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Evaluation of a Lathe with Ferrocement Bed
Evaluation of a Lathe with Ferrocement Bed M. Rahman (2),M. A. Mansur, K. H. Chua, National University of Singapore Received on January 15,1993
In an earlier study, the feasibility of using ferrocement as a replacement to cast iron in the manufacture of machine tool smctures was investigated by fabricating a prototype ferrocement bed for a centre lathe and testing the bed alone. Improved performance exhibited by the ferrocement bed in comparison with the parent cast iron bed under both static and dynamic loadings has inspired the authors to pursue the work and assess the bed in terms of the performance of the machine as a whole. A centre lathe was chosen for this purpose. The traditional cast iron bed in the machine was replaced by a ferrocement bed, designed and fabricated for accommodating all the necessary parts and attachments. The method of fabrication and installation of the bcd are described in this paper. Identical vibration and cutting tests were performed in the original as well as the modified machines. Test results indicate that the machine with ferrocement bed provides signikantly higher damping and natural frequencies, specially the torsional frequency, and results in an approximately 40% deeper cut than the original lathe before the onset of c h a m . Keywords: Machine tools, ferrocement bed, centre lathe. cutting tests. performance evaluation, vibration tests 1.
INTRODUCTION
The structure of a machine tool forms the vital link between the cutting tool and the workpiece for a metal cutting machine or between the measuring probe and the workpiece for an inspection machine. Therefore, the machining accuracy of a job. its time of production and the related cost depend largely on the performance of various suuctural components. The basic properties engineers and designers generally look for in a machine tool include, among others, a high static stiffness against bending and torsion, a good dynamic performance and a long-term dimensional stability. Cast iron, which has been the most commonly used material for this purpose since early days possesses these characteristics to an acceptable l i m i L But it has some inherent disadvantages; the primary ones being the high cost. poor torsional rigidity and damping, and the difficulty in producing the finished product. The lengthy production process consists of fabricating the intricate patterns. forming the sand mould, casting, shot blasting, rough machining, heat maimncnt, final machining, priming. filling, undercoat painting, rub down and topcoat painting. These disadvantages have led researchers to look for alternative materials either to supplement or to completely replace cast iron for the fabrication of machine tool shuctures. Attempts have already been made to use mild steel weldments, granite, hydraulic cement concrete and polymer concrete with some degree of success (1-3).
tests were conducted on individual beds in their own entity. In order to have a realistic picture of the improvements that can be achieved by using a ferrwement bed,appropriate tests should be conducted on the lathe as a whole, and this has not been covered in the previous investigations (4-6). The present study is therefore aimed at investigating whether a lathe assembled with a ferrocement bed would lead to a dynamic performance superior to its cast iron counterpart.
2.
The prototype ferrwement bed fabricated earlier (6) has been employed to pursue the work further. The geometry and dimensions of this bed. as shown in Fig. I , were selected w enable it to be assembled in a Leblond Makino Regal 400 lathe. The slideways that were fixed to the bed during casting had only been given prior rough machining. The full details of the arrangement of reinforcement, mix proportions for the matrix and the method of fabrication can be found in Ref. 6.
As a part of the present investigation, this roughly machined, as-cast ferrocement bed was machined to the desired accuracy, the exposed fmocement surface was ground and painted, and the bed was assembled in
A number of investigations (4-6) had also been carried out at the National University of Singapore to look into the feasibility of replacing cast iron with other materials, particularly with ferrocement, a relatively new development in material technology. It is a "type of thin-wall reinforced concrete commonly constructed of hydraulic cement mortar reinforced with closely spaced layers of continuous and relatively small diameter wire mesh". Ferrocement differs from the conventional reinforced concrete in that it uses fine wire mesh rather than isolated heavy rods or bars, and employs sand instead of a mixture of sand and coarse aggregates for its concrete mix. The closer dishibution and uniform dispersion of reinforcement in ferrocemnt transform the otherwise brittle mortar into a high performance material with regard to cracking, tensile strength. ductility and impact resistance. It also offers good damping, a property highly desired in the machine tool industry, thus justifying its selection as a replacement material.
In these investigations, the prototype bed for a centre lathe was fabricated using ferrocemnt as the basic material. The fcrrocement bed was nearly identical to the parent cast iron bed as far as the geometry and functional requirements were concemed. It was designed to accommodate all the auxiliaries of the existing lathe. However, the slideways were fabricated from cast iron, and later bolted and bonded (by epoxy) to the ferrocement bed for subsequent tests to evaluate its performance. Static tests carried out on both the new and the parent cast iron beds (7) indicate that the ferrocement bed is dimensionally stable and has adequate suength and stiffness to justify its use in the machine tools industry. An experimental modal analysis technique, substantiated by a finite element analysis (8) had been used to assess the dynamic performance of the bed. Dynamic tests revealed that ferrocement is a better material in terms of dynamic performance. It has high natural frequencies as well as high damping ratios. The first resonance for the ferrocement bed occurred at a frequency almost twice of that for the cast iron bed with damping values of similar magnitude. The results of these investigations thus indicate that the replacement of traditional cast iron by ferrocement, a relatively new material provides a dimensionally stable structure with adequate static strength and stiffness and results in a dramatic improvement in dynamic characteristics. However, these
Annals of the CIRP Vol. 42/1/7993
FERROCEMENT BED AND THE LATHE
Fig. 1 Geometry and dimensions of the femcement lathe bed a prototype Leblond Makino Regal 40 lathe. All these operations were carried out by Makino Asia Be. Ltd., a machine tool builder in Singapore, as part of an industrial collaboration with the National University of Singapore. The completed prototype lathe incorporating the ferrocement bed is shown in Fig.2.
Excitation and cutting tests were performed on this lathe as well as the parent Leblond Makino Regal 400 lathe containing a cast iron bed to assess the improvements resulted from the use of ferrocement bed. The technical specifications for the Regal 400 lathe are given in Table 1.
3.
TESTS FOR PERFORMANCE EVALUATION
The two identical centre lathe machines, one assembled with a fenocemnt bed and the other with a cast iron bed were used for comparative evaluation. The dynamic performance tests selected would determine the merits of one machine over the other on the basis of its susceptibility (or its resistance) to chatter, a common machine tool phenomenon (or rather a problem), the generation of which increases tool wear and power consumption,
437
A three-jaw scroll chuck of nominal site 300 mm was used. Chucking force was applied by an operator using the chuck-handle and the force was maintained at a constant value by applying maximum force at all the three jaws. 'how-away carbide tip Sandvick SNMG 120408 inserts with approach angle of 45-deg. were used. A new cutting edge was used for every cut. Spindle speeds were maintained at 654 rpm with a feed rate of 0.1 millimetre per revolution. A piezoelecuic type three-component dynamometer (by KISTLER) was used to measure the three components of a cutting force. Signals from the dynamometer were recorded into a tape. recorder and subsequently played back and analyzed through a (Fast-Fourier Transform) spechum analyzer.
Excitation Tests
Fig. 2 Prototype centre lathe with ferrocement bed
An overall view of the setup for excitation tests is shown in Fig. 4. It consists of a spectrum analyzer. ON0 S O W CF-910. an impact hammer (PCB Model K291-A), a force transducer (Model 8200 with amplifier), an accelerometer (Model 4367 with amplifier), a Hewlett Packard Desk Top Computer ,216 Series 2000 and Stability Analysis Software Package (Modal 3.0 by Smctural Measurement System Inc.).
Table 1: TECHNICAL SPECIFICATIONS OF REGAL 4(M Swing over bed (dia) Swing over cross slide (dia) Hole through spindle Typc of spindle nose (std) Type of spindle drive Spindle drive rating Spindle spced range (std) Feed range (MMPR) Thread range (nun pitch) Centre distance (mm)
390mm 240mm 57 mm L1 or D1 AC. duty rated 5 KW 45 -1800 RPM 0.05 to 2.6 0.25 to 12 760,1370 Fig. 4 Setup for excitation tests
resulting in poor product quality and limiting the rate of production. They were developed on the basis of chatter theories that the generation of chatter is largely influenced by the depth of cut, the breadth of cut and/or the dynamic characteristics of the machine-tool-workpiece system (9-1 1). The resistance of chatter can be determined either by directly measuring the limiting depth of cut at which a chatter commences or by indirectly measuring the dynamic characteristics of the machine tool concerned. These tests are broadly referred to as cutting test (direct test) and excitation or vibration test (indirect receptance measurement test).
The objectives of these experiments are to determine (a) the natural frequencies, and (b) damping ratios for various modes of vibration. e.g., the first and second torsional modes, and he first and second modes in both vertical and horizontal bending. 4.
RESULTS AND DISCUSSION
Cutting Tests Cutting Tests The procedure as suggested by Machine Tool Industries Research Association WTIRA] (12) has been followed for cutting tests. Tapered workpieces conforming to MTIRA specifications with dimensions as shown in Fig. 3 are used. These workpieces were made up of normalised carbon steel S45C.All workpieces were prepared from the same batch of material.
The results of cutting tests are presented in Table 2 in terms of the depth of cut of the workpiece at which chatter occurred, Detection of fmal chatter was made by hearing (by an experienced operator) whilst on-set of chatter was determined from the chatter marks in the workpiece.
Table 2: Results of cutting test
I
With cast iron bed
With fenocement bed
ofchatter vibration
mum
3
3.3
3.4
Average
3.3
3.4
vibration
3
4.3
5.3
4.6
5.3
Note: Approach angle = 45 deg.; Cutting speed = 80 d m i n : Feed rate = 0.1 W r e v . Fig. 3 Dimensions of the workpieces used in cutting tests
438
The above results indicate very clearly that the lathe with ferrocement bed is a more superior machine compared to the one with conventional cast iron bed as far as its dynamic performance is concerned. The Lathe machine with ferrocement bed can accommodate deeper cut before chatter takes place. Another interesting observation can be made if we look at the results during chatter. The ferrocement machine tool allows funher cut before encountering the violent vibration (final chatter), whereas there is no clear distinction between the on-set and final chatter for the case of machine with cast iron bed.
31
Figures 5 and 6 show the spectrum plots for the cutting force in the two orthogonal directions during cutting for the lathe with ferrocement. Figs. 7 and 8 represent the same spectrum plots for the parent lathe with cast iron bed. In both cases, the horizontal cutting force components were significant.
00
100 X : 210.50 200 HZ
300 Y : 0.065L LOO V
500Hz
Frequency Fig. 8 Specmm of horizontal cutting force (Cast iron)
al
In the case of ferrocement bed lathe, there are two peak amplitudes in both horizontal and vertical components of the cutting force. In both the cases, the 33 Hz peak appears to have come from the vibration of three times spindle speed at 654 rpm (i.e. 11 Hz) whereas the second peak is the resonance vibration of the structural system (workpiece-chuck-spindle). A similar trend is observed in the case of cast iron machine as well at 654 rpm.
U
3 + .-
e
d
Q
0 0
100 200 X : 308.75 HZ
300
LOO
500Hz
Y : 0.0130V
Frequency
Thus, it can be concluded from the frequency analysis of the cutting force that the frequency and damping of the fist resonance for the cast iron bed lathe is lower than those for the ferrocement bed lathe. These results are quite similar to those observed in the study of the dynamic behaviour of the beds alone (7). Excitation Tests
Fig. 5 Spectrum of vertical cutting force (Ferrocement)
The results of the excitation tests conducted on the lathe with fmocement as well as well as the cast iron beds are summarized in Table 3. The modes of vibration. and the corresponding natural frequencies and
Table 3: Natural frequencies and damping ratios of the lathes with fmocement and cast iron bed.
"0
100 200 X : 308.75 HZ
300
LOO
500Hz
Y : 0.0983V
Frequency Fig. 6 Spectrum of horizontal cutting force (Ferrocement)
Note: Tor: Torsional; H.B.: Horizontal bending; V.B.:Vertical bending
damping ratios were obtained by using the Modal 3.0 software (8).
"0
100 ZOO X : 210.50 HZ
300
100 Y : 0.03eav
500Hz
From the results presented in Table 3, it may be observed that the resonance frequency at first torsional mode of vibration for the lathe with ferrocement bed is significantly higher than the corresponding cast iron lathe. Similar improvements may also be observed in terms of damping values. These results substantiate the results of the cutting tests.
Frequency 5.
Fig. 7 Spectrum of vertical cutting force (Cast iron)
SUMMARY AND CONCLUSIONS
In this paper. a new prototype lathe has been developed with the primary objective of enhancing its dynamic performance. It is identical in every
439
respect to a conventional prototype lathe except that the cast iron bed has been replaced by a ferrocement counterpart. Cutting and excitation tests were carried out on the new as well as the conventional lathes to evaluate its performance. The results of these tests as presented and discussed in this paper may be summarized as follows:
3.
Heitmann. H. and Venkatraman, V.. Investigations on the use of prestressed and reinforced concrete as a material for machine tool sbuctures, Roc. of the 4th lMTDR Conference, IIT Madras, Dec. 1970, pp. 287-299.
(a) Using identical tapered workpieces and cumng conditions, the new lathe offers an approximately 50% deeper cut than a conventional lathe before chatter vibration initiates.
4.
Rahman, M.. Mansur. M.A. and Chua. C.H.. Evaluation of Advanced Cementitious Composites for Machine Tool Suuctures. Annual of CIRF', 3711, 1988, pp. 373-376.
(b) Workpieces cut in the lathe with ferrocement bed contain clear markings indicating on-set of chatter and final chatter. No such clear makings are left in the workpieces cut in the conventional lathe.
5.
Mansur, M. A,, and Rahman, M.. Ferrocernent as a Machine Tool Bed, Journal of Ferrocement, l8(l). 1989. pp. 19-28.
6.
Rahman, M., Mansur,M.A. and Ambrose, W. D., Design, Fabrication and Performance of a Ferrocement Machine Tool Bed, International Journal of Machine Tools and Manufacture. 27(4). 1987, pp. 431-442.
7.
Chua, K.H.. Rahrnan. M. and Mansur, M.A., Performance Evaluation of Machine Tool Sauctures Using Modal Analysis, International Journal of Analytical and Experimental Techniques in Modal Analysis, 2(1), 1987. pp. 43-49.
8.
The MTRA. A Dynamic Performance Test for Lathes, 1971.
9.
Merritt, H.E., Theory of Self-Excited Machine Tool Chatter, Contribution to Machine Tool Chatter Research 1. Journal of Engineering for Industry, Trans. of ASME. B. 87(1965), 447.
(c) Frequency analyses of the cutting force during chatter indicate higher damping values and natural frequencies for the lathe with ferrocement bed as compared to the conventional one. (d) Similarly, as revealed by the elicitation tests, the lathe with a ferrocement bed exhibits a significantly higher resonance frequency and higher damping values, especially in the torsional mode, than its cast iron counterpart From the results of these tests, it may be concluded that the replacement of the conventional cast iron bed by a ferrocement counterpart for a centre lathe results in a superior machine in terms of overall dynamic performance.
6.
ACKNOWLEDGEMENT
The authors would like to express their sincere thanks to Makino Asia Pte. Ltd for their help and cooperation in this research project
10. KoenigsberW. F. and musty. J., Machine Tool Structure, vol. 1,
Macmillan, London. 11. Tobias, S.A. and Fishwick, W.. The Chatter of Lathe Tools under Orthogonal Cutting Conditions, T ~ sASME, . VOI. 80, 1958, pp. 1079-
1088. 7. REFERENCES
1.
Morgan, G.H.. McKeon. P.A. and H.J. Renker. Materials for Machine Tool Structures, Proc., 20th International Machine Tool Design and Research Conference, Macmillan Press, 1980. pp. 429-434.
2.
Oviawe, S.E.. Fay. T.and Shumsheruddm. A.A., Development of machine tool structure using composite synthetic granite, 23rd MTDR Conference, Macmillan Press. 1983, p. 31-37.
440
12. "MODAL 3.0 OPERATING MANUAL", Structural Measurement System Inc., 1984.
In an earlier study, the feasibility of using ferrocement as a replacement to cast iron in the manufacture of machine tool smctures was investigated by fabricating a prototype ferrocement bed for a centre lathe and testing the bed alone. Improved performance exhibited by the ferrocement bed in comparison with the parent cast iron bed under both static and dynamic loadings has inspired the authors to pursue the work and assess the bed in terms of the performance of the machine as a whole. A centre lathe was chosen for this purpose. The traditional cast iron bed in the machine was replaced by a ferrocement bed, designed and fabricated for accommodating all the necessary parts and attachments. The method of fabrication and installation of the bcd are described in this paper. Identical vibration and cutting tests were performed in the original as well as the modified machines. Test results indicate that the machine with ferrocement bed provides signikantly higher damping and natural frequencies, specially the torsional frequency, and results in an approximately 40% deeper cut than the original lathe before the onset of c h a m . Keywords: Machine tools, ferrocement bed, centre lathe. cutting tests. performance evaluation, vibration tests 1.
INTRODUCTION
The structure of a machine tool forms the vital link between the cutting tool and the workpiece for a metal cutting machine or between the measuring probe and the workpiece for an inspection machine. Therefore, the machining accuracy of a job. its time of production and the related cost depend largely on the performance of various suuctural components. The basic properties engineers and designers generally look for in a machine tool include, among others, a high static stiffness against bending and torsion, a good dynamic performance and a long-term dimensional stability. Cast iron, which has been the most commonly used material for this purpose since early days possesses these characteristics to an acceptable l i m i L But it has some inherent disadvantages; the primary ones being the high cost. poor torsional rigidity and damping, and the difficulty in producing the finished product. The lengthy production process consists of fabricating the intricate patterns. forming the sand mould, casting, shot blasting, rough machining, heat maimncnt, final machining, priming. filling, undercoat painting, rub down and topcoat painting. These disadvantages have led researchers to look for alternative materials either to supplement or to completely replace cast iron for the fabrication of machine tool shuctures. Attempts have already been made to use mild steel weldments, granite, hydraulic cement concrete and polymer concrete with some degree of success (1-3).
tests were conducted on individual beds in their own entity. In order to have a realistic picture of the improvements that can be achieved by using a ferrwement bed,appropriate tests should be conducted on the lathe as a whole, and this has not been covered in the previous investigations (4-6). The present study is therefore aimed at investigating whether a lathe assembled with a ferrocement bed would lead to a dynamic performance superior to its cast iron counterpart.
2.
The prototype ferrwement bed fabricated earlier (6) has been employed to pursue the work further. The geometry and dimensions of this bed. as shown in Fig. I , were selected w enable it to be assembled in a Leblond Makino Regal 400 lathe. The slideways that were fixed to the bed during casting had only been given prior rough machining. The full details of the arrangement of reinforcement, mix proportions for the matrix and the method of fabrication can be found in Ref. 6.
As a part of the present investigation, this roughly machined, as-cast ferrocement bed was machined to the desired accuracy, the exposed fmocement surface was ground and painted, and the bed was assembled in
A number of investigations (4-6) had also been carried out at the National University of Singapore to look into the feasibility of replacing cast iron with other materials, particularly with ferrocement, a relatively new development in material technology. It is a "type of thin-wall reinforced concrete commonly constructed of hydraulic cement mortar reinforced with closely spaced layers of continuous and relatively small diameter wire mesh". Ferrocement differs from the conventional reinforced concrete in that it uses fine wire mesh rather than isolated heavy rods or bars, and employs sand instead of a mixture of sand and coarse aggregates for its concrete mix. The closer dishibution and uniform dispersion of reinforcement in ferrocemnt transform the otherwise brittle mortar into a high performance material with regard to cracking, tensile strength. ductility and impact resistance. It also offers good damping, a property highly desired in the machine tool industry, thus justifying its selection as a replacement material.
In these investigations, the prototype bed for a centre lathe was fabricated using ferrocemnt as the basic material. The fcrrocement bed was nearly identical to the parent cast iron bed as far as the geometry and functional requirements were concemed. It was designed to accommodate all the auxiliaries of the existing lathe. However, the slideways were fabricated from cast iron, and later bolted and bonded (by epoxy) to the ferrocement bed for subsequent tests to evaluate its performance. Static tests carried out on both the new and the parent cast iron beds (7) indicate that the ferrocement bed is dimensionally stable and has adequate suength and stiffness to justify its use in the machine tools industry. An experimental modal analysis technique, substantiated by a finite element analysis (8) had been used to assess the dynamic performance of the bed. Dynamic tests revealed that ferrocement is a better material in terms of dynamic performance. It has high natural frequencies as well as high damping ratios. The first resonance for the ferrocement bed occurred at a frequency almost twice of that for the cast iron bed with damping values of similar magnitude. The results of these investigations thus indicate that the replacement of traditional cast iron by ferrocement, a relatively new material provides a dimensionally stable structure with adequate static strength and stiffness and results in a dramatic improvement in dynamic characteristics. However, these
Annals of the CIRP Vol. 42/1/7993
FERROCEMENT BED AND THE LATHE
Fig. 1 Geometry and dimensions of the femcement lathe bed a prototype Leblond Makino Regal 40 lathe. All these operations were carried out by Makino Asia Be. Ltd., a machine tool builder in Singapore, as part of an industrial collaboration with the National University of Singapore. The completed prototype lathe incorporating the ferrocement bed is shown in Fig.2.
Excitation and cutting tests were performed on this lathe as well as the parent Leblond Makino Regal 400 lathe containing a cast iron bed to assess the improvements resulted from the use of ferrocement bed. The technical specifications for the Regal 400 lathe are given in Table 1.
3.
TESTS FOR PERFORMANCE EVALUATION
The two identical centre lathe machines, one assembled with a fenocemnt bed and the other with a cast iron bed were used for comparative evaluation. The dynamic performance tests selected would determine the merits of one machine over the other on the basis of its susceptibility (or its resistance) to chatter, a common machine tool phenomenon (or rather a problem), the generation of which increases tool wear and power consumption,
437
A three-jaw scroll chuck of nominal site 300 mm was used. Chucking force was applied by an operator using the chuck-handle and the force was maintained at a constant value by applying maximum force at all the three jaws. 'how-away carbide tip Sandvick SNMG 120408 inserts with approach angle of 45-deg. were used. A new cutting edge was used for every cut. Spindle speeds were maintained at 654 rpm with a feed rate of 0.1 millimetre per revolution. A piezoelecuic type three-component dynamometer (by KISTLER) was used to measure the three components of a cutting force. Signals from the dynamometer were recorded into a tape. recorder and subsequently played back and analyzed through a (Fast-Fourier Transform) spechum analyzer.
Excitation Tests
Fig. 2 Prototype centre lathe with ferrocement bed
An overall view of the setup for excitation tests is shown in Fig. 4. It consists of a spectrum analyzer. ON0 S O W CF-910. an impact hammer (PCB Model K291-A), a force transducer (Model 8200 with amplifier), an accelerometer (Model 4367 with amplifier), a Hewlett Packard Desk Top Computer ,216 Series 2000 and Stability Analysis Software Package (Modal 3.0 by Smctural Measurement System Inc.).
Table 1: TECHNICAL SPECIFICATIONS OF REGAL 4(M Swing over bed (dia) Swing over cross slide (dia) Hole through spindle Typc of spindle nose (std) Type of spindle drive Spindle drive rating Spindle spced range (std) Feed range (MMPR) Thread range (nun pitch) Centre distance (mm)
390mm 240mm 57 mm L1 or D1 AC. duty rated 5 KW 45 -1800 RPM 0.05 to 2.6 0.25 to 12 760,1370 Fig. 4 Setup for excitation tests
resulting in poor product quality and limiting the rate of production. They were developed on the basis of chatter theories that the generation of chatter is largely influenced by the depth of cut, the breadth of cut and/or the dynamic characteristics of the machine-tool-workpiece system (9-1 1). The resistance of chatter can be determined either by directly measuring the limiting depth of cut at which a chatter commences or by indirectly measuring the dynamic characteristics of the machine tool concerned. These tests are broadly referred to as cutting test (direct test) and excitation or vibration test (indirect receptance measurement test).
The objectives of these experiments are to determine (a) the natural frequencies, and (b) damping ratios for various modes of vibration. e.g., the first and second torsional modes, and he first and second modes in both vertical and horizontal bending. 4.
RESULTS AND DISCUSSION
Cutting Tests Cutting Tests The procedure as suggested by Machine Tool Industries Research Association WTIRA] (12) has been followed for cutting tests. Tapered workpieces conforming to MTIRA specifications with dimensions as shown in Fig. 3 are used. These workpieces were made up of normalised carbon steel S45C.All workpieces were prepared from the same batch of material.
The results of cutting tests are presented in Table 2 in terms of the depth of cut of the workpiece at which chatter occurred, Detection of fmal chatter was made by hearing (by an experienced operator) whilst on-set of chatter was determined from the chatter marks in the workpiece.
Table 2: Results of cutting test
I
With cast iron bed
With fenocement bed
ofchatter vibration
mum
3
3.3
3.4
Average
3.3
3.4
vibration
3
4.3
5.3
4.6
5.3
Note: Approach angle = 45 deg.; Cutting speed = 80 d m i n : Feed rate = 0.1 W r e v . Fig. 3 Dimensions of the workpieces used in cutting tests
438
The above results indicate very clearly that the lathe with ferrocement bed is a more superior machine compared to the one with conventional cast iron bed as far as its dynamic performance is concerned. The Lathe machine with ferrocement bed can accommodate deeper cut before chatter takes place. Another interesting observation can be made if we look at the results during chatter. The ferrocement machine tool allows funher cut before encountering the violent vibration (final chatter), whereas there is no clear distinction between the on-set and final chatter for the case of machine with cast iron bed.
31
Figures 5 and 6 show the spectrum plots for the cutting force in the two orthogonal directions during cutting for the lathe with ferrocement. Figs. 7 and 8 represent the same spectrum plots for the parent lathe with cast iron bed. In both cases, the horizontal cutting force components were significant.
00
100 X : 210.50 200 HZ
300 Y : 0.065L LOO V
500Hz
Frequency Fig. 8 Specmm of horizontal cutting force (Cast iron)
al
In the case of ferrocement bed lathe, there are two peak amplitudes in both horizontal and vertical components of the cutting force. In both the cases, the 33 Hz peak appears to have come from the vibration of three times spindle speed at 654 rpm (i.e. 11 Hz) whereas the second peak is the resonance vibration of the structural system (workpiece-chuck-spindle). A similar trend is observed in the case of cast iron machine as well at 654 rpm.
U
3 + .-
e
d
Q
0 0
100 200 X : 308.75 HZ
300
LOO
500Hz
Y : 0.0130V
Frequency
Thus, it can be concluded from the frequency analysis of the cutting force that the frequency and damping of the fist resonance for the cast iron bed lathe is lower than those for the ferrocement bed lathe. These results are quite similar to those observed in the study of the dynamic behaviour of the beds alone (7). Excitation Tests
Fig. 5 Spectrum of vertical cutting force (Ferrocement)
The results of the excitation tests conducted on the lathe with fmocement as well as well as the cast iron beds are summarized in Table 3. The modes of vibration. and the corresponding natural frequencies and
Table 3: Natural frequencies and damping ratios of the lathes with fmocement and cast iron bed.
"0
100 200 X : 308.75 HZ
300
LOO
500Hz
Y : 0.0983V
Frequency Fig. 6 Spectrum of horizontal cutting force (Ferrocement)
Note: Tor: Torsional; H.B.: Horizontal bending; V.B.:Vertical bending
damping ratios were obtained by using the Modal 3.0 software (8).
"0
100 ZOO X : 210.50 HZ
300
100 Y : 0.03eav
500Hz
From the results presented in Table 3, it may be observed that the resonance frequency at first torsional mode of vibration for the lathe with ferrocement bed is significantly higher than the corresponding cast iron lathe. Similar improvements may also be observed in terms of damping values. These results substantiate the results of the cutting tests.
Frequency 5.
Fig. 7 Spectrum of vertical cutting force (Cast iron)
SUMMARY AND CONCLUSIONS
In this paper. a new prototype lathe has been developed with the primary objective of enhancing its dynamic performance. It is identical in every
439
respect to a conventional prototype lathe except that the cast iron bed has been replaced by a ferrocement counterpart. Cutting and excitation tests were carried out on the new as well as the conventional lathes to evaluate its performance. The results of these tests as presented and discussed in this paper may be summarized as follows:
3.
Heitmann. H. and Venkatraman, V.. Investigations on the use of prestressed and reinforced concrete as a material for machine tool sbuctures, Roc. of the 4th lMTDR Conference, IIT Madras, Dec. 1970, pp. 287-299.
(a) Using identical tapered workpieces and cumng conditions, the new lathe offers an approximately 50% deeper cut than a conventional lathe before chatter vibration initiates.
4.
Rahman, M.. Mansur. M.A. and Chua. C.H.. Evaluation of Advanced Cementitious Composites for Machine Tool Suuctures. Annual of CIRF', 3711, 1988, pp. 373-376.
(b) Workpieces cut in the lathe with ferrocement bed contain clear markings indicating on-set of chatter and final chatter. No such clear makings are left in the workpieces cut in the conventional lathe.
5.
Mansur, M. A,, and Rahman, M.. Ferrocernent as a Machine Tool Bed, Journal of Ferrocement, l8(l). 1989. pp. 19-28.
6.
Rahman, M., Mansur,M.A. and Ambrose, W. D., Design, Fabrication and Performance of a Ferrocement Machine Tool Bed, International Journal of Machine Tools and Manufacture. 27(4). 1987, pp. 431-442.
7.
Chua, K.H.. Rahrnan. M. and Mansur, M.A., Performance Evaluation of Machine Tool Sauctures Using Modal Analysis, International Journal of Analytical and Experimental Techniques in Modal Analysis, 2(1), 1987. pp. 43-49.
8.
The MTRA. A Dynamic Performance Test for Lathes, 1971.
9.
Merritt, H.E., Theory of Self-Excited Machine Tool Chatter, Contribution to Machine Tool Chatter Research 1. Journal of Engineering for Industry, Trans. of ASME. B. 87(1965), 447.
(c) Frequency analyses of the cutting force during chatter indicate higher damping values and natural frequencies for the lathe with ferrocement bed as compared to the conventional one. (d) Similarly, as revealed by the elicitation tests, the lathe with a ferrocement bed exhibits a significantly higher resonance frequency and higher damping values, especially in the torsional mode, than its cast iron counterpart From the results of these tests, it may be concluded that the replacement of the conventional cast iron bed by a ferrocement counterpart for a centre lathe results in a superior machine in terms of overall dynamic performance.
6.
ACKNOWLEDGEMENT
The authors would like to express their sincere thanks to Makino Asia Pte. Ltd for their help and cooperation in this research project
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