A Study of Tool Wear Using Statistical Analysis of Metal Cutting Acoustic Emission

levels predicted by the theoretical relation, we have extracted data from [10] sufficient to compute the AE generated in the primary deformation zone per the first term of (1). For this purpose, Zorev's data for machining plain medium-carbon steel (40 steel) with nominal rake of ten degrees was used. For given speed and feed, cutting ratio r c and average actual rake angle a a were scaled from the experimental curves and used to compute shear angle as cos a a 4> = arctan : (2) r c -sin a a Then a measure of the predicted acoustic emission in the primary zone was computed as [ cos en. ~1 ^ M>l'l-^-7 TT A 0) sm<£ cos(0 -a a ) J where K= C 4 T k ln has been taken as constant. Referring again to the data in Summary While it is difficult, if not impossible, to isolate a single effect in an oblique cutting operation, our experimental results provide strong evidence of a link between presence of a builtup edge and increased acoustic emission activity. We have hypothesized that the primary reason for this link is the increase in effective rake angle associated with the existence of a BUE. Calculated results using published experimental data in an existing theoretical relation appear to support the hypothesis. While the theoretical equation was developed for orthogonal Cutting, it is nevertheless reasonable to apply it here to qualitatively examine our explanation of the experimental results. Finally, we note that our use of equation (3) ignores acoustic emission in the secondary and tertiary zones. The existence of a BUE would generally increase AE energy from these sources, so their inclusion should further support the experimental evidence presented here. Further investigations are needed in order to better understand the BUE and acoustic emission relationship. It is planned to explore a wider range of workpiece materials, rake angle variations, and feeds. In future cutting experiments, comprehensive cutting force data will be obtained for correlation with AE measurements and BUE observations. Ultimately, it may be necessary to employ a quick-stop mechanism to quantify the built-up edge versus cutting conditions for inclusion in a process model. References 1 Iwata, K., and Moriwaki, T., "An Application of Acoustic Emission Monitoring to In-Process Sensing of Tool Wear," Annals of the CIRP, Vol. 26, No. 1, 1977, pp. 21-26. 2 Dornfeld, D. A., and Kannatey-Asibu, E" "Acoustic Emission During Orthogonal Metal Cutting," International Journal of Mechanical Science, Vol. 22, No. 5, 1980, pp. 285-296. 3 Kannatey-Asibu, E., and Dornfeld, D. A., "Quantitative Relationships for Acoustic Emission from Orthogonal Metal Cutting," ASME JOURNAL OF ENGINEERING FOR INDUSTRY, Vol. 103, 1981, pp. 330-340. 4 Kannatey-Asibu, E., and Dornfeld, D. A., "A Study of Tool Wear Using Statistical Analysis of Metal Cutting Acoustic Emission," Wear, Vol. 76, No. 2, 1982, pp. 247-261. 5 Moriwaki, T., "Detection for Cutting Tool Fracture by Acoustic Emission Measurement," Annals of the CIRP, Vol. 29, No. 1, 1980, pp. 35-40. 6 Diei, E. N., and Dornfeld, D. A., "Acoustic Emission Sensing of Tool Wear in Face Milling," ASME JOURNAL OF ENGINEERING FOR INDUSTRY, Vol. 109, No. 3, 1987, pp. 234-240. 7 Yu, Q., and Hutton, D. V., "Liquid Coupled Acoustic Emission Measurement for Milling Operations," Proceedings of the 16th North American Manufacturing Research Conference, University of Illinois, 1988, pp. 403-407. 8 Shaw, M. C, Cook, N. H., and Smith, P. A., "The Mechanics of Three Dimensional Metal Cutting," Transactions of the ASME, Vol. 74, No. 6, 1952, pp. 1055-1064 Lee, E. H., and Shaffer, B. W" "The Theory of Plasticity Applied to a Problem of Machining," ASME Journal of Applied Mechanics, Vol. 73, 1954, pp. 405-413. 10 Zorev, N. N., Metal Cutting Mechanics, Pergamon Press, 1965, Chap. 4. 11 Shaw, Milton C, Metal Cutting Principles, Clarendon Press, Oxford, 1984. Effects of Geometric and Process Parameters on Introduction Drilling is economically one of the most important manufacturing operations [1], and drill structural properties have a direct bearing on the performance of the drilling process. The vibration signal generated during drilling may be used for drill wear and breakage detection [2][3][4][5]. Drill bit stability becomes important when drilling small holes or drilling at high speeds [6][7][8]. The importance of drill bit vibrations on hole quality has also been emphasized by numerous authors [9][10][11]. Hole quality in turn is important for the performance of the manufactured part. For example, in the aircraft industry, defective fastener holes are known to promote fatigue, and result in catastrophic failure of aircraft wings or other structural elements The early research in drilling concentrated on the investigation of drilling as an orthogonal cutting process Modeling The model and solution method used to obtain the results presented here has been described in detail in a previous paper One part of the computer program calculates drill bit cross sectional properties. First the user inputs the coordinates of certain points obtained from measurements on a drill cross section. For analysis purposes the drill cross section is approximated using 4 curves connecting 5 points along the boundary (see Results and Discussion Drill bit bending frequencies depend upon both process parameters, and geometric parameters. Process parameters considered here are the rotational speed and feedrate. Geometric parameters are the drill length, drill flute helix angle, drill cross sectional area, cross sectional area moments of inertias, and drill point geometry. Considered here are the drill cross sectional geometry, and the drill flute helix angle. For sensitivity studies in terms of these parameters a nominal drill with diameter D -9.525 mm is used at two different lengths, / = 0.1 m and, / = 0.2 m. All finite element solutions are obtained for clamped-pinned boundary conditions. Since rotating shafts with dissimilar moments of inertia have different frequencies for forward and reverse precession Drill Helix Angle. The helix angle, |8 0 , is one of the most important structural properties of drill bits, and its effect on the bending frequencies is shown in Drill Rotational Speed. In Figs. 6 and 7 the effect of spindle speed, a>o, on the bending frequencies of short and long drills is illustrated. As the spindle speed increases both forward and reverse precession frequencies in two orthogonal directions increase. This effect, as shown in where, / is the feedrate, H b the Brinell hardness of the workpiece, w the web thickness, D the drill diameter, and C x and C 2 are constants. For a given drill, thrust force increases with the feedrate. Feedrate affects the depth of cut, and the tool life. Hence, it is typically optimized with respect to tool life. However, for long and thin drills the feedrate is decided solely based on the buckling condition of the drill bit [6][7] Transverse Forces. To test the significance of the transverse cutting force components on drill bit vibrations a simulation is performed. The drill bits considered are subjected to a random transverse cutting force, well within the limits of transverse cutting forces typically measured while drilling. This random force is applied to the drill after lowpass filtering. Summary and Conclusions The predictions of a previously presented and validated [18] model are used to show the importance of various parameters on drill bit performance. In particular the effects of rotational speed, thrust force, helix angle, and cross sectional properties are discussed. The rotational speed has a direct effect on the stability of the drill bits, but this effect is not significant for standard spindle speeds and short drills. However, when slender drills are used higher rotational speeds are shown to cause instability. Thrust force also becomes important for long slender drills. It is shown that for long slender drills of dimensions readily available in the market, low feedrates should be used to prevent buckling. Helix angle affects drill bits by increasing the transverse stability of the drills. Typically an untwisted (/3 0 = 0 rad/m) drill will have a minimum bending stiffness that is half that of a twisted one. It is shown that the contribution of the helix angle to the bending stiffness becomes negligible after approximately 1.5 turns along the length. The effect of cross sectional properties of twist drills on bending frequencies is also investigated. It is found that, while retaining the basic geometry of the drill cross section, not much improvement can be achieved. The model of drill vibrations presented here is potentially useful for studies in drill design, process optimization, and process control. Although the model presented here is rather general, it may be desirable to incorporate some extensions to more complex drill geometries (e.g., multiple flutes). It is also desirable to combine this type of vibration model with a model of the cutting process in drilling (e.g., Acknowledgments The authors would like to thank Dr. James MacBain for his valuable contributions to the development of the model, and to acknowledge the financial support of the Industrial Technology Institute and TRW Inc. References 1 Khang, C. H., "Research Goals in High Speed Drilling," SME Paper No. MR77-384, 1977. 2 Braun, S., Lenz, E., and Wu, C. L., "Signature Analysis Applied to Drilling," ASME Paper No. 81-Det-9, 1981. 3 Yee, W. K., and Bloomquist, D. S., "On Line Method of Determining Tool Wear by Time Domain Analysis," SME Paper No. MR82-901, 1982. 4 Yee, K. W., "On Line Use of Drill-Up for On-Line Determination of Drill Wear," SME Paper, No. MS84-914, 1984. 5 Moore, T., and Reif, Z., "Using Vibration Data to Detect Drill Breakage in High Volume," SME Paper No. MS84-908, 1984. 6 Burnham, M. W., "An Analysis of Drill Deflection for Deep Miniature Holes," ASME Paper No. MR80-956, 1980. 7 Burnham, M. W., "The Mechanics of Drilling Small Holes," 10th North American Manufacturing Research Conference, Ontario, May 1983. 8 Wilson, A. J., "Failure of a Drill Near Critical Speed," ASME Paper No. 84-WA/ DE-22, 1984. 9 Fuji, H., Marui, E., and Satsoki, E., "Whirling Vibration in Drilling, Part 1: Cause of Vibration and Role of Chisel Edge," ASME Journal of Engineering for Industry, Vol. 100, No. 3, August 1986. 10 Fuji, H., Marui, E., and Satoski, E., "Whirling Vibrations in Drilling, Part 2: Influence of Drill Geometry, Particularly of the Drill Flank on the Initiation of Vibration," ASME JOURNAL OF ENGINEERING TOR INDUSTRY, Vol. 100, No. 3, August 1986. 11 Reinhall, P. G., and Storti, D. W., "Modeling and Analysis of the Dynamics of a Drill Penetrating a Thin Plate," ASME Paper No. 86-WA-33, 1986. 12 Rudd, J. L., and Gray, T. D., "Quantification of Fastener-Hole Quality," Journal of Aircraft, Vol. 15, No. 3, March 1978, pp. 143-147. 13 Renshaw, T., Wongwiwat, K., and Sarrantonio, A., "Comparison of Properties of Joints Prepared by Ultrasonic Welding and Other Means,'' Journal of Aircraft, Vol. 20, No. 6, June 1983, pp. 552-556. 14 Magrab, E., and Glisin, D. E., "Buckling Loads and Natural Frequencies of Twist Drills," ASME Paper No. 84-WA/ Prod-12, 1984. 15 Uisoy, A. G., "A Lumped Parameter Model for the Transverse Vibration of Drill Bits," inHardt, D. E., and Book, W. J., eds, Control of Manufacturing Process and Robotic Systems, ASME, 1983, pp. 15-25. 16 Uisoy, A. G., and Tekinalp, O., "Dynamic Modeling of Transverse Drill Bit Vibrations," Annals of the CIRP, Vol. 33, No. 1, 1984, pp. 253-258. 17 MacBain, J. C, Harding, K. G., and Tekinalp, O., "Vibration Modes and Frequencies of Twist Drills Using Laser Holographic Interferometry," Symposium on Sensors and Controls for Manufacturing, ASME Winter Annual Meeting, November 17-22, 1985, Miami Beach, Florida. 18 Tekinalp, O., and Uisoy, A-G., "Modeling and Finite Element Analysis of Drill Bit Vibrations," ASME Journal of Vibration, Acoustics, Stress and Reliability in Design, Vol. Ill, No. 2, April 1989, pp. 148-155. 19 Galloway, D. F., "Some Experiments on the Influence of Various Factors on Drill Performance," ASME Transactions, Vol. 77, July 1957, pp. 191-230. 20 Oxford, C. J., Jr., "On the Drilling of Metals-I Basic Mechanics of the Process," Transactions, Vol. 77, Feb. 1955, pp. 103-114. 21 Shaw, M. C, and Oxford, C. J., Jr., "On the Drilling of Metals^II Torque and Thrust in Drilling," ASME Transactions, Vol. 79, January 1957. 22 Williams, R. A., "A Study of the Drilling Process," ASME Journal of Engineering for Industry, November 1974, pp. 1207-1215 23 Oxford, C. J., "Review of Some Recent Developments in the Design and Application of Twist Drills," Advances in Machine Tool Design and Research, 1967, pp. 845-861. 24 Kaldor, S., and Lenz, E., "Drill Point Geometry and Optimization," ASME Journal of Engineering for Industry, Vol. 105, February 1982, pp. 173-18

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