- Email: [email protected]
Automatic Chip-Breaking Detection in Turning by Frequency Analysis of Cutting Force
Automatic Chip-Breaking Detection in Turning by Frequency Analysis of Cutting Force J. L. Andreasen, L. De Chiffre (2). Institute of Manufacturing Engineering, Technical University of Denmark Received o n January 13,1993
Abstract: An automatic system for chip breaking detection in turning has been developed for use in the laboratory. The system utilises a detection technique based on frequency analysis of the dynamic feed force component. The ability to identify chip breaking has been demonstrated using different lathes, cutting tools, workpiece materials and cutting data.
Kev Words: Turning, Chip control, Cutting forces.
Introduction Efficient chip control in operations which tend to produce long continuous chips like turning, drilling and tapping is associated with problems due to a general lack of rules for predicting chip breaking and to changes in chip breakability due to variations of the conditions.
In connection with the planning of a specific machining operation it is necessary to have access to data about chip breakability for the different cutting tools, workpiece materials and cutting data. Process variations resulting from tool wear, non uniformity of workpiece material, thermal effects, etc. cause variations in the chip form during a machining operation. Therefore continuous monitoring of the produced chips is needed to avoid unacceptable chip forms. Many efforts have been put into this very important topic within the frame of CIRP 11-31. Different chip breaking detection systems have been presented in the past. There are systems based on cutting forces 141, acoustic emission /5/ and upon infrared radiation 161. This paper presents a detection system based upon Fourier analysis of the feed force as measured by a tool force dynamometer 17-91. Cutting forces and chip breaking The forces acting on the cutting tool are influenced by the parameters involved in the chip formation process i.e.
- Workpiece material - Tool geometry - Cutting fluid - Depth of cut
Figure 1 shows a typical force-time curve during the first workpiece revolutions in a longitudinal turning operation. The resulting cutting force FREscan be divided into three components FA,F, and F,. Looking at this figure it is evident that after the transient zone small variations are present in the cutting force. These variations are caused by variations in the machine-toolworkpiece-system. The sources can be one or more of the following:
- Variations in machine tool parameters: feed drive instabilities, cutting force instabilities and variations in the dynamic compliance of the machine tool.
- Variations in workpiece parameters: geometrical variations e.g.
Annals of the CIRP Vol. 42/1/1993
Force)
z.
3-
9
2-
c4
8
1-
-1 4 F
(Radial Force)
2
0.5
1
1.5
Time (s)
Fig 1 Cutting force versus time in longitudinal turning of carbon steel.
- Variations in process parameters: variations in friction between tool and workpiece caused by variations in the cutting fluid supply or caused by tool wear.
- Variations in chip formation: built-up-edgeformation, chip segmentation and chip breaking. The nature of these sources can either be periodic or random and can have a more or less pronounced interference with each other. The amplitude of these variations depends upon the dynamic behaviour of the machine tool performing the cutting. In order to use the cutting force as a tool for detecting chip breaking, it is therefore necessary to separate or suppress the influences from other sources using a suitable signal analysis.
- Feed - Cutting speed
diameter variations or not centred inhomogeneous workpiece material.
F,(Tangential t
4-
clamping,
or
Each time a chip is broken the force will vary either due to breakage in the shear zone or due to chip hammering upon the tool shank. Successive well broken chips have nearly the same length, in other words the chip breaking frequency is nearly constant over small time records. In a Fourier analysis, the feed force spectrum from conditions with well broken chips will have a broad gaussian peak while a spectrum from conditions with snarled or ribbon type chips will display no distinct peak. Figure 2 shows the distribution of 100 chips collected from one experiment 171. The abscissa displays the chip breaking frequency. The same distribution of amplitudes can be seen in the spectrum obtained from the cutting force signal. The use of spectrum analysis gives therefore the opportunity to detect whether or not chips are broken, and at which frequency they are broken. Figure 3 shows two force spectrums obtained when chips are broken and when chips are not broken 181.
45
Fig 3 Spectrums for experiments with and without broken chips./8/ (Material: C35 steel, Speed: 250 m/min)
System description A block diagram of the chip breaking detection system developed by the authors for laboratory use is shown in figure 4. The cutting force is measured by the use of a specially designed tool force dynamometer build for mounting on the turret of a CNC lathe (figure 5). The transducers used in this dynamometer are four piezo-electric three axis load cells from Kistler. This choice together with the use of a titanium alloy tool holder fixture, gives a useable dynamic range from 0-2000Hz /lo/,although limited by resonance at 1300 Hz in the lathe.
Fig 4 Block diagram for chip breaking detection system. by chip breaking. In all the experiments presented here a Xthreshold of 40 Hz (5spectrum lines) was found suitable.
For .chip breaking detection only the feed force component FA is measured. The amplified feed force signal is band-pass filtered using a fourth order Butterworth filter in order to increase the resolution of the A/D conversion by suppressing the DC and resonant components. The signal is then input to a Bruel & Kjar 2032 signat analyzer to be A/D converted and Fast Fourier transformed in time sequences lasting 125 ms (frequency range 0-6.4 KHz). To reduce the influences from random events, the feed force spectrum is averaged from 20 instant spectrums. The averaging process is performed with a max overlap, consequently reducing the time for averaging to about 800 ms. Once created, the averaged spectrum is read out into a PCcomputer and automatically checked for chip breaking using a detection algorithm. The output from the algorithm is either a Chip breaking or a Chip breaking cannot be detected command, as the final step of the detection cycle. By letting the signal analyzer perform the averaging while the PC performs the check, a total detection cycle lasting less than 1 second obtained. There are two variables in the detection algorithm at present. One is the here named X-threshold, which is a lower limit for the width of a peak which can be considered as a peak caused
46
Fig 5 Cutting force dynamometer mounted on CNC-lathe. The other variable is the Y-threshold which is derived from the mean force by a multiplication factor. The mean value is calculated as a least square line of the amplitudes for all the frequency components. In all the experiments presented here the same Y-threshold: 1.2 was used.
Experimental Investigation The detection system has been tested using four different workpiece materials, two different lathes, with and without cutting fluid, using different cutting inserts and different cutting data. Table 1 summarises the experimental conditions. Machining experiments were carried out in longitudinal turning on a CNC-lathe using standard throw-away inserts which were
recommended by the suppliers. The inserts were all triangular, coated carbides and provided with various chip breaker shapes. Experiments were performed dry. A smaller number of experiments were repeated with a water based lubricant to prove that the principle of detection is not influenced by the presence of a cutting fluid. Some of the experiments were repeated on a conventional lathe.
Fig 6 Chip form classification
In all the experimentsa standard 25 mm square tool holder was used giving a cutting edge angle of approximately 90".The workpieces were cylindrical bars ranging from 70 to 130 mm diameter with a length of 600-700 mm. The workpieces were held in a 3-jaw chuck and supported by a live centre at the other end. For each experiment the feed force was measured and backed up on a Bruel & Kjaer type 7006 analog tape recorder, and chips were collected.
5
-
T
A EM : 6.2 x
4
Mixed :Arc + Ear
x Long : 1 3 2.1 2,3 4.3
E E
-e3
-
0
0
f2
+
0
n
+
+
H
Z
1 Z
Results and discussion The different chips were classified as either favourable or unfavourable and furthermore categorisedaccording to the I S 0 3685 standard annex G 1111 (figure 6 ) . The mean chip breaking frequency was calculated from the average weight of the chips collected from each experiment. Results from representative experiments are shown in figures 7 through 10 in the form of chip breaking diagrams with the feed as abscissa and the depth of cut as ordinate. The thin line in the diagrams displays the limit between favourable and not favourable chips as obtained by visual observation and referring to figure 6. The thick line displays the limit line as obtained automatically from the detection system, meaning that to the right and above this line the system will report Chip breaking. Conversely the system will report Chip breaking cannot be detected to the left and below this thick line. If the system worked ideally the two lines would be identical. This is not the case in any of the experiments. A transitional zone where the system is not able to detect chip breaking is present.
0
_
'
01
Z
Z
I
02
X
X
03
X
05
0 4
0.6
Feed (mm/rev)
Fig 7 Chip breaking diagram for C35 steel. (V=250 m/min, Tool: Sumitomo AC25)
*I
, +
Am: 6,2 1 A Ear:6,2
4 ':
x
Mixed : Arc+Ear
x Long : 1 3 2,l 2,3 4,3
h
E E
y3 3
./iX
Automatic Limit +
+
+
0 A.-
0
0. I
0.2
0.3
;
I I
+
d 0.4 0.5 06
Feed (mm/rev)
Fig 8 Chip breaking diagram for Ck45 steel. (V = 250 m/min, Tool: Sumifomo AC 70)
47
Conclusions A laboratory system for automatic chip breaking detection has been developed and tested. The system is able to detect chip breaking in different workpiece materials under different cutting conditions. The use of frequency analysis and an algorithm searching for a broad peak all over the frequency range makes the system reliable robust because random instances, e.g. long chip tangling with the tool, cannot be mistaken as chip breaking. Therefore the detection system is always on the safe side. At present a limitation of this technique is that chip breaking cannot be detected by the system when the chips are broken into quite different lengths. In all the experiments presented the same threshold limits were found suitable, which is a great advantage regarding the development trend towards FMS where the time for running-in i.e. estimation of threshold limits for each operation is limited. -
41 h
x"
x
0
1 0
0.1
4
1
Connected Arc :6,l Helical Shoti :4.2 x Mixed :6,1+4,2+4,3 x Long : I .3 2.3 4,3 5.1 y
~
(-
-V i a l Limit
van Luttervelt, C.A. et al.: Developments and trends in monitoring and control of machining processes. Keynote Paper, Annals of the CIRP Vol. 37/21, 1988, p. 61 1-622.
-A 0.3 0.4 0.5 0.6
-
0.2
Feed (mm/rev)
Fig 10 Chip breaking diagram for 1.4301 stainless steel. (V= 180 m/min, Tool: Seco TP15 MF2) In cutting C35 carbon steel and alloyed steel 34NiCrMo6, figure 7 and 9, this transitional zone seems to be present because the system is not able to detect ear shaped broken chips. Figure 11 shows the difference between arc and ear shaped chips. The ear shaped chips obtained in these experiments were broken into different lengths, therefore giving no detectable peak in the feed force spectrum. In cutting Ck45 carbon steel (figure 8) the ear shaped chips obtained were more uniform and therefore the system was able to detect these.
van Luttervelt, C.A. et al.: Recent developments in chip control reasearch and applications. Draft for CIRP-STC-C Keynote Paper, 1993. Druminski, R., Mainusch, M.: Sensor zur automatischen Spanbruckerkennungbeim Drehen. Zwf 74/1 1979. p. 918.
Yee, K.W.: Material dependency of chip-form detection using Acoustic Emission. 14'th NAMRC, 1986, p. 458462. Krause, W. et al.: lnfrarotsensor zur Spanbrucherkennung. Feingeratetechnik, Berlin 34 1985, 3,p. 99-100. Poulsen, J.S.: Monitoring of chip disposal in turning. M.Sc. thesis, MM-publ. 87.23, Technical University of Denmark, 1987. (In Danish)
Fig 11 a) 6.2 &c chips, b) 6.2 Ear Chips.
When cutting 1.4301 austenitic stainless steel (figure lo), the transitional zone was also present. In this case because of the presence of mixed chip forms, e.g. connected arc chips with different lengths and broken and not broken chips together. This again makes the peak in the force spectrum to disappear. However it can be seen in figure 7-10 that in none of the experiments the automatically determined limit crosses the visual limit. In other words the automatic system is always on the safe side (unfavourable chips are not being mistaken as broken chips) which indeed makes it very suitable for monitoring purpose.
48
References Kluft, W. et al.: Present knowledge of chip control. Keynote Paper, Annals of the ClRP Vol. 28/21, 1979, p. 441-455.
Andreasen, J.L.: Chip breaking detection using spectrum analysis. MSc. thesis, MM-pub1 89.02, Technical University of Denmark, 1989. (In Danish) Ibsen, F.M.: Computer software for chip breaking detection. M.Sc. thesis, MM-pub1 89.17, Technical University of Denmark, 1989. (In Danish) Hoffmann, J., Pedersen, K.B.: Design and calibration of dynamometer for CNC lathe. 14'th NAMRC 1986, p. 183188.
IS0 3685: Tool-life testing with single-point turning tools. First edition 1977, Annex G, p. 41.
Abstract: An automatic system for chip breaking detection in turning has been developed for use in the laboratory. The system utilises a detection technique based on frequency analysis of the dynamic feed force component. The ability to identify chip breaking has been demonstrated using different lathes, cutting tools, workpiece materials and cutting data.
Kev Words: Turning, Chip control, Cutting forces.
Introduction Efficient chip control in operations which tend to produce long continuous chips like turning, drilling and tapping is associated with problems due to a general lack of rules for predicting chip breaking and to changes in chip breakability due to variations of the conditions.
In connection with the planning of a specific machining operation it is necessary to have access to data about chip breakability for the different cutting tools, workpiece materials and cutting data. Process variations resulting from tool wear, non uniformity of workpiece material, thermal effects, etc. cause variations in the chip form during a machining operation. Therefore continuous monitoring of the produced chips is needed to avoid unacceptable chip forms. Many efforts have been put into this very important topic within the frame of CIRP 11-31. Different chip breaking detection systems have been presented in the past. There are systems based on cutting forces 141, acoustic emission /5/ and upon infrared radiation 161. This paper presents a detection system based upon Fourier analysis of the feed force as measured by a tool force dynamometer 17-91. Cutting forces and chip breaking The forces acting on the cutting tool are influenced by the parameters involved in the chip formation process i.e.
- Workpiece material - Tool geometry - Cutting fluid - Depth of cut
Figure 1 shows a typical force-time curve during the first workpiece revolutions in a longitudinal turning operation. The resulting cutting force FREscan be divided into three components FA,F, and F,. Looking at this figure it is evident that after the transient zone small variations are present in the cutting force. These variations are caused by variations in the machine-toolworkpiece-system. The sources can be one or more of the following:
- Variations in machine tool parameters: feed drive instabilities, cutting force instabilities and variations in the dynamic compliance of the machine tool.
- Variations in workpiece parameters: geometrical variations e.g.
Annals of the CIRP Vol. 42/1/1993
Force)
z.
3-
9
2-
c4
8
1-
-1 4 F
(Radial Force)
2
0.5
1
1.5
Time (s)
Fig 1 Cutting force versus time in longitudinal turning of carbon steel.
- Variations in process parameters: variations in friction between tool and workpiece caused by variations in the cutting fluid supply or caused by tool wear.
- Variations in chip formation: built-up-edgeformation, chip segmentation and chip breaking. The nature of these sources can either be periodic or random and can have a more or less pronounced interference with each other. The amplitude of these variations depends upon the dynamic behaviour of the machine tool performing the cutting. In order to use the cutting force as a tool for detecting chip breaking, it is therefore necessary to separate or suppress the influences from other sources using a suitable signal analysis.
- Feed - Cutting speed
diameter variations or not centred inhomogeneous workpiece material.
F,(Tangential t
4-
clamping,
or
Each time a chip is broken the force will vary either due to breakage in the shear zone or due to chip hammering upon the tool shank. Successive well broken chips have nearly the same length, in other words the chip breaking frequency is nearly constant over small time records. In a Fourier analysis, the feed force spectrum from conditions with well broken chips will have a broad gaussian peak while a spectrum from conditions with snarled or ribbon type chips will display no distinct peak. Figure 2 shows the distribution of 100 chips collected from one experiment 171. The abscissa displays the chip breaking frequency. The same distribution of amplitudes can be seen in the spectrum obtained from the cutting force signal. The use of spectrum analysis gives therefore the opportunity to detect whether or not chips are broken, and at which frequency they are broken. Figure 3 shows two force spectrums obtained when chips are broken and when chips are not broken 181.
45
Fig 3 Spectrums for experiments with and without broken chips./8/ (Material: C35 steel, Speed: 250 m/min)
System description A block diagram of the chip breaking detection system developed by the authors for laboratory use is shown in figure 4. The cutting force is measured by the use of a specially designed tool force dynamometer build for mounting on the turret of a CNC lathe (figure 5). The transducers used in this dynamometer are four piezo-electric three axis load cells from Kistler. This choice together with the use of a titanium alloy tool holder fixture, gives a useable dynamic range from 0-2000Hz /lo/,although limited by resonance at 1300 Hz in the lathe.
Fig 4 Block diagram for chip breaking detection system. by chip breaking. In all the experiments presented here a Xthreshold of 40 Hz (5spectrum lines) was found suitable.
For .chip breaking detection only the feed force component FA is measured. The amplified feed force signal is band-pass filtered using a fourth order Butterworth filter in order to increase the resolution of the A/D conversion by suppressing the DC and resonant components. The signal is then input to a Bruel & Kjar 2032 signat analyzer to be A/D converted and Fast Fourier transformed in time sequences lasting 125 ms (frequency range 0-6.4 KHz). To reduce the influences from random events, the feed force spectrum is averaged from 20 instant spectrums. The averaging process is performed with a max overlap, consequently reducing the time for averaging to about 800 ms. Once created, the averaged spectrum is read out into a PCcomputer and automatically checked for chip breaking using a detection algorithm. The output from the algorithm is either a Chip breaking or a Chip breaking cannot be detected command, as the final step of the detection cycle. By letting the signal analyzer perform the averaging while the PC performs the check, a total detection cycle lasting less than 1 second obtained. There are two variables in the detection algorithm at present. One is the here named X-threshold, which is a lower limit for the width of a peak which can be considered as a peak caused
46
Fig 5 Cutting force dynamometer mounted on CNC-lathe. The other variable is the Y-threshold which is derived from the mean force by a multiplication factor. The mean value is calculated as a least square line of the amplitudes for all the frequency components. In all the experiments presented here the same Y-threshold: 1.2 was used.
Experimental Investigation The detection system has been tested using four different workpiece materials, two different lathes, with and without cutting fluid, using different cutting inserts and different cutting data. Table 1 summarises the experimental conditions. Machining experiments were carried out in longitudinal turning on a CNC-lathe using standard throw-away inserts which were
recommended by the suppliers. The inserts were all triangular, coated carbides and provided with various chip breaker shapes. Experiments were performed dry. A smaller number of experiments were repeated with a water based lubricant to prove that the principle of detection is not influenced by the presence of a cutting fluid. Some of the experiments were repeated on a conventional lathe.
Fig 6 Chip form classification
In all the experimentsa standard 25 mm square tool holder was used giving a cutting edge angle of approximately 90".The workpieces were cylindrical bars ranging from 70 to 130 mm diameter with a length of 600-700 mm. The workpieces were held in a 3-jaw chuck and supported by a live centre at the other end. For each experiment the feed force was measured and backed up on a Bruel & Kjaer type 7006 analog tape recorder, and chips were collected.
5
-
T
A EM : 6.2 x
4
Mixed :Arc + Ear
x Long : 1 3 2.1 2,3 4.3
E E
-e3
-
0
0
f2
+
0
n
+
+
H
Z
1 Z
Results and discussion The different chips were classified as either favourable or unfavourable and furthermore categorisedaccording to the I S 0 3685 standard annex G 1111 (figure 6 ) . The mean chip breaking frequency was calculated from the average weight of the chips collected from each experiment. Results from representative experiments are shown in figures 7 through 10 in the form of chip breaking diagrams with the feed as abscissa and the depth of cut as ordinate. The thin line in the diagrams displays the limit between favourable and not favourable chips as obtained by visual observation and referring to figure 6. The thick line displays the limit line as obtained automatically from the detection system, meaning that to the right and above this line the system will report Chip breaking. Conversely the system will report Chip breaking cannot be detected to the left and below this thick line. If the system worked ideally the two lines would be identical. This is not the case in any of the experiments. A transitional zone where the system is not able to detect chip breaking is present.
0
_
'
01
Z
Z
I
02
X
X
03
X
05
0 4
0.6
Feed (mm/rev)
Fig 7 Chip breaking diagram for C35 steel. (V=250 m/min, Tool: Sumitomo AC25)
*I
, +
Am: 6,2 1 A Ear:6,2
4 ':
x
Mixed : Arc+Ear
x Long : 1 3 2,l 2,3 4,3
h
E E
y3 3
./iX
Automatic Limit +
+
+
0 A.-
0
0. I
0.2
0.3
;
I I
+
d 0.4 0.5 06
Feed (mm/rev)
Fig 8 Chip breaking diagram for Ck45 steel. (V = 250 m/min, Tool: Sumifomo AC 70)
47
Conclusions A laboratory system for automatic chip breaking detection has been developed and tested. The system is able to detect chip breaking in different workpiece materials under different cutting conditions. The use of frequency analysis and an algorithm searching for a broad peak all over the frequency range makes the system reliable robust because random instances, e.g. long chip tangling with the tool, cannot be mistaken as chip breaking. Therefore the detection system is always on the safe side. At present a limitation of this technique is that chip breaking cannot be detected by the system when the chips are broken into quite different lengths. In all the experiments presented the same threshold limits were found suitable, which is a great advantage regarding the development trend towards FMS where the time for running-in i.e. estimation of threshold limits for each operation is limited. -
41 h
x"
x
0
1 0
0.1
4
1
Connected Arc :6,l Helical Shoti :4.2 x Mixed :6,1+4,2+4,3 x Long : I .3 2.3 4,3 5.1 y
~
(-
-V i a l Limit
van Luttervelt, C.A. et al.: Developments and trends in monitoring and control of machining processes. Keynote Paper, Annals of the CIRP Vol. 37/21, 1988, p. 61 1-622.
-A 0.3 0.4 0.5 0.6
-
0.2
Feed (mm/rev)
Fig 10 Chip breaking diagram for 1.4301 stainless steel. (V= 180 m/min, Tool: Seco TP15 MF2) In cutting C35 carbon steel and alloyed steel 34NiCrMo6, figure 7 and 9, this transitional zone seems to be present because the system is not able to detect ear shaped broken chips. Figure 11 shows the difference between arc and ear shaped chips. The ear shaped chips obtained in these experiments were broken into different lengths, therefore giving no detectable peak in the feed force spectrum. In cutting Ck45 carbon steel (figure 8) the ear shaped chips obtained were more uniform and therefore the system was able to detect these.
van Luttervelt, C.A. et al.: Recent developments in chip control reasearch and applications. Draft for CIRP-STC-C Keynote Paper, 1993. Druminski, R., Mainusch, M.: Sensor zur automatischen Spanbruckerkennungbeim Drehen. Zwf 74/1 1979. p. 918.
Yee, K.W.: Material dependency of chip-form detection using Acoustic Emission. 14'th NAMRC, 1986, p. 458462. Krause, W. et al.: lnfrarotsensor zur Spanbrucherkennung. Feingeratetechnik, Berlin 34 1985, 3,p. 99-100. Poulsen, J.S.: Monitoring of chip disposal in turning. M.Sc. thesis, MM-publ. 87.23, Technical University of Denmark, 1987. (In Danish)
Fig 11 a) 6.2 &c chips, b) 6.2 Ear Chips.
When cutting 1.4301 austenitic stainless steel (figure lo), the transitional zone was also present. In this case because of the presence of mixed chip forms, e.g. connected arc chips with different lengths and broken and not broken chips together. This again makes the peak in the force spectrum to disappear. However it can be seen in figure 7-10 that in none of the experiments the automatically determined limit crosses the visual limit. In other words the automatic system is always on the safe side (unfavourable chips are not being mistaken as broken chips) which indeed makes it very suitable for monitoring purpose.
48
References Kluft, W. et al.: Present knowledge of chip control. Keynote Paper, Annals of the ClRP Vol. 28/21, 1979, p. 441-455.
Andreasen, J.L.: Chip breaking detection using spectrum analysis. MSc. thesis, MM-pub1 89.02, Technical University of Denmark, 1989. (In Danish) Ibsen, F.M.: Computer software for chip breaking detection. M.Sc. thesis, MM-pub1 89.17, Technical University of Denmark, 1989. (In Danish) Hoffmann, J., Pedersen, K.B.: Design and calibration of dynamometer for CNC lathe. 14'th NAMRC 1986, p. 183188.
IS0 3685: Tool-life testing with single-point turning tools. First edition 1977, Annex G, p. 41.