials. NIST, July, 1993, pp 159-167. [SIDornfeld, D. A.. Blum, T. "Grinding Process Feedback using Acoustic Emission"; Proc.4th Int. Grinding Conference, Dearborn, Michigan, USA, Oct. 1990 [6]Inasaki,I., Wakuda,M., "Detection of Malfunctions in Grinding Process", Procdth Word Meeting on Acoustic Emission and 1st International Conference on Acoustic Emission in Manufacturing, Sept. 1991, ASNT, Boston, Mass. p. 494-501 [7lInasaki, I., "Monitoring and Optimization of Internal Grinding Process". Annals of the CIRP, vo1.40/1, 1991, pp. 359-362. [8]Inasaki, I. "Monitoring of Dressing and Grinding Processes with Acoustic Emission Signals"; Annals of the CIRP, ~01.34/1,1985, pp.277-280. [9]Konig, W., Klumpen, T., "Monitoring and Sensor Concepts for Higher Process Reliability", 5th International Grinding-SME,Oct. 1993, Cincinnati, Ohio. [ 10]01iveira, J.F.G., et al., "Grinding Process Dominance By Means Of The Dressing Operation"; Proc. 29th International Matador Conference, Manchester UK, 1992, pp 547-560. [ I I]Oliveira, J. F. G., "Sharpness Control Of Grinding Wheels In Precision Grinding", Associate Professor Thesis (in Portuguese), S2o Carlos, SP, Brazil, 1992, 165p. [ 121 Shibata, J., Goto, T., Yamamoto, M., "Characteristics of Air Flow Around a Grinding Wheel and their Availability for Assessing the Wheel Wear". Annals of the CIRP, vol 31/1, 1982, pp.233-238. [ 131Tonshoff, H. K., Peters, J., Inasaki, I., Paul, T., "Modeling and Simulation of Grinding Process, Annals of the CIRP, ~O1.41/2. 1992. pp. 677-688. [ 14]Tonshoff, H. K., Wobker. H.G., Werner, F., "Analysis of Wheel Waviness and Grain Wear in Grinding", Transaction of NAMRIJSME , vol XXI, 1993, pp. 145-149.