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Tool wear sensors
Wear, 62 (1980) 49 - 57 0 Elsevier Sequoia S.A. Lausanne - Printed in the Netherlands
49
TOOL WEAR SENSORS*
N. H. COOK Massachusetts Institute of Technology,
Cambridge, Massachusetts (U.S.A.)
(Received December 28,1979)
Summary A state of the art review of tool wear sensing is presented. A recently developed technique is described and the need for further research effort in tool wear sensing is emphasized.
1. Introduction As we strive for higher productivity through more sophisticated controls for metal cutting machines and through greater automation of manufacturing systems, it becomes more and more desirable to be able to sense the condition of the cutting tools automatically. In this paper the state of tool wear sensing is reviewed and a recently developed technique is discussed. Tool wear sensors (TWS) can provide the information necessary for two quite different sets of decision. (1) Decisions relative to adaptive control in which feeds and speeds are continuously varied in order to achieve optimum (cost or production rate) cutting conditions at all times. For this purpose a TWS must provide continuous on-line ~fo~ation relative to wear rate or performance degradation .
(2) Decisions relative to tool-change timing and strategies. Here relatively little information is needed, i.e. it is desirable to know when a tool has progressed through 50%, 90% or 100% of its useful life. It is essential to know if a tool has failed. This type of information would be particularly useful in the operation of standard transfer lines and flexible m~ufac~r~g systems. Tool failure can be classified as follows: temperature failure where the tool temperature becomes high enough to permit gross plastic deformation at the cutting edge; fracture, either gross or by chipping, caused by fatigue or excessive forces; *Paper presented at the Symposium on Cutting Tools and Wear-related Phenomena, Lausanne, Switzerland, September 3 - 4, 19’79.
50
gradual wear of the tool faces causing gradual degradation followed by failure. For purposes of adaptive control the tool must wear gradually. For toolchange purposes failure of any type and some measure of wear progress must be sensed. Figure 1 shows the cross section of a typical worn tool where there are two major wear zones: a wear land on the relief face and a crater on the cutting face. 2. Sensing techniques Over the years many methods have been developed for tool wear sensing, none of which has achieved significant use in industry. TWS can be classified into two major groups: direct TWS where the actual tool wear is measured; indirect TWS where a parameter is measured that is correlated in some way with tool wear. 2.1. Direct methods 2.1.1. Measurement of tool geometry Changes in the geometry of the cutting tool or grinding wheel have been measured by direct mechanical gaging [ 1 - 31, profile tracers [ 4 - 61, weighing [ 71, ultrasonics [ 81, optical [ 91 and pneumatic [ 2, 10,111 methods. These tend to be off-line measurements which are not easily implemented as an automatic TWS. 2.1.2. Optical scanning Various methods have been developed, many of which are very rapid, but most must be used between cutting cycles rather than on-line [6, 12 - 201. 2.1.3. Workpiece size change Except in the case of actual failure, change of workpiece size is clearly a most important quantity which relates to part quality as well as tool wear. These methods are generally used for cylindrical surfaces (external and internal) [19, 21 - 261. The most extensive use is in grinding. 2.1.4. Measurement of distance from tool post to workpieces In a turning operation the work surface generally moves toward the tool post as wear progresses. There are inherent errors due to other causes of such motion, i.e. force, temperature and vibration. Methods involve contact measurement [ 161, ultrasonics [ 271 and air gages [ 2,3,28] . 2.1.5. Electrical techniques Various resistance measurements include measuring the contact resistance between tool and work [29,30], application of film resistors to the tool flank [27,31] and a resistive tool wear follower [ 271 in which a resistive pad wears as the tool wears.
51
2.1.6. Radioactive techniques Various methods using radioactivity have been explored; most are slow not particularly safe off-line methods [32 - 351. Generally a radioactive tool is used and the activity transferred to the chips is monitored. The microisotope method [36] in which exceedingly small quantities of activity (lo-’ Ci) are used will be discussed later. 2.1.7. Analysis of wear particles on the chips Various off-line methods have been developed for detecting tool wear debris in the chips without using radioactive tools. These methods include chemical analysis [ 311, electron microprobe analysis [ 37, 381 and radioactivation of chips.
2.2. Indirect methods The indirect measurements that have been related to tool wear include measurement of forces, vibration and sound, power input and temperature. 2.2.1. Increased force torque A great deal of work has been carried out relating forces and torques to tool wear [ 16,17,39 - 501 and ratios of force components to wear [43, 51531. In most (not all) cases forces do increase as wear progresses. However, forces also increase with variations in work hardness, depth of cut and cutting speed. Accordingly, forces per se are sensitive to more than wear. However, the ratio of two force components appears to be a fairly good indicator of wear and of failure. Clearly this would require force dynamometers capable of resolving two or more components. 2.2.2. Mechanical/vibration/sound analysis Sound and vibration analysis can provide a tool signature which will vary as wear progresses. In general the signature analysis will be specific to a given set-up and is difficult to generalize. Most methods involve spectral analysis and changes therein [ 21,42, 54 - 641. 2.2.3. Variation of power input As tools wear forces rise and therefore the electrical power input must also rise. Clearly measurement of power input tends to be less sensitive than force measurement but is easier to implement. Both magnitude and the spectral density of the power have been used [42,47,65 - 691. 2.2.4. Temperature measurement Tool temperature or tool/chip interface temperature will frequently increase somewhat as the tool wears. Perhaps more important, the tool wear rate is frequently a very strong function of interface temperature. Numerous investigations have been made [ 1, 57, 70 - 791.
52
3. Microisotope tool wear sensors A simple and safe method has been developed at the ~assachu~tts Institute of Technology for determining whether or not a tool has reached some predefined state of wear [36]. The method involves attaching or implanting a small quantity of radioactive material to the flank of the tool as shown in Fig. 1. At the end of each cutting cycle the tool is monitored with a Geiger-Muter tube to determine whether the spot is still there. When no signal is received (a) the wear land has progressed beyond the spot, (b) the tool has failed or (c) the sensing system has failed. Clearly this is a fail-safe method in which a null signal initiates action. There are many potential methods for producing a small radioactive spot (about 0.001 in in diameter) with an activity of lOPa - 1OP8 Ci; the one developed is shown in Fig_ 2. An activated tungsten wire (0.001 in in diameter) on which a thin layer of copper is plated is used as an electrode in a spark discharge process.
Fig. 1. Typical wear geometry due to gradual tool wear. Fig. 2. Schematic view of implantation method.
The best results were obtained when two successive voltage pulses were applied a few m~liseconds apart. The first pulse creates a small pit in the surface as in electro-discharge machining and welds the tungsten to the tool. The second pulse melts the weld leaving a small quantity of active material firmly attached to the tool. The purpose of the copper sheath is to provide a path of high thermal and electrical conductivity such that the second pulse will melt the weld rather than “exploding” the wire. Figure 3 shows a typical set of data obtained in a drilling operation where the entire sensing process was under microprocessor control. This process is currently undergoing field test and development. 4. Conclusion It is clear that although work on sensing has been extensive none of the methods has achieved commercial success. Most methods are not reliable and/ or costly, The need for TWS grows; there is great opportunity for invention.
53
15
.
DETECTABLE
A
BACKGROUND
D
FLANK
SOURCE
WEAR
IO
1
COUNTING RATE
FLANK WEAR (.COI IN.1
(C/SEC)
5
0
I
I
5
IO
JO
Fig. 3. Results of test 2: source counting rate, background counting rate and flank wear measurements us. number of holes drilled.
References*
5 6
7 8 9 10 11 12
S. S. Silin et al., Automatic control of the cutting process, Mach. Tool. (USSR), 42 (1). T. H. Stoferle and B. Bellman, Verschleissensoren fur Adaptive Regezungen bei der Drehbearbeitung, Werkstatt Betr., 105 (8) (1972) 577 - 582. K. Essel and W. Hansel, Development of sensors for process control systems in the field of production engineering, PDV-Rep. KFK-PDV 41, Gesellschaft fur Kernforschung mbH, Karlsruhe, May 1975. J. Deutsch, S. M. Wu and C. M. Straklowski, A new irregular surface measuring system, Int. J. Mach. Tool Des. Res., 13 (1973) 29 - 42. M. C. Shaw, P. A. Smith and N. H. Cook, Free machining steel: 1 - Tool life characteristic of resulfurized steel, Trans. Am. Sot. Mech. Eng. B, 83 (2) (1961) 163 - 193. E. Y. C. Sun and W. B. Heginbotham, A method of assessing the progress and behavior of wear patterns of tungsten carbide tools and some possibilities for interpretation, Microtecnic, 9 (2) (1957) 71 - 75. H. Takeyama and R. Murata, Basic investigation of tool wear, Am. Sot. Mech. Eng. Pap. 61-WA-92. W. Konig, W. Eversheim and M. Week, Aachener Werkzeugmaschinen - Kolloquiums, Aachen, June 1974, pp. 1679 - 1710. D. A. Gall, Adaptive control of the abrasive cut-off #operation, Ann. CZRP, 17 (1969) 395 - 399. T. Sata, T. Suto, T. Waida and H. Noguchi, In process measurements of the grinding process and its applications, Proc. Znt. Grinding Conf., 1972 - 1974, pp. 752 - 770. B. N. Colding, A Wear Relationship for Turning, Milling and Grinding, Machining Economics, Royal Institute of Technology, Stockholm, 1959. R. N. Meyer and S. M. Wu, Optical contour mapping of cutting tool crater wear, Znt. Mach. Tool Des. Res., 6 (1966) 153 - 170.
*References 80 - 103 are not cited specifically in the text but contain information of general interest.
54 13 K. C. Tsao, A. B. Hussain and S. M. Wu, Cutting tool crater wear measurement by optical comparator technique, Znt. J. Mach. Tool Des. Res., 8 (1968) 15 - 26. 14 A. M. Besuyen, The measurement of the grinding wheel wear with the Quantimet image analysing computer, Ann. CZRP, 19 (1971) 619 - 624. 15 S. Myers and S. Q. A. M. A. Hossain, Logic circuitry and vidicon measure displayed areas, Electron. Eng., (Feb. 1971) 65 _68. 16 H. Takeyama, Y. Doi, T. Mitsuoka and H. Sekiguchi, Sensors of tool life for optimization of machining advances in machine tool design and research, Proc. 8th Znt. MTDR Conf., Pergamon Press, New York, 1967. 17 K. Yamazaki, A. Yamada and S. Swami, A study on adaptive controi in an N/C milling machine, 24th General Assembly of CZRP, Kyoto, Japan, 1974. 18 K. Essel, Optimierung des Drehens durch Prozessrechnereinsatz, VDI -Z., I15 (8) (1973) 667 - 671. 19 S. Ueno, Characteristics of displacement transducers using optical fibers, Reprints: Joint Conf. of JSPE and JSME. ZZokurrikuShinetsu, No. 717-1, 1971 - 1979, p. 109. 20 S. Oboshi et at., Machining Data Acquisition System for Turning Toots, Toshiba Tungaloy Company, Japan. 21 J. Frisch and P. F. Mahr, On the use of analog digital conversion during DNC-metal cutting, Proc. North American Metal Working Research Conf., Ontario, 1973. 22 R. Tillen, Controlling workpiece accuracy, Conf. Significant Advances in Machining Technology, Cranfiekd, Sept. 1964, pp. 499 - 515. 23 T. Sade et al., In-process measurement for A/C of machines, Jpn Sot. Precis. Eng., 38 (10) (Aug. 1972) 788 - 795. 24 S. Ueno, In-process measurement in Europe and U.S., Jpn SOC. Precis. Eng., 10 (1972) 803 - 807. 25 H. R. Untenwoldt, Air gaging shows greatest progress, Microtecnic, 22 (5) (1968). 26 J. G. Wiatt, Automatic in-process machine control gaging, ASME Pap. 63”PROD.-17, 1963. 27 G. Spur and F. Leonards, Kontinuierlich Arbeitende Verschleissensoren fiir ACOSysteme bei der Drehbearbeitung, ZwF 68 10, pp. 619 ff., 1973. 28 M. Bath and R. Sharp, In-process controf of lathe improves accuracy and productivity, Advances in MTDR, Part 2, 1968 - 69, pp. 1209 - 1221. 29 A. J. Wilkinson, Constriction-resistance concept applied to wear measurement of metal-cutting tools, Proc. Inst. Electr. Eng., 118 (2) (Feb. 1971). 30 A. A. Zakaria and Y. I. El Gomayel, Tool condition monitoring for adaptive control, SME Pap. MR74-143. 31 K. Uehara, New attempts for short time tool life testing, Ann. CIRP, 22 (1) (1973). 32 B. Colding and L. G. Erwall, Wear studies of irradiated carbide cutting tools, Nucleonics, 11 (2) (1953) 46 - 49. 33 N. H. Cook and A. B. Lang, Criticism of radioactive tool life testing, Trans. Am. SOC. Mech. Eng. B, 85 (5) (1963). 34 G. Lune and P. B. Anderson, A study of the wear processes of cemented carbide cutting tools by a radioactive tracer technique, Znt. J. Mach. Tool Des. Res., 10 (1970) 79 - 93. 35 M. E. Merchant, H. Ernst and E. J. Krabaeher, Radioactive cutting tools for rapid tool-life testing, Trans. ASME, (May 1953). 36 N. H. Cook and K. Subramanian, Micro-isotope tool wear sensor, Ann. CZRP, (1978) 73 - 78. 37 I. Ham, Fundamentals of tool wear, Am. Sot. Test. Mater. Pap. MR68-617. 38 K. Uehara, On the measurement of crater wear of carbide cutting tools, Ann. CZRP, 21 (1) (1972). 39 L. V. Colwell, Methodsforsensingrate of tool wear,Ann. CIRF, 19 (1971) 647 - 651. 40 A. DeFilippi and R. Ippolito, Analysis of the correlation among: cutting force variation (us. time) - chip formation parameters - machinability, Ann. CZRP, 21 (1) (1972).
55 41 A. DeFilippi and R. Ippolito, Adaptive control in turning: cutting forces and tool wear relationships for PlO, PZO, P30 carbides, Ann. CIRP, 17 (1969) 377 - 385. 42 M. Gaudreau, An on-line technique for tool wear measurement, M. S. Thesis, Massachusetts Inst. Technol., June 1975. 43 F. K. Groszmann and A. V. Hemming, Problems involved in the development of adaptive techniques to improve the productivity of N/C lathes, Advances in MTDR, Proc. 11 th MTDR Conf., Birmingham, 1970. 44 W. Konig, K. Langhammer and H. V. Schemmel, Correlations between cutting force components and tool wear, Ann. CZRP, 21 (1) (1972). 45 G. F. Micheletti, A. DeFilippi and R. Ippolito, Tool wear and cutting forces in steel turning, Ann. CIRP, 16 (1968) 353 - 360. 46 P. Montes, Flank wear of carbide cutting tools, S.M. Thesis, Massachusetts Inst. Technol., July 1972. 47 K. Subramanian, Sensing drill wear and prediction of drill life, S.M. and M.E. Thesis, Massachusetts Inst. Technol., June 1974. 48 H. Takeyama, H. Sekiguchi and K. Takada, Direktmessungen in der Metallbearbeitung, Werkstatt Betr. 106 (9) (1973) 709 - 714. 49 K. Taraman, R. Swando and W. Yamauchi, Relationships between tool forces and flank wear, SME Pap. MR74-704. 50 H. R. Eisengrein, personal communication, 1974. 51 L. V. Colwell, Tracking tool deterioration by computer (during actual machining), Ann. CARP, 23 (1) (1974) 29 - 30. 52 L. V. Colwefl, J. C. Mazur and J. Angel& Progress Report, Di~nostic Sensing in ~aehini~g Operations, Project 320357, Univ. Michigan, May 1975. 53 E. Sakai, N. Yoda and K. Ogura, Research on tool wear sensing, JSME 740-I 5, Nov. 1974, pp. 209-212. 54 A. Edwin and T. Vlach, A new approach to tool wear monitoring, Proc. 27th Annu. Conf. and Exhibition of Instrument Society of America, Oct. 1972. 55 C. F. Micheletti, S. Rossetto and M. Ponti, Tool vibration pattern and tool life on automatic screw machine, Advances in MTDR, Vol. A, 1970, pp. 145 - 159. 56 R. C. Miller and A. Sagar, Analysis of machining sounds and its potential application to adaptive control, Cutting Tool Eng., (Feb. 1973). 57 N. F. Shillam, The on-line control of cutting conditions using direct feedback, Proc. 12th Int. MTDR Conf., Manchester, Sept. 1971, Pap. 801. 58 R. Thompson, Surface grinding vibrations and their use in grinder performance evaluation, SME Pap. MR71-274. 59 E. J. Weller and B. Welchbrodt, Listen to your tools - Tbey’re talking to you, Am. Sot. Test. Mater. Pap. MR67444. 60 E. J. Weller, H. M. Schrier and B. Welchbrodt, What sound can be expected from a worn tool, J. Eng. Znd., (Aug. 1969) 524 - 534. 61 F. W. Young, An investigation of available signals for adaptive control of machine tools, S.M. Thesis, Massachusetts Inst. Technol., May 1970. 62 A process measuring wear on a drilling tool, U.S. Patent 3,714,822 (1973), to J. Lutz. 63 Machining stability detector, U.S. Patent 3,550,107, to R. A. Thompson, S. E. Grabkowski and B. Weichbrodt. 64 Sonic worn cutting tool detector, U.S. Patent 3,548,648, to B. Weichbrodt and S. E. Grabkowski. 65 S. J. Bailey, “Energy signature” measures system changes, Control Eng., (Oct. 1973). 66 Adaptive control with the Valenite horse-power monitor-controller, Valeron Corporation. 67 Method and apparatus for monitoring condition of cutting blades, U.S. Patent 3,809,870 (1974). to R. E. Auble and G. J, Kimmet. 68 S. Malkin, personal communication, Nov. 1974. 69 G. S. Reichenbach, personal communication, Oct. 1974.
56 70 B. R. Beadle and J. G. Bollinger, Computer adaptive control of a machine tool, Ann. CIRP, 19 (1971) 61- 65. 71 R. A. Billet& Studies of cutting temperature control applied to a lathe spindle speed, Advances in MTDR, 1968, pp. 1273 - 1287. 72 L. V. Colwell, Progress Report on Temperature Sensing, Diagnostics Sensing in Machining Operations, Project 320357, Univ. Michigan, April 1975. 73 M. P. Groover and G. E. Kane, A continuing study in the determination of temperatures in metal cutting using remote thermocouples, Trans. Am. Sot. Mech. Eng., (May 1971) 603 - 608. 74 J. R. Jaeschke, R. D. Zimmerly and S. M. Wu, Automatic cutting tool temperature control, fnt. Mach. Tool Des. Res., 7 (1967) 465 - 475. 75 D. R. Olberts, A study of the effect of tool flank wear on tool-chip interface temperatures, Trans. ASME, (May 1959) 152 - 158. 76 V. Solaja and Vukelja, Identification of tool wear rate by temperature variation, Ann. CIRP, 22 (1) (1973) 117 - 119. 77 Y. M. Solomentsev et al., Using adaptive control of tool wear to optimize machining processes, Mach. Tool. (USSR), 8 (1974) 35 - 38. 78 J. D. Welch, Automatic feedrate regulation in numerically controlled contour milling, Rep. 8436-R-1, Massachusetts Inst. Technol., Cambridge, Mass., June 1960. 79 K. Gulbrandsen, personal communication, Aug. 1974. 80 N. Akgerman and J. Frisch, The use of a cutting force spectrum for tool wear compensation during turning, 12th Int. MTDR Co&, 1971, pp. 1 _ 9. 81 W. D. Cain, Automatic tool setters, SME Pap. MS. 72-629. 82 R. M. Centner and J. M. Idelsohn, Application of Adaptive Control to Manufacturing Processes, Bendix Research Lab., Michigan. 83 M. P. Groover, Adaptive control machining: current problems and future solutions, SME Pap. MS-72-131. 84 0. Hake, Die Verwendong von Radioisotipen bei Zerspavungsuntersuchungen, Dr. Thesis, Technisehe Hochschule Aachen, 1957. 85 R. F. Hill and M. Skunda, Measuring tool wear, Am. Sot. Meeh. Eng. Pap. 71-DE-42. 86 H. Lefer, Sound waves control drive speed, Power Transm. Des., (Sept. 1970) 44 45. 87 R. A. Mathias, Adaptive controlled profile milling, SME Pap. MS70-563. 88 M. Mozond, Adaptive control for turning operations, CIRP, 4th Znt. Seminar on Optimization of Manufacturing Systems, June 1972. 89 E. P. Nadenskaya, Investigation of the wear of cutting tools using radioactive isotopes, Izv. Acad. Sci., USSR. 90 H. Opitz, Verschleissuntersuehungen beim Drehen mit Radioaktiuen ~urtmeta~~werkzeugen, College International pour 1’Etude Scientifique des Techniques de Production Mecanique, March 1952. 91 E. Ratterman, A new approach to diamond wheel wear measurement, Cutting Tool Eng., (June 1971) 22 - 23. 92 The sabre blunt cutter monitoring, Mach. Prod. Eng., 119 (1971) 117 - 119. 93 G. F. Wilson, Study of radiometric method and use of the liquid scintillation technique for tool wear determination, J. Eng. fnd., (Feb. 1965). 94 P. C. Aebersold, Industrial applications of radioisotopes, Am. Sot. Mech. Eng. Pap. 60-WA-20, 1960. 95 A. Bhattacharyya and I. Ham, Design of Cutting Tools - Use of Metal Cutting Theory, Am. Sot. Tool Manuf. Eng., Dearborn, Mich., 1969. 96 J. T. Burwell and C. 0. Strang, Radioactive tracers reveal friction and wear of materials, Met. Prog., (Sept. 1951). 97 N. H. Cook, Tool wear and tool life, Am. Sot. Mech. Eng. Pap. 72-WA/PROD.-19, 1972. 98 N. H. Cook, Manufacturing Analysis, Addison-Wesley, Reading, Mass., 1966. 99 M. F. DeVries, Measuring carbide tool wear, SME Pap. I&71-921, 1971.
57 100
V. V. Kaminskaya, Trands in the development of adaptive control, Mach. Tool. (USSR), 45 (8) (1974). 101 S. Popov, Comparing two methods for measuring tool wear, Nucleonics, (Dec. 1961). 102 G. Spur and G. Pritschow, Adaptive control an spanenden Werkzeugmaschinen, VDZ-Ber. (Ver. Dtsch. Zng.), 166 (1971) 67 - 73. 103 G. Stute and G. Augsten, Adaptive control system for turning lathes, SME Pap. MS72-127, 1972.
49
TOOL WEAR SENSORS*
N. H. COOK Massachusetts Institute of Technology,
Cambridge, Massachusetts (U.S.A.)
(Received December 28,1979)
Summary A state of the art review of tool wear sensing is presented. A recently developed technique is described and the need for further research effort in tool wear sensing is emphasized.
1. Introduction As we strive for higher productivity through more sophisticated controls for metal cutting machines and through greater automation of manufacturing systems, it becomes more and more desirable to be able to sense the condition of the cutting tools automatically. In this paper the state of tool wear sensing is reviewed and a recently developed technique is discussed. Tool wear sensors (TWS) can provide the information necessary for two quite different sets of decision. (1) Decisions relative to adaptive control in which feeds and speeds are continuously varied in order to achieve optimum (cost or production rate) cutting conditions at all times. For this purpose a TWS must provide continuous on-line ~fo~ation relative to wear rate or performance degradation .
(2) Decisions relative to tool-change timing and strategies. Here relatively little information is needed, i.e. it is desirable to know when a tool has progressed through 50%, 90% or 100% of its useful life. It is essential to know if a tool has failed. This type of information would be particularly useful in the operation of standard transfer lines and flexible m~ufac~r~g systems. Tool failure can be classified as follows: temperature failure where the tool temperature becomes high enough to permit gross plastic deformation at the cutting edge; fracture, either gross or by chipping, caused by fatigue or excessive forces; *Paper presented at the Symposium on Cutting Tools and Wear-related Phenomena, Lausanne, Switzerland, September 3 - 4, 19’79.
50
gradual wear of the tool faces causing gradual degradation followed by failure. For purposes of adaptive control the tool must wear gradually. For toolchange purposes failure of any type and some measure of wear progress must be sensed. Figure 1 shows the cross section of a typical worn tool where there are two major wear zones: a wear land on the relief face and a crater on the cutting face. 2. Sensing techniques Over the years many methods have been developed for tool wear sensing, none of which has achieved significant use in industry. TWS can be classified into two major groups: direct TWS where the actual tool wear is measured; indirect TWS where a parameter is measured that is correlated in some way with tool wear. 2.1. Direct methods 2.1.1. Measurement of tool geometry Changes in the geometry of the cutting tool or grinding wheel have been measured by direct mechanical gaging [ 1 - 31, profile tracers [ 4 - 61, weighing [ 71, ultrasonics [ 81, optical [ 91 and pneumatic [ 2, 10,111 methods. These tend to be off-line measurements which are not easily implemented as an automatic TWS. 2.1.2. Optical scanning Various methods have been developed, many of which are very rapid, but most must be used between cutting cycles rather than on-line [6, 12 - 201. 2.1.3. Workpiece size change Except in the case of actual failure, change of workpiece size is clearly a most important quantity which relates to part quality as well as tool wear. These methods are generally used for cylindrical surfaces (external and internal) [19, 21 - 261. The most extensive use is in grinding. 2.1.4. Measurement of distance from tool post to workpieces In a turning operation the work surface generally moves toward the tool post as wear progresses. There are inherent errors due to other causes of such motion, i.e. force, temperature and vibration. Methods involve contact measurement [ 161, ultrasonics [ 271 and air gages [ 2,3,28] . 2.1.5. Electrical techniques Various resistance measurements include measuring the contact resistance between tool and work [29,30], application of film resistors to the tool flank [27,31] and a resistive tool wear follower [ 271 in which a resistive pad wears as the tool wears.
51
2.1.6. Radioactive techniques Various methods using radioactivity have been explored; most are slow not particularly safe off-line methods [32 - 351. Generally a radioactive tool is used and the activity transferred to the chips is monitored. The microisotope method [36] in which exceedingly small quantities of activity (lo-’ Ci) are used will be discussed later. 2.1.7. Analysis of wear particles on the chips Various off-line methods have been developed for detecting tool wear debris in the chips without using radioactive tools. These methods include chemical analysis [ 311, electron microprobe analysis [ 37, 381 and radioactivation of chips.
2.2. Indirect methods The indirect measurements that have been related to tool wear include measurement of forces, vibration and sound, power input and temperature. 2.2.1. Increased force torque A great deal of work has been carried out relating forces and torques to tool wear [ 16,17,39 - 501 and ratios of force components to wear [43, 51531. In most (not all) cases forces do increase as wear progresses. However, forces also increase with variations in work hardness, depth of cut and cutting speed. Accordingly, forces per se are sensitive to more than wear. However, the ratio of two force components appears to be a fairly good indicator of wear and of failure. Clearly this would require force dynamometers capable of resolving two or more components. 2.2.2. Mechanical/vibration/sound analysis Sound and vibration analysis can provide a tool signature which will vary as wear progresses. In general the signature analysis will be specific to a given set-up and is difficult to generalize. Most methods involve spectral analysis and changes therein [ 21,42, 54 - 641. 2.2.3. Variation of power input As tools wear forces rise and therefore the electrical power input must also rise. Clearly measurement of power input tends to be less sensitive than force measurement but is easier to implement. Both magnitude and the spectral density of the power have been used [42,47,65 - 691. 2.2.4. Temperature measurement Tool temperature or tool/chip interface temperature will frequently increase somewhat as the tool wears. Perhaps more important, the tool wear rate is frequently a very strong function of interface temperature. Numerous investigations have been made [ 1, 57, 70 - 791.
52
3. Microisotope tool wear sensors A simple and safe method has been developed at the ~assachu~tts Institute of Technology for determining whether or not a tool has reached some predefined state of wear [36]. The method involves attaching or implanting a small quantity of radioactive material to the flank of the tool as shown in Fig. 1. At the end of each cutting cycle the tool is monitored with a Geiger-Muter tube to determine whether the spot is still there. When no signal is received (a) the wear land has progressed beyond the spot, (b) the tool has failed or (c) the sensing system has failed. Clearly this is a fail-safe method in which a null signal initiates action. There are many potential methods for producing a small radioactive spot (about 0.001 in in diameter) with an activity of lOPa - 1OP8 Ci; the one developed is shown in Fig_ 2. An activated tungsten wire (0.001 in in diameter) on which a thin layer of copper is plated is used as an electrode in a spark discharge process.
Fig. 1. Typical wear geometry due to gradual tool wear. Fig. 2. Schematic view of implantation method.
The best results were obtained when two successive voltage pulses were applied a few m~liseconds apart. The first pulse creates a small pit in the surface as in electro-discharge machining and welds the tungsten to the tool. The second pulse melts the weld leaving a small quantity of active material firmly attached to the tool. The purpose of the copper sheath is to provide a path of high thermal and electrical conductivity such that the second pulse will melt the weld rather than “exploding” the wire. Figure 3 shows a typical set of data obtained in a drilling operation where the entire sensing process was under microprocessor control. This process is currently undergoing field test and development. 4. Conclusion It is clear that although work on sensing has been extensive none of the methods has achieved commercial success. Most methods are not reliable and/ or costly, The need for TWS grows; there is great opportunity for invention.
53
15
.
DETECTABLE
A
BACKGROUND
D
FLANK
SOURCE
WEAR
IO
1
COUNTING RATE
FLANK WEAR (.COI IN.1
(C/SEC)
5
0
I
I
5
IO
JO
Fig. 3. Results of test 2: source counting rate, background counting rate and flank wear measurements us. number of holes drilled.
References*
5 6
7 8 9 10 11 12
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