Acoustic Emission Based Machining Tool Condition Monitoring – An Overview

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ABSTRACT
Acoustic emission technique (AET) has been extensively researched and found to be one of the most of the most promising techniques for condition monitoring of machine tools. AE is non-intrusive, simple in operation and fast dynamic response. This paper discusses the applicability of AET for monitoring machine tool wear and breakage. This is followed by discussions on the results of two investigations of AE monitoring during tool wear occurring in turning and face milling.
Key words: Tool condition monitoring, tool wear, Acoustic emission, Machining, Turning, Face milling.
INTRODUCTION
Tool Condition Monitoring (TCM) is an important means for achieving automation in machining operations. Cutting tools used in any typical machining operation is subjected to failure. Most tools fail either by fracturing or by gradual wear. In gradual wear, the tool will reach its limit of life by either flank wear or crater wear. Fig.1 shows the various types of wear a single point cutting tool may be subjected to. These are always present in a machining operation and have regular cutting time related growth characteristics. The other types of tool wear are classified as irregular tool wear phenomena and can be generally avoided by proper selection of tool material and cutting conditions. Breakage, fracture chipping etc., come under this category. Since in any regular machining operation gradual wear takes place of which flank wear occurring on the nose and the resulting recession on the cutting edge directly affects work piece dimension and quality. This has been considered in most of the tool condition monitoring studies.


A FEW IMPORTANT ASPECTS OF ACOUSTIC EMISSION
Acoustic Emission (AE) may be defined as the class of phenomenon where transient elastic waves are generated by the rapid release of strain energy from within a material due to a rearrangement of its internal structure. Emission occurs as a release of a series of short impulsive energy packets. The energy thus released travels as a spherical wave front and can be picked from the surface of the material using highly sensitive transducers placed on the surface of the material. The mechanical wave thus picked up is converted into electrical signal, which on suitable processing and analysis can reveal useful information about the source causing the energy release. The definition of AE as given above indicates that processes that are capable of changes in the internal structure of the material such as dislocation motion, directional diffusion creep, grain secondary sliding and twinning, which result in plastic deformation, phase transformations, vacancy coalescence and decohesion of inclusions and fracture are the sources of AE.
I. The high amplitude, somewhat erratic, low frequency type called the “burst emission”, which is generally associated with surface events, such as slip line formation and surface micro cracks.
II. The low amplitude steady and high frequency type called the “continuous emission”, that is generally associated with internal mechanism activity.



ACOUSTIC EMISSION IN MACHINING
The attractiveness of AE results from its dynamic generation during the machining processes. Fig 3(a) and 3(b) suggests that the following are the possible sources of AE in a typical single point machining processes.
a) The extensive plastic straining that occurs in the primary shear zone during chip formation (location 1).
b) Rubbing between the chip underside and the tool face, secondary shear zone (location 2).
c) Rubbing between tool flank and the newly machined work piece surface, tertiary shear zone (location 3).
d) Rubbing between chips and chip breaker (location 4).
e) Cracking of work material (location 5),
f) Collision and breakage of chip (locations 6 and 7)
g) Tool fracture and
h) Possibly cutting fluids applied in the jet form under considerable pressure.