TOOL HEALTH MONITORING Presented By: Namraz P N

INTRODUCTION Flexible machining, flexible assembly, computer-aided inspection and associated manufacturing systems are proving to be superior to conventional production systems. It is the concept of CIM that will lead us to the unmanned factory of the future. Within the CIM philosophy, the entire operation, from design through production to marketing and service, will be integrated together as a global system. The objective of this paper is to (a) highlight the importance of tool health monitoring in an automated manufacturing system, (b) summarize, and critically examine, the state-of-the-art in this area, and (c) discuss the micro computer-based tool wear monitoring system that has been developed at the University of Windsor, Canada.

ROLE OF TOOL MANAGEMENT IN FMS As a subsystem of CIM, FMS integrated machine tools and automated storage and retrieval system (AS/RS) to provide flexibility for meeting varied demands Each of the machine centers within a machining cell is normally equipped with a tool magazine consisting of a large number of different tools for a variety of operations. The principal function of TMS is to ensure the availability of the right tool at the right time and at the right station for carrying out the required machining operation.

The TMS itself includes tool health monitoring (THM) of which acoustic emission (AE), or any such method, is one of the elements. Most subsystems within FMS including tool management are centered around manufacturing process. Non-availability of a tool or its improper replacement within the subsystem will halt machining operations and other dependent process, there by hindering the smooth flow of work-in- process.

TOOL HEALTH MONITORING An interruption of TMS due to excessive tool wear/fracture could be detrimental to the entire operation. The cutting tool could very well become the weakest link in the long and complicated chain of FMS. Tool monitoring is concerned with assessing the condition of the tool as machining progresses.

It involves measuring direct or indirect parameter(s) that affect tool condition (wear and fracture) and comparing with pre-set levels to initial changes in the system. Benefits of tool health monitoring include effect tool replacement policies, improved product quality and lower tool costs.

Tool health monitoring as an element of CIM

TEHNIQUES FOR TOOL HEALTH MONITORING The sharpness of a cutting tool is one of the important parameters affecting the accuracy of the machined surface. It requires the development of highly reliable tool wear sensing technique for monitoring progressive tool wear as well as sudden breakage. The method should also be capable of responding quickly to prevent any damage. Thus, the reliability and speed of response are the critical parameters of the tool monitoring system.

TOOL WEAR CONDITION MEASUREMENT Several sensing methods have been developed for tool wear measurement. Depending on the sensors used, the methods could broadly be classified into two: 1) DIRECT 2)INDIRECT

DIRECT METHODS (a) Electrical resistance method: This method measures the flank wear by sensing the electric resistance of a thin film coated on the tool flank face. (b) Optical method: In this methods the flank wear is measured directly using an ITV camera. (c) Radioactive sensing method: In this method an exceedingly small amount of radioactive material is implanted on the tool flank at a known distance from the cutting edge. As the flank wear progresses beyond this point the radioactive material is removed. At the end of each cycle the amount of radioactivity from the tool is measured to determine whether the limiting flank wear had been reached.

(d) Contact sensing method: Measurement of the tool edge position is done using touch probes. The stylus of the probe is brought in contact with the tool surface; the change in the co-ordinates measured determines the amount of tool wear .

INDIRECT METHODS Cutting force methods: In this methods, the cutting force components and the ‘flank wear are measured and the two are plotted for any correlation. . Once a complete correlation has been established, the magnitude of the cutting force, measured while cutting, is used to predict the tool wear without interrupting machining. Cutting temperature method:The temperature at the tool-work interface is sensed directly by infrared radiation or, indirectly, by thermocouples. It was found a correlation between tool wear and the variation of temperature field. Theoretical models have also been developed which predict an increase in temperature with that in wear.

Torque and power method Torque and power method. Torque and power control monitors are also used to measure the tool wear. The current, voltage and the rotational speed of the spindle drive motor are sensed to determine drive power and torque. The variation in the power consumption, or the torque generated, is related to the growth of tool wear. Control limits are set for allowable power or torque based on the maximum tool wear acceptable. The values, measured on-line, are compared with these limits and necessary action initiated to stop the spindle rotation. Vibration method. As a tool wears, its vibration characteristics change. It is this change that is monitored in the vibration method for the measurement of tool wear. It has been found that the vibration spectral density initially decreases with increase of wear, reaches minimum corresponding to a critical tool wear, and then continues to increase.

ACOUSTIC EMISSION TECHNIQUE Acoustic Emission Technique (AET) with potential for many important applications has already become an important non-destructive resting technique. Its origin lies in the phenomenon of rapid release of energy within a material in the form of a transient elastic wave resulting from dynamic changes like deformation, crack initiation and propagation, leakage etc. It is real time technique which can detect initiation and growth of cracks.

Acoustic emissions are sound or ultrasound pulses generated by events, which are detected by transducers on the surface of the specimen, which in turn generate electrical signals. The emission is thought to originate form grain boundaries sliding over one another during stressing, from plastic deformation, from inclusion cracking, from crack growth: basically, a burst of elastic energy is emitted.

There are three basic techniques behind using acoustic emission as a non destructive testing method: (1) To use several detectors and timing circuits, together with three dimensional geometry, to locate the source of the AE, to locate where stress are causing something to happen, such as a crack propagating. (2) To monitor the rate of emission during stressing, to discover any sudden changes in rate which might be indicative of the formation of new defects, such as cracks. (3) To monitor the rate of emission and attempt to relate this to the size, of a defect, such as a propagating crack, to determine the remaining sage life of a structure.

PRINCIPLE OF ACOUSTIC EMMISSION TESTING Acoustic emission inspection detects and analyses minute AE signals generated by growing discontinuities in material under a stimulus such as stress, temperature etc. Depending on the nature of energy release, two types of AE are observed. 1. Continuous 2. Burst

BLOCK DIAGRAM OF A E EQUIPMENT

MICRO COMPUTER BASED ACOUSTIC EMMISSION Acoustic emission referes to the propagation of stress waves generated within the material when it is deformed. A basic program reads the count rate and compares the cumulative count with limiting value for the given cutting condition and gives a warning signal, if required, to the operator to stop the machining and to replace the tool.

The experimental setup of system is shown in figure The experimental setup of system is shown in figure. It consist of a piezoelecrtric transducer to pick up AE signal which is then amplified and filtered using a wide band conditioning amplifier and with filter circuit respectively. A 16 bit pulse counter was built in house as a part of the development. It is plagged in the personal computer. A program writer on BASIC will analyze the count rate and give necessary alarms for the operator.

SOURCES OF METAL CUTTING During metal cutting, AE is generated by the deformation process in the cutting zone. High frequency AE signals are emitted during tool fracture which are sensitive to the type of fracture. Five different sources of AE have been identified in orthogonal metal cutting : 1. material deformation in the shear zone during chip formation 2.chip motion, sliding and sticking along the tool rake face

3.chip breaking 4.impact of broken chips on tool/workpiece, or entanglement of continuous chips with toollworkpiece 5. tool-work rubbing ,i.e. friction on the flank face.

CHARECTERISTICS OF AE SIGNATURE The AE signals emitting during metal cutting are of two types: 1. burst 2. continuous Burst : The bust type signals are generated during discontinuous or segmented chip formation and are characterized by rapid rise time and exponential decay. Continous : The continuous -type signals are generated when continuous chips are formed. These generally represent low amplitude events and are due to visco-elastic and plastic yielding.

The severity of AE depends on strain, strain rate, and volume of deformed material which, in turn, are dependent on basic cutting parameters, like rake angle, clearance angle, feed and cutting speed. The magnitude of AE signals in the beginning of machining is low which increases gradually until failures are encountered.

RELATIVE ADVANTAGE OF AE Changes in AE signal level occur almost at the instant of tool fracture where as that in the force level occurs only after the tool has broken or chipped-off. Because of this, AE based methods can detect tool fracture quickly, thus being suitable for timely action. The frequency range of AE signals is well above that of mechanical vibrations and noises, and therefore, no chance of contamination by the latter. This is, probably the biggest attraction of AE method over others. Besides the tool wear, AE technique can also detect tool fracture. AE can show through its signature whether a chipping or fracture has taken place.

In addition to detecting the occurrence of fracture, the location of fracture can also be found out. (e) According to Iwata and Moriwaki( 1977) AE signals for different cutting materials are basically similar inspite of the differences in their mechanical properties and cutting conditions. (f) The AE transducer can easily be attached to the tool shank without being an obstacle to the cutting set-up.

DEVELOPMENT OF MICROCOMPUTER BASED AE SYSTEMS The objective was to develop a low-cost AE system so that it is affordable to small and medium-size manufacturers. The system is microcomputer-based and is complete on its own. Since the capabilities of a microcomputer are normally resident in the machining centres, these can easily be tapped, if required, for implementing this system at the shop floor. The system monitors the progressive wear of a cutting tool using the count and count rate method for the analysis of AE.

It consists of a piezo-electric transducer to pickup the AE signal which is then amplified and filtered using a wide-band conditioning amplifier. A 16-bit pulse counter was built in-house as a part of the development. It is plugged into the computer. A program written on BASIC will analyse the cant rate and give necessary alarms for the operator.

CONCLUSION The present shop-floor practice of tool replacement based on fixed tool life may not be the most economic since a tool can get replaced prematurely or, in the other extreme, only after damage has been done. There is obviously a need for on-line monitoring of tool-condition. This has become more important within the context of computer-integrated manufacturing. The state-of-the-art in tool wear monitoring in general, and that of the AE technique in particular, has also been discussed.

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REFERENCES Non Destructive Testing Techniques by C.V Subramanyan Investigation of Orthogonal cutting via AE signal analysis by Dorn Feld.D.A Tool manufacturing Engineers by Drozda.T.J www.howstuffworks.com